Coupling between free-surface fluctuations, velocity fluctuations and turbulent Reynolds stresses during the upstream propagation of positive surges, bores and compression waves

Abstract

In open channel, canals and rivers, a rapid increase in flow depth will induce a positive surge, also called bore or compression wave. The positive surge is a translating hydraulic jump. Herein new experiments were conducted in a large-size rectangular channel to characterise the unsteady turbulent properties, including the coupling between free-surface and velocity fluctuations. Experiments were repeated 25 times and the data analyses yielded the instantaneous median and instantaneous fluctuations of free-surface elevation, velocities and turbulent Reynolds stresses. The passage of the surge front was associated with large free-surface fluctuations, comparable to those observed in stationary hydraulic jumps, coupled with large instantaneous velocity fluctuations. The bore propagation was associated with large turbulent Reynolds stresses and instantaneous shear stress fluctuations, during the passage of the surge. A broad range of shear stress levels was observed underneath the bore front, with the probability density of the tangential stresses distributed normally and the normal stresses distributed in a skewed single-mode fashion. Maxima in normal and tangential stresses were observed shortly after the passage of a breaking bore roller toe. The maximum Reynolds stresses occurred after the occurrence of the maximum free-surface fluctuations, and this time lag implied some interaction between the free-surface fluctuations and shear stress fluctuations beneath the surge front, and possibly some causal effect.

Impact of a large cylindrical roughness on tidal bore propagation

“…Fig. 8a), the results were close to those of Leng and Chanson (2016) obtained with the same discharge per unit width and comparable Froude numbers. Herein the focus is on the impact of the large roughness element on the velocity field.…”

“…This rapid deceleration phase was followed by a transient recirculation, previously documented in earlier studies (Docherty and Chanson 2012, Khezri and Chanson 2012, Leng and Chanson 2016. The passage of the tidal bore was linked to large fluctuations of the velocity components (data not shown).…”

“…For a Gaussian distribution of the data about the ensemble-average, the difference between third and first quartiles (d 75 -d 25 ) would be equal to 1.3 times the standard deviation. The experimental results indicated further maximum free-surface fluctuations shortly after the bore leading edge, as reported by Leng and Chanson (2016). Both visual observations and instantaneous and ensemble-averaged acoustic displacement meter data showed the negligible impact of the large roughness element on the free-surface characteristics.…”

Are breaking waves, bores, surges and jumps the same flow?

“…That is, the celerity of the roller toe fluctuated rapidly with both time and transverse distance, although in a quasi-two-dimensional manner on average (Leng and Chanson 2015b). Yet both laboratory and field observations hinted to the existence of large-scale three-dimensional bubbly structures (Leng and Chanson 2015a, Chanson 2016, Leng and Chanson 2016.…”

Hydraulic jumps and breaking bores: modelling and analysis

“…A key feature of hydraulic jumps and bores is the rapid spatial and temporal deformations of the roller free-surface, in response to the interactions between entrained air bubbles and turbulent structures. Simultaneous measurements of freesurface fluctuations and instantaneous void fraction demonstrated a coupling between free-surface fluctuations and local turbulent and two-phase flow properties ( Leng and Chanson 2016), the interplay between turbulent flow properties and free-surface deformations was rarely considered in breaking jumps and bores. It is the aim of this contribution to re-visit the hydrodynamics of hydraulic jumps and bores with a marked roller.…”