scholarly journals Turbulent and viscous sediment transport – a numerical study

2014 ◽  
Vol 37 ◽  
pp. 73-80 ◽  
Author(s):  
O. Durán ◽  
B. Andreotti ◽  
P. Claudin
Keyword(s):  
Shear Stress ◽  
Sediment Transport ◽  
Numerical Study ◽  
Fluid Velocity ◽  
Density Ratio ◽  
Shear Velocity ◽  
Transport Layer ◽  
Two Phase ◽  
Unit Surface ◽  
The Mean

Abstract. Sediment transport is studied as a function of the grain to fluid density ratio using two phase numerical simulations based on a discrete element method (DEM) for particles coupled to a continuum Reynolds averaged description of hydrodynamics. At a density ratio close to unity (typically under water), sediment transport occurs in a thin layer at the surface of the static bed, and is called bed load. Steady, or "saturated" transport is reached when the fluid borne shear stress at the interface between the mobile grains and the static grains is reduced to its threshold value. The number of grains transported per unit surface therefore scales as the excess shear stress. However, the fluid velocity in the transport layer remains almost undisturbed so that the mean grain velocity scales with the shear velocity u*. At large density ratio (typically in air), the vertical velocities are large enough to make the transport layer wide and dilute. Sediment transport is then called saltation. In this case, particles are able to eject others when they collide with the granular bed. The number of grains transported per unit surface is selected by the balance between erosion and deposition and saturation is reached when one grain is statistically replaced by exactly one grain after a collision, which has the consequence that the mean grain velocity remains independent of u*. The influence of the density ratio is systematically studied to reveal the transition between these two transport regimes. Finally, for the subaqueous case, the grain Reynolds number is lowered to investigate the change from turbulent and viscous transport.

Numerical Study of Impingement and Film Composite Cooling on Blade Leading Edge

2014 ◽  
Author(s):  
Zhao Liu ◽  
Lv Ye ◽  
Zhenping Feng
Keyword(s):  
Shear Stress ◽  
Nusselt Number ◽  
Numerical Study ◽  
Density Ratio ◽  
Leading Edge ◽  
Cooling Effectiveness ◽  
Simulation Accuracy ◽  
Film Composite ◽  
Blowing Ratio ◽  
Shear Stress Transport

In this paper a numerical study is performed to simulate the impingement and film composite cooling on the first stage rotor blade of GE-E3 engine high pressure turbine. A commercial CFD software CFX11.0 with a 3D RANS approach is adopted in the study. Firstly, by comparing with available experimental data, the relative performance of four turbulence models for numerical impingement and film composite cooling is studied, including the standard k-ε model, the RNG k-ε model, the standard k-ω model and the Shear-Stress Transport k-ω model. The Shear-Stress Transport k-ω model is chosen for the numerical study as it shows the best simulation accuracy. Then the simulations consist of five different density ratios (1.16∼4.81) and seven different blowing ratios (0.5∼3.0). The results indicate that the cooling effectiveness on pressure side is lower than that on the suction side. The cooling effectiveness increases with the increase of blowing ratio in the study range, but decreases with the increase of density ratio. On the target surface, the average Nusselt number, the circumferential averaged Nusselt number and its peak value increase with the increasing in blowing ratio, but decrease with the increase of density ratio.

A numerical study of the relaxation and breakup of an elongated drop in a viscous liquid

2009 ◽  
Vol 640 ◽  
pp. 235-264 ◽  
Author(s):  
SHAOPING QUAN ◽  
DAVID P. SCHMIDT ◽  
JINSONG HUA ◽  
JING LOU
Keyword(s):  
Viscosity Ratio ◽  
Stokes Equations ◽  
Numerical Study ◽  
Fluid Velocity ◽  
Density Ratio ◽  
Length Ratio ◽  
Tetrahedral Mesh ◽  
Vortex Rings ◽  
Breakup Process ◽  
Satellite Droplet

The relaxation and breakup of an elongated droplet in a viscous and initially quiescent fluid is studied by solving the full Navier–Stokes equations using a three-dimensional finite volume method coupled with a moving mesh interface tracking (MMIT) scheme to locate the interface. The two fluids are assumed incompressible and immiscible. The interface is represented as a surface triangle mesh with zero thickness that moves with the fluid. Therefore, the jump and continuity conditions across the interface are implemented directly, without any smoothing of the fluid properties. Mesh adaptations on a tetrahedral mesh are employed to permit large deformation and to capture the changing curvature. Mesh separation is implemented to allow pinch-off. The detailed investigations of the relaxation and breakup process are presented in a more general flow regime compared to the previous works by Stone & Leal (J. Fluid Mech., vol. 198, 1989, p. 399) and Tong & Wang (Phys. Fluids, vol. 19, 2007, 092101), including the flow field of the both phases. The simulation results reveal that the vortex rings due to the interface motion and the conservation of mass play an important role in the relaxation and pinch-off process. The vortex rings are created and collapsed during the process. The effects of viscosity ratio, density ratio and length ratio on the relaxation and breakup are studied. The simulations indicate that the fluid velocity field and the neck shape are distinctly different for viscosity ratios larger and smaller than O(1), and thus a different end-pinching mechanism is observed for each regime. The length ratio also significantly affects the relaxation process and the velocity distributions, but not the neck shape. The influence of the density ratio on the relaxation and breakup process is minimal. However, the droplet evolution is retarded due to the large density of the suspending flow. The formation of a satellite droplet is observed, and the volume of the satellite droplet depends strongly on the length ratio and the viscosity ratio.

scholarly journals Sediment transport and bedforms: a numerical study of two-phase viscous shear flow

Meccanica ◽  
2016 ◽  
Vol 51 (12) ◽  
pp. 3055-3065 ◽  
Author(s):  
F. Charru ◽  
J. Bouteloup ◽  
T. Bonometti ◽  
L. Lacaze
Keyword(s):  
Sediment Transport ◽  
Shear Flow ◽  
Numerical Study ◽  
Two Phase ◽  
Viscous Shear ◽  
Viscous Shear Flow

Numerical Study of Flow Dynamics in Human Eye Vitreous Chamber After Vitrectomy and Gas Tamponade

2015 ◽  
Author(s):  
Ali Yousefi ◽  
Omid Abouali ◽  
Ebrahim Ghoshtasbi Rad ◽  
Goodarz Ahmadi
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Two Phase Flow ◽  
Numerical Study ◽  
Maximum Shear Stress ◽  
Phase Flow ◽  
Two Phase ◽  
Human Eye ◽  
Gas Tamponade ◽  
Wall Shear

The purpose of this study is to evaluate the flow pattern and the fluid shear stress acting on the retinal wall in a human eye vitreous chamber after Vitrectomy and gas tamponade including the effect of saccadic eye movements. The correlation between the maximum shear stress induced on the retinal wall and the gas fill fraction (GF) and saccade amplitudes was investigated. In modeling the geometry of vitreous chamber cavity, the indentation of the lens was taken into account. The two-phase flow at the recovery phase of the operation was modeled numerically. Unsteady three-dimensional forms of continuity and Navier-Stokes equations were solved. Volume-of-fluid method was used to solve the two-phase flow in the eye. Saccadic motion of the eye was modeled using the dynamic mesh technique. The numerical model was validated by comparing the results with the available analytical solutions and experimental data for a spherical model. Then, numerical simulation was performed based on the deformed sphere configuration, representing a more realistic model of vitreous chamber cavity. The simulation results were compared with the available numerical studies for the spherical geometry. Then the wall shear stress on the retina was computed and compared for various gas fractions. The potential effect of wall shear stress on the retinal detachment and the need for post-operation posturing in all studied cases were discussed.

scholarly journals A NUMERICAL MODEL FOR DUNE DYNAMICS

1980 ◽  
Vol 1 (17) ◽  
pp. 95
Author(s):  
J. Sundermann ◽  
H.-J. Vollmers ◽  
W. Puls
Keyword(s):  
Sediment Transport ◽  
Path Length ◽  
Erosion Rate ◽  
Flow Model ◽  
Transport Model ◽  
Shear Velocity ◽  
Mean Flow ◽  
Sediment Transport Model ◽  
The Mean ◽  
Bed Material

A numerical sediment transport model is formulated that serves especially for the simulation of bedform mechanics. The model is based on the idea that sediment transport is determined by the erosion rate and the path length of bed material. Formulas for the erosion rate and the path length are derived from physical considerations and from measurements; they depend mainly on the local values of the shear velocity and the mean flow velocity near the bed. The behaviour of detached sediment is simulated by a Monte Carlo procedure, which is based on the mean flow velocities and the eddy viscosity. All flow properties that are needed for the sediment transport model are computed by a numerical flow model that includes two turbulence equations. Results of the flow model and the sediment transport model are compared with measured data.

scholarly journals Numerical study on particle transport and deposition in rough fractures

2020 ◽  
Vol 75 ◽  
pp. 23 ◽  
Author(s):  
Xiaoyu Wang ◽  
Jun Yao ◽  
Liang Gong ◽  
Hai Sun ◽  
Yongfei Yang ◽  
...  
Keyword(s):  
Particle Deposition ◽  
Numerical Study ◽  
Density Ratio ◽  
Sediment Distribution ◽  
Particulate Materials ◽  
Transport Distance ◽  
Clear Insight ◽  
Flow Through ◽  
The Mean ◽  
Development Engineering

The transport and deposition of particulate materials through fractures is widely involved in environmental engineering and resource development engineering. A 3D Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) coupling method was used to investigate the particle and fluid flow. The Gauss Model was applied to construct the rough surfaces. First, the numerical results were compared with the previous results and reasonable agreements were obtained. Second, the results indicated a novel flow pattern of particles in rough fractures. Then, a comprehensive particle sedimentary analysis indicated that the deposition distance of particles was inversely proportional to the particle size and density ratio. In addition, the particle deposition rates were increased by the mean roughness and there was an uneven sediment distribution impacted by roughness. Reasons for this uneven sediment distribution were analyzed in detail. Moreover, the bridge plugs of particles considering the closure of fractures were simulated as well. A part of particulate materials would be filtered at the inlet due to size effect and the transport distance of entered particles decreased significantly when the particle was large. A critical particle radius (R < 0.27 mm) that can flow through closure fracture in this work was found. This work can provide a clear insight into the migration and deposition characteristics of particles in the rough fractures underground.

scholarly journals Numerical study of sediment transport in turbulent two-phase flows around an obstacle

2017 ◽  
Vol 45 ◽  
pp. 97-122 ◽  
Author(s):  
Sonia Ben Hamza ◽  
Rim Ben Kalifa ◽  
Nejla Mahjoub Saïd ◽  
Hervé Bournot ◽  
Georges Le Palec
Keyword(s):  
Sediment Transport ◽  
Numerical Study ◽  
Two Phase Flows ◽  
Two Phase

Mechanistic Prediction of Dryout Heat Flux in Annular Two-Phase Flow

2002 ◽  
Author(s):  
Tomio Okawa ◽  
Saeyun Kim ◽  
Isao Kataoka ◽  
Masanori Naitoh
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Annular Flow ◽  
Density Ratio ◽  
Droplet Deposition ◽  
Simple Correlation ◽  
Two Phase ◽  
Transition Length ◽  
The Mean ◽  
Liquid Film Dryout

Incorporating the recently developed correlations for the rates of droplet deposition and entrainment with the film flow model, critical heat flux due to liquid film dryout in steam-water annular flow was predicted. In the present calculations, the predicted critical heat flux is affected not only by the deposition and entrainment rates but also by the entrainment fraction at the transition to annular flow (Transition quality was estimated by Wallis’s correlation). For this reason, the entrainment fraction at the transition was correlated in terms of dimensionless transition length to annular flow and density ratio. A simple correlation for the occurrence of dryout was also proposed. It was demonstrated by numerical simulations that the present method is to predict the available 1,340 data of dryout heat flux with the mean square relative error of 0.068.

Flow Dynamics During Machine Perfusion Preservation of Livers

2001 ◽  
Author(s):  
Saurin P. Purohit ◽  
Joshua Nelson ◽  
Jian Zhang ◽  
Mark G. Clemens ◽  
Charles Y. Lee
Keyword(s):  
Shear Stress ◽  
Vascular Resistance ◽  
Fluid Velocity ◽  
Microscopy Study ◽  
Machine Perfusion ◽  
Mean Flow ◽  
Hypothermic Machine Perfusion ◽  
Heart Beating ◽  
Donor Shortage ◽  
The Mean

Abstract Hypothermic machine perfusion preservation (MPP) has the potential to relieve the current donor shortage problem by providing superior preserved tissue and viable non-heart-beating donor tissue. For the liver, MPP has not improved preservation. Currently, the major cause of damage associated with MPP of livers is unknown. An intravital microscopy study was conducted to investigate the state of sinusoidal perfusion during 24-hour MPP. Fluorescein isothiocynate (FITC)-labeled albumin was utilized to mark the microvascular space while FITC-labeled red blood cells were used to determine the fluid velocity. The results showed that there was an increase in vascular resistance (&gt; 275%) when the liver was perfused with a UW solution for 24 hours at 5°C and with a flow rate of 5 ml/min. This vascular resistance further increased (&gt; 425%) during rewarming (for 1 hour, at 37°C and 15 ml/min). The mean flow velocities increased during initial MPP from 236 ±16 μm/s (mean ± standard error) to 434 ± 20 μm/s and the mean shear stress values increased from 5.3 ± 0.8 dynes/cm2 to 6.5 ± 0.8 dynes/cm2, after 24 hours of MPP the mean flow velocity values and shear stress values decreased (223 ± 13 μm/s and 3.3 ± 0.8 dynes/cm2) respectively. The reason for this was detected by the FITC-labeled albumin, in the tissue. It was evident that these areas (after 24 hours of MPP) also displayed increased blockage. It also appeared from the micrographs and the histology study that the blockage occurred as a result of endothelial cells rounding after 24 hours of MPP. The cells remained rounded even after rewarming the tissue. This could be a mechanism of damage to the liver during 24-hour of MPP.

Aerosol particle transport and deposition in vertical and horizontal turbulent duct flows

2000 ◽  
Vol 406 ◽  
pp. 55-80 ◽  
Author(s):  
HAIFENG ZHANG ◽  
GOODARZ AHMADI
Keyword(s):  
Aerosol Particle ◽  
Deposition Rate ◽  
Particle Transport ◽  
Particle Deposition ◽  
Flow Direction ◽  
Fluid Velocity ◽  
Deposition Velocity ◽  
Density Ratio ◽  
Shear Velocity ◽  
Brownian Diffusion

Aerosol particle transport and deposition in vertical and horizontal turbulent duct flows in the presence of different gravity directions are studied. The instantaneous fluid velocity field is generated by the direct numerical simulation of the Navier–Stokes equation via a pseudospectral method. A particle equation of motion including Stokes drag, Brownian diffusion, lift and gravitational forces is used for trajectory analysis. Ensembles of 8192 particle paths are evaluated, compiled, and statistically analysed. The results show that the wall coherent structure plays an important role in the particle deposition process. The simulated deposition velocities under various conditions are compared with the available experimental data and the sublayer model predictions. It is shown that the shear velocity, density ratio, the shear-induced lift force and the flow direction affect the particle deposition rate. The results for vertical ducts show that the particle deposition velocity varies with the direction of gravity, and the effect becomes more significant when the shear velocity is small. For horizontal ducts, the gravitational sedimentation increases the particle deposition rate on the lower wall.

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