Turbulence in mobile-bed streams

This experimental study is devoted to quantify the near-bed turbulence parameters in mobile-bed flows with noncohesive bed-load sediment transport and to compare them with those in clear-water flows. A reduction in magnitude of near-bed turbulence level due to the decrease of flow velocity relative to particle velocity transporting particles results in an excessive near-bed damping in Reynolds shear stress (RSS) distributions. The bed particles are associated with the momentum provided from the flow to maintain their motion overcoming the bed resistance. It leads to a reduction in RSS distributions over the entire flow depth. In the logarithmic law, the von Kármán coefficient decreases, and the virtual bed and the zero-velocity levels move up in presence of bed-load transport. The friction factor decreases with bed-load transport substantiating the concept of reduction of flow resistance. The traversing length of an eddy decreases and its size increases in mobile-bed flows, as compared to those in clear-water flows. The third-order correlations suggest that during the bed-load transport, a streamwise acceleration inclining downward is prevalent. It is 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 in presence of bed-load transport. The TKE-budget reveals that for the bed-load transport the pressure energy diffusion rate near the bed changes sharply to a negative magnitude, implying a gain in turbulence production. According to the quadrant analysis, sweep events in mobile-bed flows are the principal mechanism of bed-load transport. Using a Gram-Charlier series expansion based on the exponential distribution, the universal probability density functions (PDFs) for turbulence parameters given by Bose and Dey (2010) have been successfully applied in mobile-bed flows.