On the relation between fronts and high-shear layers in wall turbulence

Direct numerical simulation results of turbulent channel flow are analyzed in order to examine the relation between two kinds of near-wall flow struct ures, namely the instantaneous shear layers and the fronts which are derive d from two-point statistics of the streamwise velocity component. The near- wall shear layers are analyzed by flow visualizations and conditional sampl ing, while the fronts are examined by means of space-time correlations and spatial two-point correlation functions. The present study focuses on the a nalysis of the propagation speed and the spatial shape of the structures. C oncerning the propagation speed it is shown that the results obtained from flow visualizations are in close agreement with the propagation velocities derived from space-time correlation functions. The comparison of VISA resul ts for the instantaneous shear with spatial structures obtained from two-po int correlations of the streamwise velocity and the sheer gives evidence th at the fronts are intimately related to the pronounced near-wall shear laye rs.