About Active and Passive Multispecies in a Flow Simulation

When you enable multispecies flow, the app solves the momentum, mass, and energy conservation equations as it does in most other simulations. In addition, it solves a transport equation that governs the evolution of the mass fraction, $y_{i}$ , of each of the $n$ species involved in the simulation. The transport equation takes the following form:

\int_{Ω}^{} \frac{ⅆ}{ⅆ t} (ρ y_{i}) ⅆ V + \int_{\partial Ω}^{} ρ y_{i} u_{j} n_{j} ⅆ A = \int_{\partial Ω}^{} ρ D_{i} \frac{\partial y_{i}}{\partial x_{j}} n_{j} ⅆ A + \int_{Ω}^{} \dot{ω_{i}} ⅆ V,

where $\partial Ω$ is the boundary of the integration domain, and $n_{i}$ is the normal vector to the boundary of the integration domain.

$y_{i} = \frac{m_{i}}{m_{T}}$: is the mass fraction of the ith species,
$m_{i}$: is the mass of ith species,
$m_{T} = \sum_{i = 1}^{n} m_{i}$: is the total mass,
$ρ$: is the density of the mixture,
$D_{i}$: is the mass diffusivity of the ith species in the mixture,
$\dot{ω_{i}}$: is the rate of mass production or destruction per unit of volume for the ith species,
$u_{i}$: is the velocity vector,
$d V$: is the differential volume,
$d A$: is the differential area,
$Ω$: is the integration domain,
$\partial Ω$: is the boundary of the integration domain, and
$n_{i}$: is the normal vector to the boundary of the integration domain.

Mass conservation dictates that the app has to solve only $n - 1$ species equations because $\sum_{i - 1}^{n} y_{i} = 1$ . In this formulation the app assumes that Fick's law holds; therefore, it is not required to solve for the diffusion velocities.

Active Multispecies

When you enable active multispecies, the app computes the fluid properties using each species involved in the simulation. In this case, the overall fluid properties are the combined properties of the mixture, and all the species involved in the simulation affect the evolution of momentum and energy. As a result, the fluid materials you include in an active multispecies simulation must have all of the following material properties defined:

Density
Dynamic viscosity
Heat conductivity
Diffusivity
Specific heat
Gas constant (specified by activating the Equation of State, or EOS)

Use active multispecies if you are interested in simulating the evolution of a fluid mixture that exhibits changes in the mass fraction of its components as it flows.

Passive Multispecies

When you enable passive multispecies, the app calculates mass, momentum, and energy using the fluid properties of the background fluid only. The app includes the flow of other species in the simulation but disregards them for calculations of these quantities.

This simplification is known as the passive scalar approach, and it is valid in dilute mixtures when the changes in the fluid mixture are negligible. A negligible quantity in this context means that the concentration of one component overwhelms the concentration of the rest of the mixture. In this approach, you must specify all of the following material properties for all the species in the simulation:

Density
Dynamic viscosity
Heat conductivity
Diffusivity