COMSOL 6.4 - Glossary of Terms

Glossary of Terms

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

angular coefficient method

A method for solving molecular flows that computes the total flux arriving at a point by integrating the flux arriving from all other surface elements visible from that point.

Boltzmann equation

A partial differential equation that describes the evolution of the density and velocity distribution function for a gas in both space and time.

Boltzmann BGK equation

A form of the Boltzmann equation with a simplified collision term involving a single relaxation time. The equation was introduced in a paper by Bhatnagar, Gross, and Krook.

Fluid flow that is well described by approximating the liquid as a continuum with the Navier–Stokes equations.

The release of a molecule or atom from a solid surface to which it was previously attached. Desorption involves breaking the chemical bond between the adsorbed species and the surface.

Transport of material resulting from the random motion of molecules in the presence of a concentration gradient. In the context of vacuum systems, this term can refer to the transient diffusive process that occurs when gas molecules within the walls of the system diffuse into the chamber after it is initially pumped down. Once a constant concentration gradient is present in the chamber walls, the diffusive transport through the walls is often referred to in the context of permeation.

discrete velocity method

A kinetic method for solving the Boltzmann equation, usually with a simplified collision term (for example, the Boltzmann BGK equation). The method removes the velocity dependence of the distribution function by discretizing the velocity space and solving separate convection equations for each of the discrete velocities. These equations are coupled together by the scattering term.

A kinetic method for solving the Boltzmann equation, which computes the trajectories of large numbers of randomized particles.

The process by which gas is transported through a hole that is small compared to the mean free path.

equilibrium function

A function that describes the equilibrium concentration of molecules belonging to one of the discrete velocities used in the lattice Boltzmann method.

free molecular flow

The flow of gas molecules through a geometry that is much smaller than the mean free path (Knudsen number, Kn>10). In the free molecular flow regime, the gas molecules collide with the walls of the geometry much more frequently than they collide with themselves.

A dimensionless number that provides a measure of how rarefied a gas flow is; in other words, it is the mean free path of the gas molecules compared to the length scale of the flow. The following equation defines the Knudsen number Kn where λ is the mean free path of the molecules and L is a length scale characteristic to the flow.

A layer of rarefied fluid flow that occurs within a few mean free paths of the walls in a gas flow. The continuum Navier–Stokes equations break down in this layer.

lattice Boltzmann method

A discrete velocity method which is optimized for near continuum flows.

A three-step process in which gases, especially hydrogen, can enter a vacuum system from the outside. The gas first adsorbs on the outer surface of a vacuum system, then it diffuses through the walls of the vessel, and finally it desorbs from the interior wall of the system.

A term describing the set of discrete velocities employed in the lattice Boltzmann method.

Standard cubic centimeters per minute. A unit of flow in which the quantity of gas flowing is converted into a volumetric flow rate under standardized conditions.

Fluid flow that occurs when the Knudsen number, Kn, is in the range 0.01<Kn<0.1. As a result of rarefaction effects in the Knudsen layer the no slip boundary condition fails. The flow outside the Knudsen layer can be represented by the continuum Navier–Stokes equations provided that an appropriate slip boundary condition is used for the fluid flow and the correct temperature jump boundary condition is applied at the interface.

sticking coefficient

The probability that molecules incident on a surface are adsorbed.

transitional flow

Fluid flow that occurs when the Knudsen number, Kn, is in the range 0.1<Kn<10. In this regime, the flow is so rarefied that continuum equations break down completely. However, collisions between the molecules are still important so free molecular flow is not applicable.