Aliasing Effect and Mitigation Strategy in Combined CFD-CAA Simulations

Many aeroacoustics problems are solved by combined CFD-CAA simulations in which the time step typically cannot be too small due to the limited computational resource. This situation results in an identified aliasing error in spectral analysis, so an anti-aliasing operation prior to sampling is necessary. In this study, an anti-aliasing filter in a time-domain is designed based on the compact finite difference scheme. This filter is applied on the Navier-Stokes solver directly, prior to sampling for CAA analysis. A pipe flow with oscillating inlet velocity validates this mitigation strategy. The result shows the fake peak induced by the aliasing error is removed while not affecting other signature peaks. The filter is applied to more complicated cases with signature peaks in predicting acoustic fields of a vortex whistle. The acoustic field around the vortex whistle with constant and variable inlet flow rates is simulated and the aliasing peak is successfully removed. Although the peak magnitudes are decreased slightly by the filter, the signature frequencies are not changed. Therefore, the simulation with anti-aliasing filter can predict the acoustic feature without the aliasing error, even if the time step is not sufficiently small, and thus reduce the simulation time significantly.