Some investigations into the spanwise non-uniformity of nominally two-dimensional incompressible boundary layers downstream of gauze screens

Abstract

Summary. This Report describes investigations on spanwise non-uniformity in boundary-layer flow at low subsonic speeds over a range of experimental conditions, covering particularly the effect of varying number and open-air ratio of screens, type of honeycomb, flow contraction, surface curvature, and downstream position of measuring plane. Regions with laminar and turbulent boundary layers were investigated. Bradshaw\\'s investigations into the effect of open-area ratio are confirmed. The conclusion is drawn that in a boundary-layer tunnel, where spanwise uniformity is essential, at least the two downstream screens should have open-area ratios greater than 0.57. The Report also recommends the use of a high- quality honeycomb downstream of the screens. Whilst open-area ratios greater than 0.57 are also advisable for new general-purpose tunnels, the Report suggests that the cost of changing the screens of existing tunnels is scarcely justifiable economically.