Keywords

1 Introduction

Jet is one of the typical flow modes occurring in nature and industry [1]. Depending on the state of the jet flow at the nozzle exit, behavior such as vortex generation and coalescence changes greatly [2]. In this study, we control the jet flow ejected from nozzle by an induced flow of the coaxial type DBD-PA (DBD: Dielectric Barrier Discharge, PA: Plasma Actuator). The coaxial type DBD-PA is an axisymmetric nozzle, and the jet is ejected from this nozzle. When AC high voltage is applied to DBD-PA, the induced flow from DBD-PA is generated. DBD-PA is driven by burst modulation control and the induced flow is intermittently generated. This intermittently generated induced flow controls the jet. The coaxial type DBD-PA controls the jet by controlling vortex generation in the jet.

2 Experimental Details

Figure 1 is the coaxial type DBD-PA. DBD-PA is configured two copper electrodes and a dielectric. Electrodes place on the front and back of the dielectric. The coaxial type DBD-PA nozzle is made of machinable ceramics of dielectric constant 9 and the two electrodes are made of phosphor bronze of 0.5 mm thickness. The coaxial type DBD-PA nozzle of diameter is d = 10 mm. And the converging nozzle with a contraction ratio of 6.25 attaches to the tip the coaxial type DBD-PA nozzle. Applying AC high voltage to the coaxial type DBD-PA occur dielectric barrier discharge. As shown in Fig. 2, the induced flow that is used jet diffusion control and diffusion promotion is generated from the nozzle inner electrode side.

Fig. 1
figure 1

The coaxial type DBD-PA

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Fig. 2
figure 2

Flow and DBD plasma

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Figure 3 shows an over view of the experimental apparatus. In this study, we conduct experiments in the jet at Reynolds number Re = 10,000. Compressor supplies Air to experimental apparatus. Then, the mass flow controller adjusts air flow rate. Passing through the seeding generator, adjusted air is mixed tracer particles for visualization. Here, tracer particles are corn oil with a particle size of about 1 μm. To rectify the flow of adjusted air, pass through an air filter with a filtration degree of 5 μm and a pipe for run-up. This pipe for run-up of an inner diameter is 25 mm and of a length is about 1.2 m. After that, air jet out vertically upward from the coaxial type DBD-PA nozzle attached the converging nozzle.

Fig. 3
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Experimental apparatus

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Visualization of the jet is conducted by laser sheet scattered light technique with YAG laser. And the jet is photographed by the high speed camera. Here, frame rate of the high speed camera is 12,000 fps. The function generator generates AC waveform. Here, AC waveform is sine wave. The high voltage amplifier boosts this AC waveform. The high voltage amplifier boosts AC high voltage Vp-p (peak to peak) = 16 kV. The AC high voltage waveform is applied to the coaxial type DBD-PA.

Figure 4 shows burst modulation control parameter of AC high voltage applied to the coaxial type DBD-PA. Here, f burst : the frequency of the on-off cycle of AC high voltage waveform of burst modulation control. Burst ratio of burst modulation control in all experimental conditions is 50%. The f burst is determined on basis of the natural vortex frequency f n . Here, f n : the frequency of naturally generated vortices of a free jet at plasma off. The f n at Re = 10,000 is searched by measured frequency of flow velocity fluctuation of the free jet with laser doppler velocimeter. As a result, the f n of Re = 10,000 is 1984 Hz. In addition, this f n is confirmed by counted the naturally generated vortices. The high-speed camera photographs the free jet at Re = 10,000 of XY plane. The naturally generated vortices of the free jet are counted at a height x/d = 0.5 from nozzle exit using this moving image for 1.0 s. f base : AC high voltage driving frequency is about 7 kHz. This study is investigated effect on the jet from change in the f burst . The influence of change in the f burst is evaluated by searched the f vortex . Here, f vortex : frequency of generated vortices of plasma excited jet. The f vortex is searched by counted the generated vortices of plasma excited jet at a height x/d = 0.5 from nozzle exit using moving image for 0.1 s.

Fig. 4
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Burst modulation control parameter

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3 Results and Discussion

Figure 5 shows the searched result of f vortex at Re = 10,000. The vertical axis of graph is the ratio of f burst and f vortex , the horizontal axis of graph is the ratio of f n and f burst . This graph can be divided into three sections of f burst < f vortex , f burst = f vortex , f burst > f vortex . Within f burst = f vortex (f vortex /f burst = 1), the induced flow of the coaxial type DBD-PA synchronizes f burst and f vortex [3]. This phenomenon is the phenomenon of lock-in [2]. Driving the coaxial type DBD-PA with burst modulation, it is possible to generate an axisymmetric vortex within frequency of burst modulation control in which the phenomenon of lock-in occurs. Consequently, the jet is controlled by the coaxial type DBD-PA within frequency of burst modulation control in which the phenomenon of lock-in occurs.

Fig. 5
figure 5

Relationship between f vortex /f burst and f burst /f n

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Figure 6 shows XY plane of jet visualization image at Re = 10,000. Figure 6a is the free jet of plasma off. The free jet is not controlled. The free jet generates vortices in an unsteady cycle. In Fig. 6b is f burst = 0.3f n = 595 Hz. This condition generates a large vortex and a small vortex near the nozzle of the jet in an unsteady cycle. We consider that the induced flow of the coaxial type DBD-PA generates a large vortex and Shear layer instability of jet generates a small vortex. In Fig. 6c–e, the jet is controlled by driving the coaxial type DBD-PA frequency of burst modulation control in which the phenomenon of lock-in occurs. Figure 6c is f burst = 0.9f n = 1786 Hz, the phenomenon of lock-in begins with this f burst . Figure 6d is f burst = 1.0f n = 1984 Hz, this f burst is the natural vortex frequency. When the coaxial type DBD-PA driving condition is f burst = 0.9f n or 1.0f n , these conditions generate large vortices near the nozzle of the jet in a steady cycle and these vortices coalescence develop large-scale vortex ring compared to other experimental conditions (Fig. 6a, b, e, f). Figure 6e is f burst = 1.4f n = 2778 Hz, the phenomenon of lock-in ends with this f burst . This condition generates a small vortex near the nozzle of the jet in a steady cycle. Figure 6f is f burst = 2.0f n = 3968 Hz. This condition generates a small vortex near the nozzle of the jet in an unsteady cycle. We consider that frequency of the induced flow of the coaxial type DBD-PA is higher than other conditions and the generation of vortex can not follow.

Fig. 6
figure 6

XY plane of jet visualization image at Re = 10,000. a Plasma off. b f burst = 0.3f n = 595 Hz. c f burs = 0.9f n = 1786 Hz d f burst = 1.0f n = 1984 Hz. e f burst = 1.4f n = 2778 Hz. f f burst = 2.0f n = 3968 Hz

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4 Conclusions

The coaxial type DBD-PA is driven by burst modulation control and the intermittently generated induced flow control the jet. As a result, following knowledge is obtained.

  1. 1.

    By the phenomenon of lock-in, the induced flow of the coaxial type DBD-PA synchronizes vortex generation in the jet in a specific burst modulation control frequency range. Consequently, the jet is controlled by the coaxial type DBD-PA within frequency of burst modulation control in which the phenomenon of lock-in occurs.

  2. 2.

    Driving the coaxial type DBD-PA with burst modulation control generates a large vortex near the nozzle of the jet in a steady cycle and these vortices coalescence develop large-scale vortex ring.