Comparison of noise mechanisms in high and low reynolds number high-speed jets

Direct numerical simulation results from a low Reynolds number jet and experimental results from a high Reynolds number jet were analyzed and compared to investigate the influence of the Reynolds number on the mechanisms generating jet noise. The direct numerical simulation results are for a Mach 0.9, low Reynolds number (Re_D ∼ 3600) axisymmetric jet, and the experimental results are for an ideally expanded, Mach 1.3, high Reynolds number (Re₃ ∼ 1.06 × 10⁶) axisymmetric jet. Previous experimental work on the high Reynolds number jet using a three-dimensional far-field microphone array, located at 30 deg with respect to the downstream jet axis, estimated the source location and the time of generation of each large amplitude sound wave. Simultaneously with the far-field sound measurements, the source region of the flow field was visualized using a MHz rate imaging system. In the current work, this technique is used with the direct numerical simulation data and the results are compared with the experimental data. There are many similarities between the two results including the far-field acoustic spectrum, coherence, and average waveform as well as the mean noise source location. The few differences can be attributed to the limited range of turbulence scales and the laminar initial shear layer of the low Reynolds number jet. The main conclusion is that the rapid breakdown of the large-scale structures appears to be important, perhaps the main mechanism of jet noise, independent of the Reynolds number. Right before the breakdown, the structures seem to contract in size, tilt, and eventually disintegrate. To offer a possible explanation for the observed noise mechanism, a simple one-dimensional wave packet model is shown to create more noise in the far field when truncated to simulate a breakdown.