Response of bed-load to turbulence in mobile-bed streams

This work aims at quantifying experimentally the near-bed turbulence parameters in mobile-bed streams with noncohesive bed-load and comparing them with those in clear-water flows. A reduction of flow velocity relative to particle velocity transporting particles produces a decrease in magnitude of near-bed turbulence level and in turn an excessive near-bed damping in Reynolds shear stress (RSS) distributions over the entire flow depth. In presence of bed-load transport, the parameters of the logarithmic law change: the von Kármán coefficient decreases, whereas the virtual bed and the zero-velocity levels increase. The friction factor decreases, indicating a reduction of flow resistance. In mobile-bed flows, the traversing length of an eddy diminishes and its size increases with respect to those in clear-water flows. The third-order correlations show that a downward streamwise acceleration prevails, being associated with a streamwise diffusion of vertical Reynolds normal stress (RNS) and a downward diffusion of streamwise RNS. The streamwise and the downward vertical fluxes of turbulent kinetic energy (TKE) increase and its budget shows that the magnitude of the pressure energy diffusion rate near the bed becomes sharply negative, with increasing turbulence production. Quadrant analysis shows that sweep events are the principal mechanism of bed-load transport. The universal probability density functions (PDFs) for turbulence parameters given by Bose and Dey (2010) have been successfully applied in mobile-bed flows.