A study of convection velocities in a zero pressure gradient turbulent boundary layer

Time-resolved DPIV measurements performed in wall parallel planes at several wall normal locations in a turbulent boundary layer (TBL) are used to illuminate the distribution of wall parallel velocities in a three-dimensional energy spectrum over streamwise, spanwise, and temporal wavelengths. To our knowledge, this is the first time this type of spectral distribution has been reported. Slices of the 3D spectrum can give insight into the propagation of different scales in the flow as well as the streamwise and spanwise extent of dominant scales. Measurements were performed at three wall normal locations, y⁺ = 34,108, and 278, in a zero pressure gradient TBL at Re _τ = 470. Two high speed cameras placed side-by-side in the streamwise direction give a 10δ streamwise field of view with a time step of Δt⁺ = 0.5 between consecutive fields. Far from the wall the convection velocities of all scales are very close to the local mean velocity in agreement with the work of Dennis and Nickels,¹ while at y ⁺ = 34 it was found that all measured scales in the flow convect faster than the local mean in agreement with Krogstad et. al.² The variation of the convection velocity with scale and distance from the wall will be discussed.