Computations of unsteady pressure in fluid flows for acoustic analyses.

Abstract

Two problems involving flow computations of unsteady pressure are undertaken. The first study examines plane wave propagation in a uniform shear flow comparing exact and approximate treatment of fluid convection. Unlike the approximate treatment, significant wave strengthening and nonzero acoustic perturbation vorticity are found in the exact convection formulation. The second, and much more extensive, research effort explores the application of a vortex boundary-element method (VBEM) to aerodynamic noise problems. Two novel methods are developed and tested for use with the VBEM to provide an efficient means of computing pressure at arbitrary locations within the flow field, including the body surface. Both methods are two to three orders of magnitudes faster computationally than the direct Poisson approach. These pressure methods are exploited in computations of flow past a cube yawed at zero and thirty degrees to the streamwise direction at Reynolds numbers of 500 and 1500. An additional pressure method is developed for use in the VBEM code but is found to be acceptable only for fully attached flow, failing at points of pressure discontinuity such as sharp corners. As partial validation of the VBEM computations, an experimental study of shedding frequencies in the near wake of a non-yawed cube is conducted. Wake shedding frequencies predicted by the VBEM agree with these experimental data. The streamwise extent and the symmetric nature of the wake separation zone in the time-averaged flow results further indicate the flow prediction accuracy. These new, highly efficient pressure techniques are particularly useful to the vortex method practitioners who need to determine surface pressures continually in an unsteady flow for acoustic analyses. Sample pressure spectra for the cube are provided in the work to illustrate this capability.