Noise Characteristics and Shock Structures of Under-Expanded Jet Array Configurations

Results on noise characteristics and flow-visualization shock structures of 8 × 8 arrays of jets for stagnation pressure ratios in the range 2.4 – 22, and spacing-to-diameter ratios of 1.4 – 4, are presented. Three regimes of noise generation were observed for the arrays tested: regime of weak interactions at low pressure ratios, where the noise spectrum is essentially similar to that for individual jets in the array; regime of combined jet at high pressure ratios, where noise spectrum is similar to that for a single large jet; and, regime of strong interactions, which appears between these two regimes and where individual jets as well as the combined jet significantly contribute to noise generation. The domain of each of these regimes is determined by pressure ratio and spacing-todiameter ratio for the array. The influence of neighboring jets on shock structures of individual jets in the array appears to be insignificant in flow-visualization images at low pressure ratios; however, with increasing pressure ratio, the shock structures increase in size and increasingly influence their neighbors. Large shocks, which span the width of the array and similar to that for a single jet, were observed in the combined jet regime. The combined jet, which is formed by merger of individual jets from the array, dominates noise generation when Mach number in the mixed stream is greater than 0.75. The noise characteristics of the jet arrays correlate well with their corresponding form of shock structures in the three regions described.