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.