Search

Some basic characteristics of a classic hydraulic jump flow over a pebbled rough bed, as well as on a smooth bed as a reference, are presented in this experimental study. For the experiments, an inflow Froude number Fr1 from 1.54 to 4.94 and inflow Reynolds number Re1 from 42 000 to 230 000 were considered. Visual observations and measurements suggested some differences between the formation of a hydraulic jump on rough and smooth bed configurations, including different air entrainment processes, larger vortical structures in the roller length and stronger backward flow in the upper layer. Furthermore, the jump roller and aerated flow lengths were shorter on a pebbled rough bed than on a smooth bed, while the dimensionless advection velocity of large vortices was the same for both bed types. The instantaneous jump toe perimeter showed the largest variation at the largest Fr1 and was generally larger on rough bed than on smooth bed. Larger oscillations of the free-surface profile were observed on smooth bed, highlighting that roughness resulted in smaller free-surface oscillations, suggesting the higher rate of energy dissipation.

At hydraulic structures, some strong interactions may develop between fast flowing waters and the air adjacent to the water in motion that enhance the air-water transfer of atmospheric and volatile gases in the flow. In turn, in-stream structures may contribute to the aeration and re-oxygenation during overflow. The present study aims to characterize the aeration performance of a steep stepped weir, based upon a detailed physical investigation of air-water interfacial properties across a relatively wide range of discharges. The data showed a strong fragmentation of the air-water flows, a very broad range of entrained bubbles and drops, and a large amount of particle clustering. The results implied a monotonic increase in re-aeration with increasing rate of energy dissipation, while the largest aeration efficiency was observed on the horizontal step weir chute, with the smallest on the 1V:2.33H inclined downward steps. Altogether, the study showed that a steep stepped chute can make a sizeable contribution to the re-oxygenation of the waters, although the downward inclined steps reduce the re-aeration performances.

Partially-filled pipe flows are commonly observed in urban hydraulics, sewers and road crossings. The occurrence of a compression wave in the confined space may result from flash flooding, transient operation or accidental blockage, inducing explosive conditions. In this study, the propagation of a compression wave was studied in a relatively large-size laboratory flume with a semi-circular cross-section. The unsteady flow properties were recorded to understand how the circular cross-sectional shape impacted onto the surging water propagation. Both free-surface and velocity data indicated a marked impact of the compression wave passage, with large instantaneous fluctuations comparable and sometimes larger than observations of compression waves in rectangular channels.

During the recent decades, a number of overflow embankment protection systems were implemented. One design is the Minimum Energy Loss (MEL) weir, developed to pass large flood events with minimum energy loss and low erosion. Several MEL weirs have successfully operated for decades in Australian catchments affected by heavy tropical and sub-tropical rainfalls with very flat gradients. Their historical performances are discussed and confirmed the design capability to pass large floods with small afflux and very small energy loss and little environmental impact including no significant erosion at the abutments. During a major flood, visual and quantitative observations were undertaken at the Chinchilla MEL weir. On the smooth converging chute, the observations showed that the inception of self‐aeration was a three‐dimensional process with a gradual change in free‐surface roughness, from a smooth glassy free-surface to a very-rough choppy surface. An optical technique (OF) was implemented to derive the contour maps of surface velocities based upon video movies taken from a sturdy tripod. The self-aerated flow velocity data were close to the backwater equation in the self‐aerated region, while highlighting regions of high- and low-velocities across the chute. The OF data showed large streamwise surface velocity fluctuations in the aerated flow region, consistent with the broad literature on self-aerated flow measurements. The ratio of transverse to streamwise surface turbulence intensity indicated a strong anisotropy of the free-surface turbulence.

In open channel flows, a hydraulic jump is a rapid shift from a supercritical flow to a subcritical flow with a sharp rise in the elevation of the free surface, which is associated with strong energy dissipation, air entrainment, and spray. The present research aims to investigate the air–water flow properties in a hydraulic jump on rough pebbled bed. A range of discharges from 0.06 to 0.1 m3/s, corresponding to inflow Froude numbers Fr1 between 1.7 and 2.84 and Reynolds numbers Re1 from 140,000 to 220,000 were considered. The basic parameters of two-phase flow such as the void fraction and bubble count rate are presented. Close to the jump toe, the mean void fraction on rough bed was larger than on smooth bed, while farther downstream larger magnitudes were observed on smooth bed. No significant difference was observed between rough and smooth beds in terms of the maximum void fraction. In the turbulent shear layer, the bubble count rate was comparatively larger on rough bed than on smooth bed, while the longitudinal distribution decayed for both bed configurations.

When the Froude number F of a free-surface flow ranges between 0.3 and 3, the flow is unstable and frequently characterised by free surface undulations, with the undular hydraulic jump being a seminal flow in hydro-environmental mechanics. The presence of the free surface undulations significantly affects the flow field, with major velocity and pressure field redistributions between successive crests and troughs. All the current theoretical models to simulate undular hydraulic jumps are limited to two-dimensional flow conditions, ignoring all the relevant three-dimensional flow effects, namely shock-wave drag, turbulent breaking and turbulent stresses. These aspects are critically accounted for in this review article, where a depth-averaged Boussinesq model which approximately accounted for 3D flow effects was presented, constituting the first attempt in this line. The model predictions were compared with experimental results from different sources for F1

Breaking bores are commonly observed in a number of natural processes, often associated with the presence of a transient mixture of air and water, with intense recirculation, air bubble entrainment, and splashing. Two-phase flow measurements in such highly unsteady flows cannot be based on long-duration measurements and require novel ensemble-statistical approaches based on multiple repetitions. Detailed measurements of air–water flow properties were then conducted in a breaking bore with Fr1 = 2.4 using an array of multiple dual-tip phase-detection probes. Based on an extensive experimental program, inclusive of 2000 tests at a single position and 100 tests at multiple elevations, a detailed sensitivity analysis was conducted on the necessary number of repetitions to obtain physically meaningful and statistically reliable air–water flow properties. The results led to a robust methodology to estimate ensemble-statistical values, including confidence intervals and residual error. In addition, these results provided a detailed characterization of the behavior of air–water flow properties in highly unsteady flows, including void fraction, number of interfaces, and bubble chord time/length. Despite the transient nature, all physical processes showed consistent behaviors with theoretical models and other stationary flows, including hydraulic jumps and plunging jets. Overall, this study provided two-phase flow characteristics that go beyond the limitations imposed by the unsteady nature of the flow, proving thoroughly the importance of large datasets for the estimation of air–water flow properties in highly unsteady flows.

Hydraulic structures are man-made waterworks interacting with the rainfall run-off to store and convey water, or mitigate the impact of run-off. Current approaches in hydraulic structure design ten...

The Authors developed an interesting discussion on measurements in highly-unsteady transient free-surface flows. Using the dam break wave propagation on a dry bed as an example, they discussed the ...

A culvert is a covered road structure constructed to pass flood and drainage. The engineering design principle of culverts focuses on optimising flood capacity at the lowest possible cost, resulting in high flow velocity through the barrel. With modern engineering design being more environmentally aware, culverts are considered common barriers for fish migrating upstream due to the excessive barrel velocity. The migration of fish upriver is important for their breeding and feeding activities, contributing to a stable population and species diversity. Improved or novel design for a fish-friendly culvert is needed to ensure well-being of local fish community. This article is a review of recent developments in seeking a solution for a better fish-friendly culvert design. A major common feature of the reviewed studies is the application of hybrid modelling, combining laboratory experiments and numerical Computational Fluid Dynamics (CFD) modelling together. Various design alternatives were compared for pros and cons: channel roughening, installation of speed-reducing structures (e.g., baffles or beams), and simply widening the channel by adding more culvert units (boxes). Overall, channel widening stands to be a better design choice due to its suitability for both flood passage and fish passage, with little impact on operation safety and a reduced maintenance cost.

Culverts are important hydraulic structures for delivering a range of valuable socioeconomic services; however, they are also known to have negative impacts on freshwater and estuarine river system...

Low-level river crossings and culverts deliver valuable transportation and hydraulic control services to the society, but have negative impacts in terms of upstream fish passage. Recently, full-hei...

The water waves resulting from the collapse of a dam are important unsteady free surface flows in civil and environmental engineering. Considering the basic case of ideal dam break waves in a horizontal and rectangular channel the wave patterns observed experimentally depends on the initial depths downstream (hd) and upstream (ho) of the dam. For r = hd/ho above the transition domain 0.4–0.55, the surge travelling downstream is undular, a feature described by the dispersive Serre–Green–Naghdi (SGN) equations. In contrast, for r below this transition domain, the surge is broken and it is well described by the weak solution of the Saint–Venant equations, called Shallow Water Equations (SWE). Hybrid models combining SGN–SWE equations are thus used in practice, typically implementing wave breaking modules resorting to several criteria to define the onset of breaking, frequently involving case-dependent calibration of parameters. In this work, a new set of higher-order depth-averaged non-hydrostatic equations is presented. The equations consist in the SGN equations plus additional higher-order contributions originating from the variation with elevation of the velocity profile, modeled here with a Picard iteration of the potential flow equations. It is demonstrated that the higher-order terms confer wave breaking ability to the model without using any empirical parameter, such while, for r > 0.4–0.55, the model results are essentially identical to the SGN equations but, for r

A positive surge is an unsteady open channel flow motion characterised by a sudden rise in water surface elevation. The literature is focused primarily on rectangular channels. Herein, the free-surface features of positive surges propagating upstream were investigated in an asymmetrical canal. The surge propagation was a three-dimensional unsteady flow motion, resulting in a complicated transient secondary motion compared to positive surge propagating in rectangular canals. The present results may be relevant to surge propagation in man-made trapezoidal canals and natural irregular-shaped channels.

In plunging jets and at hydraulic jumps, large amounts of air bubbles are entrained at the impingement of the liquid jet into the receiving body. Air is entrapped and advected into a turbulent shear layer with strong interactions between the air bubble advection process and momentum shear flow. In this new physical study, air–water flow measurements were systematically repeated with identical inflow length, inflow depth and inflow velocity in a vertical supported jet (PJ) and a horizontal hydraulic jump (HJ). Detailed measurements were conducted with the same instrumentation. Both similarities and differences were observed between the two multiphase gas–liquid shear flows. Visual observations showed a key difference in the outer region, with a buoyancy-driven flow in the plunging jet with negligible void fraction, versus a strong recirculation motion with uncontrolled interfacial aeration in the hydraulic jump. Differences were also observed at the impingement perimeter, in terms of fluctuation frequencies and amplitudes, for identical inflow conditions. Both flow conditions yielded intense local singular air entrainment and close results were observed in terms of void fraction, bubble count rate, bubble chord sizes and interfacial area in the shear layer, in both the plunging jet and hydraulic jump. The transfer of momentum between impinging jet and receiving water, as well as the effect of buoyancy, were however affected by the flow geometry.

A compression wave is an unsteady rapidly-varied open channel flow motion, characterised by an increase in water depth. A detailed investigation was conducted in a prismatic asymmetrical channel, to better understand the physical processes observed in tidal/tsunami-bore affected estuaries with irregular topography. The prismatic channel was equipped with a 1 V:5H transverse slope across the full channel width. The free-surface data presented three-dimensional unsteady flow features, and the velocity measurements showed a drastic impact of the surge on the flow field, with strong three-dimensional features. With the arrival of the surge front, the transverse velocity component became large in the shallow-water section, indicating some unsteady secondary motion and recirculation during and shortly after the passage of the surge. The Reynolds shear stress data were associated with large turbulent stresses and shear stress fluctuations. For d1/D

Channels with longitudinal beams have been studied for decades in chemical engineering, environmental and sanitary engineering, aeronautics, astronautics, biology and geology. In the current study, a combination of physical and numerical Computational Fluid Dynamics (CFD) modelling was undertaken to test whether an asymmetrical channel equipped with a sidewall longitudinal rib could provide flow conditions conducive to upstream fish passage. The study focused on small‐bodied fish and juveniles of larger fish, typically less than 100 mm in total length. A detailed hydrodynamic study was conducted in an asymmetrical rectangular channel equipped with a sidewall square rib in a culvert barrel channel. Both free‐surface velocity and boundary shear stress measurements showed strong secondary currents of Prandtl's second kind. The channel asymmetry contributed to intense secondary motion, associated with turbulent dissipation. The channel design provided a small well‐defined highly turbulent low‐velocity zone beneath the rib. In the context of hydraulic structure designs, uttermost care must be considered because of manufacturing, installation and operational practices. In many instances, alternative engineering designs with small baffles and asymmetrical appurtenance should be preferred to assist with fish passage in hydraulic structures.

A hydraulic jump is a stationary transition from an upstream supercritical to a downstream subcritical flow. Hydraulic jumps with relatively low Froude numbers may be observed downstream of low-head hydraulic structures and their flow properties have not been not well documented. In this study, the hydraulic properties were investigated experimentally in weak hydraulic jumps with an inflow Froude number Fr1 = 2.1 and inflow depths 0.012 m

Transient motion, turbulence and bubble dynamics make any velocity quantification extremely difficult in unsteady gas–liquid flows. In the present study, novel Eulerian and Lagrangian techniques of velocimetry were developed, using both intrusive and non-intrusive measurements. The selected unsteady gas–liquid flow was a breaking bore, featured with a transient motion, air entrainment and coherent structures. Intrusively, Eulerian probe measurements resulted to the development of a single bubble event detection (SBED) technique in unsteady air–water flows. Non-intrusively, the motion of air–water pattern was detected using a novel particle tracking velocimetry (PTV). Both velocities obtained using SBED and PTV techniques were validated against the established optical flow (OF) results, achieving consistent velocity data among the three techniques. The filtering criteria of the SBED and PTV techniques were discussed, showing the best options in the breaking bore. It is concluded that the most robust velocity measurements in gas–liquid flow are achieved with consistent velocity data between the SBED, PTV and OF techniques.

Lagrangian field data in tidal shallow waters are rare, but valuable for the understanding of the spatiotemporal structure of flow and particle transport. The response of drifters to the wind and water flow in tidal shallow water was examined using correlation, spectral, and coherence analyses. Under moderate wind conditions (0–4 m/s), floating drifter motions in bounded sheltered water are affected by wind through low-frequency induced wind current rather than direct wind drag, when only a small portion of the drifter is unsubmerged. The field validation of both high- and low-resolution drifters with surface measured velocity from a fixed acoustic Doppler current profiler is good in the streamwise direction. The correlation between the drifter and fixed instrument velocities is low in the cross-stream direction due to strong spatial variability of the flow field. The evaluation shows that drifters are applicable to studying the flow dynamics of tidal water bodies in relation to small-scale processes.

A culvert is a covered channel designed to pass water through an embankment. The recognition of the adverse ecological impacts of culverts on upstream fish passage is driving the development of new culvert design guidelines, with a focus on small-bodied fish species seeking low velocity zones to minimise energy expenditure. Herein a hybrid modelling technique was applied, combining physical modelling, one-dimensional theoretical calculation and three-dimensional computational fluid dynamics modelling. The results reveal fundamental turbulent processes that may affect small-body-mass fish navigability and provide new insights for the development of standard box culvert design guidelines. Systematic validations were performed to a wide range of initial conditions and smooth barrel geometries. A physical relationship was derived from numerical and experimental data of past and present studies, correlating the dimensionless flow area with a normalised local velocity V/Vmean.

Quantitative real-time observations of a tidal bore in a macro-tidal estuary are difficult and dangerous, particularly in large estuaries. Mathematical and numerical models have been used to predict tidal bore advances; however, to date, there have been no validations of large-scale flow patterns. A marine radar can provide valuable real-time information on tidal bore propagation. In this paper, a template matching method using a cross-correlation algorithm was explored to estimate the evolution and celerity of a tidal bore with medium resolution marine radar images. The Qiantang River tidal bore was recorded at two different geographical locations. Characteristic flow patterns were derived and analysed, including temporal changes over a relatively large-scale area. The experimental results showed that the radar-derived celerity and calculated height of the tidal bore were consistent with visual observations in this estuarine zone.

Low-level river crossings can have negative impacts on freshwater ecosystems, including blocking upstream fish passage. In order to restore upstream fish passage in culverts, we developed physically-based design methods to yield cost-effective culvert structures in order to maintain or restore waterway connectivity for a range of small-bodied fish species. New guidelines are proposed for fish-friendly multi-cell box culvert designs based upon two basic concepts: (1) the culvert design is optimised for fish passage for small to medium water discharges, and for flood capacity for larger discharges, and (2) low-velocity zones in the culvert barrel are defined in terms of a percentage of the wetted flow area where the local longitudinal velocity component is less than a characteristic fish speed linked to swimming performances of targeted fish species. This approach is novel and relies upon an accurate physically-based knowledge of the entire velocity field in the barrel, specifically the longitudinal velocity map, because fish tend to target low-velocity zone (LVZ) boundaries. The influence of the relative discharge threshold Q1/Qdes, characteristic fish swimming speed Ufish, and percentage of flow area on the size of box culvert structures was assessed. The results showed that the increase in culvert size and cost might become significant for a smooth culvert barrel with Ufish 0.3, when providing 15% flow area with 0

Image-based velocimetry captured a great interest in physical modelling of highly-aerated flows, for example in stepped spillways. The present study investigated the performance of a novel filtering technique based upon a detailed systematic comparison with intrusive phase-detection probe data. All measurements were conducted in a large-size stepped spillway. A sensitivity analysis provided recommendations in terms of optimum sampling and processing parameters for optical flow measurements in high-velocity air-water flows.

The characterization of high-velocity air–water flows is crucial for the design of hydraulic structures including stepped spillways. Recent experimental advances have shown that LIDAR technology ca...

A compression wave in a canal is an unsteady motion characterised by a rapid rise of water surface elevation, resulting in an unsteady turbulent flow. In the present study, the hydrodynamics of unsteady compression wave flows were experimentally investigated using an array of two profiling velocimeters, sampled simultaneously. The two-dimensional cross-correlation data in the y-z plane, formed by the two sampling profiles, showed the existence of large scale coherent structures underneath the free-surface, resembling the shape of hairpin vortex. The length scales tended to increase during and after the compression wave passage compared to those during the initially steady flows. Both strain rate and vorticity showed larger values at lower water column near the channel bed, and during the rapid deceleration phase associated with immediate wave passage. The propagation of the compression wave was a dynamically-active process, with large scale transient coherent motions, vortical structures and intensive turbulent mixing occurring underneath.

During the last decades, concerns regarding the ecological impact of standard culverts have led to some design evolution. The installation of baffles along the culvert barrel yields smaller velocities and larger water depths in the barrel, potentially more suitable for upstream fish passage, albeit with a decrease in discharge capacity. Small triangular corner baffles were proposed to facilitate the upstream passage of small-body-mass fish, without compromising the discharge capacity of the culvert at design flow. Although fish benefited from low velocity regions for resting and sheltering, a small fraction of small-body-mass fish were observed to become disoriented by the adverse effect of flow reversal regions in the wake of plain baffles. This study presents the hydrodynamic testing of small ventilated triangular corner baffles for standard box culverts. The baffle ventilation was introduced to reduce the impact of negative wake behind the baffles. Two designs were tested: a baffle with three holes and a brush baffle. Detailed modelling in a near-full-scale culvert barrel showed that the ventilated corner baffles created a smaller negative wake region. A lesser negative velocity magnitude was observed behind the ventilated baffles, in comparison to plain baffles, for the same flow rate, baffle height and spacing. With ventilated corner baffles, the longitudinal distribution of low-velocity zone was more uniform, yielding a better longitudinal connectivity for upstream passage, compared to plain baffles. A comparison between various boundary treatments suggested however that the requirements for continuous, sizeable low positive velocity zone suitable to small-bodied fish might be better fulfilled with an asymmetrically roughened culvert barrel than with triangular baffles, even with ventilation.

Small tide dominated estuaries are affected by both large scale flow structures manifested as oscillatory residual currents and small scale bed generated turbulence. The present understanding of the effects and relative contributions of these factors are inadequate due to limited systematic observations. A field study was conducted in a small sub-tropical estuary in which high frequency (50 Hz) turbulence data were recorded continuously for about 48 h. A triple decomposition technique was introduced to isolate the contributions of tides, slow fluctuations and ‘true’ turbulence in the flow field. A striking feature of the data set was the slow fluctuations which exhibited large velocity amplitudes up to 50% of the tidal amplitude under neap tide conditions. The skewness of the fast ‘true’ turbulence were within the range of −4 and +2. The analysis of the fast fluctuations revealed that variation in the turbulence characteristics are linked with the tidal velocity suggesting that the dispersion and mixing are dependent on some large scale flow properties. The results also indicated enhanced mixing associated with the slow fluctuation in the flow field. The results provided useful sets of information about the properties of slow and fast fluctuating components and their variations through tidal cycles in a micro-tidal estuary.

In free-surface flows, the interactions between high-velocity liquid and atmosphere may lead to strong gas-liquid mixing and complex multiphase flow interactions. In this study, the void fraction power spectrum density (PSD) is used to provide an alternative view of the air-water flow composition. The high frequency range of the PSD curve reflects contributions of small chord times, while the low frequency range contains contributions of both small and large chord times. Likewise, the interactions between the smallest bubbles contribute approximately uniformly to the entire frequency range of the spectrum, while any interaction involving large bubbles/drops will be modulated by 1/f2. It is shown that the void fraction spectra are a powerful tool in providing an alternative view of the air-water flow composition.

Stepped spillways have been used worldwide as both primary and secondary spillway systems. Some key features of prototype operations include high Reynolds number overflows, high rate of energy dissipation, highly turbulent flows, and intense free-surface aeration. Fundamentally, the stepped invert profile generates intense turbulent dissipation during the spill associated with a significant reduction of kinetic energy in the free-surface flow, as well as strong self-aeration. The present study focused on the effects of inclined downward steps on the air-water flow properties, flow resistance, rate of energy dissipation and aeration efficiency of a 45° stepped chute (1V:1H). some physical modelling was conducted in a relatively large facility operating with Reynolds numbers 2.8103 < Re < 1106. Detailed air-water flow properties were systematically investigated with Reynolds numbers within 2.3105 < Re < 8.0105 with three inclined downward step slopes, i.e. horizontal (0°), 1V:5H (11.3°), and 1V:2.33H (23.3°). The introduction of downward step slope changed the cavity shape, impacting onto the cavity recirculation motion. While the recirculation was three-dimensional for all step geometries, the motion appeared to be more irregular with the elongated cavity shape, compared to that observed with flat horizontal steps (/k = 2.0). In high-velocity self-aerated chute flows, detailed air-water flow measurements constitute a basic requirement for reliable estimates of flow resistance and head losses, and a complete theoretical development was proposed for the total drag force and rate of energy dissipation. The total drag may be derived from momentum and energy considerations, although both approaches necessitate to take into count the non-uniform air-water velocity and pressure distributions. The systematic observations on stepped chute with horizontal and inclined downward steps presented very close air-water flow properties. Despite negligible differences at the millimetric and macro-scales, the performances of the whole stepped chute was substantially impacted by the downward step slope. The inclined downward stepped chute design was less efficient in terms of rate of energy dissipation and dissolved oxygen aeration efficiency. Importantly, the study demonstrated the importance and significance of detailed air-water flow measurements, using a panoply of complementary metrologies to obtain a full description of the physical multiphase processes.

A spillway is a conveyance structure designed to pass the flood waters and safely dissipate their turbulent kinetic energy, before the spill waters re-join the natural river system. The construction of steps on the spillway chute may contribute to some energy dissipation, in turn reducing the size of the downstream energy dissipator. The stepped chute flow is the locus of very intense turbulent dissipation, as well as some strong self-aeration. An unique opportunity for field observations was the Hinze Dam Stage 3 (Australia) with superb physical and optical access. Between 2013 and 2021, four major flood events were documented, with detailed observations conducted for six flow rates within 20.9 m3/s < Q < 334 m3/s, corresponding to dimensionless discharges 0.44 < dc/h < 2.82 and Reynolds numbers ranging from 0.68107 to 10.8107. Some uniquely novel aspects of the research included a series of systematic observations of a full-scale prototype stepped spillway, operating with a relatively wide range of unit discharges 1.71 < q < 27.3 m2/s, by a world-renown hydraulic expert. The observations provided new information on the basic hydraulic flow patterns, the inception of free-surface aeration and the surface velocity field. The location of and water depth at the inception point were recorded and successfully compared to the literature, including large size stepped spillway model data and limited prototype data. The free-surface immediately upstream of the air entrainment inception region presented elongated air-surface features. A novel conceptual model of a 3D entrapment mechanism was proposed, based upon the prototype observations. A robust optical flow (OF) technique was applied to video movies taken from a downstream observation platform. The OF data provided physically meaningful surface velocities in the nonaerated flow region upstream of the inception of free-surface aeration, with streamwise velocities close to ideal fluid flow calculations. Large streamwise surface velocity fluctuations were observed in the non-aerated flow region, while the spanwise surface velocity fluctuations were much smaller. Overall, the current study detailed the world's first successful detailed velocity measurements in a prototype stepped spillway. It presented the proof of concept (POC) for future field observations in high-velocity chute spillways, while the limitations were discussed and future enhancements were proposed.

A hydraulic jump is a stationary transition from an upstream torrential flow to a downstream fluvial motion. Most jumps involve a vigorously tumbling flow region, called roller, where much kinetic energy is being lost. In the present study, both the hydraulic properties and air-water flow characteristics were investigated experimentally in hydraulic jumps with a small Froude number Fr1 = 2.1 and several Reynolds numbers 0.078×105 0.4, the air bubble motion in the roller was mostly driven by buoyancy and the two-phase flow processes were dominated by interfacial aeration/de-aeration through the pseudo-free-surface. Similarity and scale effects were tested in terms of a wide range of hydraulic and air-water flow properties in a hydraulic jump with constant Froude and Morton numbers, i.e. Fr1 = 2.1 and Mo = 2.5.10^-11, with different Reynolds numbers. Altogether, more than two dozen parameters were tested systematically under Froude similar conditions. All the data demonstrated that the selection of relevant (air-water) flow property(ies) used to assess similarity and scale effects is uppermost important. Furthermore the concept of similarity and scale effects must be linked to specific flow conditions. In a hydraulic jump at low Froude number Fr1 = 2.1, the present finding demonstrated that many hydraulic jump properties could not be extrapolated from laboratory study to full scale hydraulic structures without substantial scale effects. These findings have profound implications for civil, environmental and sanitary engineering designs, commonly operating with Reynolds numbers in excess of 10^6.

Hydraulic jumps are commonly employed as energy dissipators to guarantee long-term operation of hydraulic structures. Thus, a comprehensive and in-depth understanding of their main features is fundamental. In this context, the current study focused on a hydraulic jump with low Froude numbers (Fr1 = 2,1 and 2.4) and a relatively high Reynolds number (Re = 2×105). Experimental tests employed dual-tip phase-detection probes to provide a comprehensive characterisation of the main air-water flow properties of the hydraulic jump in terms of void fraction, bubble count rate and interfacial velocities. Importantly, this research focused on the air-water flow property distribution across the channel width, revealing lower values of void fraction and bubble count rate next to the sidewalls as compared to the channel centreline. Such a spatial variability in the transverse direction questions whether data near the walls may be representative of the flow behaviour in the centreline, raising the issue of sidewall effects in image-based techniques. These findings provide helpful information to both researchers and practitioners for a better understanding of the physical process, leading to an optimised design of hydraulic structures. Herein, ensemble-averaged turbulence statistics were extracted from the optical flow data. The results suggested that the flow region near the roller toe and in the shear layer dissipated the largest proportion of turbulent kinetic energy. The Hinze scale was derived, showing that the bubble-turbulence interplay followed both sub-Hinze and super-Hinze break-up schemes.

The hydrodynamic instabilities of open-channel flow past sequential lateral cavities were physically modelled in a relatively large facility. The range of free-surface oscillation was about one third of the mean water depth. High-frequency sampling of free-surface elevations confirmed the existence of fluctuations at a quasi-constant frequency, less than that of gravity standing waves. The trajectories of neutrally-buoyant particles were recorded within a cavity. The data showed a wide range of residence times with complicated motion patterns. The current study emphasised the effect of full-height sidewall baffles on open-channel flow instabilities while further analyses need to be performed to provide reference for the design of baffled waterways.

In air-water flows, the presence of bubbles hinders the application of traditional velocimetry techniques, such as particle image velocimetry, acoustic Doppler velocimetry and laser Doppler velocimetry, etc. Image-based velocimetry recently received much attention, particularly the optical flow (OF) technique, which detects the apparent motion between consecutive images. In the present study, the optical flow techniques were applied to different high-velocity air-water flows, including stationary hydraulic jump, steady plunging jet flow, and unsteady breaking bore. All experiments were conducted in large-size facilities. A set of parameters was found to work for different air-water flows. The two-dimensional averaged interfacial velocity distributions were obtained, and compared favourably with the intrusive phase-detection probe measurements.

Partially-filled pipe flows are commonly observed in civil, environmental and sanitary engineering. The propagation of a positive surge in a confined space might create explosive conditions. Herein the propagation of a compression wave was investigated physically in a relatively large-size semi-circular channel. The unsteady turbulent characteristics were recorded to understand how the sudden cross sectional shape change impacted onto the transient surge propagation and unsteady flow characteristics.

A breaking bore is an unsteady two-phase flow, characterised with intense air entrainment, large-scale turbulence and a highly fluctuating roller free surface. The combination of the presence of air bubbles and the unsteadiness of the flow constitutes a major challenge for experimental measurements of the bore roller velocity field in the bore roller. A novel single bubble event detection (SBED) technique was developed to provide pseudo-instantaneous interfacial velocities in the breaking roller, using an array of dual-tip phase-detection probes. The concept of the SBED technique is based upon cross-correlation analyses for each single bubble event travelling through both leading and trailing tips of the phase-detection probe. Several filtering criteria were combined with the SBED technique to improve its performance. This paper presents a detailed illustration of the SBED technique and its application to a breaking bore.