Assessing the Effects of Near Wall Corrections of the Force Acting on Particles in Gas-Solid Channel Flows

2005 ◽  
Author(s):  
Boris Arcen ◽  
Anne Tanie`re ◽  
Benoiˆt Oesterle´
Keyword(s):  
Channel Flow ◽  
Lift Force ◽  
Particle Concentration ◽  
Turbulent Channel Flow ◽  
Shear Velocity ◽  
Solid Particles ◽  
Wall Region ◽  
Wall Effects ◽  
Strong Shear ◽  
Near Wall

The importance of using the lift force and wall-corrections of the drag coefficient for modeling the motion of solid particles in a fully-developed channel flow is investigated by means of direct numerical simulation (DNS). The turbulent channel flow is computed at a Reynolds number based on the wall-shear velocity and channel half-width of 185. Contrary to most of the numerical simulations, we consider in the present study a lift force formulation that accounts for the weak and strong shear as well as for the wall effects (hereinafter referred to as optimum lift force), and the wall-corrections of the drag force. The DNS results show that the optimum lift force and the wall-corrections of the drag together have little influence on most of the statistics (particle concentration, mean velocities, and mean relative and drift velocities), even in the near wall region.

scholarly journals Large eddy simulation of turbulent channel flow using differential equation wall model

2018 ◽  
Vol 15 (2) ◽  
pp. 75-89
Author(s):  
Muhammad Saiful Islam Mallik ◽  
Md. Ashraf Uddin
Keyword(s):  
Differential Equation ◽  
Flow Field ◽  
Channel Flow ◽  
Turbulent Channel Flow ◽  
Wall Region ◽  
Computational Domain ◽  
Wall Model ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Near Wall

A large eddy simulation (LES) of a plane turbulent channel flow is performed at a Reynolds number Re? = 590 based on the channel half width, ? and wall shear velocity, u? by approximating the near wall region using differential equation wall model (DEWM). The simulation is performed in a computational domain of 2?? x 2? x ??. The computational domain is discretized by staggered grid system with 32 x 30 x 32 grid points. In this domain the governing equations of LES are discretized spatially by second order finite difference formulation, and for temporal discretization the third order low-storage Runge-Kutta method is used. Essential turbulence statistics of the computed flow field based on this LES approach are calculated and compared with the available Direct Numerical Simulation (DNS) and LES data where no wall model was used. Comparing the results throughout the calculation domain we have found that the LES results based on DEWM show closer agreement with the DNS data, especially at the near wall region. That is, the LES approach based on DEWM can capture the effects of near wall structures more accurately. Flow structures in the computed flow field in the 3D turbulent channel have also been discussed and compared with LES data using no wall model.

Pipe-Wall Friction in Vertical Sand-Slurry Flows

2005 ◽  
Author(s):  
Vaclav Matousek
Keyword(s):  
Lift Force ◽  
Pipe Wall ◽  
Fine Sand ◽  
Coarse Sand ◽  
Solid Particles ◽  
Wall Friction ◽  
Wall Region ◽  
Pressure Drops ◽  
Frictional Pressure Drop ◽  
Near Wall

Friction due to the presence of solid particles suspended in a flow is a result of processes in a relatively thin layer near the pipe wall. Pipe-wall friction generated by particles in permanent contact with pipe wall is relatively well understood. However, very little is known about the friction deriving from sporadic contact (collisions) of particles with the wall. This friction is a major contributor to the frictional pressure drop in many slurry pipeline applications. The paper describes results of extensive laboratory tests of vertical flows of different sand fractions (fine, medium and coarse sands) carried out in the Laboratory of Dredging Engineering of the Delft University. In order to identify mechanisms that govern the solid-particle friction at the pipe wall the paper analyses friction conditions in observed vertical flows. The effects of particle-particle interactions and particle-liquid interactions on the pipe-wall friction are evaluated. One of the interesting phenomena observed in the laboratory was that frictional pressure drops in highly-concentrated flows at high velocities are lower for slurries of medium sand and coarse sand than for slurries of fine sand. The observed trend is believed to be associated with the liquid–lift force acting on solid particles traveling near a pipe wall. This off-wall force seems to be the most effective for medium to coarse particles traveling in highly concentrated mixture in the near-wall region. Thus pressure drops due to the presence of solids in non-stratified flows seem to be primarily produced by the combined effect of the Bagnold collisional force (force that colliding particles exert against the pipe wall) and liquid lift force acting on solid particles in the near-wall zone of the slurry flow.

Correlation between Temperature and Velocity Fluctuations in the Near-Wall Region of Rotating Turbulent Channel Flow

2012 ◽  
Vol 29 (5) ◽  
pp. 054702 ◽  
Author(s):  
Zi-Xuan Yang ◽  
Gui-Xiang Cui ◽  
Chun-Xiao Xu ◽  
Zhao-Shun Zhang ◽  
Liang Shao
Keyword(s):  
Channel Flow ◽  
Turbulent Channel Flow ◽  
Wall Region ◽  
Velocity Fluctuations ◽  
Near Wall

Evolution of the scalar dissipation rate downstream of a concentrated line source in turbulent channel flow

2014 ◽  
Vol 749 ◽  
pp. 227-274 ◽  
Author(s):  
E. Germaine ◽  
L. Mydlarski ◽  
L. Cortelezzi
Keyword(s):  
Scalar Field ◽  
Channel Flow ◽  
Line Source ◽  
Dissipation Rate ◽  
Turbulent Channel Flow ◽  
Scalar Fields ◽  
Small Scale ◽  
Wall Region ◽  
Small Scales ◽  
Near Wall

AbstractThe dissipation rate,$\varepsilon _{\theta }$, of a passive scalar (temperature in air) emitted from a concentrated source into a fully developed high-aspect-ratio turbulent channel flow is studied. The goal of the present work is to investigate the return to isotropy of the scalar field when the scalar is injected in a highly anisotropic manner into an inhomogeneous turbulent flow at small scales. Both experiments and direct numerical simulations (DNS) were used to study the downstream evolution of$\varepsilon _{\theta }$for scalar fields generated by line sources located at the channel centreline$(y_s/h = 1.0)$and near the wall$(y_s/h = 0.17)$. The temperature fluctuations and temperature derivatives were measured by means of a pair of parallel cold-wire thermometers in a flow at$Re_{\tau } = 520$. The DNS were performed at$Re_{\tau } = 190$using a spectral method to solve the continuity and Navier–Stokes equations, and a flux integral method (Germaine, Mydlarski & Cortelezzi,J. Comput. Phys., vol. 174, 2001, pp. 614–648) for the advection–diffusion equation. The statistics of the scalar field computed from both experimental and numerical data were found to be in good agreement, with certain discrepancies that were attributable to the difference in the Reynolds numbers of the two flows. A return to isotropy of the small scales was never perfectly observed in any region of the channel for the downstream distances studied herein. However, a continuous decay of the small-scale anisotropy was observed for the scalar field generated by the centreline line source in both the experiments and DNS. The scalar mixing was found to be more rapid in the near-wall region, where the experimental results exhibited low levels of small-scale anisotropy. However, the DNS, which were performed at lower$Re_{\tau }$, showed that persistent anisotropy can also exist near the wall, independently of the downstream location. The role of the mean velocity gradient in the production of$\varepsilon _{\theta }$(and therefore anisotropy) in the near-wall region was highlighted.

scholarly journals Geometric study of Lagrangian and Eulerian structures in turbulent channel flow

2011 ◽  
Vol 674 ◽  
pp. 67-92 ◽  
Author(s):  
YUE YANG ◽  
D. I. PULLIN
Keyword(s):  
Reynolds Number ◽  
Channel Flow ◽  
Inclination Angle ◽  
Turbulent Channel Flow ◽  
Small Scale ◽  
Wall Region ◽  
Sweep Angle ◽  
Multi Scale ◽  
Geometric Study ◽  
Near Wall

We report the detailed multi-scale and multi-directional geometric study of both evolving Lagrangian and instantaneous Eulerian structures in turbulent channel flow at low and moderate Reynolds numbers. The Lagrangian structures (material surfaces) are obtained by tracking the Lagrangian scalar field, and Eulerian structures are extracted from the swirling strength field at a time instant. The multi-scale and multi-directional geometric analysis, based on the mirror-extended curvelet transform, is developed to quantify the geometry, including the averaged inclination and sweep angles, of both structures at up to eight scales ranging from the half-height δ of the channel to several viscous length scales δν. Here, the inclination angle is on the plane of the streamwise and wall-normal directions, and the sweep angle is on the plane of streamwise and spanwise directions. The results show that coherent quasi-streamwise structures in the near-wall region are composed of inclined objects with averaged inclination angle 35°–45°, averaged sweep angle 30°–40° and characteristic scale 20δν, and ‘curved legs’ with averaged inclination angle 20°–30°, averaged sweep angle 15°–30° and length scale 5δν–10δν. The temporal evolution of Lagrangian structures shows increasing inclination and sweep angles with time, which may correspond to the lifting process of near-wall quasi-streamwise vortices. The large-scale structures that appear to be composed of a number of individual small-scale objects are detected using cross-correlations between Eulerian structures with large and small scales. These packets are located at the near-wall region with the typical height 0.25δ and may extend over 10δ in the streamwise direction in moderate-Reynolds-number, long channel flows. In addition, the effects of the Reynolds number and comparisons between Lagrangian and Eulerian structures are discussed.

scholarly journals Large-eddy simulation and wall modelling of turbulent channel flow

2009 ◽  
Vol 631 ◽  
pp. 281-309 ◽  
Author(s):  
D. CHUNG ◽  
D. I. PULLIN
Keyword(s):  
Shear Stress ◽  
Large Eddy Simulation ◽  
Channel Flow ◽  
Reynolds Shear Stress ◽  
Turbulent Channel Flow ◽  
Streamwise Velocity ◽  
Wall Region ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Near Wall

We report large-eddy simulation (LES) of turbulent channel flow. This LES neither resolves nor partially resolves the near-wall region. Instead, we develop a special near-wall subgrid-scale (SGS) model based on wall-parallel filtering and wall-normal averaging of the streamwise momentum equation, with an assumption of local inner scaling used to reduce the unsteady term. This gives an ordinary differential equation (ODE) for the wall shear stress at every wall location that is coupled with the LES. An extended form of the stretched-vortex SGS model, which incorporates the production of near-wall Reynolds shear stress due to the winding of streamwise momentum by near-wall attached SGS vortices, then provides a log relation for the streamwise velocity at the top boundary of the near-wall averaged domain. This allows calculation of an instantaneous slip velocity that is then used as a ‘virtual-wall’ boundary condition for the LES. A Kármán-like constant is calculated dynamically as part of the LES. With this closure we perform LES of turbulent channel flow for Reynolds numbers Reτ based on the friction velocity uτ and the channel half-width δ in the range 2 × 103 to 2 × 107. Results, including SGS-extended longitudinal spectra, compare favourably with the direct numerical simulation (DNS) data of Hoyas & Jiménez (2006) at Reτ = 2003 and maintain an O(1) grid dependence on Reτ.

Topology of fine-scale motions in turbulent channel flow

1996 ◽  
Vol 310 ◽  
pp. 269-292 ◽  
Author(s):  
Hugh M. Blackburn ◽  
Nagi N. Mansour ◽  
Brian J. Cantwell
Keyword(s):  
Strain Rate ◽  
Channel Flow ◽  
Velocity Gradient ◽  
Outer Region ◽  
Turbulent Channel Flow ◽  
Principal Strain ◽  
Wall Region ◽  
Fine Scale ◽  
Stable Focus ◽  
Topological Features

An investigation of topological features of the velocity gradient field of turbulent channel flow has been carried out using results from a direct numerical simulation for which the Reynolds number based on the channel half-width and the centreline velocity was 7860. Plots of the joint probability density functions of the invariants of the rate of strain and velocity gradient tensors indicated that away from the wall region, the fine-scale motions in the flow have many characteristics in common with a variety of other turbulent and transitional flows: the intermediate principal strain rate tended to be positive at sites of high viscous dissipation of kinetic energy, while the invariants of the velocity gradient tensor showed that a preference existed for stable focus/stretching and unstable node/saddle/saddle topologies. Visualization of regions in the flow with stable focus/stretching topologies revealed arrays of discrete downstream-leaning flow structures which originated near the wall and penetrated into the outer region of the flow. In all regions of the flow, there was a strong preference for the vorticity to be aligned with the intermediate principal strain rate direction, with the effect increasing near the walls in response to boundary conditions.

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