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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Active control of boundary layer instabilities

Author: LI, Yong

Year: 2005

Supervisor(s): Mike Gaster

The active control of spatially evolving three-dimensional instability waves in the boundary layer of a flat plate has been investigated both numerically and experimentally. The boundary layer was excited by various computer-generated controlled perturbations in a very low free-stream turbulence wind tunnel. Sensors were used to detect the on-coming disturbances and control signals were sent to the downstream actuators to generate counteracting disturbances in order to suppress the amplification of the original ones. The development of the disturbances in the boundary layer was measured by means of hot-wire anemometry.

The key element of active control was the determination of the transfer function of the control system connecting the upstream detector and the downstream actuator, in order to achieve the cancellation of disturbances. The two- and three-dimensional transfer functions were obtained by solving the Orr-Sommerfeld equations. With the chosen transfer functions, it was shown that the amplitudes of the disturbances in the boundary layer could be significantly suppressed. To simplify the control problem in practice some control strategies using approximate transfer functions were used. It was shown that with the simple control transfer function the same result could be achieved as by the full control mode.

Both off-line and real-time control on various wall perturbations was carried out in the experimental investigation. The main aim of the off-line control was to validate the theoretical model of different control modes and to choose the most suitable control mode for real-time control. Good agreement between the experimental results and the numerical modelling was obtained. The experimental results of the real-time control showed that the control strategy could be developed to stop the growth of the three-dimensional Tollmien-Schlichting waves.