Required field
Required field
Required field

# Model

Under the Model tab, additional parameters that define the physics of the simulation are defined.

For convenience, the setup parameters will be divided into two main categories: computational fluid dynamics (CFD) and finite element analysis (FEA).

## Computational Fluid Dynamics

### Gravity

For fluid flow analysis, gravity is defined via a vector. The global coordinate system applies for the gravity direction.

### Passive Species

In case passive species are defined in the global settings, both Turbulent Schmidt number and Diffusion coefficient can be specified in the model section.

$$(Sc_t)$$ Turb. Schmidt number

The turbulent Schmidt number describes the ratio between the rates of turbulent transport of momentum and the turbulent transport of mass (or any passive scalar).

Diffusion coefficients

The Diffusion coefficient is a parameter highlighted in Fick’s First Law:

$$J = -D \frac {d\psi}{dx}$$

where:

$$J$$ is the diffusion flux;

$$D$$ is the diffusion coefficient;

$$\frac {d\psi}{dx}$$ is the spatial gradient of the substance’s concentration.

Therefore, the diffusion coefficient is the factor of proportionality between the flux and the spatial gradient of a diffusing species$$^1$$.

### Delta Coefficient

This setting is available for the OPENFOAM® solvers whenever a large eddy simulation (LES) turbulence model is chosen.

In an LES simulation, only the large eddies are fully resolved. Resolving eddies of all scales results in an enormous computational effort. For this reason, the Delta coefficient is used as a filter for the subgrid-scale.

The Delta coefficient will be related to the local cell size. The smaller eddies that are filtered out are modeled using a RANS approach. Meanwhile, large eddies continue to be fully resolved.

### Surface Tension

For a multiphase analysis, the surface tension between the phases can be defined here.

## Finite Element Analysis

### Geometric Behavior

The Geometric behavior determines if large strains in the system should be taken into account or not.

Setting the geometric behavior to Linear will ignore large strains. This is a valid simplification in case only small deformations are expected. The Nonlinear configuration might require more iterations and can lead to instability of the simulation.

### Gravity

In the case of structural analysis, gravity magnitude and direction must be specified separately. Furthermore, the gravity magnitude can also be defined as a function of time via table and formula input.

Last updated: July 28th, 2020

What's Next

part of: Simulation Setup