Advanced Acoustic Assessment of Small-Scale Military-Style Nozzles with Chevrons

Abstract

With the emergence of more powerful fighter aircraft, supersonic jet noise reduction devices are being intensively researched. Small scale measurements are a crucial step in evaluating the potential of noise reduction concepts at an early stage in the design process. With this in mind, the present study provides an acoustic assessment of small-scale military-style nozzles with chevrons. Comparisons are made between the present measurements and those made by NASA at moderate scale. Measurements made with baseline nozzles (without chevrons) show excellent agreement with NASA data establishing the accuracy of the scaling methodology. The effect of chevrons on supersonic jets is then investigated for cold jets, highlighting the crucial role of the jet operating conditions on the effects of chevrons on the jet flow and subsequent acoustic benefits. At low Reynolds numbers (small scale) the penetration of the chevrons in the jet flow is the most important chevron parameter in reducing the generated noise. A small scale heat simulated jet is investigated in the overexpanded condition and shows no substantial noise reduction from the chevrons. This is contradictory to moderate scale measurements. The discrepancy is attributed to a Reynolds number low enough to sustain an annular laminar boundary layer in the nozzle that separates in the over-expanded flow condition. Transition of the boundary layer to turbulent flow is induced with inner roughness of the nozzle and results in more noise reduction with the chevrons. The resulting effect is comparable to results from NASA, seemingly validating the hypothesis made that jets of too low Reynolds number cannot be used directly to observe and measure chevron noise reduction. These results are important in assessing the limitations of small scale measurements in this particular jet noise reduction method.