Thermal effects on supersonic underexpanded jet noise

To investigate the influence and underlying mechanisms of thermal effects induced by high total temperatures on the flow and acoustic fields of supersonic underexpanded jets, numerical simulations were conducted on a Mach number 1.95 supersonic underexpanded jet using a hybrid approach that combines Large Eddy Simulation (LES) with the Ffowcs Williams-Hawkings (FW-H) acoustic analogy. By comparing flow and acoustic data under varying total temperature conditions, the impacts of thermal effects on jet evolution and far-field radiated noise were analyzed. The results indicate that thermal effects accelerate the development of the jet shear layer, shorten the potential core length by 14.2%, and enhance turbulent fluctuations along the nozzle lip line upstream of the potential core. Consequently, thermal effects increase the peak far-field overall sound pressure level, shift the peak radiation direction upstream, and move the source locations of broadband Shock-Associated Noise (BBSAN) upstream. Subsequently, by designing comparative cases with controlled jet parameters, the thermal effect was decoupled into a temperature effect (constant-velocity heating) and a velocity effect (constant-temperature accelerating), revealing their differential impacts on BBSAN generation: the temperature effect enhances the shear layer turbulence upstream of the potential core, and by altering the jet Mach number, it shortens the shock-cell length and weakens the shock-wave intensity, leading to a decreased peak sound pressure level and an increased peak frequency of the BBSAN; conversely, the velocity effect also enhances the upstream shear layer turbulence, but it elongates the shock cells and strengthens the shock-wave structure, resulting in a significantly increased peak sound pressure level and a decreased peak frequency. Under the combined influence of these two effects, the overall variations in BBSAN are relatively minor. Furthermore, within the thermal effect, the velocity effect dominates the impact on large-scale turbulent mixing noise, thereby significantly enhancing it.