Effect of Filter Width on Sound of Variable Density Jets

Abstract

The e ect of ltering on the sound spectrum in variable density jets is investigated. Prediction of aeroacoustics using large-eddy simulations is becoming increasingly popular. A direct evaluation of the sound eld is, however, often computationally too expensive. The sound in the near far eld of variable density jets was therefore computed using a hybrid method developed by Ewert & Schr oder. The power spectral density (PSD) and overall sound pressure levels (OASPL) of the reference direct numerical simulation (DNS) are compared to ltered DNS and large-eddy simulation (LES) of the variable density jet at a density ratio of s=0.14, relating the jet to the free-stream density. The jet develops under a global instability, which determines the low Strouhal number range of the PSD and results in similar shapes of the spectrum for di erent radiation angles. Unlike the pure LES, the ltered elds show only slightly reduced levels at all Strouhal numbers. Good agreement is found for low radiation angles, but an up to 10dB reduction is observed for sideline and backward radiation. The LES simulation showed higher values at low Strouhal numbers, which could be attributed to a non-universal side-jet phenomenon, which may be encountered in strongly heated jets. A further simulation using a reduced part of the available time series without this event lead to PSD values comparable to that of the DNS.