The evolution of the most energetic modes in a high subsonic Mach number turbulent jet

Low dimensional techniques are applied to the compressible turbulent mixing layer in the sound source regions of the flow from a cold (75°F) Mach 0.85 jet (z/D=3 to 8) using POD and Fourier decomposition. Measurements are acquired along the streamwise cross plane (r, θ) using a multi-component PIV system with an azimuthal grid spacing of 10° to prevent aliasing of the Fourier-azimuthal modes. The decomposition is performed using single, two and three component forms of the POD (u-streamwise, v-radial, w-azimuthal) applied in radius. Fourier decomposition is applied along the azimuthal direction because of the mean periodic nature of axisymmetric flows. The relative distribution of energy from this joint technique is shown to be consistent with Glauser & George⁵ and Jung et al⁸ who used the scalar (u) form in the incompressible axisymmetric mixing layer, and Ukeiley et al¹⁸ who used a vector form (streamwise and radial component) in the compressible Mach 0.30 & 0.60 axisymmetric jet. The dominant Fourier-azimuthal modes in the current investigation at z/D=3 and z/D= 8 are m = 5 and m = 2, respectively, and is similar to the previous findings whereby the mean energy shifts to lower modes with the growth of the mixing layer. Using the findings from this low-dimensional analysis, a Modified form of Bonnet et al's² Complementary Technique is employed to reconstruct temporally, the evolution of the joint technique's expansion coefficients via Adrian's¹ Linear Stochastic Estimation.