Active Control of Turbulent Flow Separation by Surface Plasma

射频辉光放电被用于微电子器件制造、臭氧产生和气体激光激发。在大气压力下操作的这种排放已经显示出产生用于流动控制的射流。我们实验室最近获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是一种简单的器件，没有运动部件或管道，具有高的频率响应，因此具有航空应用的现实可能性。在2000年的最后一年里，已经对翼型和涡轮机叶片进行了试验，以控制转捩、表面摩擦阻力和气流分离。然而，仍然缺乏关于表面等离子体物理学和相关流体动力学的信息，以充分利用装置进行流量控制。壁面等离子体射流的产生机理目前还不清楚，在流动控制中等离子体激发的最佳条件也不清楚。这些正是我们提出这项研究的原因，这样我们就可以推进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。在分离点前放置表面等离子体作动器以延迟流动分离，可以研究对静态失速的控制。在这里，时间平均升力和阻力应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的面积来控制升力面上的动态失速，甚至可以从分离中恢复。这种方法的新奇之处在于，不需要对物体表面上的流动分离进行实时检测，因为涡流周期性地从圆柱体表面脱落。另外，圆柱绕流是一个研究较多的课题，因此有足够的数据库来验证我们的基线测量结果。PIV（Particle Image Velocimetry）系统是近年来流体力学研究中常用的一种流动测量技术，它可以获得光片平面内的完整速度场。利用PIV系统，在激光光片照射下，用数码相机在很短的时间内拍摄流场中的小颗粒。每个流动粒子的移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，将以1kHz的频率以1600x1200像素的全相机分辨率捕获流图像8秒，激光器产生20 mJ的能量。所有这些设备将从EPSRC工程仪器贷款库中提供，用于本研究。PIV测量将得到其他技术的补充，如热线测量和流动可视化，这将使我们的结果充满信心，增加对流动分离过程中旋涡结构的洞察力，并提供更好的理解，其中可以进行表面等离子体流分离控制的机制。

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.