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An experimental investigation of an incompressible turbulent boundary layer in the vicinity of separation

  • P. Dengel (a1) and H. H. Fernholz (a1)

Abstract

The following is an investigation into the effects of small changes in the static pressure distribution on the development of an axisymmetric, incompressible, turbulent boundary layer with incipient separation. The pressure distribution was closely controlled to study three cases, in which the skin friction was either approximately zero, slightly negative, or slightly positive along a fixed length. Mean flow and turbulence structure in air were measured using pulsed-wire and hot-wire anemometry.

These measurements show characteristic properties of steady turbulent boundary layers both on the verge of separation and with a long, shallow separation bubble. There is an asymptotic velocity defect law near separation. A linear relationship between χW, the wall value of the reverse-flow parameter, and the form parameter H12 suggests the importance of χW in characterizing the boundary layer. The occurrence of the first reverse-flow events coincides with the vanishing of the logarithmic law, the asymptotic mean velocity profile, and a sudden drop in the values of the skewness SW and the flatness FW of the skin friction. This implies that the presence of instantaneous reverse flow is associated with a complete change in the nature of the near-wall flow, well upstream of mean separation. As the three cases were investigated in a single test section under closely controlled conditions with the same experimental techniques, this data set is well suited to a sensitivity study. It is possible to show the effect of small changes in the upstream pressure gradient on the separation region and to distinguish the effect of mean reverse flow from that of the adverse pressure gradient. This effect of the reverse flow is displayed most clearly in a plateau in $\overline{u^{\prime 2}}$ near the wall and in unusual behaviour of the skewness and the flatness profiles over the inner half of the boundary layer.

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An experimental investigation of an incompressible turbulent boundary layer in the vicinity of separation

  • P. Dengel (a1) and H. H. Fernholz (a1)

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