Showing papers in "Journal of Hydraulic Research in 2016"
TL;DR: In this paper, the authors assess the recent trends in the numerical meshless method smoothed particle hydrodynamics, with particular focus on its potential use in modelling free-surface flows.
Abstract: This paper assesses some recent trends in the novel numerical meshless method smoothed particle hydrodynamics, with particular focus on its potential use in modelling free-surface flows. Due to its Lagrangian nature, smoothed particle hydrodynamics (SPH) appears to be effective in solving diverse fluid-dynamic problems with highly nonlinear deformation such as wave breaking and impact, multi-phase mixing processes, jet impact, sloshing, flooding and tsunami inundation, and fluid–structure interactions. The paper considers the key areas of rapid progress and development, including the numerical formulations, SPH operators, remedies to problems within the classical formulations, novel methodologies to improve the stability and robustness of the method, boundary conditions, multi-fluid approaches, particle adaptivity, and hardware acceleration. The key ongoing challenges in SPH that must be addressed by academic research and industrial users are identified and discussed. Finally, a roadmap is propose...
326 citations
TL;DR: In this paper, entrainment and mixing in lock-exchange gravity currents are investigated by large eddy simulations, varying the initial excess density driving the motion and the aspect ratio r of the initial water depth to the lock length.
Abstract: Entrainment and mixing in lock-exchange gravity currents are investigated by large eddy simulations. Nine cases are analysed, varying the initial excess density driving the motion and the aspect ratio r of the initial water depth to the lock length. Laboratory experiments are also performed and a fair agreement between numerical simulations and measurements is found. Mixing between the gravity current and the ambient fluid, in both the slumping and self-similar phases, is studied for a range of entrainment parameters, gravity current fractional area and using an energy budget method. The entrainment is found to increase as r decreases. The occurrence of irreversible mixing is detected during the entire development of the flow, i.e. both in the slumping and self-similar phases. A higher amount of mixing is observed as r decreases and the initial excess density increases.
90 citations
TL;DR: The reliability of the affinity law to predict the behaviour of a machine under variable speeds is discussed, and the results of this model are compared with an experimental database which includes the performance curves of five PATs operating at different speeds.
Abstract: The use of pumps operating as turbines (PATs) is attractive to optimize the equipment costs of small hydropower plants. Unfortunately, the lack of information on the performance of PATs restricts the wide use of this technology. If a single characteristic curve is available, the behaviour of a PAT can be predicted by the application of the turbomachinery affinity law. In this paper, the reliability of the affinity law to predict the behaviour of a machine under variable speeds is discussed, and the results of this model are compared with an experimental database which includes the performance curves of five PATs operating at different speeds. The results show that the difference between the theoretical model and the experimental results is significant. Therefore, a new model, based on a relaxation of the affinity equations, has been proposed, in order to minimize the errors between the predicted and measured characteristic curves.
74 citations
TL;DR: This paper presents the first study on application of the Horn–Schunck method to aerated stepped spillway flows and it is found that the image pyramid approach has the most significant effect on the accuracy compared to other image processing techniques.
Abstract: Optical flow estimation is known from Computer Vision where it is used to determine obstacle movements through a sequence of images following an assumption of brightness conservation. This paper presents the first study on application of the optical flow method to aerated stepped spillway flows. For this purpose, the flow is captured with a high-speed camera and illuminated with a synchronized LED light source. The flow velocities, obtained using a basic Horn–Schunck method for estimation of the optical flow coupled with an image pyramid multi-resolution approach for image filtering, compare well with data from intrusive conductivity probe measurements. Application of the Horn–Schunck method yields densely populated flow field data sets with velocity information for every pixel. It is found that the image pyramid approach has the most significant effect on the accuracy compared to other image processing techniques. However, the final results show some dependency on the pixel intensity distribution...
69 citations
TL;DR: In this paper, the size-scale effect, minimum upstream head, and Weber number limits were investigated for four piano key weirs with geometric model scales of 1:1, 1:7,1:15, and 1:25.
Abstract: With inertia and gravity representing the dominant forces for most open channel flow applications (e.g. weir flow), Froude similitude is commonly used for scaling hydraulic performance data from the model to prototype structures. With weir flow, as the upstream head decreases, however, the relevance of surface tension and viscosity forces can increase to the point when the model and prototype similitude is not fully achieved through Froude scaling. Such discrepancies are referred as size-scale effects, and among other things, can result in variations in the head–discharge relationship, nappe trajectory, and air entrainment. Published criteria for avoiding significant size-scale effects for free flow over linear weirs have suggested that minimal heads of ∼0.02 to 0.07 m be respected, independently of the model size. In this study, the size-scale effect, minimum upstream head, and Weber number limits are investigated for four piano key weirs with geometric model scales of 1:1, 1:7, 1:15, and 1:25.
63 citations
TL;DR: In this paper, the authors investigated erosion events in a turbulent open channel flow laden with monodisperse spherical particles and found that a particle collision and a subsequent sweep event on time scales of several bulk units is responsible for the erosion.
Abstract: This paper investigates erosion events in a turbulent open channel flow laden with monodisperse spherical particles. The data were generated in a previous study using direct numerical simulations with a phase-resolving immersed boundary method. The particles have a mobility below their nominal threshold of incipient motion and settle onto the rough bed that consists of a hexagonally packed layer of spheres with the same size. Conditioned averaging is employed to extract the characteristic features of an erosion event, defining a suitable criterion for detection and accounting for possible asymmetry of flow structures. The highly resolved dataset provides detailed insight into the key-mechanisms of erosion. The results show that a particle collision, together with a subsequent sweep event on time scales of several bulk units, is responsible for the erosion.
58 citations
TL;DR: In this article, the effects of sediment size, tailwater depth, flow intensity, and weir height on local scour at submerged weirs in sand-bed channels were investigated.
Abstract: Submerged weirs are river training structures that are used for raising upstream water level, bed stabilization and reducing flow velocity. This paper presents an experimental study of local scour at submerged weirs in sand-bed channels. Two types of tests (coarse sand tests and fine sand tests) were conducted to investigate the effects of sediment size and tailwater depth on scouring at submerged weirs. The flow regimes over the weir are found to be independent of the sediment size, and the transition flow regime boundary can be expressed as a function of upstream Froude number and the ratio of weir height to tailwater depth. New equations, including the effects of sediment size, tailwater depth, flow intensity, and weir height, are proposed for prediction of equilibrium scour depths both upstream and downstream of the submerged weir. A new design method is given for estimating the maximum scour depths at the weir.
54 citations
TL;DR: In this paper, the authors investigated the effect of the transients on the turbine performance and the formation and mitigation processes for the rotating vortex ropes and their effects on the forces exerted on the runner.
Abstract: The paper addresses unsteady pressure measurements on the blades and stationary parts of a Kaplan turbine model (Porjus U9) during load variation. The turbine was studied in various load acceptance and load rejection scenarios in off-cam mode to investigate the effect of the transients on the turbine performance. The formation and mitigation processes for the rotating vortex ropes and their effects on the forces exerted on the runner were also investigated. The results show a smooth transition during load variations between high load and the best efficiency point, at which no rotating vortex ropes form in the draft tube. However, load variation to part load resulted in a draft tube surge and the formation of a rotating vortex rope with two fluctuating components: rotating and plunging. The rotating vortex ropes began to form at the end of the draft tube cone during the closure of the guide vanes and travelled upstream with further guide vane closure. The plunging mode induced flow oscillation thro...
51 citations
TL;DR: In this article, the wave speed and the bulk viscosity for the draft tube of a reduced scale physical model of a Francis turbine were derived based on experimental identification of the hydraulic natural frequency of the test rig.
Abstract: The operation of Francis turbines at off-design conditions may cause the development of a cavitation vortex rope in the draft tube cone, acting as a pressure excitation source. The interactions between this excitation source and the hydraulic system at the natural frequency may result in resonance phenomena, causing serious hydro-mechanical oscillations. One-dimensional draft tube models for the simulation and prediction of part load resonances require an accurate modelling of the wave speed and the bulk viscosity for the draft tube flow. This paper introduces a new methodology for determining these two hydroacoustic parameters in the draft tube of a reduced scale physical model of a Francis turbine, based on experimental identification of the hydraulic natural frequency of the test rig. Finally, dimensionless numbers are derived to define both the wave speed and bulk viscosity for different operating points of the turbine.
48 citations
TL;DR: In this paper, an improved model (Oildroplets) is introduced to calculate the droplet size distribution in underwater oil jets and plumes, where new theoretical modifications were made to the breakup and coalescence closures in an existing framework.
Abstract: In deepwater oil well blowouts, accurate calculations of droplet sizes are important in predicting the fate and transport of oil. The current knowledge limits the ability of available models to calculate the wide range of droplet sizes that exist in deepwater oil jets and plumes. In this paper, an improved model (Oildroplets) is introduced to calculate the droplet size distribution in underwater oil jets. Breakup and coalescence are the key processes in controlling the droplet sizes calculated in Oildroplets. New theoretical modifications were made to the droplet breakup and coalescence closures in an existing framework that works for a wide range of release conditions. The model Oildroplets is used to predict the droplet size distribution in field and laboratory experiments, where the droplet sizes vary from very small (µm) to large (mm). The comparison of model-computed results with experimental data shows good agreement.
42 citations
TL;DR: In this paper, the Fourier transform of the pressure probe signals accompanied by optical detection provided the frequencies and amplitudes of the detected single and twin ropes, which were compared between the two draft tubes in a range of Reynolds number from 105 to 5'×'105.
Abstract: We report on parallel experimental studies of twin vortex ropes in laboratory models of an elbow and a conical hydroturbine draft tube focusing on the unstable cyclic switch-overs between the single and twin helices. The measurements involve high-speed visualization, pressure pulsation recordings and laser-beam detecting of the precessing helices. The Fourier transform of the pressure probe signals accompanied by optical detection provided the frequencies and amplitudes of the detected single and twin ropes, which were compared between the two draft tubes in a range of Reynolds number from 105 to 5 × 105. The experiments showed constant, though different, Strouhal numbers both for the single and twin ropes. The single-rope Strouhal numbers normalized with the swirl number for the two draft tubes collapse onto a unique value of 1.1 independent of Reynolds number. The ratio of the helix frequencies in the twin- and single rope regimes was 1.15 and 1.4, respectively, for the elbow and the conical model.
TL;DR: In this paper, a side-discharge valve based transient generator that can produce two types of pseudorandom binary signals: a maximum length binary signal and an inverse repeat signal is presented.
Abstract: This paper presents the original design of a side-discharge valve based transient generator that can produce two types of pseudorandom binary signals: a maximum length binary signal and an inverse repeat signal. These two signals are both wide bandwidth, persistent and periodic, but the inverse repeat signal has the advantageous property that it is antisymmetric within each period. The two signals are used to extract the frequency response function of a single water pipeline in the laboratory. The experimental results demonstrate that the frequency response function extracted by the inverse repeat signal is closer to the theoretical linear results as obtained from the transfer matrix method due to it being able to cancel the effect of even-order nonlinearities. The customized transient generator is then applied to a pipeline with a leak. The location of the leak is successfully determined using the first three resonant peaks as extracted by the inverse repeat signal.
TL;DR: In this paper, an experimental characterization of the hydrodynamics of a mixing interface at an open channel confluence is presented, where both a confluence and a cylinder set-up were studied in order to validate the bluff body analogy, which has been proposed to characterize the mixing interface.
Abstract: An experimental characterization of the hydrodynamics of a mixing interface at an open channel confluence is presented. In the laboratory experiments, both a confluence and a cylinder set-up were studied in order to validate the bluff body analogy, which has been proposed to characterize the mixing interface. The experimental characterization included flow visualization and the computation of the mean flow field, time scales of the coherent structures, and turbulent kinetic energy. The comparison among the two configurations confirms the validity of the analogy as similar features were found for the mean flow field and time scales of the coherent structures. However, differences in the length of the stagnation zone, the flow velocity deficit, and the turbulence intensity were observed. These differences should be taken into account when the bluff body analogy is used to characterize the confluence hydrodynamics and to quantify the mixing at the confluence interface.
TL;DR: In this article, a semi-empirical velocity formula for cross-flow turbines was proposed and validated against the numerical solutions for crossflow turbines with different geometries and boundary conditions, and the results confirmed the previous hydrodynamic analysis and thus can be used in the design of the cross-flowing turbines as well as for reducing the number of simulations needed to optimize the turbine geometry.
Abstract: A numerical and experimental study was carried out for validation of a previously proposed design criterion for a cross-flow turbine and a new semi-empirical formula linking inlet velocity to inlet pressure. An experimental test stand was designed to conduct a series of experiments and to measure the efficiency of the turbine designed based on the proposed criterion. The experimental efficiency was compared to that from numerical simulations performed using a RANS model with a shear stress transport (SST) turbulence closure. The proposed semi-empirical velocity formula was also validated against the numerical solutions for cross-flow turbines with different geometries and boundary conditions. The results confirmed the previous hydrodynamic analysis and thus can be employed in the design of the cross-flow turbines as well as for reducing the number of simulations needed to optimize the turbine geometry.
TL;DR: In this article, the authors used a smart debris device for direct measurement of the impact acceleration forces associated with tsunami-borne debris that impact inland structures, which will lead to advanced predictive capabilities of such forces for use in design guidelines.
Abstract: This paper presents new experimental techniques utilizing a smart debris device for direct measurement of the impact acceleration forces associated with tsunami-borne debris that impact inland structures. The resulting experimental data will lead to advanced predictive capabilities of such forces for use in design guidelines. The measured debris acceleration data were used to calculate impact forces (mass × acceleration). An image processing technique was used to detect the debris impact angle. The debris impact tests were conducted using a disc-shaped smart debris device with masses of 550, 800 and 1000 g. For calculation of the debris force it was found necessary to include the mass of entrained water. The impact acceleration was found to be a function of debris mass, velocity, and contact duration. An equation is developed to allow estimation of the debris velocity for a known distance between the debris pick-up location by a tsunami and the structure.
TL;DR: In this paper, an improved theoretical model linking screw performance to screw geometry and flow conditions is presented, taking into account leakages, friction losses and variable fill levels, and an optimal screw immersion level is proposed.
Abstract: The generation of renewable energy with Archimedes screw generators (ASG) transforming potential energy of fluid flow into mechanical energy is a growing technology suitable for low-head hydraulic sites. This paper presents an improved theoretical model linking screw performance to screw geometry and flow conditions. This model takes into account leakages, friction losses and variable fill levels. The modelled values of torques and efficiencies are in a fairly good agreement with experimental results obtained for a laboratory-scale screw. The downstream screw immersion is shown to impact ASG efficiency and an optimal immersion level is proposed. It has been found that fluid friction on the screw is not negligible. The analysis shows that a single value of the friction coefficient is suitable for modelling the screw performance under various flow conditions. The leakage phenomenon at under-filling flow conditions and friction forces in complex turbulent flows need to be further studied.
TL;DR: In order to reduce the computational effort required for simulating realistic scenarios, the authors have exploited the use of Graphics Processing Units in combination with non-trivial optimization procedures.
Abstract: Computational tools may help engineers in the assessment of sediment transport during the decision-making processes. The main requirements are that the numerical results have to be accurate and simulation models must be fast. The present work is based on the 2D shallow water equations in combination with the 2D Exner equation. The resulting numerical model accuracy was already discussed in previous work. Regarding the speed of the computation, the Exner equation slows down the already costly 2D shallow water model as the number of variables to solve is increased and the numerical stability is more restrictive. In order to reduce the computational effort required for simulating realistic scenarios, the authors have exploited the use of Graphics Processing Units in combination with non-trivial optimization procedures. The gain in computing cost obtained with the graphic hardware is compared against single-core (sequential) and multi-core (parallel) CPU implementations in two unsteady cases.
TL;DR: In this article, a meshless method is used to simulate free-surface fluid flows containing solid particles, motivated by the need to simulate river ice dynamics problems, and the computed results using the SPH-DEM model agree quantitatively with the expected behaviour in the test cases.
Abstract: A meshless method is used to simulate free-surface fluid flows containing solid particles, motivated by the need to simulate river ice dynamics problems. A smoothed particle hydrodynamics model (SPH), with an arbitrary Lagrangian–Eulerian formulation for the fluid phase, is two-way coupled with the discrete element method (DEM) for the solid phase. Validation test cases include a bouncing sphere on a level surface, a collapse of a granular column, wedge entry into still water and solids of different densities falling into still water. The computed results using the SPH-DEM model agree quantitatively with the expected behaviour in the test cases. Numerical convergence is demonstrated for the wedge entry validation case. The SPH-DEM model is then used to simulate the stability of floating ice blocks approaching a stationary cover and ice accumulation upstream of an obstruction. The results show promise to serve as a useful quantitative engineering tool.
TL;DR: In this article, the Navier-Stokes-Forchheimer (NSF) equation is solved for laminar vegetated flow and then the obtained velocity distribution is modified for turbulent flow.
Abstract: Vegetated flows are typical in many aquatic systems such as natural and man-made wetlands, and therefore attract significant attention of researchers and engineers. This study first solves the Navier–Stokes–Forchheimer (NSF) equation for laminar vegetated flow and then modifies the obtained velocity distribution for turbulent flow. It demonstrates that (i) for flows through emergent and over submerged vegetation, the laminar velocity distributions are expressed by the Jacobi elliptic functions for which the parabolic law is recovered for zero vegetation; (ii) for flow through emergent vegetation, the laminar velocity distribution exhibits a typical boundary-layer profile, while its turbulent counterpart is simply uniform; and (iii) for flow over submerged vegetation, both laminar and turbulent velocity distributions are similar to those in conventional channel flows for the water layer, but both are approximated by hyperbolic sine laws for the vegetation layer. The laminar solutions meet the NSF e...
TL;DR: In this article, an analytical model for the analysis of wave excitation forces and added mass and damping coefficients is proposed, based on a linearized velocity potential theory, which is validated by a comparison of the simulation results with the available data.
Abstract: In this paper, the diffraction and radiation problem of multiple two-dimensional rectangular bodies floating on a layer of water of finite depth with waves is studied. An analytical model for the analysis of wave excitation forces and added mass and damping coefficients is proposed, based on a linearized velocity potential theory. Expressions for velocity potentials are obtained by the method of separation of variables, in which unknown coefficients are determined by utilizing the eigen-function expansion matching method. The model is validated by a comparison of the simulation results with the available data. The validated model is then utilized to examine the effect of the structure width, structure draft, spacing between adjacent structures, and structure numbers on wave transmission coefficient of floating structures.
TL;DR: In this article, the interaction between flow field and plant motion in open-channel flows with submerged flexible vegetation was explored, and a strong correlation between plant motion and flow velocity was observed in the mixing-layer zone (near the vegetation edge) only.
Abstract: This paper explores the interaction between flow field and plant motion in open-channel flows with submerged flexible vegetation. Experiments were performed in a 10 m-long and 40 cm-wide tilting flume. Flows with submerged vegetation were studied under a range of conditions by varying lengths of flexible vegetation models (5.0, 7.0, 9.0 and 10.5 cm). Flow velocity components and the tip motion of waving plants were measured simultaneously using particle image velocimetry (PIV) and particle tracking velocimetry (PTV) techniques. The applied techniques are capable of investigating the interaction between flow field and plant motion. We examined quantitatively how plant motions were coupled to strong oscillations in flow velocity associated with the “monami” phenomenon as well as its vertical extent. A strong correlation between flow velocity and plant motion was observed in the mixing-layer zone (near the vegetation edge) only. Analysis of cross-correlation and coherence functions enabled estimation...
TL;DR: In this paper, a model is presented to demonstrate that free surface oscillatory spatial correlation patterns result from individual surface features oscillating vertically as they advect over space and time.
Abstract: Understanding the dynamic free surface of geophysical flows has the potential to enable direct inference of the flow properties based on measurements of the free surface. An important step is to understand the inherent response of free surfaces in depth-limited flows. Here a model is presented to demonstrate that free surface oscillatory spatial correlation patterns result from individual surface features oscillating vertically as they advect over space and time. Comparison with laboratory observations shows that these oscillating surface features can be unambiguously explained by simple harmonic motion, whereby the oscillation frequency is controlled by the root-mean-square water surface fluctuation, and to a lesser extent the surface tension. This demonstrates that the observed “complex” wave pattern can be simply described as an ensemble of spatially and temporally distributed oscillons. Similarities between the oscillon frequency and estimated frequency of near-bed bursting events suggest that...
TL;DR: In this article, a particle image velocimetry technique is employed to measure velocity fields under regular gravity waves propagating at a constant depth, based on the measurements, the Eulerian mean current velocity is determined.
Abstract: An original method is proposed to extract drift velocity induced by waves propagating in a wave flume. The particle image velocimetry technique is employed to measure velocity fields under regular gravity waves propagating at a constant depth. Based on the measurements, the Eulerian mean current velocity is determined. The instantaneous velocity fields are integrated to calculate particle trajectories and the Lagrangian time-averaged vertical distribution of mass-transport velocity. The Stokes drift velocity is determined by subtracting the Eulerian mean current from the Lagrangian mass-transport velocity. The results are in good agreement with theoretical second-order irrotational solution and confirm applicability of the weakly nonlinear solution for calculating transport processes associated with propagating ocean waves.
TL;DR: In this article, an analytical model for the transport of bio-coated sediment (bio-sediment) particles is developed, where experiments were conducted cultivating bio-coat around fine sediment particles in an experimental flume for a specific period of time.
Abstract: In this study, an analytical model for the transport of biofilm-coated sediment (bio-sediment) particles is developed. Experiments were conducted cultivating biofilm-coat around fine sediment particles (silt type) in an experimental flume for a specific period of time. The particle tracking velocimetry method was used to detect the saltation trajectory, the size of bio-flocs, and the flow velocity. A concentration meter was used to measure the concentrations of near-bed and suspended sediment transport. It was observed that the ratio of saltation length to saltation height for bio-flocs is greater than that for uncontaminated (without a biofilm-coat) sediment particles. The experimental data were used to calibrate the analytical model. A relationship for the bed-load transport rate of bio-sediments as a function of transport stage and particle parameters is proposed. Also, a formula for the reference concentration is given. Then, a computational scheme of bio-sediment transport (both bed-load and ...
TL;DR: In this article, a probabilistic polynomial chaos approach is proposed to solve for randomness in frequency domain using the transfer matrix method with results of comparable accuracy to those calculated by using a model employing the traditional method of characteristics.
Abstract: This paper presents a novel probabilistic approach based on the polynomial chaos expansion that can model the uncertainty propagation from the beginning of a waterhammer simulation and not as an afterthought. Uncertainties are considered in pipe diameter, friction coefficient, and wave speed, as well as internal boundary conditions of leaks and blockages. The polynomial chaos expansion solver results are in an excellent agreement with those calculated by using a model employing the traditional method of characteristics. The probabilistic polynomial chaos approach has the advantage of being robust and more efficient than other non-intrusive methods such as Monte Carlo simulation, which requires thousands of iterations for sharp solutions. The polynomial chaos approach is further extended to solve for randomness in frequency domain using the transfer matrix method with results of comparable accuracy. With further developments, this probabilistic approach can be integrated within existing network mod...
TL;DR: In this paper, the influence of several parameters on the incipient motion of gravels has been carefully examined from theoretical and experimental standpoints from both analytical and experimental perspectives. But the effects of the grain Reynolds number and relative submergence are negligible for turbulent flows over hydraulically rough boundaries.
Abstract: The influence of several parameters on the incipient motion of gravels has been carefully examined from theoretical and experimental standpoints. The effect of the longitudinal bed slope is significant and well described with existing methods. This analysis shows that the effects of the grain Reynolds number and relative submergence are negligible for turbulent flows over hydraulically rough boundaries. Laboratory experiments for homogeneous gravels on a plane bed provided detailed measurements of both mean flow velocity and velocity fluctuations for the analysis of the lift and drag coefficients. This analysis of the lift and drag coefficients enables a better evaluation of the Shields parameter for incipient motion as a function of the angle of repose . The theoretical developments presented in this paper are corroborated by the results of our laboratory experiments as well as many others available in the literature.
TL;DR: In this paper, a local time stepping strategy is proposed for mixed flow modeling, which adopts the Preissmann slot approach. But the results of several tests show that local time-stopping can reduce run time significantly, compared to the conventional global time-step, especially when only a small region of the domain is surcharged.
Abstract: Mixed flows in closed conduits are characterized by waves, celerity values of which lie within a range of up to two orders of magnitude due to the simultaneous occurrence of free-surface and pressurized flow. If an explicit numerical scheme is used to simulate these phenomena, the time step necessary to guarantee stability is considerably restricted by pressure wave celerity, and thus the computational efficiency is reduced. In order to address this specific problem this paper proposes the application of the local time stepping strategy to a finite-volume scheme for mixed flow modelling, which adopts the Preissmann slot approach. The results of several tests show that local time stepping reduces run time significantly, compared to the conventional global time stepping, especially when only a small region of the domain is surcharged. The accuracy and mass conservation in the proposed approach are not impaired. Moreover, in the free-surface region of the flow the accuracy slightly improves.
TL;DR: In this paper, an analytical solution approach for the steady two-dimensional suspended sediment transport in channels using the generalized integral transform technique (GITT) is proposed, which greatly extends the domain of analytical solutions for sediment transport problems.
Abstract: An analytical solution approach is proposed for the steady two-dimensional suspended sediment transport in channels using the generalized integral transform technique (GITT). Previous analytical solutions are limited by advection velocity distribution, eddy diffusivity distribution, or boundary conditions. The contribution of the new approach is to eliminate these limitations. It solves the suspended sediment transport equation with arbitrary advection velocity and eddy diffusivity distributions. The new solution approach also considers realistic boundary conditions at the free surface and the bed. Compared with previous analytical solutions as well as numerical solutions from a high resolution spectral code, the new approach is of high accuracy. It greatly extends the domain of analytical solutions for suspended sediment transport problems.
TL;DR: In this paper, a new runner surrogate model is introduced, where the relative flow angle is expressed via the so-called swirl-free velocity profile (represented with two parameters) and the model provides swirling flow profiles consistent with the hydraulic turbine operation.
Abstract: Operating hydraulic turbines over an extended range of discharge values requires an optimized runner that minimizes the weighted-average draft tube losses. It is shown that the draft tube losses can be minimized before actually designing the (new or refurbished) runner blades by optimizing the runner outlet flow. For this, a new runner surrogate model is introduced, where the relative flow angle is expressed via the so-called swirl-free velocity profile (represented with two parameters). Given a swirl-free velocity profile, as well as a turbine discharge value, the model provides swirling flow profiles consistent with the hydraulic turbine operation. With this swirling flow as inlet condition, the draft tube flow computation provides the relationship between the swirl-free velocity parameters and the hydraulic loss. It is shown that this approach can be used for optimizing the runner for a range of turbine operating regimes.
TL;DR: In this article, the double-averaged Navier-Stokes equations were solved using the Reynolds stress closure model for a depth-limited flow with submerged vegetation in a rectangular channel.
Abstract: This paper reports a numerical investigation of the characteristics of mean flow and turbulence statistics for a depth-limited flow with submerged vegetation in a rectangular channel. To achieve this, the double-averaged Navier–Stokes equations were solved using the Reynolds stress closure model. A zone with increased velocity was observed at the corner between sidewall and free surface, thus forming two velocity maxima, one at the centre of the channel and one at the corner. This is consistent with previous experimental observations. The velocity maximum at the corner can be attributed to the enlarged bottom vortex caused by submerged vegetation, which transports high-momentum fluids from the centre to the corner of the channel. The velocity maximum at the corner was found to occur in the flow with submerged vegetation regardless of roughness density and relative submergence. The velocity maximum at the corner affects the mean flow, but has a negligible effect on turbulence statistics.