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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  [2302 journals]
• Numerical modeling of $$30^{\circ }$$ 30 ∘ and $$45{^\circ }$$ 45
∘ inclined dense turbulent jets in stationary ambient
• Abstract: Dispersion of turbulent jets in shallow coastal waters has numerous engineering applications. The accurate forecasting of the complex interaction of these jets with the ambient fluid presents significant challenge and has yet to be fully elucidated. In this paper, numerical simulation of $$30{^\circ }$$ and $$45{^\circ }$$ inclined dense turbulent jets in stationary water have been conducted. These two angles are examined in this study due to lower terminal rise heights for $$30{^\circ }$$ and $$45{^\circ }$$ , this is critically important for discharges of effluent in shallow waters compared to higher angles. Mixing behavior of dense jets is studied using a finite volume model (OpenFOAM). Five Reynolds-Averaged Navier–Stokes turbulence models are applied to evaluate the accuracy of CFD predictions. These models include two Linear Eddy Viscosity Models: RNG $$k-\varepsilon$$ , and realizable $$k-\varepsilon$$ ; one Nonlinear Eddy Viscosity Model: nonlinear $$k-\varepsilon$$ ; and two Reynolds Stress Models: LRR and Launder–Gibson. Based on the numerical results, the geometrical characteristics of the dense jets, such as the terminal rise height, the location of centerline peak, and the return point are investigated. The mixing and dilution characteristics have also been studied through the analysis of cross-sectional concentration and velocity profiles. The results of this study are compared to various advanced experimental and analytical investigations, and comparative figures and tables are discussed. It has been observed that the LRR turbulence model as well as the realizable $$k-\varepsilon$$ model predicts the flow more accurately among the various turbulence models studied herein.
PubDate: 2015-06-01

• Two dimensional river flow and sediment transport model
• Abstract: This work presents the development and calibration of a two dimensional river flow, sediment transport and bed evolution model that couples the active-layer and multiple size-class approach for sediment transport modeling with an enhanced advection algorithm. The governing equations are solved using the fractional step method. This resulted in three successive steps (advection, diffusion and continuity) for the flow equations, and two steps (advection and diffusion) for the sediment transport and bed evolution equations. The focus of the research is the improvement of the advection computation (both in water and sediment computation) by reducing the limitation it imposes on the time step. Overcoming this restriction is enabled by introducing modifications to the characteristic method. Implementing these adjustments allows the characteristic curve to extend throughout multiple computational cells. After evaluating the advection computation for both flow and sediment transport by comparison of the proposed algorithm with various classical methods, the developed model is assessed using field measurements conducted on the Danube River. Analysis of the measured and computed values confirmed the developed model’s reliability.
PubDate: 2015-06-01

• Longitudinal dispersion of turbulent oscillatory pipe flows
• Abstract: In the present study, we examine the longitudinal dispersion of oscillatory pipe flows in the turbulent range which is not well covered before. An analytical analysis was first performed using the homogenization approach (i.e. multiple scale perturbation analysis) to predict the magnitude of the longitudinal dispersion induced by a turbulent oscillatory flow forced by a sinusoidal pressure gradient inside a circular pipe. An axisymmetric co-axial eddy viscosity model was adopted to resolve the radial distribution of velocities and turbulent shear stresses. Based on the derived kinematic characteristics, the longitudinal dispersion coefficient for the turbulent oscillatory pipe flow was then quantified. The results demonstrated that a dimensionless parameter $$\alpha$$ , which is the ratio of the oscillatory velocity amplitude divided by the frequency and pipe radius, determines the flow structure as well as the magnitude of the induced longitudinal dispersion coefficient. Experiments were also conducted to quantify the longitudinal dispersion coefficient under different frequencies and oscillatory velocity magnitudes. The measurement approaches were based on the non-invasive laser imaging techniques of particle image velocimetry and planar laser induced fluorescence. The experimental conditions covered a relatively wide range of boundary Reynolds number $$(\hbox {Re}_\delta )$$ from 100 to 1,000, and included both laminar and turbulent flow regimes. The results showed that when the flow enters the conditional turbulence regime, i.e. $$\hbox {Re}_\delta \ge 500$$ , the longitudinal dispersion coefficient increases drastically. The analytical predictions based on the homogenization approach in the present study agree well with the measured longitudinal dispersion coefficients.
PubDate: 2015-06-01

• Modeling assessment of a saltwater intrusion and a transport time scale
response to sea-level rise in a tidal estuary
• Abstract: This paper documents a modeling investigation to comprehend the effect of future sea-level rise (SLR) on estuarine salinity and transport time scales, including the residence time and the water age of dissolved substances in a partially mixed estuary. A three-dimensional semi-implicit Eulerian–Lagrangian finite-element model was established and applied to the Tamsui River estuarine system and the adjacent coastal sea in northern Taiwan. The modeling results indicated reasonable agreement with the observed water levels, tidal currents, and salinity. The model was then applied to calculate the salt intrusion, residence time, and water age between the baseline (without SLR) and different scenarios, including SLRs of 0.34, 1.05, and 1.40 m for the year 2100. The numerical model results reveal that the average salt content and salt intrusion length will increase as the sea level rises. The 1 psu isohaline moves toward upstream reaches with an increase in SLR. The results reveal that the maximum increment of tidal-averaged and depth-averaged salinity would be 1.1, 2.4, and 3.0 psu, respectively, for the SLRs of 0.34, 1.05 and 1.40 m at the middle estuary under mean flow conditions. The regression between salt intrusion length and freshwater discharge are established corresponding to different SLR scenarios. The residence time of the entire Tamsui River system would increase from 6.3 to 23 % compared to the baseline under low flow conditions. The concentration of dissolved substances would have a longer transport time from upstream to downstream because water volume increases with SLR. This indicates that the water age will increase in the main Tamsui River estuary as the sea level rises.
PubDate: 2015-06-01

• Coherent structures and their interactions in smooth open channel flows
• Abstract: The study presents experimental results of coherent structures and their interactions in a smooth open channel flow based on measurement of instantaneous two-dimensional velocity vectors with particle image velocimetry. The sampled data were analyzed through techniques of ensemble average, vortex extraction, and proper orthogonal decomposition (POD). Redistribution of turbulent kinetic energy is observed in the near-surface region. The spanwise vortices, which are closely related to hairpin vortices, exhibit a clear dependence on Reynolds number of the flow. Hairpin vortex packets and long streamwise vortices are identified as typical large-scale and super-scale coherent structures, respectively, and their interaction is revealed by examining the relationship between the population density of spanwise vortices and the coefficient functions of the first POD mode. Interactions between large-scale and super-scale structures have been recognized to support the hypothesis of closed-loop feedback cycle.
PubDate: 2015-06-01

• Erratum to: Turbulent velocity, sediment motion and particle trajectories
under breaking tidal bores: simultaneous physical measurements
• PubDate: 2015-06-01

• Environmental hydraulic engineering: a perspective
• PubDate: 2015-06-01

• Influence of jet aeration on pressures around a block embedded in a plunge
pool bottom
• Abstract: The influence of the air entrained by water jets on the dynamic pressures applied on the bottom of a plunge pool and inside underlying fissures was analyzed with systematic experiments. The large experimental facility reproduced aerated high-velocity jets up to 22.1 m/s impinging on a pool and impacting on an instrumented cubic block embedded on the bottom. Plunging and submerged jets are compared, as well as jet impingement on the center or on the side of the block. A relationship is proposed to describe the time-averaged pressures at stagnation as a function of the relative pool depth, considering pressure measurements in this position as well as recent experimental evidence on the jet centerline velocity decay. Air bubbles influence the dynamic pressures on the rock bottom by reducing jet momentum, but also by reducing the jet dissipation rates in the water pool. These two processes are opposed. The reduction of momentum, consequence of a jet with a lower apparent density, results in lower pressures, while lower jet dissipation in the pool results in higher kinetic energy of the jet impacting the bottom and higher pressures. Finally, the spectral contents show that the resonance frequencies of aerated jets are shifted as a consequence of wave celerity reduction caused by lower mean densities inside the fissures, which is an evidence of the presence of air bubbles.
PubDate: 2015-06-01

• Turbulent velocity, sediment motion and particle trajectories under
breaking tidal bores: simultaneous physical measurements
• Abstract: A tidal bore is a hydrodynamic discontinuity propagating upstream in an estuarine zone with a funnel shape as the tide starts rising under spring tidal conditions. The transient sediment motion beneath tidal bores was investigated in laboratory under controlled flow conditions by measuring simultaneously the fluid and sediment particle velocities. Although no sediment transport was observed in the initially steady flow and in undular bores, a transient sheet flow motion was observed beneath the breaking bores. The sediment transport was initiated during the passage of the bore roller toe by the large longitudinal pressure gradient force, and the sediment particles were subjected to large horizontal accelerations. About 5 % of all particles were accelerated in excess of 1 g. The sediments were advected upstream with an average velocity close to the instantaneous fluid velocity. The time evolution of instantaneous particle velocity for each trajectory was analysed, using the starting point of particle trajectory corresponding to the entrainment point, and the end point to the particle stoppage point. The present data provided some quantitative data in terms of force terms acting on sediment particles beneath a tidal bore and their trajectory characteristics.
PubDate: 2015-06-01

• Non-Newtonian power-law gravity currents propagating in confining
boundaries
• Abstract: The propagation of viscous, thin gravity currents of non-Newtonian liquids in horizontal and inclined channels with semicircular and triangular cross-sections is investigated theoretically and experimentally. The liquid rheology is described by a power-law model with flow behaviour index $$n$$ , and the volume released in the channel is taken to be proportional to $$t^{\alpha }$$ , where $$t$$ is time and $$\alpha$$ is a non-negative constant. Some results are generalised to power-law cross-sections. These conditions are representative of environmental flows, such as lava or mud discharges, in a variety of conditions. Theoretical solutions are obtained in self-similar form for horizontal channels, and with the method of characteristics for inclined channels. The position of the current front is found to be a function of the current volume, the liquid rheology, and the channel inclination and geometry. The triangular cross-section is associated with the fastest or slowest propagation rate depending on whether $$\alpha <\alpha _c$$ or $$\alpha >\alpha _c$$ . For horizontal channels, $$\alpha _c=n/(n+1)<1$$ , whereas for inclined channels, $$\alpha _c=1$$ , irrespective of the value of $$n$$ . Experiments were conducted with Newtonian and power-law liquids by independently measuring the rheological parameters and releasing currents with constant volume ( $$\alpha =0$$ ) or constant volume flux ( $$\alpha =1$$ ) in right triangular and semicircular channels. The experimental results validate the model for horizontal channels and inclined channels with $$\alpha =0$$ . For tests in inclined channels with $$\alpha =1$$ , the propagation rate of the current front tended to lower values than predicted, and different flow regimes were observed, i.e., uniform flow with normal depth or instabilities resembling roll waves at an early stage of development. The theoretical solution accurately describes current propagation with time before the transition to longer roll waves. An uncertainty analysis reveals that the rheological parameters are the main source of uncertainty in the experiments and that the model is most sensitive to their variation. This behaviour supports the use of carefully designed laboratory experiments as rheometric tests.
PubDate: 2015-06-01

• Velocity profiles in a real vegetated channel
• Abstract: Abstract Most of the studies regarding vegetation effects on velocity profiles are based on laboratory experiments. The main focus of this paper is to show how the laboratory knowledge established for submerged vegetation applies to real-scale systems affected by vegetation growth (mainly Ranunculus fluitans). To do so, experiments are conducted at two gage stations of an operational irrigation system. The analysis of first- and second-order fluctuations of velocities is based on field measurements performed by micro-acoustic doppler velocimeter during 8 months, completed with flow measurement campaigns in different seasons. The Reynolds stresses are used to determine shear velocities and deflected plant heights. Then, the modified log–wake law (MLWL), initially derived from laboratory flume experiments, is applied with a unique parametrisation for the whole set of velocity profiles. The MLWL, along with a lateral distribution function, is used to calculate the discharge and to show the influence of vegetation height on the stage–discharge relationships.
PubDate: 2015-05-21

• A recycling method for the large-eddy simulation of plumes in the
atmospheric boundary layer
• Abstract: A method for the large-eddy simulation (LES) of dispersion and mixing of passive scalars is developed and evaluated. The new method addresses the requirements of tracking the evolution of plumes for large distances from their sources while attaining a low computational cost. To reduce computational cost, the velocity and thermodynamic fields are solved on a doubly periodic domain in the horizontal directions. In contrast, when the plume reaches the downstream end of the computational domain, it is reintroduced at the upstream plane but as a different scalar field. The same procedure is repeated when the new scalar field reaches the downstream boundary. By using several scalar fields to describe the evolution of a single plume, the simulation is computationally cheaper since the same velocity and thermodynamic fields are reused, or recycled, when computing the plume evolution. The recycling method is verified by showing that low-order plume statistics are identical to a single-domain LES. Three cases of dispersion and mixing from a point ground source in diverse boundary layer conditions (stable, convectively unstable, and shallow cumulus convection) are considered. Moreover, the LES results are compared with the predictions a Gaussian plume model, which is found to perform satisfactorily in all cases when accurate information about the state of the boundary layer is provided.
PubDate: 2015-05-17

• Large eddy simulations of 45° inclined dense jets
• Abstract: Submerged inclined dense jets (negatively buoyant jets) occur in many engineering applications such as brine discharges from seawater desalination plants and de-cooling water discharges from liquefied natural gas plants, and their mixing behavior needs to be examined in details for the environmental impact analysis. In the present study, a detailed numerical investigation was performed using the large eddy simulation (LES) approach with both the Smagorinsky and Dynamic Smagorinsky sub-grid scale (SGS) models to simulate the characteristics of the inclined dense jet with 45° inclination. The numerical predictions included the jet trajectory, geometrical characteristics, jet spread and eddy structures. Experimental measurements were also obtained for the validation of the LES predictions, and data from existing studies in the literature were included for comparison. Overall, the LES predictions were able to reproduce the geometric characteristics of the inclined dense jet in a satisfactory manner in most aspects. The dilution was however generally underestimated, which was attributed primarily to the inability of the SGS models to reproduce the convective mixing induced by the buoyancy-induced instability using the adopted grid spacing in the bottom half of the inclined dense jet.
PubDate: 2015-05-16

• Measurements of sediment pickup rate over dune-covered bed
• Abstract: Laboratory experiments were conducted to measure sediment pickup rate over two-dimensional fixed dunes. Measurements were performed over both stoss and lee sides of the dune with sediments of D 50 = 0.23, 0.44 and 0.86 mm. Flow velocity and turbulence were also measured by using an acoustic Doppler velocimeter. By analysing the experimental data, an empirical sediment pickup function based on depth-averaged flow parameters was proposed to estimate the pickup rate over the dune.
PubDate: 2015-05-16

• Tsunami forcing by a low Froude number landslide
• Abstract: The waves generated by a submarine landslide, of great concern to coastal communities, exhibit strong dependence on the landslide motion along the sea floor. A series of two-dimensional physical experiments investigate the waves generated by a solid block landslide moving along a horizontal boundary, allowing measurement of both onshore- and offshore-propagating waves using the laser-induced fluorescence technique. This technique provides high-quality free surface measurements over the entire length of the experimental flume, and hence a data set that can be used to validate numerical models for this idealised scenario. The landslide motion is provided by a mechanical system, allowing testing of a range of landslide accelerations and terminal velocities. The landslide Froude number governs the overall behaviour of the wave field. At lower Froude numbers, the waves are almost entirely generated by the landslide acceleration and deceleration, and the offshore- and onshore-propagating wave groups contain approximately equal energy. Interactions between the landslide and the offshore-propagating waves become more important as the Froude number increases. Two inviscid-irrotational models demonstrate the importance of dispersive effects for tsunamis generated by a submarine landslide, and correctly predict the behaviour of the entire wave field at low Froude numbers. The predictions in the vicinity of the landslide worsen with increasing Froude number, due to the linear free surface conditions used by the models. Lower Froude numbers appear to be more representative of previous sloping-boundary experimental geometries, although rigid block landslides still represent an idealisation of a field scenario.
PubDate: 2015-05-07

in a linearly stratified environment
• Abstract: We present results of laboratory experiments conducted to study the evolution, growth, and spreading rate of a dispersed particle-laden plume produced by a constant inflow into a density varying environment. Particles having mean size, $$d_p=100\ \upmu$$ m, density $$\rho _p=2500 \ {\text{ kg/m}}^3$$ , volume fraction, $$\phi _v =$$ 0–0.7 % , were injected along with the lighter buoyant fluid into a linearly stratified medium. It was observed that a particle-laden plume intruding at the neutral density layer is characterized by four spreading regimes: (i) radial momentum flux balanced by the inertia force; (ii) inertia buoyancy regime; (iii) fluid-particle inertia regime, and (iv) viscous buoyancy regime. Regimes (i), (ii), and (iv) are similar to that of a single-phase plume, for which $$\phi _v = 0\,\%$$ . The maximum height, $$Z_m$$ , for $$\phi _v > 0\,\%$$ was observed to be consistently lower than the single-phase case. An empirical parameterization was developed for the maximum height for particle-laden case, and was found to be in very good agreement with the experimental data. In the inertia buoyancy regime, the radial spread of the plume, $$R_f$$ , for $$\phi _v > 0\,\%$$ , advanced in time as $$R_f \propto t^{0.68 \pm 0.02}$$ which is slower compared to the single-phase plume that propagates at $$R_f \propto t^{0.74 \pm 0.02}$$ . Due to the presence of particles, ‘particle fall out’ effect occurs, which along with the formation of a secondary umbrella region inhibits the spreading rate and results in slower propagation of the particle-laden plume. The effect of particles on spreading height of plume, $$Z_s$$ , and thickness of the plume, $$h_p$$ , were also studied, and these results were compared with the single-phase case. Overall from these experiments, it was found that the evolution, growth, and spread of dispersed particle-laden plume is very different from that of the single-phase plume, and presence of low concentration of particles ( $$\phi _v < 1\,\%$$ ) could have significant effects on the plume dynamics.
PubDate: 2015-05-03

• Rapid operation of a Tainter gate: generation process and initial upstream
surge motion
• Abstract: In water supply channels, the brusque operation of control gates may induce large unsteady flow motion called surges. Such a rapid operation of gates must often be restricted, although it may be conducted to scour silted channels and sewers. Herein a physical study was conducted under controlled flow conditions to study the turbulent mixing in the very-close vicinity of a rapidly opening/closing Tainter gate, with a focus on the unsteady transient mixing induced by the gate operation. The data suggested that the negative/positive surge generation was associated with large instantaneous free-surface fluctuations. The velocity measurements indicated significant variations in longitudinal velocity during the surge generation, as well as large fluctuations of all velocity components. The processes were associated with large Reynolds stress levels. A succession of rapid closure and opening of undershoot gates provided optimum conditions to scour silted canals, and the present results gave some detailed insights into the physical processes.
PubDate: 2015-04-29

• Twin inclined jets in crossflow: experimental investigation of different
flow regimes and jet elevations
• Abstract: The present experimental study is dedicated to unfolding the mixing process generated by double-inclined, variably elevated jets in crossflow. Twin tandem jets in crossflow are very common in the industry and are closely dependent on several parameters. Detailing the induced interactions in such a model would ultimately enhance our understanding and help optimize related applications. The jets handled in the present work are placed three diameters apart, arranged in line with the oncoming crossflow and sent at variable levels (h = 0, 1, 2, 5 cm) from the ground of a wind tunnel and are discharged from elliptic discharging cross sections. Elliptic jets are of particular interest in applications like industrial and boat chimneys, and are practically easily obtained by razing circular cylinders at the desired inclination and height. Particle image velocimetry and coupled charge device CCD camera were used. The dynamic and turbulent behavior of the resulting flowfield was characterized in terms of streamlines, velocity components and vortical structures. The obtained data helped highlight the impact of jets’ elevation over their mixing mechanism among the surrounding free stream: establishment of the resulting flowfield dynamics and settlement of the induced vortical structures. The mixing process evolution was also considered under different flow regimes. For the matter an injection ratio (defined as jets to the mainstream’s velocity ratio) raging between 0.67, 1 and 1.29 was considered. These cases correspond to jets dynamically dominated by, equivalent to or more dominant than the mainstream. The impact of jets’ elevation combined to that of the injection ratio affects both the developed vortical structures and the established dynamic fields, which in turn highly affects the induced mixing process.
PubDate: 2015-04-19

• Advent of sheet flow in suction affected alluvial channels
• Abstract: The presence of suction (flow of water from channel to ground water) affects the channel hydrodynamics and increases the bed shear stress. At high bed shear stress in alluvial channels made of the non-cohesive material, sediment transport occurs as sheet flow layer of high sediment concentration. The sediment transport in the form of sheet flow has been observed in the present study when suction was applied to the non-transporting channels designed on incipient motion condition. The erosion of the channel banks contributed to the sheet flow because of the increased channel bed shear stress. An empirical relation for the thickness of sheet flow layer has been developed which includes suction as independent parameter along with others.
PubDate: 2015-04-19

• RANS simulation of neutral atmospheric boundary layer flows over complex
terrain by proper imposition of boundary conditions and modification on
the k - ε model
• Abstract: In this study, a modelling methodology is proposed for RANS simulations of neutral Atmospheric Boundary Layer (ABL) flows on the basis of the standard k-ε model, which allows the adoption of an arbitrary shear stress model. This modelling methodology is first examined in the context of an open flat terrain in an empty domain to ascertain there are no substantial changes in the prescribed profiles. The results show that relatively good homogeneity can be achieved with this modelling methodology for various sets of inflow boundary profiles. In addition, to extend the solutions derived from the standard k-ε model to RNG k-ε model, the RNG k-ε model is in detail assembly and tuned. Finally, the topographic effects on surface wind speeds over a complex terrain are assessed with the combined use of the proposed methodology and the modified RNG model. The numerical results are in good agreement with wind tunnel testing results and long-term field observations. A discussion of the effects of horizontal homogeneity and turbulence models on the simulated wind flows over a complex terrain is also given.
PubDate: 2015-04-18

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