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 Environmental Fluid Mechanics   [SJR: 0.437]   [H-I: 24]   [2 followers]  Follow         Hybrid journal (It can contain Open Access articles)    ISSN (Print) 1573-1510 - ISSN (Online) 1567-7419    Published by Springer-Verlag  [2281 journals]
• Effect of upstream flow regime on street canyon flow mean turbulence
statistics
• Abstract: Abstract The effect on the flow over a street canyon (lateral length/height, L/h $$=$$ 30) of using either 3D (cube) or 2D (rectangular block) upstream roughness arrays, of the same height as the canyon, has been studied for two streamwise canyon width to height aspect ratios (AR $$=$$ W/h) of 1 and 3, in a wind tunnel using Particle Image Velocimetry. The mean streamwise velocity, shear stress, turbulent intensities and length scales, together with shear layer boundaries and mass fluxes across the canyon opening are presented for different combinations of skimming and wake-interference regimes using different upstream roughness and canyon configurations. These results show significant trends with canyon aspect ratio and roughness array plan area packing density $$(\uplambda _{\mathrm{p}})$$ with respect to 2D and 3D configurations. The mean streamwise velocity for configurations of equal $$\uplambda _{\mathrm{p}}$$ is higher in 3D than 2D configurations, while the spatially averaged shear stress is shown to be lower in 3D than 2D configurations. The relative contribution to the total turbulent kinetic energy (TKE) demonstrates that staggered and aligned arrays or 2D and 3D arrays do not produce similar profiles of TKE. Finally, the integral length scale is larger in 2D cases than 3D cases of equal $$\uplambda _{\mathrm{p}}$$ . Urban air quality is a significant concern for human health. By investigating the influence of upstream roughness on canyon flow one can determine which cases or flow regimes in both the upstream roughness and canyon will result in decreased ventilation and negatively effect the air quality of urban areas. From the present work decreased ventilation occurs in the skimming flow regime and is lowest in the case of upstream 2D bar roughness with $$\uplambda _{\mathrm{p}} = 50~\%$$ and canyon AR $$=$$ 1.
PubDate: 2015-08-01

• Bedload transport for heterogeneous sediments
• Abstract: Abstract A large panel of instruments was deployed in the Eastern English Channel to measure the evolution of bedload fluxes during a tidal cycle for two different sites. The first one was characterized by a sandy bed with a low dispersion in size while the other study site implied graded sediments with grain sizes ranging from fine sands to granules. The in situ results obtained were compared with predictions of total bedload fluxes by classical models. A good agreement was found for homogeneous sediments with these formulas. In the case of size heterogeneous sediments, a fractionwise approach, involving a hiding-exposure coefficient and a hindrance factor, provided better predictions of bedload fluxes, but still some discrepancies were noticed. Present results revealed that the consideration of particle shape in formulas through the circularity index enhanced the estimations of bedload transport rates. A new adjustment of Wu et al.’s (J Hydraul Res 38:427–434, 2000) formula was proposed and a very good agreement was obtained between the measured and predicted values.
PubDate: 2015-08-01

• Effects of a sharp change of emergent vegetation distributions on
thermally driven flow over a slope
• Abstract: Abstract In this study, thermally driven flow within a sharp change of rooted emergent vegetation distributions is discussed. A conceptual and linear model is developed to study the interferences and competitions between two forcing mechanisms (vegetation shading and sloping bottom effects) on circulation patterns. The two forcing mechanisms can counteract or reinforce each other according to the presence of emergent vegetation in shallow or deep regions. For tall vegetation in shallows and open water in deep regions, vegetation shading leads to opposite temperature gradients against those from a slope. A critical vegetation blockage $$B_{\textit{critical}}$$ is found to cause a minimum horizontal exchange flowrate and divide the flow regime as topographic or vegetation shading dominated. In clear water, opposite circulation from the bottom is produced and gradually spreads to the whole water column. On the contrary, in turbid water, the opposite circulation emerges from the water surface and gradually propagates to the sloping bottom. The circulation in turbid water is more easily affected by a sharp change of vegetation distributions rather than clear water. For open in shallows and tall vegetation in deep water, pressure gradients from vegetation shading can enhance buoyancy from the sloping bottom and lead to greater horizontal exchange flowrates. Due to a sharp change of emergent vegetation distributions, horizontal exchange flowrates are possibly increased and greater than those without vegetation. In short, circulation patterns and strength can be significantly affected by a sharp change of emergent vegetation distributions.
PubDate: 2015-08-01

• Experimental study of wave dynamics in coastal wetlands
• Abstract: Abstract This paper presents laboratory experiments of wave-driven hydrodynamics in a three-dimensional laboratory model of constructed coastal wetlands. The simulated wetland plants were placed on the tops of conically-shaped mounds, such that the laboratory model was dynamically similar to marsh mounds constructed in Dalehite Cove in Galveston Bay, Texas. Three marsh mounds were placed in the three-dimensional wave basin of the Haynes Coastal Engineering Laboratory at Texas A&M University, with the center of the central wetland mound located in the center of the tank along a plane of symmetry in the alongshore direction. The experiments included two water depths, corresponding to emergent and submerged vegetation, and four wave conditions, typical of wind-driven waves and ocean swell. The wave conditions were designed so that the waves would break on the offshore slope of the wetland mounds, sending a strong swash current through the vegetated patches. Three different spacings between the wetland mounds were tested. To understand the effects of vegetation, all experiments were repeated with and without simulated plants. Measurements were made throughout the nearshore region surrounding the wetland mounds using a dense array of acoustic Doppler velocimeters and capacitance wave gauges. These data were analyzed to quantify the significant wave height, phase average wave field and phase lags, wave energy dissipation over the vegetated patches, mean surface water levels, and the near-bottom current field. The significant wave height and energy dissipation results demonstrated that the bathymetry is the dominant mechanism for wave attenuation for this design. The presence of plants primarily increases the rate of wave attenuation through the vegetation and causes a blockage effect on flow through the vegetation. The nearshore circulation is most evident in the water level and velocity data. In the narrowest mound spacing, flow is obstructed in the channel between mounds by the mound slope and forced over the wetlands. The close mound spacing also retains water in the nearshore, resulting in a large setup and lower flows through the channel. As the spacing increases, flow is less obstructed in the channel. This allows for more refraction of waves off the mounds and deflection of flow around the plant patches, yielding higher recirculating flow through the channel between mounds. An optimal balance of unobstructed flow in the channel, wave dissipation over the mounds, and modest setup in the nearshore results when the edge-to-edge plant spacing is equal to the mound base diameter.
PubDate: 2015-08-01

• Flow patterns around two neighboring patches of emergent vegetation and
possible implications for deposition and vegetation growth
• Abstract: Abstract The flow around two neighboring, circular, vegetation patches of equal diameter $$(D)$$ was investigated using computational fluid dynamics. Depending on the patches’ transverse and longitudinal center-to-center spacing $$(T$$ and $$L$$ , respectively), several distinct flow patterns were observed. The patterns are compared to flow near an isolated patch. The key flow patterns were interpreted in terms of implications for deposition. Deposition maps were calculated for two different threshold velocities: $$0.5U_{0}$$ and $$0.7U_{0}$$ , where $$U_{0}$$ is the free stream velocity. When the two patches were far away from each other, the interaction of their wakes was weak, and the flow and deposition pattern around each patch resembled that of a single, isolated patch. When the patches were very close, wake interaction took place, resulting in additional deposition along the centerline between the two patches, but further downstream than the deposition in line with each patch. For some intermediate patch spacings, the wake of the upstream patch was dramatically shortened, relative to an isolated patch, and the wake of the downstream patch was lengthened. The results show that flow distribution is influenced by interaction between neighboring vegetation patches and suggest that this may create feedbacks that influence the evolution of vegetated landscapes.
PubDate: 2015-08-01

• Towards aeraulic simulations at urban scale using the lattice Boltzmann
method
• Abstract: Abstract The lattice Boltzmann method (LBM) is an innovative approach in computational fluid dynamics (CFD). Due to the underlying lattice structure, the LBM is inherently parallel and therefore well suited for high performance computing. Its application to outdoor aeraulic studies is promising, e.g. applied on complex urban configurations, as an alternative approach to the commonplace Reynolds-averaged Navier–Stokes and large eddy simulation methods based on the Navier–Stokes equations. Emerging many-core devices, such as graphic processing units (GPUs), nowadays make possible to run very large scale simulations on rather inexpensive hardware. In this paper, we present simulation results obtained using our multi-GPU LBM solver. For validation purpose, we study the flow around a wall-mounted cube and show agreement with previously published experimental results. Furthermore, we discuss larger scale flow simulations involving nine cubes which demonstrate the practicability of CFD simulations in building external aeraulics.
PubDate: 2015-08-01

• Shallow-water solutions for gravity currents in non-rectangular cross-area
channels with stratified ambient
• Abstract: Abstract We consider the propagation of a high-Reynolds-number gravity current in a horizontal channel along the horizontal coordinate $$x$$ . The current is of constant density, $$\rho _c$$ , and the ambient has a linear stable stratification, from $$\rho _{b}$$ at the bottom $$z=0$$ to $$\rho _{o}$$ at the top $$z=H$$ . The cross-section of the channel is given by the quite general $$-f_1(z)\le y \le f_2(z)$$ for $$0 \le z \le H$$ . We develop a one-layer shallow-water (SW) formulation, and we implement it for the solution of a gravity current of fixed volume released from a lock (we focus on a “heavy” bottom current with $$\rho _{c}\ge \rho _{b}$$ ). The dependent variables are the position of the interface, $$h(x,t)$$ , and the speed (averaged over the area of the current), $$u(x,t)$$ , where $$t$$ is the time. The non-rectangular cross-section geometry enters the formulation via $$f(h)$$ and integrals of $$f(z), z f(z)$$ and $$z^2 f(z)$$ , where $$f(z) = f_1(z) + f_2(z)$$ is the width of the channel. For a given geometry $$f(z)$$ , the free input parameters are (1) the height ratio $$H/h_0$$ of ambient to lock; and (2) the stratification parameter $$S = (\rho _{b}- \rho _{o})/(\rho _{c}- \rho _{o})$$ . In general, the SW equations of motion are a hyperbolic PDE system which we solve by a finite-difference method. However, we show that the initial motion displays a “slumping” stage with constant speed of propagation; this can be calculated exactly by the method of characteristics. An analytical solution for the long-time self-similar propagation is also presented for $$S=1$$ and the power-law $$f(z) = bz^\alpha$$ cross section profile. Solutions are presented for various stratification and typical geometries: power-law ( $$f(z) = b z^\alpha$$ , where $$b, \alpha$$ are positive constants), power-law B ( $$f(z) = b( H-z)^\alpha$$ ), trapezoidal, and circle. In general, the increase of the stratification parameter $$S$$ causes a reduction of the speed of propagation, but the details depend on the geometry of the cross section. We also show that, upon a simple transformation, the solutions for the bottom (“heavy”) current can be used for the prediction of top (“light”) currents The present solutio...
PubDate: 2015-08-01

• Erratum to: Shallow-water solutions for gravity currents in
non-rectangular cross-area channels with stratified ambient
• PubDate: 2015-08-01

• Aeration and air–water mass transfer on stepped chutes with
embankment dam slopes
• Abstract: Abstract Stepped spillway flows are characterised by significant free-surface aeration downstream of the inception point of air entrainment. The stepped design is advantageous for applications which require large energy dissipation and strong flow aeration. While the energy dissipation rate for embankment stepped spillways was studied previously, the optimum design for aeration and air–water mass transfer is not known. Herein new air–water flow experiments were conducted on several stepped spillways with embankment dam slopes using with phase-detection intrusive probes. The present data were compared with previous studies in terms of energy dissipation, flow aeration and air–water mass transfer. The air–water mass transfer was calculated based upon air–water flow measurements in terms of dissolved oxygen. The re-oxygenation rates were compared with previous studies comprising both conductivity and direct dissolved oxygen measurements. The comparison highlighted a monotonic increase of aeration efficiency with energy dissipation rate. All data were in good agreement independent of channel slopes, stepped configuration and sensor size. The data confirmed the effects of strong air–water interactions within the bulk of the flow for both energy dissipation and re-oxygenation performances.
PubDate: 2015-08-01

• Response to: “Comment on: effect of flow confinement on the
hydrodynamics of circular impinging jets: implications for erosion
assessment” by Spyros Beltaos
• PubDate: 2015-08-01

• Aeration performances of a gabion stepped weir with and without capping
• Abstract: Abstract The stepped spillway design has been used for more than 3,300 years. A simple structure is the gabion stepped weir. A laboratory study was performed herein in a large size facility. Three gabion stepped weirs were tested with and without capping, as well as a flat impervious stepped configuration. For each configuration, detailed air–water flow measurements were conducted systematically for a range of discharges. The observations highlighted the seepage flow through the gabions and the interactions between seepage and overflow. The air–water flow properties showed that the air concentration, bubble count rate and specific interface data presented lower quantitative values in the gabion stepped weir, compared to those on the impervious stepped chute, while higher velocities were measured at the downstream end of the gabion stepped chute. The re-oxygenation rate was deduced from the integration of the mass transfer equation using air–water interfacial area and velocity measurements. The aeration performances of the gabion stepped weir were lesser than on the flat impervious stepped chute, but for the lowest discharge. For the two configurations with step capping, the resulting flow properties were close to those on the impervious stepped configuration.
PubDate: 2015-08-01

• Comment on: “Effect of flow confinement on the hydrodynamics of
circular impinging jets: implications for erosion assessment”; by
Seyed Mohammad Ghaneeizad, Joseph F. Atkinson, and Sean J. Bennett,
Environ Fluid Mech: DOI 10.1007/s10652-014-9354-3
• Abstract: Abstract This brief comment clarifies certain aspects concerning the distribution of the radial wall shear stress that is generated by jets impinging normally on flat plates.
PubDate: 2015-08-01

• Definition of mixing zones in rivers
• Abstract: Abstract The adequate definition of the mixing zone generated by the discharge of an effluent is of great importance, as it serves as support for the environmental authorities on the decision-making about the authorization of the discharge. The evolution of the mixing zone of an effluent is affected by different kind of phenomena as temporal variations on the hydrodynamic conditions, spatial variations in the geomorphology and bathymetry of the receiving water, etc. The correct definition of the mixing zone should take into account these factors, for which the use of mathematical modelling is needed. The turbulent hydrodynamic processes in the near field of the discharge and in the far field occur at different spatial and temporal scales. The mathematical model needs to be able of simulating the hydrodynamic and transport processes on both fields. The present paper proposes a methodology to be followed when the prediction of the extents of the mixing zone generated by an effluent discharged into a river is needed. The methodology consists of the obtaining of the necessary hydrodynamic and pollutant concentration data on the effluent and on the receiving water; the building of the adequate calculation meshes for the modelling; the calibration and validation of the model; and finally the definition of the critical conditions for the prediction of the behaviour of the mixing zone. An example of application of the proposed methodology is shown for a real case, the discharge of the WWTP of Casar de Periedo town in the Saja River, Cantabria, Spain, for which field data have been measured. The prediction of the extents of the mixing zone for this case was made using a hydrodynamic two-dimensional depth-averaged long wave model jointly with a transport model. In order to simulate the near field and the far field jointly, an Embedded mesh system was built. For the Embedded mesh system, it was needed the establishment of conditions for the information exchange between the meshes.
PubDate: 2015-07-28

• Mid-depth oil concentration due to vertical oil dispersion in a regular
wave field
• Abstract: Abstract An experimental program is organized to investigate the vertical oil dispersion of surface oil spills in a regular wave field. Various waves characteristics and different volumes of oil spills are tested to assess the oil concentration variations at two sampling stations. It is found that the oil concentration due to vertical oil dispersion follows an ascending diagram to reach a maximum and then decreases while oil slick passes the location. The maximum mid-depth oil concentration (Cmax) at the farther sampling station is 30–50 % less than the concentration at the closer sampling station to the spill location. A 50 % increase in oil spill volume causes 30–60 % growth in oil concentrations. The relations between oil concentration and important parameters such as wave characteristics, amount of spilled oil and the distance of sampling stations from the spill location are indicated and also oil concentration variations are quantified. Two equations are derived through statistical analysis of the obtained experimental data, which estimate the magnitude and time of maximum oil concentration.
PubDate: 2015-07-18

• Computation of the Basset force: recent advances and environmental flow
applications
• Abstract: Abstract When numerically integrating the equation describing the motion of a particle in a carrier fluid, the computation of the Basset (history) force becomes by far the most expensive and cumbersome, as opposed to forces such as drag, virtual mass, lift, buoyancy and Magnus. The expression representing the Basset force constitutes an integro-differential term whose standard integrand is singular when the upper integration limit is enforced. These shortcomings have led some researchers to either disregard or outright neglect the contribution of the Basset force to the total force, even in those cases where it may yield to important errors in the determination of particle trajectories in the computation of sediment transport and other environmental flows. This work is devoted to review four recent contributions associated with the computation of the Basset force, and to compare their proposals to diminish the inherent problems of the term integration. All papers, except one, use variants of a window-based approach; the most recent contribution, in turn, employs a specialized quadrature to increase the accuracy of the computation. An analysis was carried out to compare CPU computation times, rates of convergence and accuracy of the approximations versus a known analytical solution. All methods provide sound solutions to the issues associated with the computation of the Basset force; further, a road map to select the best solution for each given problem is provided. Finally, we discuss the implications of the techniques for the simulation of sediment transport processes and other environmental flows.
PubDate: 2015-07-17

• Revisiting the flow regimes for urban street canyons using the numerical
Green’s function
• Abstract: Abstract Vortex interactions within a two-dimensional street canyon are analysed using the numerical Green’s function. On account of the inhomogeneity of the domain, vortex interactions are asymmetric: the influence of a street-level vorticity source on the roof-level shear layer differs from that of the latter on the street level. Consequently the magnitudes of the induced vertical velocities are maximised at different aspect ratios. It is argued that the transition from isolated roughness to wake interference is related to the onset of strong long-range interactions while the transition from wake interference to skimming flow is related to the weakening of these interactions. The Green’s function analysis is verified using three-dimensional large-eddy simulations.
PubDate: 2015-07-16

• Modeling investigation of asymmetric tidal mixing and residual circulation
in a partially stratified estuary
• Abstract: Abstract A three-dimensional hydrodynamic model was applied to the Danshuei River estuarine system in northern Taiwan to investigate the influence of flood-ebb, spring-neap tidal cycles, and salinity distribution on tidal mixing, residual circulation, stratification, and tidal asymmetry. The model was validated using observational data collected in 2008. The results from the model agreed well with observations of water surface elevation, tidal currents, and salinity. It was found that the depth-averaged tidal current during flood tide is weaker with a shorter duration than that during ebb tide in the estuary, which was attributed to tidal asymmetry. Vertical profiles of salinity, flow, eddy diffusivity, and Richardson number also showed a marked asymmetry between flood and ebb tides. Bottom boundary stresses were higher during flood tides than during ebb tides, resulting in more mixing occurrence and consequently decreasing the Richardson numbers. The tidally averaged salinity was more stratified during neap tides than during spring tides because the presence of the stronger vertical diffusivity and turbulent kinetic energy during spring tides. The modeling results also confirmed that the residual circulation was stronger during neap tides than during spring tides.
PubDate: 2015-06-26

• Dressler’s theory for curved topography flows: iterative derivation,
transcritical flow solutions and higher-order wave-type equations
• Abstract: Abstract The Dressler equations are a system of two non-linear partial differential equations for shallow fluid flows over curved topography. The theory originated from an asymptotic stretching method formulating the equations of motion in terrain-fitted curvilinear coordinates. Apparently, these equations failed to produce a transcritical flow profile changing from sub- to supercritical flow conditions. Further, wave-like motions over a flat bottom are excluded because the bed-normal velocity component is not accounted for. However, the theory was found relevant for several environmental flow problems including density currents over mountains and valleys, seepage flow in hillslope hydrology, the development of antidunes, the formation of geological deposits from hyper-concentrated flows, and shallow-water flow modeling in hydraulics. In this work, Dressler’s theory is developed in an alternative way by a systematic iteration of the stream and potential functions in terrain-fitted coordinates. The first iteration was found to be the former Dressler’s theory, whereas a second iteration of the governing equations results in velocity components generalizing Dressler’s theory to wave-like motion. Dressler’s first-order theory produces a transcritical flow solution over topography only if the total head is fixed by a minimum value of the specific energy at the transition point. However, the theory deviates from measurements under subcritical flow conditions, given that the bed-normal velocity component is significant. A second iteration to the velocity field was used to produce a second-order differential equation that resembles the cnoidal-wave theory. It accurately produces flow over an obstacle including the critical point and the minimum specific energy as part of the numerical solution. The new cnoidal-wave model compares well with the theory of a Cosserat surface for directed fluid sheets, whereas the Saint-Venant theory appears to be poor.
PubDate: 2015-06-06

• Hydrodynamics of suspended canopies with limited length and width
• Abstract: Abstract Experimental measurements and numerical simulations are carried out to determine the hydrodynamics induced by suspended canopies of limited width and height for canopies with six different densities and canopy element arrangements and two different upstream velocities. Measurements of velocity are obtained using acoustic Doppler velocimetry and the drag force via a load cell. Numerical simulation results using OpenFOAM agree very well with the experimental data and are used to investigate the generated flow fields in detail. The bulk features of the flow are similar to those of other canopies, including emergent and submerged canopies, but the finite dimensions of the canopy results in flow patterns that differ from suspended canopies of essentially infinite width. The detailed hydrodynamics of the flow are controlled by the blockage of the suspended canopy which depends both on the canopy density and the lateral spacing between consecutive longitudinal rows of canopy elements. Increased flow blockage results in increases in the drag coefficient from 0.72 to 1.4, reduction in the flow rate inside the canopy from 58 to 98 % (of the diverted flow, 20–43 % is diverted below the canopy) and increases in the steady wake zone length from 0.6 to 4 times the canopy length. Flow blockage has relatively little effect on the length of the upstream adjustment and total wake zones at 1.09 and 7 times the canopy length respectively. The flow also depends only weakly on the upstream velocity.
PubDate: 2015-06-02

• Diffusion of bedload particles in open-channel flows: distribution of
travel times and second-order statistics of particle trajectories
• Abstract: Abstract The motion of bedload particles is diffusive and occurs within at least three scale ranges: local, intermediate and global, each of which with a distinctly different diffusion regime. However, these regimes, extensions of the scale ranges and boundaries between them remain to be better defined and quantified. These issues are explored using a Lagrangian model of saltating grains over the uniform fixed bed. The model combines deterministic particle motion dynamics with stochastic characteristics such as probability distributions of step lengths and resting times. Specifically, it is proposed that a memoryless exponential distribution is an appropriate model for the distribution of rest periods while the probability that a particle stops after a current jump follows a binomial distribution, which is a distribution with lack of memory as well. These distributions are incorporated in the deterministic Lagrangian model of saltating grains and extensive numerical simulations are conducted for the identification of the diffusive behavior of particles at different time scales. Based on the simulations and physical considerations, the local, intermediate, and global scale ranges are quantified and the transitions from one range to another are studied for a spectrum of motion parameters. The obtained results demonstrate that two different time scales should be considered for parameterization of diffusive behavior within intermediate and global scale ranges and for defining the local–intermediate and intermediate–global boundaries. The simulations highlight the importance of the distributions of the step lengths and resting times for the identification of the boundaries (or transition intervals) between the scale ranges.
PubDate: 2015-06-02

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