The control of transient disturbances in a flat plate boundary layer through active wall motion

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Abstract
Experimental results are presented demonstrating the evolution and control of a large amplitude and highly three‐dimensional disturbance in a laminar boundary layer. A localized disturbance was introduced into a laminar boundary layer at Reδ* =1095 by means of a flexible membrane mounted on a flat plate. For low amplitudes, the disturbance initially decayed, leaving a slowly growing wave packet comprised of linearly unstable Orr–Sommerfeld modes. At higher amplitudes, the disturbance grew rapidly, leading to a turbulent spot. The high‐amplitude disturbance was acted upon by an active wall counterdisturbance consisting of a traveling bump at the wall. The counterdisturbance was generated by an array of eight small flexible membranes mounted next to one another, flush with the wall and activated in rapid succession. The breakdown of the disturbance to a turbulent spot is shown to be delayed by the active wall motion by approximately 50δ*. The active wall motion is shown to inhibit the development of the initial disturbance and to decrease the amplification rate of spanwise vorticity by reducing the level of cross‐stream stretching ∂w/∂z.

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