Turbulent jet noise in the absence of coherent structures

Abstract

© 2017 American Physical Society.Sound radiation from a subsonic turbulent jet is examined after a hypothetical removal of the near-field coherent structures in the axisymmetric component of the velocity fluctuations. With the help of a well-validated database of large-eddy simulation, the near-field coherent structures are extracted using a discrete wavelet transform (DWT), and their spatial structures are examined using a proper orthogonal decomposition (POD). The acoustic far field is calculated using Lighthill's acoustic analogy. It is shown that the coherent part extracted by DWT accounts for most of the fluctuation energy in the axisymmetric component of axial velocity, whereas the incoherent part, assumed to have a Gaussian probability distribution, has little energy. After the coherent part is removed, the axisymmetric component of the sound is found to be significantly reduced, around 7 dB in the overall sound pressure level at 30 deg with respect to the jet axis. The reduction is mostly at low Strouhal numbers (St<0.4, based on the speed of sound and the nozzle exit diameter). The first few POD modes of the near-field coherent part, which capture most of the fluctuation energy, are found to be characterized by large-scale wavy structures. After these POD modes are removed, the axisymmetric component of the sound pressure level is also reduced considerably, by around 5 dB/St at St=0.2. The results suggest that there is a causal link between the axisymmetric components of the near-field hydrodynamic fluctuations and far-field low-angle jet noise, although the axisymmetric mode constitutes only a small proportion of total fluctuation energy. It is also suggested that not only the large-scale wavy structures in low POD modes but also the smaller scale structures in higher POD modes need to be included for jet noise modeling, because they are both shown to be efficient at sound radiation.