Flow Patterns and Free-Surface Dynamics in Hydraulic Jump on Pebbled Rough Bed

Some basic characteristics of a classic hydraulic jump flow over a pebbled rough bed, as well as on a smooth bed as a reference, are presented in this experimental study. For the experiments, an inflow Froude number Fr1 from 1.54 to 4.94 and inflow Reynolds number Re1 from 42 000 to 230 000 were considered. Visual observations and measurements suggested some differences between the formation of a hydraulic jump on rough and smooth bed configurations, including different air entrainment processes, larger vortical structures in the roller length and stronger backward flow in the upper layer. Furthermore, the jump roller and aerated flow lengths were shorter on a pebbled rough bed than on a smooth bed, while the dimensionless advection velocity of large vortices was the same for both bed types. The instantaneous jump toe perimeter showed the largest variation at the largest Fr1 and was generally larger on rough bed than on smooth bed. Larger oscillations of the free-surface profile were observed on smooth bed, highlighting that roughness resulted in smaller free-surface oscillations, suggesting the higher rate of energy dissipation.