Controlling turbulent boundary layers through the actuation of a compliant structure

Abstract

The control of turbulent boundary layers through spanwise wall forcing has been the subject of extensive numerical and experimental investigation in recent years. It has been shown that the benefits of drag reduction and potential power saving are enhanced when the forcing takes the form of a streamwise travelling wave of spanwise velocity. When this wave has a certain non-dimensional frequency and wavenumber, large turbulent skin-friction drag reductions of almost 50% and potential net power savings of 38% can be achieved. While there are numerous direct numerical simulations (DNS) showing these trends, an experimental validation was lacking for boundary layer flows. The work presented details the design and manufacture of an active surface, capable of discretising these complex waveforms to bring about turbulent flow-control in a wind tunnel experiment. Through the optimal design of a compliant structure, based on the Kagome lattice geometry, an adaptive framework was developed which is capable of discretising waveforms through controlled local deformation. By combining this compliant structure with a pre-tensioned membrane, an active surface is produced. The surface, 3 m in length, is then driven pneumatically, producing surface travelling waves of variable wavenumber and phase velocity. Photogrammetric and vibrometer measurements of the static and dynamic performance of actuated surface are presented. Constant temperature anemometer measurements of the boundary layer were taken with and without the active surface applying forcing. A linear interpolation of the viscous sub-layer was performed to assess changes in wall shear-stress. For certain forcing parameters, a drag reduction of 20% was measured.