Influence of chamfers on broadband orifice noise in a water-pipe flow

A model based on incompressible Large Eddy Simulation (LES) is proposed for the prediction of broadband noise generated by an orifice in a water-pipe flow. The model results are compared to measured wall pressure fluctuations for two orifice geometries with similar drag coefficients; a sharp-edged orifice and a narrower orifice with an upstream chamfer. Both orifices have a thickness equal to half the orifice diameter. RANS and LES fail to reproduce the drag coefficient for the orifice with upstream chamfer. However, the plane-wave model with acoustic source from LES predicts the observed Power Spectrum Density (PSD) of the acoustic pressure wall-pressure within a factor 3. At critical Reynolds numbers one observes lines in the PSD of the narrower orifice, also observed for the same orifice with sharp upstream edge. This might be due to the onset of cavitation. Experiments with a thin orifice indicate a behaviour, at Strouhal numbers below unity, similar to that the thicker chamfered orifice. At higher Strouhal numbers the PSD of the thin orifice is two orders of magnitude lower. A simplified sound source model using the measured drag coefficients as input predicts the order of magnitude of the PSD of acoustic fluctuations.