Statistics of particle dispersion in direct numerical simulations of wall-bounded turbulence: Results of an international collaborative benchmark test (original) (raw)
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A Modeling Study on Particle Dispersion in Wall-Bounded Turbulent Flows
Advances in Applied Mathematics and Mechanics
Three physical mechanisms which may affect dispersion of particle's motion in wall-bounded turbulent flows, including the effects of turbulence, wall roughness in particle-wall collisions, and inter-particle collisions, are numerically investigated in this study. Parametric studies with different wall roughness extents and with different mass loading ratios of particles are performed in fully developed channel flows with the Eulerian-Lagrangian approach. A low-Reynolds-number k-epsilon turbulence model is applied for the solution of the carrier-flow field, while the deterministic Lagrangian method together with binary-collision hard-sphere model is applied for the solution of particle motion. It is shown that the mechanism of inter-particle collisions should be taken into account in the modeling except for the flows laden with sufficiently low mass loading ratios of particles. Influences of wall roughness on particle dispersion due to particle-wall collisions are found to be con...
A CFD model for particle dispersion in turbulent boundary layer flows
Nuclear Engineering and Design - NUCL ENG DES, 2008
In Lagrangian particle dispersion modeling, the assumption that turbulence is isotropic everywhere yields erroneous predictions of particle deposition rates on walls, even in simple geometries. In this investigation, the stochastic particle tracking model in Fluent 6.2 is modified to include a better treatment of particle–turbulence interactions close to walls where anisotropic effects are significant. The fluid rms velocities in the boundary layer are computed using fits of DNS data obtained in channel flow. The new model is tested against correlations for particle removal rates in turbulent pipe flow and 90° bends. Comparison with experimental data is much better than with the default model. The model is also assessed against data of particle removal in the human mouth–throat geometry where the flow is decidedly three-dimensional. Here, the agreement with the data is reasonable, especially in view of the fact that the DNS fits used are those of channel flows, for lack of better al...
Assessment of a New Fluent Model for Particle Dispersion in Turbulent Flows
In Lagrangian particle dispersion modeling, the assumption that turbulence is isotropic everywhere yields erroneous predictions of particle deposition rates on walls, even in simple geometries. In this investigation, the stochastic particle tracking model in FLUENT 6.2 is modified to include a better treatment of particle-turbulence interactions close to walls where anisotropic effects are significant. The fluid rms velocities in the boundary layer are computed using fits of DNS data obtained in channel flow. The new model is tested against correlations for particle removal rates in turbulent pipe flow and 90 o bends. Comparison with experimental data is much better than with the default model. The model is also assessed against data of particle removal in the human mouth-throat geometry where the flow is decidedly three dimensional. Here, the agreement with the data is reasonable, especially in view of the fact that the DNS fits used are those of channel flows, for lack of better alternatives. The CFD Best Practice Guidelines are followed to a large extent, in particular by using multiple grid resolutions and at least second order discretization schemes.
The accurate prediction of particle transport is a primary safety issue. Tracking particles in Lagrangian fashion can naturally be performed with CFD tools which provide the right framework to follow the paths of particles in complex geometries. The presence of turbulent structures in the fluid complicates the particle tracking problem considerably, because particle trajectories are no longer deterministic and additional modeling of the velocity fluctuations is needed. In the present investigation, a Lagrangian continuous random walk (CRW) model is developed to predict turbulent particle dispersion in wall-bounded flows with prevailing inhomogeneous turbulence. The particle model uses 3D mean flow data from the Fluent CFD code, as well as Eulerian statistics from Direct Numerical Simulation (DNS) databases. The turbulent fluid velocities are based on the non-dimensional Langevin equation. The model predictions are compared to the DNS data by Marchioli et al. (2007) who produced detailed statistics of velocity and transfer rates for classes of particles having Stokes numbers between 0.2 and 125 and dispersed in a parallel channel flow with Re τ =150. The model is in very good agreement with the DNS data for the various measures of particle dispersion. The predicted deposition rates are also in good agreement with the widely used experimental correlation of McCoy and Hanratty (1977) and Liu and Agarwal (1974).
Acta Mechanica, 2020
The purpose of this paper is to understand the capability and consistency of large eddy simulation (LES) in Eulerian-Lagrangian studies aimed at predicting inertial particle dispersion in turbulent wall-bounded flows, in the absence of ad hoc closure models in the Lagrangian equations of particle motion. The degree of improvement granted by LES models is object of debate, in terms of both accurate prediction of particle accumulation and local particle segregation; therefore, we assessed the accuracy in the prediction of the particle velocity statistics by comparison against direct numerical simulation (DNS) of a finer computational mesh, under both one-way and two-way coupling regimes. We performed DNS and LES at friction Reynolds number Re τ = 180 in smooth and rough channels, tracking particles with different inertia, with the aim to conduct a parametric study that examines the accuracy of particle statistics obtained from LES computations. The issue has been widely analysed in turbulent flow bounded by smooth walls, whereas the effect of rough boundaries on momentum coupled two-phase flows has been much less investigated until now. The action of the roughness of the wall is studied in terms of both turbulence modification and particle interaction with the wall surface due to particle rebounding off the solid boundary, without the introduction of a virtual rebound model. Results show that resolved LES adequately predicts particle-induced changes in both fluid and particle statistics in rough channels, at least for the range of parameters considered here. 1 Introduction Particle-laden turbulent flows can be found in various fields of engineering technical processes which occur in chemical engineering, pneumatic transport of particles, sediment transport, pollutant dispersion in air or water for environmental applications, transport of contaminants in industrial processes, air pollution control and many more. Particle dynamics in turbulent two-phase flows are very complex due to the interaction with a wide range of length and timescales of motion. As a consequence, the spatial distribution of the solid phase is strongly nonhomogeneous [1-5] since it is a result of the interaction between particles and vortical coherent structures, the
Comparison of turbulent particle dispersion models in turbulent shear flows
Brazilian Journal of Chemical Engineering, 2007
This work compares the performance of two Lagrangian turbulent particle dispersion models: the standard model (e.g., that presented in Sommerfeld et al. (1993)), in which the fluctuating fluid velocity experienced by the particle is composed of two components, one correlated with the previous time step and a second one randomly sampled from a Wiener process, and the model proposed by Minier and Peirano (2001), which is based on the PDF approach and performs closure at the level of acceleration of the fluid experienced by the particle. Formulation of a Langevin equation model for the increments of fluid velocity seen by the particle allows capturing some underlying physics of particle dispersion in general turbulent flows while keeping the mathematical manipulation of the stochastic model simple, thereby avoiding some pitfalls and simplifying the derivation of macroscopic relations. The performance of both dispersion models is tested in the configurations of grid-generated turbulence (Wells and Stock (1983) experiments), simple shear flow (Hyland et al., 1999) and confined axisymmetric jet flow laden with solids (Hishida and Maeda (1987) experiments).
International Journal of Multiphase Flow
This study is concerned with the statistics of vertical turbulent channel flow laden with inertial particles for two different volume concentrations (Φ V = 3 × 10 −6 and Φ V = 5 × 10 −5) at a Stokes number of St + = 58.6 based on viscous units. Two independent direct numerical simulation models utilizing the point-particle approach are compared to recent experimental measurements, where all relevant nondimensional parameters are directly matched. While both numerical models are built on the same general approach, details of the implementations are different, particularly regarding how two-way coupling is represented. At low volume loading, both numerical models are in general agreement with the experimental measurements, with certain exceptions near the walls for the wall-normal particle velocity fluctuations. At high loading, these discrepancies are increased, and it is found that particle clustering is overpredicted in the simulations as compared to the experimental observations. Potential reasons for the discrepancies are discussed. As this study is among the first to perform one-to-one comparisons of particle-laden flow statistics between numerical models and experiments, it suggests that continued efforts are required to reconcile differences between
International Journal of Multiphase Flow, 2008
A Lagrangian continuous random walk (CRW) model is developed to predict turbulent particle dispersion in arbitrary wall-bounded flows with prevailing anisotropic, inhomogeneous turbulence. The particle tracking model uses 3D mean flow data obtained from the Fluent CFD code, as well as Eulerian statistics of instantaneous quantities computed from DNS databases. The turbulent fluid velocities at the current time step are related to those of the previous time step through a Markov chain based on the normalized Langevin equation which takes into account turbulence inhomogeneities. The model includes a drift velocity correction that considerably reduces unphysical features common in random walk models. It is shown that the model satisfies the well-mixed criterion such that tracer particles retain approximately uniform concentrations when introduced uniformly in the domain, while their deposition velocity is vanishingly small, as it should be. To handle arbitrary geometries, it is assumed that the velocity rms values in the boundary layer can locally be approximated by the DNS data of fully developed channel flows. Benchmarks of the model are performed against particle deposition data in turbulent pipe flows, 90°b ends, as well as more complex 3D flows inside a mouth-throat geometry. Good agreement with the data is obtained across the range of particle inertia.
Lagrangian particle dispersion in turbulent flow over a wall mounted obstacle
International Journal of Heat and Fluid Flow, 2009
Large-eddy simulations (LES) of particle-laden turbulent flows are presented in order to investigate the effects of particle response time on the dispersion patterns of a space developing flow with an obstruction, where solid particles are injected inside the wake of an obstacle [Vincont, J.Y., Simoens, S., Ayrault M., Wallace, J.M., 2000. Passive scalar dispersion in a turbulent boundary layer from a line source at the wall and downstream of an obstacle. J. Fluid Mech. 424, 127-167]. The numerical method is based on a fully explicit fractional step approach and finite-differences on Cartesian grids, using the immersed boundary method (IBM) to represent the existence of solid obstacles. Two different turbulence models have been tested, the classical Smagorinsky turbulence model and the filtered structure function model. The dispersed phase was modelled either by an Eulerian approach or a Lagrangian particle tracking scheme of solid particles with Stokes numbers in the range St = 0-25, assuming one-way coupling between the two phases. A very good agreement was observed between the Lagrangian and Eulerian approaches. The effect of particle size was found to significantly differentiate the dispersion pattern for the inhomogeneous flow over the obstacle. Although in homogeneous flows like particle-laden turbulent channels near-wall particle clustering increases monotonically with particle size, for the examined flow over an obstacle, preferential concentration effects were stronger only for an intermediate range of Stokes numbers.
Quantification of Particle and Fluid Scales in Particle-Laden Turbulent Channel Flow
Volume 1: Symposia, Parts A and B, 2006
In this work, we study the dispersion of inertial particles in fully-developed turbulent channel flow to evaluate the relationship between particle and fluid time scales, and to identify suitable scales for parametrization of near-wall particle behavior. Direct Numerical Simulation (DNS) and Lagrangian particle tracking are used to build a complete and homogeneous dataset which covers a large target parameter space and includes statistics of particle velocity and particle concentration at steady state. Our results show that the Lagrangian integral time scale of the fluid is adequate to characterize particle wall deposition and that such fluid time scale will be different when sampled at the position of either fluid particles or inertial particles. Differences become particularly evident in the range 5 < St < 25. These observations can be crucial to improve the accuracy of engineering models for particle deposition.