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# Global Settings

Settings like the turbulence model, time dependency, and material behavior can be defined as part of the Global Settings of a simulation. The global settings of a simulation are accessed by clicking on the first entry of a simulation tree:

In this documentation page, we will go through the parameters that are defined in the global settings. For convenience, these parameters will be divided into two main categories: Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA).

## Computational Fluid Dynamics

### 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 energy dissipation, mixing, heat transfer, and drag being increased$$^1$$. Turbulence modeling is the construction and use of a model to predict the effects of turbulence.

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

Currently, the following models are supported:

For further information on turbulence, please visit this SimWiki article.

### Time Dependency

There are two variants of simulation: Steady-state and Transient. 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.

Note

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

### Algorithm

In SimScale, the following algorithms are available:

• For transient analysis with turbulence: PISO, PIMPLE;
• Lastly, for transient laminar analysis: ICO.

These algorithms are responsible for the pressure-velocity coupling. For additional information, readers are referred to this page$$^2$$.

### Passive Species

Passive scalar transport is available for incompressible and convective heat transfer analysis.

Passive scalars allow you to simulate the transport of a scalar quantity within a 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. Refer to this documentation page for more information about passive species.

### Compressible

This parameter is exclusive to convective heat transfer analysis. The user can enable or disable this parameter via an on/off toggle.

When the toggle is off, the Boussinesq approximation is used. This approximation is valid for small temperature variations within the domain. This assumption is often used, for example, in natural convection simulations.

When Compressible is toggled on, the resulting density variations within the domain are calculated.

### Based On

This setting is exclusive for compressible transient analysis. Pressure-based indicates that the solver being used in the analysis is rhoPimpleFoam$$^3$$.

Heat transfer through radiation takes place in the 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.

Both convective and conjugate heat transfer analysis types support radiation. Visit this dedicated page for more information.

### Local Time Stepping

This setting is exclusive to multiphase analysis, which is inherently a transient analysis type. With the Local time stepping option enabled, it’s possible to accelerate the simulation towards a steady-state. As a result, faster computing times and smaller result data size.

This option is commonly used in ship hull resistance analysis, to accelerate the simulation towards a steady-state showing wave patterns.

## Finite Element Analysis

### Nonlinear Analysis

In case the model’s 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

Heat transfer and thermomechanical analysis can be performed as Steady-state and Transient simulations. Transient analysis accounts for time-dependent effects, whereas steady-state analysis returns only the converged steady-state solution.

### Inertia Effect

In transient thermomechanical analysis, Inertia effects are only considered when the simulation type is set to Dynamic. Otherwise, if set to Static, inertia effects won’t be taken into account.

Last updated: July 1st, 2020

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