Effects of Spatial Filtering on Sound Radiation from a Subsonic Axisymmetric Jet

Abstract

The effects of spatial filtering on the sound generated from a subsonic axisymmetric jet were investigated by filtering near-field flow variables obtained from a direct numerical simulation. This is useful to assess the accuracy of the large-eddy simulation (LES) technique for predicting aerodynamically generated sound. Lighthill's acoustic analogy in the frequency domain was employed to predict the far-field sound. The direct numerical simulation results were in excellent agreement with recently published results for the same jet (Mitchell, B. E., Lele, S. K., and Moin, P., Direct Computation of the Sound Generated by Vortex Pairing in an Axisymmetric Jet, Journal of Fluid Mechanics, Vol. 383, 1999, pp. 113-142). To handle the effects of domain truncation errors on the Lighthill source term, a windowing function was employed. Predictions of the far-field sound using Lighthill's acoustic analogy were in good agreement with the simulation results at low frequencies, even for shallow angles from the jet axis. Significant discrepancies were observed at high frequencies. It was found that low-frequency sound was dominant and the effects of filtering on the low-frequency sound were negligible. In addition, the sound levels computed from both the filtered and unfiltered source terms were in good agreement with the directly computed results. Filtering reduced the small-scale fluctuations in the near-field and, as expected, decreased the magnitude of the source term for the high-frequency sound. A model was developed and tested to predict the subgrid contribution to the Lighthill tensor in cases where, as in LES, only relatively large-scale flow structures are resolved.