The three-dimensional character of a nominally two-dimensional separated turbulent shear flow

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Abstract
Extensive experiments were performed in the highly turbulent shear layer and in the reverse-flow region downstream of a normal plate followed by a long splitter plate in its plane of symmetry. Whereas the earlier measurements of Ruderich & Fernholz (1986) and Castro & Haque (1987) concentrated on the plane of symmetry (x, y-plane, z = 0), the present investigation reveals the full extent of the three-dimensional behaviour of the flow by presenting data in the whole flow field. Spanwise measurements of five components of the Reynolds stress tensor, of integral timescales, and of mean and fluctuating values of the skin friction were carried out by the pulsed-wire technique. Integral lengthscales and spectra outside the reverse-flow region were measured using hot wires. In order to keep three-dimensional effects as small as possible, the aspect ratio was large (model width was 2.55 times the length of the reverse-flow region) and the blockage ratio and free-stream turbulence intensity were low. Despite these efforts the flow investigated here must be considered three-dimensional and classified as both pressure and shear driven. There appears to be a connection between the shape of the reverse-flow region and the level of the fluctuating velocities. Measurements of power spectral densities revealed no flapping motion of the separated turbulent shear layer.