Authors:Meysam Fazeli; Ammar Safaie; Mirmosadegh Jamali 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: 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:Hang Wang; Hubert Chanson Pages: 1087 - 1110 Abstract: A physical study of hydraulic jump is often undertaken using down-scaled Froude-similar models with Reynolds numbers much smaller than in prototype (e.g. spillway stilling basins). The potential viscous scale effects may affect a number of physical processes including turbulence development and air entrainment, thus challenging the extrapolation of laboratory data to the prediction of prototype conditions or justification of numerical modelling. This paper presents an experimental study of hydraulic jumps with a particular focus on the scale effects in terms of free-surface fluctuation and deformation, bubble advection and diffusion, bubble-turbulence interaction and turbulence dissipation. A broad range of free-surface, air–water flow and turbulence properties were measured systematically for Froude numbers from 3.8 to 10 and Reynolds numbers from 2.1 × 104 to 1.6 × 105. Based upon self-similarities in the longitudinal evolution of a number of characteristic flow properties, the analytical expressions of time-averaged roller surface profile, void fraction distribution and longitudinal velocity distribution were derived for given Froude number. The roller surface dynamics were found free of scale effects in terms of fluctuation amplitudes but the characteristic frequencies were scale-sensitive. While some air–water flow parameters such as bubble count rate, bubble chord time distribution and bubble grouping behaviour could only be correctly quantified at full-scale prototype conditions, the aeration level and turbulent scales might be estimated with satisfactory accuracy for engineering applications given a model Reynolds number no less than 4 × 10 to 6 × 104. PubDate: 2016-12-01 DOI: 10.1007/s10652-016-9466-z Issue No:Vol. 16, No. 6 (2016)

Authors:D. Termini Pages: 1111 - 1127 Abstract: Vegetation is a critical component of aquatic ecosystems and exerts an important role in river morphodynamics. This paper describes the results of laboratory study carried out to investigate the role of vegetation on flow field and erosion process developing in non-equilibrium situations. Particular attention is paid to scour evolving downstream of a rigid bed and to the effect of a vegetated carpet used as a protective measure against scouring. The reported experimental study shows that the introduction of the vegetated carpet results in a more favourable configuration than in its absence determining reduced values of flow velocity, turbulence intensity and bed shear stress, and modifying their patterns, downstream of the rigid bed. Consequently, the vegetated carpet protects the bed against the erosive action of flow and determines a beneficial effect with respect the stability of the structure. Experiments also show that a scour hole forms downstream of the vegetated carpet but the geometrical characteristics (length and depth) of such a scour hole are reduced by about 50 %compared to those obtained downstream of the rigid bed in absence of vegetation. PubDate: 2016-12-01 DOI: 10.1007/s10652-016-9467-y Issue No:Vol. 16, No. 6 (2016)

Authors:Yu-hang Guo; Bao-zhi Pan; Wen-bin Liu Pages: 1129 - 1141 Abstract: Relative permeability and resistivity index are important parameters in petrophysics experiments and reservoir evaluations. According to the results of previous studies, there exists a relationship between resistivity index and relative permeability in in situ reservoir condition and a lot of transformation models were established. In this paper, we compared the relative permeability which was calculated from the resistivity index measured in different conditions (in situ reservoir condition and conventional condition), and the result shows that the relative permeabilities are similar. If core samples are fragile and not resistant to high temperature or high pressure, it is possible to use resistivity index measured in conventional condition to get the relative permeability which provides a convenient way for experimental measurement and reservoir evaluation. In practical applications, the results show that the pore structure make a great influence on the relationship between wetting phase tortuosity ratio and resistivity index. Therefore, in this study we took the difference of pore structure into account when deciding model parameters by core analysis, and the verification results are basically consistent with the laboratory measurement results. PubDate: 2016-12-01 DOI: 10.1007/s10652-016-9469-9 Issue No:Vol. 16, No. 6 (2016)

Authors:Michał Korycki; Lech Łobocki; Andrzej Wyszogrodzki Pages: 1143 - 1171 Abstract: Results of large-eddy simulations of stably stratified atmospheric flow around an isolated, complex-shaped tall building are presented. The study focuses on the identification of flow structures in the building wake in high and low Froude number regimes. A qualitative comparison of results with available literature data and existing theories is presented. In addition to flow structures identified in earlier studies such as the horseshoe and recirculation eddy vortices, we analyze a stationary disturbance akin to mountain gravity wave, and a complex vortex structure associated with this wave, consisting of multiple symmetric pairs of vortices. The Froude number appears to be the principal parameter controlling the structure of the wake, waves and vortex pattern. PubDate: 2016-12-01 DOI: 10.1007/s10652-016-9470-3 Issue No:Vol. 16, No. 6 (2016)

Authors:Victor Dupuis; Sébastien Proust; Céline Berni; André Paquier Pages: 1173 - 1193 Abstract: Open channel flows subjected to a longitudinal transition in roughness, from bed friction to emergent cylinder drag and vice versa, are investigated experimentally in an 18-m-long laboratory flume. These are compared to uniform flows subject to (1) bed roughness only and (2) an array of emergent vertical cylinders installed on bed roughness. The near-bed region is investigated in detail for uniform flows through the cylinder array. The water column can be divided into two parts: a region of constant velocity and a boundary layer near the channel bed. In the latter region, a local increase in velocity, or velocity bulge, is observed in line of a cylinder row. The velocity bulge may be related to the disorganization of the von Kármán vortex street by the bed-induced turbulence, resulting in reduced momentum loss in the cylinder wake. The boundary layer height is found to be independent of water depth and bed roughness (smooth or rough bottom). Strong oscillations of the free surface (seiching) are observed. Oscillation amplitude is dependent on the longitudinal position within the cylinder array and is found to decrease with decreasing array length. When water depth/boundary layer height ratio is close to unity, the disorganization of the von Kármán vortex street throughout the water column prevents seiching from occurring. In the case of roughness transition flows, the water depth is found to vary only upstream of the change in roughness. Vertical profiles of velocity and turbulence are self-similar upstream of the transition and collapse with the uniform flow profiles. Downstream of the roughness change, velocity and turbulence vary over a distance of 35–50 times the water depth. Roughness transition flows show that seiching is lowered by flow non-uniformity. A 1D momentum equation integrating bed friction and drag force exerted by the cylinder array predicts accurately the water surface profile (0.9 % mean relative error). The computed profiles show that upstream of the transition, flow depth varies over a distance of about 2600 times the uniform water depth of the upstream roughness. The 1D equation is solved analytically for zero bed friction. PubDate: 2016-12-01 DOI: 10.1007/s10652-016-9471-2 Issue No:Vol. 16, No. 6 (2016)

Authors:Juan Pablo Toro; Fabián A. Bombardelli; Joongcheol Paik; Inês Meireles; António Amador Pages: 1195 - 1221 Abstract: In this work we address the mean flow and turbulence statistics in the non-aerated region of a stepped spillway by using two different numerical strategies in two dimensions. First, we present results regarding the flow in a large portion of the spillway, simulated with a volume of fluid (VoF) method to capture the position of the free surface (case A). Numerically-obtained data are in very good agreement with particle image velocimetry (PIV) data; further, results suggest that profiles of mean velocity, turbulent kinetic energy (TKE) and dissipation rate of TKE at the step edges are approximately self-similar. It was also found that values of TKE and dissipation rate of TKE in the boundary layer development region follow universal similarity laws which are valid for open-channel flows. In addition, the field of simulated dimensionless pressure and pressure distributions at the step edges are qualitatively similar to those reported in a recent experimental work. Second, additional simulations were developed as a pressure-driven flow for only a portion of the spillway (case B). This was possible due to prior knowledge of the water depths. We show that, despite the fact that the pressure field can not be interpreted as in case A, the numerical simulations closely reproduce the experimental data regarding averaged velocity, vorticity, and the turbulence statistics. It was also found that turbulence intensity profiles in the intermediate region are consistent with published experimental results for open-channel flows. These numerical results offer new avenues for the simulation of portions of stepped spillways to assess the physics at the inception point of air entrainment with more sophisticated turbulence closures. PubDate: 2016-12-01 DOI: 10.1007/s10652-016-9472-1 Issue No:Vol. 16, No. 6 (2016)

Authors:Denise Hertwig; Gopal Patnaik; Bernd Leitl 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: 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: 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

Authors:Snehasis Kundu; Koeli Ghoshal Abstract: This paper presents a mathematical model to investigate type II profile of suspension concentration distribution (i.e., the concentration profile where the maximum concentration appears at some distance above the bed surface) in a steady, uniform turbulent flow through open-channels. Starting from the mass and momentum conservation equations of two-phase flow, a theoretical model has been derived. The distribution equation is derived considering the effects of fluid lift force, drag force, particle inertia, particle–particle interactions, particle velocity fluctuations and drift diffusion. The equation is solved numerically and is compared with available experimental data as well as with other models existing in the literature. Good agreement between the observed value and computed result, and minimum error in comparison to other models indicate that the present model can be applied in predicting particle concentration distribution for type II profile for a wide range of flow conditions. The proposed model is also able to show the transition from type I profile to type II profile. PubDate: 2016-12-27 DOI: 10.1007/s10652-016-9498-4

Authors:C. Cenedese; R. Nokes; J. Hyatt Abstract: In nature, density driven currents often flow over or within a bottom roughness: a sea breeze encountering tall buildings, a shallow flow encountering aquatic vegetation, or a dense oceanic current flowing over a rough bottom. Laboratory experiments investigating the mechanisms by which bottom roughness enhances or inhibits entrainment and dilution in a lock-exchange dense gravity current have been conducted. The bottom roughness has been idealized by an array of vertical, rigid cylinders. Both spacing (sparse vs. dense configuration) and height of the roughness elements compared with the height of the current have been varied. Two-dimensional density fields have been obtained. Experimental results suggest that enhancement of the entrainment/dilution of the current can occur due to two different mechanisms. For a sparse configuration, the dense current propagates between the cylinders and the entrainment is enhanced by the vortices generated in the wake of the cylindrical obstacles. For a dense configuration, the dense current rides on top of the cylinders and the dilution is enhanced by the onset of convective instability between the dense current above the cylinders and the ambient lighter water between the cylinders. For low values of the ratio of the cylinder to lock height \(\lambda \) the dense current behavior approaches that of a current over a smooth bottom, while the largest deviations from the smooth bottom case are observed for large values of \(\lambda \) . PubDate: 2016-12-26 DOI: 10.1007/s10652-016-9501-0

Authors:Elizabeth Smith; Evgeni Fedorovich; Alan Shapiro Abstract: This study focuses on the inertial oscillation aspect of the nocturnal low-level jet (NLLJ). In the context of the Ekman model solutions, conceptual NLLJ inertial oscillation analytical frameworks proposed by Blackadar in 1957 and Shapiro and Fedorovich and van de Wiel et al. in 2010 are compared. Considering a NLLJ produced via direct numerical simulation over flat terrain with no baroclinic influence as a reference case, the deficiencies of each framework in representing a realistic NLLJ are assessed. The Blackadar theory results in unrealistic wind profiles near the surface. While extensions of Blackadar’s framework by Shapiro and Fedorovich and van de Wiel et al. produce more realistic NLLJs, the simpler approach taken by van de Wiel et al. does not describe the NLLJ wind hodograph at later times sufficiently in qualitative terms. PubDate: 2016-12-22 DOI: 10.1007/s10652-016-9502-z

Authors:Y. Jiang; X. Liu Abstract: The dynamics of density current over a bottom covered by macro-roughness elements were investigated by laboratory experiments and a computational model using large eddy simulations. The macro-roughness considered had significant size in comparison with the scale of density current. Five different roughness conditions were considered, namely flat bottom (for reference), half spheres, fine gravels, medium gravels, and large gravels. These bottom conditions had variations in roughness element size, shape, angularity, and spatial configuration. The density current was a lock-exchange type with a density difference of 1% between the two fluids initially separated by a gate in the middle. In the computational model, the roughness was captured using two different methods depending on the size of the roughness elements. For the large roughness elements, i.e., the half spheres and the medium and large gravels, an immersed boundary method was used to resolve the surface of each gravel, which was obtained through 3D laser scanning. The realistic and physically correct placement of these scanned objects in the simulation domain was achieved using a computer tool which can detect the collision of rigid bodies and simulate their dynamics. For the fine gravels, a rough wall function was used. The computational model was validated with the data measured in the experiments, including front position and velocity, and point velocity measurement within the current. The results show that density currents over macro-roughness have distinct behavior from those over a smooth boundary. The characteristics (size, angularity, and pavement pattern) of the macro-roughness play a key role in the current development. Macro-roughness significantly retards the front propagation and enhances entrainment. PubDate: 2016-12-21 DOI: 10.1007/s10652-016-9500-1

Abstract: Windage, the additional direct, wind-induced drift of material floating at the free surface of the ocean, plays a crucial role in the surface transport of biological and contaminant material. Lagrangian coherent structures (LCS) uncover the hidden organizing structures that underlie material transport by fluid flows. Despite numerous studies in which LCS ideas have been applied to ocean surface transport scenarios, such as oil spills, debris fields and biological material, there has been no consideration of the influence of windage on LCS. Here we investigate and demonstrate the impact of windage on ocean surface LCS via a case study of the ocean surrounding the UNESCO World Heritage Ningaloo coral reef coast in Western Australia. We demonstrate that the inclusion of windage is necessary when applying LCS to the study of surface transport of any floating material in the ocean. PubDate: 2016-12-16 DOI: 10.1007/s10652-016-9499-3

Abstract: This study implemented three analytical models to investigate the lateral distribution of depth-averaged streamwise velocity in a rectangular channel with lateral, unevenly-distributed, flexible submerged vegetation. Secondary flow, vegetation drag, and turbulent shear were introduced into the momentum equations to represent the interaction between vegetation and flow. Comparison of model results and experimental data indicated that predictions were improved with a mixing layer model, which considers the secondary flow term in the mixing region, particularly for channels with a high aspect ratio. The research established a relationship between the vegetation drag coefficient and the Reynolds number. A sensitivity analysis of the dimensionless eddy viscosity coefficient and bed friction factor indicated that the coefficient had as significant an impact as the second factor on the lateral velocity profile. A reasonable dimensionless eddy viscosity coefficient is essential to predict velocity accurately. PubDate: 2016-10-04 DOI: 10.1007/s10652-016-9485-9

Authors:Moisés Brito; João Fernandes; João Bento Leal Abstract: The main goal of this study is the 3D numerical simulation of river flows with submerged vegetated floodplains. Since, vegetation layers are usually dense and present a large spatial heterogeneity they are here represented as a porous media. Standard semi-empirical relations drawn for porous beds packed with non-spherical particles are used to estimate the porous media parameters based on the averaged geometry of the vegetation elements. Thus, eliminating the uncertainty arising from a bulk drag coefficient approach and allowing the use of a coarser mesh. The free flow is described by Reynolds-averaged Navier–Stokes (RANS) equations, whereas the porous media flow is described by the volumetric-average of RANS equations. The volume-of-fluid method and an anisotropic explicit algebraic Reynolds stress model are used for free-surface and turbulence closure, respectively. The simulation approach is validated against results by other authors featuring vegetated flows in horizontal and rectangular open-channel. The computed results show that the time-averaged streamwise velocity and Reynolds shear stress vertical profiles are properly simulated. The validated approach was applied to simulate compound open-channel flows with submerged vegetated floodplains and compared with data obtained in an experimental facility. The results show that the proposed porous media approach is adequate to simulate flows with submerged vegetation on the floodplains. PubDate: 2016-09-09 DOI: 10.1007/s10652-016-9481-0

Authors:Nino Krvavica; Vanja Travaš; Nevenka Ožanić Abstract: The interfacial friction and entrainment were investigated in a microtidal salt-wedge estuary. A detailed sampling campaign was conducted in the Rječina River estuary in Croatia from January 2014 to June 2015. The observed vertical profiles of salinity s and temperature T confirmed the presence of a highly stratified estuary, represented by an upper layer of freshwater separated from a lower salt-wedge by a sharp density interface. The entrainment rate E across the interface was estimated by a two-layer box-model, based on the observed freshwater flow rate Q and layer-averaged salinity. Interfacial friction factor \(\lambda _i\) was estimated by fitting the results of a numerical model to the observed interface depths. For this purpose we applied a numerical two-layer shallow water model extended to account for irregular non-prismatic cross sections of the channel. We found that in microtidal conditions, the strength of the stratification is reduced with increasing Q. Furthermore, we found that as Q increases, so does the shear velocity, the interfacial friction factor and the vertical mixing across the interface. More detail analysis showed that E may be parametrized by bulk non-dimensional parameters, in particular, a combination of bulk Richardson number Ri and average friction factor \(\lambda\) , which accounts for the channel bed friction and the interfacial friction. On the other hand, \(\lambda _i\) can be linked to a combination of Reynolds number Re and bulk Richardson Ri. Contrary to previous studies, we showed that in field conditions, \(\lambda _i\) may increase with Re. PubDate: 2016-08-26 DOI: 10.1007/s10652-016-9480-1