Experiments on the influence of surface curvature on laminar to turbulent transition in three-dimensional boundary-layer flows

Kurzfassung

The development of streamwise vortices and transition of a three-dimensional boundary-layer flow on a curved surface is studied experimentally. The model is a cylindrical wing shaped body with concave profile segments. It is set at a sweep angle of 30 o. In order to enable comparision with theory quasi two-dimensional flow conditions are approximated by means of contoured end plates. The streamwise pressure gradient leads to the amplificaiton of both Goertler and crossflow instabilities. The disturbance development is measured by means of hot-wire anemometry. Surface hot-films are used for sensoring transition. Streamwise vortices are found to be strongly amplified along the concave segment in the presence of strong and weak crossflow as well. No characteristic difference is found for the streamwise vortices developing in the two cases. Remarkably, on the convex segment the streamwise vortices are damped, even in the region of strong crossflow instability. Apart from the stationary modes are also traced. A significant difference is observed between the disturbance interactions, appearing on the swept curved model and the swept flat plate used in former experiments. On the curved model, transition occurs under swept conditions earlier than under unswept conditions. This becomes most obvious on the concave profile segment.