Effect of Reynolds number, near-wall perturbation and turbulence on smooth open-channel flows

Abstract

Three series of experiments were conducted to study the effect of Reynolds number, near-wall perturbation and turbulence on the velocity characteristics of smooth open-channel flows. Measurements were carried out using a laser Doppler anemometer. The variables of interest include mean velocity, turbulence, velocity probability density functions and Gram-Charlier series coefficients. For the range of depth Reynolds number studied (23×103 < Re h <72×103), the turbulence intensity and Reynolds shear stress profiles show that the effect of Reynolds number can be significant in open-channel flows. However, at identical distances from the bed, the velocity probability density functions are relatively insensitive to Reynolds number effects. The coefficients of the Gram-Charlier series expansion are also independent of Reynolds number for wall normal distances y+ <300. In the case of the flow with the near-wall perturbation, the mean velocity profile at the farthest downstream station more-or-less recovers to the undisturbed state, whereas the turbulence intensity profiles do not completely recover. From the various profiles obtained downstream of the perturbation, the wall-normal distance corresponding to 0.01<y/h<0.1 requires a greater longitudinal distance to recover to the undisturbed state. As noticed from the velocity defect profiles, the effects of near-wall disturbance begin to penetrate into the flow with increasing distance from the perturbation. The presence of higher levels of turbulence influences the skin friction coefficient, the extent of collapse with the log–law and the wake parameter.