The potential of controlling shockwave–boundary layer interactions(SWBLIs) in air by plasma aerodynamic actuation is demonstrated. Experiments are conducted in a Mach 3 in-draft air tunnel. The separation-inducing shock is generated with a diamond-shaped shockwave generator located on the wall opposite to the surface electrodes, and the flow properties are studied with schlieren imaging and static wall pressure probes. The measurements show that the separation phenomenon is weakened with the plasma aerodynamic actuation, which is observed to have significant control authority over the interaction. The main effect is the displacement of the reflected shock. Perturbations of incident and reflected oblique shocks interacting with the separation bubble in a rectangular cross section supersonic test section are produced by the plasma actuation. This interaction results in a reduction of the separation bubble size, as detected by phase-lock schlieren images.The measured static wall pressure also shows that the separation-inducing shock is restrained. Our results suggest that the boundary layer separation control through heating is the primary control mechanism.
The plasma synthetic jet is a novel active flow control method because of advantages such as fast response,high frequency and non-moving parts,and it has received more attention recently,especially regarding its application to high-speed flow control.In this paper,the experimental characterization of the plasma synthetic jet actuator is investigated.The actuator consists of a copper anode,a tungsten cathode and a ceramic shell,and with these three parts a cavity can be formed inside the actuator.A pulsed-DC power supply was adopted to generate the arc plasma between the electrodes,through which the gas inside was heated and expanded from the orifice.Discharge parameters such as voltage and current were recorded,respectively,by voltage and current probes.The schlieren system was used for flow visualization,and jet velocities with different discharge parameters were measured.The schlieren images showed that the strength of plasma jets in a series of pulses varies from each other.Through velocity measurement,it is found that at a fixed frequency,the jet velocity hardly increases when the discharge voltage ranges from 16 kV to 20 kV.However,with the discharge voltage fixed,the jet velocity suddenly decreases when the pulse frequency rises above 500 Hz,whereas at other testing frequencies no such decrease was observed.The maximum jet velocity measured in the experiment was up to110 m/s,which is believed to be effective for high-speed flow control.
Plasma flow control(PFC) is a new kind of active flow control technology, which can improve the aerodynamic performances of aircrafts remarkably. The flow separation control of an unmanned air vehicle(UAV) by nanosecond discharge plasma aerodynamic actuation(NDPAA) is investigated experimentally in this paper. Experimental results show that the applied voltages for both the nanosecond discharge and the millisecond discharge are nearly the same, but the current for nanosecond discharge(30 A) is much bigger than that for millisecond discharge(0.1 A). The flow field induced by the NDPAA is similar to a shock wave upward, and has a maximal velocity of less than 0.5 m/s. Fast heating effect for nanosecond discharge induces shock waves in the quiescent air. The lasting time of the shock waves is about 80 μs and its spread velocity is nearly 380 m/s. By using the NDPAA, the flow separation on the suction side of the UAV can be totally suppressed and the critical stall angle of attack increases from 20° to 27° with a maximal lift coefficient increment of 11.24%. The flow separation can be suppressed when the discharge voltage is larger than the threshold value, and the optimum operation frequency for the NDPAA is the one which makes the Strouhal number equal one. The NDPAA is more effective than the millisecond discharge plasma aerodynamic actuation(MDPAA) in boundary layer flow control. The main mechanism for nanosecond discharge is shock effect. Shock effect is more effective in flow control than momentum effect in high speed flow control.
闭式等离子体可以克服等离子体隐身技术在开放环境中等离子体难以维持及能耗过大的问题。针对等离子体隐身应用，设计了一种封闭式的等离子体发生装置，选用微秒脉冲电源，以氩气为工质气体，在低气压环境下进行了放电实验。采用发射光谱法，测量了密闭腔体内部厚度方向上的Ar谱线强度，并将碰撞-辐射模型用于分析等离子体参数的分布规律。当放电参数确定时，给定电子温度和电子密度，可通过碰撞-辐射模型计算得到2p能级上的布居分布比值，将其与从光谱数据中得到的布居分布比值进行比较，当差异值最小时，即可确定相应的等离子体参数。通过对电子温度在1～5 eV 范围内的2p9和2p1能级布居分布比值进行计算，分析了碰撞-辐射模型计算可能存在的误差。实验结果表明，在厚度方向上，封闭式腔体中的等离子体电子密度达到1011 cm^-3量级且呈一定的梯度分布，但变化幅度不大，其分布情况有利于等离子体隐身技术的应用。
Electrical characteristics and optical emission spectrum of the radio frequency（RF） surface dielectric barrier discharge（SDBD） plasma actuation are investigated experimentally in this paper. Influences of operating pressure, duty cycle and load power on the discharge are analyzed. When the operating pressure reaches 30 kPa, the discharge energy calculated from the Charge–Voltage（Q–V） Lissajous figure increases significantly, while the effective capacitance decreases remarkably. As the duty cycle of the applied voltage increases, the voltage–current waveforms, the area of Q–V loop and the capacity show no distinct changes. Below 40 W, effective capacitance increases with the increase of load power, but it almost remains unchanged when load power is between 40 W and 95 W. The relative intensity I391.4peak/I380.5peak changes little as the operating pressure varies from 4 kPa to 100 kPa, while it rises evidently with the pressure below 4 kPa, which indicates that the RF discharge mode shifts from filamentary discharge to glow discharge at around 4 kPa. With the increase of load power, the relative intensity I391.4peak/I380.5peak rises evidently. Additionally, the relative intensity I371.1peak/I380.5peak is insensitive to the pressure,the duty cycle, and the load power.
