Conditional turbulence characteristics in water-worked and screeded gravel-bed flows

The conditional double-averaged streamwise velocity, spatially averaged (SA) turbulence and dispersive quantities in flows over a water-worked gravel bed (WGB), and a screeded gravel bed (SGB) were studied. The effects of the temporal and advective bursting events on the WGB and SGB flows were analyzed by measuring the instantaneous flow field using a particle image velocimetry system. The flow conditions (flow rate and flow depth) were maintained the same in both the beds. The SA turbulence and dispersive quantities studied were the SA Reynolds shear and normal stresses, SA turbulent kinetic energy fluxes, dispersive shear and normal stresses, and dispersive kinetic energy fluxes. In experimental beds, owing to the water work, the WGB exhibited a spatially organized roughness structure as compared with a randomly organized roughness structure in the SGB, resulting in a higher WGB roughness than the SGB one. Analysis of the temporal bursting events revealed that the conditional SA turbulence quantities corresponding to the temporal sweeps were dominant within the roughness layer, whereas those corresponding to the temporal ejections govern above the roughness layer. Analysis of the advective bursting events indicated that the dispersive quantities involving advective sweeps were the key mechanism within the roughness layer, whereas those involving advective ejections were the prevailing mechanism in the main flow layer. The comparison of the results in the WGB and SGB flows elucidated that owing to the higher WGB bed roughness than the SGB one, the conditional SA turbulence and dispersive quantities in the WGB were greater than those in the SGB.Within the roughness layer, the bursting events in the WGB were more persistent and frequent than those in the SGB.