Measurements of interacting turbulent shear layers in the near wake of a flat plate

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Abstract
Measurements of velocity fluctuations in the wake of a thin non-lifting aerofoil are presented: in one set of experiments the flow was symmetrical, while in the other the upper surface of the aerofoil was roughened to increase the surface shear stress. Measurements were confined to the near wake, where the disturbed region lies within the inner layers of the original boundary layers; thus the boundary-layer thickness is not a relevant length scale. The practical relevance of the experiment is to the prediction of flow over aerofoils, where only the initial region of the wake significantly affects the aerofoil pressure distribution. Temperature-conditioned sampling techniques were used, one boundary layer at a time being heated so that fluid from each boundary layer could be traced within the wake. In contrast to the behaviour of merging shear layers in ducts and jets, the wake interaction involves significant fine-scale mixing; the results reveal a three-layer structure, with a fine-scale inner wake of mixed fluid separating two layers in which structural changes are confined to the region of time sharing, or internal intermittency, between mixed and unmixed fluid. The implications of the results for calculation methods are discussed.