Optimum Design of Wind Tunnel Contractions

Abstract

A method for the design of low-speed wind tunnel contractions is presented. An optimum contraction is considered to be the shortest one that satisfies flow quality requirements in the test section; i.e., avoids any boundary-layer separation and supplies flow to the test section with a specified degree of uniformity. It is shown that by optimizing duct wall curvature distribution, it is possible to reduce contraction length to about one-half of that presently used in practice for a given test section flow quality. For example, a contraction with an area ratio of eight for a large wind tunnel can be as short as one inlet radius. The present method closely relates the shape and length of the contraction to the degree of flow quality required in the test section, to the contraction ratio, to the inlet flow Reynolds number, and to the viscous flow conditions in the settling chamber upstream of the contraction inlet. Presented are data to guide the design of a contraction of any combination of these parameters. The results of the study indicate that the avoidance of turbulent boundary-layer separation near the exit need not be a design driver; rather, the possibility of boundary-laye r relaminarizati on near the exit needs attention.