An investigation of turbulence structure in a low Reynolds number incompressible turbulent boundary layer.

摘要

Hot-wire anemometry measurements in an incompressible turbulent boundary-layer flow over a flat plate at zero pressure gradient were made using two X-probes simultaneously. The experiment was performed in the large Atmospheric Boundary-Layer Wind Tunnel at the University of California, Davis. The 7.32 meter long flat plate installed within the wind tunnel generated approximately 20 cm thick boundary layer, with {dollar}Rsb{lcub}theta{rcub}{dollar} {dollar}approx{dollar} 4,000. Mean velocity and turbulence intensity data very close to the wall {dollar}(ysp{lcub}+{rcub} geq 1){dollar} were measured with a single hot wire to improve the measurement resolution. Space-time correlations of {dollar}uspprime{dollar} and {dollar}vspprime{dollar} velocities and of their instantaneous product were obtained with a pair of X-wires. The mean convection velocities, the extent in space, the mean inclination angles, and coherence characteristics of the {dollar}uspprime{dollar}, {dollar}vspprime{dollar} and {dollar}uspprime vspprime{dollar} large-scale structures are presented. (The {dollar}uspprime vspprime{dollar} results are presented for the first time.) The {dollar}uspprime{dollar} structure is inclined at a small angle (19{dollar}spcirc{dollar}) to the wall, while the {dollar}vspprime{dollar} and {dollar}uspprime vspprime{dollar} structures propagate almost at wall-normal directions. Each of the {dollar}uspprime{dollar} and {dollar}vspprime{dollar} structures appears elongated in the direction of the corresponding velocity fluctuation and is limited to {dollar}delta{dollar}-extent in the other two directions. The similarity between the {dollar}vspprime{dollar} and {dollar}uspprime vspprime{dollar} structures suggests that the {dollar}uspprime vspprime{dollar} might mainly be a consequence of the motion of the {dollar}vspprime{dollar} structure. Finally, a possible explanation for the differences between the {dollar}uspprime vspprime{dollar}, {dollar}vspprime{dollar} and the {dollar}uspprime{dollar} structures is the existence of different coherent structure scales, one dominating {dollar}uspprime{dollar} and the other dominating {dollar}vspprime{dollar} and {dollar}uspprime vspprime{dollar}.