Keywords

1 Introduction

Active flow control (AFC) with periodic actuation has been proven to bring aerodynamic benefits (e.g. Greenblatt and Wygnanski [1], Barros et al. [2], and Chabert et al. [3]). These benefits stem from the power and mass flow rate reductions obtained compared to steady actuation. The success of periodic actuation is rooted in its exploitation of the flow instabilities, contrary to steady actuation which attempts to alter the flow topology by momentum injection.

For aircraft, circulation control in combination with high-lift devices offers several advantages over traditional high-lift configurations. The basic concept of circulation control involves the Coanda principle, where energy is introduced into the flow by means of a thin jet ejected tangentially from a slit near the rounded trailing edge. The main advantage of circulation control is an increased lift output, which makes shorter take-offs and landings possible. This technology was first patented by Davidson [4] in 1960 and has since been repeatedly investigated [5,6,7]. A circulation control wing (CCW) with steady jets, even at very small mass flow rates, has been shown to yield lift coefficients that are comparable or superior to conventional high-lift systems [8, 9]. A particular variant of circulation control is the Coanda flap, where the objective is to keep the flow attached over a highly deflected flap by blowing a jet tangentially over its specially designed upper surface. This concept has been previously investigated and geometrically optimized in several previous studies [10,11,12].

Efficiency requirements demand that the lift gained through the use of circulation control to be as large as possible in comparison to the momentum coefficient of the blown jet, which is usually acquired by engine bleed. This ratio is referred to as the lift gain factor. An increase in the lift gain factor can be achieved through periodic blowing. Two studies during the mid-1970s investigated pulsed blowing associated with circulation control [13, 14]. Results from these experiments indicated that pulsed blowing reduced the mass requirements for CCW. More recently, periodic blowing on a circulation control wing with circular trailing edge was examined [15], where a 50% reduction in the required mass flow for a required lift coefficient was achieved. It is worth to note that the benefits of periodic excitation targeting flow instabilities have also been demonstrated in other flow control applications, such as pulsed actuation over a flap (e.g. Petz and Nitsche [16]) and acoustic excitation (e.g. Greenblatt and Wygnanski [1], Seifert et al. [17]).

In this study, we summarize our main experimental activities on a high-lift configuration with Coanda actuation within the Collaborative Research Center CRC 880. The investigations are based on two experiments in the large water tunnel and in the low speed wind tunnel facility at the Technische Universität Braunschweig. The periodic actuation is performed using two specially-designed high-speed proportional valves and 33 individually controlled piezo-ceramic lips segments for the water and wind tunnel experiments, respectively. The results demonstrate the benefits of superimposing a periodic actuation component onto the steady one, where up to \(\varDelta C_l=0.6\) lift increase compared to the corresponding steady blowing is observed.

2 Experimental Setup

Two experimental measurements are conducted in the wind and the water tunnel. This section describes both models, their corresponding actuators, and the facilities. Also detailed are the instrumentation and measurement techniques.

2.1 Models

The water tunnel experimental model is a modified two-dimensional DLR-F15 airfoil with a chord length of \(c=300\>\)mm and a span of \(1\>\)m. It features a highly deflected Coanda-flap and a drooped leading edge, as seen in Fig. 1. The Coanda-flap has a length of \(c_\mathrm{fl}=0.25\cdot c\) (i.e. \(c_\mathrm{fl}=75\>\)mm) and is deflected by \(65^{\circ }\) for a landing configuration. The Coanda jet is blown over the flap shoulder through a \(0.00067\cdot c=0.2\>mm\) slit. The jet is supplied through a plenum inside the model, which is connected on both sides of the model to a KSB Movitec VF 32-7 PD multi-stage inline-pump installed outside the tunnel that delivers flow rates up to \(10\>\)l/s at \(8\>\)bar pressure.

The droop nose shape was reached through a parametric study that maximized the lift [18]. The geometry is morphed from the clean nose by deflecting the leading edge down by \(90^{\circ }\) over a length of \(0.2\>c\) and increasing the leading edge thickness by 60%. The reference coordinate system, shown in Fig. 1a, is that of the clean airfoil, where the leading edge coincides with the origin. Henceforth, all subsequent dimensions are with respect to this reference coordinate system.

Fig. 1
figure 1

a Schematic and b a picture of the experimental model installed in the water tunnel

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The wind tunnel experimental model is the same two-dimensional modified DLR-F15 airfoil (Fig. 2) but adapted and scaled for the wind tunnel. It has a chord length of \(c=600\>\)mm, a \(1.3\>\)m span, and a flap length of \(c_\mathrm{fl}= 150\>\)mm. The jet is supplied through a plenum inside the model, which is connected on both sides of the model to a PoleStar PST120 refrigeration air dryer installed outside the tunnel and the building compressor, that can delivers flow rates up to \(1000\>\)l/s at \(3\>\)bar pressure. To minimize side wall effects, the flow is kept attached near the tunnel walls using two separate Coanda actuation from a separate pneumatic feed.

Fig. 2
figure 2

The experimental model installed in the wind tunnel test section

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2.2 Facilities

The water tunnel experiment is carried out in the Großer Wasserkanal Braunschweig (GWB) at the Technische Universität Braunschweig [19]. The facility is a Göttingen-type closed return tunnel, with a \(6\>\)m long and \(1\>\)m \(\times 1\>\)m test section. The flow is driven by a \(1.5\>\)m diameter one-stage axial pump powered by a variable frequency drive \(160\>\)kW electric motor, yielding flow velocities up to \(6\>\)m/s in the test section. To inhibit cavitation, the tunnel can be pressurized up to \(2\>\)bar above ambient pressure. Although Reynolds numbers of up to Re\(=\frac{V_\infty \>c}{\nu _\infty }=2.5\times 10^6\) are possible, the Reynolds number in the current study is limited to \(1.5\times 10^6\) at \(4.5\>\)m/s due to concerns about exceeding the load limit of the model. The subscript \(\infty \) denotes freestream conditions.

The wind tunnel experiment is carried out in the “Modell-Unterschallkanal Braunschweig” (MUB) at the Institute of Fluid Mechanics of the Technische Universität Braunschweig, which is a low speed, closed circuit tunnel with interchangeable test sections. The test section used in this study has a cross section of \(1.3 \times 1.3 \times 5\>\) meters, which allows for a maximum flow velocity of \(60\>\)m/s when empty. The current experiment is conducted at a reduced velocity of \(\approx 50\>\)m/s corresponding to a chord Reynolds number of \(1.75\times 10^6\). The wind tunnel is driven by a \(300\>\)kW variable speed DC motor. The tunnel temperature is kept constant by a water-cooled heat exchanger in the settling chamber.

2.3 Actuation

Two different types of actuators are employed in the two experiments. The water tunnel experiment uses two specially-modified high-speed proportional valves that yield spanwise-uniform actuation. On the other hand, the wind tunnel experiment relies on 33 individually-controllable adaptive lip segments that can generate both spanwise-uniform and spanwise-varied actuation, such as traveling waves. Here, we detail both types of actuators.

2.3.1 High-Speed Proportional Valves

Due to the lack of commercially-available high-frequency proportional valves for water, two custom-made valves are purposely constructed (see Fig. 3a). The valves build on the body of Danfoss EV210B, where the electromagnetic coils are lengthened and the plug stems are extended and modified. To keep the valves from overheating, each electromagnetic coil is wrapped in a water cooling coil, which in turn is encased in thermally conductive epoxy. The valves allow high flow rates of up to \(3\>\)l/s and actuation frequencies of up to \(30\>\)Hz. The flow rate fluctuation levels are however dependent on the actuation frequency. The amplitude of dynamic flow actuation decreases with frequency as the plug stem inertia becomes increasingly more significant. This amplitude drop can be compensated (to some extent) by increasing the power. The valves are powered by an ET-System LAB/SMS-690 digitally controlled power supply (\(0-90\>\)V & \(0-67\>\)A), which yields a total power of \(6\>\)kW. The power output is controlled through an analog input signal (\(0-10\>\)V) produced by a frequency generator in real time. The power supply can faithfully reproduce a sinusoidal input signal up to frequencies of \(50\>\)Hz. To minimize hydraulic dampening, the two valves are attached on either side of the airfoil model close to both plenum inlets. Two Prandtl probes installed between the valves and the model plenum are used to measure the flowrate and thus the actuation intensity in real time.

Fig. 3
figure 3

a The high-speed proportional valves and b the adaptive lip segments used in the two experiments

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2.3.2 Adaptive Lip Segments

Figure 3b shows the 33 adaptive lip segment actuators used for the wind tunnel experiment. Actuation motion is performed through a piezoelectric stack actuator acting with compliant leverage to deflect the lip. Each lip segment is equipped with a high voltage piezoelectric stack actuator (type P-010.40 PI Ceramic) with a \(56\>\)mm length and a \(10\>\)mm diameter. Each actuator generates a free displacement of 60\(\>\mu m\) and has a blocking force of \(2.2\>\)kN. The lever mechanism translates this stroke into a \(0.6\>\)mm movement at the tip in the unloaded case and \(0.4\>\)mm in the loaded case (\(1\>\)bar plenum pressure). The flexure hinge is composed of a carbon fiber laminate with a thickness of \(0.5\>\)mm. To reduce the weight of the lip, the main body is made of aluminum whereas the lip tip is made of hardened steel to provide sufficient stiffness against the aerodynamic loads. The lip segments can operate up to a frequency of \(300\>\)Hz. Each segment has a \(29\>\)mm width, which allows well-resolved spanwise-varied wave actuation. For more details about the adaptive lip segments, the reader is referred to El Sayed et al.  [20] and Wierach et al.  [21].

2.4 Instrumentation and Measurement Technique

Since the main objective of this work is to examine the possible lift gains brought by superimposing periodic actuation, all presented results from both experiments are based on surface pressure measurements and the integrated lift coefficients.

In the water tunnel experiment, the pressure distributions are measured using 64 pressure taps along the mid-span section connected to high precision (0.1% full-scale error) Keller PD-X33 pressure transducers. Each ensemble is sampled at \(100\>\)Hz for 5 seconds. Besides the pressure taps, the model is instrumented with two real-time miniature piezo-resistive pressure Keller sensors with \(300\>\)kHz possible eigenfrequency that are flush mounted onto the Coanda flap. These real-time pressure sensors provide the input for the closed-loop control measurements.

In the wind tunnel experiment, the pressure signals are collected from 196 surface pressure taps drilled mainly along three spanwise locations. The pressure measurements are acquired using a temperature compensated DTC Initium pressure measurement system with an ESP-64HD pressure scanner. The scanner has a \(7\>\)kPa pressure range and a full-scale accuracy of 0.03 %. For each measurement 400 samples are acquired at a sampling frequency of 100 Hz. The wake dynamics are captured by eight time-resolved XCQ-093 Kulite pressure sensors distributed around the flap mid-span. The Kulites have a pressure range of \(35\>\)kPa and an accuracy of 0.1% of the full scale. For each measurement 4000 samples are acquired at a sampling frequency of \(1\>\)kHz. The Kulite signals are also used as input for the machine-learning control [20].

2.5 Test Cases

Periodic actuation is characterized by three parameters, the actuation frequency, the blowing intensity, and the blowing amplitude. The actuation frequency \(f^a\) is usually presented in its non-dimensional form \(F^+\), defined as [1]

$$\begin{aligned} F^+=\frac{f^a\>c_\mathrm{fl}}{V_\infty }\>, \end{aligned}$$
(1)

where \(c_\mathrm{fl}\) is the flap chord length. For both experiments, the actuation frequency can reach a maximum of 0.8 at Re\(=1.5\times 10^6\), imposed by mechanical limitations. The blowing intensity of a circulation control wing is usually characterized by the non-dimensional momentum coefficient \(c_\mu \) [22]. The momentum coefficient was first introduced by Poisson-Quinton and Lepage [23] as the blown jet thrust normalized by the product of the free stream dynamic pressure and a reference area

$$\begin{aligned} c_\mu =\frac{\rho _\mathrm{jet}\>V^2_\mathrm{jet}\>h\>b}{\frac{1}{2}\rho _\infty \> V^2_\infty \>S_{ref}}\>, \end{aligned}$$
(2)

where \(V_\mathrm{jet}\) is the jet nozzle velocity averaged accross the exit, h is the slit height, b the slit width, and \(S_{ref}\) is the projected clean wing area. For sinusoidal spanwise-uniform actuation, the instantaneous momentum coefficient can be expressed as

$$\begin{aligned} c_\mu =C_\mu + C_{\mu 1}\>\cos (2\pi f^a\>t)=C_\mu + C_{\mu 1}\>\cos \left( \frac{2\pi V_\infty }{c_\mathrm{fl}} F^+\>t\right) \>, \end{aligned}$$
(3)

where \(C_\mu \) is the time-averaged mean steady component, and \(C_{\mu 1}\) is the oscillation amplitude.

For the wind tunnel experiment, the additional degrees of freedom from the 33 individually-controlled lip segments allow for spanwise-varied actuation. Two types of motions are investigated, the traveling wave motion which is prescribed by the individual lip segments movement (\(b_i\)) as

$$ b_i(t)=\frac{A}{2} (1+\cos (\omega t-kz_i)) \>, $$

and the standing wave defined as,

$$ b_i(t)= \frac{A}{2} (1+\sin (kz_i) \cos (\omega t))\>, $$

where \(z_i\) denotes the \(i-\)th lip along the spanwise direction, A is the normalized wave amplitude, \(\omega \) is the angular frequency, \(k = 2\pi /\lambda \) is the wavenumber, with \(\lambda \) the wavelength. The wave amplitude A is non-dimensionalized by the maximum possible amplitude, which is the amplitude that almost completely closes the jet slit. Hence, values close to one correspond to amplitudes that are close to the maximum amplitude. To avoid damaging the actuators, the maximum allowed amplitude is limited to \(A = 0.9\). The measurements are acquired for a range of momentum coefficients, starting from the natural unactuated case \(C_\mu =0\) and up to \(C_\mu =0.065\), where the flow is fully attached to the flap.

3 Results

In this section we present results from both experiments, with more focus on the water tunnel experiment. This is due to the better quality results achieved in the water tunnel, which shows more spanwise-uniform jet exit velocity distribution. The better jet uniformity is simply due to the single lip segment used in the water tunnel as opposed to the 33 lip segments in the wind tunnel, which require very careful and tedious adjustments.

3.1 Steady Blowing

For circulation control wings, increasing steady blowing has been repeatedly shown to increase the lift coefficient [24, 25]. This can be explained from the mechanisms behind the Coanda flap. The plane wall jet that emerges from a slit follows the surface and, depending on its momentum, increasingly attaches the flow over the flap. Steady mean equations of motion that take curvature into account prove that equilibrium is maintained by the centrifugal force being balanced by the pressure gradient that is normal to the mean streamlines. The mechanism causing separation is the entrainment of ambient fluid that is slowing the jet down, subject to the adverse pressure gradient from the flap knuckle shape at the hinge line toward the trailing edge [26]. Hence, increasing the actuation intensity causes the separation to gradually recede over the flap up to \({C_\mu } = 0.06\) where the flow becomes fully attached. This gradual mitigation of separation and the added circulation result in steady lift gains, as shown in Fig. 4 for a range of steady blowing intensities.

Fig. 4
figure 4

Lift coefficient over a range of angles of attack with increasing momentum coefficients at Re\(=1.5\times 10^6\). Results are from the water tunnel experiment

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3.2 Spanwise-Synchronous Actuation

The benefits of superimposing a periodic component on the steady actuation component are shown in Fig. 5 and 7 for a range of angles of attack and mean blowing intensities. In the figures, the lift gain \(\varDelta C_l\) distributions are presented with respect to steady blowing for a range of actuation frequencies and amplitudes. The contour lines are generated using the Gaussian process regression model. The limitation in the actuation range is clear in all test cases, where the highest achieved actuation amplitude decreases with increasing frequency. In all test cases, the optimal actuation amplitude remains mostly unchanged at \(C_{\mu ,1}\approx 0.015\). It is not clear what the significance of this amplitude is. Moreover, the highest lift gains are achieved at amplitudes that do not require the complete closing/opening of the valves, demonstrating the advantage of having actuation with independently-controlled steady and unsteady components.

Fig. 5
figure 5

Lift gain distributions with respect to steady blowing at \(\alpha =0^{\circ }\) for a range of actuation frequencies and amplitudes for \(C_{\mu }\in [0.012, 0.055]\). Results are from the water tunnel experiment

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Figure 5 presents the lift gain distributions relative to steady blowing at \(\alpha =0^{\circ }\) for a range of actuation frequencies and amplitudes for \(C_{\mu }\in [0.012, 0.055]\) for the water tunnel experiment. The addition of a periodic component to actuation has a different effect on the lift gains depending on the mean blowing intensity. For \(C_{\mu } < 0.04\), the superposition of unsteady actuation component yields mostly lift gains, with the highest gains of \(\varDelta C_l=0.6\) achieved at \(C_{\mu }=0.012\) for \(F^+_\mathrm{opt}=0.136\) and \(C_{\mu , 1}=0.015\). The response topology of the lift gain in that range is mostly unchanged. Here, the optimum actuation frequency is close to (but not at) the sub-harmonic of the natural shedding frequency, which stands at \(St=\frac{f\>c_\mathrm{fl}}{V_\infty }=0.25\), where f is the main natural shedding frequency of the flow [27]. Surprisingly, no special sensitivity is observed at the natural shedding frequency or its harmonics. At \(C_{\mu }=0.04\), the response starts to exhibit a new topology with two distinct regions of lift gains instead of only one. The two regions are split along \(F^+\approx 0.155\), with the highest lift gain achieved by the left peak at \(F^+\approx 0.1\). Further increase in the mean blowing intensity gradually reduces the lift gains and the magnitude of the left peak. Simultaneously, the optimal actuation frequency shifts to the right peak at \(F^+\approx 0.4\). At \(C_{\mu }=0.055\), where the flow is fully attached, the response surface is dominated by lift losses across most frequency/amplitude combinations with only minor lift gains.

Fig. 6
figure 6

Peak lift gains and the optimal actuation frequency for a range of mean blowing intensities \(C_{\mu }\) for \(\alpha =0^{\circ }\)

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The highest lift gain and the optimal actuation frequency distributions for a range of mean blowing intensities are presented in Fig. 6. As the figure shows, the lift gains gradually decrease with increasing mean momentum coefficient. This is expected, as the flow over the flap becomes more attached with higher forcing. It is worth to note that even if the lift gains were highest at lowest \(C_{\mu }\), the overall lift is low and is of no interest to a STOL-capable aircraft. Corroborating the observations in Fig. 5, the optimal actuation frequency shows a slight decrease between \(C_{\mu }=0.015\) and 0.04 followed by a sudden jump at \(C_{\mu }=0.04\) and a subsequent clustering around \(F^+_\mathrm{opt}\approx 0.4\). We believe this sudden shift at \(C_{\mu }=0.04\) is due to the flow response at this momentum coefficient transitioning from mainly separated to mainly attached flow.

Figure 7 presents the lift gain distributions relative to steady blowing at \(C_{\mu }=0.035\) for three angles of attack \(\alpha = 0, 5\) and \(10^{\circ }\) for the water tunnel experiment. As can be seen, the lift gain topology does not significantly change with changing angle of attack. The highest lift gains do however decrease, but are all achieved at approximately the same actuation setting. It is worth to note, that all three measured angles lie in the linear regime of the lift (c.f. Fig. 4), where the flow behavior, and hence its response, is not expected to change significantly. This is partially due to the rounded droop nose, which significantly reduces the pressure peak near the leading edge and renders the boundary layer more receptive to actuation over a range of angle of attacks. We note that two-dimensional experiments on maximum lift contains several uncertainties, such as side-wall boundary layer interactions, blockage effects, and downwash at the center section of the wing, which become more significant with higher angles of attack. We believe that comparisons of maximum lift coefficients with and without dynamic blowing requires high-fidelity simulations by e.g. hybrid scale-resolving approaches such as those performed by Francois et al.  [28].

Fig. 7
figure 7

Lift gain distributions with respect to steady blowing at \(C_{\mu }=0.035\) for a range of actuation frequencies and amplitudes for a \(\alpha =0^{\circ }\), b \(\alpha =5^{\circ }\) and c \(\alpha =10^{\circ }\). Results are from the water tunnel experiment

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3.3 Spanwise-Varied Actuation

Spanwise-varied actuation yields further lift gains than spanwise synchronous forcing. Fig. 8 presents the lift coefficient for an angle of attack range for (a) \(C_{\mu }=0.015\), and (b) \(C_{\mu }=0.065\) at \(\alpha =0^{\circ }\) from the wind tunnel experiment. The figures show the lift vs. angle of attack distribution for the no-blowing, the constant blowing, the optimal standing wave, the optimal traveling wave, and the synchronized spanwise-uniform actuation cases.

For \(C_{\mu }=0.015\), steady actuation yields a lift gain of \(\varDelta C_l \approx 0.5\) with respect to no-blowing across the angle of attack range. Superimposing spanwise-synchronous sinousoidal actuation at the optimal actuation frequency \(F^+=0.15\) yields a lift increase of \(\varDelta C_l \approx 0.3\) at \(\alpha =0^{\circ }\), which is much lower than the lift gain achieved in the water tunnel for the same blowing intensity (c.f. Fig. 6). This discrepancy is attributed to spanwise disturbances in the jet exit velocity distribution for the wind tunnel experiment, and to the implementation differences between the wind and water tunnel actuation mechanisms. Uniform jet velocity distribution for the wind tunnel experiment is very difficult to achieve with the 33 lip segments. Whereas \(\approx 10\%\) spanwise jet dynamic pressure variations are achieved for the water tunnel experiment, only \(\approx 20\%\) for the wind tunnel are realized. Similar to the observations reported in Sect. 3.2, the lift gains decrease with angle of attack. Unlike the water tunnel experiment, the lift gains reach zero at \(\alpha =10^{\circ }\) and become lift loses for \(\alpha >10^{\circ }\). Spanwise-varied actuation further increases the lift, with traveling wave yielding more gains than standing wave. Optimal actuation is achieved at \(\omega =197\) and 329 rad/s and \(k=13.7\) and 6.3 rad/m for the standing and traveling wave, respectively. As the figure shows, lift gains of \(\varDelta C_l \approx 0.1\) and 0.3 are realized with respect to optimal spanwise-synchronous blowing at \(\alpha =0^{\circ }\) for the best standing and traveling wave, respectively. Similar to spanwise-synchronous actuation, these lift gains decrease with higher angles of attack.

Fig. 8
figure 8

Lift coefficient for a range of angles of attack for a \(C_{\mu }=0.015\), and b \(C_{\mu }=0.065\) at \(\alpha =0^{\circ }\). Results are from the wind tunnel experiment

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For \(C_{\mu }=0.065\), steady actuation yields a significant lift increase of \(\varDelta C_l \approx 2.7\) across the angle of attack range. Superimposing spanwise-uniform or spanwise-varied actuation at the respective optimal settings yields similar lift gain of \(\varDelta C_l \approx 0.5\) across all pre-stall angles. All three optimal actuation motions are performed at \(F^+\approx 0.59\). It is not clear why all three actuation types yield similar lift gains and how these large gains are achieved for this attached case. Future investigations on an improved flap with new and stiffer lip segments shall try to shed light on these observations.

4 Conclusions

This manuscript summarizes research within the Collaborative Research Center CRC 880 on the quantification of the lift gains from superimposing a periodic spanwise-synchronous and spanwise-varied actuation component on a steady component for a high-lift configuration with a highly deflected Coanda flap. The assessment relies on pressure measurements in two experiments, in a water and in a wind tunnel. Steady actuation is shown to increase the lift coefficient significantly up to \(C_{l,max} \approx 5\) at Re\(=1.75\times 10^6\). This high maximum lift coefficient enables short take-off and landing (STOL) capabilities.

Further lift gains are achieved through the superposition of a spanwise-synchronous forcing component. Superimposed periodic forcing yields lift increases throughout a large actuation parameter range. The highest lift gain with respect to the reference steady case reaches \(\varDelta C_l = 0.6\). The highest gains are achieved at an actuation frequency of \(F^+ \approx 0.15\), which does not constitute a harmonic or a sub-harmonic to the natural shedding frequency of the flow. The highest aerodynamic gains are achieved at mid-range amplitudes. This demonstrates the advantage of having actuation with independently-controlled steady and unsteady components.

The introduction of spanwise-variation in the form of a standing and traveling wave is demonstrated to bring additional lift gains with respect to both steady blowing and spanwise-synchronized actuation. The optimal standing and traveling waves are relatively slow with wavenumber \(k=13.7\) and 6.3 rad/m for the standing and traveling wave, respectively. The complex interactions that result from such three-dimensional actuation are not fully understood, especially at high blowing intensities. Further investigations and time-resolved PIV measurements are required to identify the underlying phenomena leading to the lift increase/decrease.