BOUNDARY-CONDITIONS FOR DIRECT COMPUTATION OF AERODYNAMIC SOUND GENERATION

Accurate computation of the far-field sound along with the near-field source terms associated with a free shear flow requires that the Navie r-Stokes equations be solved using accurate numerical differentiation and time-marching schemes, with nonreflecting boundary conditions. Non reflecting boundary conditions have been developed for two-dimensional linearized Euler equations by Giles. These conditions are modified fo r use with nonlinear Navier-Stokes computations of open flow problems. At an outflow, vortical structures are found to produce large reflect ions due to nonlinear effects; these reflection errors cannot be impro ved by increasing the accuracy of the linear boundary conditions. An e xit zone just upstream of an outflow where disturbances are significan tly attenuated through grid stretching and filtering is developed for use with the nonreflecting boundary conditions; reflections from vorti cal structures are decreased by 3 orders of magnitude. The accuracy an d stability of the boundary conditions are investigated in several mod el flows that include sound radiation by an energy source in a uniform ly sheared viscous flow, the propagation of vortices in a uniform flow , and the spatial evolution of a compressible mixing layer.