LIDAR Observations of Free-Surface Time and Length Scales in Hydraulic Jumps

A hydraulic jump is a complex, rapidly varied flow phenomenon occurring in the transition from supercritical to subcritical flows. Hydraulic jumps are characterized by instationary motions including jump toe oscillations and free-surface fluctuations. A light detection and ranging (LIDAR) instrument was used to continuously record the time-varying free-surface features of fully aerated hydraulic jumps with fully developed inflow conditions and high spatial and temporal resolution. The free-surface features were analyzed with two processing methods considering a freely moving hydraulic jump and a case where each instantaneous hydraulic jump profile was shifted to the mean jump toe position. The results provided detailed dimensionless distributions of the mean profiles, as well as free-surface fluctuations and jump toe oscillations that increased with increasing Froude numbers. The high spatial resolution data provided continuous free-surface time and length scales along the hydraulic jumps. Largest free-surface scales were observed close to the jump toe linked with longitudinal oscillations of the hydraulic jumps, and the dimensionless free-surface scales increased with the Froude number. The present study provides new insights into free-surface turbulent structures, highlighting the strong relationship between jump toe movements and free-surface fluctuations in hydraulic jumps.