UNSTEADY-FLOW TESTING IN A LOW-CORRECTION WIND TUNNEL

Abstract

The paper describes the application of a passive low-correction wind tunnel to unsteady-flow model testing. Designed for two-dimensional (2-D) low-speed testing, the test-section has transverse airfoil-slatted side walls separating it from outer plenum chambers. The uniform spacing of the airfoil slats determines a key parameter, the open-area ratio (OAR). Two series of tests involving unsteady flow are considered, both with steady, uniform low-turbulence approach flow. In the first series, the stationary test models represent 2-D bluff bodies, three sizes of normal flat plate and four sizes of circular cylinder, the unsteady flow arising from the Kármán vortex street created in the wake of each body. Each model was tested at the same constant Reynolds number and the wake Strouhal numberSwas measured in a full range of tunnel wall configurations 0≤OAR≤1, i.e. from solid walls to open jet. The results showed that, for each model,Swas far too high in the presence of solid walls and too low in an open jet, while varying nearly linearly over the range of OAR. The interesting feature was that for each model shape the results for all sizes converged at a particular OAR to a value ofSvery close to the accepted free-air value. This success of the airfoil-slatted test-section in correcting unsteady-flow wall effects prompted the second test series to find out if oscillating test models could produce similarly successful results. The tests were on two relatively large sizes of NACA0015 airfoil in plunging oscillation, and instantaneous pressure distributions were measured for different values of oscillation amplitude and frequency over the full range of OAR. It was found that values of pressure, lift and moment coefficient close to theoretical free-air values were obtained over a small range of OAR, whereas, again, values were much too high or low, respectively, in the presence of solid walls or open jets.