Influence of orifice thickness and chamfer on broadband noise in a water circuit

An incompressible large-eddy simulation (LES) is used to predict broadband noise generation by an orifice in a water circuit. Flow conditions are chosen in order to avoid cavitation. The model's results are compared to measured wall pressure fluctuations for three orifices: a thin sharp-square-edge orifice, a thick sharp-square-edge orifice and a thick narrower orifice with an upstream chamfer. Reynolds-averaged Navier-Stokes (RANS) and large-eddy simulations (LES) fail to reproduce the steady-flow drag coefficient of the thick sharp-edge orifice and of the upstream-chamfered orifice. Large differences are found in the steady-flow drag coefficient of different samples of the thick orifice. An edge-tone like instability is observed in the near-field pressure fluctuations of one of the thick sharp-square-edge orifices, which does not appear in the other samples and is suppressed by the upstream chamfer. However, there are only minor differences in far-field radiated sound for these thick sharp-edged orifices. A plane-wave model with acoustic source from the LES predicts the power spectral density (PSD) of the acoustic pressures within a factor 3. At low Strouhal numbers (based on the orifice diameter and the cross-sectional averaged velocity in the orifice) the thin orifice behaves similarly to the thicker strongly chamfered orifices, with thin central cylindrical section. At higher Strouhal numbers the measured and predicted acoustic-pressure fluctuations due to these orifices are two orders of magnitude lower than those of thick orifices with sharp square edges. As orifices are used for the control of the flow distribution through complex water-cooling networks, the good results for thick chamfered narrow orifices call for a further study to find shapes that minimize the sound production at constant static pressure drop.