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

本研究项目的总体目标是利用涡流发生器射流对后向台阶流中的再附着过程和扩散器流动中的分离过程进行闭环控制。首先，构建了后向台阶再附着过程的控制系统。当喷射射流的跨距为射流直径的20倍时，得到了二维再附着过程。在闭环系统中，选择再附着区的表面静压作为控制反馈参数，将射流与自由流的速度比指定为操纵变量。该系统具有较快的响应速度，时间常数为100毫秒量级。此外，还对近壁流动进行了阶跃响应，以求取其暂态特性。其次，对扩散器内流动分离的抑制机理进行了研究。扩散器的开启角度可根据“无失速”、“明显失速”和“短暂失速”三种流型在8~15度之间变化。采用微型壁面流量传感器测量正向流动分数。结果表明，涡流发生器射流可以有效地防止扩散器内的失速。虽然较高的射流速度比可以获得分离控制，但过大的射流速度比会导致压力损失，从而降低扩散器的效率。结果表明，由于分离点在空间上是固定的，与扩散器流动中的分离过程相比，再附着过程更容易控制。控制系统取决于扩散器中的失速状态。

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