A Study of a Closed-Loop Control System of the Separation and Reattachment Process by Using the Vortex Generator Jets

本课题的总体目标是利用涡发生器射流开发一种用于后向阶梯流中的再附着过程和扩压流中的分离过程的闭环控制系统。首先，构建了后面向步长再附着过程的控制系统。当射流展向节距为射流直径的20倍时，得到了二维再附着过程。在闭环控制系统中，选取再附着区表面静压作为控制反馈参数，射流与自由流速度比作为被控变量。该系统的响应速度更快，时间常数约为100毫秒。为了寻找瞬态特性，还对近壁流动进行了阶跃响应。其次，对扩散器内流动分离的抑制机理进行了研究。扩压器的开口角度可以根据“未失速”、“明显失速”和“暂时失速”三种流动状态从8度到15度进行调整。采用微壁流量传感器测量正向流量。结果表明，涡发生器的射流可以防止扩散器中的失速。射流速度比高虽然可以实现分离控制，但射流速度比过高会造成压力损失，降低扩散器的效率。结果表明，当分离点在空间上固定时，再附着过程比扩散流中的分离过程更容易控制。控制系统依赖于扩散器的失速状态

The general goal of the present research project is to develop a closed loop control system of the reattachment process in backward facing step flow and the separation process in the diffuser flow by using the vortex generator jets. At first, the control system of the reattachment process over the backward facing step was constructed. Two-dimensional reattachment process was obtained when the spanwise pitch of the injected jets was 20 times a jet diameter. The surface static pressure in the reattachment region was selected as the control feedback parameter and the jet to the free stream velocity ratio was specified as the manipulated variable in the closed loop control system. The system had a faster response with a time constant of an order of 100 msec. A step response of the near-wall flow was also conductedin order to find transient characteristics. Next, the suppression mechanism of the flow separation in the diffuser was examined. An opening angle of the diffuser could be changed from 8 to 15 deg corresponding to three flow regimes, "Unstalled", "Appreciable Stall" and "Transitory Stall". A micro wall flow sensor was used for measurement of forward flow fraction. It was found that the vortex generator jets prevented the stall in the diffuser. Although the separation control was obtained resulting from the higher jet velocity ratio, excessive jet velocity ratio introduced lower diffuser effectiveness due to the pressure loss. It was concluded that the easier control system of the reattachment process was obtained compared with the separation process in the diffuser flow as the separation point was fixed in space. The control system depended on the stall regime in the diffuser