Divergence-free Wavelets for Coherent Vortex Extraction in 3D homogeneous isotropic turbulence

In this paper we investigate the use of divergence-free wavelet bases for the Coherent Vortex Extraction (CVE) of turbulent flows. We begin with a short presentation of the construction of 3D divergence-free biorthogonal wavelets. Then we apply the CVE decomposition to a homogeneous isotropic turbulent flow, computed by a direct numerical simulation at resolution 240 and upsampled to N = 256. At first, the CVE is applied to the vorticity field. Using the divergencefree wavelets for a vorticity field makes sense since the vorticity also verifies an incompressibility condition when the velocity does. The coherent part of the vorticity field is reconstructed from the largest wavelet coefficients, corresponding to 3 % N, while the complement constitutes the incoherent part. We show that the coherent part corresponds to the vortex tubes of the flow and retains most of the energy and enstrophy. These results are then compared to those obtained using non–divergent free wavelets, both orthogonal and biorthogonal. Then we also apply the CVE method, using divergence-free wavelets, to decompose the velocity field and subsequently compute the corresponding vorticity fields. The results show that the decomposition of velocity exhibit large smooth vortex structures in contrast to what is obtained with the decomposition of the vorticity.