Simulation Of Low Flows Research Articles

How can man‐made water reservoirs be accounted for in a lumped rainfall‐runoff model?

The evaluation of diffuse impacts far downstream of man‐made reservoirs is complex but useful for water resources management and impact studies. The influence of reservoirs is usually modeled using a spatially distributed approach. To date lumped approaches have not been used to address this problem, probably because the lumped mode is seen as a major obstacle to accounting for water works that are at specific locations within the watershed. Here we propose a way to explicitly account for artificial storages in a lumped hydrological model. The method was tested on a large set of 46 watersheds (in France, the United States and Brazil) influenced by 26 reservoirs representing various types of management objectives. The results indicate that the method developed can effectively account for reservoirs despite the lumped modeling approach, with significant improvements in the simulation of low flows, but more limited for high flows.

Low-Flows in Deterministic Modelling: A Brief Review

Deterministic hydrological models are limited in their ability to model low-flows, but as they are increasingly being used for low-flow studies, a review is timely if they are to be used for this purpose. The representations of the physical processes that govern low-flows are described for a selection of models developed and/or used in Canada. The models vary in the extent to which they incorporate low-flow processes, such as drawdown of storage in lakes, stream channels and wetlands, riparian evapotranspiration, freeze-up and bank storage. The representation of groundwater in the models is not physically-based and the hydrologic models must be coupled with distributed groundwater models to answer questions about groundwater withdrawals. Most models have not been rigorously tested for low-flow simulations. Considerable work remains to be done, especially in the adequate representation of low-flow processes, and in evaluation of models for low-flow studies by using low-flow specific evaluation criteria.

Numerical Simulation of Low Current Vacuum Arc Supersonic Flow

Based on a two-dimensional axisymmetric magneto-hydrodynamic (MHD) model, low current vacuum arc (LCVA) characteristics are studied. The influence of cathode process under different axial magnetic fields and different anode radii on LCVA characteristics is also simulated. The results show that the influence of both cathode process and anode radii on LCVA is significant. The sign of anode sheath potentials can change from negative to positive with the decrease of anode radii. The simulation results are in part verified by experimental results. Especially, as the effect of ion kinetic energy is considered, ion temperature is improved significantly; which is in agreement with experimental results.

A fractional step method for solving the compressible Navier–Stokes equations

This paper proposes a fractional step method for the calculation of compressible Navier–Stokes equations. The purpose of this study is to develop a robust and efficient numerical method for the simulation of low Mach number flows in which the poorly distributed eigenvalues usually result in the numerical difficulties. The method takes advantage of the pressure-based and the density-based methods. It thus accelerates the numerical convergence by adjusting the eigenvalues of the Jacobian matrix. In order to control the numerical instability and the spurious wave reflections at the numerical boundaries, a characteristic boundary condition is formulated. This numerical method as well as the boundary condition treatment extend the traditional fractional step method to the simulation of compressible Navier–Stokes equations. The performance and the accuracy of the method to calculate the flows with large amplitude of acoustic waves and strong heat transfer have been demonstrated by the direct numerical simulation of three distinct cases: a one-dimensional Euler equation with large amplitude of acoustic waves, an adiabatic turbulent boundary layer, and a turbulent boundary layer with heated wall.

한강 본류에의 적용을 위한 FLDWAV 모형의 개선

FLDWAV model was modified such that it can adequately simulate the effect of Jamsil and Singok submerged weirs in the main reach of the Han River. The enhanced model combines weir-type discharge equations for overflow at fixed weir and Manning equation for fluvial-type flow at the movable weir. Equations for weir overflow include those for submerged weir flow and free overflow. Gates of the movable weir may be open or closed for the simulation. In order to test the simulation capabilities, the enhanced model was applied for various flow conditions at submerged weirs. Backwater effect due to Jamsil and Singok submerged weirs were well simulated. Simulations were carried out for spring and neap tides extracted from artificial tide generated by combining tidal constituents. Simulation results cleared indicated that tidal effect extends further upstream as the flood discharge decreases. Low flow simulation capabilities of the enhanced model was tested. Discontinuities of water surface elevation due to the submerged weirs were successively simulated.

Solutions for variable density low Mach number flows with a compressible pressure-based algorithm

Two difficulties are always encountered in the simulation of variable density low Mach number flows by the primitive variable methods: (1) the decoupling of pressure and velocity caused by the extremely small velocity; (2) the spurious oscillations, nonsmooth solutions and sharp resolution caused by discontinuities, making that the flow cannot be successfully simulated by low-ordered schemes. In this paper, a pressure-based compressible flow solver with a scheme of high-resolution is developed to overcome these difficulties. In the present work, we adopt pressure correction algorithm to overcome the decoupling and the MUSCL scheme to predict the discontinuities at low Mach number. We applied the proposed compressible flow solver to simulate the compressible flows in a planar nozzle, arc bumps and the lid-driven cavity and found that the numerical results are in good agreement with those reported in the previous works, indicating that the numerical algorithm developed in this work is a reliable and accurate tool for studying thermal variable density low Mach number flows.

Kinetically reduced local Navier-Stokes equations: An alternative approach to hydrodynamics

An alternative approach, the kinetically reduced local Navier-Stokes (KRLNS) equations for the grand potential and the momentum, is proposed for the simulation of low Mach number flows. The Taylor-Green vortex flow is considered in the KRLNS framework, and compared to the results of the direct numerical simulation of the incompressible Navier-Stokes equations. The excellent agreement between the KRLNS equations and the incompressible nonlocal Navier-Stokes equations for this nontrivial time-dependent flow indicates that the former is a viable alternative for computational fluid dynamics at low Mach numbers.

A Velocity Extraction Method in Molecular Dynamic Simulation of Low Speed Nanoscale Flows

A new algorithm to extract the velocity caused by the external forces inmolecular dynamic simulation of nanoscale flow problems is proposed. The flowvelocity, an important component in these type of problems, is usually obtained from theaverage value in the time space because the accumulation of the thermal velocity willapproach zero when the time period is large, but this method is not always suitable,especially when the flow velocity is much smaller than the thermal velocity. Based on theidea of the linear accumulation of the flow velocity, in this study a new algorithm isderived to extract the flow velocity. This algorithm can be used to calculate nanoscaleflow problem no matter whether the value of the flow velocity is big or small. Using thisnew algorithm, the 2-D liquid flow of argon in nanochannels was simulated. Thenumerical result demonstrates the effectiveness of the new algorithm.

Artificial neural networks and high and low flows in various climate regimes

An algorithm coupling linear least squares and simplex optimization (LLSSIM) is used to examine the ability of a three-layer feedforward artificial neural network (ANN) to simulate the high and low flows in various climate regimes over a mountainous catchment (the Mesochora catchment in central Greece). The plot of the long-term annual catchment pseudo-precipitation (rain plus snowmelt) simulated by the snow accumulation and ablation model (SAA) of the US National Weather Service (US NWS) showed trends of three climatically distinct periods, described by clearly descending, rising and moderately descending segments in pseudo-precipitation. The ANN model was calibrated for each of the three climate types and each was validated against the others. A set of statistical measures and graphs adapted for high and low flows showed the robustness of the ANN model under various climates and transient conditions. The ANN model proved capable of simulating well the daily high and low flows when it is calibrated for increasing pseudo-precipitation and validated for moderately decreasing pseudo-precipitation. For the entire period, the ANN model provided a better simulation of high and low flows than the conceptual soil moisture accounting (SMA) model of the US NWS, which was also employed in this study. Because the ANN is not a physically-based model, it is by no means a substitute for the SMA model. However, it is concluded that the ANN approach is an effective alternative for daily high- and low-flow simulation and forecasting in climatically varied regimes, particularly in cases where the internal dynamics of the catchment do not require an explicit representation.

Calculation of sound generation and radiation from instationary flows

The Lattice–Boltzmann method (LBM) for simulation of low Mach number flows is evaluated for the application in flow acoustics. By linearization and von-Neumann analysis quantitative measures for the accuracy of phase speed and attenuation of low amplitude sound waves in presence of a mean flow are derived. It is shown that only phase errors are relevant when simulating sound waves in the audible frequency range in air. For the two dimensional 9 bit model and the three dimensional 19 bit model the phase speed error is below 0.1% (1%) as long as the wave is resolved with at least 34 (12) points per wavelength. The LBM is applied to the problem of a Helmholtz resonator under a grazing flow and to the trailing edge noise generation problem. The results clearly demonstrate the ability to reproduce relevant flow acoustic effects.

HYDROLOGICAL MODELING OF THE IROQUOIS RIVER WATERSHED USING HSPF AND SWAT

ABSTRACT: The performance of two popular watershed scale simulation models — HSPF and SWAT — were evaluated for simulating the hydrology of the 5,568 km2 Iroquois River watershed in Illinois and Indiana. This large, tile drained agricultural watershed provides distinctly different conditions for model comparison in contrast to previous studies. Both models were calibrated for a nine-year period (1987 through 1995) and verified using an independent 15-year period (1972 through 1986) by comparing simulated and observed daily, monthly, and annual streamflow. The characteristics of simulated flows from both models are mostly similar to each other and to observed flows, particularly for the calibration results. SWAT predicts flows slightly better than HSPF for the verification period, with the primary advantage being better simulation of low flows. A noticeable difference in the models' hydrologic simulation relates to the estimation of potential evapotranspiration (PET). Comparatively low PET values provided as input to HSPF from the BASINS 3.0 database may be a factor in HSPF's overestimation of low flows. Another factor affecting baseflow simulation is the presence of tile drains in the watershed. HSPF parameters can be adjusted to indirectly account for the faster subsurface flow associated with tile drains, but there is no specific tile drainage component in HSPF as there is in SWAT. Continued comparative studies such as this, under a variety of hydrologic conditions and watershed scales, provide needed guidance to potential users in model selection and application.

Efficient Large-eddy Simulations of Low Mach Number Flows Using Preconditioning and Multigrid

In the present paper, an implicit time accurate approach combined with multigrid, preconditioning and residual smoothing is used for the large-eddy simulation (LES) of low Mach number flow. In general, due to the restriction imposed on the time step by the physics of the flow, the advantage of an implicit method over an explicit one for LES is not obvious. It is shown that for the test cases considered in this paper, the present approach allows an efficiency gain of a factor 4–7 compared to the use of a purely explicit approach. The efficiency varies according to the test case, grid clustering, physical time step and requested residual drop. Numerical difficulties are catalogued and mitigatory procedures are introduced. Several problems with available experimental and DNS data are employed to verify the efficiency of the method.

Improvement of a parsimonious model for streamflow simulation

Hydrologists have been struggling over the past decades to improve rainfall-runoff models. As a consequence, models proposed 20–30 years ago still keep evolving as progress is made in the understanding of catchment hydrological behaviour. Here we present the GR4J model, a daily lumped rainfall-runoff model which is the result of a continuous improvement process over the last 15 years. The article provides the mathematical formulation of a new four-parameter version of the model. Model performance is assessed on a large sample of catchments: compared to other rainfall-runoff models, the GR4J performance is among the best ones. It also gives better results than the previous three-parameter model version, especially in the simulation of low flows. The tests indicate that a four-parameter structure corresponds to the maximum level of complexity that could be afforded in the model. Adding more free parameters did not bring significant improvements. The gain in model robustness with this new version should enhance the confidence in the practical use of this simple model for water engineering and resource management. The discussion underlines the potential limits introduced in the modelling process when one relies on a priori concepts in building a model structure and it stresses the value of large catchment samples to assess models.

Vers une amélioration d'un modèle global pluie-débit au travers d'une approche comparative

Click to increase image sizeClick to decrease image sizeThe study examined and evaluated 38 structures of lumped rainfall-runoff models on 429 catchments located in France, the United-States, Australia, the Ivory Coast and Brazil. All structures were evaluated at a daily time-step in the same testing framework and fed with the same amount of data. Their parameters were calibrated using a local optimisation search procedure that proved able to locate satisfactory sets of optimum parameters. Model performances in terms of quality of observed streamflow simulation were assessed with several quantitative criteria, including new formulations. Tests results indicate that a low number of free parameters (3 to 5) is sufficient to achieve a satisfactory modelling of the catchment hydrological behaviour, the increasing number of degrees of freedom in the model structure limiting model robustness. Furthermore model structure plays a key role in the quality of simulations. Some mathematical tools used in different structures appear complementary and their association in a single model may improve streamflow simulations. The structure of the simple GR3J model was thus modified into a more satisfactory four-parameter structure, with improved low flow simulations. Such a model can be used to solve many issues of water resources management and engineering.

Numerical Simulation of Low Reynolds Number Flows in In-Line Tube Banks

Numerical Simulation of Low Reynolds Number Flows in In-Line Tube Banks

A novel numerical scheme for reactive flows at low Mach numbers

For the simulation of low Mach number reactive flows with significant density changes a novel numerical scheme is proposed which contains modified lattice BGK model in combination with the conventional convective-diffusion solvers for equations of temperature and species. Together with boundary-fitting formulation and local 2nd order grid refinement (embedded grids) the scheme enables the accurate consideration of low Mach number combustion in complex geometry in the wide range of Reynolds and Damköler numbers on the simplest Cartesian grids.

A Novel Lattice BGK Approach for Low Mach Number Combustion

An extended lattice Boltzmann (BGK) model is presented for the simulation of low Mach number flows with significant density changes. For applications to reactive flows this new model is coupled with a finite-difference scheme for solving the transport equations of energy and species. With a boundary fitting formulation and local grid refinement the scheme enables accurate and efficient computations of low Mach number reactive flows in complex geometry on the simplest Cartesian grids. Examples of reactive flows around porous burners are presented.

Large-Scale Motions in the Marine Atmospheric Surface Layer

Multi-level turbulent wind data from the Riso Air-Sea Experiments (RASEX) were used to examine the structure of large-scale motions in the marine atmospheric surface layer. The quadrant technique was used to identify flux events (ejections/sweeps). Ejections, which appear to occur in groups, are seen to occur first at the upper level, moving successively to lower levels with small time delays. A strong correlation between events at different heights suggests that they may all be part of a single large structure. Cross-correlation between velocity signals was used to estimate orientation of the structure using Taylor's hypothesis. The inclination of this structure is shallow (≃ 15°) near the surface and increases with height. Spatial representations of the fluctuating wind vectors show a structure that is strikingly similar to conceptual models of transverse vortices and shear layers seen in laboratory flows and direct numerical simulation (DNS) of low Reynolds number flows. Spatial visualization of velocity fluctuations during other time periods and conditions clearly shows the existence of shear layers, transverse vortices, plumes, and downdrafts of various sizes and strengths. A quantitative analysis shows an increase in the frequency of shear related events with increasing wind speed.

Lattice-BGK Model for Low Mach Number Combustion

For the simulation of low Mach number reactive flows with significant density changes a modified lattice-BGK model in combination with the conventional convective-diffusion solvers for equations of temperature and species is proposed. Together with boundary-fitting conditions and local grid refinement the scheme enables the accurate consideration of low Mach number combustion in complex geometry as the flows around porous burners or droplets combustion.

Viscous flow with large fluid-fluid interface displacement

The arbitrary Lagrange–Euler (ALE) kinematic description has been implemented in a 3D transient finite element program to simulate multiple fluid flows with fluid–fluid interface or surface displacements. The description of fluid interfaces includes variable interfacial tension, and the formulation is useful in the simulation of low and intermediate Reynolds number viscous flow. The displacement of two immiscible Newtonian fluids in a vertical (concentric and eccentric) annulus and a (vertical and inclined) tube is simulated. © 1998 John Wiley & Sons, Ltd.