1 Introduction

The most useful model to describe the heat conduction is based on the Fourier law that proposes a linear relation between the heat flux vector and the gradient of temperature. If we combine this equation with the usual energy equation, we obtain the existence of thermal waves propagating with an unbounded speed. That is, a thermal perturbation at one point is instantaneously felt at any other point of the space for every distance. It is clear that this effect contradicts the causality principle. For this reason, a big deal trying to overcome this paradox has been developed in the last and current centuries. It seems that the first works in this aspect correspond to Cattaneo and Maxwell (Cattaneo 1958). They proposed the introduction of a relaxation time in the Fourier law. Recently, in the 1990s decade, Green and Naghdi proposed several alternative models (Green and Naghdi 1992, 1993). In fact, they proposed these new theories in the context of the thermoelasticity and the main difference concerning the classical theory corresponds to the thermal effects. The most general is the so-called type III and it contains the Fourier model as a limit case. It is also worth recalling the type II which is also called without energy dissipation. It also corresponds to another limit case of the type III theory.

A big interest has also been developed to understand models with microstructure. In fact, Eringen (1999) contributed in an important way in this sense in the last century. An interesting case for these models corresponds to those where microtemperatures are taken into account. That is, among the microstructure effects, we can consider the microtemperatures. First contribution on this kind of materials came back to the one by Grot (1969) and some people used them to study several problems (Riha 1975, 1976; Verma et al. 1979). We recall the contribution (Iesan and Quintanilla 2000) as a new reborn of the interest for this kind of questions, because many works have studied this kind of problems recently (see (Casas and Quintanilla 2005a; Chirita et al. 2013; Ciarletta et al. 2010; Ieşan 2007; Iesan 2018; Iesan and Quintanilla 2018; Magaña and Quintanilla 2018; Quintanilla 2011, 2013) among others). In the last 20 years, there has been a big deal of people interested in the study of elastic materials with microtemperatures.

Cowin (1985), Cowin and Nunziato (1983), Nunziato and Cowin (1979) proposed a mathematical theory to model elastic materials with voids. Since these contributions, many people have been interested in the study of thermoelastic materials with voids and the quantity of contributions involving this model is huge (Bazarra and Fernández 2018; Casas and Quintanilla 2005b; Feng and Apalara 2019; Feng and Yin 2019; Fernández and Masid 2016, 2017a, b; Ieşan and Quintanilla 2014; Kumar and Vohra 2017, 2019; Kumar et al. 2016; Magaña and Quintanilla 2006, 2017; Ramos et al. 2020). It is worth noting that the model has become useful to understand the behavior of elastic materials with small distributed pores and we can find them in the study of biological materials as bones or in the study of soils, woods, ceramics, or rocks. It is also worth noting the structural similarity (from the mathematical point of view) of the system of equations for the poro-elasticity with the equations of the Timoshenko beam (see, for instance, Almeida Júnior and Ramos 2017).

In the present paper, we want to joint these three basic ideas: on one side, we consider the type III theory; on the second aspect, we consider microtemperatures; and on the third side, we consider porous aspects. First contribution concerning the three aspects at the same time can be seen at Magaña and Quintanilla (2020). There, the authors consider the system of equations that we can obtain from the studies (Aouadi et al. 2018; Ieşan 2007; Ieşan and Quintanilla 2009). Here, we continue the research started in Magaña and Quintanilla (2020), introducing a fully discrete approximation based on the finite-element method and the implicit Euler scheme, proving a discrete stability property and a priori error estimates, and performing some one- and two-dimensional numerical simulations to demonstrate the accuracy and the behavior of the discrete solutions.

We think that it is relevant to point out that the behavior of the thermoelastic materials in the context of the type III theory has been revealed different from the classical theory based on the Fourier law. We can cite several contributions (Leseduarte et al. 2010; Magaña and Quintanilla 2018; Miranville and Quintanilla 2020, 2019; Magaña and Quintanilla 2020) where we have detected relevant differences in the behavior of the solutions corresponding to this kind of materials. The main reason is that, when we consider type III theory, new coupling terms appear which are not present when we consider the theory based on the classical Fourier law. At the same time, when we consider microtemperatures, there are also more new coupling terms which are not present in the case of the Fourier theory with microtemperatures. Therefore, our system is more complex from the mathematical point of view, and then, new and strong difficulties could appear when we consider the new theory. Furthermore, what we will develop here cannot be a direct extension of the classical theory, but we could consider new aspects in our study.

2 Mathematical and variational formulations

First, we describe the problem (see Magaña and Quintanilla 2020 for further details). Let \(\varOmega \) be a bounded domain in \(\mathbb {R}^d\) (for \(d=1,2,3\)) with boundary smooth enough to allow the application of the divergence theorem. We will use the standard notation where “, i” means the partial derivative with respect to the variable \(x_i\), a superposed dot represents time derivative and summation on repeated indices is assumed. Moreover, let [0, T], \(T>0\), be the time interval of interest.

Let us denote \(\varvec{u}=(u_i)_{i=1}^d\), \(\varphi \), \(\theta \), and \(\varvec{M}=(M_i)_{i=1}^d\) the displacement, the volume fraction, the temperature, and the microtemperatures, respectively.

Since we are interested in the thermoelastic theory of type III with voids and microtemperatures, the corresponding thermo-mechanical problem is the following (see Aouadi et al. 2018; Iesan and Quintanilla 2000; Ieşan and Quintanilla 2009; Magaña and Quintanilla 2020):

Problem

P. Find the displacement \(\varvec{u}:\overline{\varOmega }\times [0,T]\rightarrow \mathbb {R}^d\), the volume fraction \(\varphi :\overline{\varOmega }\times [0,T]\rightarrow \mathbb {R}\), the thermal displacement \(\tau :\overline{\varOmega }\times [0,T]\rightarrow \mathbb {R}\), and the microthermal displacement \(\varvec{R}:\overline{\varOmega }\times [0,T]\rightarrow \mathbb {R}^d\), such that:

$$\begin{aligned}&\displaystyle \rho \ddot{u}_i = (A_{ijkl} u_{k,l} -a_{ij} \theta +\zeta _{ij}\varphi + B_{ijkl} R_{k,l})_{,j}\quad \hbox {in}\quad \varOmega \times (0,T)\nonumber \\& \quad \hbox {for }i=1,\ldots ,d,\end{aligned}$$
(1)
$$\begin{aligned}&\displaystyle J \ddot{\varphi }= (A_{ij} \varphi _{,j}- \alpha _{ij}\dot{R}_i +H_{ij} \tau _{,i} )_{,j} - \zeta _{ij} u_{i,j} + \kappa \dot{\tau }- F_{ij} R_{i,j} - \xi \varphi \nonumber \\&\quad \hbox {in}\quad \varOmega \times (0,T), \end{aligned}$$
(2)
$$\begin{aligned}&\displaystyle c \ddot{\tau }= -a_{ij} \dot{u}_{i,j} + (H_{ij} \varphi _{,i})_{,j}- (d_{ij} \dot{R}_i)_{,j}+ (K_{ij} \tau _{,i} + K^*_{ij} \dot{\tau }_{,i})_{,j}- b_{ij}\dot{R}_{i,j} \nonumber \\&\displaystyle \qquad \qquad -\kappa \dot{\varphi }+ (A^{1}_{ij} M_i)_{,j} \quad \hbox {in}\quad \varOmega \times (0,T),\end{aligned}$$
(3)
$$\begin{aligned}&\displaystyle c_{ij}\ddot{R}_{j}= (B_{klij} u_{k,l} - b_{ij} \dot{\tau }+ F_{ij} \varphi +C_{ijkl} R_{k,l})_{,j} - d_{ij} \dot{\tau }_{,j} + (C^*_{ijkl} \dot{R}_{k,l})_{,j} \nonumber \\&\displaystyle \qquad \qquad - \alpha _{ij} \dot{\varphi }_{,j} - A^{2}_{ij} \theta _{,j} - A^{3}_{ij} M_j \quad \hbox {in}\quad \varOmega \times (0,T) \quad \hbox {for }i=1,\ldots ,d, \end{aligned}$$
(4)
$$\begin{aligned}&\displaystyle u_i(\varvec{x},0)=u_i^0(\varvec{x}),\quad \dot{u}_i(\varvec{x},0)=v_i^0(\varvec{x}),\quad \varphi (\varvec{x},0)=\varphi ^0(\varvec{x}) \text { for } \varvec{x}\in \varOmega ,\end{aligned}$$
(5)
$$\begin{aligned}&\displaystyle \dot{\varphi }(\varvec{x},0)=e^0(\varvec{x}),\quad \tau (\varvec{x},0)=\tau ^0(\varvec{x}),\quad \dot{\tau }(\varvec{x},0)=\theta ^0(\varvec{x})\text { for } \varvec{x}\in \varOmega ,\end{aligned}$$
(6)
$$\begin{aligned}&R_i(\varvec{x},0)=R_i^0(\varvec{x}), \quad \dot{R_i}(\varvec{x},0)=M_i^0(\varvec{x}) \text { for } \varvec{x}\in \varOmega ,\end{aligned}$$
(7)
$$\begin{aligned}&\displaystyle u_i(\varvec{x},t)=\varphi (\varvec{x},t)=\tau (\varvec{x},t)=R_i(\varvec{x},t)=0\,\, \text { for } \,\,\varvec{x}\in \partial \varOmega , \,\, t\in [0,T]. \end{aligned}$$
(8)

Here, \(\tau \) is the thermal displacement introduced by Green and Naghdi and \(\varvec{R}=(R_i)_{i=1}^d\) are the microthermal displacements, defined, respectively, by:

$$\begin{aligned} \tau (\varvec{x},t)= \tau ^0(\varvec{x}) + \int _0^t \theta (\varvec{x},s) \, \mathrm{d}s,\quad R_i(\varvec{x},t)=R_i^0(\varvec{x}) + \int _0^t M_i(\varvec{x},s) \, \mathrm{d}s. \end{aligned}$$

As usual, \(\rho \) denotes the mass density, J the product of the mass density by the equilibrated inertia, and c the thermal capacity. \(A_{ijkl}\) is the elastic tensor, and \(a_{ij}\), \(\zeta _{ij}\), and \(B_{ijkl}\) are, respectively, the coupling tensors between the displacement and the temperature, the displacement and the volume fraction, and the displacement and the microtemperatures. \(A_{ij}\), \(A^{(1)}_{ij}\), \(A^{(2)}_{ij}\), \(A^{(3)}_{ij}\), \(\alpha _{ij}\), \(H_{ij}\), \(F_{ij}\), \(d_{ij}\), and \(b_{ij}\) are other coupling tensors between the variables. \(K_{ij}\) is the tensor introduced by Green and Naghdi, and it is usually called rate conductivity, \(K_{ij}^*\) is the thermal conductivity tensor, \(c_{ij}\) is a typical tensor of the theories with microtemperatures, and, finally, \(C_{ijkl}\) and \(C_{ijkl}^*\) are the specific type III tensors with microtemperatures.

The following symmetries are assumed (see Aouadi et al. 2018; Magaña and Quintanilla 2020):

$$\begin{aligned} \left. \begin{array}{l} A_{ijkl}=A_{klij},\quad A_{ij}=A_{ji},\quad K_{ij}=K_{ji}, \quad K^*_{ij}=K^*_{ji}, \quad C_{ijkl}=C_{klij},\\ C^*_{ijkl}=C^*_{klij}, \quad c_{ij}=c_{ji}, \quad A^3_{ij}=A^3_{ji},\quad H_{ij}=H_{ji},\quad B_{ijkl}=B_{klij}. \end{array}\right\} \end{aligned}$$
(9)

From the second law of thermodynamics the following inequality must be satisfied (see Iesan and Quintanilla 2000):

$$\begin{aligned} K^*_{i,j} \xi _i \xi _j + (A^1_{ij}+A^2_{ij}) \eta _i \xi _j + A^3_{ij}\eta _i \eta _j + C^*_{ijkl} \eta _{ij} \eta _{kl} \ge K_0 (\xi _i \xi _i + \eta _i \eta _i + \eta _{ij} \eta _{ij}), \end{aligned}$$
(10)

for a positive constant \(K_0\) and for each pair of vectors \(\xi _i\) and \(\eta _i\) and for each tensor \(\eta _{ij}\).

We will also impose some assumptions over the constitutive coefficients. For each vector \(\xi _i\), each pair of tensors \(\xi _{ij}\) and \(\eta _{ij}\) and each real number l, the following inequalities are assumed:

$$\begin{aligned} \left. \begin{array}{l} A_{ijkl}\xi _{ij} \xi _{kl}+2 B_{ijkl} \xi _{ij}\eta _{kl}+ C_{ijkl} \eta _{ij}\eta _{kl} +2 \zeta _{ij} \xi _{ij} l + \xi l^2 \\ \qquad \ge C_0 (\xi _{ij}\xi _{ij} +\eta _{ij}\eta _{ij}+ l^2),\\ A_{ij} \xi _i \xi _j + 2 H_{ij} \xi _{i}\eta _j + K_{ij} \eta _i \eta _j \ge C_1(\xi _i \xi _i + \eta _i \eta _j ), \\ c_{ij} \xi _{i}\xi _{j} \ge C_2 \xi _{i} \xi _{i},\quad \rho \ge \rho _0>0,\quad J\ge J_0>0, \quad c\ge c_0 >0,\\ \end{array}\right\} \end{aligned}$$
(11)

for positive constants \(J_0, c_0, C_0, C_1, C_2\), and \(\rho _0\). The first two conditions proposed here can be interpreted with the help of the stability theory for thermoelastic materials. The physical meaning of the assumptions in the third line of (11) is clear.

First, we show that the energy of the system is dissipative.

Proposition 1

Let us define the energy of the system \(\mathcal {E}(t)\) as follows:

$$\begin{aligned} \displaystyle \mathcal {E}(t)= & {} \frac{1}{2}\Big \{\rho (v_i(t),v_i(t))_Y+J\Vert e(t)\Vert _Y^2+c\Vert \theta (t)\Vert _Y^2+ (c_{ij}M_j(t),M_i(t))_Y \nonumber \\&+(A_{ijkl}u_{i,j}(t),u_{k,l}(t))_Y +(B_{ijkl}u_{i,j}(t),R_{k,l}(t))_Y+\xi \Vert \varphi (t)\Vert _Y^2 \nonumber \\&+(C_{ijkl}R_{i,j}(t),R_{k,l}(t))_Y+(\zeta _{ij}u_{i,j}(t),\varphi (t))_Y +(A_{ij}\varphi _{,i}(t),\varphi _{,j}(t))_Y\nonumber \\&+ (H_{ij}\varphi _{,i}(t),\tau _{,j}(t))_Y+(K_{ij}\tau _{,i}(t),\tau _{,j}(t))_Y\Big \}, \end{aligned}$$
(12)

where we have used the notation \(Y=L^2(\varOmega )\) and \((\cdot ,\cdot )_Y\) for the usual scalar product in this space. Then, this energy is dissipative.

Proof

We note that, from the previous definition after a direct calculation, we find that:

$$\begin{aligned}&\displaystyle \mathcal {E}'(t)=-\int _\varOmega \Big (K_{ij}^*\theta _{,i}\theta _{,j}+(A_{ij}^1+A_{ij}^2)M_i\theta _{,j}+A_{ij}^3M_iM_j \Big )\, \mathrm{d}v\\&\quad \displaystyle -\int _{\varOmega }C_{ijkl}^*M_{i,j}M_{k,l}\, \mathrm{d}v, \end{aligned}$$

and using assumption (10), we then conclude that the energy is always dissipative.

Now, we recall the following existence and uniqueness result (Magaña and Quintanilla 2020).

Theorem 1

Under assumptions (9)–(11), if the following regularity on the initial conditions hold:

$$\begin{aligned} \varvec{u}^0,\varvec{v}^0,\,\varvec{R}^0,\,\varvec{M}^0\in [H^2(\varOmega )]^d,\quad \varphi ^0,\, e^0,\,\tau ^0,\,\theta ^0\in H^2(\varOmega ), \end{aligned}$$

then there exists a unique solution to Problem P with the regularity:

$$\begin{aligned} \varvec{u},\, \varvec{R}\in C^1([0,T];V)\cap C^2([0,T];H),\quad \varphi ,\,\tau \in C^1([0,T];E)\cap C^2([0,T];Y). \end{aligned}$$

To obtain the exponential decay of the solutions to Problem P, we will assume that, for every tensor \(\xi _{ij}\) and every vector \(\zeta _i\):

$$\begin{aligned} B_{klij} \xi _{kl} \xi _{ij} \ge C \xi _{ij} \xi _{ij}, \quad H_{ij} \zeta _{i} \zeta _j \ge C^* \zeta _i \zeta _i, \end{aligned}$$
(13)

for two positive constants C and \(C^*\).

Even if the above assumptions are quite natural, we need to impose also two more technical conditions on some of the tensors. Let us suppose that there exist two constants, \(m_1\) and \(m_2\), such that:

$$\begin{aligned} a_{ij} = m_1 \zeta _{ij} \quad \text { and } \quad \alpha _{ij}=m_2 \zeta _{ij}. \end{aligned}$$
(14)

Notice that, for isotropic and homogeneous materials, assumptions (14) are satisfied whenever the corresponding constitutive parameter is different from zero, because, in this case, \(\zeta _{ij}=\zeta \delta _{ij}\) for a constant \(\zeta \ne 0\) (\(\delta _{ij}\) denotes the Kronecker delta).

Therefore, we have the following (see Magaña and Quintanilla 2020).

Theorem 2

Under the assumptions of Theorem 1 and (13)–(14), the solution to Problem P is asymptotically stable; that is, there exist two positive constants M and \(\alpha \), such that:

$$\begin{aligned} \Vert \mathcal {E}(t)\Vert \le M \Vert \mathcal {E}(0)\Vert e^{-\alpha t}, \end{aligned}$$

where the energy of the system \(\mathcal {E}\) was defined in (12).

Finally, to provide the numerical approximation of Problem P in the next section, we will obtain the variational formulation of this problem. Thus, let \(H=[L^2(\varOmega )]^d\) and \(Q=[L^2(\varOmega )]^{d\times d}\), and denote by \((\cdot ,\cdot )_H\) and \((\cdot ,\cdot )_Q\) the respective scalar products in these spaces, with corresponding norms \(\Vert \cdot \Vert _H\) and \(\Vert \cdot \Vert _Q\). Moreover, let us define the variational spaces \(E=H^1_0(\varOmega )\) and \(V=[H^1_0(\varOmega )]^d\).

Then, applying Green’s formula to Eqs. (1)–(4) and using boundary conditions (8), we have the following weak problem.

Problem

VP. Find the velocity \(\varvec{v}:[0,T]\rightarrow V\), the volume fraction speed \(e:[0,T]\rightarrow E\), the temperature \(\theta :[0,T]\rightarrow E\), and the microtemperatures \(\varvec{M}:[0,T]\rightarrow V\), such that \(\varvec{v}(0)=\varvec{v}^0\), \(e(0)=e^0\), \(\theta (0)=\theta ^0\), \(\varvec{M}(0)=\varvec{M}^0\) and, for a.e. \(t\in (0,T)\):

$$\begin{aligned}&\rho (\dot{v}_i(t) ,w_i)_Y +(A_{ijkl}u_{k,l} (t), w_{i,j})_Y= (a_{ij}\theta (t),w_{i,j})_Y -(B_{ijkl}R_{k,l}(t),w_{i,j})_Y \nonumber \\&\qquad -(\zeta _{ij}\varphi (t),w_{i,j})_Y \quad \forall \varvec{w}=(w_i)_{i=1}^d\in V,\end{aligned}$$
(15)
$$\begin{aligned}&J(\dot{e}(t),r)_Y+(A_{ij}\varphi _{,j}(t),r_{,i})_Y+\xi (\varphi (t),r)_Y=(\alpha _{ij}M_i(t),r_{,j})_Y+\kappa (\theta (t),r)_Y\nonumber \\&\qquad -(H_{ij}\tau _{,i}(t),r_{,j})_Y -(\zeta _{ij}u_{i,j}(t),r)_Y-(F_{ij}R_{i,j}(t),r)_Y\quad \forall r\in E, \end{aligned}$$
(16)
$$\begin{aligned}&c(\dot{\theta }(t),z)_Y+(K_{ij}^*\theta _{,i}(t),z_{,j})_Y+(K_{ij}\tau _{,i}(t),z_{,j})_Y =(d_{ij}M_i(t),z_{,j})_Y \nonumber \\&\qquad - (A_{ij}^1M_i(t),z_{,j})_Y -(b_{ij}M_{i,j}(t),z)_Y-(a_{ij}v_{i,j}(t),z)_Y-\kappa (e(t),z)_Y \nonumber \\&\qquad -(H_{ij}\varphi _{,i}(t),z_{,j})_Y\quad \forall z\in E,\end{aligned}$$
(17)
$$\begin{aligned}&(c_{ij} \dot{M}_j(t),\xi _i)_Y+(C_{ijkl} R_{k,l}(t),\xi _{i,j})+(C_{ijkl}^*M_{k,l}(t),\xi _{i,j})_Y=(b_{ij}\theta (t),\xi _{i,j})_Y \nonumber \\&\qquad -(B_{klij}u_{k,l}(t),\xi _{i,j})_Y -(F_{ij}\varphi (t),\xi _{i,j})_Y-(d_{ij}\theta _{,j} (t),\xi _i)_Y-(\alpha _{ij}e_{,j}(t),\xi _{i})_Y \nonumber \\&\qquad -(A_{ij}^2\theta _{,j}(t),\xi _i)_Y-(A_{ij}^3 M_j (t),\xi _i)_Y\quad \forall \varvec{\xi }=(\xi _i)_{i=1}^d\in V, \end{aligned}$$
(18)

where we recall that the displacement, the volume fraction, the thermal displacement, and the microthermal displacements are then recovered from relations:

$$\begin{aligned}&\varvec{u}(t)=\int _0^t\varvec{v}(s)\, \mathrm{d}s+ \varvec{u}^0,\quad \varphi (t)=\int _0^t e(s)\, \mathrm{d}s+\varphi ^0,\end{aligned}$$
(19)
$$\begin{aligned}&\tau (t)=\int _0^t \theta (s)\, \mathrm{d}s+\tau ^0,\quad \varvec{R}(t)=\int _0^t\varvec{M}(s)\, \mathrm{d}s+ \varvec{R}^0. \end{aligned}$$
(20)

3 Fully discrete approximations: an a priori error analysis

In this section, we now consider a fully discrete approximation of Problem VP. This is done in two steps. First, we assume that the domain \(\overline{\varOmega }\) is polyhedral and we denote by \({\mathcal {T}}^h\) a regular triangulation in the sense of Ciarlet (1993). Thus, we construct the finite-dimensional spaces \(V^h\subset V\) and \(E^h\subset E\) given by:

$$\begin{aligned}& V^h=\{\varvec{z}^h\in [C(\overline{\varOmega })]^d \; ; \; \varvec{z}^h_{|Tr}\in [P_1(Tr)]^d\quad \forall Tr\in \mathcal {T}^h, \quad \varvec{z}^h=\varvec{0}\quad \hbox {on}\quad \partial \varOmega \},\end{aligned}$$
(21)
$$\begin{aligned}& E^h=\{\eta ^h\in C(\overline{\varOmega }) \; ; \; \eta ^h_{|Tr}\in P_1(Tr) \quad \forall Tr\in \mathcal {T}^h, \quad \eta ^h=0 \quad \hbox {on}\quad \partial \varOmega \}, \end{aligned}$$
(22)

where \(P_1(Tr)\) represents the space of polynomials of degree less or equal to one in the element Tr, i.e., the finite-element spaces \(V^h\) and \(E^h\) are composed of continuous and piecewise affine functions. Here, \(h>0\) denotes the spatial discretization parameter. Moreover, we assume that the discrete initial conditions, denoted by \(\varvec{u}^{0h}\), \(\varvec{v}^{0h}\), \(\varphi ^{0h}\), \(e^{0h}\), \(\tau ^{0h}\), \(\theta ^{0h}\), \(\varvec{R}^{0h}\) and \(\varvec{M}^{0h}\), are given by:

$$\begin{aligned} \begin{array}{l} \varvec{u}^{0h}=\mathcal {P}_1^{h} \varvec{u}^0,\quad \varvec{v}^{0h}=\mathcal {P}_1^{h} \varvec{v}^0,\quad \varphi ^{0h}=\mathcal {P}_2^{h} \varphi ^0,\quad e^{0h}=\mathcal {P}_2^{h} e^0,\\ \tau ^{0h}=\mathcal {P}_2^{h} \tau ^0,\quad \theta ^{0h}=\mathcal {P}_2^{h} \theta ^0,\quad \varvec{R}^{0h}=\mathcal {P}_1^{h} \varvec{R}^0,\quad \varvec{M}^{0h}=\mathcal {P}_1^{h} \varvec{M}^0, \end{array} \end{aligned}$$
(23)

where \(\mathcal {P}_1^{h}\) and \(\mathcal {P}_2^{h}\) are the classical finite-element interpolation operators over \(V^h\) and \(E^h\), respectively (see, e.g., Ciarlet 1993).

Second, we consider a partition of the time interval [0, T], denoted by \(0=t_0<t_1<\cdots < t_N=T\). In this case, we use a uniform partition with step size \(k=T/N\) and nodes \(t_n=n\,k\) for \(n=0,1,\dots ,N\). For a continuous function z(t), we use the notation \(z_n=z(t_n)\) and, for the sequence \(\{z_n\}_{n=0}^N\), we denote by \(\delta z_n=(z_n-z_{n-1})/k \) its corresponding divided differences.

Therefore, using the backward Euler scheme, the fully discrete approximations are considered as follows.

Problem

VP\(^{hk}\). Find the discrete velocity \(\varvec{v}^{hk}=\{\varvec{v}^{hk,n}\}_{n=0}^N\subset V^h\), the discrete volume fraction speed \(e^{hk}=\{e^{hk,n}\}_{n=0}^N\subset E^h\), the temperature \(\theta ^{hk}=\{\theta ^{hk,n}\}_{n=0}^N\subset E^h\), and the microtemperatures \(\varvec{M}^{hk}=\{\varvec{M}^{hk,n}\}_{n=0}^N\subset V^h\), such that \(\varvec{v}^{hk,0}=\varvec{v}^{0h}\), \(e^{hk,0}=e^{0h}\), \(\theta ^{hk,0}=\theta ^{0h}\), \(\varvec{M}^{hk,0}=\varvec{M}^{0h}\), and, for \(n=1,\ldots , N\):

$$\begin{aligned}&\rho (\delta v^{hk,n}_i ,w_i^h)_Y +(A_{ijkl}u_{k,l}^{hk,n} , w_{i,j}^h)_Y= (a_{ij}\theta ^{hk,n},w_{i,j}^h)_Y -(B_{ijkl}R_{k,l}^{hk,n},w_{i,j}^h)_Y \nonumber \\&\qquad -(\zeta _{ij}\varphi ^{hk,n},w_{i,j}^h)_Y \quad \forall \varvec{w}^h=(w_i^h)_{i=1}^d\in V^h,\end{aligned}$$
(24)
$$\begin{aligned}&J(\delta {e}^{hk,n},r^h)_Y+(A_{ij}\varphi ^{hk,n}_{,j},r_{,i}^h)_Y+\xi (\varphi ^{hk,n},r^h)_Y=(\alpha _{ij}M^{hk,n}_i,r^h_{,j})_Y \nonumber \\&\qquad -(H_{ij}\tau ^{hk,n}_{,i},r^h_{,j})_Y -(\zeta _{ij}u^{hk,n}_{i,j},r^h)_Y+\kappa (\theta ^{hk,n},r^h)_Y \nonumber \\&\qquad -(F_{ij}R^{hk,n}_{i,j},r^h)_Y\quad \forall r^h\in E^h,\end{aligned}$$
(25)
$$\begin{aligned}&c(\delta {\theta }^{hk,n},z^h)_Y+(K_{ij}^*\theta ^{hk,n}_{,i},z_{,j}^h)_Y+(K_{ij}\tau ^{hk,n}_{,i},z^h_{,j})_Y =(d_{ij}M^{hk,n}_i,z^h_{,j})_Y \nonumber \\&\qquad - (A_{ij}^1M^{hk,n}_i,z^h_{,j})_Y-(b_{ij}M^{hk,n}_{i,j},z^h)_Y -(a_{ij}v^{hk,n}_{i,j},z^h)_Y-\kappa (e^{hk,n},z^h)_Y \nonumber \\&\qquad -(H_{ij}\varphi ^{hk,n}_{,i},z^h_{,j})_Y\quad \forall z^h\in E^h,\end{aligned}$$
(26)
$$\begin{aligned}&(c_{ij} \delta M^{hk,n}_j,\xi ^h_i)_Y+(C_{ijkl} R^{hk,n}_{k,l},\xi _{i,j}^h)+(C_{ijkl}^*M^{hk,n}_{k,l},\xi ^h_{i,j})_Y+(A_{ij}^3 M^{hk,n}_j ,\xi ^h_i)_Y \nonumber \\&\qquad = (b_{ij}\theta ^{hk,n},\xi ^h_{i,j})_Y -(F_{ij}\varphi ^{hk,n},\xi _{i,j}^h)_Y-(d_{ij}\theta ^{hk,n}_{,j} ,\xi ^h_i)_Y-(\alpha _{ij}e^{hk,n}_{,j},\xi ^h_{i})_Y \nonumber \\&\qquad -(A_{ij}^2\theta ^{hk,n}_{,j},\xi ^h_i)_Y -(B_{klij}u^{hk,n}_{k,l},\xi ^h_{i,j})_Y\quad \forall \varvec{\xi }^h=(\xi ^h_i)_{i=1}^d\in V^h, \end{aligned}$$
(27)

where the discrete displacement, the discrete volume fraction, the discrete thermal displacement, and the discrete microthermal displacement are then recovered from relations:

$$\begin{aligned}&\varvec{u}^{hk,n}=k\sum _{j=1}^n \varvec{v}^{hk,j}+ \varvec{u}^{0h},\quad \varphi ^{hk,n}=k\sum _{j=1}^n e^{hk,j}+\varphi ^{0h},\end{aligned}$$
(28)
$$\begin{aligned}&\tau ^{hk,n}=k\sum _{j=1}^n\theta ^{hk,j}+\tau ^{0h},\quad \varvec{R}^{hk,n}=k\sum _{j=1}^n \varvec{M}^{hk,j}+ \varvec{R}^{0h}. \end{aligned}$$
(29)

The existence of a unique solution to Problem VP\(^{hk}\) can be easily proved using Lax–Milgram lemma and taking into account assumptions (11)–(14).

The aim of this section is to provide the numerical analysis of Problem VP. First, we have the following discrete stability result.

Lemma 1

Under the assumptions of Theorem 2, it follows that the sequences \(\{\varvec{u}^{hk},\varvec{v}^{hk},\varphi ^{hk},e^{hk},\tau ^{hk},\theta ^{hk},\varvec{R}^{hk}, \varvec{M}^{hk}\}\) generated by Problem \(VP^{hk}\) satisfy the stability estimate:

$$\begin{aligned}&\Vert \varvec{v}^{hk,n}\Vert ^2_H+\Vert \nabla \varvec{u}^{hk,n}\Vert _Q^2+ \Vert e^{hk,n}\Vert ^2_Y+ \Vert \nabla \varphi ^{hk,n}\Vert _H^2+\Vert \varphi ^{hk,n}\Vert _Y^2+ \Vert \theta ^{hk,n}\Vert ^2_Y\\&\quad \displaystyle +\Vert \nabla \tau ^{hk,n}\Vert _H^2+\Vert \varvec{M}^{hk,n}\Vert ^2_H +\Vert \nabla \varvec{R}^{hk,n}\Vert _Q^2\le C, \end{aligned}$$

where C is a positive constant assumed to be independent of the discretization parameters h and k.

Proof

First, if we take as a test function \(w_i^h=v_i^{hk,n}\) in discrete variational equation (24), we find that:

$$\begin{aligned}&\rho (\delta v^{hk,n}_i ,v^{hk,n}_i)_Y +(A_{ijkl}u_{k,l}^{hk,n} , v^{hk,n}_{i,j})_Y= (a_{ij}\theta ^{hk,n},v^{hk,n}_{i,j})_Y \\&\quad -(B_{ijkl}R_{k,l}^{hk,n},v^{hk,n}_{i,j})_Y -(\zeta _{ij}\varphi ^{hk,n},v^{hk,n}_{i,j})_Y. \end{aligned}$$

Thus, taking into account that:

$$\begin{aligned} \displaystyle (\delta v^{hk,n}_i ,v^{hk,n}_i)_Y\ge \frac{1}{2k}\left\{ \Vert \varvec{v}^{hk,n}\Vert ^2_H-\Vert \varvec{v}^{hk,n-1}\Vert _H^2\right\} , \end{aligned}$$

we find that:

$$\begin{aligned}& \displaystyle \frac{\rho }{2k}\left\{ \Vert \varvec{v}^{hk,n}\Vert ^2_H-\Vert \varvec{v}^{hk,n-1}\Vert _H^2\right\} +\frac{1}{2k}\Big \{(A_{ijkl}u_{k,l}^{hk,n} , u^{hk,n}_{i,j})_Y-(A_{ijkl}u_{k,l}^{hk,n-1} , u^{hk,n-1}_{i,j})_Y\nonumber \\&\displaystyle \qquad \qquad + (A_{ijkl}(u_{k,l}^{hk,n}-u_{k,l}^{hk,n-1}), u^{hk,n}_{i,j}-u_{i,j}^{hk,n-1})_Y\Big \} \nonumber \\&\qquad = (a_{ij}\theta ^{hk,n},v^{hk,n}_{i,j})_Y -(B_{ijkl}R_{k,l}^{hk,n},v^{hk,n}_{i,j})_Y -(\zeta _{ij}\varphi ^{hk,n},v^{hk,n}_{i,j})_Y. \end{aligned}$$
(30)

Second, taking \(r^h=e^{hk,n}\) as a test function in (25), we have:

$$\begin{aligned}&J(\delta {e}^{hk,n},e^{hk,n})_Y+(A_{ij}\varphi ^{hk,n}_{,j},e^{hk,n}_{,i})_Y+\xi (\varphi ^{hk,n},e^{hk,n})_Y=(\alpha _{ij}M^{hk,n}_i,e^{hk,n}_{,j})_Y\\&\quad -(H_{ij}\tau ^{hk,n}_{,i},e^{hk,n}_{,j})_Y -(\zeta _{ij}u^{hk,n}_{i,j},e^{hk,n})_Y+\kappa (\theta ^{hk,n},e^{hk,n})_Y-(F_{ij}R^{hk,n}_{i,j},e^{hk,n})_Y, \end{aligned}$$

and using the estimates:

$$\begin{aligned} J(\delta {e}^{hk,n},e^{hk,n})_Y\ge \frac{J}{2k}\left\{ \Vert e^{hk,n}\Vert ^2_Y-\Vert e^{hk,n-1}\Vert _Y^2\right\} , \end{aligned}$$

we obtain:

$$\begin{aligned}& \displaystyle \frac{J}{2k}\left\{ \Vert e^{hk,n}\Vert ^2_Y-\Vert e^{hk,n-1}\Vert _Y^2\right\} +\frac{1}{2k}\Big \{ (A_{ij}\varphi ^{hk,n}_{,j},\varphi ^{hk,n}_{,i})_Y- (A_{ij}\varphi ^{hk,n-1}_{,j},\varphi ^{hk,n-1}_{,i})_Y\nonumber \\&\qquad + (A_{ij}(\varphi ^{hk,n}_{,j}-\varphi ^{hk,n-1}_{,j}),\varphi ^{hk,n}_{,i}-\varphi ^{hk,n-1}_{,i})_Y\Big \} \nonumber \\&\qquad +\frac{\xi }{2k}\Big \{\Vert \varphi ^{hk,n}\Vert _Y^2-\Vert \varphi ^{hk,n-1}\Vert _Y^2+\Vert \varphi ^{hk,n}-\varphi ^{hk,n-1}\Vert _Y^2\Big \} \nonumber \\&\quad =(\alpha _{ij}M^{hk,n}_i,e^{hk,n}_{,j})_Y-(H_{ij}\tau ^{hk,n}_{,i},e^{hk,n}_{,j})_Y-(\zeta _{ij}u^{hk,n}_{i,j},e^{hk,n})_Y \nonumber \\&\qquad +\kappa (\theta ^{hk,n},e^{hk,n})_Y-(F_{ij}R^{hk,n}_{i,j},e^{hk,n})_Y. \end{aligned}$$
(31)

Third, choosing \(z^h=\theta ^{hk,n}\) as a test function in (26), it follows that:

$$\begin{aligned}&c(\delta {\theta }^{hk,n},\theta ^{hk,n})_Y+(K_{ij}^*\theta ^{hk,n}_{,i},\theta ^{hk,n}_{,j})_Y +(K_{ij}\tau ^{hk,n}_{,i},\theta ^{hk,n}_{,j})_Y =(d_{ij}M^{hk,n}_i,\theta ^{hk,n}_{,j})_Y\\&\quad - (A_{ij}^1M^{hk,n}_i,\theta ^{hk,n}_{,j})_Y-(b_{ij}M^{hk,n}_{i,j},\theta ^{hk,n})_Y -(a_{ij}v^{hk,n}_{i,j},\theta ^{hk,n})_Y\\&\quad -(H_{ij}\varphi ^{hk,n}_{,i},\theta ^{hk,n}_{,j})_Y-\kappa (e^{hk,n},\theta ^{hk,n})_Y. \end{aligned}$$

Keeping in mind that

$$\begin{aligned} c(\delta {\theta }^{hk,n},\theta ^{hk,n})_Y\ge \frac{c}{2k}\left\{ \Vert \theta ^{hk,n}\Vert ^2_Y-\Vert \theta ^{hk,n-1}\Vert _Y^2\right\} , \end{aligned}$$

we find that:

$$\begin{aligned}& \displaystyle \frac{c}{2k}\left\{ \Vert \theta ^{hk,n}\Vert ^2_Y-\Vert \theta ^{hk,n-1}\Vert _Y^2\right\} +(K_{ij}^*\theta ^{hk,n}_{,i},\theta ^{hk,n}_{,j})_Y +\frac{1}{2k}\Big \{(K_{ij}\tau ^{hk,n}_{,i},\tau ^{hk,n}_{,j})_Y \nonumber \\&\qquad \displaystyle -(K_{ij}\tau ^{hk,n-1}_{,i},\tau ^{hk,n-1}_{,j})_Y+(K_{ij}(\tau ^{hk,n}_{,i}-\tau ^{hk,n-1}_{,i}),\tau ^{hk,n}_{,j}-\tau ^{hk,n-1}_{,j})_Y\Big \} \nonumber \\&=(d_{ij}M^{hk,n}_i,\theta ^{hk,n}_{,j})_Y - (A_{ij}^1M^{hk,n}_i,\theta ^{hk,n}_{,j})_Y-(b_{ij}M^{hk,n}_{i,j},\theta ^{hk,n})_Y -(a_{ij}v^{hk,n}_{i,j},\theta ^{hk,n})_Y \nonumber \\&\qquad -(H_{ij}\varphi ^{hk,n}_{,i},\theta ^{hk,n}_{,j})_Y-\kappa (e^{hk,n},\theta ^{hk,n})_Y . \end{aligned}$$
(32)

Finally, taking \(\xi _i^h=M_i^{hk,n}\) as a test function in (27), we obtain:

$$\begin{aligned}&(c_{ij} \delta M^{hk,n}_j,M^{hk,n}_i)_Y+(C_{ijkl} R^{hk,n}_{k,l},M^{hk,n}_{i,j})_Y+(C_{ijkl}^*M^{hk,n}_{k,l},M^{hk,n}_{i,j})_Y \\&\quad +(A_{ij}^3 M^{hk,n}_j ,M^{hk,n}_i)_Y-(B_{klij}u^{hk,n}_{k,l},M^{hk,n}_{i,j})_Y\\&\quad - (b_{ij}\theta ^{hk,n},M^{hk,n}_{i,j})_Y +(F_{ij}\varphi ^{hk,n},M^{hk,n}_{i,j})_Y\\&\quad +(d_{ij}\theta ^{hk,n}_{,j} ,M^{hk,n}_i)_Y +(\alpha _{ij}e^{hk,n}_{,j},M^{hk,n}_{i})_Y+(A_{ij}^2\theta ^{hk,n}_{,j},M^{hk,n}_i)_Y=0, \end{aligned}$$

and, since, using (11), it follows that:

$$\begin{aligned} \displaystyle (c_{ij}\delta M^{hk,n}_j ,M^{hk,n}_i)_Y\ge \frac{C_2}{2k}\left\{ \Vert \varvec{M}^{hk,n}\Vert ^2_H-\Vert \varvec{M}^{hk,n-1}\Vert _H^2\right\} , \end{aligned}$$

we have:

$$\begin{aligned}& \displaystyle \frac{C_2}{2k}\left\{ \Vert \varvec{M}^{hk,n}\Vert ^2_H-\Vert \varvec{M}^{hk,n-1}\Vert _H^2\right\} + (C_{ijkl}^*M^{hk,n}_{k,l},M^{hk,n}_{i,j})_Y+\frac{1}{2k}\Big \{(C_{ijkl} R^{hk,n}_{k,l},R^{hk,n}_{i,j})_Y \nonumber \\&\qquad \displaystyle -(C_{ijkl} R^{hk,n-1}_{k,l},R^{hk,n-1}_{i,j})_Y+(C_{ijkl}(R^{hk,n}_{k,l}-R^{hk,n-1}_{k,l}),R^{hk,n}_{i,j}-R^{hk,n-1}_{i,j})_Y\Big \} \nonumber \\&\displaystyle =(B_{klij}u^{hk,n}_{k,l},M^{hk,n}_{i,j})_Y + (b_{ij}\theta ^{hk,n},M^{hk,n}_{i,j})_Y -(F_{ij}\varphi ^{hk,n},M^{hk,n}_{i,j})_Y-(d_{ij}\theta ^{hk,n}_{,j} ,M^{hk,n}_i)_Y \nonumber \\&\qquad \displaystyle -(\alpha _{ij}e^{hk,n}_{,j},M^{hk,n}_{i})_Y-(A_{ij}^2\theta ^{hk,n}_{,j},M^{hk,n}_i)_Y. \end{aligned}$$
(33)

Combining now estimates (30)–(33), after easy algebraic manipulations, we find that:

$$\begin{aligned}& \displaystyle \frac{\rho }{2k}\left\{ \Vert \varvec{v}^{hk,n}\Vert ^2_H-\Vert \varvec{v}^{hk,n-1}\Vert _H^2\right\} +\frac{1}{2k}\Big \{(A_{ijkl}u_{k,l}^{hk,n} , u^{hk,n}_{i,j})_Y-(A_{ijkl}u_{k,l}^{hk,n-1} , u^{hk,n-1}_{i,j})_Y \\&\displaystyle \qquad + (A_{ijkl}(u_{k,l}^{hk,n}-u_{k,l}^{hk,n-1}), u^{hk,n}_{i,j}-u_{i,j}^{hk,n-1})_Y\Big \} \\&\qquad +\displaystyle \frac{J}{2k}\left\{ \Vert e^{hk,n}\Vert ^2_Y-\Vert e^{hk,n-1}\Vert _Y^2\right\} +\frac{1}{2k}\Big \{ (A_{ij}\varphi ^{hk,n}_{,j},\varphi ^{hk,n}_{,i})_Y- (A_{ij}\varphi ^{hk,n-1}_{,j},\varphi ^{hk,n-1}_{,i})_Y \\&\qquad + (A_{ij}(\varphi ^{hk,n}_{,j}-\varphi ^{hk,n-1}_{,j}),\varphi ^{hk,n}_{,i}-\varphi ^{hk,n-1}_{,i})_Y\Big \}\\&\qquad +\frac{\xi }{2k}\Big \{\Vert \varphi ^{hk,n}\Vert _Y^2-\Vert \varphi ^{hk,n-1}\Vert _Y^2+\Vert \varphi ^{hk,n}-\varphi ^{hk,n-1}\Vert _Y^2\Big \}\\&\qquad \displaystyle +\frac{c}{2k}\left\{ \Vert \theta ^{hk,n}\Vert ^2_Y-\Vert \theta ^{hk,n-1}\Vert _Y^2\right\} +(K_{ij}^*\theta ^{hk,n}_{,i},\theta ^{hk,n}_{,j})_Y +\frac{1}{2k}\Big \{(K_{ij}\tau ^{hk,n}_{,i},\tau ^{hk,n}_{,j})_Y\\&\qquad \displaystyle -(K_{ij}\tau ^{hk,n-1}_{,i},\tau ^{hk,n-1}_{,j})_Y+(K_{ij}(\tau ^{hk,n}_{,i}-\tau ^{hk,n-1}_{,i}),\tau ^{hk,n}_{,j}-\tau ^{hk,n-1}_{,j})_Y\Big \} \\&\qquad \displaystyle +\frac{C_2}{2k}\left\{ \Vert \varvec{M}^{hk,n}\Vert ^2_H-\Vert \varvec{M}^{hk,n-1}\Vert _H^2\right\} + (C_{ijkl}^*M^{hk,n}_{k,l},M^{hk,n}_{i,j})_Y\\&\qquad \displaystyle +(\zeta _{ij}u^{hk,n}_{i,j},e^{hk,n})_Y+(A_{ij}^3M_j^{hk,n},M_i^{hk,n})_Y+\frac{1}{2k}\Big \{(C_{ijkl} R^{hk,n}_{k,l},R^{hk,n}_{i,j})_Y\\&\qquad \displaystyle -(C_{ijkl} R^{hk,n-1}_{k,l},R^{hk,n-1}_{i,j})_Y+(C_{ijkl}(R^{hk,n}_{k,l}-R^{hk,n-1}_{k,l}),R^{hk,n}_{i,j}-R^{hk,n-1}_{i,j})_Y\Big \}\\&\qquad +(B_{ijkl}R_{k,l}^{hk,n},v^{hk,n}_{i,j})_Y +(B_{klij}u^{hk,n}_{k,l},M^{hk,n}_{i,j})_Y+(\zeta _{ij}\varphi ^{hk,n},v^{hk,n}_{i,j})_Y\\&\qquad +(H_{ij}\tau ^{hk,n}_{,i},e^{hk,n}_{,j})_Y+(H_{ij}\varphi ^{hk,n}_{,i},\theta ^{hk,n}_{,j})_Y +((A_{ij}^1+A_{ij}^2)\theta ^{hk,n}_{,j},M^{hk,n}_i)_Y\\&\qquad \le C\Big (\Vert e^{hk,n}\Vert _Y^2+\Vert \varphi ^{hk,n}\Vert _Y^2+\Vert \nabla \varvec{R}^{hk,n}\Vert _Q^2\Big )+\epsilon \Vert \nabla \varvec{M}^{hk,n}\Vert _Q^2, \end{aligned}$$

where \(\epsilon >0\) is a positive constant assumed small enough, and C is a generic constant, whose value may change from line to line, and it is independent of the discretization parameters h and k.

Keeping in mind assumptions (10), we find that:

$$\begin{aligned}&(C_{ijkl}^*M^{hk,n}_{k,l},M^{hk,n}_{i,j})_Y+((A_{ij}^1+A_{ij}^2)\theta ^{hk,n}_{,j},M^{hk,n}_i)_Y+ (K_{ij}^*\theta ^{hk,n}_{,i},\theta ^{hk,n}_{,j})_Y \\&\quad + (A_{ij}^3M_j^{hk,n},M_i^{hk,n})_Y \ge C(\Vert \nabla \theta ^{hk,n}\Vert _H^2+\Vert \nabla \varvec{M}^{hk,n}\Vert _Q^2+\Vert \varvec{M}^{hk,n}\Vert _H^2). \end{aligned}$$

Observing that:

$$\begin{aligned}&\displaystyle (B_{ijkl}R_{k,l}^{hk,n},v^{hk,n}_{i,j})_Y +(B_{klij}u^{hk,n}_{k,l},M^{hk,n}_{i,j})_Y=\frac{1}{k}\Big \{ (B_{ijkl}R_{k,l}^{hk,n},u^{hk,n}_{i,j})_Y\\&\quad -(B_{ijkl}R_{k,l}^{hk,n-1},u^{hk,n-1}_{i,j})_Y+ \displaystyle (B_{ijkl}(R_{k,l}^{hk,n}-R_{k,l}^{hk,n-1}),u^{hk,n}_{i,j}-u^{hk,n-1}_{i,j})_Y\Big \},\\&\displaystyle (\zeta _{ij}\varphi ^{hk,n},v^{hk,n}_{i,j})_Y+(\zeta _{ij}u^{hk,n}_{i,j},e^{hk,n})_Y=\frac{1}{k}\Big \{ (\zeta _{ij}\varphi ^{hk,n},u^{hk,n}_{i,j})_Y-(\zeta _{ij}\varphi ^{hk,n-1},u^{hk,n-1}_{i,j})_Y\\&\quad + (\zeta _{ij}(\varphi ^{hk,n}-\varphi ^{hk,n-1}),u^{hk,n}_{i,j}-u^{hk,n-1}_{i,j})_Y\Big \},\\&\quad \displaystyle (H_{ij}\tau ^{hk,n}_{,i},e^{hk,n}_{,j})_Y+(H_{ij}\varphi ^{hk,n}_{,i},\theta ^{hk,n}_{,j})_Y=\frac{1}{k}\Big \{ (H_{ij}\tau ^{hk,n}_{,i},\varphi ^{hk,n}_{,j})_Y-(H_{ij}\tau ^{hk,n-1}_{,i},\varphi ^{hk,n-1}_{,j})_Y\\&\quad + (H_{ij}(\tau ^{hk,n}_{,i}-\tau ^{hk,n-1}_{,i}),\varphi ^{hk,n}_{,j}-\varphi ^{hk,n-1}_{,j})_Y\Big \}, \end{aligned}$$

using assumptions (11), it follows that:

$$\begin{aligned}&\displaystyle (A_{ijkl}(u_{k,l}^{hk,n}-u_{k,l}^{hk,n-1}), u^{hk,n}_{i,j}-u_{i,j}^{hk,n-1})_Y+2(B_{ijkl}(R_{k,l}^{hk,n}-R_{k,l}^{hk,n-1}),u^{hk,n}_{i,j}-u^{hk,n-1}_{i,j})_Y\\&\quad +(C_{ijkl}(R^{hk,n}_{k,l}-R^{hk,n-1}_{k,l}),R^{hk,n}_{i,j}-R^{hk,n-1}_{i,j})_Y +2(\zeta _{ij}(\varphi ^{hk,n}-\varphi ^{hk,n-1}),u^{hk,n}_{i,j}-u^{hk,n-1}_{i,j})_Y\\&\quad +\xi \Vert \varphi ^{hk,n}-\varphi ^{hk,n-1}\Vert _Y^2\ge 0,\\&(A_{ij}(\varphi ^{hk,n}_{,j}-\varphi ^{hk,n-1}_{,j}),\varphi ^{hk,n}_{,i}-\varphi ^{hk,n-1}_{,i})_Y+ 2(H_{ij}(\tau ^{hk,n}_{,i}-\tau ^{hk,n-1}_{,i}),\varphi ^{hk,n}_{,j}-\varphi ^{hk,n-1}_{,j})_Y\\&\quad +(K_{ij}(\tau ^{hk,n}_{,i}-\tau ^{hk,n-1}_{,i}),\tau ^{hk,n}_{,j}-\tau ^{hk,n-1}_{,j})_Y\ge 0. \end{aligned}$$

Therefore, multiplying the above estimates by k and summing up to n, we have:

$$\begin{aligned}& \rho \Vert \varvec{v}^{hk,n}\Vert ^2_H+(A_{ijkl}u_{k,l}^{hk,n} , u^{hk,n}_{i,j})_Y+J \Vert e^{hk,n}\Vert ^2_Y+ (A_{ij}\varphi ^{hk,n}_{,j},\varphi ^{hk,n}_{,i})_Y\\&\qquad +\xi \Vert \varphi ^{hk,n}\Vert _Y^2+c \Vert \theta ^{hk,n}\Vert ^2_Y\\&\qquad \displaystyle + (K_{ij}\tau ^{hk,n}_{,i},\tau ^{hk,n}_{,j})_Y +C_2 \Vert \varvec{M}^{hk,n}\Vert ^2_H+(C_{ijkl} R^{hk,n}_{k,l},R^{hk,n}_{i,j})_Y+2(B_{ijkl}R_{k,l}^{hk,n},u^{hk,n}_{i,j})_Y\\&\qquad \displaystyle + 2(\zeta _{ij}\varphi ^{hk,n},u^{hk,n}_{i,j})_Y+2(H_{ij}\tau ^{hk,n}_{,i},\varphi ^{hk,n}_{,j})_Y \\&\le Ck\sum _{j=1}^n\Big (\Vert e^{hk,j}\Vert _Y^2+\Vert \varphi ^{hk,j}\Vert _Y^2+\Vert \nabla \varvec{R}^{hk,j}\Vert _Q^2\Big )+C\Big ( \Vert \varvec{v}^{0h}\Vert ^2_H+\Vert \nabla \varvec{u}^{0h}\Vert _Q^2+\Vert e^{0h}\Vert ^2_Y\\&\qquad \displaystyle + \Vert \nabla \varphi ^{0h}\Vert _H^2 +\Vert \varphi ^{0h}\Vert _Y^2+\Vert \theta ^{0h}\Vert ^2_Y + \Vert \nabla \tau ^{0h}\Vert _H^2 +\Vert \varvec{M}^{0h}\Vert ^2_H+ \Vert \nabla \varvec{R}^{0h}\Vert _Q^2\Big ). \end{aligned}$$

Finally, using again assumptions (11), we obtain:

$$\begin{aligned}&\displaystyle (A_{ijkl}u_{k,l}^{hk,n} , u^{hk,n}_{i,j})_Y+2(B_{ijkl}R_{k,l}^{hk,n},u^{hk,n}_{i,j})_Y+(C_{ijkl} R^{hk,n}_{k,l},R^{hk,n}_{i,j})_Y+2(\zeta _{ij}\varphi ^{hk,n},u^{hk,n}_{i,j})_Y\\&\quad +\xi \Vert \varphi ^{hk,n}\Vert _Y^2\ge C\Big (\Vert \nabla \varvec{u}^{hk,n}\Vert _Q^2+\Vert \nabla \varvec{R}^{hk,n}\Vert _Q^2+\Vert \varphi ^{hk,n}\Vert _Y^2\Big ),\\&(A_{ij}\varphi ^{hk,n}_{,j},\varphi ^{hk,n}_{,i})_Y+2(H_{ij}\tau ^{hk,n}_{,i},\varphi ^{hk,n}_{,j})_Y\\&\quad + (K_{ij}\tau ^{hk,n}_{,i},\tau ^{hk,n}_{,j})_Y\ge C\Big (\Vert \nabla \varphi ^{hk,n}\Vert _H^2+\Vert \nabla \tau ^{hk,n}\Vert _H^2\Big ), \end{aligned}$$

and so:

$$\begin{aligned}& \Vert \varvec{v}^{hk,n}\Vert ^2_H+\Vert \nabla \varvec{u}^{hk,n}\Vert _Q^2+ \Vert e^{hk,n}\Vert ^2_Y+ \Vert \nabla \varphi ^{hk,n}\Vert _H^2+\Vert \varphi ^{hk,n}\Vert _Y^2+ \Vert \theta ^{hk,n}\Vert ^2_Y+\Vert \nabla \tau ^{hk,n}\Vert _H^2\\&\qquad \displaystyle +\Vert \varvec{M}^{hk,n}\Vert ^2_H +\Vert \nabla \varvec{R}^{hk,n}\Vert _Q^2\\&\le Ck\sum _{j=1}^n\Big (\Vert e^{hk,j}\Vert _Y^2+\Vert \varphi ^{hk,j}\Vert _Y^2+\Vert \nabla \varvec{R}^{hk,j}\Vert _Q^2\Big )+C\Big ( \Vert \varvec{v}^{0h}\Vert ^2_H+\Vert \nabla \varvec{u}^{0h}\Vert _Q^2+\Vert e^{0h}\Vert ^2_Y\\&\qquad \displaystyle + \Vert \nabla \varphi ^{0h}\Vert _H^2 +\Vert \varphi ^{0h}\Vert _Y^2+\Vert \theta ^{0h}\Vert ^2_Y + \Vert \nabla \tau ^{0h}\Vert _H^2 + \Vert \varvec{M}^{0h}\Vert ^2_H+ \Vert \nabla \varvec{R}^{0h}\Vert _Q^2\Big ). \end{aligned}$$

Therefore, applying a discrete version of Gronwall’s inequality (see, e.g., Campo et al. 2006), we deduce the desired stability property.

Now, our aim will be to obtain a priori error estimates on the numerical errors from the approximations given in Problem \(VP^{hk}\). We have the following.

Theorem 3

Let the assumptions of Theorem 2 still hold. If we also assume that:

$$\begin{aligned} a_{ij}(\varvec{x})=a_{ij},\quad \alpha _{ij}(\varvec{x})=\alpha _{ij}\quad \hbox {for all}\quad \varvec{x}\in \varOmega , \end{aligned}$$
(34)

and if we denote by \((\varvec{v},e,\theta ,\varvec{M})\) and \((\varvec{v}^{hk},e^{hk},\theta ^{hk},\varvec{M}^{hk})\) the respective solutions to problems VP and VP\(^{hk}\), then we have the following a priori error estimates for all \(\varvec{w}^{h}=\{\varvec{w}^{h,n}\}_{n=0}^N,\,\varvec{\xi }^h=\{\varvec{\xi }^{h,n}\}_{n=0}^N\subset V^h\) and \(r^h=\{r^{h,n}\}_{n=0}^N,\, z^h=\{z^{h,n}\}_{n=0}^N\subset E^h\):

$$\begin{aligned}&\displaystyle \max _{0\le n\le N}\Big \{\Vert \varvec{v}^n-\varvec{v}^{hk,n}\Vert ^2_H+\Vert \nabla (\varvec{u}^n-\varvec{u}^{hk,n})\Vert _Q^2 + \Vert e^n-e^{hk,n}\Vert ^2_Y+\Vert \nabla (\varphi ^n-\varphi ^{hk,n})\Vert _H^2 \nonumber \\&\qquad \displaystyle + \Vert \theta ^n-\theta ^{hk,n}\Vert ^2_Y +\Vert \varphi ^n-\varphi ^{hk,n}\Vert _Y^2+\Vert \nabla (\tau ^n-\tau ^{hk,n})\Vert _H^2 + \Vert \varvec{M}^n-\varvec{M}^{hk,n}\Vert ^2_H \nonumber \\&\qquad \displaystyle +\Vert \nabla (\varvec{R}^n-\varvec{R}^{hk,n})\Vert _Q^2\Big \} \nonumber \\&\qquad \displaystyle \le Ck\sum _{j=1}^n\Big (\Vert \dot{\varvec{v}}^j-\delta \varvec{v}^j\Vert _H^2+\Vert \nabla (\dot{\varvec{u}}^j-\delta \varvec{u}^j)\Vert _Q^2+\Vert \nabla (\varvec{v}^j-\varvec{w}^{h,j})\Vert _Q^2+\Vert \varvec{v}^j-\varvec{w}^{h,j}\Vert _H^2 \nonumber \\&\qquad \displaystyle +\Vert \dot{e}^j-\delta e^j\Vert _Y^2+\Vert \nabla (\dot{\varphi }^j-\delta \varphi ^j)\Vert _H^2+\Vert \nabla (e^j-r^{h,j})\Vert _H^2 +\Vert e^j-r^{h,j}\Vert _Y^2\nonumber \\&\qquad \displaystyle +\Vert \dot{\theta }^j-\delta \theta ^j\Vert _Y^2+\Vert \nabla (\dot{\tau }^j-\delta \tau ^j)\Vert _H^2\nonumber \\&\qquad +\Vert \nabla (\theta ^j-z^{h,j})\Vert _H^2\nonumber \\&\qquad +\Vert \theta ^j-z^{h,j}\Vert _Y^2+\Vert \dot{\varvec{M}}^j-\delta \varvec{M}^j\Vert _H^2 \nonumber \\&\qquad \displaystyle +\Vert \nabla (\dot{\varvec{R}}^j-\delta \varvec{R}^j)\Vert _Q^2+\Vert \nabla (\varvec{M}^j-\varvec{\xi }^{h,j})\Vert _Q^2+\Vert \varvec{M}^j-\varvec{\xi }^{h,j}\Vert _H^2\Big ) \nonumber \\&\qquad \displaystyle +\frac{C}{k}\sum _{j=1}^{N-1}\Big \{\Vert \varvec{v}^j-\varvec{w}^{h,j}-(\varvec{v}^{j+1}-\varvec{w}^{h,j+1})\Vert _H^2+ \Vert e^j-r^{h,j}-(e^{j+1}-r^{h,j+1})\Vert _Y^2 \nonumber \\&\qquad \displaystyle +\Vert \varvec{M}^j-\varvec{\xi }^{h,j}-(\varvec{M}^{j+1}-\varvec{\xi }^{h,j+1})\Vert _H^2+ \Vert \theta ^j-z^{h,j}-(\theta ^{j+1}-z^{h,j+1})\Vert _Y^2\Big \} \nonumber \\&\qquad \displaystyle +C\max _{0\le n\le N}\Big \{\Vert \varvec{v}^n-\varvec{w}^{h,n}\Vert _H^2+\Vert e^n-r^{h,n}\Vert _Y^2+\Vert \theta ^n-z^{h,n}\Vert _Y^2+\Vert \varvec{M}^n-\varvec{\xi }^{h,n}\Vert _H^2\Big \} \nonumber \\&\qquad \displaystyle +C\Big (\Vert \varvec{v}^0-\varvec{v}^{0h}\Vert ^2_H+\Vert \nabla (\varvec{u}^0-\varvec{u}^{0h})\Vert _Q^2+\Vert e^0-e^{0h}\Vert ^2_Y \nonumber \\&\qquad +\Vert \nabla (\varphi ^0-\varphi ^{0h})\Vert _H^2+\Vert \xi ^0-\xi ^{0h}\Vert _Y^2 \nonumber \\&\qquad \displaystyle +\Vert \theta ^0-\theta ^{0h}\Vert ^2_Y+\Vert \nabla (\tau ^0-\tau ^{0h})\Vert _H^2 + \Vert \varvec{M}^0-\varvec{M}^{0h}\Vert ^2_H +\Vert \nabla (\varvec{R}^0-\varvec{R}^{0h})\Vert _Q^2\Big ), \end{aligned}$$
(35)

where \(C>0\) is a positive constant assumed to be independent of the discretization parameters but depending on the continuous solution.

Remark 1

We note that assumption (34) implies that these terms are homogeneous. Such conditions are found, for instance, in the case that the material is homogeneous and isotropic, that is:

$$\begin{aligned} a_{ij}(\varvec{x})=a \delta _{ij},\quad \alpha _{ij}(\varvec{x})=\alpha \delta _{ij}\quad \hbox {for all}\quad \varvec{x}\in \varOmega , \end{aligned}$$

where a and \(\alpha \) are constants and \(\delta _{ij}\) represents the Kronecker symbol.

Proof

First, we will derive the estimates for the velocity. Thus, subtracting variational equation (15) at time \(t_n\) for a test function \(\varvec{w}=\varvec{w}^h\in V^h\) and discrete variational equation (24), it follows that, for all \(\varvec{w}^h=(w_i^h)_{i=1}^d\in V^h\):

$$\begin{aligned} \begin{array}{l} \rho (\dot{v}^n_i-\delta v^{hk,n}_i ,w_i^h)_Y +(A_{ijkl}(u_{k,l}^n-u_{k,l}^{hk,n}) , w_{i,j}^h)_Y- (a_{ij}(\theta ^n-\theta ^{hk,n}),w_{i,j}^h)_Y \\ \qquad \qquad +(B_{ijkl}(R^n_{k,l}-R_{k,l}^{hk,n}),w_{i,j}^h)_Y +(\zeta _{ij}(\varphi ^n-\varphi ^{hk,n}),w_{i,j}^h)_Y=0 , \end{array} \end{aligned}$$

and so, for all \(\varvec{w}^h=(w_i^h)_{i=1}^d\in V^h\):

$$\begin{aligned}&\rho (\dot{v}^n_i-\delta v^{hk,n}_i ,v_i^n-v^{hk,n}_i)_Y\\&\qquad +(A_{ijkl}(u_{k,l}^n-u_{k,l}^{hk,n}) , v_{i,j}^n-v^{hk,n}_{i,j})_Y\\&\qquad - (a_{ij}(\theta ^n-\theta ^{hk,n}),v_{i,j}^n-v^{hk,n}_{i,j})_Y \\&\qquad +(B_{ijkl}(R^n_{k,l}-R_{k,l}^{hk,n}),v_{i,j}^n-v^{hk,n}_{i,j})_Y\\&\qquad +(\zeta _{ij}(\varphi ^n-\varphi ^{hk,n}),v_{i,j}^n-v^{hk,n}_{i,j})_Y \\&= \rho (\dot{v}^n_i-\delta v^{hk,n}_i ,v^n_i-w_i^h)_Y\\&\qquad +(A_{ijkl}(u_{k,l}^n-u_{k,l}^{hk,n}) , v_{i,j}^n-w_{i,j}^h)_Y\\&\qquad - (a_{ij}(\theta ^n-\theta ^{hk,n}),v_{i,j}^n-w_{i,j}^h)_Y \\&\qquad +(B_{ijkl}(R^n_{k,l}-R_{k,l}^{hk,n}),v_{i,j}^n-w_{i,j}^h)_Y\\&\qquad +(\zeta _{ij}(\varphi ^n-\varphi ^{hk,n}),v_{i,j}^n-w_{i,j}^h)_Y. \end{aligned}$$

Keeping in mind that:

$$\begin{aligned}&\displaystyle (\dot{v}^n_i-\delta v^{hk,n}_i ,v^n_i-v^{hk,n}_i)_Y\ge (\dot{v}^n_i-\delta v^n_i,v^n_i-v^{hk,n}_i)_Y\\&\quad +\frac{1}{2k}\left\{ \Vert \varvec{v}^n-\varvec{v}^{hk,n}\Vert ^2_H-\Vert \varvec{v}^{n-1}-\varvec{v}^{hk,n-1}\Vert _H^2\right\} ,\\&(A_{ijkl}(u_{k,l}^n-u_{k,l}^{hk,n}) , v_{i,j}^n-v^{hk,n}_{i,j})_Y= (A_{ijkl}(u_{k,l}^n-u_{k,l}^{hk,n}) , \dot{u}_{i,j}^n-\delta u^{n}_{i,j})_Y\\&\quad + (A_{ijkl}(u_{k,l}^n-u_{k,l}^{hk,n}) , \delta u_{i,j}^n-\delta u^{hk,n}_{i,j})_Y,\\&\displaystyle (A_{ijkl}(u_{k,l}^n-u_{k,l}^{hk,n}) , \delta u_{i,j}^n-\delta u^{hk,n}_{i,j})_Y=\frac{1}{2k}\Big \{(A_{ijkl}(u_{k,l}^n-u_{k,l}^{hk,n}), u_{i,j}^n-u^{hk,n}_{i,j})_Y\\&\qquad \displaystyle -(A_{ijkl}(u_{k,l}^{n-1}-u_{k,l}^{hk,n-1}) , u_{i,j}^{n-1}-u^{hk,n-1}_{i,j})_Y\\&\quad \displaystyle +(A_{ijkl}(u_{k,l}^n-u_{k,l}^{hk,n}- (u_{k,l}^{n-1}-u_{k,l}^{hk,n-1})) , u_{i,j}^n-u^{hk,n}_{i,j}-(u_{i,j}^{n-1}-u_{i,j}^{hk,n-1}))_Y\Big \}, \end{aligned}$$

where we used the notations \(\delta \varvec{v}^n=(\varvec{v}^n-\varvec{v}^{n-1})/k\) and \(\delta \varvec{u}^n=(\varvec{u}^n-\varvec{u}^{n-1})/k,\) we obtain the following estimates for the velocity:

$$\begin{aligned}&\displaystyle \frac{\rho }{2k}\left\{ \Vert \varvec{v}^n-\varvec{v}^{hk,n}\Vert ^2_H-\Vert \varvec{v}^{n-1}-\varvec{v}^{hk,n-1}\Vert _H^2\right\} +\frac{1}{2k}\Big \{(A_{ijkl}(u_{k,l}^n-u_{k,l}^{hk,n}) , u_{i,j}^n-u^{hk,n}_{i,j})_Y \nonumber \\&\displaystyle \qquad \displaystyle -(A_{ijkl}(u_{k,l}^{n-1}-u_{k,l}^{hk,n-1}) , u_{i,j}^{n-1}-u^{hk,n-1}_{i,j})_Y \nonumber \\&\qquad \displaystyle +(A_{ijkl}(u_{k,l}^n-u_{k,l}^{hk,n}- (u_{k,l}^{n-1}-u_{k,l}^{hk,n-1})) , u_{i,j}^n-u^{hk,n}_{i,j}-(u_{i,j}^{n-1}-u_{i,j}^{hk,n-1}))_Y\Big \} \nonumber \\&\qquad \displaystyle +(B_{ijkl}(R^n_{k,l}-R_{k,l}^{hk,n}),v_{i,j}^n-v^{hk,n}_{i,j})_Y +(\zeta _{ij}(\varphi ^n-\varphi ^{hk,n}),v_{i,j}^n-v^{hk,n}_{i,j})_Y \nonumber \\&\qquad \displaystyle - (a_{ij}(\theta ^n-\theta ^{hk,n}),v_{i,j}^n-v^{hk,n}_{i,j})_Y \nonumber \\&\displaystyle \le C\Big (\Vert \dot{\varvec{v}}^n-\delta \varvec{v}^n\Vert _H^2+\Vert \nabla (\dot{\varvec{u}}^n-\delta \varvec{u}^n)\Vert _Q^2+\Vert \nabla (\varvec{v}^h-\varvec{w}^h)\Vert _Q^2+\Vert \nabla (\varvec{u}^{n}-\varvec{u}^{hk,n})\Vert _Q^2\nonumber \\&\qquad \displaystyle +\Vert \varvec{v}^n-\varvec{w}^h\Vert _H^2 +\Vert \theta ^n-\theta ^{hk,n}\Vert _Y^2+\Vert \varphi ^n-\varphi ^{hk,n}\Vert _Y^2+\Vert \nabla (\varvec{R}^n-\varvec{R}^{hk,n})\Vert _Q^2 \nonumber \\&\qquad \displaystyle +(\delta {v}^n_i-\delta v^{hk,n}_i ,v^n_i-w_i^h)_Y\Big ) \quad \forall \varvec{w}^h=(w_i^h)_{i=1}^d\in V^h. \end{aligned}$$
(36)

Now, we will derive the estimates on the volume fraction speed. Therefore, subtracting variational equation (16) at time \(t_n\) for all \(r=r^h\in E^h\) and discrete variational equation (25), we get:

$$\begin{aligned} \begin{array}{l} J(\dot{e}^n-\delta {e}^{hk,n},r^h)_Y+(A_{ij}(\varphi ^n_{,j}-\varphi ^{hk,n}_{,j}),r_{,i}^h)_Y+\xi (\varphi ^n-\varphi ^{hk,n},r^h)_Y\\ \qquad -(\alpha _{ij}(M^n_i-M^{hk,n}_i),r^h_{,j})_Y+(H_{ij}(\tau ^n_{,i}-\tau ^{hk,n}_{,i}),r^h_{,j})_Y +(\zeta _{ij}(u^n_{i,j}-u^{hk,n}_{i,j}),r^h)_Y\\ \qquad -\kappa (\theta ^n-\theta ^{hk,n},r^h)_Y +(F_{ij}(R^n_{i,j}-R^{hk,n}_{i,j}),r^h)_Y=0 \quad \forall r^h\in E^h, \end{array} \end{aligned}$$

and so, for all \(r^h\in E^h\):

$$\begin{aligned} \begin{array}{lll} J(\dot{e}^n-\delta {e}^{hk,n},e^n-e^{hk,n})_Y+(A_{ij}(\varphi ^n_{,j}-\varphi ^{hk,n}_{,j}),e^n_{,i}-e^{hk,n}_{,i})_Y+\xi (\varphi ^n-\varphi ^{hk,n},e^n-e^{hk,n})_Y\\ \qquad -(\alpha _{ij}(M^n_i-M^{hk,n}_i),e^n_{,j}-e^{hk,n}_{,j})_Y +(H_{ij}(\tau ^n_{,i}-\tau ^{hk,n}_{,i}),e^n_{,j}-e^{hk,n}_{,j})_Y\\ \qquad +(\zeta _{ij}(u^n_{i,j}-u^{hk,n}_{i,j}),e^n-e^{hk,n})_Y-\kappa (\theta ^n-\theta ^{hk,n},e^n-e^{hk,n})_Y\\ \qquad +(F_{ij}(R^n_{i,j}-R^{hk,n}_{i,j}),e^n-e^{hk,n})_Y\\ = J(\dot{e}^n-\delta {e}^{hk,n},e^n-r^{h})_Y+(A_{ij}(\varphi ^n_{,j}-\varphi ^{hk,n}_{,j}),e^n_{,i}-r^{h}_{,i})_Y+\xi (\varphi ^n-\varphi ^{hk,n},e^n-r^{h})_Y\\ \qquad -(\alpha _{ij}(M^n_i-M^{hk,n}_i),e^n_{,j}-r^{h}_{,j})_Y +(H_{ij}(\tau ^n_{,i}-\tau ^{hk,n}_{,i}),e^n_{,j}-r^{h}_{,j})_Y\\ \qquad +(\zeta _{ij}(u^n_{i,j}-u^{hk,n}_{i,j}),e^n-r^{h})_Y-\kappa (\theta ^n-\theta ^{hk,n},e^n-r^{h})_Y\\ \qquad \qquad +(F_{ij}(R^n_{i,j}-R^{hk,n}_{i,j}),e^n-r^{h})_Y. \end{array} \end{aligned}$$

Taking into account that:

$$\begin{aligned}&\displaystyle (\dot{e}^n-\delta e^{hk,n} ,e^n-e^{hk,n})_Y\ge (\dot{e}^n-\delta e^n,e^n-e^{hk,n})_Y\\&\displaystyle \qquad +\frac{1}{2k}\left\{ \Vert e^n-e^{hk,n}\Vert ^2_Y-\Vert e^{n-1}-e^{hk,n-1}\Vert _Y^2\right\} ,\\&(A_{ij}(\varphi _{,j}^n-\varphi _{,j}^{hk,n}) , e_{,i}^n-e^{hk,n}_{,i})_Y= (A_{ij}(\varphi _{,j}^n-\varphi _{,j}^{hk,n}) , \dot{\varphi }_{,i}^n-\delta \varphi ^{n}_{,i})_Y\\&\qquad + (A_{ij}(\varphi _{,j}^n-\varphi _{,j}^{hk,n}) , \delta \varphi _{,i}^n-\delta \varphi ^{hk,n}_{,i})_Y, \\&\displaystyle (A_{ij}(\varphi _{,j}^n-\varphi _{,j}^{hk,n}) , \delta \varphi _{,i}^n-\delta \varphi ^{hk,n}_{,i})_Y=\frac{1}{2k}\Big \{(A_{ij}(\varphi _{,j}^n-\varphi _{,j}^{hk,n}), \varphi _{,i}^n-\varphi ^{hk,n}_{,i})_Y\\&\qquad \displaystyle -(A_{ij}(\varphi _{,j}^{n-1}-\varphi _{,j}^{hk,n-1}) , \varphi _{,i}^{n-1}-\varphi ^{hk,n-1}_{,i})_Y\\&\qquad \displaystyle +(A_{ij}(\varphi _{,j}^n-\varphi _{,j}^{hk,n}- (\varphi _{,j}^{n-1}-\varphi _{,j}^{hk,n-1})) , \varphi _{,i}^n-\varphi ^{hk,n}_{,i}-(\varphi _{,i}^{n-1}-\varphi _{,i}^{hk,n-1}))_Y\Big \},\\&\displaystyle (\varphi ^n-\varphi ^{hk,n},e^n-e^{hk,n})_Y= (\varphi ^n-\varphi ^{hk,n},\dot{\varphi }^n-\varphi ^{hk,n})_Y+\frac{1}{2k}\Big \{\Vert \xi ^n-\xi ^{hk,n}\Vert _Y^2\\&\qquad \displaystyle - \Vert \xi ^{n-1}-\xi ^{hk,n-1}\Vert _Y^2+\Vert \xi ^n-\xi ^{hk,n}-(\xi ^{n-1}-\xi ^{hk,n-1})\Vert _Y^2\Big \}, \end{aligned}$$

where we used the notations \(\delta e^n=(e^n-e^{n-1})/k\) and \(\delta \varphi ^n=(\varphi ^n-\varphi ^{n-1})/k,\) we have the following estimates for the volume fraction speed, for all \(r^h\in E^h\):

$$\begin{aligned}&\displaystyle \frac{J}{2k}\left\{ \Vert e^n-e^{hk,n}\Vert ^2_Y-\Vert e^{n-1}-e^{hk,n-1}\Vert _Y^2\right\} + \frac{1}{2k}\Big \{(A_{ij}(\varphi _{,j}^n-\varphi _{,j}^{hk,n}) , \varphi _{,i}^n-\varphi ^{hk,n}_{,i})_Y \nonumber \\&\qquad \displaystyle -(A_{ij}(\varphi _{,j}^{n-1}-\varphi _{,j}^{hk,n-1}) , \varphi _{,i}^{n-1}-\varphi ^{hk,n-1}_{,i})_Y \nonumber \\&\qquad \displaystyle +(A_{ij}(\varphi _{,j}^n-\varphi _{,j}^{hk,n}- (\varphi _{,j}^{n-1}-\varphi _{,j}^{hk,n-1})) , \varphi _{,i}^n-\varphi ^{hk,n}_{,i}-(\varphi _{,i}^{n-1}-\varphi _{,i}^{hk,n-1}))_Y\Big \} \nonumber \\&\qquad \displaystyle +\frac{\xi }{2k}\Big \{\Vert \xi ^n-\xi ^{hk,n}\Vert _Y^2 - \Vert \xi ^{n-1}-\xi ^{hk,n-1}\Vert _Y^2+\Vert \xi ^n-\xi ^{hk,n}-(\xi ^{n-1}-\xi ^{hk,n-1})\Vert _Y^2\Big \} \nonumber \\&\qquad \displaystyle -(\alpha _{ij}(M^n_i-M^{hk,n}_i),e^n_{,j}-e^{hk,n}_{,j})_Y +(H_{ij}(\tau ^n_{,i}-\tau ^{hk,n}_{,i}),e^n_{,j}-e^{hk,n}_{,j})_Y \nonumber \\&\qquad \displaystyle +(\zeta _{ij}(u^n_{i,j}-u^{hk,n}_{i,j}),e^n-e^{hk,n})_Y-\kappa (\theta ^n-\theta ^{hk,n},e^n-e^{hk,n})_Y \nonumber \\&\qquad \displaystyle +(F_{ij}(R^n_{i,j}-R^{hk,n}_{i,j}),e^n-e^{hk,n})_Y \nonumber \\&\displaystyle \le C\Big (\Vert \dot{e}^n-\delta e^n\Vert _Y^2+\Vert \nabla (\dot{\varphi }^n-\delta \varphi ^n)\Vert _H^2+\Vert \nabla (e^h-r^h)\Vert _H^2+\Vert \nabla (\varphi ^{n}-\varphi ^{hk,n})\Vert _H^2 \nonumber \\&\qquad \displaystyle +\Vert e^n-r^h\Vert _Y^2 +\Vert \theta ^n-\theta ^{hk,n}\Vert _Y^2+\Vert \varphi ^n-\varphi ^{hk,n}\Vert _Y^2+\Vert \nabla (\varvec{R}^n-\varvec{R}^{hk,n})\Vert _Q^2 \nonumber \\&\qquad \displaystyle +\Vert \nabla (\varvec{u}^n-\varvec{u}^{hk,n})\Vert _Q^2+\Vert \varvec{M}^n-\varvec{M}^{hk,n}\Vert _H^2+\Vert \nabla (\tau ^n-\tau ^{hk,n})\Vert _H^2\nonumber \\&\qquad \displaystyle +(\delta {e}^n-\delta e^{hk,n} ,e^n-r^h)_Y\Big ). \end{aligned}$$
(37)

Now, we will obtain the error estimates on the temperature. Subtracting variational equation (17) at time \(t=t_n\) for a test function \(z=z^h\in E^h\) and discrete variational equation, we find that, for all \(z^h\in E^h\):

$$\begin{aligned} \begin{array}{l} c(\dot{\theta }^n-\delta {\theta }^{hk,n},z^h)_Y+(K_{ij}^*(\theta ^n_{,i}-\theta ^{hk,n}_{,i}) ,z_{,j}^h)_Y+(K_{ij}(\tau ^n_{,i}-\tau ^{hk,n}_{,i}),z^h_{,j})_Y \\ \qquad -(d_{ij}(M^n_i-M^{hk,n}_i),z^h_{,j})_Y + (A_{ij}^1(M^n_i-M^{hk,n}_i),z^h_{,j})_Y+(b_{ij}(M^n_{i,j}-M^{hk,n}_{i,j}),z^h)_Y \\ \qquad +(a_{ij}(v^n_{i,j}-v^{hk,n}_{i,j}),z^h)_Y+(H_{ij}(\varphi ^n_{,i}-\varphi ^{hk,n}_{,i}),z^h_{,j})_Y-\kappa (e^n-e^{hk,n},z^h)_Y=0, \end{array} \end{aligned}$$

and so, for all \(z^h\in V^h\), it follows that:

$$\begin{aligned}&c(\dot{\theta }^n-\delta {\theta }^{hk,n},\theta ^n-\theta ^{hk,n})_Y+(K_{ij}^*(\theta ^n_{,i}-\theta ^{hk,n}_{,i}) ,\theta ^n_{,j}-\theta ^{hk,n}_{,j})_Y\\&\quad +(K_{ij}(\tau ^n_{,i}-\tau ^{hk,n}_{,i}),\theta ^n_{,j}-\theta ^{hk,n}_{,j})_Y\\&\qquad -(d_{ij}(M^n_i-M^{hk,n}_i),\theta ^n_{,j}-\theta ^{hk,n}_{,j})_Y+ (A_{ij}^1(M^n_i-M^{hk,n}_i),\theta ^n_{,j}-\theta ^{hk,n}_{,j})_Y\\&\qquad +(b_{ij}(M^n_{i,j}-M^{hk,n}_{i,j}),\theta ^n-\theta ^{hk,n})_Y\\&\qquad +(a_{ij}(v^n_{i,j}-v^{hk,n}_{i,j}),\theta ^n-\theta ^{hk,n})_Y\\&\qquad +(H_{ij}(\varphi ^n_{,i}-\varphi ^{hk,n}_{,i}),\theta ^n_{,j}-\theta ^{hk,n}_{,j})_Y-\kappa (e^n-e^{hk,n},\theta ^n-\theta ^{hk,n})_Y\\&\quad =c(\dot{\theta }^n-\delta {\theta }^{hk,n},\theta ^n-z^h)_Y\\&\qquad +(K_{ij}^*(\theta ^n_{,i}-\theta ^{hk,n}_{,i}) ,\theta ^n_{,j}-z_{,j}^h)_Y+(K_{ij}(\tau ^n_{,i}-\tau ^{hk,n}_{,i}),\theta ^n_{,j}-z^h_{,j})_Y \\&\qquad -(d_{ij}(M^n_i-M^{hk,n}_i),\theta ^n_{,j}-z^h_{,j})_Y+ (A_{ij}^1(M^n_i-M^{hk,n}_i),\theta ^n_{,j}-z^h_{,j})_Y\\&\qquad +(b_{ij}(M^n_{i,j}-M^{hk,n}_{i,j}),\theta ^n-z^h)_Y +(a_{ij}(v^n_{i,j}-v^{hk,n}_{i,j}),\theta ^n-z^h)_Y\\&\qquad +(H_{ij}(\varphi ^n_{,i}-\varphi ^{hk,n}_{,i}),\theta ^n_{,j}-z^h_{,j})_Y-\kappa (e^n-e^{hk,n},\theta ^n-z^h)_Y. \end{aligned}$$

Keeping in mind that:

$$\begin{aligned}&\displaystyle (\dot{\theta }^n-\delta \theta ^{hk,n} ,\theta ^n-\theta ^{hk,n})_Y\ge (\dot{\theta }^n-\delta \theta ^n,\theta ^n-\theta ^{hk,n})_Y\\&\qquad +\frac{1}{2k}\left\{ \Vert \theta ^n-\theta ^{hk,n}\Vert ^2_Y-\Vert \theta ^{n-1}-\theta ^{hk,n-1}\Vert _Y^2\right\} ,\\&\qquad (K_{ij}(\tau _{,j}^n-\tau _{,j}^{hk,n}) , \theta _{,i}^n-\theta ^{hk,n}_{,i})_Y= (K_{ij}(\tau _{,j}^n-\tau _{,j}^{hk,n}) , \dot{\tau }_{,i}^n-\delta \tau ^{n}_{,i})_Y\\&\qquad + (K_{ij}(\tau _{,j}^n-\tau _{,j}^{hk,n}) , \delta \tau _{,i}^n-\delta \tau ^{hk,n}_{,i})_Y,\\&\qquad \displaystyle (K_{ij}(\tau _{,j}^n-\tau _{,j}^{hk,n}) , \delta \tau _{,i}^n-\delta \tau ^{hk,n}_{,i})_Y=\frac{1}{2k}\Big \{(K_{ij}(\tau _{,j}^n-\tau _{,j}^{hk,n}), \tau _{,i}^n-\tau ^{hk,n}_{,i})_Y\\&\qquad \displaystyle -(K_{ij}(\tau _{,j}^{n-1}-\tau _{,j}^{hk,n-1}) , \tau _{,i}^{n-1}-\tau ^{hk,n-1}_{,i})_Y\\&\qquad \displaystyle +(K_{ij}(\tau _{,j}^n-\tau _{,j}^{hk,n}- (\tau _{,j}^{n-1}-\tau _{,j}^{hk,n-1})) , \tau _{,i}^n-\tau ^{hk,n}_{,i}-(\tau _{,i}^{n-1}-\tau _{,i}^{hk,n-1}))_Y\Big \},\\&\qquad (a_{ij}(v^n_{i,j}-v^{hk,n}_{i,j}),\theta ^n-z^h)_Y=-(a_{ij}(v^n_{i}-v^{hk,n}_i),\theta ^n_{,j}-z^h_{,j})_Y, \end{aligned}$$

where we used the notations \(\delta \theta ^n=(\theta ^n-\theta ^{n-1})/k\) and \(\delta \tau ^n=(\tau ^n-\tau ^{n-1})/k\) and assumption (34), we get the following estimates for the temperature, for all \(z^h\in E^h\):

$$\begin{aligned}&\displaystyle \frac{1}{2k}\left\{ \Vert \theta ^n-\theta ^{hk,n}\Vert ^2_Y-\Vert \theta ^{n-1}-\theta ^{hk,n-1}\Vert _Y^2\right\} +(K_{ij}^*(\theta ^n_{,i}-\theta ^{hk,n}_{,i}) ,\theta ^n_{,j}-\theta ^{hk,n}_{,j})_Y \nonumber \\&\qquad \displaystyle +\frac{1}{2k}\Big \{(K_{ij}(\tau _{,j}^n-\tau _{,j}^{hk,n}) , \tau _{,i}^n-\tau ^{hk,n}_{,i})_Y -(K_{ij}(\tau _{,j}^{n-1}-\tau _{,j}^{hk,n-1}) , \tau _{,i}^{n-1}-\tau ^{hk,n-1}_{,i})_Y \nonumber \\&\qquad \displaystyle +(K_{ij}(\tau _{,j}^n-\tau _{,j}^{hk,n}- (\tau _{,j}^{n-1}-\tau _{,j}^{hk,n-1})) , \tau _{,i}^n-\tau ^{hk,n}_{,i}-(\tau _{,i}^{n-1}-\tau _{,i}^{hk,n-1}))_Y\Big \} \nonumber \\&\qquad -(d_{ij}(M^n_i-M^{hk,n}_i),\theta ^n_{,j}-\theta ^{hk,n}_{,j})_Y+ (A_{ij}^1(M^n_i-M^{hk,n}_i),\theta ^n_{,j}-\theta ^{hk,n}_{,j})_Y \nonumber \\&\qquad +(b_{ij}(M^n_{i,j}-M^{hk,n}_{i,j}),\theta ^n-\theta ^{hk,n})_Y +(a_{ij}(v^n_{i,j}-v^{hk,n}_{i,j}),\theta ^n-\theta ^{hk,n})_Y \nonumber \\&\qquad +(H_{ij}(\varphi ^n_{,i}-\varphi ^{hk,n}_{,i}),\theta ^n_{,j}-\theta ^{hk,n}_{,j})_Y-\kappa (e_n-e^{hk,n},\theta ^n-\theta ^{hk,n})_Y \nonumber \\&\displaystyle \le C\Big (\Vert \dot{\theta }^n-\delta \theta ^n\Vert _Y^2+\Vert \nabla (\dot{\tau }^n-\delta \tau ^n)\Vert _H^2+\Vert \nabla (\theta ^n-z^h)\Vert _H^2+\Vert \nabla (\tau ^{n}-\tau ^{hk,n})\Vert _H^2 \nonumber \\&\qquad \displaystyle +\Vert \theta ^n-z^h\Vert _Y^2 +\Vert e^n-e^{hk,n}\Vert _Y^2+\Vert \nabla (\varphi ^n-\varphi ^{hk,n})\Vert _H^2+\Vert \varvec{M}^n-\varvec{M}^{hk,n}\Vert _H^2 \nonumber \\&\qquad \displaystyle +\Vert \varvec{v}^n-\varvec{v}^{hk,n}\Vert _H^2 +(\delta {\theta }^n-\delta \theta ^{hk,n} ,\theta ^n-z^h)_Y+\Vert \theta ^n-\theta ^{hk,n}\Vert _Y^2\Big ) \nonumber \\&\qquad \displaystyle +\epsilon \Vert \nabla (\theta ^n-\theta ^{hk,n})\Vert _H^2+\epsilon \Vert \nabla (\varvec{M}^n-\varvec{M}^{hk,n}\Vert _Q^2, \end{aligned}$$
(38)

where \(\epsilon >0\) is assumed small enough.

Finally, we will obtain the error estimates on the microtemperatures. Therefore, we subtract variational equation (18) at time \(t_n\) for a test function \(\varvec{\xi }=\varvec{\xi }^h\in V^h\) and discrete variational equation (27) to obtain the following estimates, for all \(\varvec{\xi }^h=(\xi ^h_i)_{i=1}^d\in V^h:\)

$$\begin{aligned} \begin{array}{l} (c_{ij} (\dot{M}^n_j- \delta M^{hk,n}_j),\xi ^h_i)_Y+(C_{ijkl} (R^n_{k,l}-R^{hk,n}_{k,l}),\xi _{i,j}^h)+(C_{ijkl}^*(M^n_{k,l}-M^{hk,n}_{k,l}),\xi ^h_{i,j})_Y\\ \qquad +(A_{ij}^3 (M^n_j-M^{hk,n}_j) ,\xi ^h_i)_Y- (b_{ij}(\theta ^n-\theta ^{hk,n}),\xi ^h_{i,j})_Y \\ \qquad +(F_{ij}(\varphi ^n-\varphi ^{hk,n}),\xi _{i,j}^h)_Y+(d_{ij}(\theta ^n_{,j}-\theta ^{hk,n}_{,j}) ,\xi ^h_i)_Y+(\alpha _{ij}(e^n_{,j}-e^{hk,n}_{,j}),\xi ^h_{i})_Y \\ \qquad +(A_{ij}^2(\theta ^n_{,j}-\theta ^{hk,n}_{,j}),\xi ^h_i)_Y +(B_{klij}(u^n_{k,l}-u^{hk,n}_{k,l}),\xi ^h_{i,j})_Y=0, \end{array} \end{aligned}$$

and so, for all \(\varvec{\xi }^h=(\xi _i^h)_{i=1}^d\in V^h\), it follows that:

$$\begin{aligned} \begin{array}{l}(c_{ij}(\dot{M}^n_j- \delta M^{hk,n}_j),M^n_i-M^{hk,n}_i)_Y+(C_{ijkl} (R^n_{k,l}-R^{hk,n}_{k,l}),M^n_{i,j}-M^{hk,n}_{i,j})\\ \qquad +(C_{ijkl}^*(M^n_{k,l}-M^{hk,n}_{k,l}),M^n_{i,j}-M^{hk,n}_{i,j})_Y+(A_{ij}^3 (M^n_j-M^{hk,n}_j) ,M^n_i-M^{hk,n}_i)_Y\\ \qquad - (b_{ij}(\theta ^n-\theta ^{hk,n}),M^n_{i,j}-M^{hk,n}_{i,j})_Y +(F_{ij}(\varphi ^n-\varphi ^{hk,n}),M^n_{i,j}-M^{hk,n}_{i,j})_Y\\ \qquad +(d_{ij}(\theta ^n_{,j}-\theta ^{hk,n}_{,j}) ,M^n_i-M^{hk,n}_i)_Y+(\alpha _{ij}(e^n_{,j}-e^{hk,n}_{,j}),M^n_{i}-M^{hk,n}_{i})_Y\\ \qquad +(A_{ij}^2(\theta ^n_{,j}-\theta ^{hk,n}_{,j}),M^n_i-M^{hk,n}_i)_Y+(B_{klij}(u^n_{k,l}-u^{hk,n}_{k,l}),M^n_{i,j}-M^{hk,n}_{i,j})_Y\\ \quad =(c_{ij}(\dot{M}^n_j- \delta M^{hk,n}_j),M^n_i-\xi ^h_i)_Y+(C_{ijkl} (R^n_{k,l}-R^{hk,n}_{k,l}),M^n_{i,j}-\xi ^h_{i,j})\\ \qquad +(C_{ijkl}^*(M^n_{k,l}-M^{hk,n}_{k,l}),M^n_{i,j}-\xi ^h_{i,j})_Y+(A_{ij}^3 (M^n_j-M^{hk,n}_j) ,M^n_i-\xi ^h_i)_Y\\ \qquad - (b_{ij}(\theta ^n-\theta ^{hk,n}),M^n_{i,j}-\xi ^h_{i,j})_Y +(F_{ij}(\varphi ^n-\varphi ^{hk,n}),M^n_{i,j}-\xi ^h_{i,j})_Y\\ \qquad +(d_{ij}(\theta ^n_{,j}-\theta ^{hk,n}_{,j}) ,M^n_i-\xi ^h_i)_Y+(\alpha _{ij}(e^n_{,j}-e^{hk,n}_{,j}),M^n_{i}-\xi ^h_{i})_Y\\ \qquad +(A_{ij}^2(\theta ^n_{,j}-\theta ^{hk,n}_{,j}),M^n_i-\xi ^h_i)_Y +(B_{klij}(u^n_{k,l}-u^{hk,n}_{k,l}),M^n_{i,j}-\xi ^h_{i,j})_Y. \end{array} \end{aligned}$$

Keeping in mind that:

$$\begin{aligned} \begin{array}{l} \displaystyle (c_{ij}(\dot{M}^n_j-\delta M^{hk,n}_j) ,M^n_i-M^{hk,n}_i)_Y\ge (c_{ij}(\dot{M}^n_j-\delta M^n_j),M^n_i-M^{hk,n}_i)_Y\\ \qquad \displaystyle +\frac{C_2}{2k}\left\{ \Vert \varvec{M}^n-\varvec{M}^{hk,n}\Vert ^2_H-\Vert \varvec{M}^{n-1}-\varvec{M}^{hk,n-1}\Vert _H^2\right\} ,\\ (C_{ijkl}(R_{k,l}^n-R_{k,l}^{hk,n}) , M_{i,j}^n-M^{hk,n}_{i,j})_Y= (C_{ijkl}(R_{k,l}^n-R_{k,l}^{hk,n}) , \dot{R}_{i,j}^n-\delta R^{n}_{i,j})_Y\\ \qquad + (C_{ijkl}(R_{k,l}^n-R_{k,l}^{hk,n}) , \delta R_{i,j}^n-\delta R^{hk,n}_{i,j})_Y,\\ \displaystyle (C_{ijkl}(R_{k,l}^n-R_{k,l}^{hk,n}) , \delta R_{i,j}^n-\delta R^{hk,n}_{i,j})_Y=\frac{1}{2k}\Big \{(C_{ijkl}(R_{k,l}^n-R_{k,l}^{hk,n}) , R_{i,j}^n-R^{hk,n}_{i,j})_Y\\ \qquad \displaystyle -(C_{ijkl}(R_{k,l}^{n-1}-R_{k,l}^{hk,n-1}) , R_{i,j}^{n-1}-R^{hk,n-1}_{i,j})_Y\\ \qquad \displaystyle +(C_{ijkl}(R_{k,l}^n-R_{k,l}^{hk,n}- (R_{k,l}^{n-1}-R_{k,l}^{hk,n-1})) , R_{i,j}^n-R^{hk,n}_{i,j}-(R_{i,j}^{n-1}-R_{i,j}^{hk,n-1}))_Y\Big \},\\ (\alpha _{ij}(e^n_{,j}-e^{hk,n}_{,j}),M^n_{i}-\xi ^h_{i})_Y=-(\alpha _{ij}(e^n-e^{hk,n}),M^n_{i,j}-\xi ^h_{i,j})_Y, \end{array} \end{aligned}$$

where we used the notations \(\delta \varvec{M}^n=(\varvec{M}^n-\varvec{M}^{n-1})/k\) and \(\delta \varvec{R}^n=(\varvec{R}^n-\varvec{R}^{n-1})/k\), and assumption (34), we obtain the following estimates for the microtemperatures, for all \(\varvec{\xi }^h=(\xi _i^h)_{i=1}^d\in V^h:\)

$$\begin{aligned}&\displaystyle \frac{C_2}{2k}\left\{ \Vert \varvec{M}^n-\varvec{M}^{hk,n}\Vert ^2_H-\Vert \varvec{M}^{n-1}-\varvec{M}^{hk,n-1}\Vert _H^2\right\} +(C_{ijkl}^*(M^n_{k,l}-M^{hk,n}_{k,l}),M^n_{i,j}-M^{hk,n}_{i,j})_Y \nonumber \\&\qquad \displaystyle +\frac{1}{2k}\Big \{(C_{ijkl}(R_{k,l}^n-R_{k,l}^{hk,n}) , R_{i,j}^n-R^{hk,n}_{i,j})_Y\nonumber \\&\qquad -(C_{ijkl}(R_{k,l}^{n-1}-R_{k,l}^{hk,n-1}) , R_{i,j}^{n-1}-R^{hk,n-1}_{i,j})_Y\nonumber \\&\qquad \displaystyle +(C_{ijkl}(R_{k,l}^n-R_{k,l}^{hk,n}- (R_{k,l}^{n-1}-R_{k,l}^{hk,n-1})) , R_{i,j}^n-R^{hk,n}_{i,j}-(R_{i,j}^{n-1}-R_{i,j}^{hk,n-1}))_Y\Big \} \nonumber \\&\qquad \displaystyle +(A_{ij}^3 (M^n_j-M^{hk,n}_j) ,M^n_i-M^{hk,n}_i)_Y \nonumber \\&\qquad - (b_{ij}(\theta ^n-\theta ^{hk,n}),M^n_{i,j}-M^{hk,n}_{i,j})_Y +(F_{ij}(\varphi ^n-\varphi ^{hk,n}),M^n_{i,j}-M^{hk,n}_{i,j})_Y \nonumber \\&\qquad +(d_{ij}(\theta ^n_{,j}-\theta ^{hk,n}_{,j}) ,M^n_i-M^{hk,n}_i)_Y-(\alpha _{ij}(e^n-e^{hk,n}),M^n_{i,j}-M^{hk,n}_{i,j})_Y \nonumber \\&\qquad +(A_{ij}^2(\theta ^n_{,j}-\theta ^{hk,n}_{,j}),M^n_i-M^{hk,n}_i)_Y+(B_{klij}(u^n_{k,l}-u^{hk,n}_{k,l}),M^n_{i,j}-M^{hk,n}_{i,j})_Y \nonumber \\&\displaystyle \le C\Big (\Vert \dot{\varvec{M}}^n-\delta \varvec{M}^n\Vert _H^2+\Vert \nabla (\dot{\varvec{R}}^n-\delta \varvec{R}^n)\Vert _Q^2+\Vert \nabla (\varvec{M}^h-\varvec{\xi }^h)\Vert _Q^2+\Vert \nabla (\varvec{u}^{n}-\varvec{u}^{hk,n})\Vert _Q^2\nonumber \\&\qquad \displaystyle +\Vert \varvec{M}^n-\varvec{\xi }^h\Vert _H^2 +\Vert \theta ^n-\theta ^{hk,n}\Vert _Y^2+\Vert \varphi ^n-\varphi ^{hk,n}\Vert _Y^2+\Vert \nabla (\varvec{R}^n-\varvec{R}^{hk,n})\Vert _Q^2 \nonumber \\&\qquad \displaystyle +\Vert e^n-e^{hk,n}\Vert _Y^2+(\delta {M}^n_i-\delta M^{hk,n}_i ,M^n_i-\xi _i^h)_Y+\Vert \varvec{M}^n-\varvec{M}^{hk,n}\Vert _H^2\Big ) \nonumber \\&\qquad \displaystyle +\epsilon \Vert \nabla (\theta ^n-\theta ^{hk,n})\Vert _H^2+\epsilon \Vert \nabla (\varvec{M}^n-\varvec{M}^{hk,n})\Vert _Q^2, \end{aligned}$$
(39)

where \(\epsilon >0\) is assumed small enough.

Using assumption (10), we find that:

$$\begin{aligned} \begin{array}{l} (C_{ijkl}^*(M^n_{k,l}-M^{hk,n}_{k,l}),M^n_{i,j}-M^{hk,n}_{i,j})_Y+((A_{ij}^1+A_{ij}^2)(\theta ^n_{,j}-\theta ^{hk,n}_{,j}),M^n_i-M^{hk,n}_i)_Y\\ \qquad + (K_{ij}^*(\theta ^n_{,i}-\theta ^{hk,n}_{,i}),\theta ^n_{,j}-\theta ^{hk,n}_{,j})_Y+ (A_{ij}^3(M^n_j-M_j^{hk,n}),M^n_i-M_i^{hk,n})_Y\\ \qquad \ge K_0(\Vert \nabla (\theta ^n-\theta ^{hk,n})\Vert _H^2+\Vert \nabla (\varvec{M}^n-\varvec{M}^{hk,n})\Vert _Q^2+\Vert \varvec{M}^n-\varvec{M}^{hk,n}\Vert _H^2).\end{array} \end{aligned}$$

Therefore, combining estimates (36)–(39), it follows that:

$$\begin{aligned}&\displaystyle \frac{\rho }{2k}\left\{ \Vert \varvec{v}^n-\varvec{v}^{hk,n}\Vert ^2_H-\Vert \varvec{v}^{n-1}-\varvec{v}^{hk,n-1}\Vert _H^2\right\} +\frac{1}{2k}\Big \{(A_{ijkl}(u_{k,l}^n-u_{k,l}^{hk,n}) , u_{i,j}^n-u^{hk,n}_{i,j})_Y \nonumber \\&\displaystyle \qquad \displaystyle -(A_{ijkl}(u_{k,l}^{n-1}-u_{k,l}^{hk,n-1}) , u_{i,j}^{n-1}-u^{hk,n-1}_{i,j})_Y \nonumber \\&\qquad \displaystyle +(A_{ijkl}(u_{k,l}^n-u_{k,l}^{hk,n}- (u_{k,l}^{n-1}-u_{k,l}^{hk,n-1})) , u_{i,j}^n-u^{hk,n}_{i,j}-(u_{i,j}^{n-1}-u_{i,j}^{hk,n-1}))_Y\Big \} \nonumber \\&\qquad \displaystyle +(B_{ijkl}(R^n_{k,l}-R_{k,l}^{hk,n}),v_{i,j}^n-v^{hk,n}_{i,j})_Y +(B_{klij}(u^n_{k,l}-u^{hk,n}_{k,l}),M^n_{i,j}-M^{hk,n}_{i,j})_Y \nonumber \\&\qquad \displaystyle + \frac{J}{2k}\left\{ \Vert e^n-e^{hk,n}\Vert ^2_Y-\Vert e^{n-1}-e^{hk,n-1}\Vert _Y^2\right\} + \frac{1}{2k}\Big \{(A_{ij}(\varphi _{,j}^n-\varphi _{,j}^{hk,n}) , \varphi _{,i}^n-\varphi ^{hk,n}_{,i})_Y \nonumber \\&\qquad \displaystyle -(A_{ij}(\varphi _{,j}^{n-1}-\varphi _{,j}^{hk,n-1}) , \varphi _{,i}^{n-1}-\varphi ^{hk,n-1}_{,i})_Y \nonumber \\&\qquad \displaystyle +(A_{ij}(\varphi _{,j}^n-\varphi _{,j}^{hk,n}- (\varphi _{,j}^{n-1}-\varphi _{,j}^{hk,n-1})) , \varphi _{,i}^n-\varphi ^{hk,n}_{,i}-(\varphi _{,i}^{n-1}-\varphi _{,i}^{hk,n-1}))_Y\Big \}\nonumber \\&\qquad \displaystyle +\frac{\xi }{2k}\Big \{\Vert \xi ^n-\xi ^{hk,n}\Vert _Y^2 - \Vert \xi ^{n-1}-\xi ^{hk,n-1}\Vert _Y^2+\Vert \xi ^n-\xi ^{hk,n}-(\xi ^{n-1}-\xi ^{hk,n-1})\Vert _Y^2\Big \}\nonumber \\&\qquad \displaystyle +(H_{ij}(\tau ^n_{,i}-\tau ^{hk,n}_{,i}),e^n_{,j}-e^{hk,n}_{,j})_Y+(H_{ij}(\varphi ^n_{,i}-\varphi ^{hk,n}_{,i}),\theta ^n_{,j}-\theta ^{hk,n}_{,j})_Y\nonumber \\&\qquad \displaystyle +(\zeta _{ij}(u^n_{i,j}-u^{hk,n}_{i,j}),e^n-e^{hk,n})_Y +(\zeta _{ij}(\varphi ^n-\varphi ^{hk,n}),v_{i,j}^n-v^{hk,n}_{i,j})_Y\nonumber \\&\qquad \displaystyle +(F_{ij}(R^n_{i,j}-R^{hk,n}_{i,j}),e^n-e^{hk,n})_Y+(F_{ij}(\varphi ^n-\varphi ^{hk,n}),M^n_{i,j}-M^{hk,n}_{i,j})_Y\nonumber \\&\qquad \displaystyle + \frac{1}{2k}\left\{ \Vert \theta ^n-\theta ^{hk,n}\Vert ^2_Y-\Vert \theta ^{n-1}-\theta ^{hk,n-1}\Vert _Y^2\right\} +\frac{1}{2k}\Big \{(K_{ij}(\tau _{,j}^n-\tau _{,j}^{hk,n}) , \tau _{,i}^n-\tau ^{hk,n}_{,i})_Y \nonumber \\&\qquad \displaystyle -(K_{ij}(\tau _{,j}^{n-1}-\tau _{,j}^{hk,n-1}) , \tau _{,i}^{n-1}-\tau ^{hk,n-1}_{,i})_Y \nonumber \\&\qquad \displaystyle +(K_{ij}(\tau _{,j}^n-\tau _{,j}^{hk,n}- (\tau _{,j}^{n-1}-\tau _{,j}^{hk,n-1})) , \tau _{,i}^n-\tau ^{hk,n}_{,i}-(\tau _{,i}^{n-1}-\tau _{,i}^{hk,n-1}))_Y\Big \} \nonumber \\&\qquad \displaystyle +\frac{C_2}{2k}\left\{ \Vert \varvec{M}^n-\varvec{M}^{hk,n}\Vert ^2_H-\Vert \varvec{M}^{n-1}-\varvec{M}^{hk,n-1}\Vert _H^2\right\} \nonumber \\&\displaystyle +\frac{1}{2k}\Big \{(C_{ijkl}(R_{k,l}^n-R_{k,l}^{hk,n}) , R_{i,j}^n-R^{hk,n}_{i,j})_Y -(C_{ijkl}(R_{k,l}^{n-1}-R_{k,l}^{hk,n-1}) , R_{i,j}^{n-1}-R^{hk,n-1}_{i,j})_Y \nonumber \\&\qquad \displaystyle +(C_{ijkl}(R_{k,l}^n-R_{k,l}^{hk,n}- (R_{k,l}^{n-1}-R_{k,l}^{hk,n-1})) , R_{i,j}^n-R^{hk,n}_{i,j}-(R_{i,j}^{n-1}-R_{i,j}^{hk,n-1}))_Y\Big \}\nonumber \\&\displaystyle \le C\Big (\Vert \dot{\varvec{v}}^n-\delta \varvec{v}^n\Vert _H^2+\Vert \nabla (\dot{\varvec{u}}^n-\delta \varvec{u}^n)\Vert _Q^2+\Vert \nabla (\varvec{v}^h-\varvec{w}^h)\Vert _Q^2+\Vert \nabla (\varvec{u}^{n}-\varvec{u}^{hk,n})\Vert _Q^2 \nonumber \\&\qquad \displaystyle +\Vert \varvec{v}^n-\varvec{w}^h\Vert _H^2 +\Vert \theta ^n-\theta ^{hk,n}\Vert _Y^2+\Vert \varphi ^n-\varphi ^{hk,n}\Vert _Y^2+\Vert \nabla (\varvec{R}^n-\varvec{R}^{hk,n})\Vert _Q^2 \nonumber \\&\qquad \displaystyle +(\delta {v}^n_i-\delta v^{hk,n}_i ,v^n_i-w_i^h)_Y+\Vert \dot{e}^n-\delta e^n\Vert _Y^2+\Vert \nabla (\dot{\varphi }^n-\delta \varphi ^n)\Vert _H^2+\Vert \nabla (e^h-r^h)\Vert _H^2 \nonumber \\&\qquad \displaystyle +\Vert \nabla (\varphi ^{n}-\varphi ^{hk,n})\Vert _H^2+\Vert e^n-r^h\Vert _Y^2 +\Vert \varvec{M}^n-\varvec{M}^{hk,n}\Vert _H^2+\Vert \nabla (\tau ^n-\tau ^{hk,n})\Vert _H^2 \nonumber \\&\qquad \displaystyle +(\delta {e}^n-\delta e^{hk,n} ,e^n-r^h)_Y+\Vert \dot{\theta }^n-\delta \theta ^n\Vert _Y^2+\Vert \nabla (\dot{\tau }^n-\delta \tau ^n)\Vert _H^2+\Vert \nabla (\theta ^n-z^h)\Vert _H^2 \nonumber \\&\qquad \displaystyle +\Vert \theta ^n-z^h\Vert _Y^2 +\Vert e^n-e^{hk,n}\Vert _Y^2 +\Vert \varvec{v}^n-\varvec{v}^{hk,n}\Vert _H^2 +(\delta {\theta }^n-\delta \theta ^{hk,n} ,\theta ^n-z^h)_Y \nonumber \\&\qquad \displaystyle +\Vert \dot{\varvec{M}}^n-\delta \varvec{M}^n\Vert _H^2+\Vert \nabla (\dot{\varvec{R}}^n-\delta \varvec{R}^n)\Vert _Q^2+\Vert \nabla (\varvec{M}^h-\varvec{\xi }^h)\Vert _Q^2+\Vert \varvec{M}^n-\varvec{\xi }^h\Vert _H^2 \nonumber \\&\qquad \displaystyle +(\delta {M}^n_i-\delta M^{hk,n}_i ,M^n_i-\xi _i^h)_Y\Big )\quad \forall \varvec{w}^h,\,\varvec{\xi }^h\in V^h,\, r^h,\,z^h\in E^h. \end{aligned}$$
(40)

Now, we observe that:

$$\begin{aligned} \begin{array}{l} \displaystyle (B_{ijkl}(R^n_{k,l}-R_{k,l}^{hk,n}),\delta u_{i,j}^n-\delta u^{hk,n}_{i,j})_Y +(B_{klij}(u^n_{k,l}-u^{hk,n}_{k,l}),\delta R^n_{i,j}-\delta R^{hk,n}_{i,j})_Y\\ \quad \displaystyle =\frac{1}{k}\Big \{(B_{ijkl}(R^n_{k,l}-R_{k,l}^{hk,n}), u_{i,j}^n- u^{hk,n}_{i,j})_Y-(B_{ijkl}(R^{n-1}_{k,l}-R_{k,l}^{hk,n-1}), u_{i,j}^{n-1}- u^{hk,n-1}_{i,j})_Y\\ \qquad \displaystyle +(B_{ijkl}(R^n_{k,l}-R_{k,l}^{hk,n}-(R^{n-1}_{k,l}-R_{k,l}^{hk,n-1})), u_{i,j}^n- u^{hk,n}_{i,j}-(u_{i,j}^{n-1}- u^{hk,n-1}_{i,j}))_Y\Big \}, \\ \displaystyle (\zeta _{ij}(u^n_{i,j}-u^{hk,n}_{i,j}),\delta \varphi ^n-\delta \varphi ^{hk,n})_Y +(\zeta _{ij}(\varphi ^n-\varphi ^{hk,n}),\delta u_{i,j}^n-\delta u^{hk,n}_{i,j})_Y\\ \quad \displaystyle =\frac{1}{k}\Big \{(\zeta _{ij}(u^n_{i,j}-u^{hk,n}_{i,j}), \varphi ^n-\varphi ^{hk,n})_Y-(\zeta _{ij}(u^{n-1}_{i,j}-u^{hk,n-1}_{i,j}), \varphi ^{n-1}-\varphi ^{hk,n-1})_Y\\ \qquad \displaystyle + (\zeta _{ij}(u^n_{i,j}-u^{hk,n}_{i,j}-(u^{n-1}_{i,j}-u^{hk,n-1}_{i,j})), \varphi ^n-\varphi ^{hk,n}-(\varphi ^{n-1}-\varphi ^{hk,n-1}))_Y\Big \}, \\ (H_{ij}(\tau ^n_{,i}-\tau ^{hk,n}_{,i}),\delta \varphi ^n_{,j}-\delta \varphi ^{hk,n}_{,j})_Y +(H_{ij}(\varphi ^n_{,i}-\varphi ^{hk,n}_{,i}),\delta \tau ^n_{,j}-\delta \tau ^{hk,n}_{,j})_Y\\ \quad \displaystyle =\frac{1}{k}\Big \{ (H_{ij}(\tau ^n_{,i}-\tau ^{hk,n}_{,i}), \varphi ^n_{,j}- \varphi ^{hk,n}_{,j})_Y -(H_{ij}(\tau ^{n-1}_{,i}-\tau ^{hk,n-1}_{,i}), \varphi ^{n-1}_{,j}-\varphi ^{hk,n-1}_{,j})_Y\\ \qquad \displaystyle + (H_{ij}(\tau ^n_{,i}-\tau ^{hk,n}_{,i}-(\tau ^{n-1}_{,i}-\tau ^{hk,n-1}_{,i})), \varphi ^n_{,j}- \varphi ^{hk,n}_{,j}-(\varphi ^{n-1}_{,j}- \varphi ^{hk,n-1}_{,j}))_Y\Big \} , \end{array} \end{aligned}$$

so using again assumptions (11), it follows that:

$$\begin{aligned} \begin{array}{l} \displaystyle (A_{ijkl}(u_{k,l}^n-u_{k,l}^{hk,n}-(u_{k,l}^{n-1}-u_{k,l}^{hk,n-1})), u_{i,j}^n-u^{hk,n}_{i,j}-(u_{i,j}^{n-1}-u_{i,j}^{hk,n-1}))_Y\\ \qquad +2(B_{ijkl}(R_{k,l}^{n}-R_{k,l}^{hk,n}-(R_{k,l}^{n-1}-R_{k,l}^{hk,n-1})),u^{n}_{i,j}-u^{hk,n}_{i,j}-(u^{n-1}_{i,j}-u^{hk,n-1}_{i,j}))_Y\\ \qquad +(C_{ijkl}(R^{n}_{k,l}-R^{hk,n}_{k,l}-(R^{n-1}_{k,l}-R^{hk,n-1}_{k,l})),R^n_{i,j}-R^{hk,n}_{i,j}-(R^{n-1}_{i,j}-R^{hk,n-1}_{i,j}))_Y\\ \qquad +2(\zeta _{ij}(\varphi ^n-\varphi ^{hk,n}-(\varphi ^{n-1}-\varphi ^{hk,n-1})),u^n_{i,j}-u^{hk,n}_{i,j}-(u^{n-1}_{i,j}-u^{hk,n-1}_{i,j}))_Y\\ \qquad +\xi \Vert \varphi ^n-\varphi ^{hk,n}-(\varphi ^{n-1}-\varphi ^{hk,n-1})\Vert _Y^2\ge 0,\\ \qquad (A_{ij}(\varphi ^n_{,j}-\varphi ^{hk,n}_{,j}-(\varphi ^{n-1}_{,j}-\varphi ^{hk,n-1}_{,j})),\varphi ^n_{,i}-\varphi ^{hk,n}_{,i}-(\varphi ^{n-1}_{,i}-\varphi ^{hk,n-1}_{,i}))_Y\\ \qquad + 2(H_{ij}(\tau ^n_{,i}-\tau ^{hk,n}_{,i}-(\tau ^{n-1}_{,i}-\tau ^{hk,n-1}_{,i})),\varphi ^n_{,j}-\varphi ^{hk,n}_{,j}-(\varphi ^{n-1}_{,j}-\varphi ^{hk,n-1}_{,j}))_Y\\ \qquad +(K_{ij}(\tau ^n_{,i}-\tau ^{hk,n}_{,i}-(\tau ^{n-1}_{,i}-\tau ^{hk,n-1}_{,i})),\tau ^n_{,j}-\tau ^{hk,n}_{,j}-(\tau ^{n-1}_{,j}-\tau ^{hk,n-1}_{,j}))_Y\ge 0. \end{array} \end{aligned}$$

Multiplying estimates (40) by k and summing up to n, we have:

$$\begin{aligned}&\displaystyle \Vert \varvec{v}^n-\varvec{v}^{hk,n}\Vert ^2_H+(A_{ijkl}(u_{k,l}^n-u_{k,l}^{hk,n}) , u_{i,j}^n-u^{hk,n}_{i,j})_Y\\&\quad +2(B_{ijkl}(R^n_{k,l}-R_{k,l}^{hk,n}),u_{i,j}^n-u^{hk,n}_{i,j})_Y \\&\qquad \displaystyle + \Vert e^n-e^{hk,n}\Vert ^2_Y+(A_{ij}(\varphi _{,j}^n-\varphi _{,j}^{hk,n}) , \varphi _{,i}^n-\varphi ^{hk,n}_{,i})_Y+\Vert \xi ^n-\xi ^{hk,n}\Vert _Y^2 \\&\qquad \displaystyle +2(H_{ij}(\tau ^n_{,i}-\tau ^{hk,n}_{,i}),\varphi ^n_{,j}-\varphi ^{hk,n}_{,j})_Y+2(\zeta _{ij}(u^n_{i,j}-u^{hk,n}_{i,j}),e^n-e^{hk,n})_Y \\&\qquad \displaystyle +(K_{ij}(\tau _{,j}^n-\tau _{,j}^{hk,n}) , \tau _{,i}^n-\tau ^{hk,n}_{,i})_Y + \Vert \theta ^n-\theta ^{hk,n}\Vert ^2_Y+ \Vert \varvec{M}^n-\varvec{M}^{hk,n}\Vert ^2_H\\&\qquad \displaystyle +(C_{ijkl}(R_{k,l}^n-R_{k,l}^{hk,n}) , R_{i,j}^n-R^{hk,n}_{i,j})_Y\\&\displaystyle \le Ck\sum _{j=1}^n\Big (\Vert \dot{\varvec{v}}^j-\delta \varvec{v}^j\Vert _H^2+\Vert \nabla (\dot{\varvec{u}}^j-\delta \varvec{u}^j)\Vert _Q^2+\Vert \nabla (\varvec{v}^j-\varvec{w}^{h,j})\Vert _Q^2+\Vert \nabla (\varvec{u}^{j}-\varvec{u}^{hk,j})\Vert _Q^2\\&\qquad \displaystyle +\Vert \varvec{v}^j-\varvec{w}^{h,j}\Vert _H^2 +\Vert \theta ^j-\theta ^{hk,j}\Vert _Y^2+\Vert \varphi ^j-\varphi ^{hk,j}\Vert _Y^2+\Vert \nabla (\varvec{R}^j-\varvec{R}^{hk,j})\Vert _Q^2\\&\qquad \displaystyle +(\delta {v}^j_i-\delta v^{hk,j}_i ,v^j_i-w_i^{h,j})_Y+\Vert \dot{e}^j-\delta e^j\Vert _Y^2+\Vert \nabla (\dot{\varphi }^j-\delta \varphi ^j)\Vert _H^2+\Vert \nabla (e^j-r^{h,j})\Vert _H^2\\&\qquad \displaystyle +\Vert \nabla (\varphi ^{j}-\varphi ^{hk,j})\Vert _H^2+\Vert e^j-r^{h,j}\Vert _Y^2 +\Vert \varvec{M}^j-\varvec{M}^{hk,j}\Vert _H^2+\Vert \nabla (\tau ^j-\tau ^{hk,j})\Vert _H^2\\&\qquad \displaystyle +(\delta {e}^j-\delta e^{hk,j} ,e^j-r^{h,j})_Y+\Vert \dot{\theta }^j-\delta \theta ^j\Vert _Y^2+\Vert \nabla (\dot{\tau }^j-\delta \tau ^j)\Vert _H^2+\Vert \nabla (\theta ^j-z^{h,j})\Vert _H^2\\&\qquad \displaystyle +\Vert \theta ^j-z^{h,j}\Vert _Y^2 +\Vert e^j-e^{hk,j}\Vert _Y^2 +\Vert \varvec{v}^j-\varvec{v}^{hk,j}\Vert _H^2 +(\delta {\theta }^j-\delta \theta ^{hk,j} ,\theta ^j-z^{h,j})_Y \\&\qquad \displaystyle +\Vert \dot{\varvec{M}}^j-\delta \varvec{M}^j\Vert _H^2+\Vert \nabla (\dot{\varvec{R}}^j-\delta \varvec{R}^j)\Vert _Q^2+\Vert \nabla (\varvec{M}^j-\varvec{\xi }^{h,j})\Vert _Q^2+\Vert \varvec{M}^j-\varvec{\xi }^{h,j}\Vert _H^2\\&\qquad \displaystyle +(\delta {M}^j_i-\delta M^{hk,j}_i ,M^j_i-\xi _i^{h,j})_Y\Big )\\&\quad +C\Big (\Vert \varvec{v}^0-\varvec{v}^{0h}\Vert ^2_H+\Vert \nabla (\varvec{u}^0-\varvec{u}^{0h})\Vert _Q^2+\Vert e^0-e^{0h}\Vert ^2_Y \\&\qquad \displaystyle +\Vert \nabla (\varphi ^0-\varphi ^{0h})\Vert _H^2+\Vert \xi ^0-\xi ^{0h}\Vert _Y^2+\Vert \theta ^0-\theta ^{0h}\Vert ^2_Y\\&\qquad +\Vert \nabla (\tau ^0-\tau ^{0h})\Vert _H^2 + \Vert \varvec{M}^0-\varvec{M}^{0h}\Vert ^2_H\\&\qquad \displaystyle +\Vert \nabla (\varvec{R}^0-\varvec{R}^{0h})\Vert _Q^2\Big ) \quad \forall \varvec{w}^h,\,\varvec{\xi }^h\in V^h,\, r^h,\,z^h\in E^h. \end{aligned}$$

Finally, using assumptions (11), we find that:

$$\begin{aligned}&\displaystyle (A_{ijkl}(u_{k,l}^n-u_{k,l}^{hk,n}), u_{i,j}^n-u^{hk,n}_{i,j})_Y +2(B_{ijkl}(R_{k,l}^{n}-R_{k,l}^{hk,n}),u^{n}_{i,j}-u^{hk,n}_{i,j})_Y\\&\quad +(C_{ijkl}(R^{n}_{k,l}-R^{hk,n}_{k,l}),R^n_{i,j}-R^{hk,n}_{i,j})_Y +2(\zeta _{ij}(\varphi ^n-\varphi ^{hk,n}),u^n_{i,j}-u^{hk,n}_{i,j})_Y\\&\quad +\xi \Vert \varphi ^n-\varphi ^{hk,n}\Vert _Y^2\ge C\Big (\Vert \nabla (\varvec{u}^n-\varvec{u}^{hk,n})\Vert _Q^2+\Vert \nabla (\varvec{R}^n-\varvec{R}^{hk,n})\Vert _H^2+\Vert \varphi ^n-\varphi ^{hk,n}\Vert _Y^2\Big ),\\&\quad (A_{ij}(\varphi ^n_{,j}-\varphi ^{hk,n}_{,j}),\varphi ^n_{,i}-\varphi ^{hk,n}_{,i})_Y + 2(H_{ij}(\tau ^n_{,i}-\tau ^{hk,n}_{,i}),\varphi ^n_{,j}-\varphi ^{hk,n}_{,j})_Y\\&\quad +(K_{ij}(\tau ^n_{,i}-\tau ^{hk,n}_{,i}),\tau ^n_{,j}-\tau ^{hk,n}_{,j})_Y\ge C\Big (\Vert \nabla (\varphi ^n-\varphi ^{hk,n})\Vert _H^2+\Vert \nabla (\tau ^n-\tau ^{hk,n})\Vert _H^2\Big ). \end{aligned}$$

Therefore, taking into account that:

$$\begin{aligned}&\displaystyle k\sum _{j=1}^n (\delta \varvec{v}^j-\delta \varvec{v}^{hk,j},\varvec{v}^j-\varvec{w}^{h,j})_H=\sum _{j=1}^n (\varvec{v}^{j}-\varvec{v}^{hk,j}-(\varvec{v}^{j-1}-\varvec{v}^{hk,j-1}),\varvec{v}^j-\varvec{w}^{h,j})_H\\&\displaystyle \qquad =(\varvec{v}^n-\varvec{v}^{hk,n},\varvec{v}^n-\varvec{w}^{h,n})_H+(\varvec{v}^{0h}-\varvec{v}^0,\varvec{v}^1-\varvec{w}^{h,1})_H\\&\displaystyle \qquad \qquad + \sum _{j=1}^{n-1} (\varvec{v}^j-\varvec{v}^{hk,j},\varvec{v}^j-\varvec{w}^{h,j}-(\varvec{v}^{j+1}-\varvec{w}^{h,j+1}))_H,\\&\displaystyle k\sum _{j=1}^n (\delta e^j-\delta e^{hk,j},e^j-r^{h,j})_Y=\sum _{j=1}^n (e^{j}-e^{hk,j}-(e^{j-1}-e^{hk,j-1}),e^j-r^{h,j})_Y\\&\displaystyle \qquad =(e^n-e^{hk,n},e^n-r^{h,n})_Y+(e^{0h}-e^0,e^1-r^{h,1})_Y\\&\displaystyle \qquad \qquad + \sum _{j=1}^{n-1} (e^j-e^{hk,j},e^j-r^{h,j}-(e^{j+1}-r^{h,j+1}))_Y,\\&\displaystyle k\sum _{j=1}^n (\delta \theta ^j-\delta \theta ^{hk,j},\theta ^j-z^{h,j})_Y=\sum _{j=1}^n (\theta ^{j}-\theta ^{hk,j}-(\theta ^{j-1}-\theta ^{hk,j-1}),\theta ^j-r^{h,j})_Y\\&\displaystyle \qquad =(\theta ^n-\theta ^{hk,n},\theta ^n-r^{h,n})_Y+(\theta ^{0h}-\theta ^0,\theta ^1-r^{h,1})_Y\\&\displaystyle \qquad \qquad + \sum _{j=1}^{n-1} (\theta ^j-\theta ^{hk,j},\theta ^j-r^{h,j}-(\theta ^{j+1}-r^{h,j+1}))_Y,\\&\displaystyle k\sum _{j=1}^n (\delta \varvec{M}^j-\delta \varvec{M}^{hk,j},\varvec{M}^j-\varvec{\xi }^{h,j})_H\\&\qquad =\sum _{j=1}^n (\varvec{M}^{j}-\varvec{M}^{hk,j}-(\varvec{M}^{j-1}-\varvec{M}^{hk,j-1}),\varvec{M}^j-\varvec{\xi }^{h,j})_H\\&\displaystyle \qquad =(\varvec{M}^n-\varvec{M}^{hk,n},\varvec{M}^n-\varvec{\xi }^{h,n})_H+(\varvec{M}^{0h}-\varvec{M}^0,\varvec{M}^1-\varvec{\xi }^{h,1})_H\\&\displaystyle \qquad \qquad + \sum _{j=1}^{n-1} (\varvec{M}^j-\varvec{M}^{hk,j},\varvec{M}^j-\varvec{\xi }^{h,j}-(\varvec{M}^{j+1}-\varvec{\xi }^{h,j+1}))_H, \end{aligned}$$

applying again a discrete version of Gronwall’s inequality (see Campo et al. 2006), we derive error estimates (35).

We remark that these a priori error estimates can be used to obtain the convergence order of the approximations given by Problem VP\(^{hk}\). Thus, as an example, we have the following result which states the linear convergence of the algorithm under suitable additional regularity conditions.

Corollary 1

Let the assumptions of Theorem 2 hold. Therefore, if we assume the following additional regularity:

$$\begin{aligned} \begin{array}{l} \varvec{u},\,\varvec{R}\in H^3(0,T;H)\cap W^{1,\infty }(0,T;[H^2(\varOmega )]^d)\cap H^2(0,T;V),\\ \theta ,\,\tau \in H^2(0,T;Y)\cap L^{\infty }(0,T;H^2(\varOmega ))\cap H^1(0,T;E), \end{array} \end{aligned}$$

it follows that the approximations obtained by Problem VP\(^{hk}\) are linearly convergent; that is, there exists a positive constant C, independent of the discretization parameters h and k, such that:

$$\begin{aligned} \begin{array}{l} \displaystyle \max _{0\le n\le N}\Big \{ \Vert \varvec{v}^n-\varvec{v}^{hk,n}\Vert _H+\Vert \nabla (\varvec{u}^n-\varvec{u}^{hk,n})\Vert _Q + \Vert e^n-e^{hk,n}\Vert _Y+\Vert \nabla (\varphi ^n-\varphi ^{hk,n})\Vert _H \\ \qquad \displaystyle + \Vert \theta ^n-\theta ^{hk,n}\Vert _Y +\Vert \varphi ^n-\varphi ^{hk,n}\Vert _Y+\Vert \nabla (\tau ^n-\tau ^{hk,n})\Vert _H + \Vert \varvec{M}^n-\varvec{M}^{hk,n}\Vert _H\\ \qquad +\Vert \nabla (\varvec{R}^n-\varvec{R}^{hk,n})\Vert _Q\Big \}\le C(h+k).\end{array} \end{aligned}$$

4 Numerical results

In this final section, we will present some numerical results obtained in one- and two-dimensional examples.

In the numerical resolution, we assume that the material is homogeneous and isotropic, and therefore, the tensors defined in Problem P can be simplified. In particular, we will assume the following form for all of them:

$$\begin{aligned}&A_{ijkl} u_{k,l}w_{i,j}=(\lambda +\mu ) u_{i,i}w_{i,i}+\mu u_{i,j}w_{i,j} \quad \hbox {for all}\quad \varvec{u}=(u_i)_{i=1}^d,\, \varvec{w}=(w_i)_{i=1}^d\in V,\\&a_{ij}\theta w_{i,j}=a\theta w_{i,i}\quad \hbox {for all}\quad \theta \in E,\, \varvec{w}=(w_i)_{i=1}^d\in V,\\&B_{ijkl}R_{k,l}w_{i,j}=BR_{i,j}w_{i,j}\quad \hbox {for all}\quad \varvec{R}=(R_i)_{i=1}^d,\, \varvec{w}=(w_i)_{i=1}^d\in V,\\&\zeta _{ij}\varphi w_{i,j}=\zeta \varphi w_{i,i} \quad \hbox {for all}\quad \varphi \in E,\, \varvec{w}=(w_i)_{i=1}^d\in V,\\&A_{ij}\varphi _{,j}r_{,i}=A\varphi _{,i}r_{,i} \quad \hbox {for all}\quad \varphi ,\, r\in E,\\&\alpha _{ij} M_i r_{,j}=\alpha M_i r_{,i} \quad \hbox {for all}\quad \varvec{M}=(M_i)_{i=1}^d\in V,\, r\in E,\\&H_{ij}z_{,i}r_{,j}= H z_{,i}r_{,i} \quad \hbox {for all}\quad z,\, r\in E,\\&F_{ij}R_{i,j}r= F R_{i,i}r \quad \hbox {for all}\quad \varvec{R}=(R_i)_{i=1}^d\in V,\, r\in E,\\&K_{ij}^*\theta _{,j}z_{,i}=K^*\theta _{,i}z_{,i} \quad \hbox {for all}\quad \theta ,\, z\in E,\\&K_{ij}\tau _{,j}z_{,i}=K\tau _{,i}z_{,i} \quad \hbox {for all}\quad \tau ,\, z\in E,\\&d_{ij}M_iz_{,j}=D M_i z_{,i} \quad \hbox {for all}\quad \varvec{M}=(M_i)_{i=1}^d\in V,\, z\in E,\\&A^1_{ij}M_iz_{,j}=A^1 M_i z_{,i} \quad \hbox {for all}\quad \varvec{M}=(M_i)_{i=1}^d\in V,\, z\in E,\\&b_{ij}M_iz_{,j}=b M_i z_{,i} \quad \hbox {for all}\quad \varvec{M}=(M_i)_{i=1}^d\in V,\, z\in E,\\&C_{ijkl} R_{k,l}w_{i,j}=C_1 R_{i,i}w_{i,i}+C_2 R_{i,j}w_{i,j}\quad \hbox {for all}\quad \varvec{R}=(R_i)_{i=1}^d,\, \varvec{w}=(w_i)_{i=1}^d\in V,\\&C_{ijkl}^* R_{k,l}w_{i,j}=C_1^* R_{i,i}w_{i,i}+C_2^* R_{i,j}w_{i,j}\quad \hbox {for all}\quad \varvec{R}=(R_i)_{i=1}^d,\, \varvec{w}=(w_i)_{i=1}^d\in V,\\&A^2_{ij}\theta _{,j}w_i=A^2 \theta _{,i} w_i \quad \hbox {for all}\quad \theta \in E,\, \varvec{w}=(w_i)_{i=1}^d\in V,\\&A^3_{ij}M_jw_i=A^3 M_{i} w_i \quad \hbox {for all}\quad \varvec{M}=(M_i)_{i=1}^d,\, \varvec{w}=(w_i)_{i=1}^d\in V,\\&c_{ij}M_j\xi _i=c^*M_i\xi _i \quad \hbox {for all}\quad \varvec{M}=(M_i)_{i=1}^d,\, \varvec{\xi }=(\xi _i)_{i=1}^d\in V. \end{aligned}$$

Hence, using these tensors, discrete problem \(VP^{hk}\) leads to a linear system for a variable U in an adequate product space which is solved using classical Cholesky’s method. This numerical scheme was implemented on a 3.2 Ghz PC using MATLAB, and a typical 1D run (\(h=k=0.01\)) took about 0.622 s of CPU time; meanwhile, a typical 2D run took about 3.66 s of CPU time.

4.1 Numerical convergence and asymptotic behavior in a one-dimensional problem

As a simpler one-dimensional case, we will consider the following one-dimensional version of Problem P using the isotropic and homogeneous expressions given above. We note that, in some cases, the coefficients are collected together, because they lead to the same term.

Problem

P\(^{1D}\). Find the displacement \(u:[0,1]\times [0,1]\rightarrow \mathbb {R}\), the volume fraction \(\varphi :[0,1]\times [0,1]\rightarrow \mathbb {R}\), the thermal displacement \(\tau :[0,1] \times [0,1]\rightarrow \mathbb {R}\), and the microthermal displacement \(R:[0,1]\times [0,1]\rightarrow \mathbb {R}\), such that:

$$\begin{aligned}& \rho \ddot{u}=\mu u_{xx}+\zeta \varphi _{x}-a\dot{\tau }_x+BR_{xx}+F_1\quad \hbox {in}\quad (0,1)\times (0,1),\\& J\ddot{\varphi }= A\varphi _{xx} -\zeta u_{x}-\xi \varphi +\kappa \dot{\tau }+H\tau _{xx}-F R_x-\alpha \dot{R}_x+F_2\quad \hbox {in}\quad (0,1)\times (0,1),\\& c\ddot{\tau }=K\tau _{xx}+H\varphi _{xx}-a\dot{u}_x-\kappa \dot{\varphi }+A^1\dot{R}_x +K^*\dot{\tau }_{xx}-b\dot{R}_x-D\dot{R}_x+F_3 \\&\qquad \qquad \hbox {in}\quad (0,1)\times (0,1),\\& c^*\ddot{R}=CR_{xx}+Bu_{xx}+F\varphi _x-\alpha \dot{\varphi }_x-D\dot{\tau }_x+C^*\dot{R}_{xx}-A^3\dot{R}-b\dot{\tau }_{x}-A^2\dot{\tau }_x+F_4\\&\qquad \qquad \hbox {in}\quad (0,1)\times (0,1),\\& u(0,t)=u(1,t)=\varphi (0,t)=\varphi (1,t)=0 \quad \hbox {for a.e.}\quad t\in (0,1),\\& \tau (0,t)=\tau (1,t)=R(0,t)=R(1,t)=0 \quad \hbox {for a.e.}\quad t\in (0,1), \\&u(x,0)=\dot{u}(x,0)=\varphi (x,0)=\dot{\varphi }(x,0)=x(x-1)\quad \hbox {for a.e. } x\in (0,1),\\& \tau (x,0)=\dot{\tau }(x,0)=R(x,0)=\dot{R}(x,0)=x(x-1)\quad \hbox {for a.e. } x\in (0,1), \end{aligned}$$

where the artificial volume forces \(F_i\), \(i=1,2,3,4\), are given by: for \((x,t)\in (0,1)\times (0,1)\)

$$\begin{aligned} \begin{array}{l} F_{1}(x,t)=e^t \left( x(x-1)-8\right) ,\\ F_{2}(x,t)=e^t \left( 8x-8\right) ,\\ F_{3}(x,t)=e^t \left( 4x(x-1)-14+4x\right) ,\\ F_{4}(x,t)=e^t \left( 3x(x-1)-14+6x\right) , \end{array} \end{aligned}$$

and we used the following data in the simulations:

$$\begin{aligned} \begin{array}{l} \rho =1,\quad \mu =2,\quad \zeta =1,\quad a=1,\quad B=2,\quad J=1,\quad A=1,\quad \xi =2,\quad \kappa =3,\\ H=1,\quad F=2,\quad \alpha =1, \quad c=1,\quad K=3,\quad K^*=3,\quad D=1,\quad b=1,\\ c^*=2,\quad C=3, \quad C^*=2,\quad A^1=1,\quad A^3=1,\quad A^2=2. \end{array} \end{aligned}$$

The exact solution to Problem \(P^{1D}\) can be easily calculated and it has the following form, for \((x,t)\in (0,1)\times (0,1)\):

$$\begin{aligned} u(x,t)=\varphi (x,t)=\tau (x,t)=R(x,t)=e^tx(x-1). \end{aligned}$$

The numerical errors, given by:

$$\begin{aligned} \begin{array}{l} \displaystyle \max _{0\le n\le N}\Big \{ \Vert v^n-v^{hk,n}\Vert _Y+\Vert (u^n-u^{hk,n})_x\Vert _Y + \Vert e^n-e^{hk,n}\Vert _Y+\Vert (\varphi ^n-\varphi ^{hk,n})_x\Vert _Y \\ \qquad \displaystyle + \Vert \theta ^n-\theta ^{hk,n}\Vert _Y +\Vert \varphi ^n-\varphi ^{hk,n}\Vert _Y+\Vert (\tau ^n-\tau ^{hk,n})_x\Vert _Y \\ \qquad + \Vert M^n-M^{hk,n}\Vert _Y+\Vert (R^n-R^{hk,n})_x\Vert _Y\Big \}, \end{array} \end{aligned}$$

and obtained for different discretization parameters h and k, are depicted in Table 1. Moreover, the evolution of the error depending on the parameter \(h+k\) is plotted in Fig. 1. We notice that the convergence of the algorithm is clearly observed, and the linear convergence, stated in Corollary 1, is achieved.

Table 1 Example 1: Numerical errors for some h and k
Fig. 1
figure 1

Example 1: Asymptotic constant error

If we assume now that there are not volume forces, and we use the following data:

$$\begin{aligned} \begin{array}{l} T=20,\quad \rho =0.5,\quad \mu =7,\quad \zeta =1,\quad a=1,\quad B=2,\quad J=10,\quad A=1,\\ \xi =5,\quad \kappa =3,\quad H=1,\quad F=0.1,\quad \alpha =1, \quad c=1,\quad K=3,\quad K^*=3, \\ D=1,\quad b=1,\quad c^*=2,\quad C=3,\quad C^*=5,\quad A^1=2,\quad A^3=1,\quad A^2=2, \end{array} \end{aligned}$$

and the initial conditions:

$$\begin{aligned} u^0(x)=v^0(x)=R^0(x)=M^0(x)=x(x-1) \hbox { for }x\in (0,1),\quad \varphi ^0=e^0=\tau ^0=\theta ^0=0, \end{aligned}$$

taking the discretization parameters \(h=k=10^{-3}\), the evolution in time of the discrete energy \(\mathcal {E}^{hk,n}\), defined as:

$$\begin{aligned} \begin{array}{l} \displaystyle \mathcal {E}^{hk,n}=\frac{1}{2}\Big \{\rho \Vert v^{hk,n}\Vert _Y^2+J\Vert e^{hk,n}\Vert _Y^2+c\Vert \theta ^{hk,n}\Vert _Y^2+ c^* \Vert M^{hk,n}\Vert _Y^2+A^1\Vert u_x^{hk,n}\Vert ^2_Y\\ \qquad \qquad +(Bu_x^{hk,n},R_x^{hk,n})_Y +C\Vert R_x^{hk,n}\Vert _Y^2+(\zeta u_x^{hk,n},\varphi ^{hk,n})_Y+\xi \Vert \varphi ^{hk,n}\Vert _Y^2 \\ \qquad \qquad +A^2\Vert \varphi _x^{hk,n}\Vert _Y^2+ (H\varphi _x^{hk,n},\tau _x^{hk,n})_Y +K\Vert \tau _x^{hk,n}\Vert _Y^2\Big \},\end{array} \end{aligned}$$
Fig. 2
figure 2

Example 1: Evolution of the discrete energy in natural and semi-log scales

is plotted in Fig. 2. As can be seen, it converges to zero and an exponential decay seems to be achieved.

Fig. 3
figure 3

Example 2: Norms of the displacement (left) and microtemperatures (right) at final time over the deformed mesh (multiplied by 10)

4.2 Numerical results in a two-dimensional problem

For this second example, the square domain \([0,1]\times [0,1]\) is considered, assumed to be clamped on its vertical boundaries \(\{0,1\}\times [0,1]\) and traction-free on the rest of the boundary.

Fig. 4
figure 4

Example 2: Microthermal displacement and volume fraction at final time

The following data have been employed in this simulation:

$$\begin{aligned} \begin{array}{l} \varOmega =(0,1)\times (0,1),\quad T=1,\quad \rho =1,\quad \lambda =1,\quad \mu =1,\quad \zeta =1,\quad a=1,\\ B=2,\quad J=1,\quad A=1,\quad \xi =2,\quad \kappa =3,\quad H=1,\quad F=2,\quad \alpha =1, \quad c=1,\\ K=3,\quad K^*=3,\quad D=2,\quad b=2,\quad c^*=2,\quad C_1=2,\quad C_2=2,\quad C^*_1=1,\\ C_2^*=1,\quad A^1=2,\quad A^2=1,\quad A^3=1,\end{array} \end{aligned}$$

and the initial conditions:

$$\begin{aligned} \begin{array}{l} \varvec{u}^0=\varvec{v}^0=\varvec{R}^0=\varvec{M}^0=\varvec{0},\quad \tau ^0=\theta ^0=0,\\ \varphi ^0(x,y)=e^0(x,y)=x(x-1) \quad \hbox {for all}\quad (x,y)\in (0,1)\times (0,1). \end{array} \end{aligned}$$

Taking the time discretization parameter \(k=0.01\), in Fig. 3, we plot the norm of both the displacement (left) and microtemperatures (right) at the final time and over the deformed mesh. As expected, due to the clamping conditions, the displacement and the microtemperatures, which are generated by the volume fraction, have a similar behavior.

Moreover, in Fig. 4 we plot the microthermal displacement (left) and the volume fraction (right) at the final time. We note that the volume fraction, even if it has a quadratic behavior, changes its sign, being now positive. Thus, in Fig. 5, the evolution in time of the volume fraction at middle point \(\varvec{x}=(0.5,0.5)\) is shown. As we can see, it starts to increase after some time and it seems to converge to a steady state.

Fig. 5
figure 5

Example 2: Evolution in time of the volume fraction at point \(\varvec{x}=(0.5,0.5)\)