Simulation Of Open Channel Flow Research Articles

風応力が作用する気液界面におけるガス交換機構の直接数値シミュレーション

We carried out a direct numerical simulation of open-channel flow with wind shear stress to investigate numerically the effect of the wind stress on the air-water gas transfer. The numerical results show the wind shear to affect significantly turbulent eddies close to the air-water interface. Spatial patterns of the surface divergence and the interfacial gas flux are stretched in the wind direction,and the similarity between both patterns is found to decrease with increase of the wind stress. The cross-correlation coefficients between the gas flux and the spanwise vorticity are also obviously changed by the presence of the wind stress. The gas transfer velocity in this case doesn't agree with the solution of the surface divergence model. This suggests that the other dynamical mechanism promotes the gas transfer at the wind-sheared interface.

Spatial and temporal scales of force and torque acting on wall-mounted spherical particles in open channel flow

Data from direct numerical simulation of open channel flow over a geometrically rough wall at a bulk Reynolds number of Reb = 2900, generated by Chan-Braun et al. [“Force and torque acting on particles in a transitionally rough open-channel flow,” J. Fluid Mech. 684, 441–474 (2011)]10.1017/jfm.2011.311 are further analysed with respect to the time and length scales of force and torque acting on the wall-mounted spheres. For the two sizes of spheres in a square arrangement (11 and 49 wall units in diameter, yielding hydraulically smooth and transitionally rough flow, respectively), the spatial structure of drag, lift, and spanwise torque is investigated. The auto-correlation and spectra in time as well as the space-time correlation and convection velocities are presented and discussed. It is found that the statistics of spanwise particle torque are similar to those of shear stress at a smooth wall. Particle drag and lift are shown to differ from spanwise particle torque, exhibiting considerably smaller time and length scales; the convection velocities of drag and lift are somewhat larger than those of spanwise torque. Furthermore, correlations between the flow field and particle-related quantities are presented. The spatial structure of the correlation between streamwise velocity and drag/spanwise torque features elongated shapes reminiscent of buffer-layer streaks. The correlation between the pressure field and the particle drag exhibits two opposite-signed bulges on the upstream and downstream sides of a particle.

A mesh-free method boundary condition technique in open channel flow simulation

The mesh-free particle method has been successfully used in simulation of various types of flow. Small particle distance is typically used for representing the form of the simulation domain, requiring a large amount of computing time, accordingly, and, thus, seriously affecting the efficiency of the mesh-free method. In order to optimize the efficiency and accuracy of the mesh-free method in simulating open channel flow, a new approach based on the assigning velocity to ghost particles next to the solid boundary is developed. This ghost particle velocity correction also considers the bed roughness, the flow depth and the particle distance all together so that larger particle distance can be used in open channel flow simulations with the mesh-free method, requiring less simulation time, while the correct velocity distribution over depth is still maintained. Experimental cases are used to compare the numerical results to show the advantages of using this improved method.

Modeling Flow in an Open Channel with Heterogeneous Bed Roughness

AbstractPhysical and numerical simulations of open channel flow over a heterogeneously roughened bed are examined. The velocity field is mapped at four different cross sections by using an acoustic Doppler velocimeter and the boundary shear stress obtained from both the velocity data and application of a Preston tube. These data illustrate that in the current arrangement the roughness elements dominate the physics of the flow and act as a source of vorticity, which is initially manifested as local boundary shear stress and, in turn, affects the secondary flow structures and momentum transfer in the channel. This paper presents the first attempt at using the Shiono–Knight method to model velocity and boundary shear stress distributions in a heterogeneous open channel. Appropriate advice concerning values of calibration parameters and discretization of the cross section is given. The combined physical/numerical approach provides an insight into the changing properties of the flow structure within a nonunifo...

Direct numerical simulations of stratified open channel flows

We carry out numerical simulations of wall-bounded stably stratified flows. We mainly focus on how stratification affects the near-wall turbulence at moderate Reynolds numbers, i.e. Reτ = 360. A set of fully-resolved open channel flow simulations is performed, where a stable stratification has been introduced through a negative heat flux at the lower wall. In agreement with previous studies, it is found that turbulence cannot be sustained for h/L values higher than 1.2, where L is the so-called Monin-Obukhov length and h is the height of the open channel. For smaller values, buoyancy does not re-laminarize the flow, but nevertheless affects the wall turbulence. Near-wall streaks are weakly affected by stratification, whereas the outer modes are increasingly damped as we move away from the wall. A decomposition of the wall-normal velocity is proposed in order to separate the gravity wave and turbulent flow fields. This method has been tested both for open channel and full channel flows. Gravity waves are likely to develop and to dominate close to the upper boundary (centerline for full channel). However, their intensity is weaker in the open channel, possibly due to the upper boundary condition. Moreover, the presence of internal gravity waves can also be deduced from a correlation analysis, which reveals (together with spanwise spectra) a narrowing of the outer structures as the stratification is increased.

MPS-Based Mesh-Free Particle Method for Modeling Open-Channel Flows

Dealing with large deformation and fragmentation of geometries and interfaces (e.g., free surfaces), the regular mesh-based Eulerian methods, such as finite-element and finite-difference methods, have difficulties in fluid-flow modeling. Recently, studies have focused on a new generation of numerical methods called mesh-free particle (Lagrangian) methods. In this study, a mesh-free particle method based on the moving-particle semi-implicit (MPS) particle-interaction model has been developed for simulation of open-channel flow. The model is able to simulate viscous fluid flow with large deformation and fragmentation of free surface in practical fields. Moreover, the model is capable of modeling open-channel problems with both inflow and outflow and inconstant numbers of particles. The model has been validated and applied to some common sample problems. The results show the reasonable accuracy of the model. The final model is capable of modeling free-surface deformation and fragmentation as well as accurate...

Force and torque acting on particles in a transitionally rough open-channel flow

AbstractDirect numerical simulation of open channel flow over a geometrically rough wall has been performed at a bulk Reynolds number of ${\mathit{Re}}_{b} \approx 2900$. The wall consisted of a layer of spheres in a square arrangement. Two cases have been considered. In the first case the spheres are small (with diameter equivalent to $10. 7$ wall units) and the limit of the hydraulically smooth flow regime is approached. In the second case the spheres are more than three times larger ($49. 3$ wall units) and the flow is in the transitionally rough flow regime. Special emphasis is given to the characterisation of the force and torque acting on a particle due to the turbulent flow. It is found that in both cases the mean drag, lift and spanwise torque are to a large extent produced at the top region of the particle surface. The intensity of the particle force fluctuations is significantly larger in the large-sphere case, while the trend differs for the fluctuations of the individual components of the torque. A simplified model is used to show that the torque fluctuations might be explained by the spheres acting as a filter with respect to the size of the flow scales which can effectively generate torque fluctuations. Fluctuations of both force and torque are found to exhibit strongly non-Gaussian probability density functions with particularly long tails, an effect which is more pronounced in the small-sphere case. Some implications of the present results for sediment erosion are briefly discussed.

Large Eddy Simulation of open channel flows with non-submerged vegetation

Results of several Large Eddy Simulations (LES) of open channel flows with non-submerged vegetation are presented in this article. It is shown that the vegetation can make the flow structure in the mainstream direction uniform for both supercritical and subcritical flows. For subcritical flows, the LES results of the ensemble-average of time-averaged velocity distributions at four vertical sections around a single plant are in good agreement with measurements. The velocity sees double peaks at the upper and lower positions of flows. For supercritical flows, the ensemble-average velocities see some discrepancy between LES and measurement results. Some secondary flow eddies appear near the single plant, and they just locate in the positions of the double peaks in stream-wise velocity profiles. It is also found that the vegetation drag coefficient deceases as the Froude number increases.

Structure of Turbulent Channel Flow under Spanwise System Rotation

Direct numerical simulation of turbulent channel flow under spanwise system rotation is performed. When system rotation is imposed on turbulent Poiseuille flow, the Reynolds number decreases in one side of a channel, while it increases in the other side. Hence, the effects of the Reynolds number as well as spanwise system rotation cannot be negligible in turbulent Poiseuille flow. To exclude the Reynolds-number effects, we performed the numerical simulation of open channel flow where the free-slip boundary condition is imposed on one wall, and the Reynolds number defined by the friction velocity and channel width can be always constant. When system rotation is imposed to enhance the spanwise vorticity of the mean shear, large-scale turbulence is attenuated and the mean velocity gradient increases over an entire channel. In contrast, when system rotation is imposed in the opposite sense to the mean spanwise vorticity, the mean velocity is leveled and small-scale turbulence is markedly decreased. We also investigate the effects of system rotation on the longitudinal vortical structure typically observed in near-wall turbulence.

A third-order explicit central scheme for open channel flow simulations

A third-order explicit time-stepping model for the simulations in open channels flow is presented. The time discretization of one-dimensional Saint- Venant's basic equations is obtained by Taylor series expansion, formulated in multi-stage approach, whereas the spatial discretization is obtained by finite difference central scheme. First- and second-order models have been obtained by using piecewise constant and piecewise linear MUSCL (monotone upwind scheme for conservation laws) approximations. Afterwards, the third-order model proposed is obtained by a piecewise quadratic polynomial approximation. Using Fourier's linear analysis, the stability and the accuracy of the scheme are investigated. The numerical results, for predicting dam-break in a horizontal and frictionless channel and for the representation of the hydraulic jump in prismatic and non-prismatic channels, with non-uniform bottom slope, are evaluated, and compared with the corresponding analytical and measured solutions.

Improved treatment of source terms in TVD scheme for shallow water equations

A number of high-resolution schemes have been recently developed to solve the homogeneous form of the shallow water equations. However, most approximate Riemann solvers experience difficulties with natural river applications if the irregular bed topography is not handled correctly. Based on the finite-difference flux-limited total variation diminishing (TVD) scheme, this paper develops a simple approach to handle the source terms for the one-dimensional open channel flow simulation with rapidly varying bed topography. Conclusions on the validity of the operator-splitting approach, the eigenvector-projection approach, and the proposed approach are presented. Analytical solution, experimental data, and available numerical result comparisons are shown to demonstrate the accuracy, robustness, stability, simplicity, and applicability of the proposed model.

Improved treatment of source terms in TVD scheme for shallow water equations

A number of high-resolution schemes have been recently developed to solve the homogeneous form of the shallow water equations. However, most approximate Riemann solvers experience difficulties with natural river applications if the irregular bed topography is not handled correctly. Based on the finite-difference flux-limited total variation diminishing (TVD) scheme, this paper develops a simple approach to handle the source terms for the one-dimensional open channel flow simulation with rapidly varying bed topography. Conclusions on the validity of the operator-splitting approach, the eigenvector-projection approach, and the proposed approach are presented. Analytical solution, experimental data, and available numerical result comparisons are shown to demonstrate the accuracy, robustness, stability, simplicity, and applicability of the proposed model.

Assessing the credibility of a series of computational fluid dynamic simulations of open channel flow

AbstractRecent developments in numerical algorithms have enabled the construction of three‐dimensional models for the prediction of flows in open channels. These advances encompass improvements in both numerical solutions and the process representation required for an accurate system definition. However, to date, there is still little agreement on how to assess systematically and report the credibility of these simulations. This paper addresses this problem by adopting a Grid Convergence Index approach. The results indicate, for two simple hypothetical cases, a zero‐degree confluence and a meander bend, that the numerical code can be verified to an acceptable numerical standard. However, it is shown that this does not mean that verification is complete, as the literature implies, as whilst the discretization resolution may be sufficient to verify one of the model variables it does not imply that every variable has converged. Furthermore, the scheme may still be insufficient to capture all the processes of interest that are operating within the chosen environment. Copyright © 2003 John Wiley & Sons, Ltd.

Direct Numerical Simulation of Open Channel Flow with Buoyancy

AbstractFree surface flow is important in chemical reactions, heat exchange, atmospheric science and industrial technology. In terms of fundamental research, turbulence near a free surface is important in transport phenomenon. However, free surface effects exhibit some nonlinear behavior, so that numerical methods are usually employed instead of analytical approaches. In this investigation, direct numerical simulation is applied to turbulent channel flow with a free surface. The Navier‐Stokes equations are solved using a finite difference method, and an interpolation scheme is applied to the convective term. Grid resolution is 128x65x128 and Reynolds number based on the friction velocity and channel elevation is 150. Slip wall approximations have been used extensively. This means that the free surface has no stress, velocity or deformation. In order to solve such problems, low Froude number approximations have recently been presented and these are used to calculate the free surface flow. In turbulent flow, dissipation and production are essential qualities, Reynolds‐stresses balances and pressure‐strain correlations are examined numerically. It is shown that the main difference between free surface and slip wall approximations is in pressure‐strain correlations near the surface.

Sample Size Determination in Open-Channel Inverse Problems

Open-channel flow simulations require values of friction parameters that are determined through formulating their inverse problems using a sample of historic events. However, there is a risk of the parameter values being biased toward the events used because of the sample size of historic events. Traditional sample-size determination approaches are known to suffer from a conflict between reliability and costs. In addition, there is no objective approach to determine a minimum size. These problems are solved through formulating the new “confidence calculation method,” which builds on the following: (1) the data points of a “parent sample” of friction parameters are resampled into “subsample populations,” where their “subsample means” contain latent information governed by the “central limit theorem”; (2) a rigorously derived formula makes it possible to calculate standard deviations of subsample populations, replacing direct resampling operations with mathematical ones; and (3) the method is parameterized ...

A numerical study of free-surface turbulence in channel flow

Direct numerical simulations of open-channel flow indicate that turbulence at the free surface contains large-scale persistent structures. They are ‘‘upwellings’’ caused by impingement of bursts emanating from the bottom boundary; ‘‘downdrafts’’ in regions where adjacent upwellings interact, and whirlpool-like ‘‘attached vortices’’ which form at the edge of upwellings. The attached vortices are particularly long-lived in the sense that once formed, unless destroyed by other upwellings, they tend to interact with each other and dissipate only slowly. If turbulence generation at the bottom wall is turned off by changing the boundary condition to free slip, then the upwellings (related to bursts) and downdrafts no longer form. The dominant structures at the free surface become the attached vortices which pair, merge, and slowly dissipate. In the central regions, as expected, the structure remains three dimensional throughout the decay process. Near the free surface, the structure appears to be quasi- two dimensional, as indicated by quantitative measures such as energy spectra, interwave number energy transfer, invariants of the anisotropy tensor, and length scales. In the decaying case, the quasi-two-dimensional region increases in thickness, with decay time, though the structure in the central regions of the flow remains three dimensional.

Turbulent simulation of open channel flow at low Reynolds number

A numerical technique for simulating turbulent flows in which the free surface is allowed to undergo arbitrarily large deformations and is subject only to a maximum slope limit is applied to turbulent open channel flow at a Reynolds number of approximately 3000 based on the surface velocity and depth. The test problem has been extensively studied in the literature and allows detailed comparisons to be made. It is found that the method is in general agreement with published results and can be used for a more extensive examination of turbulent fluid mechanics at a free surface.

A dissipative finite element model for discontinuous unsteady flow in open channel

The purpose of this paper is to present a dissipative finite-element formulation applicable to discontinuous one-dimensional open-channel flow. The traditional Galerkin approximation seems to be inappropriate for simulation of open-channel flow when discontinuities are introduced in the flow domain; the solutions show undesirable oscillations in the vicinity of the discontinuity. The method presented herein is a weighted residual formulation based on discontinuous weighting functions which are not identical to the shape functions; it provides smoother and more accurate results than the standard Galerkin approximation, offering to the finite-element method the ability of simulating steep wave fronts. The dissipative model is applied to flow with surges in a rectangular horizontal channel. Computational results are compared with experimental data and with results obtained by finite-difference schemes.

Two dimension-curvilinear grid for open channel flow simulation

A mathematical model for two-dimensional flow simulation in an open channel is developed. The model is obtained through the use of a stretched curvilinear grid which defines points where velocity and surface elevation are simulated. Simulation is achieved by numerically integrating the Navier-Stokes mass and momentum equations using centred finite difference approximations of derivatives. The model uses an implicit discretization scheme, the Newton-Raphson iterative technique and a customized Gauss elimination solving algorithm. The computer program developed for this model was tested for uniform, non-uniform and unsteady flow conditions. The results have been found consistent with theoretical solutions and/or field measurements. Limitation and verification of the model is also outlined.