Modification To The NASA Langley 4- By 7-Meter Tunnel For Improved Rotorcraft Aerodynamics And Acoustics

A series of investigations have been conducted to improve the mean flow characteristics throughout the circuit and the turbulence intensity in the test section of the NASA Langley 4- by 7-Meter Tunnel. Although the velocity profile in the test section indicated a fairly uniform flow pattern, asymmetric flow existed in the downstream diffusers. This resulted in flow separation upstream and downstream of the drive fan. Flow deflec­tors were installed to displace the airstream and produce more symmetrical velocity distributions. These flow deflectors proved to be extremely effective in providing more uniform flow entering the drive fan and alleviating diffuser flow separation. Currently, the closed test section configuration has good turbulence characteristics. Unfortunately, the open test section configuration has periodic flow pulsations occurring in three distinct speed regions which generate very high turbulence levels. These periodic flow pulsa­tions make it very difficult to set rotorcraft model conditions and acquire detailed time-dependent rotor wake velocity characteristics. Modification of either the test section entrance using triangular shaped vanes or the test section exit by an alternate flow collector geometry proved to be effective In alleviating these flow pulsa­tions. The alternate flow collector geometry had mini­mal effects on the size of the uniform core and the tunnel drive power required; whereas, the triangular vanes had detrimental effects on both of these para­meters.