TURBULENCE MEASUREMENTS AROUND A MILD SEPARATION BUBBLE AND DOWNSTREAM OF REATTACHMENT

This paper describes the behaviour of a turbulent boundary layer on a smooth, axisymmetric body exposed to an adverse pressure gradient of s ufficient strength to cause a short region of mean reverse flow ('sepa ration'). The pressure distribution is tailored such that the boundary layer reattaches and then develops in a nominally zero pressure gradi ent. Hot-wire and pulsed-wire measurements are presented over the sepa rated region and downstream of reattachment. The response of the turbu lence quantities to separation and to reattachment is discussed, with emphasis on the relaxation behaviour after reattachment. Over the sepa ration bubble, the response is characteristic of that seen by other wo rkers: the Reynolds stresses in the inner region are reduced and stres s peaks develop away from the wall. At reattachment, the skewness of t he fluctuating wall shear stress vanishes, as it is known to do at sep aration. After reattachment, the outer-layer stresses decay towards le vels typical of unperturbed boundary layers. But the inner-layer relax ation is unusual. As the viscous wall stress increases downstream of r eattachment, the recovery does not start at the wall and travel outwar d via the formation of an 'internal' layer, the process observed in ma ny other relaxing flows. In fact, the inner layer responds markedly mo re slowly than the outer layer, even though response times are shortes t near the wall. It is concluded that the large-scale, outer structure s in the turbulent boundary layer survive the separation process and i nterfere with the regeneration of Reynolds stresses in the inner regio n after reattachment. This behaviour continues for at least six bubble lengths (20 boundary-layer thicknesses) after reattachment and is bel ieved to have profound implications for our understanding of the inter action between inner and outer layers in turbulent boundary layers.