Abstract:
The inaccurate nozzle-exit conditions due to the difficulties in measurements make the numerical simulation insufficient to predict the jet development of the laboratory nozzle. Moreover, there is a greater discrepancy for the full-scale turbulent jet. Therefore, the effects of inflow conditions are of great interest again in jet noise recently. Large eddy simulations (LES) of a round jet at
Ma=0.75 and
ReD=8.7×10
5 are carried out so as to study the inflow forcing effects on the predicted flow and noise results. A modified multi-mode linear instability forcing method for the inflow is proposed and employed so as to trigger the turbulence. As a comparison, calculations without inflow forcing and with a vortex-ring forcing are also carried out. Numerical results show that both the vortex-ring forcing and the multi-mode liner instability forcing methods are efficient to damage the azimuthal stability of the large scale ring vortices appeared near nozzle exit. This leads to an accelerated flow transition to turbulent shear-layer, which is the exact state of the flow in realistic nozzles. The predicted spectra of the axial and radial velocity fluctuations are compared. It is found that the peak related to flow transition is less distinct for jets with force. Besides, the initial turbulent fluctuations are at higher level with the proposed force compared with the vortex-ring forcing method, resulting in a faster flow transition. Comparison of the predicted sound far fields shows that with the inflow forcing methods, the low frequency noises caused by vortex pairing during transition are suppressed. As a result, theoretical data well match the experimental data for turbulent jets.