Experimental investigation of the pressure-velocity correlation of a M=0.6 axisymmetric jet

A quantitative measure of the strength of the pressure-velocity correlation of a Mach 0.6, axisymmetric jet, with an exit nozzle diameter of 50.8mm is examined. The exit flow temperature is held constant at a temperature of 25°C, and is pressure and temperature balanced with ambient conditions. The fluctuating pressure field is sampled by an azimuthal array of (15) dynamic transducers, evenly spaced at 24°. These are held fixed and positioned just outside the shear layer near the jet exit at z/D=0.875, and 1.75R from the centerline, where the pressure field has been shown to be hydrodynamic. The instantaneous velocity measurements are simultaneously acquired using a multi-component LDA system who's measurement volume is traversed along several radial and streamwise locations within the potential core, and mixing layer regions of the flow. From this multi-point evaluation, the cross-correlation between the near-field pressure array, and streamwise component of the velocity field are examined as a function of radial, streamwise, and also azimuthal separation. The results illustrate a coherence on the order of 25% between the near field pressure and the velocity field. Analysis of the coherency spectra illustrates the frequency band of the correlations and suggests that the potential core and mixing layer regions of the flow are, in general, governed by the high and low frequency motions of the flow, respectively. The azimuthal modal distribution of the cross-correlation shows the dominance of the column mode of the jet, with no higher modes exhibited within the potential core region, and only modes 1 & 2 within the shear layer.