# Computational Fluid Dynamics (CFD)¶

**Computational Fluid Dynamics (CFD)** software programs compute fluid
flows (such as gases or liquids).
The input is boundary conditions, such as the known properties of the
gas on boundary surfaces surrounding the problem space (time-dependent
problems also include the starting conditions at time t=0).
The output are gas properties within the bounded volume, such
as pressure, temperature, velocity, and density as functions of position
(and time if time-dependent), typically represented as volume meshes.
The CFD computation is based on the Navier-Stokes equations
(or, more simply, inviscid Euler equations).
There are many approaches to CFD computation based on what gas flow conditions are
assumed in the Navier-Stokes equations and how the meshing and solving are done.
In general, CFD can involve complex interactions
of various parameters and be computationally intensive,
with various degrees of difficulty
depending on fluid dynamics factors,
such as whether the flow is time-dependent or steady state,
turbulent or laminar, viscous or inviscid,
supersonic/compressible or incompressible,
3D, or constrained in some way (e.g. constant temperature).

SIMION is not itself a CFD program. However, it can utilize gas flow data from
other CFD programs for the purpose of particle tracing in those gas flows
(see *Ion-Gas Collisions*), combined with SIMION computed electromagnetic
fields. Some users have used programs like Fluent, ANSYS CFX, and various others.

Moreover, the third-party Virtual Device Hydrodynamics program (“Virtual Jet”), which specializes in computation of supersonic/compressible gas flows (such as in ESI sources) over wide pressure ranges, includes special optional functions for integrating with SIMION, including exporting its calculated gas flows directly to regular SIMION PA files for use in SIMION, a custom SIMION workbench user program for integrating those flows into SIMION particle tracing, and a 3D editor that can define not just the boundary conditions for the gas flow calculation but also simultaneously the electrode geometries for the SIMION electric field calculation (exporting them as GEM files).

Note

This page is abridged from the full SIMION 8.1.1 “Supplemental Documentation” (Help file). The following additional sections can be found in the full version of this page accessible via the “Help > Supplemental Documentation” menu in SIMION 8.1.1:- Mesh Import
- Visualizing Gas Flows
- Converting to/from .CSV or .DAT
- Information on Specific CFD Programs