Influence of nozzle spacing and diameter on acoustic radiation from supersonic jets in closely spaced arrays

Acoustic emissions were characterized for fourteen, 8 × 8 arrays of axisymmetric supersonic jets experimentally. The nozzle diameters ranged from 3.2 mm (1/8 in.) to 6.4 mm (1/4 in.) and the hole-to-hole spacing (S) over hole diameter (d), or the S/d ratios ranged from 1.44 to 3. The arrays were tested at several net pressure ratios ranging from 2 to 24. It was found that up to a critical net pressure ratio, the arrays radiated ultrasonic frequencies. Beyond this critical net pressure ratio the characteristic frequency decreased to lie within the audible range. Frequency response plots of the sound pressure indicate a broadband frequency peak generated by the turbulent mixing noise of the jet. At lower net pressure ratio (NPR) values, this broadband peak is similar to a single jet within the jet array. However, as the NPR continues to increase this frequency peak shifts to lower values which are similar to a single jet with an equivalent exit area of the entire array. Dimensional analysis revealed that at a critical net pressure ratio a dramatic reduction in the characteristic Strouhal number occurred. A small increase in the characteristic acoustic pressure was also observed at net pressure ratios below the critical net pressure ratio and a larger increase was observed at higher net pressure ratios. The critical net pressure ratio appeared to be a linear function of S/d for the nozzle arrays. A linear curve fit was applied to the measured critical net pressure ratio and this was compared to a theoretical model prediction. The experimental results revealed that the critical net pressure ratio is well predicted by the models.