DESIGN OF A VARIABLE CONTRACTION FOR A FULL-SCALE AUTOMOTIVE WIND-TUNNEL

The design of a three-dimensional contraction for a projected full-scale aero-acoustic automotive wind tunnel is described. The use of two interchangeable outlet sections (overall contraction ratio 5.5 and 16.3, respectively) allows either full-scale testing at a test section speed of 45 m/s or 1/4-model-scale testing up to 75 m/s. In order to achieve the shortest possible contraction lengths, unconventional contraction contours are used. The contours were designed using a three-dimensional potential flow method in combination with a two-dimensional boundary-layer method. The final contraction designs were checked experimentally in 1/8-scale pilot-tunnel tests. Although the results show that the design goals have been fully met, flow visualization experiments revealed re-laminarization and flow separation in the corners near the throat for both contraction models. However, a re-calculation of the boundary-layer development for the actual 1/8-model-scale Reynolds numbers confirmed that both effects are caused by a Reynolds number problem. For the full-scale contractions it can therefore be expected that neither re-laminarization nor boundary-layer separation will occur.