A new steady blockage correction method has been developed for the wall interference correction of closed test-section subsonic wind tunnels based on the nonlinear relationship between separation blockage and separation drag. This method can be applied continuously from the linear lift-slope region to the highly nonlinear post-stall region by on-line processing. It was found that, in the post-stall region, the separation blockage takes a major portion of the blockage for dynamic pressure correction, while the lift interference effect is small. The present method was validated by comparing the results with the classical method based on the test results of a bluff body and the measured-boundary-condition method. It was shown that the present method is in good agreement with the measured-boundary-condition method, enabling better wall corrections than the bluff body method in both near-stall and post-stall regions.
The wall interference effects on unsteady flows around a circular cylinder and an oscillating NACA0012 airfoil in closed test-section wind tunnels have been numerically investigated by solving the compressible Navier-Stokes equations. The numerical scheme is based on a node-based finite-volume method with Roe's flux=difference splitting and an implicit time-integration method coupled with dual time-step sub-iteration. The computed results for the cylinder showed that the favorable pressure gradient in the accelerated flow region of the cylinder is enhanced by the wall interference, and as a result the fluctuation of lift and drag is augmented. The drag further increases because of the low base pressure. The computed results of the oscillating airfoil having a thin wake showed that the lift curve slope is increased and the magnitude of hysteresis loop is reduced by the interference effects. Since the vortex around the airfoil is generated and convected downstream faster than the free-air condition, the phase of lift, drag and pitching moment coefficients was shifted. The pressure on the test section wall shows harmonic terms having the shedding/oscillating frequency contained in the wall effect.
For unsteady blockage correction, the pressure distribution along the test section wall was decomposed into Fourier series and a quasi-steady method based on a doublet-panel method was applied to each Fourier coefficient. The unsteady correction for a complete test period was accomplished by recombining each corrected terms. The corrected results by the present method correlated well with free-air conditions for tunnel blockage area ratios acceptable by linear correction theory.
