Identifying the most energetic modes of the pressure near-field region of a Mach 0.85 axisymmetric jet

The near-field pressure region of a Mach 0.85 axisymmetric jet with an exit nozzle diameter of 50.8mm, is examined experimentally using an azimuthal array of 15 equidistantly spaced (24°) transducers positioned just outside the jet shear layer. The exit flow temperature is held constant at a temperature of 27°C, and is pressure and temperature balanced with ambient conditions. The transducer array is traversed downstream through the end of the potential core. Examination of the Fourier-azimuthal decomposition reveals the presence of only the first three modes (0,1, &2), with a downstream contribution of low-frequency energy only. A second set of experiments fixes 7 transducers, spaced at (48°) in azimuth, near the jet exit at z/D=0.875. A separate azimuthal array is then fitted with another 7 transducers, positioned in similar fashion, and again traversed downstream through the end of the potential core. The cross-correlations between the array fixed at the jet lip and the downstream array, exhibit magnitudes on the order of 60%, extending through the end of the potential core where the correlation falls to 20%. The modal decomposition of the cross-correlations suggest a column mode dominance. The decay in magnitude is in direct relation with the decay in the contribution of mode-0.