Free-surface Channel Flow Research Articles

Free-surface channel flow around a square cylinder

The free-surface channel flow around a square cylinder is analysed, over a wide range of blocking ratios, using three-dimensional simulations. The state of the flow is characterised in terms of the Froude number upstream and downstream of the square cylinder. The simulations confirm the presence of the subcritical and choked states, and provide new insight into the supercritical state and band-gap through an analysis of how the momentum flux varies with Froude number along the channel. The influence of the blocking ratio on the flow state and drag force is analysed and shows the significant rise of drag in the choked regime.

Application of video imagery techniques for low cost measurement of water surface velocity in open channels

Developments in digital video recording technology make the video imagery tools more popular for velocity measurement in water flows. This has especially been of large interest due to its inherent advantage of non-contact nature which is quite handy in extreme flow conditions. Particle Image Velocimetry (PIV), Particle Tracking Velocimetry (PTV) and Large Scale Particle Tracking Velocimetry (LSPTV) are applied to free surface channel flow for water surface velocity measurement. Experiments are conducted to measure either a single point velocity applying PTV or velocity profiles across the channel width applying PIV on the water surface in a rectang typical velocities of nearly 1 andular tilting flume for various flow conditions. Technical issues regarding tracer particle size and type, travel distance, lighting, recording speed, camera position, image distortion and state of flow are discussed. Measured data is compared to computational results obtained from a numerical model involving a non-linear turbulence model capable of predicting turbulence driven secondary flows. Confirmation of reasonable match between computational and experimental results whereby applying mutual collaboration of them for discharge measurement has been attested. In addition to discharge, boundary roughness has also been predicted as an outcome of the numerical solution.

Pulsation effects on pollutant and sediment transport in free-surface flow

A series of numerical simulations are performed to study the pollutant and sediment transport in free surface channel flow. The present paper examines the dispersion of passive contaminants injected from a time periodic source in a fully developed turbulent flow. More precisely, the pulsation effects on the distribution behaviors of dissolved and particulate pollutants are analyzed and discussed. Simulations are carried out using a commercial Computational Fluid Dynamic (CFD) code, Fluent 6.3, which is based on the finite volume approach. The standard k−ε turbulence closure model is selected to simulate the turbulence generation and the Volume of Fluid (VOF) method is used to accurately capture the time varying free surface. The Discrete Phase Model (DPM) is used for capturing the movement of particles. Numerical results show that increasing pulsation amplitude and decreasing frequency generates higher dispersive effects in the concentration profiles of a dissolved pollutant. It is also concluded that, unlike dissolved substances, the particle transportation can be enhanced only for certain combinations of the pulsation amplitude and frequency due to the synchronization of the particle’s movement with the oscillating potential.•Increasing pulsation amplitude and decreasing frequency generates higher dispersive effects.•Particle transportation can be enhanced only for certain amplitude-frequency combinations.

Force acting on a square cylinder fixed in a free-surface channel flow

AbstractWe describe an experimental study of the forces acting on a square cylinder (of width $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}b$) which occupies 10–40 % of a channel (of width $w$), fixed in a free-surface channel flow. The force experienced by the obstacle depends critically on the Froude number upstream of the obstacle, ${\mathit{Fr}}_1$ (depth $h_1$), which sets the downstream Froude number, ${\mathit{Fr}}_2$ (depth $h_2$). When ${\mathit{Fr}}_1<{\mathit{Fr}}_{1c}$, where ${\mathit{Fr}}_{1c}$ is a critical Froude number, the flow is subcritical upstream and downstream of the obstacle. The drag effect tends to decrease or increase the water depth downstream or upstream of the obstacle, respectively. The force is form drag caused by an attached wake and scales as $\overline{F_{D}}\simeq C_D \rho b u_1^2 h_1/2$, where $C_D$ is a drag coefficient and $u_1$ is the upstream flow speed. The empirically determined drag coefficient is strongly influenced by blocking, and its variation follows the trend $C_D=C_{D0}(1+C_{D0}b/2w)^2$, where $C_{D0}=1.9$ corresponds to the drag coefficient of a square cylinder in an unblocked turbulent flow. The r.m.s. lift force is approximately 10–40 % of the mean drag force and is generated by vortex shedding from the obstacle. When ${\mathit{Fr}}_1={\mathit{Fr}}_{1c}\, (<1)$, the flow is choked and adjusts by generating a hydraulic jump downstream of the obstacle. The drag force scales as $\overline{F}_D\simeq C_K \rho b g (h_1^2-h_2^2)/2$, where experimentally we find $C_K\simeq 1$. The r.m.s. lift force is significantly smaller than the mean drag force. A consistent model is developed to explain the transitional behaviour by using a semi-empirical form of the drag force that combines form and hydrostatic components. The mean drag force scales as $\overline{F_{D}}\simeq \lambda \rho b g^{1/3} u_1^{4/3} h_1^{4/3}$, where $\lambda $ is a function of $b/w$ and ${\mathit{Fr}}_1$. For a choked flow, $\lambda =\lambda _c$ is a function of blocking ($b/w$). For small blocking fractions, $\lambda _c= C_{D0}/2$. In the choked flow regime, the largest contribution to the total drag force comes from the form-drag component.

A liquid metal flume for free surface magnetohydrodynamic experiments

We present an experiment designed to study magnetohydrodynamic effects in free surface channel flow. The wide aspect ratio channel (the width to height ratio is about 15) is completely enclosed in an inert atmosphere to prevent oxidization of the liquid metal. A custom-designed pump reduces entrainment of oxygen, which was found to be a problem with standard centrifugal and gear pumps. Laser Doppler velocimetry experiments characterize velocity profiles of the flow. Various flow constraints mitigate secondary circulation and end effects on the flow. Measurements of the wave propagation characteristics in the liquid metal demonstrate the surfactant effect of surface oxides and the damping of fluctuations by a cross-channel magnetic field.

A moving unstructured staggered mesh method for the simulation of incompressible free-surface flows

A new moving staggered mesh discretization for the numerical simulation of incompressible flow problems involving free-surfaces is presented. The method uses the staggered mesh to obtain speed and conservation properties. Mesh motion provides a high quality mesh in the interior and detailed resolution of the free-surface motion on the surface. Mesh flipping allows for optimal mesh connectivity to be maintained. The method uses an exact projection procedure which reduces the number of unknowns as well as satisfying the continuity constraint without solving a pressure Poisson equation. The implementation of surface tension forces in the staggered mesh framework is discussed. The resulting method is tested against analytical solutions for liquid sloshing and free-surface channel flow. It is also demonstrated on the cases of droplet collision, three-dimensional sloshing, and turbulence next to a free-surface.

HIGHER ORDER NUMERICAL METHODS EVALUATION FOR THE COMPUTATION OF ONE DIMENSIONAL FREE SURFACE SHALLOW WATER FLOWS

International Journal of Computational Engineering ScienceVol. 03, No. 01, pp. 13-55 (2002) No AccessHIGHER ORDER NUMERICAL METHODS EVALUATION FOR THE COMPUTATION OF ONE DIMENSIONAL FREE SURFACE SHALLOW WATER FLOWSA. I. DELISA. I. DELISDepartment of Applied Mathematics, University of Crete, 71409 Heraklion, Crete, Greece Search for more papers by this author https://doi.org/10.1142/S1465876302000538Cited by:3 PreviousNext AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsRecommend to Library ShareShare onFacebookTwitterLinked InRedditEmail AbstractSeveral explicit high resolution TVD methods and their detailed application to steady and unsteady one-dimensional free surface channel flow problems are presented. Some of these methods are widely presented in the literature. Their evaluation is based on computations performed for steady state benchmark problems with analytical solutions and for a wide range of cases, which include friction, nonuniform bed slopes, hydraulic jumps and non-prismatic channel cross sections. Results for the classical benchmark problem of an idealised dam-break are also presented. The results obtained by the various TVD methods are compared with each other and the exact solutions, providing insight into the limitations of the methods along with their relative performances in robustness and accuracy. Some subtle points of the methods and the effects of selecting different options for certain methods are emphasized.Keywords:Finite DifferencesShallow WaterTVDComputational Hydraulics Engineering FiguresReferencesRelatedDetailsCited By 3A finite volume method parallelization for the simulation of free surface shallow water flowsA.I. Delis and E.N. Mathioudakis1 Jul 2009 | Mathematics and Computers in Simulation, Vol. 79, No. 11Application of the HLL Approximate Riemann Solver to One-Dimensional Open Channel Flows with Irregular GeometryXinya Ying and Sam S. Y. Wang26 April 2012Improved implementation of the HLL approximate Riemann solver for one-dimensional open channel flowsXinya Ying and Sam S.Y. Wang26 April 2010 | Journal of Hydraulic Research, Vol. 46, No. 1 Recommended Vol. 03, No. 01 Metrics History KeywordsFinite DifferencesShallow WaterTVDComputational Hydraulics EngineeringPDF download

Large eddy simulation of free surface shallow-water flow

Shallow-water flow with free surface frequently occurs in ambient water bodies, in which the horizontal scale of motion is generally two orders of magnitude greater than the water depth. To accurately predict this flow phenomenon in more detail, a three-dimensional numerical model incorporating the method of large eddy simulation (LES) has been developed and assessed. The governing equations are split into three parts in the finite difference solution: advection, dispersion and propagation. The advection part is solved by the QUICKEST scheme. The dispersion part is solved by the central difference method and the propagation part is solved implicitly using the Gauss–Seidel iteration method. The model has been applied to free surface channel flow for which ample experimental data are available for verification. The inflow boundary condition for turbulence is generated by a spectral line processor. The computed results compare favourably with the experimental data and those results obtained by using a periodic boundary condition. The performance of the model is also assessed for the case in which anisotropic grids and filters with horizontal grid size of the order of the water depth are used for computational efficiency. The coarse horizontal grid was found to cause a significant reduction in the large-scale turbulent motion generated by the bottom turbulence, and the turbulent motion is predominately described by the sub-grid scale (SGS) terms. The use of the Smagorinsky model for SGS turbulence in this situation is found inappropriate. A parabolic mixing length model, which accounts for the filtered turbulence, is then proposed. The new model can reproduce more accurately the flow quantities. Copyright © 2000 John Wiley & Sons, Ltd.

Myths, Math, and Physics of Free-Surface Vorticity

The boundary equations for the production of vorticity are examined in the case of a free surface. Mathematical manipulation of the equations leads to interesting results which are given physical interpretation through examination of three flows: a surface wave, an accelerated free-surface channel flow, and the vortex ring interaction with a free surface. The predicted phenomena are unusual, and require abandonment of preconceptions regarding the notion that vorticity is angular momentum. The flux and appearance of vorticity in free-surface flows is shown to be a consequence of necessary viscous accelerations at the boundary even for otherwise irrotational subsurface flow. The deformation of the free surface is the critical factor separating the free-surface problem from the vortex image analogy.

Bistable Flow Patterns in a Free Surface Water Channel

A series of experiments is reported on a free surface water tunnel with a slot inlet centered at mid-depth in which the flow exhibited equally probable bistable flow patterns. The two flow fields are strongly asymmetric even when the geometry is symmetric and consists of a long stall on one side of the inlet and a short stall on the opposite side. Flow visualization and laser-Doppler velocimetry were performed to examine the flow structure of both stable states in detail. Results showed that the mean flow and turbulence structure of the two bistable states are largely mirror images of each other within the separation zone. The effect of the wall on the flow is minor, only very close to the wall/free surface did the difference in wall constraints cause the two flow patterns to diverge. After reattachment both flows relax to free-surface channel flow. It is shown that the bistable flow pattern results from the interaction of the free shear layers and that only strong disturbances in the free shear layers can cause the flow in one stable state to switch to the other stable state. Bistable flow exists for geometries with asymmetry up to 10 percent for the expansion ratio studied.

Structural features of turbulent flow over smooth and rough boundaries

An experimental study of boundary-layer turbulence in a free surface channel flow is described. Attention is concentrated on the effects of different surface roughness conditions on the turbulence structure in the boundary region. Hydrogen bubble flow tracers and medium high-speed motion photography were used to obtain an instantaneous visual and quantitative description of the flow field. In particular it proved possible to record instantaneous longitudinal and vertical velocity profiles from which distributions of the instantaneous Reynolds stress contribution were computed.Two well-defined intermittent features of the flow structure were visually identified close to the boundary. These consisted of fluid ejection phases, previously reported by Kline et al. (1967) for smooth boundary flow, and fluid inrush phases. Conditional averaging of the instantaneous velocity data yielded quantitative confirmation that ejection phases corresponded with ejection of low momentum fluid outwards from the boundary whilst inrush phases were associated with the transport of high momentum fluid inwards towards the boundary. Inrush and ejection events were present irrespective of the surface roughness condition.Conditional averaging also indicated that both inrush and ejection sequences correlate with an extremely high contribution to Reynolds stress and hence turbulence production close to the boundary. Indeed the present results, taken with those from previous studies, suggest that turbulence production is dominated by the joint contribution from the inrush and ejection events. It is emphasized that these structural features are intermittent, forming important linked elements of a randomly repeating cycle of wall-region turbulence production which is apparently driven by some violent three-dimensional instability mechanism.Whilst the most coherent effects of the observed inrush phases appear to be mainly confined to a region close to the boundary, the influence of the ejection phases is far more extensive. The ejected low momentum fluid elements, drawn from the viscous sublayer and from between the interstices of the roughness elements, travel outwards from the boundary into the body of the flow and give rise to very large positive contributions to Reynolds stress at points remote from the boundary. This effect is sufficiently strong to prompt the suggestion that the ejection process could represent a universal and dominant mode of momentum transport outside the immediate wall region and possibly extending across the entire thickness of the boundary layer.A structural model based on the present observations is seen to exhibit consistency with many commonly visualized features and recorded average properties of turbulent boundary-layer flows in general.