Continuous measurements of time-varying free-surface profiles in aerated hydraulic jumps with a LIDAR

Hydraulic jumps are a complex hydraulic phenomenon characterised by a water surface discontinuity from supercritical to subcritical flows. Along the jump roller, high turbulence, flow aeration and three-dimensional motions are key characteristics which are associated with jump toe oscillations and free-surface fluctuations. Previous studies have provided significant insights into the characterisation of the free-surface and jump toe characteristics in hydraulic jumps. However, a continuous observation of the time-varying free-surface has been so far limited to fixed data points along the hydraulic jump. Herein, novel experiments were conducted with a LIDAR measuring the continuous free-surface profiles in aerated hydraulic jumps providing detailed spatial and temporal free-surface characteristics of aerated hydraulic jumps. The free-surface observations provided basic statistics of the free-surface including mean profiles and standard deviations of the free-surface fluctuations as well as their characteristic frequencies. The effect of the jump toe oscillations on the free-surface properties was analysed for two conditions using a free moving jump toe and a fixed jump toe position. The results showed a strong effect of the jump toe oscillations on the free-surface features close to the jump toe. Based upon the characteristic frequencies, three characteristic regions in hydraulic jumps were identified. Overall the comparison with previous studies showed good agreement highlighting that a LIDAR is a suitable instrument to measure the free-surface characteristics in aerated hydraulic jumps.