The experimental control of turbulent boundary layers using streamwise travelling waves of spanwise wall velocity, produced using a novel active surface, is outlined in this paper. The innovative surface comprises a pneumatically 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 reduction of 21.5% was recorded in the boundary layer with Reτ = 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 simulations. The drag reduction was determined with constant temperature hot-wire measurements of the mean velocity gradient in the viscous sublayer, while simultaneous 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 investigations at higher Reynolds numbers.
This paper details the design, manufacture and testing of an adaptive structure based on the kagome lattice geometry -a pattern with well documented interesting structural characteristics. The structure is used to produce in-plane travelling waves of variable length and speed in a flat surface. The geometry and dimensions, as well as the location and compliance of boundary conditions, were optimized numerically, and a pneumatically-actuated working demonstrator was manufactured. Static and dynamic photogrammetric and force measurements were taken. The structure was found to be capable of producing dynamic planar waveforms of variable wavelength with large strains. The lattice structure was then surfaced with a pre-tensioned membrane skin allowing these waveforms to be produced over a continuous plane.
Linear algebra methods for determining modes of kinematic and static indeter- minacy in jointed frames are extended to reveal modes of compliance in oth- erwise rigid assemblies. These modes are extracted from a structural model, based on nite elements, via a singular value decomposition and yield the ways in which a structure can be most easily deformed. This modal approach also allows for the formulation of a reduced-order structural model, whereby relevant modes are selected and used as the basis for the optimisation of a complaint structure. The method detailed is shown to be a useful design tool, demon- strated by its application to a structure based on the Kagome lattice geometry. For certain frameworks, rst order e ects produce tightening under actuation. As a result, a scheme to adjust the modes to take nonlinear e ects into account is also given
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