Coherent vorticity extraction in turbulent boundary layers using orthogonal wavelets

High-resolution data obtained by direct numerical simulation of turbulent boundary layers are analysed by means of orthogonal wavelets. The data, originally given on a Chebychev grid, are first interpolated onto an adapted dyadic grid. Then, they are decomposed using a wavelet basis, which accounts for the anisotropy of the flow by using different scales in the wall-normal direction and in the planes parallel to the wall. Thus the vorticity field is decomposed into coherent and incoherent contributions using thresholding of the wavelet coefficients. It is shown that less than 1% of the coefficients retain the coherent structures of the flow, while the majority of the coefficients corresponds to a structureless, i.e., noise-like background flow. Scaleand direction-dependent statistics in wavelet space quantify the flow properties at different wall distances.