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

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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