Convection velocities in a turbulent boundary layer

Abstract

The convection velocities of ejections, sweeps, and shear layers have been studied in a turbulent boundary layer at Reθ=1409 by means of two-point correlations. Except in the buffer layer, where the convection velocities are reasonably constant, considerable variations in Uc are observed, both with respect to the distance from the wall and the scale of the event. The largest-scale motions (of the order of the boundary layer thickness) are convected at velocities close to the local mean. However, the velocities drop significantly as the scales are reduced for all types of events studied. Hence, the concept of a unique structure convection velocity does not appear to be very useful, since it must depend on the state of evolution of the structure. The propagation velocity also appears to be different for different parts of the event. It is found that for all scales and positions, the convection velocity for ejections is distinctly lower than that for sweeps, suggesting that the mechanisms responsible for the two types of events are quite different.