# CFD Module Glossary

This glossary contains application-specific terms used in the CFD Module software and documentation. For finite element modeling terms, mathematical terms, and geometry and CAD terms, see the COMSOL Multiphysics glossary.

The glossary content is copyright © COMSOL, Inc. For permission to reproduce content, contact .

### absorption

Uptake of a gas into the bulk of a liquid. Gas absorption takes place for example in theC liquid of a scrubber tower where an up-streaming gas is washed by a down-going flow of a scrubber solution.

Attachment of a molecule or atom to a solid surface. Adsorption involves a chemical bond between the adsorbed species and the surface.

### anisotropy

Variation of a transport property in different directions in a material. Is often obtained from homogenization of regular structures, for example, monolithic structures in tubular reactors.

### Arrhenius rate equation

Expression that relates the rate constant of a chemical reaction to the exponential of the temperature.

### batch reactor

Reactor characterized by its operation, which means that the reactor does reaches steady state.

### bipolar plate

Electrically conducting plate connected to the anode on one side and to the cathode on the other side in an electrochemical cell.

### Brinkman equations

Extension of Darcy’s law in order to include the transport of momentum through shear in porous media flow.

### Butler-Volmer equation

Expression that relates the reaction rate of an electron transfer reaction on an electrode surface to the exponential of the overpotential. The equation can be derived from the Arrhenius rate equation by accounting for the contribution of the electric potential to the activation energy.

### boundary layer

Region in a fluid close to a solid surface. This region is characterized by large gradients in velocity and is often treated with approximative methods, because it is difficult to geometrically resolve the large gradients found there.

### continuous reactor

Reactor that operates without interruption. This type of reactor is characterized by its steady-state operation.

### diffusion layer

Fictitious layer in a fluid close to a solid surface where a chemical reaction takes place. The flux of species perpendicular to the surface in this layer is dominated by diffusion.

### Darcy’s law

Equation that gives the velocity vector as proportional to the pressure gradient. Often used to describe flow in porous media.

### electroneutrality condition

Condition that states that the sum of charges in a control volume in an electrolyte should be zero.

### electrophoresis

Migration of charged electrolyte ions in an electric field.

### electroosmosis

Onset of a flow due to the application of an external electric field or due to the formation of an electric field created by ion transport in membranes, for example.

### Euler flow

Flow at high velocities, where incompressibility of the fluid is of importance whereas the influence of viscous momentum transport is negligible.

### Fick’s law

The first law relates the concentration gradients to the diffusive flux of a solute infinitely diluted in a solvent. The second law introduces the first law into a differential material balance for the solute.

### fully developed laminar flow

Laminar flow along a channel or pipe that only has velocity components in the main direction of the flow. The velocity profile perpendicular to the flow does not change downstream in the flow.

### Hagen-Poiseuille equation

See Poiseuille’s law.

### heterogeneous reaction

Reaction that takes place at the interface between two phases.

### homogeneous reaction

Reaction that takes place in the bulk of a solution.

### law of the wall

See wall function.

### monolithic reactor

Catalytic reactor made of one single piece of solid material. Incorporates a catalytic structure in its often porous structure.

### Navier-Stokes equations

Equations for the momentum balances coupled to the equation of continuity for a Newtonian incompressible fluid.

### Nernst-Planck equation

Equation that describes the flux of an ion through diffusion, convection, and migration in an electric field. The equation is valid for diluted electrolytes.

### Newtonian flow

Flow characterized by a constant viscosity or a viscosity that is independent of the shear rate in the fluid.

### Poiseuille’s law

Equation that relates the mass rate of flow in a tube as proportional to the pressure difference per unit length and to the fourth power of the tube radius. The law is valid for fully developed laminar flow.

### RANS

Reynolds-averaged Navier-Stokes, which implies time averaging of the velocity fluctuations in turbulent flow. The Reynolds’ stresses obtained by this averaging have to be expressed with an additional set of equations. Turbulence models like the k-ε and k-ω models belong to this class.

### specific surface area

Internal surface area of a porous structure given in area per unit volume, which yields the unit one over unit length. Often used to characterize the structure of porous catalysts.

### streamline-diffusion stabilization

A numerical technique for stabilization of the numeric solution to a convection-dominated PDE by artificially adding diffusion in the direction of the streamlines.

### wall function

Semi-empirical expression for the anisotropic flow close to a solid surface used in turbulence models. Often based on negligible variations in pressure gradient in the direction tangential to the surface.