Turbulence of non-uniform open channel flows and mean velocity scaling

Abstract:

Turbulence and mean velocity distributions are well studied for uniform flows, however fully turbulent non-uniform open channel flows over rough gravel beds are an understudied class of realistic flows with significant geomorphologic importance. The two-fold objectives of this study are to investigate the effects of flow deceleration on multi-layer turbulence and determine velocity scales that produce self-similar profiles using turbulent boundary layer theory. Turbulence statistics were calculated from data collected using a three-dimensional acoustic Doppler velocimeter in backwater region produced by placing cylindrical obstruction downstream of measuring location. Results indicate inner turbulent processes are set according to shear velocity while outer layer of turbulence is not. Mean streamwise velocity profiles of 101 published datasets including 2211 data points from non-uniform decelerating rivers and laboratory flumes are scaled using inner, outer and 2 mixed methods. Theoretical arguments indicate the outer velocity scale determined from the AIP definition of equilibrium provides a more appropriate scaling for the velocity deficit of non-uniform open channel flows than inner scaling determined from Clauser equilibrium. Scaling results of mean profiles show outer scaling collapses data better than inner scaling. Mixed scaling approaches, which account for upstream conditions and the effect of bed roughness felt throughout the flow, provide a better scaling choice for this class of flows than either inner or outer scales alone. Results of this analysis provide insight on the behavior of turbulence in decelerating fully turbulent open channel flow over a rough gravel bed and provides a scaling useful for predicting velocity distributions.