Authors:Hang Wang; Frédéric Murzyn Pages: 7 - 25 Abstract: Abstract A hydraulic jump is a turbulent shear flow with a free-surface roller. The turbulent flow pattern is characterised by the development of instantaneous three-dimensional turbulent structures throughout the air–water column up to the free surface. The length and time scales of the turbulent structures are key information to describe the turbulent processes, which is of significant importance for the improvement of numerical models and physical measurement techniques. However, few physical data are available so far due to the complexity of the measurement. This paper presents an investigation of a series of characteristic turbulent scales for hydraulic jumps, covering the length and time scales of turbulent flow structures in bubbly flow, on free surface and at the impingement point. The bubbly-flow turbulent scales are obtained for Fr = 7.5 with 3.4 × 104 < Re < 1.4 × 105 in both longitudinal and transverse directions, and are compared with the free-surface scales. The results highlight three-dimensional flow patterns with anisotropic turbulence field. The turbulent structures are observed with different length and time scales respectively in the shear flow region and free-surface recirculation region. The bubbly structures next to the roller surface and the free-surface fluctuation structures show comparable length and time scales, both larger than the scales of vortical structures in the shear flow and smaller than the scales of impingement perimeter at the jump toe. A decomposition of physical signals indicates that the large turbulent scales are related to the unsteady motion of the flow in the upper part of the roller, while the high-frequency velocity turbulence dominates in the lower part of the roller. Scale effects cannot be ignored for Reynolds number smaller than 4 × 104, mainly linked to the formation of large eddies in the shear layer. The present study provides a comprehensive assessment of turbulent scales in hydraulic jump, including the analyses of first data set of longitudinal bubbly-flow integral scales and transverse jump toe perimeter integral scales. PubDate: 2017-02-01 DOI: 10.1007/s10652-016-9451-6 Issue No:Vol. 17, No. 1 (2017)

Authors:Gangfu Zhang; Hubert Chanson Pages: 27 - 46 Abstract: Abstract In high-velocity chute flows, free-surface aeration is often observed. The phenomenon is called self-aeration or white waters. When the turbulent shear stresses next to the free-surface are large enough, air bubbles are entrained throughout the entire air–water column. A rapidly-varied flow region is observed immediately downstream of the inception point of free-surface aeration. An analytical solution of the air diffusion equation is proposed and the results compare well with new experimental data. Both experiments and theory indicate that the flow bulking spans over approximately 3–4 step cavities downstream of the inception point of free-surface aeration on a stepped chute. Further downstream the void fraction distributions follow closely earlier solutions of the air diffusion equation. The application of the diffusion equation solution to prototype and laboratory data shows air bubble diffusivities typically larger than the momentum transfer coefficient. The result highlights however a marked decrease in the ratio of air bubble diffusivity to eddy viscosity with increasing Reynolds number. The finding might indicate some limitation of laboratory investigations of air bubble diffusion process in self-aerated flows and of their extrapolation to full-scale prototype applications. PubDate: 2017-02-01 DOI: 10.1007/s10652-015-9442-z Issue No:Vol. 17, No. 1 (2017)

Authors:Pierre Lubin; Hubert Chanson Pages: 47 - 77 Abstract: Abstract The flow structure in the aerated region of the roller generated by breaking waves remains a great challenge to study, with large quantities of entrained air and turbulence interactions making it very difficult to investigate in details. A number of analogies were proposed between breaking waves in deep or shallow water, tidal bores and hydraulic jumps. Many numerical models used to simulate waves in the surf zone do not implicitly simulate the breaking process of the waves, but are required to parameterise the wave-breaking effects, thus relying on experimental data. Analogies are also assumed to quantify the roller dynamics and the energy dissipation. The scope of this paper is to review the different analogies proposed in the literature and to discuss current practices. A thorough survey is offered and a discussion is developed an aimed at improving the use of possible breaking proxies. The most recent data are revisited and scrutinised for the use of most advanced numerical models to educe the surf zone hydrodynamics. In particular, the roller dynamics and geometrical characteristics are discussed. An open discussion is proposed to explore the actual practices and propose perspectives based on the most appropriate analogy, namely the tidal bore. PubDate: 2017-02-01 DOI: 10.1007/s10652-016-9475-y Issue No:Vol. 17, No. 1 (2017)

Authors:A. Khosronejad; P. Diplas; F. Sotiropoulos Pages: 79 - 109 Abstract: Abstract Bendway weirs are one of the most practical in–stream rock structures utilized to protect the outer bend of meandering streams and rivers from erosion. We present development of a simulation-based paradigm for effective design of bendway weir structures to enhance meandering stream bank stability and control lateral migration. To do so, we employ the St. Anthony Falls Laboratory Virtual StreamLab (VSL3D) code to elucidate the flow and sediment transport phenomena induced by interaction of flow, mobile bed, and in–stream structures in large rivers under prototype conditions. We carried out numerous numerical experiments to systematically simulate various arrangements of bendway weir in two river test-beds and gaining insights into the physical mechanisms via which such bendway weirs modify turbulent flow, sediment transport and scour processes. The so-gained physical insights are then taken into account to develop a set of practical physics-based design criteria for optimal placement of bendway weirs in large rivers. PubDate: 2017-02-01 DOI: 10.1007/s10652-016-9452-5 Issue No:Vol. 17, No. 1 (2017)

Authors:Christian González; David H. Richter; Diogo Bolster; Samuel Bateman; Joseph Calantoni; Cristián Escauriaza Pages: 111 - 137 Abstract: Abstract At the smallest scales of sediment transport in rivers, the coherent structures of the turbulent boundary layer constitute the fundamental mechanisms of bedload transport, locally increasing the instantaneous hydrodynamic forces acting on sediment particles, and mobilizing them downstream. Near the critical threshold for initiating sediment motion, the interactions of the particles with these unsteady coherent structures and with other sediment grains, produce localized transport events with brief episodes of collective motion occurring due to the near-bed velocity fluctuations. Simulations of these flows pose a significant challenge for numerical models aimed at capturing the physical processes and complex non-linear interactions that generate highly intermittent and self-similar bedload transport fluxes. In this investigation we carry out direct numerical simulations of the flow in a rectangular flat-bed channel, at a Reynolds number equal to Re = 3632, coupled with the discrete element method to simulate the dynamics of spherical particles near the bed. We perform two-way coupled Lagrangian simulations of 48,510 sediment particles, with 4851 fixed particles to account for bed roughness. Our simulations consider a total of eight different values of the non-dimensional Shields parameter to study the evolution of transport statistics. From the trajectory and velocity of each sediment particle, we compute the changes in the probability distribution functions of velocities, bed activity, and jump lengths as the Shields number increases. For the lower shear stresses, the intermittency of the global bedload transport flux is described by computing the singularity or multifr actal spectrum of transport, which also characterizes the widespread range of transport event magnitudes. These findings can help to identify the mechanisms of sediment transport at the particle scale. The statistical analysis can also be used as an ingredient to develop larger, upscaled models for predicting mean transport rates, considering the variability of entrainment and deposition that characterizes the transport near the threshold of motion. PubDate: 2017-02-01 DOI: 10.1007/s10652-016-9476-x Issue No:Vol. 17, No. 1 (2017)

Authors:Liqun Lyu; Zhaoyin Wang; P. Cui Pages: 139 - 158 Abstract: Abstract A typical two-phase debris flow exhibits a high and steep flow head consisting of rolling boulders and cobbles with intermittent or fluctuating velocity. The relative motion between the solid phase and the liquid phase is obvious. The motion of a two-phase debris flow depends not only on the rheological properties of the flow, but also on the energy transmission between the solid and liquid phases. Several models have been developed to study two-phase debris flows. An essential shortcoming of most of these models is the omission of the interaction between the two phases and identification of the different roles of the different materials in two-phase debris flows. The tracer particles were used for the velocity of solid phase and the velocity of liquid phase was calculated by the water velocity on the surface of the debris flow in the experiments. This paper analyzed the intermittent feature of two-phase debris flows based on videos of debris flows in the field and flume experiments. The experiments showed that the height of the head of the two-phase debris flow increased gradually in the initiation stage and reached equilibrium at a certain distance from the start of the debris flow. The height growth and the velocity of the flow head showed fluctuating characteristics. Model equations were established and the analyses proved that the average velocity of the two-phase debris flow head was proportional to the flood discharge and inversely proportional to the volume of the debris flow head. PubDate: 2017-02-01 DOI: 10.1007/s10652-016-9483-y Issue No:Vol. 17, No. 1 (2017)

Authors:Jasim Imran; Sadia M. Khan; Carlos Pirmez; Gary Parker Pages: 159 - 186 Abstract: Abstract The scaling problem associated with the modeling of turbidity currents has been recognized but is yet to be explored systematically. This paper presents an analysis of the dimensionless governing equations of turbidity currents to investigate the scale effect. Three types of flow conditions are considered: (i) conservative density current; (ii) purely depositional turbidity current; and (iii) mixed erosional/depositional turbidity current. Two controlling dimensionless numbers, the Froude number and the Reynolds number, appear in the non-dimensional governing equations. When densimetric Froude similarity is satisfied, the analysis shows that the results would be scale-invariant for conservative density current under the rough turbulent condition. In the case of purely depositional flows, truly scale-invariant results cannot be obtained, as the Reynolds-mediated scale effects appear in the bottom boundary conditions of the flow velocity and sediment fall velocity. However, the scale effect would be relatively modest. The Reynolds effect becomes more significant for erosional or mixed erosional/depositional turbidity currents as Reynolds-mediated scale effects also appear in the sediment entrainment relation. Numerical simulations have been conducted at three different scales by considering densimetric Froude scaling alone as well as combined densimetric Froude and Reynolds similarity. Simulation results confirm that although the scaling of densimetric Froude number alone can produce scale-invariable results for conservative density currents, variations occur in the case of turbidity currents. The results become scale invariant when densimetric Froude and Reynolds similarities are satisfied simultaneously. PubDate: 2017-02-01 DOI: 10.1007/s10652-016-9488-6 Issue No:Vol. 17, No. 1 (2017)

Authors:M. Ben Meftah; D. Malcangio; F. De Serio; M. Mossa Abstract: Abstract The discharge of brackish water, as a dense jet in a natural water body, by the osmotic power plants, undergoes complex mixing processes and has significant environmental impacts. This paper focuses on the mixing processes that develop when a dense round jet outfall perpendicularly enters a shallow flowing current. Extensive experimental measurements of both the salinity and the velocity flow fields were conducted to investigate the hydrodynamic jet behavior within the ambient current. Experiments were carried out in a closed circuit flume at the Coastal Engineering Laboratory (LIC) of the Technical University of Bari (Italy). The salinity concentration and velocity fields were analyzed, providing a more thorough knowledge about the main features of the jet behavior within the ambient flow, such as the jet penetration, spreading, dilution, terminal rise height and its impact point with the flume lower boundary. In this study, special attention is given to understand and confirm the conjecture, not yet experimentally demonstrated, of the development and orientation of the jet vortex structures. Results show that the dense jet is almost characterized by two distinct phases: a rapid ascent phase and a gradually descent phase. The measured flow velocity fields definitely confirm the formation of the counter-rotating vortices pair, within the jet cross-section, during both the ascent and descent phases. Nevertheless, the experimental results show that the counter-rotating vortices pair of both phases (ascent and descent) are of opposite rotational direction. PubDate: 2017-02-17 DOI: 10.1007/s10652-017-9515-2

Authors:Oscar Castro-Orgaz; Hubert Chanson Abstract: Abstract Dam-break flood waves are associated with major environmental disasters provoked by the sudden release of water stored in reservoirs. Ritter found in 1892 an analytical solution to the wave structure of an ideal fluid released during an instantaneous dam failure, propagating over initially dry horizontal terrain. This solution, though ideal, hence frictionless, is widely used to test numerical solutions of the Shallow Water Equations (SWE), and as educational tool in courses of fluid mechanics, given that it is a peculiar case of the Riemann problem. However, the real wave structure observed experimentally differs in a major portion of the wave profile, including the positive and negative fronts. Given the importance of an accurate prediction of the dam break wave, the positive and negative wave portions originating from the breaking of a dam with initially dry land on the tailwater reach are revisited in this work. First, the propagation features of the dry-front are investigated using an analytical boundary-layer type model (Whitham/Dressler/Chanson model) constructed matching an (outer) inviscid dynamic wave to an (inner) viscous diffusive wave. The analytical solution is evaluated using an accurate numerical solution of the SWE produced using the MUSCL-Hancock finite-volume method, which is tested independently obtaining the solution based on the discontinuous Galerkin finite-element method. The propagation features of the negative wave are poorly reproduced by the SWE during the initial stages of dam break flows, and, thus, are then investigated using the Serre–Green–Naghdi equations for weakly-dispersive fully non-linear water waves, which are solved using a finite volume-finite difference scheme. PubDate: 2017-02-09 DOI: 10.1007/s10652-017-9512-5

Authors:Ignacio J. Moncho-Esteve; Frederik Folke; Manuel García-Villalba; Guillermo Palau-Salvador Abstract: Abstract This paper presents large eddy simulation of turbulent flow in a meandering open channel with smooth wall and rectangular cross-section. The Reynolds number based on the channel height is 40,000 and the aspect ratio of the cross-section is 4.48. The depth-averaged mean stream-wise velocity agree well to experimental measurements. In this specific case, two interacting cells are formed that swap from one bend to the other. Transport and mixing of a pollutant is analysed using three different positions of release, e.g. on the inner bank, on the outer bank and on the centre of the cross section. The obtained depth-average mean concentration profiles are reasonably consistent with available experimental data. The role of the secondary motions in the mixing processes is the main focus of the discussion. It is found that the mixing when the scalar is released on the centre of the cross-section is stronger and faster than the mixing of the scalar released on the sides. When the position of release is close to a bank side, the mixing is weaker and a clear concentration of scalar close to the corresponding side-wall can be observed in both cases. PubDate: 2017-02-02 DOI: 10.1007/s10652-017-9513-4

Authors:P. Angelidis; D. Kalpakis; V. Gyrikis; N. Kotsovinos Abstract: Abstract We consider the problem of the vertically upwards disposal of heavy brine sewage from a two-dimensional diffuser in a lighter, homogeneous, motionless and shallow ambient sea. The rejected high salinity water of seawater desalination plants for urban and agricultural uses is such a case of a two dimensional fountain. The disposal of brine sewage produces a negative buoyant jet due to its initial momentum, which impinges on the free surface, spreads laterally on it and then sinks downwards, because of the negative buoyancy. Laboratory experiments and dimensional considerations are used in this paper in order to investigate the spreading behavior (width) of the vertical fountain which impinges on the free surface of the shallow ambient fluid. The experimental results have been used to derive an equation relating the width at the free surface with the initial parameters of the flow. In addition, the experimentally measured dilution of the heavier brine sewage on the recipient’s surface is compared with the dilution which was calculated by a numerical simulation of a well-known commercial software package, CORJET (a CORMIX sub model). PubDate: 2017-01-16 DOI: 10.1007/s10652-017-9511-6

Authors:Debasish Pal; Koeli Ghoshal Abstract: Abstract Here we propose a theoretical model to compute the suspended grain-size distribution in fluvial environment. We derive the model based on the Reynolds averaged Navier–Stokes equation and the continuity equation of sediment phase. The model includes the effects of secondary current and stratification which are the cause of complex interaction between turbulence and grain-size distribution in the sediment-laden flow. Due to an immense importance of particle–particle and particle-turbulence interactions near the channel bed, we include their impacts in the boundary condition of the model. The present model has noteworthy contribution to demonstrate the phenomena of suspended grain-size distribution in the real world. Reported experimental data in literature shows well agreement with the numerical solution computed from the suggested model. The better computational accuracy of the present model is ascertained when the upper bound of calculated error between observed experimental data and computed values is found to be lowest for our model in comparison to a large number of existing models developed from different mathematical viewpoints. PubDate: 2017-01-12 DOI: 10.1007/s10652-017-9510-7

Authors:Cynthia E. Bluteau; Roger Pieters; Gregory A. Lawrence Abstract: Abstract Laboratory experiments have been performed to investigate the effects of salt exclusion on the behaviour of lakes with salinities up to 8 g L−1. At these salinities the freezing temperature is less than the temperature of maximum density and, unlike sea-ice, a reverse temperature stratification forms beneath the ice that can support at least some of the excluded salt. Temperature time series at four depths showed that salt exclusion drives cascades of localised overturning, while the persistence of reverse temperature stratification indicated that mixing was not complete. While our array of temperature sensors had insufficient spatial resolution to provide full details of the flow, we hypothesize that: at salinities of 1 and 2 g L−1 salt is released relatively uniformly and forms a layer of elevated salinity immediately below the ice, which supports double-diffusive salt-fingering; and at salinities of 4 and 8 g L−1, salt plumes penetrate the reverse stratification. After the ice melted, a relatively fresh surface layer formed above a more saline layer, sufficient to suppress spring turnover. Our measurements compare favourably with field observations from lakes, and highlight the importance of salt exclusion on biogeochemical processes in lakes. PubDate: 2017-01-12 DOI: 10.1007/s10652-016-9508-6

Authors:Meysam Fazeli; Ammar Safaie; Mirmosadegh Jamali Abstract: Abstract This paper is concerned with an analysis of image processing data to identify interfacial waves at the interface between two fluid layers in a laboratory flume. The interfacial waves are excited through a non-linear resonant interaction with a surface wave traveling in a wave flume filled with a two-layer fluid. A spatial harmonic analysis is proposed to extract information about the nonlinear evolution of the constituent waves from the interface oscillation data. The analysis is capable of handling different stages of the resonance in the wave flume and gives accurate and consistent results on time variations of the wave amplitudes. Contrary to a temporal harmonic analysis, the proposed method does not require prescription of a functional form for the amplitude variations. The experimental measurements are compared to the theoretical predictions and available methods. The proposed method provides an efficient solution for detecting these waves and capturing their behaviors. PubDate: 2017-01-04 DOI: 10.1007/s10652-016-9506-8

Authors:Thomas Ducrocq; Ludovic Cassan; Jacques Chorda; Hélène Roux Abstract: Abstract This paper investigates flows around a free surface piercing cylinder with Froude number F > 0.5 and Reynolds number around Re = 50,000. The aim of this work is to gain a better understanding of the flow behaviour in environmental systems such as fishways. The advances are based upon experimental and numerical results. Several flow discharges and slopes are tested to obtain both subcritical and supercritical flows. The drag force exerted on the cylinder is measured with the help of a torque gauge while the velocity field is obtained using particle velocimetry. For the numerical part, two URANS turbulence models are tested, the k- \(\omega\) SST and the RNG k- \(\varepsilon\) models using the OpenFOAM software suite for subcritical cases, and then compared with the corresponding experimental results. With fishways applications in mind, the changes in drag coefficient \(C_d\) versus Froude number and water depth are studied and experimental correlations proposed. We conclude that the most suitable URANS turbulence model for reproducing this kind of flow is the k- \(\omega\) SST model. PubDate: 2017-01-03 DOI: 10.1007/s10652-016-9505-9

Authors:Denise Hertwig; Gopal Patnaik; Bernd Leitl Abstract: Abstract Essential prerequisites for a thorough model evaluation are the availability of problem-specific, quality-controlled reference data and the use of model-specific comparison methods. The work presented here is motivated by the striking lack of proportion between the increasing use of large-eddy simulation (LES) as a standard technique in micro-meteorology and wind engineering and the level of scrutiny that is commonly applied to assess the quality of results obtained. We propose and apply an in-depth, multi-level validation concept that is specifically targeted at the time-dependency of mechanically induced shear-layer turbulence. Near-surface isothermal turbulent flow in a densely built-up city serves as the test scenario for the approach. High-resolution LES data are evaluated based on a comprehensive database of boundary-layer wind-tunnel measurements. From an exploratory data analysis of mean flow and turbulence statistics, a high level of agreement between simulation and experiment is apparent. Inspecting frequency distributions of the underlying instantaneous data proves to be necessary for a more rigorous assessment of the overall prediction quality. From velocity histograms local accuracy limitations due to a comparatively coarse building representation as well as particular strengths of the model to capture complex urban flow features with sufficient accuracy are readily determined. However, the analysis shows that further crucial information about the physical validity of the LES needs to be obtained through the comparison of eddy statistics, which is focused on in part II. Compared with methods that rely on single figures of merit, the multi-level validation strategy presented here supports conclusions about the simulation quality and the model’s fitness for its intended range of application through a deeper understanding of the unsteady structure of the flow. PubDate: 2016-12-31 DOI: 10.1007/s10652-016-9507-7

Authors:Denise Hertwig; Gopal Patnaik; Bernd Leitl Abstract: Abstract Time-dependent three-dimensional numerical simulations such as large-eddy simulation (LES) play an important role in fundamental research and practical applications in meteorology and wind engineering. Whether these simulations provide a sufficiently accurate picture of the time-dependent structure of the flow, however, is often not determined in enough detail. We propose an application-specific validation procedure for LES that focuses on the time dependent nature of mechanically induced shear-layer turbulence to derive information about strengths and limitations of the model. The validation procedure is tested for LES of turbulent flow in a complex city, for which reference data from wind-tunnel experiments are available. An initial comparison of mean flow statistics and frequency distributions was presented in part I. Part II focuses on comparing eddy statistics and flow structures. Analyses of integral time scales and auto-spectral energy densities show that the tested LES reproduces the temporal characteristics of energy-dominant and flux-carrying eddies accurately. Quadrant analysis of the vertical turbulent momentum flux reveals strong similarities between instantaneous ejection-sweep patterns in the LES and the laboratory flow, also showing comparable occurrence statistics of rare but strong flux events. A further comparison of wavelet-coefficient frequency distributions and associated high-order statistics reveals a strong agreement of location-dependent intermittency patterns induced by resolved eddies in the energy-production range. The validation concept enables wide-ranging conclusions to be drawn about the skill of turbulence-resolving simulations than the traditional approach of comparing only mean flow and turbulence statistics. Based on the accuracy levels determined, it can be stated that the tested LES is sufficiently accurate for its purpose of generating realistic urban wind fields that can be used to drive simpler dispersion models. PubDate: 2016-12-30 DOI: 10.1007/s10652-016-9504-x

Authors:Chao Yan; Heidi M. Nepf; Wei-Xi Huang; Gui-Xiang Cui Abstract: Abstract Predicting flow and mass transport in vegetated regions has a broad range of applications in ecology and engineering practice. This paper presents large eddy simulation (LES) of turbulent flow and scalar transport within a fully developed open-channel with submerged vegetation. To properly represent the scalar transport, an additional diffusivity was introduced within the canopy to account for the contribution of stem wakes, which were not resolved by the LES, to turbulent diffusion. The LES produced good agreement with the velocity and concentration fields measured in a flume experiment. The simulation revealed a secondary flow distributed symmetrically about the channel centerline, which differed significantly from the circulation in a bare channel. The secondary circulation accelerated the vertical spread of the plume both within and above the canopy layer. Quadrant analysis was used to identify the form and shape of canopy-scale turbulent structures within and above the vegetation canopy. Within the canopy, sweep events contributed more to momentum transfer than ejection events, whereas the opposite occurred above the canopy. The coherent structures were similar to those observed in terrestrial canopies, but smaller in scale due to the constraint of the water surface. PubDate: 2016-12-29 DOI: 10.1007/s10652-016-9503-y