Turbulent characteristics of free surface and velocity transition from free hydraulic jump to open-channel flow

A hydraulic jump represents a localized open-channel flow phenomenon marked by intense turbulence and significant velocity discontinuity. The transition from the highly turbulent roller region to the far-field uniform flow constitutes a critical hydraulic challenge requiring detailed investigation. This study experimentally examines the hydraulic and turbulent characteristics of the free surface and velocity field from the jump toe to the far-field region using an integrated instrumentation approach: acoustic displacement meters for free-surface dynamics, a double-tip conductivity phase-detection probe for air–water interface tracking, and an acoustic Doppler velocimeter for velocity measurements. Observations revealed that high-frequency roller-induced surface fluctuations attenuate downstream, with the fluctuation frequency stabilizing to match the jump toe oscillation frequency beyond 2Lj. The tailwater waves have a characteristic wavelength three times the downstream flow depth and propagate at a celerity faster than the mean downstream flow velocity. Longitudinal mean velocities require approximately 2Lj to approach uniformity, while the turbulent velocities, Reynolds stresses, and turbulence kinetic energy need 3Lj to attain quasi-constant values, indicating slower turbulence decay compared to mean flow stabilization. A critical practical implication is that the free surface achieves a steady state faster than turbulent flow parameters. To minimize roller-induced disturbances in downstream flows, a minimum distance of 3Lj should be maintained.