Two-point correlations of the near and far-field pressure in a transonic jet

To better understand the relationship between the near-field pressure and the acoustic far-field, multi-point measurements of the near-field pressure around the periphery of a cold Mach 0.85 round jet are compared to simultaneous multi-point farfield acoustic pressure measurements. The results indicate that the near-field pressure is low dimensional and the instantaneous contribution from both azimuthal mode 0 and 1 is sufficient to accurately recover the dynamics of the near-field pressure. Correlations of the acoustic far-field with the contribution of each azimuthal mode to the near field pressure, however, indicate that only azimuthal mode 0 is well correlated with the far-field pressure, suggesting that the acoustic source in the jet is predominantly axisymmetric. The correlation of the higher azimuthal pressure modes with the far-field acoustic pressure is extremely poor suggesting that the axisymmetric source is weakened by the presence of higher azimuthal modes in the near field of the jet.