Assessment of Mach Number Correction for Sidewall Interference in Transonic Airfoil Testing

Effects of test section sidewalls have been investigated in a two-dimensional transonic wind tunnel. The facility has recently been modified to improve the flow two dimensionality for airfoil testing. A procedure where two Mach number corrections for top and bottom wall interference and for sidewall interference are combined is applied to data obtained in the modified facility. Corrected data for two different airfoils are compared with free-air computations and other experimental results, and satisfactory agreement is observed when a proper correction for sidewall effects is selected. Sidewall correction methods proposed are then applied to various configurations of airfoil model. It is found that the applicability depends on the ratio of test section height to airfoil chord rather than model aspect ratio for the range of aspect ratio tested. To reduce threedimensional effects, a device for removing sidewall boundary layers has been installed upstream of the model. The reduction of the boundary-layer thickness by the device, however, has no effect on Mach number correction, and the significant usefulness of the device has not been determined. These results suggest that the optimum experimental arrangement for transonic airfoil testing should be reconsidered for each wind tunnel to provide truly two-dimensional data.