Active Control of Turbulent Flow Separation by Surface Plasma

射频辉光放电被应用于微电子器件制造、臭氧生成和气体激光激发。这种在大气压力下运行的排放已被证明可以产生用于流量控制的射流。我们实验室最近取得的一些结果清楚地证实了这些说法。表面等离子体致动器结构简单，无运动部件和导管，具有较高的频率响应特性，在航空领域应用具有现实的可能性。在过去的一年里，已经对翼型和涡轮叶片进行了测试，以可能控制过渡、表面摩擦阻力和流动分离。然而，表面等离子体物理和相关流体动力学方面的信息仍然缺乏，无法充分利用这些装置进行流动控制。表面等离子体产生壁面射流的机理尚不清楚，流动控制中等离子体激发的最佳条件也不清楚。这正是我们提出这项研究的原因，这样我们就可以提高我们对表面等离子体在许多航空应用中的理解，特别是流动分离控制。在本研究中，我们将研究静态和动态失速时流动分离的主动控制。静态失速的控制可以通过在分离点前放置表面等离子体致动器来研究，以延迟流动分离。在这里，时间平均升力和阻力应该表明分离控制的有效性。利用表面等离子体作动器可以通过减小分离区域的面积，甚至从分离中恢复来控制升力表面上的动态失速。这种方法的新颖之处在于，由于涡流周期性地从圆柱体表面脱落，因此不需要实时检测体表面的流动分离。此外，圆柱周围的流动是许多研究人员研究的主题，因此有足够的数据库来帮助验证我们的基线测量。PIV （Particle Image velocity metry，粒子图像测速）系统是近年来流体动力学研究中常用的一种流量测量技术，它可以获得光片平面上的整个速度场。在PIV系统中，用数码相机在短时间间隔内拍摄到由激光片照射的流体中的小颗粒。每个流粒子的运动距离和方向给出速度矢量，从而全局映射速度场。在我们的研究中，流图像将以1 kHz的频率在1600x1200像素的全相机分辨率下拍摄8秒，激光产生20兆焦耳的能量。所有这些设备将从EPSRC工程仪器贷款池中为本研究提供。PIV测量将得到其他技术的补充，如热线测量和流动可视化，这将给我们的结果带来信心，增加对流动分离过程中螺旋结构的了解，并更好地理解利用表面等离子体进行流动分离控制的机制。

Radio frequency glow discharges are been used in microelectronic device fabrication, ozone generation and in gas laser excitation. Such discharges operating at atmospheric pressure have been shown to produce jet flows to be used for flow control. Some of recent results obtained from our laboratory clearly confirmed these claims. Surface plasma actuators are simple device with no moving parts or ducting, which have high frequency response and thus have a realistic possibility for aeronautical applications. Already, tests have been conducted for airfoils and turbine blades for possible control of transition, skin-friction drag and flow separation in the last year of so. However, there is still a lack of information on surface plasma physics and associated fluid dynamics to fully utilise the devices for flow control. The production mechanism of wall jets by surface plasma is not well understood, nor is the optimum condition for plasma excitation in flow control. These are precisely the reasons why we propose this research, so that we can advance our understanding on surface plasma for many aeronautical applications, flow separation control in particular.In this investigation we would like to study active control of flow separation during static and dynamics stall. Control of static stall can be investigated by placing surface plasma actuators before the separation point over a circular cylinder with a view to delay flow separation. Here, the time averaged lift and drag forces should indicate the effectiveness of separation control. Control of dynamics stall over a lifting surface can be carried out by reducing the area of separation region or even to recover from separation by using surface plasma actuator. Novelty of this approach is that a real-time detection of flow separation over the body surface is not required, as the vortices are periodically shed from the cylinder surface. Besides, the flow around a circular cylinder is a subject that has been studied by many researchers, therefore there are enough database to help validate our baseline measurements.PIV (Particle Image Velocimetry) system is becoming a common flow measurement technique in fluid dynamic research in recent years, where an entire velocity field in a light-sheet plane can be obtained. With PIV system, small particles in the flow shone by the laser light sheet are photographed in a short interval with a digital camera. The distance and direction of movement of each flow particle gives the velocity vector, thereby globally mapping the velocity field. In our study, flow images will be captured at 1 kHz at a full camera resolution of 1600x1200 pixels for 8 seconds, with 20 mJ of energy being produced by the laser. All of these equipments will be made available from EPSRC Engineering Instrument Loan Pool for this study. The PIV measurements will be complimented by other techniques, such as hot-wire measurements and flow visualisation, which will give confidence in our results, add insight into vortical structures during flow separation and provide better understanding of the mechanisms in which flow separation control with surface plasma can be carried out.