A Theoretical and Experimental Study of the Exhaust Noise Environment of a Proposed High Reynolds Number Tunnel

Abstract

The growing interest in noise pollution has presented designers of modern aerodynamic test facilities with new problems and interesting and complex challenges. Due to the sheer magnitude of their power output, many existing aerodynamic facilities and some of those designed for the future are enormous sources of sound. As part of a feasibility and preliminary design study of a High Reynolds Number Tunnel (HIRT) currently being performed by ARO, Inc., at the Arnold Engineering Development Center (AEDC), an investigation has been conducted to (a) predict the acoustic environments which may be anticipated from the operation of HIRT and (b) perform a comprehensive experimental study using a scale-model facility in order to obtain acoustic data over the anticipated range of operational variables for HIRT. One important feature of the facility which necessitated this study is the open-circuit blowdown design which causes the starting shock wave and high-speed jet flow to exhaust into the atmosphere. A comprehensive analysis of the noise-generating mechanisms associated with the facility has been conducted, although based in some cases on engineering approximations and acoustic models of the flow environment. These data were used to define those design features of the facility which would produce predominant noise source mechanisms. Based on the analytical findings, a one-thirteenth-scale model of HIRT was designed using precise scaling of the important features of HIRT to retain, within practical limits, those design features which contribute to the noise characteristics of the facility. Utilizing the model facility, acoustic tests were performed over a broad range of operational variables representative of the operational range of the full-scale facility. Free-field acoustic measurements were taken along various polar rays around the exhaust stack of the facility to define the noise levels and spectra, directivity patterns, and propagational characteristics. Comparisons are made between the acoustic predictions and the experimental measurements and show the applicability of current noise prediction methods (which are based on jet and rocket exhaust flow) to noise problems associated with aerodynamic test facilities.