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As people spend most of their time in an indoor environment, it is important to predict indoor pollutant level in order to assess health risks. As particles are an important pollutant indoors, it is of great interest to study the airflow pattern and particle dispersion in buildings. This study uses large eddy simulation (LES) to predict three-dimensional and transient turbulent flows and a Lagrangian model to compute particle trajectories in a room.The motion of three different types of solid particles in a decaying homogeneous isotropic turbulent airflow is calculated. By comparing the computed results with the experimental data from the literature, the computational method used in this investigation is found to be successful in predicting the airflow and particle trajectories in terms of the second-order statistics, such as the mean-square displacement and turbulent intensity.This Lagrangian model is then applied to the study of particles' dispersion in a ventilated cavity with a simplified geometry for two ventilation scenarios. It is shown that light particles follow the air flow in the room and many particles are exhausted, while heavier particles deposit to the floor or/and are exhausted. Practical implicationsThe results of this paper can be used to study dispersion of infectious diseases in enclosed spaces in which virus or bacteria are often attached to particles and transported to different rooms in a building through ventilation systems. In most of studies, the virus or bacteria have been considered to be gaseous phase so there is no slip between virus/bacteria and air. The results in this paper show that heavier particles are submitted to gravity and are sensitive to the ventilation strategy. KeywordsRoom airflow, particle, large eddy simulation, Lagrangian model, computational fluid dynamics Practical implicationsThe results of this paper can be used to study dispersion of infectious diseases in enclosed spaces in which virus or bacteria are often attached to particles and transported to different rooms in a building through ventilation system. In most studies, the virus or bacteria have been considered Béghein, C., Jiang, Y. and Chen, Q. 2005. "Using large eddy simulation to study particle motions in a room," Indoor Air, 15, 281-290.2 to be gaseous phase so there is no slip between virus/bacteria and air. The results in this paper show that heavier particles are submitted to gravity and are sensitive to the ventilation strategy.

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The aim of this communication is to show the ability of POD to compute the instantaneous flow velocity when applying the Lagrangian technique to predict particle dispersion. The instantaneous flow velocity at the particle's location is obtained by solving a low-order dynamical model, deduced by a Galerkin projection of the Navier-Stokes equations onto each POD eigenfunction and it is coupled with the particle's equation of motion. This technique is applied to particle dispersion in a three-dimensional lid driven cavity. It yields a substantial decrease in computing time in comparison with LES computation and it enables treating different cases of particle dispersion Keywords: reduced order models, proper orthogonal decomposition, particle dispersion, computational fluid dynamics 1. Introduction. Nowadays people spend a plenty of time in an indoor environment and it is thus important to predict indoor air quality in order to assess health risks. Particle dispersion is a crucial issue of indoor air quality. Indeed particle dispersion problems can be encountered when considering paint sprays, tobacco smoke, and dust inhalation. This problem can be solved numerically by using a Lagrangian technique, which requires the knowledge of the instantaneous fluid velocity at the particle's location. The instantaneous velocity can be obtained in three different ways. The first one is Direct Numerical Simulation [3,7,17]. The mesh should be fine enough to solve the smallest scales of motion. This approach is therefore very time consuming and cannot be applied to building simulation. Another approach is to use Reynolds Averaged Navier Stokes models to compute the time averaged turbulent flow, and to introduce a stochastic model that provides the fluctuating velocity field [8,9,16]. These models are generally formulated for homogeneous turbulence and their extension to more complex flows is a hard task. The last approach consists in using Large Eddy Simulation (LES) to generate the instantaneous flow information [4,19,20]. Although the large-scale structures only are computed, applying LES to compute particle dispersion in a room is nevertheless a computationally intensive technique. Since the instantaneous velocity field and the particle dispersion are computed at the same time, the time step is imposed by the particles's equation of motion, and thus a great number of time steps (and therefore a long computing time) are necessary.To reduce the computing time, there is another alternative still not much used. It consists in building a low-order dynamical system that provides the fluid flow dynamics. This reduced-order model can be obtained by Proper Orthogonal Decomposition (POD) that is based on the construction of an optimal "physical" basis (in energetic norm sense). A Galerkin projection of the Navier-Stokes equations onto each POD basis function yields a system of differential equations (reduced-order model) that is coupled with particles' equation of motion. This principle was already applied to investigate the motion of bu...

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