PROPAGATION VELOCITY OF PERTURBATIONS IN TURBULENT CHANNEL FLOW

A database obtained from direct numerical simulation of a turbulent channel flow is analyzed to extract the streamwise component of the propagation velocity V of velocity, vorticity, and pressure fluctuations from their space-time correlations. A surprising result is that V is approximately the same as the local mean velocity for most of the channel, except for the near-wall region. For y+ less-than-or-equal-to 15, V is virtually constant, implying that perturbations of all flow variables propagate like waves near the wall. In this region, V is 55% of the centerline velocity U(c) for velocity and vorticity perturbations and 75% of U(c) for pressure perturbations. This is equal to U at y+ = 15 for velocity and vorticity perturbations, and equal to U at y+ = 20 for pressure perturbations, indicating that the dynamics of the near-wall turbulence is controlled by turbulence structures present near y+ congruent-to 15-20. Scale dependence of V is also examined by analyzing the bandpass-filtered flow fields. This paper contains comprehensive documentation on the propagation velocities, which should prove useful in the evaluation of Taylor's hypothesis. An attempt has been made to explain some of the data in terms of the current understanding of organized structures.