The Effects of Heating on Noise Generation in Subsonic Transitional Jets

Large eddy simulation was performed for two jets, i.e. a cold jet and its heated counterpart, with the same exit Mach number. The far-field noise was predicted by Kirchhoff's integral method based on the near-field pressure fields on a controlled surface. The directivity and intensity of the far-field noise are found to be in good agreement with existed experiments. At polar angles < 70°, heating enhances the noise intensity, while the noise intensity for the cold jet at angles > 70° is higher. For the heated jet, the potential core length is shorter, and radial spreading is much faster. Moreover, the peak locations of the fluctuation intensity of the velocity on the centreline and nozzle lip line move upstream in the hot jet, and more rapid decay occurs. Based on the mean flow of simulation, linear parabolized stability equations (LPSE) are solved to study spatial evolution of instability waves. In the low-frequency range, m = 0 mode that absent in direct forcing is dominant in acoustic field, but the peak of N-factor of m = 0 mode predicted by LPSE is not the maximum. This should be associated with nonlinear interaction for instability waves with opposite signs of m.