# Compressible Fluid Flow Analysis¶

The **compressible fluid flow analysis** could be used to
run CFD simulations where density variations have a significant
influence on the system. Commonly, when the flow velocities exceed ~30
% of the speed of sound, compressible effects start to gain importance.

In the following, compressible simulation setup is discussed.

## Turbulence model¶

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 *Reynolds numbers*. In order to simulate
a turbulent fluid flow an appropriate turbulence model should be chosen. Currently,
these models are supported:

- Reynolds-averaged Navier–Stokes (RANS)
- k-Epsilon
- kOmega-SST

- Large eddy simulation (LES)
- Smagorinsky
- Spalart-Allmaras

## Time dependency¶

There are two variants of simulation: *Steady-state* and *Transient* flow.
In order to account for time-dependent effects, consider a transient simulation.
If you are only interested in the converged steady state solution, where the flow
condition does not change over time, consider a steady-state simulation. Steady-state
simulations are computationally less demanding.

## Solver¶

Depending on the chosen turbulence and time-dependency, different solvers are available. Each combination of choices corresponds to one of the OpenFOAM® solvers. The complete list of available solvers are presented in table below.

turbulence model | Time dependency | solver | OpenFOAM® solver |
---|---|---|---|

Laminar | Transient | pressure-based | rhoPimpleFoam |

density-based | rhoCentralFoam | ||

Steady-state | - | rhoSimpleFoam | |

RANS | Transient | pressure-based | rhoPimpleFoam |

Steady-state | - | rhoSimpleFoam | |

LES | Transient | pressure-based | rhoPimpleFoam |

## Domain¶

In order to perform a **compressible** flow simulation on a given
*domain* you have to discretize your
geometry by creating a mesh. Details of CAD handling and Meshing are described
in the *Pre-processing* section.

After a mesh is assigned to the simulation, it is possible to use
domain-related entities associated with the mesh in setting up the
simulation. Additionally, one can view the mesh or define new entities,
e.g. a *Topological Entity Set*, to facilitate the simulation setup
process. Details of each step are described in the following sections:

## Model¶

For compressible simulations, the thermal properties of the system must be defined.

Moreover, if LES turbulence model is being used, LES delta coefficient should be set as well.

## Initial and boundary conditions¶

In a **compressible** simulation, the computational domain will
be solved for three fields: pressure (p), velocity (U), and temperature (T).
Depending on the choice of solver, additional turbulent transport
quantities may be included. As a general rule for CFD simulations,
all field conditions must be well-defined on all boundaries. Therefore,
it is very important to define appropriate initial and boundary conditions
for **all** variables on every boundary.

Important

Initial and boundary conditions must be specified for all variables on every boundary.

It is recommended to set the **initial conditions** close to the expected
solution to avoid potential convergence problems. For this analysis type,
variables could be initialized using the following methods:

Finally, the following **boundary conditions** are available for each variable:

## Advanced concepts¶

Based on the choice of solver, the following *Advanced concepts* are available in a compressible analysis:

## Numerics¶

Numerical settings play an important role in simulation configuration. Ideally, they could enhance stability and robustness of the simulation. Since the optimal combination is not always trivial to find, default values are tried to be as meaningful and relevant as possible. However, all numerical setting are made available for users to have full control over the simulation. These settings are divided into three categories:

- Properties
All properties regarding the iterative solvers of velocity and pressure equations are set here. Relaxation factors, residual controls, and solver-specific tweaks are among these settings. However, depending on the solver (e.g. PIMPLE, PISO, ...), these settings will be adjusted. For each field, a

*Help*message is provided on the platform.

- Solver
In this part, linear solvers used in computing each variable could be chosen separately. Upon choosing a solver, a set of preconditioners/smoothers and their tolerances become available. To assist with selecting the best solver, a

*Help*message is provided for each field.

- Numerical Schemes
These schemes determine how each term in the governing equations should be discretized. Schemes categorized in the following groups:

- Time differentiation
- Gradient
- Divergence
- Laplacian
- Interpolation
- Surface-normal gradient

## Simulation Control¶

The *Simulation Control* settings define the general controls over the simulation.
Number of iterations, simulation interval, timestep size, and several other
setting could be set. The following controls are available:

## Result Control¶

*Result Control* allows users to define extra simulation result outputs. Each result
control item provides data that requires additional calculation. The following result
control items are available:

## Disclaimer¶

**Compressible flow analysis** is performed using the OPENFOAM® software.
See our *Third-party software section*
for further information.

This offering is not approved or endorsed by OpenCFD Limited, producer and distributor of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks. OPENFOAM® is a registered trade mark of OpenCFD Limited, producer and distributor of the OpenFOAM software.