Effects of spatial filtering on sound radiation from a subsonic axisymmetric jet

The effects of spatial filtering on the sound generated from a subsonic axi symmetric jet were investigated by filtering near-field how variables obtai ned from a direct numerical simulation. This is useful to assess the accura cy of the large-eddy simulation (LES) technique for predicting aerodynamica lly generated sound. Lighthill's acoustic analogy in the frequency domain w as employed to predict the far-field sound. The direct numerical simulation results were in excellent agreement with recently published results for th e 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 acousti c analogy were in good agreement with the simulation results at low frequen cies, even for shallow angles from the jet axis. Significant discrepancies were observed at high frequencies. It was found that low frequency sound wa s dominant and the effects of filtering on the low-frequency sound were neg ligible. In addition, the sound levels computed from both the filtered and unfiltered source terms were in good agreement,vith the directly computed r esults. Filtering reduced the small-scale fluctuations in the near-field an d, as expected, decreased the magnitude of the source term for the high-fre quency sound. A model was developed and tested to predict the subgrid contr ibution to the Lighthill tensor in cases where, as in LES, only relatively large-scale flow structures are resolved.