Environmental Fluid Mechanics Research Papers (original) (raw)

2025, Environmental Fluid Mechanics

Lake Nyos, a deep crater lake, located in the north-west of Cameroon, was permanently stratified below 50 m depth due to subaquatic sources supplying warm, salty and CO 2 -enriched water into the deepest reaches. The high CO 2 content in these source waters caused the 1986 limnic eruption. The deep inflowing water is denser than the hypolimnetic water and maintains the stability of the water column, which is double-diffusively stratified. During the dry season in Feb 2002, cooling triggered the formation of a doublediffusive (DD) staircase, a sequence of homogeneously mixed layers separated by distinct stable interfaces. The initiation of the staircase was slightly below the permanent chemocline at ~ 50 m depth, from where the staircase expanded vertically in a diffusion-type manner for ~ 750 days to a maximal vertical extension of ~ 37 m. The staircase pattern caused the upward heat fluxes to increase which depleted the driving temperature gradient. Subsequently, the density ratio increased and reduced the upward heat flux divergence until DD progressively weakened and finally the staircase structure eroded. Based on 39 CTD profiles, we describe the DD phenomenon, explain the three distinct phases of this unique DD event, which lasted for ~ 850 days, and discuss the vertical extension of the DD zone in relation to the rates of new layer formation and layer decay. To our knowledge, this is the only observation over the entire lifespan-"from birth to death"-of a DD event in a natural water body.

2025, Environmental Fluid Mechanics

A mechanism of formation of small-scale inhomogeneities in spatial distributions of aerosols and droplets associated with clustering instability in the atmospheric turbulent flow is discussed. The particle clustering is a consequence of a spontaneous breakdown of their homogeneous space distribution due to the clustering instability, and is caused by a combined effect of the particle inertia and a finite correlation time of the turbulent velocity field. In this paper a theoretical approach proposed in Phys. Rev. E 66, 036302 ( ) is further developed and applied to investigate the mechanisms of formation of small-scale aerosol inhomogeneities in the atmospheric turbulent flow. The theory of the particle clustering instability is extended to the case when the particle Stokes time is larger than the Kolmogorov time scale, but is much smaller than the correlation time at the integral scale of turbulence. We determined the criterion of the clustering instability for the Stokes number larger than 1. We discussed applications of the analyzed effects to the dynamics of aerosols and droplets in the atmospheric turbulent flow.

2025, Conference Paper

Brine, the byproduct of the desalination process, is a type of wastewater and must be disposed of safely. Marine outfall systems are used to dispose of brine, which is discharged into the ambient environment through the discharge ports of the diffuser pipes. A proper hydraulic design of the diffuser pipe is crucial since it ensures sufficient dilution of the discharged wastewater. Jet behavior must be thoroughly analyzed for a successful diffuser design. In this context, jet trajectories are vital since they are directly related to the dilution efficiency of the wastewater discharged. In assessing the trajectories of dense jets from single-side diffusers, there are three main geometrical features: maximum rise height, impact distance, and near-field distance. In this study, the maximum rise height is modeled among these three features. Both classical regression analysis and teaching-learning-based algorithm (TLBO) were used in this concept. This study aims to demonstrate the feasibility of using TLBO in modeling the maximum rise height under dynamic ambient conditions with co-flowing ambient currents. For this purpose, current speed, Froude number, and port spacing were independent variables affecting the maximum rise height. In addition, different statistical indices (e.g. absolute mean error, relative error) were used to evaluate the performance of the models. The results of this evaluation revealed the success of TLBO in modeling the maximum rise height.

2025

We present two general interactive PC-based modeling and visualization software systems developed for the study of two types of environmental water flows: buoyant jet mixing and urban drainage problems. VISJET (http://www.aoe-water.hku.hk/visjet) is arguably the most robust software with advanced graphics for the prediction of mixing and transport of effluent discharges into a stratified crossflow. The prediction engine is a Lagrangian model for buoyant jets with three-dimensional trajectories, and is based on extensive basic experiments and turbulence model calculations. It can be used in outfall design and environmental impact assessment, and as an educational or training tool. VISFLOOD (http://www.aoe-water.hku.hk/visflood) is based on the numerical solution of the Saint-Venant equations, and caters for the simulation of unsteady flood propagation in urban drainage systems. Both software systems are fully interactive with data interrogation; the 3D visualization is fully integrat...

2025

Haute école d'ingénierie et d'architecture Fribourg (HEIA), HES-SO, Switzerland

2025

The initial development of negatively buoyant jets has been investigated experimental and numerically, more specifically the role played by gravity in the development of the leading vortex ring. A classical pistoncylinder arrangement has been used to produce the negatively buoyant jets. Under the experimental conditions considered in this work, the Froude number, Fr, which compares the jet momentum and the buoyancy flux is the most important parameter characterizing the dynamics of the flow. When the value of this parameter is sufficiently small the initial vortex ring generated at the start of the motion is pushed upwards by the gravity force before it can entrain enough vorticity to acquire a self induced velocity. However when the Froude number exceeds a critical value, Frc ∼ 1, the vortex ring can travel downwards and entrainins vorticity from the trailing jet during a longer time. Total and vortex circulation, as well as the trajectory of the leading vortex have been measured to clarify the effect of gravity on the distribution of vorticity during the initial development of negatively buoyant jets.

2025, Proc. Instn Civ. Engrs Wat. Marit. & Energy, UK, Vol. 112, Mar., pp. 20-30

At hydraulic structures (e.g. weirs, spillways), air-water transfer of atmospheric gases (e.g. oxygen) occurs by self-aeration along the chute and by flow aeration in the hydraulic jump at the downstream end of the structure. In this Paper, experimental data are re-analysed and compared with a numerical method to predict the free-surface aeration. Aeration at hydraulic jumps is also examined and compared with existing correlations. Calculations of free-surface aeration and empirical correlations for hydraulic jump aeration are combined to predict the dissolved oxygen content downstream of weirs and spillways. Good agreement between calculations and prototype data is obtained. The results indicate that self-aeration might contribute to a large part of the oxygenation taking place at hydraulic structures for small water discharges. For large discharges, the reduction or the disappearance of free-surface aeration affects substantially the aeration efficiency.

2025, Environmental Fluid Mechanics

OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.

2025

This paper is mainly about the areas of non-Newtonian fluid mechanics which are not investigated or not appropriately and sufficiently investigated. In fact, this should also include emerging areas of research in the field of non-Newtonian fluid mechanics due to new scientific and technological developments and advancements. The purpose of the paper is to highlight and draw the attention to these areas so that researchers (especially the young researchers and newcomers to research such as PhD students) invest their resources and efforts in these areas instead of investing in other areas which are previously investigated and hence they are of less priority from this aspect. Apart from the obvious benefit of "leveling up" in research, the attention to these rather neglected and nonexplored areas of research can be beneficial at the scientific and individual levels since it can lead to breakthroughs and new discoveries in these areas of research by inspecting and assessing their potentials and impacts at the theoretical and practical levels and probing their beneficial applications. We will also provide a brief discussion about the possibility of introducing novel tools and methods in these areas of research (and in non-Newtonian fluid mechanics research in general) as well as highlighting some of the existing limitations of the past and current research in the field of non-Newtonian fluid mechanics (noting that this discussion should help in achieving the ultimate objective of this investigation).

2025, 30th IAHR Biennial Congress, Thessaloniki, Greece, J. GANOULIS and P. PRINOS Ed., Vol. C2, pp. 237-244 (ISBN 960-243-597-6)

Although field observations of sudden spillway releases highlighted strong aeration of dam break wave flows, no study investigated these two-phase flow characteristics. New original experiments were conducted down a 24 m long stepped waterways. Dam break wave propagation showed an equivalent flow resistance similar to steady flow conditions (f ~ 0.05). Detailed unsteady air-water flow measurements demonstrated very strong aeration of the wave leading edge : C mean ~ 0.8 down to 0.4 for (t-t s)* g/d o < 1.2 while the air-water flow structure was very coarse. Void fraction distributions compared well with analytical models.

2025

This paper presents a two-dimensional model to study the sediment deposition from marine outfall jets. The introduced unidirectional coupling (fluid-sediment) is an appropriate choice in the case of low-concentrated particle-laden jets such as municipal wastewater discharge, where the concentration of particles is small enough and does not affect the hydrodynamic development of the jet in the nearfield. The sedimentation model takes advantage of the preferential concentration phenomenon. The deposition criterion states that the deposition of sediments begins when the vertical component of the entrainment velocity becomes smaller than the settling velocity. Once the deposition process begins, it is controlled by the settling velocity, entrainment velocity, volume flux, and sediment concentration. The deposition along the jet trajectory is expressed by an ordinary differential equation coupled with the liquid phase equations. Experiments of Lane-Serff and Moran [Sedimentation from Buoyant jets. J Hyd Eng. 2005;131(3):166-174], Cuthbertson and Davies [Deposition from particle-laden, round, turbulent, horizontal, buoyant jets in stationary and coflowing receiving fluids. J Hydr Eng. 2008;134(4):390-402], and Lee [Mixing of horizontal sediment laden jets [dissertation]. Hong Kong: University of Hong Kong; 2010], chosen from bibliography, are used to validate the model. These experiments cover the cases of horizontal and inclined buoyant jets in stationary ambient, horizontal buoyant jets in co-flow current and nonbuoyant horizontal jets in stationary ambient. Good agreement between the experiments and the obtained simulations is revealed.

2025, Proc. Intl Symp. : Waves - Physical and Numerical Modelling, IAHR, Vancouver, Canada, M. ISAACSON and M. QUICK Edit., Vol. 2, pp. 783-792 (ISBN 0- 88865-364-6).

The aeration characteristics of plunging breakers in the deep sea are described using a similarity with plunging jets. A large two-dimensional jet facility provides new experimental data on the number and sizes of the entrained bubbles and the resulting airwater interface area. The results enable new calculations of the air-water gas transfer contribution of the plunging breakers. The results are consistent with experimental field observations and emphasise the role of plunging breakers in the aeration process of the oceans.

2025, Environmental Fluid Mechanics

We report a semi-analytical theory of wave propagation through a vegetated water. Our aim is to construct a mathematical model for waves propagating through a lattice-like array of vertical cylinders, where the macro-scale variation of waves is derived from the dynamics in the micro-scale cells. Assuming infinitesimal waves, periodic lattice configuration, and strong contrast between the lattice spacing and the typical wavelength, the perturbation theory of homogenization (multiple scales) is used to derive the effective equations governing the macro-scale wave dynamics. The constitutive coefficients are computed from the solution of micro-scale boundary-value problem for a finite number of unit cells. Eddy viscosity in a unit cell is determined by balancing the time-averaged rate of dissipation and the rate of work done by wave force on the forest at a finite number of macro stations. While the spirit is similar to RANS scheme, less computational effort is needed. Using one fitting parameter, the theory is used to simulate three existing experiments with encouraging results. Limitations of the present theory are also pointed out.

2025, Physics of Fluids

We study the three-dimensional turbulent Kolmogorov flow, i.e., the Navier–Stokes equations forced by a single-low-wave-number sinusoidal force in a periodic domain, by means of direct numerical simulations. This classical model system is a realization of anisotropic and non-homogeneous hydrodynamic turbulence. Boussinesq's eddy viscosity linear relation is checked and found to be approximately valid over half of the system volume. A more general quadratic Reynolds stress development is proposed, and its parameters are estimated at varying the Taylor scale-based Reynolds number in the flow up to the value 200. The case of a forcing with a different shape, here chosen Gaussian, is considered, and the differences with the sinusoidal forcing are emphasized.

2025

In this paper we continue our previous investigation about energy minimization in the flow of fluids through tubes and networks of interconnected tubes of various geometries. We will show that the principle of energy minimization holds independent of the geometry of the tubes and networks of such interconnected tubes and independent of the type of fluid in such geometries where in this regard we consider generalized Newtonian fluids. We consider in this investigation the flow of Newtonian fluids through tubes and networks of interconnected tubes of elliptical, rectangular, equilateral triangular and concentric circular annular cross sectional geometries. We also consider a combination of geometric factor with a fluid type factor by showing that the principle of energy minimization holds in the flow of some non-Newtonian fluids (namely power law, Ellis and Ree-Eyring fluids) through tubes and networks of interconnected tubes of elliptical cross sections. The relevance of this study extends beyond tubes and networks of fluid flow to include for instance porous media and electrical networks.

2025

In this paper we continue our previous investigation about the use of stress function in the flow of generalized Newtonian fluids through conduits of circular and noncircular (or/and multiply connected) cross sections where we visualize the stages of yield in the process of flow of viscoplastic fluids through tubes of elliptical, rectangular, triangular and annular cross sections. The purpose of this qualitative investigation is to provide an initial idea about the expected yield development in the process of flow of yield-stress fluids through tubes of some of the most common non-circular (and non-simply-connected) cross sectional geometries.

2025, Modelling of high-velocity free-surface flows: a revised interfacial area approach

The specific interface area transport equation is derived from the first principles, and the closure model for the turbulent flow is proposed. The flow is modelled by a two-phase method providing velocities of both phases at the bubbly, spray and the intermediate large scale interface (LSI) regimes. The interface area is transported by the interface velocitya linear combination of corresponding phase velocities and a turbulent drift velocity. The resulting equation includes the source terms due to the bubbles (droplets) breakup and coalescence, turbulent splashes at the interface. A model is proposed for the source term in the LSI regime. The model is implemented in the developer's version of the Simcenter STAR-CCM+® CFD code. The new model is applied to self-aerating water flows down stepped chutes; the results are in reasonable agreement with the experimental data.

2025

In this paper we continue our previous investigation about the use of stress function in the flow of generalized Newtonian fluids through conduits of circular and noncircular (or/and multiply connected) cross sections where we inspect the flow of Ree-Eyring fluids in tubes of elliptical cross sections. We derive analytical expressions for the flow velocity profile and for the volumetric flow rate. The obtained analytical expressions were tested against the available analytical expressions for the special cases of Newtonian flow in circular tubes, Newtonian flow in elliptical tubes and Ree-Eyring flow in circular tubes and the results were identical. The obtained analytical expressions were also tested for sensible trends, tendencies and correlations and they passed all these tests.

2025, Environmental Fluid Mechanics

The measurements taken during the Vertical Transport and Mixing Experiment (VTMX, October, 2000) on a northeastern slope of Salt Lake Valley, Utah, were used to calculate the statistics of velocity fluctuations in a katabatic gravity current in the absence of synoptic forcing. The data from ultrasonic anemometer-thermometers placed at elevations 4.5 and 13.9 m were used. The contributions of small-scale turbulence and waves were isolated by applying a high-pass digital (Elliptical) filter, whereupon the filtered quantities were identified as small-scale turbulence and the rest as internal gravity waves. Internal waves were found to play a role not only at canonical large gradient Richardson numbers (Ri g > 1), but sometimes at smaller values (0.1 < Ri g < 1), in contrast to typical observations in flatterrain stable boundary layers. This may be attributed, at least partly, to (critical) internal waves on the slope, identified by Princevac et al. , which degenerate into turbulence and help maintain an active internal wave field. The applicability of both Monin-Obukhov (MO) similarity theory and local scaling to filtered and unfiltered data was tested by analyzing rms velocity fluctuations as a function of the stability parameter z/L, where L is the Obukhov length and z the height above the ground. For weaker stabilities, z/L < 1, the MO similarity and local scaling were valid for both filtered and unfiltered data. Conversely, when z/L > 1, the use of both scaling types is questionable, although filtered data showed a tendency to follow local scaling. A relationship between z/L and Ri g was identified. Eddy diffusivities of momentum K M and heat K H were dependent on wave activities, notably when Ri g > 1. The

2025, Water

Vegetation plays a vital role in the flow characteristics of natural open channels, such as rivers. Typically, vegetation density is higher in the lower layer and sparser in the upper layer of these channels. In this research, Ansys Fluent and the k–ϵ model have been employed to simulate various vegetation configurations to capture intricate flow complexities within vegetation regions. Numerical analysis demonstrated that the numerical results align with anticipated Turbulence Kinetic Energy data obtained from analytical and experimental studies. Our findings revealed that double-layer vegetation induces a more intricate flow distribution. In the spaces between vegetation zones, Turbulence Kinetic Energy decreases due to the resistance imposed by the vegetation patches. This resistance has positive implications for sustaining aquatic life and facilitating sediment deposition, promoting a more environmentally sustainable outcome.

2025

Floods are one of the most important factors on the bridge scour process. However, the uncertainty related to discharge is high due to the presence of clustering effects, the use of outdated rating curves, and the practical issue of measuring at extreme conditions. In this context, employing the best scour model with an uncertain discharge input leads to unreliable scour estimations. The goal of this research seeks to quantify the scour uncertainty due to the discharge uncertainty using stochastic tools and the BRISENT model [Pizarro et al., 2017] for discharge and scour analysis, respectively. To this aim, we examine several stations covering small and large temporal scales of the river discharge. These stations are selected under the criterion of ensuring low human influence on the natural process. The stochastic structure of discharge is modeled fitting the Hurst-Kolmogorov (HK) behavior in terms of the climacogram and a discharge generator was constructed based on the assumption...

2025, Environmental Fluid Mechanics

In this paper, details, and results of three-dimensional numerical modeling of flow around the semi-conical piers vertically mounted on the bed in a channel, are presented. For flow simulation, 3-D Navier-Stokes equations are solved numerically using the finite volume method and large eddy simulation (LES). In this study, the semi-conical piers with different side slope angles are tested, and the flow around them is compared with the cylindrical reference pier. Flow structures, vortex shedding behind piers, horseshoe vortices, instantaneous and time-averaged flow structures are presented and discussed. Numerical model results show that the semi-conical piers are eventuated remarkable reduction (up to 25%) in downward flow velocity in the upstream side of the piers, and much more reduction (up to 46%) in bed shear stresses in comparison with the cylindrical pier. Moreover, the model results showed some decrease in vortex shedding frequency for the semi-conical piers compared to the cylindrical pier. We report on numerical results of large eddy simulation of the flow around semi-conical piers with different side slopes. This research is significant because of the effect of these piers on the: reduction of the downward flow and the bed shear stress around the piers. reduction of vortex shedding frequency for the semi-conical piers compared to the cylinder. different behavior of the horseshoe vortices at the upstream compared to the cylinder.

2025, Journal of Hydraulic Research

Solitary waves are often used to replicate tsunami waves and, therefore, the study of their propagation and interactions with structures could help to understand the impacts of tsunami waves on infrastructures. This paper presents a numerical model that uses the level-set framework for simulating the propagation of solitary waves and their interactions with structures using the immersed boundary method. The model imposes a set of appropriate boundary conditions to prescribe the incoming solitary waves within the sharp-interface immersed boundary method. Turbulence is modelled using the large-eddy simulation. Experimental tests are conducted to measure the free-surface elevation and velocity field evolution of solitary waves, as they propagate downwave interacting with two different wall-mounted structures. Analysis of the computed and experimental results revealed interesting dynamics of primary vortex-tubes forming near the sharp edges of structures. It is shown that the primary vortices eventually split to form pairs of asymmetrical doublets.

2025, Environmental Fluid Mechanics

Computational modeling of open-channel flows is usually carried out using the rigid-lid (RL) assumption to prescribe the water surface. Such an approach deems more appropriate for sub-critical flow regimes with Froude number ≪ 1. When the Froude number is locally high, e.g. flows around hydraulic structures, the free-surface fluctuations affect the induced hydrodynamics. Indeed, prior investigations on the effect of the RL assumption on large-eddy simulation (LES) of open-channel flows around bridge abutments revealed that such an assumption may influence both the first and second order turbulence statistics. In this work, we investigate the effect of the RL assumption on bed-morphodynamics calculations. The fully coupled flow and bed morphodynamics virtual flow simulator (VFS-Geophysics) model is herein employed to numerically investigate the sediment transport around a laboratory-scale model of abutment. Numerical simulations using a mass-conserving level-set (LS) method and a RL assumption are performed to evaluate the influence of free-surface elevation on the computed bed-morphodynamics at a Reynolds number of 7.9 × 10 4 . We also conducted an experimental study to observe the scour development around the abutment model. The instantaneous scour patterns and bed profiles computed with the LS and RL are compared with the measurements. The mean absolute errors of bed-profile predictions at or near quasi-equilibrium of bed evolution, for the LS and RL computed results, are about 4.1% and 4.7%, respectively. Despite the differences in flow field computations of the two methods, the computed bed morphology appeared relatively insensitive to the two numerical approaches. However, the computational cost of the coupled LES-LS-morphodynamics is at least one order of magnitude greater than that of the LES-RL-morphodynamics.

2025, Journal of Hydraulic Research

2025, Environmental Fluid Mechanics

2025, Journal of Hydraulic Research

2025, Environmental Fluid Mechanics

Lake water level fluctuations provide an important role in flushing shallow coastal embayments in the Great Lakes, especially if the embayment has a resonant response. Specifically, long-period waves (of periods 4-30 min) can excite resonance in coastal embayments, which greatly increases the flushing rates. We describe how resonance can explain the difference in responses of three shallow ðdepth %2 mÞ coastal embayments of Lake Ontario and Lake Huron to similar long-period waves. Higher frequency water level fluctuations were analyzed to determine the most influential frequencies within the embayments. Observations in two adjacent embayments in Lake Huron show dramatic differences between their amplified responses to identical forcing, while in Frenchman's Bay the oscillations are damped for the whole forcing spectrum. We model the water level response of the shallow coastal embayments to lake long-period wave forcing using a driven Helmholtz harmonic resonator. We compare and find favourable agreement ðR 2 ¼ 78 %Þ between the amplification of water level fluctuations predicted by our model and field values for nearly enclosed embayments, where the Helmholtz mode dominates the energy of the oscillations. Additionally, strong peaks corresponding to the first three natural modes are observed in the water level oscillations of one of the Lake Huron embayments. This embayment has a wider entrance and its stronger amplified response can be explained using an analytical model based on an asymptotic theory of nonlinear resonance of free long-period oscillations induced by wind waves.

2024, Lecturas matemáticas

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2024, Environmental Fluid Mechanics

In this laboratory study, propagation behaviour, particle deposition patterns, and suspension characteristics of non-cohesive particle-driven gravity currents formed under constant-volume release conditions were investigated. The experimental gravity currents were created in a two-dimensional lock exchange type tank using two different particles (silicon carbide and glass beads) with four different median diameters. Video imaging and image processing techniques were utilized to monitor the current propagation, laser diffraction size analysis and dry weighing techniques were utilized to examine the size and mass characteristics of the deposits and suspensions, and acoustic Doppler velocimetry was utilized for flow velocity measurements for turbulence analysis. Our observations showed that the experimental gravity currents experienced two different propagation phases based upon the particle settling regimes. The first propagation phase was named as the propagation with the turbulence-dominated settling (TDS) and the later propagation phase was named as the propagation with gravity-dominated settling (GDS). It is found that a critical turbulent Reynolds number value (estimated to be O(1)) delineates the settling regimes, hence determines the transition between the propagation phases. With increasing particle settling velocity, the observed propagation phases in our experimental currents showed increasing deviations from the slumping, inertia-buoyancy, and viscous-buoyancy propagation phases that have been reported for homogeneous constant-volume gravity currents with no or negligible settling in the literature. Propagation observations showed that the initial median particle diameters of the currents have negligible effect on the current propagation characteristics during the TDS phase, but become important during the GDS phase. The currents with smaller initial median particle diameters propagated faster and a longer distance in the GDS phase than their counterparts with larger median particle diameters. The deposited particle characteristics indicated that particles of different sizes settle at similar speeds during the TDS phase due to turbulent mixing and the settling speed becomes dependent on the particle size during the GDS phase. As a result, size sorting of the deposited particles became more pronounced during the GDS phase. At the earlier stages of propagation, the vertical profiles of suspended particle concentrations in the current head showed some extent of vertical uniformity due to turbulent mixing around the half height of the current head. On the other hand, at the later stages of propagation, suspended particle concentration profiles exhibited an exponential profile. Deposited and suspended particle characteristics showed that horizontal particle sorting, that is size grading of particles in the flow Extended author information available on the last page of the article Environmental Fluid Mechanics 1 3 direction, was more pronounced than the vertical particle sorting, that is size grading of particles at different elevations within the current head.

2024, Environmental Fluid Mechanics

2024

Driftwood is one of the important physical components in mountainous rivers which causes severe hazards due to the clogging of bridges, culverts, and narrow sections during floods. Therefore, the understanding of driftwood dynamics and mitigation measures are crucial for managing wood in rivers. Open check dams are the most commonly used engineering measure for preventing driftwood from reaching downstream areas. Nevertheless, these open check dams frequently lose their sediment transport function when they are blocked by sediment and driftwood, especially during major flood events. This paper proposes a new type of open check dam for preventing from clogging. Thus, flume experiments were conducted to examine the influence of different types of open check dams on the characteristics of driftwood deposition. For the model with wood length (LWD)=16.5 cm, wood diameter (D) =15 mm, and wood number (N)=172, the highest trapping efficiency was observed with 90.1% and 87.2% retention rates for the classical debris flow breaker and curved footed open check dams, respectively. Laboratory tests showed that through this proposed design, woody debris blockage in a very short time was prevented from the accumulation of woods beside the dam. In addition to this, most of the sediment passed through the check dam and most of the driftwood got trapped. It can be briefly stated that the geometrical design of the structure plays an important role and can be chosen carefully to optimize trapping efficiency. By designing this type of open check dams in mountain river basins, it may provide a better understanding of the driftwood accumulation and basis for the optimal design of these structures. Further development of the solution proposed in this work can pave the way for designing different types of open check dams for effective flood management.

2024, Environmental fluid mechanics

In high-velocity free-surface flows, the interactions between the fast-flowing flow and the atmosphere can lead to strong air-water mixing through the free-surface. The flow resistance may be derived from momentum and energy considerations. This was undertaken through some theoretical development based upon an extension of traditional integral approaches to air-water flows, with the re-analyses of detailed air-water measurements in both prototype spillways and large-size physical models. Both the momentum and energy approaches yielded close results in terms of the Darcy-Weisbach friction factor. Based upon the same data sets, the absence of correction coefficients was found to underestimate the residual energy, compared to the detailed calculations using the relevant air-water kinetic energy and pressure correction coefficients. The finding has basic design implications, which are discussed.

2024, Environmental fluid mechanics

In this work, vegetation effects on the characteristics of radiation-induced natural convection (isotherms, circulation patterns, exchange flow rate) in sloping water bodies are investigated numerically. The water body consists of (a) a sloping vegetated region (with a bottom slope equal to 0.1) and (b) a deep region with a horizontal bottom. The vegetation of porosity 0.85 (typical of aquatic plants found in lakes) has a length equal to the length of the sloping region. It can block (totally or partially) the radiation and as a result a nonuniform (differential) heating is developed along the free surface of the water body. The Volume-Averaged Navier-Stokes equations together with the Volume-Averaged Energy equation are solved numerically in the vegetated region. The radiation-induced natural convection in a water body with only a sloping region (with no vegetation) is also considered for validation purposes since numerical and scaling analysis results are available in literature. The results indicate significant vegetation effects on the thermal and flow patterns especially for vegetation which blocks completely surface heating.

2024, Proceedings of 24th Australasian Fluid Mechanics Conference AFMC2024, Canberra, Australia, 1-5 December, Paper AFMC2024-017, 8 pages

A culvert is a covered channel designed to pass water through an embankment. The adverse ecological impacts of road crossings on upstream fish passage have driven the development of new design guidelines with a focus on small-bodied native fish species and juveniles of larger fish. Recent studies recommended the usage of low velocity zones (LVZ) in the culvert for upstream fish passage. To date, there is a limited knowledge on the hydrodynamics in the LVZ, particularly the turbulence characteristics, despite its relevance to fish kinematics. New research was carried out to study the turbulence and secondary motion in low velocity zone in a rectangular culvert. Detailed turbulence measurements were undertaken in a near full-scale box culvert barrel. Velocity and Reynolds stress measurements showed some strong secondary motion of Prandtl's second kind. The turbulent characteristics, including burst events and secondary currents, were carefully detailed, presenting a novel insight into the turbulence of fish's preferential swimming zone.

2024, Environmental Fluid Mechanics

The tidal bore of the Garonne River (France) was investigated on 29 August, 31 August and 27 October 2015, during which instantaneous velocity measurements were performed continuously at highfrequency (200 Hz). The tidal bore occurrence had a marked effect on the flow field and turbulent Reynolds stress data, indicating large shear stresses, together with large and rapid fluctuations, during the bore passage and the early flood tide. Like many natural process flows, the tidal bore flow motion was dominated by coherent structure activities and turbulent events, with significant impact on the natural systems including in terms of sediment processes. Herein a new turbulent event analysis was developed for the highly-unsteady rapidly-varied tidal bore flow. The analysis was based upon basic concepts, in which turbulent bursting events were defined in terms of the instantaneous relative turbulent flux, and the method was extended to the rapidly-varied, highly-unsteady tidal bore motion. The turbulent event data showed relatively close results for most fluxes during the tidal bores. The event duration showed some tidal trend, with longer turbulent events immediately after the tidal bore passage, occurring simultaneously with major sediment erosion processes. The field data set and analyses suggested that a turbulent event analysis may be applicable to highly-unsteady rapidly-varied flows, providing quantitative details into the turbulent bursts that are responsible for major mixing and sedimentary processes.

2024, HAL (Le Centre pour la Communication Scientifique Directe)

A tidal bore is a series of waves propagating upstream as the tidal flow turns to rising, and the bore front corresponds to the leading edge of the tidal wave in a funnel shaped estuarine zone with macro-tidal conditions. Some field observations were conducted in the tidal bore of the Garonne River on 7 June 2012 in the Arcins channel, a few weeks after a major flood. Despite the high initial water level and strong fluvial current, the bore front exhibited a sharp discontinuity in terms of free-surface elevation: the bore front was 0.45 m and 0.52 m high on the morning and afternoon respectively. The tidal bore was a flat undular bore with a Froude number close to unity: Fr 1 = 1.02 and 1.19 in the morning and afternoon respectively. The field observations highlighted a number of unusual features on the morning of 7 June 2012 when detailed free-surface and velocity measurements were conducted simultaneously. These included (a) a slight rise in water elevation starting about 70 s prior to the front, (b) a flow reversal about 50 s after the bore front, (c) some large fluctuations in suspended sediment concentration (SSC) about 100 s after the bore front and (d) a transient water elevation lowering about 10 minutes after the bore front passage. The measurements of water temperature and salinity showed nearly identical results before and after the tidal bore: there was no evidence of saline or thermal front.

2024, Environmental Fluid Mechanics

This paper summarizes recent advances in vegetation hydrodynamics and uses the new concepts to explore not only how vegetation impacts flow and transport, but also how flow feedbacks can influence vegetation spatial structure. Sparse and dense submerged canopies are defined based on the relative contribution of turbulent stress and canopy drag to the momentum balance. In sparse canopies turbulent stress remains elevated within the canopy and suspended sediment concentrations are comparable to that in unvegetated regions. In dense canopies turbulent stress is reduced by canopy drag and suspended sediment concentration is also reduced. Further, for dense canopies, the length-scale of turbulence penetration into the canopy,  e , is shown to predict both the roughness height and the displacement height of the overflow profile. In a second case study, the relation between flow speed and spatial structure of a seagrass meadow gives insight into the stability of different spatial structures, defined by the area fraction covered by vegetation. In the last case study, a momentum balance suggests that in natural channels the total resistance is set predominantly by the area fraction occupied by vegetation, called the blockage factor, with little direct dependence on the specific canopy morphology.

2024, Bulletin of the American Physical Society

Submitted for the DFD15 Meeting of The American Physical Society Flow over and within large-scale porous topography: Impact of surface heterogeneity on turbulence structure ALI M. HAMED, Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, PRATEEK RANJAN, MATTHEW J. SADOWSKI, University of Illinois at UrbanaChampaign, HEIDI M. NEPF, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, LEONARDO P. CHAMORRO, Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign — An experimental investigation of the flow within and above model canopies was carried out to determine the effect of canopy height heterogeneity on the structure and spatial distribution of the turbulence. Two 800 mm long models with 20% blockage were placed in a 2.5 m long refractive-index-matching channel. The first model (base case) is constituted of equal height (h) square bar elements arranged in a stagger...

2024, Environmental Fluid Mechanics

2024, Research Report No. CE149, Dept. of Civil Engineering, University of Queensland, Australia, Aug., 68 pages (ISBN 0 86776 5828)

In open channels, the transition from a rapid to fluvial flow is called a hydraulic jump. It is characterised by large scale turbulence, air bubble entrainment and energy dissipation. The autho'6 have investigated the air bubble entrainment at hydraulic jumps with partially developed inflow conditions. The air-water flow is characterised by a turbulent shear region with a large air content which contributes to the enhancement of the air-water interface area and air-water gas transfer in the hydraulic jump. Further a new gas transfer model is presented. Based upon physical evidence, the model enables to predict the dissolved gas contents and water quality downstream of hydraulic jumps with partially developed inflows.

2024, Environmental Fluid Mechanics

Motivated by shallow ocean waves propagating over coral reefs, we investigate the drift velocities due to surface wave motion in an effectively inviscid fluid that overlies a saturated porous bed of finite depth. Previous work in this area either neglects the large-scale flow between layers (Phillips in Flow and reactions in permeable rocks, Cambridge University Press, Cambridge, 1991) or only considers the drift above the porous layer (Monismith in Ann Rev Fluid Mech 39:37–55, 2007). Overcoming these limitations, we propose a model where flow is described by a velocity potential above the porous layer and by Darcy’s law in the porous bed, with derived matching conditions at the interface between the two layers. Both a horizontal and a novel vertical drift effect arise from the damping of the porous bed, which requires the use of a complex wavenumber k. This is in contrast to the purely horizontal second-order drift first derived by Stokes (Trans Camb Philos Soc 8:441–455, 1847) whe...

2024, The EGU General Assembly

in a trapezoidal cross section channel, to investigate uni and bi-directional exchange flows. Both rotating and non rotating experiments, with a non-erodible and erodible bed layer were considered. Experimental measurements focused on obtaining high-resolution velocity and density fields in different vertical planes spanning the width of the channel using 2D Particle Image Velocimetry, Laser Induced Fluorescence, ADV and micro-conductivity probes in several channel sections. Preliminary results from the study are presented herein and consider the exchange flow dynamics at the interface, with particular focus on the observed cross-channel variations in the counter-flowing water masses and layer thickness. Further analyses are ongoing to improve understanding of the interfacial mixing and entrainment/detrainment processes, along with the bedmorphological changes initiated by interaction mechanisms between the erodible bottom boundary and the uni-or bi-directional exchange flows.

2024

, in a trapezoidal cross section channel, to investigate uni and bi-directional exchange flows. Both rotating and non rotating experiments, with a non-erodible and erodible bed layer were considered. Experimental measurements focused on obtaining high-resolution velocity and density fields in different vertical planes spanning the width of the channel using 2D Particle Image Velocimetry, Laser Induced Fluorescence, ADV and micro-conductivity probes in several channel sections. Preliminary results from the study are presented herein and consider the exchange flow dynamics at the interface, with particular focus on the observed cross-channel variations in the counter-flowing water masses and layer thickness. Further analyses are ongoing to improve understanding of the interfacial mixing and entrainment/detrainment processes, along with the bedmorphological changes initiated by interaction mechanisms between the erodible bottom boundary and the uni-or bi-directional exchange flows.

2024, The EGU General Assembly