Xinqian Leng

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

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.

Upstream Propagation of Surges and Bores: Free-Surface Observations

In a free-surface flow, a sudden increase in water depth induces a positive surge, also called compression wave or bore. Herein a physical study was conducted in relatively large-size rectangular channel with a smooth bed. The upstream propagation of breaking and undular bores were investigated with a broad range of Froude numbers Fri ranging from 1.1 to 2.3. Both instantaneous and ensemble-averaged free-surface measurements were performed nonintrusively. The observations showed the occurrence of undular bores for 1 < Fr1 1.2 to 1.3, breaking bores for Fr1 > 1.4 to 1.5, and breaking bores with secondary waves for 1.2-1.3 > Fr1 1.4-1.5. The propagation of a breaking bore was associated with an upward free-surface curvature immediately before the roller toe for Fri < 2, and an abrupt increase in free-surface elevation with the passage of the breaking roller. The propagation of undular bores was characterized by a smooth upward free-surface curvature, followed by a smooth first wave crest and a train of secondary quasi-periodic undulations. For all tidal bores, the passage of the bore front was always associated with large free-surface fluctuations, occurring slightly after the arrival of the front. During the generation process, the positive surge formed very rapidly and the surge celerity increased very rapidly, reaching maximum values excess of the fully-developed bore celerity. With increasing time, the surge decelerated and the bore propagated at an early constant celerity for (xgate —x)/xgate > 10.

Rapid operation of a Tainter gate: generation process and initial upstream surge motion

Abstract In water supply channels, the brusque operation of control gates may induce large unsteady flow motion called surges. Such a rapid operation of gates must often be restricted, although it may be conducted to scour silted channels and sewers. Herein a physical study was conducted under controlled flow conditions to study the turbulent mixing in the very-close vicinity of a rapidly opening/closing Tainter gate, with a focus on the unsteady transient mixing induced by the gate operation. The data suggested that the negative/positive surge generation was associated with large instantaneous free-surface fluctuations. The velocity measurements indicated significant variations in longitudinal velocity during the surge generation, as well as large fluctuations of all velocity components. The processes were associated with large Reynolds stress levels. A succession of rapid closure and opening of undershoot gates provided optimum conditions to scour silted canals, and the present results gave some detailed insights into the physical processes.

Unsteady turbulence in expansion waves in rivers and estuaries: an experimental study

A sudden decrease in water depth, called a negative surge or expansion wave, is characterised by a gentle change in free-surface elevation. Some geophysical applications include the ebb tide flow in macro-tidal estuaries, the rundown of swash waters and the retreating waters after maximum tsunami runup in a river channel. The upstream propagation of expansion waves against an initially steady flow was investigated in laboratory under controlled flow conditions including detailed free-surface velocity and Reynolds stress measurements. Both non-intrusive free-surface measurements and intrusive velocity measurements were conducted for relatively large Reynolds numbers with two types of bed roughness. The data showed that the propagation of expansion waves appeared to be a relatively smooth lowering to the water surface. The wave leading edge celerity data showed a characteristic trend, with a rapid acceleration immediately following the surge generation, followed by a deceleration of the leading edge surge towards an asymptotical value:

Tidal bore hydrodynamics and sediment processes: 2010–2016 field observations in France

(\mathrm{U}+\mathrm{V}_\mathrm{o})/(\mathrm{g}\times \mathrm{d}_\mathrm{o})^{1/2}=1

Breaking bore: physical observations of roller characteristics

(U+Vo)/(g×do)1/2=1 for both smooth and rough bed experiments. The results indicated that the bed roughness had little to no effect, within the experimental flow conditions. Relatively large fluctuations in free-surface elevation, velocity and turbulent shear stress were recorded beneath the leading edge of the negative surge for all flow conditions. The instantaneous turbulent shear stress levels were significantly larger than the critical shear stress for sediment erosion. The present results implied a substantial bed erosion during an expansion wave motion.

Turbulent advances of a breaking bore: preliminary physical experiments

ABSTRACT A tidal bore is a natural estuarine phenomenon forming a positive surge in a funnel-shaped river mouth during the early flood tide under spring tide conditions and low freshwater levels. The bore passage may induce some enhanced turbulent mixing, with upstream advection of suspended material. Herein, the flow field and turbulence characteristics of tidal bores were measured using both numerical computational fluid dynamics (CFD) and physical modeling. This joint modeling approach, combined with some theoretical knowledge, led to some new understanding of turbulent velocity field, turbulent mixing process, Reynolds stress tensor, and tidal bore hydrodynamics. Thétis is a CFD model using the volume of fluid technique to model the free-surface and Large Eddy Simulation (LES) technique for the turbulence modeling. Physical data sets were used to map the velocity and pressure field and resolve some unusual feature of the unsteady flow motion. A discussion will be provided to explain why a detailed validation process, involving a physical knowledge of the flow, is crucial. Comparison of the numerical model results and experimental data over broad ranges of conditions for the same flow is mandatory. The validation process from two-dimensional to three-dimensional will be commented and difficulties will be highlighted.

Unsteady velocity profiling in bores and positive surges

ABSTRACT A tidal bore is a compressive wave, advancing upstream in an estuary when the flood tidal flow starts. It is observed when a macro-tidal flood flow enters the funnel shaped river mouth with shallow waters. Its upriver propagation impacts the natural system, with sediment scouring and suspension. The tidal bores of the Garonne and Sélune Rivers in France were extensively investigated between 2010 and 2016. Instantaneous velocity measurements were conducted continuously at high-frequency (50 to 200 Hz) during each bore event. In the Garonne River, instantaneous sediment concentration data were obtained and the sediment properties were systematically tested. The nature of the observations was comprehensive, regrouping hydrodynamics and turbulence, sedimentology and suspended sediment transport. The key outcomes show that the tidal bore occurrence has a marked effect on the velocity field and suspended sediment processes, including a sudden flow deceleration and flow reversal during the bore passage. The turbulent Reynolds stresses present large instantaneous amplitudes, with rapid fluctuations, during the tidal bore. The sediment flux data imply considerable mass transport rates during the first hour of flood tide. This unique review of field data further shows a number of common features, as well as the uniqueness of each individual event.