# Model

In the model section additional parameters that define the physics of the simulation are specified.

## Gravity

Depending on the analysis type, gravity either needs to be defined as vector or as a combination of magnitude and direction.

In case of structural analyses, gravity magnitude and direction must be specified separately. Here, the gravity magnitude can also be defined dynamically dependent on the simulation time.

Fluid flow analysis only allow for a fixed vector-based gravity definition.

## Geometric Behavior

In structural analysis the **Geometric behavior** determines of large strains in the system should be taken into account or not. Setting the geometric behavior to ‘**Linear**‘ will ignore large strains, which is a valid simplification of the problem in case only small deformations are to be expected. The ‘**Nonlinear**‘ mode might require more iterations and can lead to instability of the simulation.

## Passive Species

In case passive species are defined in the global simulation settings, the **diffusion coefficient** and the** turbulent Schmidt number** can be defined in the Model section. The fluids **kinematic viscosity** needs to be defined as part of the material properties.

### 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

t.b.d.

## Surface tension

In case of a multiphase simulation the surface tension between the phases can be defined here.

## Reynolds scaling factor

For LBM-based simulations (Incompressible LBM), a **Reynolds scaling factor** can be defined. This factor is used to scale the fluid density in order to keep both mesh size low as well as the Reynolds number in a compatible range for the used turbulence model. E.g. when comparing LBM results with experimental data from wind-tunnel testing, the applied scaling factor should follow the scale of the test model (e.g. for a wind-tunnel model size of 1:250, a scaling factor of 0.004 should be used for the full size model in SimScale).

A scaling factor of 1 together with a large model size might lead to an excessively fine mesh in order to resolve the turbulence effects properly.