Wavelet analysis of coherent vorticity near the turbulent/non-turbulent interface in a turbulent planar jet

Abstract

Coherent vorticity near the turbulent/non-turbulent (T/NT) interface is investigated by using direct numerical simulation of a planar jet. The coherent vorticity extraction (CVE) method based on the orthogonal wavelet decomposition of vorticity is applied to the planar jet for extracting the coherent vorticity. We analyze the conditional statistics conditioned on the distance from the T/NT interface. The coherent vorticity is reconstructed from small number of wavelet coefficients. Nevertheless, the coherent vorticity contains most of enstrophy in the planar jet. Furthermore, the characteristics of the vorticity field are well captured even near the T/NT interface by the coherent vorticity. The coherent velocity obtained by the Biot–Savart relation shows that the large-scale motions, such as induced flow and engulfing motion in the non-turbulent region, are also well represented by the coherent field. The enstrophy transport equation is decomposed into coherent and incoherent parts by the CVE for investigating the role of the coherent vorticity in the enstrophy transport mechanism. The conditional average of the enstrophy transport equation shows that the enstrophy production and dissipation associated with the incoherent vorticity are small compared with the coherent contributions. The enstrophy diffusion near the T/NT interface, which causes the local entrainment of non-turbulent fluids, arises from the coherent vorticity. Thus, most of the enstrophy transport mechanism near the T/NT interface is well captured by the coherent vorticity extracted by the CVE method.