CFD Glossary: Key Terms in Computational Fluid Dynamics

A plain-language reference for the terms that appear in every CFD project, from meshing to post-processing. Each definition is short on purpose: enough to understand what the term means and why it matters, with a link to a tutorial when there is one that shows it in practice.

Jump to: A · B · C · D · E · F · G · H · I · K · L · M · N · O · P · R · S · T · U · V · W · Y

A

Aspect ratio
How stretched a cell is: the ratio between its longest and shortest dimension. High aspect ratios are acceptable inside a boundary layer, where the flow changes in one direction only, and harmful where gradients act in every direction. See mesh quality metrics.

B

Bias factor
A control that grades the element size along an edge, placing small cells at one end and larger ones at the other, so resolution goes where the gradients are. See the bias factor tutorial.

Body of Influence
A dummy body used only to refine the mesh inside its volume. It never takes part in the physics, which makes it the tool for refining wakes, jets and other regions that do not coincide with a surface of the geometry. See Body of Influence.

Boundary conditions
The values or relations imposed on the edges of the domain: an inlet velocity, an outlet pressure, a wall temperature, a symmetry plane. They define the problem as much as the geometry does, and most unphysical results trace back to one of them.

Boundary layer
The thin region next to a wall where the velocity drops from the free stream value to zero. Friction, heat transfer and separation are all decided there, which is why it receives most of the mesh effort.

C

CFD
Computational Fluid Dynamics: solving the equations of fluid motion numerically on a discretised domain, to predict velocities, pressures, temperatures and the forces they produce.

Conjugate heat transfer
Heat transfer computed simultaneously in solid and fluid regions, with the temperature and the heat flux coupled across their interface rather than imposed on it. See heat transfer between plates.

Convergence
The point where the solution stops changing with further iterations. Low residuals are a symptom, not a proof: the reliable test is that the quantities you care about have stopped moving and the mass and energy balances close. See convergence criteria.

Courant number
The number of cells a fluid particle crosses in one time step. Transient runs, and interface capturing in particular, need it kept low so information does not jump across several cells per step. See the lock exchange case.

D

DDPM
The Dense Discrete Phase Model: a particle tracking approach that accounts for the volume the particles occupy, so it remains valid in fluidised beds and heavily loaded flows where the standard model breaks down. See DDPM in Fluent.

Discretisation
Replacing the continuous equations with algebraic ones written on the cells of a mesh. The scheme chosen for each term decides the accuracy and the stability of the solution.

Drag coefficient
The drag force made dimensionless with the dynamic pressure and a reference area. The value a solver reports depends entirely on the reference values you enter, so a wrong area gives a wrong coefficient even with a perfect flow field. See reference values.

E

Element order
Whether cells carry nodes only at their corners (linear) or also at the midpoints of their edges (quadratic). Quadratic elements follow curved geometry better at a higher cost per cell. See element order.

F

Finite volume method
The discretisation used by most CFD solvers: the domain is split into control volumes and the equations are enforced as balances of flux through their faces, which conserves mass, momentum and energy by construction.

First layer height
The thickness of the cell adjacent to a wall. It fixes the y+ value the turbulence model sees, so it should be calculated rather than guessed. Use the Y+ and first layer thickness calculator.

G

Growth rate
The ratio between the thickness of one inflation layer and the one below it. Values around 1.2 keep the transition smooth; larger ones save cells at the risk of an abrupt jump into the surrounding mesh.

H

Hexahedral mesh
A mesh made of six-sided cells. It needs fewer cells than a tetrahedral one for the same resolution and aligns better with the flow, but requires a sweepable geometry. See hexahedral meshing.

I

Inflation layers
Thin prism cells stacked along the walls to resolve the boundary layer. They are defined by the first layer height, the number of layers and the growth rate. See the inflation tutorial.

K

k-epsilon
A two-equation RANS turbulence model, robust and inexpensive, that performs well in free shear flows and less well near walls with separation.

k-omega SST
A two-equation RANS model that blends k-omega near walls with k-epsilon in the free stream. It is the usual default for external aerodynamics and flows with adverse pressure gradients.

L

LES
Large Eddy Simulation: the large turbulent structures are resolved directly and only the small ones are modelled. More accurate than RANS for unsteady separated flows, and far more expensive. See LES on PitzDaily.

M

Mesh independence
The state in which refining the mesh no longer changes the result you care about. A mesh independence study, refining in steps and comparing that result, is what shows your answer comes from the physics and not from the cell size. See mesh independence.

Mesh motion (sliding mesh)
A transient approach in which a cell zone physically rotates at each time step and slides against its neighbours. It captures blade passing effects that a steady rotating frame averages away. See mesh motion.

MRF (Moving Reference Frame)
A steady approach to rotating machinery: the zone around the rotor is solved in a rotating frame, adding centrifugal and Coriolis terms instead of moving the mesh. See MRF in Fluent.

MultiZone
A meshing method that decomposes the geometry automatically, filling sweepable regions with hexahedra and the rest with tetrahedra or prisms. See MultiZone with inflation.

Mushy zone
In solidification and melting models, the region where the material is partly solid. Its resistance to flow is set by a constant that affects how fast the phase front advances. See solidification.

N

Navier–Stokes equations
The equations of conservation of momentum for a viscous fluid. Together with conservation of mass and, when heat is involved, energy, they are what a CFD solver actually solves.

O

OpenFOAM
An open source C++ toolbox for CFD, driven by text dictionaries instead of a graphical interface, with no licence cost and no limit on parallel cores. See OpenFOAM vs Ansys CFX.

Orthogonal quality
A mesh metric measuring how well the face normals line up with the vectors joining neighbouring cell centres. Low values degrade the accuracy of the gradients and slow convergence.

P

Periodic boundary
A pair of boundaries treated as if they were connected, so a single sector of a repeating geometry represents the whole. It requires identical meshes on both faces. See Match Control.

Polyhedral mesh
A mesh of cells with many faces and neighbours. It usually needs fewer cells than a tetrahedral mesh and converges more easily, thanks to the extra neighbours in each gradient calculation. See mesh types compared.

Prism cell
A cell with triangular faces on top and bottom. Stacked along a wall, prisms form the inflation layers of most meshes.

R

RANS
Reynolds-Averaged Navier–Stokes: the flow is split into a mean and a fluctuating part, and the effect of turbulence on the mean is modelled. It is the workhorse of industrial CFD because it is affordable.

Residuals
The remaining imbalance in the discretised equations at each iteration. They show whether the solver is progressing, but reaching a threshold does not by itself mean the solution has converged.

Reynolds number
The ratio between inertial and viscous forces. It tells you whether a flow is laminar or turbulent and is the first number to compute before choosing a model or a mesh.

S

Sizing
The controls that decide the cell size in a mesh: a global element size, curvature and proximity refinement, and local controls on bodies, faces and edges. See sizing controls.

Skewness
How far a cell departs from its ideal shape. Highly skewed cells reduce accuracy and are a common cause of divergence, so the worst value in a mesh is worth checking before solving.

Steady vs transient
A steady simulation looks for the state the flow settles into; a transient one follows its evolution in time. Oscillating residuals in a steady run are often the flow telling you it is genuinely unsteady. See steady and transient compared.

T

Tetrahedral mesh
A mesh of four-faced cells that can fill any geometry automatically, at the cost of more cells and poorer alignment with the flow than hexahedra.

Turbulence model
The set of equations that represents the effect of turbulence without resolving every eddy. The choice, from k-epsilon to SST to LES, decides both the accuracy and the mesh the case requires.

U

Under-relaxation
A factor that limits how much a variable is allowed to change between iterations. Lowering it stabilises a difficult case at the cost of slower convergence.

V

VOF (Volume of Fluid)
A multiphase approach for immiscible fluids with a sharp interface. Each cell stores the volume fraction of each phase, and the interface appears where that fraction changes. See free surface with VOF.

W

Wall function
An empirical relation that bridges the region between the wall and the first cell, so the boundary layer does not have to be fully resolved. It only works when the first cell sits in the y+ range the function was built for.

Y

y+
The dimensionless distance from the wall to the centre of the first cell. It tells you whether that cell sits in the viscous sublayer or further out, and each turbulence model expects a particular range. Calculate your first layer height with the Y+ calculator.

Missing a term? The list grows with the tutorials on the site. Browse the full tutorial library or start with the PitzDaily case if you are new to CFD.