3.2.4. The Emission Model Setup

Due to the common HVAC operation during driving and brief natural ventilation when getting off, it is reasonable to expect a very low concentration of contaminants left in the cabin. Moreover, the existence of contaminants has no impact on the airflow and concentration dispersion. Under such reasonable assumptions, defining the user-defined source (UDS) equation is computationally less expensive compared to the multi-component Eulerian approach when modeling the gas transport [51]. Therefore, the unsteady variation, convection, diffusion, and generation in the domain were calculated using a UDS coupled with the user-defined functions (UDFs) based on the existing flow parameters.

$$\frac{\partial}{\partial t}(\rho S) + \nabla \cdot (\rho S U - \Gamma \nabla S) = 0 \tag{13}$$

where ρ is the density of air, S is a scalar representing the contaminant concentration, Γ is the molecular diffusivity of S.
