Feedback control of the laminar-turbulent transition onset in a boundary layer by suction

The boundary layer on a flat plate is laminar in a region close to the leading edge and becomes turbulent further downstream. The streamwise position of the onset of the transition depends on different parameters among them the magnitude of the incident velocity. We assume that the flat plate is equipped with a porous zone which allows a suction of the boundary layer. Again; the onset of transition depends strongly on the suction velocity. Acting on this velocity allows an active control of the position of the transition. Only the transition location is assumed to be measured. The paper develops a controller to keep the transition point fixed in a slightly unsteady flow by controlling the suction velocity on the panel. The aerodynamical modelling is presented and approximations are physically justified. Low cost numerical simulation aspects are discussed. A low order simplified model of the flow is identified from numerical simulations. It is used for the control law design. The system is then computationally tested. Feasibility of the active control is proved, but the closed loop behavior is not conform to the one expected. We conjecture that the simplified model does not retain sufficiently rich information to allow good previsions of the closed loop propel-ties. Correlatively the closed loop seems to be very sensitive to different varying quantities such as delay, unmodelized actuator and sensor dynamics, and control sampling period. (C) 1998 Elsevier Science Inc. All rights reserved.