Session Y08: Particle-Laden Flows: Simulations (11:30am - 12:15pm CST)

Ensuring consistent particle drag predictions between Euler-Euler, Euler-Lagrange, and sub-grid informed drag models

Accurately predicting the forces on particles is essential when performing simulations of particle-laden flow. Mean drag models are typically used in both Euler-Euler (EE) and Euler-Lagrange (EL) simulations since sub-grid information is limited. While these mean drag models are well suited to predict the average force experienced by a fixed array of particles, their use in EE and EL frameworks is a necessary, albeit crude, approximation. Such simulation ignore the effects of relative particle motion and sub-grid volume fraction variations on the drag experienced by the particles. Here, these effects are studied, and a force consistency relation is introduced to ensure that drag models used for EE and EL are consistent with sub-grid informed drag predictions.   

Stochastic methods for capturing dispersion in particle-laden flows

Owing to a balance between speed and resolution, Eulerian--Lagrangian (EL) methods have gained substantial traction for modeling strongly-coupled particle-laden flows where the solids dynamics are intimately linked to the carrier fluid flow. However, existing drag force closures developed for EL methods typically capture the mean fluid-particle force experienced by an assembly of particles. Therefore, the variance in drag force, arising from neighbor-induced fluid disturbances, is generally ignored, with implications on the accuracy in quantitatively predicting particle velocity variance and dispersion. Here we provide a detailed account of stochastic approaches that may be utilized in EL methods to account for neighbor-induced drag force fluctuations. The frameworks correspond to Langevin equations for the particle position (PL), particle velocity (VL), and fluctuating drag force (FL). Rigorous derivations of the particle velocity variance and dispersion resulting from each method are obtained. It is shown that the FL method allows for the most complex behavior, enabling control of both granular temperature and dispersion. The FL framework considered here acts as a foundation for improving EL methods by accounting for the statistics of the unresolved neighbor-induced flow.   

Discrete Green's function method for estimating undisturbed fluid velocity in particle-laden flows

In recent years, the undisturbed fluid velocity has been identified as a key model quantity in the prediction of two-way coupled particle motion and resulting fluid energetics. A number of recent methods have found promise in estimating the undisturbed fluid velocity primarily in unbounded settings. However, the need for accurate procedures in the context of walls is starting to receive attention. We examine one such procedure based on the method of discrete Green's functions. Formally valid, and exact in the zero Reynolds number limit, we demonstrate in this regime that accurate motion of a particle near a wall can be computed using the present procedure. An issue surrounding the use of this method at finite Reynolds number is addressed by extending the theory to an Oseen-like discrete Green's function. The resulting expression can be derived exactly from the discrete equations. By relating the forms of the Stokes- and Oseen-discrete Green's functions, we propose a more general discrete Green's function formulation applicable outside the low particle Reynolds number regime.   

Role of Pulsatility on Aerosol Dispersion in Expiratory Flows

With an expected second wave of COVID 19 in the near future, there is an immediate need to develop a better understanding of factors contributing to dispersion of contagion carrying droplets during expiratory events. Although single-pulse expiratory events have been widely studied in the past, this work seeks to quantify the effects of pulsatility (multiple expulsions during a single event) on the underlying flow physics. We hypothesize that a pulsatile jet (mimicking for example a real cough or continuous speech) could increase entrainment and carry droplets farther than a single puff of turbulent jet due to vortex-vortex interactions. In this talk, direct numerical simulations (DNS) of turbulent pulsatile jets coupled with Lagrangian particle tracking of micron-sized droplets will be presented to investigate the role of secondary and tertiary expulsions on aerosol dispersion. Flow developing in the trachea is first approximated by DNS of a fully-developed turbulent pipe flow laden with 10-micron droplets and then utilized as an inflow boundary condition when examining pulsatility. The volumetric flowrate of the incoming turbulence is modulated according to a damped sine wave that controls the number of pulses, its duration, and peak amplitude.