CORRELATION OF NEAR FIELD PRESSURE WITH COHERENT STRUCTURES IN AN EXCITED MACH 1.3 JET

Large Eddy Simulations (LES) were performed of a Mach 1.3 cold jet excited by plasma actuators arranged on the periphery of a round nozzle exit to understand the effect of excitation on the near field sound and shear layer struc-tures. The flow is excited with the axisymmetric (m=0) mode at 0.525 % duty cycle. The time period between pulses was varied from a very low value, where the response is ex-pected to be similar to that due to an impulse, to a value near the preferred column mode value where the response is highly non-linear. Simulation data was collected at points on three arrays radiating 8.6 ◦ to the horizontal, originating at radial distances of 1.126D, 2.126D and 5.76D respec-tively. Detailed auto and two-point correlations of measure-ments on these rakes have been examined. In the first set of observations, different points were considered on Array 1. The auto-correlations clearly show the existence and growth of structures with excitation. At the lowest frequency, the effect of individual pulses is distinct, confirming that at this value, the response is similar to that of an impulse. At the highest frequency, the interaction between structures yields an almost sinusoidal value. In the second set of observa-tions, two point correlations of each point on the innermost array were obtained with the downstream point of the outer-most array where the signal is purely acoustic. The analysis indicate that the highest correlation occurs with the region immediately downstream of the potential core collapse as well as with the point immediately beneath (sideline). The features observed in the statistical analysis are connected to the phase averaged flow structure which includes a series of rollers and ribs and associated dilatational field.