Self-noise mechanisms in a laminar-inflow axial turbine stage

The airfoil self-noise defines the lower bound of turbomachinery noise, and its generation mechanisms are closely related to the boundary layer instabilities and vortex shedding patterns. This paper studies the airfoil self-noise generation mechanisms of a laminar inflow axial turbine stage employing large eddy simulation and dynamic mode decomposition, and the spectral characteristics of both the laminar-boundary-layer-vortex-shedding (LBL-VS) noise and the turbulent-boundary-layer-trailing-edge (TBL-TE) noise are calculated by the Ffowcs Williams-Hawkings equation. The results indicate that the stator self-noise generating mechanism is based on the periodic vortex shedding process and its strong broadband noise radiation is attributed to the self-sustaining hydrodynamic-acoustic feedback loop. The rotor self-noise generating mechanism instead is based on the vortex dipole, which is mainly influenced by the streamwise vortexes in the suction side boundary layer. For the LBL-VS noise generated by the stator, the aerodynamic source point of the acoustic feedback loop is proved to be the maximum velocity point, since this location shows the strongest hydrodynamic and acoustic coherences with the trailing edge vortex shedding. The periodic vortex street is responsible for the peak tone. The acoustic feedback loop is believed not to cause direct harmonic sound emission but to raise the far-field noise level by enhancing the periodic vortex shedding process. For the TBL-TE noise generated by the rotor, the vortex street develops into the vortex dipole due to the unequal phase velocities of the vortical structures shed from the opposite blade sides, presenting a poor hydrodynamic self-sustainability and a low acoustic radiation.