Properties of the Tones Emerging in the Near-Field Pressure Spectra of Hot High-Speed Jets

The influence of temperature on the acoustic tones emerging in the near-nozzle pressure spectra of high-speed jets is investigated using large-eddy simulations. The jets have nozzle-exit temperatures equal to 1, 1.5 and 2.25 times the ambient temperature and velocities yielding acoustic and jet Mach numbers Ma and Mj, relative to the speeds of sound in the ambient medium and in the jet flow, respectively, between Ma=0.30 and 2 and between Mj~0.50 and 1.5. For the hot jets, acoustic tones appear near the nozzle for sufficiently high nozzle-exit velocities, as for the isothermal jets. As the jet temperature increases, at a fixed acoustic Mach number, the tones are weaker and appear at higher Strouhal numbers. At a fixed jet Mach number, on the contrary, their characteristics do not change much. However, their levels increase by a few dB, their Strouhal numbers slightly decrease, and for Mⱼ < 1 their widths are larger. These results are consistent with the properties of the upstream-propagating free-stream guided jet waves for a vortex-sheet model. Finally, at a fixed jet Mach number, the tone prominence does not vary significantly with the temperature, suggesting that the mechanisms responsible for the tones are of same nature and of similar strength in isothermal and hot jets.