Gravel-Bed Hydrodynamics: Double-Averaging Approach: Fid-Bau Portal

Gravel-Bed Hydrodynamics: Double-Averaging Approach

Dey, Subhasish / Das, Ratul

Abstract

This study focuses on the double-averaged (DA) turbulence characteristics and the local flow characterization in the flow zones over and within the flow-gravel-bed interface. The cumulative running averaged (CRA) value of a turbulence quantity for a given interval (in this study, half of the gravel size) of flow measurement is the prerequisite to determine a complete DA value of that quantity. The CRA turbulence quantities within the interfacial sublayer require a longer distance to become spatially independent. A method is proposed to fit the log law for the DA velocity over the bed and a polynomial law within the interfacial sublayer. The form-induced shear stress is 40% of the DA Reynolds shear stress occurring at the virtual bed level. The spatial velocity fluctuations diminish more rapidly than the temporal velocity fluctuations below the virtual bed level. In the inertial subrange of turbulent kinetic energy (TKE), the eddy size decreases significantly near the virtual bed level, and in the dissipation range, it increases slightly. The TKE fluxes directed downward and downstream compose the sweep events that are the prevailing mechanism within the interfacial sublayer. It is also confirmed by the quadrant analysis done for the bursting events. The form-induced energy fluxes below the crest level are comparable with the TKE fluxes. In the TKE budget, the form-induced diffusion rate is significant within the interfacial sublayer. The quantities constituting the TKE budget attain their peaks above the virtual bed level and reduce sharply below it. However, the pressure energy diffusion rate induces suction near the crest. The local flow characterization is done by separating the local velocity profiles into log, S-shaped, and accelerated classes. Unlike the velocity profiles per class, the profiles of turbulence quantities per class are almost indistinguishable within the interfacial sublayer. A comparison of the results of this study with those of previous studies shows more dissimilarities than similarities.