Background Noise Measurements from Jet Exit Vanes Designed to Reduce Flow Pulsations in an Open-Jet Wind Tunnel.

Abstract : Many open-jet wind tunnels experience pulsations of the flow which are typically characterized by periodic low-frequency velocity and pressure variations. One method of reducing these fluctuations is to install vanes around the perimeter of the jet exit to protrude into the flow. Although these vanes have been shown to be effective in reducing the fluctuation content, they can also increase the test section background noise level. This report presents the results of an experimental acoustic program in the Langley 4- by 7- Meter Tunnel to evaluate the effect on tunnel background noise of such modifications to the jet exit nozzle. Noise levels for the baselilne tunnel cnfiguration are compared with those for three jet exit nozzle. Noise levels for the baseline tunnel configuration are compared with those fodr three jet exit nozzle modifications including an enhanced noise reduction configuration that minimizes the effect of the vanes on the background noise. Although noise levels for this modified vanes on the background noise. Although the noise levels for this modified vane configuration were comparable to baseline tunnel background noise levels in this facility, installation of these modified vanes in an acoustic tunnel may be of concern because the noise levels for the vanes could be well above background noise levels in a quiet facility. Keywords: Wind tunnels (open-jet), low speed; Flow pulsations; Flow surging; Jet exit vanes; Turbulence; Wind tunnel nozzles; Background noise.