Prediction of Extrathoracic Aerosol Deposition using RANS-Random Walk and LES Approaches

Computational Fluid Dynamics (CFD) has emerged as a powerful and economical alternative to empiricism in the prediction of aerosol deposition inside the extrathoracic airways (ETA). In RANS-type treatments, a main difficulty is the specification of turbulent fluid fluctuations experienced by the particles, and hence recent research has concentrated on Large Eddy Simulations (LES) in conjunction with Lagrangian Particle Tracking (LPT). While providing close agreement with data, LES/LPT approaches are extremely time consuming, thus the motivation to investigate whether better Lagrangian stochastic models can help RANS-based treatments achieve comparable accuracy. In this study, the RANS-RSM model is used to obtain the mean carrier flow field, whereas turbulent fluid velocities are defined through a stochastic Continuous Random Walk (CRW) model based on the normalized Langevin equation. With extensive validation against flow field and particle deposition data, we demonstrate that RANS, combined with the Langevin CRW provides accuracy which compares very favorably with the more computationally intensive LES approaches.     

A new model for the simulation of particle resuspension by turbulent flows based on a stochastic description of wall roughness and adhesion forces

We propose in this paper a new model aiming at simulating particle reentrainment in turbulent flows using stochastic Lagrangian methods. The resuspension model presented here emphasizes the role played by surface roughness in the reentrainment process, both in the stochastic calculation of adhesion forces based on a random model of large-scale and fine-scale wall asperities and in a newly proposed kinetic scenario of resuspension. The whole model has been implemented in a dedicated code and statistics of interest are obtained through Monte Carlo simulations. A step-by-step validation process is carried out by first assessing the adhesion-force sub-model, before analyzing the ability of the model to predict particle onset along the wall as measured in recent experimental studies. The complete particle resuspension model is then validated by comparing numerical outcomes to experimental data, where it is seen that the model is able to capture the various phenomena quite well. The present work follows a precedent study devoted to the modeling of particle deposition [Guingo, M., & Minier, J.-P. (2007). A stochastic model of coherent structures in boundary layers for the simulation of particle deposition in turbulent flows. In: Proceedings of the 6th international conference on multiphase flow. Leipzig, Germany; Guingo, M., & Minier, J.-P. (2008). A stochastic model of coherent structures for particle deposition in turbulent flows. Physics of Fluids 20, 053303].     

The influence of an anisotropic Langevin dispersion model on the prediction of micro- and nanoparticle deposition in wall-bounded turbulent flows

This paper deals with the issues of stochastic dispersion models and associated best practice responses for the investigation of micro- and nanoparticle deposition in turbulent flows. For such applications, Reynolds averaged turbulence models are widely used in combination with particle Lagrangian tracking, due to their relative simplicity and computational efficiency. Such approaches imply to generate the instantaneous velocity of the fluid at particle location to reproduce the effect of turbulence on particle transport. The default dispersion model used in most CFD codes is an eddy lifetime model, which frequently overestimates the deposition rates. In this work, a simple method is proposed to implement a three-dimensional stochastic dispersion model based on the Langevin equation in the Fluent^® commercial code. Comparisons are provided between this model, complemented by the simulation of Brownian effects, and available numerical data obtained using either an eddy lifetime model or a simple Langevin model. Computations are carried out in horizontal and vertical channel flows and in circular pipe flows as well. The use of the proposed anisotropic Langevin model is shown to improve the accuracy of deposition prediction in the whole range of particle inertia.     

EVALUATION OF LAGRANGIAN PARTICLE DISPERSION MODELS IN TURBULENT FLOWS

This work evaluates the performance of Lagrangian turbulent particle dispersion models based on the Langevin equation. A family of Langevin models, extensively reported in the open literature, decompose the fluctuating fluid velocity seen by the particle in two components, one correlated with the previous time step and a second one randomly sampled from a Wiener process, i.e., the closure is at the level of the fluid velocity seen by the particle. We will call those models generically the “standard model.” On the other hand, the model proposed by Minier and Peirano (2001) is considered; this approach is based on the probability density function (PDF) and performs the closure at the level of the acceleration of the fluid seen by the particle. The 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 simple the mathematical manipulation of the stochastic model, avoiding some pitfalls, and simplifying the derivation of macroscopic relations. The performance of the previous dispersion models is evaluated in the configurations of grid-generated turbulence (Snyder and Lumley, 1971; Wells and Stock, 1983), simple shear flow (Hyland et al., 1999), and confined axisymmetric jet flow laden with solids (Hishida and Maeda, 1987).     

Effect of Sub-Grid Scales on Large Eddy Simulation of Particle Deposition in a Turbulent Channel Flow

Large-eddy simulations (LES) of particle transport and deposition in turbulent channel flow were presented. Particular attention was given to the effect of subgrid scales on particle dispersion and deposition processes. A computational scheme for simulating the effect of subgrid scales (SGS) turbulence fluctuation on particle motion was developed and tested. Large-eddy simulation of Navier-Stokes equations using a finite volume method was used for finding instantaneous filtered fluid velocity fields of the continuous phase in the channel. Selective structure function model was used to account for the subgrid-scale Reynolds stresses. It was shown that the LES was capable of capturing the turbulence near wall coherent eddy structures.

The Lagrangian particle tracking approach was used and the transport and deposition of particles in the channel were analyzed. The drag, lift, Brownian, and gravity forces were included in the particle equation of motion. The Brownian force was simulated using a white noise stochastic process model. Effects of SGS of turbulence fluctuations on deposition rate of different size particles were studied. It was shown that the inclusion of the SGS turbulence fluctuations improves the model predictions for particle deposition rate especially for small particles. Effect of gravity on particle deposition was also investigated and it was shown that the gravity force in the stream wise direction increases the deposition rate of large particles.     

A second-order-moment–Monte-Carlo model for simulating swirling gas–particle flows

A second-order moment (SOM) gas-phase turbulence model, combined with a Monte-Carlo (MC) simulation of stochastic particle motion using Langevin equation to simulate the gas velocity seen by particles, is called an SOM–MC two-phase turbulence model. The SOM–MC model was applied to simulate swirling gas–particle flows with a swirl number of 0.47. The prediction results are compared with the PDPA measurement data and those predicted using the Langevin-closed unified second-order moment (LUSM) model. The comparison shows that both models give the predicted time-averaged flow field of particle phase in general agreement with those measured, and there is only slight difference between the prediction results using these two models. In the near-inlet region, the SOM-MC model gives a more reasonable distribution of particle axial velocity with reverse flows due to free of particle numerical diffusion, but it needs much longer computation time. Both models underpredict the gas and particle fluctuation velocities, compared with those measured. This is possibly caused by the particle–wall and particle–particle interaction in the near-wall region, and the effect of particles on dissipation of gas turbulence, which is not taken into account in both models.     

A numerical study of particle motion within the human larynx and trachea

In this paper, particle trajectories are calculated using a stochastic model for turbulent fluctuations incorporated into the particle momentum equation, in combination with the time-averaged solutions of flow fields in the larynx and trachea. The manner in which turbulence may affect overall deposition is investigated through illustrative numerical experiments of the effects of flow rate, initial particle location, density, and size, from which results are given in the form of probability density histograms of final particle locations (i.e. deposition sites). The histogram bins are defined in a unique manner that highlight the deposition mechanisms associated with turbulent dispersion. It is observed that turbulence may play a key role in enhancing particle deposition in the larynx and trachea.     

Aerosol Particle Removal and Re-entrainment in Turbulent Channel Flows – A Direct Numerical Simulation Approach

Aerosol particle removal and re-entrainment in turbulent channel flows are studied. The instantaneous fluid velocity field is generated by the direct numerical simulation (DNS) of the Navier – Stokes equation via a pseudospectral method. Particle removal mechanisms in turbulent channel flows are examined and the effects of hydrodynamic forces, torques and the near-wall coherent vorticity are discussed. The particle resuspension rates are evaluated, and the results are compared with the model of Reeks. The particle equation of motion used includes the hydrodynamic, the Brownian, the shearinduced lift and the gravitational forces. An ensemble of 8192 particles is used for particle resuspension and the subsequent trajectory analyses. It is found that large-size particles move away roughly perpendicular to the wall due to the action of the lift force. Small particles, however, follow the upward flows formed by the near-wall eddies in the low-speed streak regions. Thus, turbulent near-wall vortical structures play an important role in small particle resuspension, while the lift is an important factor for reentrainment of large particles. The simulation results suggests that small particles (with τ _p ⁺ ≤ 0.023) primarily move away from the wall in the low-speed streaks, while larger particles (with τ _p ⁺ ≥ 780) are mostly removed in the high-speed streaks.     

Modeling particle deposition and distribution in a chamber with a two-equation Reynolds-averaged Navier–Stokes model

A Lagrangian simulation for aerosol particle transport and deposition in a chamber is developed. The eddy interaction model (EIM) is adopted to generate the instantaneous turbulent fluctuating velocity field. It is found that a satisfactory result can be obtained only when the near-wall grid is sufficiently fine and the near-wall turbulent kinetic energy is damped effectively according to its component normal to the wall. Seven particle size groups ranging from 0.01 to are studied. A comparison between the current numerical model and a semi-empirical expression indicates that improved deposition fraction results were obtained. The particle deposition and particle fate in the chamber are also presented.     

Simulation of Particle Deposition in an Idealized Mouth with Different Small Diameter Inlets

The deposition of monodisperse particles (1.0-12.5 w m diameter) in an idealized mouth geometry has been studied numerically for three different inlet diameters (3.0, 5.0, and 8.0 mm). The continuous phase flow is solved using a RANS (Reynolds Averaged Navier-Stokes) k m y turbulence model at an inhalation flow rate of 16.3, 21.7, and 32.2 L/min. The particulate phase is simulated using a random-walk/Lagrangian stochastic eddy-interaction model (EIM). When optimized near-wall corrections are included in the EIM, the particle deposition results in the idealized mouth geometry are in relatively good agreement with measured data obtained in separate experiments. Without the near-wall corrections in the EIM, poor agreement with experiment is seen.     

Aerosol Particle Removal and Re-entrainment in Turbulent Channel Flows - A Direct Numerical Simulation Approach

Aerosol particle removal and re-entrainment in turbulent channel flows are studied. The instantaneous fluid velocity field is generated by the direct numerical simulation (DNS) of the Navier - Stokes equation via a pseudospectral method. Particle removal mechanisms in turbulent channel flows are examined and the effects of hydrodynamic forces, torques and the near-wall coherent vorticity are discussed. The particle resuspension rates are evaluated, and the results are compared with the model of Reeks. The particle equation of motion used includes the hydrodynamic, the Brownian, the shearinduced lift and the gravitational forces. An ensemble of 8192 particles is used for particle resuspension and the subsequent trajectory analyses. It is found that large-size particles move away roughly perpendicular to the wall due to the action of the lift force. Small particles, however, follow the upward flows formed by the near-wall eddies in the low-speed streak regions. Thus, turbulent near-wall vortical structures play an important role in small particle resuspension, while the lift is an important factor for reentrainment of large particles. The simulation results suggests that small particles (with τ_p⁺ ≤ 0.023) primarily move away from the wall in the low-speed streaks, while larger particles (with τ_p⁺ ≥ 780) are mostly removed in the high-speed streaks.     

稀疏气固两相湍流边界层拟序结构变动特性

应用PIV两相同时测量方法,对壁面Reynolds数为430的水平槽道稀疏气固两相湍流边界层拟序结构变动特性进行了研究。选取质量载荷为10^-4~10^-3的110 μm聚乙烯颗粒作为离散相。结果表明,低载荷颗粒仍能显著改变湍流拟序结构,进而影响宏观湍流属性。颗粒重力沉降形成的粗糙壁面增强了壁面附近湍流猝发行为,导致黏性底层中的气相法向脉动速度和雷诺剪切应力显著增大。颗粒与壁面的碰撞加强了低速流体上抛、削弱了高速流体下扫,同时增强了轨道交叉效应,从而抑制了湍流拟序结构发展,显著减小了黏性底层以上区域的法向脉动速度和雷诺剪切应力。此外,颗粒惯性还减小了黏性底层厚度、增大了流向速度梯度,导致气相流向脉动速度峰值增大,且其对应位置也更加靠近壁面。     

The dependence of particle deposition velocity on particle inertia in turbulent pipe flow

Recent measurements of particle deposition velocities on the walls of a pipe in turbulent flow (Liu and Agarwal, 1974) show a decline with increasing particle size beyond a critical particle size. A stochastic model of particle deposition is presented which explains this result. As in other models, the deposition process is composed of turbulent diffusion, together with inertial projection through the boundary layer; in this model, both processes are particle inertia dependent, in opposing ways. The observed decline is due to the increased fractional penetration of the boundary layer with increasing particle size being insufficient to compensate for the reduced rate of transport to that region.

A simple expression is given for the particle deposition velocity in terms of the r.m.s. velocity at that point and the fractional penetration of the boundary layer. The inertial dependence of the particle velocity is expressed in terms of the particle's response to the turbulent velocity fluctuations of its neighbouring fluid by relating the velocity spectral densities of the particle and fluid using a linear dimensionless form of the equation of motion of the particle. The fractional penetration of the boundary layer is based on Stokes' drag with a quiescent fluid.

The deposition profile shows good agreement with the experiments of Liu and Agarwal.     

Particle Detachment from Rough Surfaces in Turbulent Flows

Two flow structure-based models for particle resuspension from rough surfaces in turbulent streams are developed. It is assumed that the real area of contact is determined by elastic deformation of asperities and the effect of topographic properties of surfaces are included. The JKR adhesion model is used to analyze the behaviour of individual asperities. The theories of rolling and sliding detachment are used and the flow-induced resuspension is studied. The effects of the near-wall coherent eddies, and turbulence urst/inrush motion are included in the model development. The critical shear velocities needed to detach different sized particles from rough surfaces under various conditions are evaluated and discussed. The model predictions are compared with the available experimental data and good agreement is obtained.     

Application of a modified Eulerian model to study the particle deposition on a vertical surface in turbulent flow

In this study the v2-f model was used with the two-phase Eulerian approach to predict the particle deposition rate on a vertical surface in a turbulent flow. The standard Eulerian particle model was adopted from the literature and modified, considering the majority of particle transport mechanisms in the particle deposition rate. The performance of the modified model was examined by comparing the rate of particle deposition on a vertical surface with the experimental and numerical data in a turbulent channel flow available in the literature. The model took into account the effects of drag force, lift force, turbophoretic force, electrostatic force, inertia force and Brownian/turbulent diffusion on the particle deposition rate. Electrostatic forces due to mirror charging and charged particles under the influence of an electric field were considered. The predictions of the modified particle model were in good agreement with the experimental data. It was observed that when both electrostatic forces are present they are the dominant factor in the deposition rate in a wider range of particle sizes.     

Validation of the Langevin particle dispersion model against experiments on turbulent mixing in a T-junction

The fluctuating fluid velocities seen by particles entrained in a turbulent fluid have recently been modeled using a stochastic model based normalized Langevin Continuous Random Walk (CRW). This model has been quite successful in predicting particle dispersion in mildly complex flows. In the present study, we aim at validating the CRW model further against data collected in a challenging 3D geometry. We consider turbulent fluid mixing downstream of a T-junction using a hybrid Euler-Lagrange approach whereby tracer particle trajectories are computed and mixing of the streams deduced from the relative concentration of particles originating from the two inlet branches of the Tee. In a first simulation, RANS Reynolds Stress Model (RSM) is used to obtain the mean flow field, whereas the fluid fluctuations are specified from a CRW. Simulation results are compared to experimental data on mixing of two isothermal streams consisting of tap and de-ionized water, respectively. It is found that RSM-CRW yields strong under-prediction of the mixing. Closer look at the results shows that the Reynolds stresses, which are required inputs to the CRW, are poorly predicted with RSM. Detached Eddy Simulations (DES) are subsequently performed to provide the mean flow field, and the DES-CRW model predictions are found to compare quite well with the experimental data.     

Micro-particle transport and deposition in a human oral airway model

Laminar-to-turbulent air flow for typical inhalation modes as well as micro-particle transport and wall deposition in a representative human oral airway model have been simulated using a commercial finite-volume code with user-enhanced programs. The computer model has been validated with experimental airflow and particle deposition data sets. For the first time, accurate local and segmentally averaged particle deposition fractions have been computed under transitional and turbulent flow conditions. Specifically, turbulence that occurs after the constriction in the oral airways for moderate and high-level breathing can enhance particle deposition in the trachea near the larynx, but it is more likely to affect the deposition of smaller particles, say, St<0.05. Particles released around the top/bottom zone of the inlet plane more easily deposit on the curved oral airway surface. Although more complicated geometric features of the oral airway may have a measurable effect on particle deposition, the present simulations with a relatively simple geometry exhibit the main features of laminar-transitional-turbulent particle suspension flows in actual human oral airways. Hence, the present model may serve as the “entryway” for simulating and analyzing airflow and particle deposition in the lung.     

不可压缩湍流反应流动的直接模拟和RANS-SOM燃烧模型的检验

The second-order moment combustion model, proposed by the authors is validated using the direct numerical simulation （DNS） of incompressible turbulent reacting channel flows. The instantaneous DNS results show the near-wall strip structures of concentration and temperature fluctuations. The DNS statistical results give the budget of the terms in the correlation equations, showing that the production and dissipation terms are most important. The DNS statistical data are used to validate the closure model in RANS second-order moment （SOM） combustion model. It is found that the simulated diffusion and production terms are in agreement with the DNS data in most flow regions, except in the near-wall region, where the near-wall modification should be made, and the closure model for the dissipation term needs further improvement. The algebraic second-order moment （ASOM） combustion model is well validated by DNS.