Large-Eddy Simulation of Particulate Resuspension and Transport Under Influences of Human-Body Motion in an Indoor Setting

Abstract

A methodology is presented for simulating particulate resuspension and transport under influences of human-body motion in an indoor setting. The simulations in this study mirror experiments performed by the U.S. Environmental Protection Agency (EPA), which funded the present study, and the Research Triangle Institute (RTI) at the EPA test facility in Cary, NC.

A large-eddy simulation (LES) framework is implemented to obtain the time-dependent flow field within a room. An artificial compressibility method with low-diffusion upwinding and weighted essentially non-oscillatory (WENO) variable extrapolation is employed to obtain an incompressible Navier-Stokes solution. Unresolved fluctuations are accounted for by a Smagorinsky sub-grid scale stress model. A human body is modeled as an immersed boundary within the Cartesian grid domain. This body is comprised of several immersed components, representing separate body parts. Interpolation methods force the fluid and particle properties near the immersed surface to respond to the motion of the bodies, which is governed by prescribed rate laws.

The particle phase is assumed to be dilute, and thus, does not affect the solution of the carrier fluid. An Eulerian viewpoint is taken to model the particle fields, requiring separate solutions for each size of particle simulated. Size classes are determined by taking sectional averages of a lognormal probability density function, extracted from experimental data. The motion of the particle fields, subject to hydrodynamic drag forces, is determined by solving mass and momentum conservation equations for each size class. A second-order TVD upwind scheme is used for the advection of particle fields, and a point-implicit sub-iteration method is used for time-advancement.

The present simulations involve a human body walking and stamping its feet for about 20 seconds — causing particles initially contained within a carpet to resuspend — then standing still for the remainder of the simulation. In order to account for the porous structure of the carpet, Darcy-type resistance terms are applied to the solution of the carrier fluid. Micro-scale surface effects acting on the particles, such as van der Waals and electrostatic forces, are modeled by applying a size-dependent sticking force to particles contained by the carpet. This sticking force is approximated in a parametric fashion by comparing simulated particle emission factors with those obtained experimentally. Effects of an HVAC system are also modeled by applying inflow boundary conditions of measured velocity at two known vent locations. These simulations are performed on a computational domain of approximately 5.4 million grid points and are mapped to 36 Intel Xeon processors on an IBM Blade Center Linux Cluster using the MPI message passing standard.

The simulations produced similar levels of particulate mass resuspension to those observed experimentally. Results indicated that a large majority of the particles resuspended originated from regions of the carpet very near where the immersed-body "feet" penetrated, while particles elsewhere in the room were mostly undisturbed. Despite the fact that most of the mass resuspended was due to large particles, much more small-particle mass remained airborne over the duration of the 7-minute simulations due to much lower settling rates. A relatively "well-mixed" state was achieved in the room after about 3 minutes of physical time. This made it possible to identify steady particle-decay trends over the last few minutes of the simulations in order predict concentrations in the room beyond this extent of time.