Air-water interface dynamic and free surface features in hydraulic jumps

Following previous experimental works (Mouaze et al., J Fulids Engng. ASME 127(6) (2005) 1191-1193; Murzyn et al., Int. J. Multiphase Flow, 31(1) (2005) 141-154) on gas fraction characterisation in hydraulic jumps, the present paper aims to investigate free surface properties developing in these flows. Indeed, air-water interface exhibits a wide range of frequencies that free surface probes are not always enough accurate to follow. As a solution, two new free surface wire gauges (thin wires, phi = 0.05 mm, 1 mm apart) have been built and calibrated. Accurate measurements of free surface fluctuations have been obtained with frequency resolution up to 12 Hz. Two sets of experiments have been made depending on Froude (Fr) and Weber (We) numbers. Experimental results show that free surface mean and turbulent profiles exhibit discontinuities at the front of the toe characterized by a sudden high level of turbulence downstream followed by a zone of dissipation. Furthermore, free surface length scales have been estimated from correlation measurements. Similar features are found according to Fr and We. Good agreement is shown with results deduced from a video analysis technique (Mouaze et al., J. Fulids Engng. ASME 127(6) (2005) 1191-1193). Lastly, comparison is made with Brocchini and Peregrine (Brocchini and Peregrine, J Fluid Mech. 449 (2001 a) 225-254; 449 (2001b) 255-290) depicting similar shapes for behaviour of vertical velocity fluctuations (w') as a function of longitudinal free surface length scale.