Characterisation of free-surface turbulence in hydraulic jump roller at low inflow Froude number

This study investigates the free-surface turbulence and roller toe dynamics in hydraulic jumps at low inflow Froude numbers (Fr 1 = 2.1–3.8) and high Reynolds numbers (Re = 1.36×10 5 –2.46×10 5 ). Experiments were conducted in a large-scale facility using ultrasonic displacement sensors and high-speed digital imaging to capture both time-averaged and turbulent flow features. The measured free-surface profiles agreed closely with visual observations, and both conjugate depth ratio and dimensionless roller length increased with increasing Froude numbers. The free-surface turbulence peaked in the upstream half of the roller, with dominant fluctuation frequencies between 1.6 Hz and 4 Hz. The roller toe data analysis revealed slightly lower normalised fluctuations along the centreline than near sidewalls, reflecting sidewall–turbulence interactions. A video-based edge detection technique quantified the jump toe perimeter variability, showing that its standard deviation increased linearly with increasing Froude numbers. The results provide new insights into the unsteady structure of hydraulic jumps, supporting improved measurement approaches and hydraulic structure design. Free-surface and internal turbulence in hydraulic jump with relatively low inflow Froude number (Fr 1 = 3.0, Re = 2.0×10 5 ). • Free-surface turbulence at hydraulic jumps with breaking roller can be extreme • Hydraulic jumps at low inflow Froude numbers (Fr 1 = 2.1–3.8) and high Reynolds numbers (Re = 1.36×10 5 –2.46×10 5 ) were investigated • Digital cameras and ultrasonic displacement meters were used in a large-size facility. • Roller toe perimeter fluctuations and indentation coefficients quantified via image-based edge detection