Tunnel-induced gradients and their effect on drag

A wake-induced drag increment is discussed that is independent of the horizontal buoyancy, gradient times volume, form usually used to correct wind-tunnel data. It is shown that the latter form should be used only in connection with externally imposed gradients. An analysis is provided that shows how, contrary to traditional thinking, products of image-induced gradients with volume-related quantities sum to zero. It is demonstrated experimentally that the size of a separation bubble can be reduced significantly by tunnel effects. The wake-induced drag increment corrects for this.