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Design Handbook on Animal Research Facilities Using Static Microisolators 
to the movement of the fluid. 
Boundary condition(s): a term that means either a set of conditions that define a physical 
problem, or a plane at which a known solution is applied to governing equations. 
Boundary layer: a very narrow region next to a solid object in a moving fluid, containing high 
gradients in velocity. 
CFD (computational fluid dynamics): the study of the behavior of fluids using computers to 
solve the equations that govern fluid flow. 
Clustering: increasing the number of grid points in a region to better resolve a geometric or flow 
feature; increasing the local grid resolution. 
Continuum: having properties that vary continuously with position. The air in a room can be 
thought of as a continuum because any cube of air will behave much like any other cube of air. 
Convection: a more generic description of the advection process. 
Convergence: a state that is achieved when the imbalances in governing equations fall below an 
acceptably low level during the solution process. 
Diffusion: the process by which a quantity spreads from one point to another due to the existence 
of a gradient in that variable. 
Diffusion, molecular: spreading of a quantity due to molecular interactions within the fluid. 
Diffusion, turbulent: spreading of a quantity due to increased mixing rates exhibited by turbulent 
flows. In the majority of situations, turbulent diffusion far exceeds molecular diffusion. 
Divergence: a situation that occurs when the imbalances in governing equations reach 
unacceptably high levels during the solution process 
Eddy viscosity: an additional viscosity that is produced due to the effects of fluid turbulence. 
Eddy diffusivity: an additional diffusivity that is produced due to the effects of fluid turbulence. 
Far-field distance: the approximate distance from the surface of the body to the farthest point in 
the Computational Domain. For example, "The wing simulation had a far-field distance of 15 
wing chords." 
Gauss Siedel equation solve: a method by which linear equations are solved on a cell-by-cell 
basis. 
