The Royal Aircraft Establishment 4 ft x 3 ft experimental low turbulence wind tunnel. Part 1 general flow characteristics

Abstract

The 4 x 3 ft Wind Tunnel was erected as a model of larger tunnels to investigate unconventional design features directed towards obtaining a high standard of flow. Diffusers of 5 deg cone angle are used, except for the rapid expansion through three wireTgauze screens up to tile maximum section. The contraction ratio is 31.2 : 1 and nine screens are fitted in the maximum section. A speed control is used operating independently of the fan by means of a by-pass duct. The velocity distribution across the working-section is constant to ± ¼ per cent. The standard deviation of the velocity with time measured over a period of 50 sec is 0.03 per cent. The flow in the diffusers shows no tendency to separate and the velocity distribution approaching tile first screen is very satisfactory. The installation of cascades with gap/chord ratio of ¼ gives uniform outlet flow without appreciable increase in the pressure drop. There is no separation in the rapid expansion of the bulge, but the flow in the contraction cone is not satisfactory. A longer contraction would have been advantageous. The power factor has been measured as 0.27 with.all screens fitted but could be improved slightly if all the leaks were sealed. The speed control is satisfactory in operation.