Experimental control of turbulent boundary layers with in-plane travelling waves

The experimental control of turbulent boundary layers using stream- wise travelling waves of spanwise wall velocity, produced using a novel active surface, is outlined in this paper. The innovative surface comprises a pneu- matically actuated compliant structure based on the kagome lattice geometry, supporting a pre-tensioned membrane skin. Careful design of the structure enables waves of variable length and speed to be produced in the flat surface in a robust and repeatable way, at frequencies and amplitudes known to have a favourable influence on the boundary layer. Two surfaces were developed, a preliminary module extending 152 mm in the streamwise direction, and a longer one with a fetch of 2 . 9 m so that the boundary layer can adjust to the new surface condition imposed by the forcing. With a shorter, 1 . 5 m portion of the surface actuated, generating an upstream-travelling wave, a drag re- duction of 21 . 5% was recorded in the boundary layer with Re τ = 1125. At the same flow conditions, a downstream-travelling produced a much smaller drag reduction of 2 . 6%, agreeing with the observed trends in current simula- tions. The drag reduction was determined with constant temperature hot-wire measurements of the mean velocity gradient in the viscous sublayer, while si- multaneous laser Doppler vibrometer measurements of the surface recorded the wall motion. Despite the mechanics of the dynamic surface resulting in some out-of-plane motion (which is small in comparison to the in-plane streamwise movement), the positive drag reduction results are encouraging for future in- vestigations at higher Reynolds numbers