The behaviour of turbulence in unsteady open channel flow

An investigation into the behaviour of the turbulence during laboratory simulations of floods in rivers with mild bed slopes was undertaken. Computer control of the flow rate into the flume enabled reproducible flood waves to be generated. To rigorously model the energy gradients in a long channel, an interactive sluice gate control was developed for the downstream end of the flume. Mean flow unsteadiness effects on the turbulence were evaluated by considering different duration hydrographs with similar shapes and magnitudes. The investigation was limited to the longitudinal component of turbulence, as a one-component laser Doppler anemometer was employed for the determination of point velocities. Flow visualisation using a dye plume supplemented velocity data. It was observed that for events having a shorter duration the peak turbulent intensity had a greater magnitude, and occurred relatively earlier on the rising limb of the flood. The turbulent energy peak coincided with the maximum flow rate divergence. For increasing flow divergence magnitude, which only occurs on the rising limb, the production of turbulence was larger than dissipation, with the transport of turbulence providing an additional sink for turbulent energy. After the depth had peaked the flow experienced pseudo-equilibrium conditions, where the transport mechanism was insignificant and the rate of production approximated dissipation. A feature of the falling limb was a period of inactivity, in which the magnitudes of production and dissipation were at minimum. A second -5/3 slope region was observed in the energy spectra. The length scale associated with an energy source for this double structure was two orders of magnitude larger than the Kolmogorov dissipation length scale. Decay times for flow structures of this size are similar to the duration of these hydrographs. It is possible that the unsteady flow created vortex structures that persisted for some time after the flow which generated them had moved downstream. These vortex structures, which provide a turbulence memory mechanism, and the state of pseudo-equilibrium on the falling limb are responsible for residual turbulent energy in the flow throughout the falling limb and immediately following the passing of the flood wave. In addition, it is suggested that mean flow controls both the production and dissipation of turbulence, with the dissipation of turbulent kinetic energy being controlled by the diffusion of momentum during low speed streaks. The Kolmogorov scale may be interpreted as defining the critical damping condition along these streaks where Reynolds stresses balance viscous forces