# Global Settings

Settings like the turbulence model, time dependency, and material behavior can ge defined as part of the Global Settings of a simulation. The global settings of a simulation can be found in the settings panel of the simulation parent tree node.

Depending on the chosen analysis type, some of the following settings might be available:

## Nonlinear analysis

In case the models displacement response to a given load can be assumed to be linear (usually the case for small loads or displacements), keep this setting disabled. Otherwise, when the relationship between applied forces and the displacement response can’t be assumed to be linear, enable the nonlinear analysis setting.

## Time dependency

There are two variants of simulation: Steady-state and Transient. In order to account for time-dependent effects, consider a transient simulation. If you are only interested in the converged steady state solution, consider a steady-state simulation. Steady-state simulations are computationally less demanding.

If you are running a high-speed flow simulation in which the compressibility of the fluid needs to be taken into account, depending on the selected turbulence model, a pressure-based and/or a density-based solver might be available. Find out more.

In case of a multiphase analysis, the time dependency setting will always be set to transient.

## Turbulence model

Turbulence modeling is an important issue in many CFD simulations. Virtually all engineering applications are turbulent and hence require a turbulence model. When turbulence is present, it usually dominates all other flow phenomena, which results in the energy dissipation, mixing, heat transfer and drag being increased . Turbulence modeling is the construction and use of a model in order to predict the effects of turbulence.

The Navier–Stokes equations govern the velocity and pressure of a fluid flow. In a turbulent flow, the transport quantities may be decomposed into a mean part and a fluctuating part. The Reynolds-averaged Navier-Stokes (RANS) equation is obtained by averaging the equations which govern the mean flow. There is a closure problem which needs to be solved to avoid the velocity fluctuations due to non-linearity of the Navier–Stokes equations . In turbulence modeling we only need to know the effect of turbulence on the mean flow.

The common turbulence models used in CFD applications are RANS-based models, especially two equation models. SimScale makes available the some of the most commonly used models in industrial and research applications.

A turbulence model should be chosen in accordance to the flow regime. In a Laminar flow, associated with low Reynolds numbers, viscous effects dominate the flow and turbulence can be neglected. This flow regime is characterized by regular flow layers.

On the other hand, a Turbulent flow is characterized by chaotic and irregular patterns that are associated with high Reynolds numbers. In order to simulate a turbulent fluid flow an appropriate turbulence model should be chosen. Currently, these models are supported:

## Passive species

Passive scalars allow you to simulate the transport of a scalar quantity within an incompressible fluid flow. The core assumption of this is that the species that is transported within the flow does not affect the fluid flow (therefore passive). This is a valid assumption for example for the transport of oxygen within a water flow. It is important to note that, scalar transport does not assume any physical dimensions for passive quantities.

## Analysis Type Specific Settings

### Compressible (only for Convective Heat Transfer)

In Convective Heat Transfer analyses, the fluid can be modelled either with compressibility or without. Compressibility effects usually only play a role for high-speed flows above ~ Mach 0.3.

### Radiation (only for Convective Heat Transfer)

Heat transfer through radiation takes place in form of electromagnetic waves and it can be calculated in the simulation. This phenomenon becomes more important when the temperatures involved in the simulation are large.

### Inertia effect (only for Thermomechanical)

In Thermomechanical analysis, inertia effects are only considered when the simulation type is set to ‘Dynamic’.

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