Volume I - Section VII - Glossary of Terms 
Page VH - 5 
Gradient: the amount by which a variable changes in space or time. 
Grid resolution: the amount of grid points located in a physical area, for example, “The grid uses 
20 grid points to resolve the boundary layer.” 
Near-wall spacing: the distance of the closest point to the surface of a body, an especially 
important parameter in viscous flow simulations. 
Normal stress: the force/unit area that results from one body directly striking another. For 
instance, slamming your fist down upon a table will cause pain due to normal stress on your hand 
and the table. Pressure is always a normal stress. 
Reynolds number: a nondimensional number that is used to indicate how turbulent the flow of a 
fluid. 
Reynolds stress: the averaged product of two velocity components in turbulence modeling, in 
which an instantaneous velocity is broken down into mean and fluctuating components. 
Reynolds flux: a Reynolds flux, as in a Reynolds stress, is the average product of two fluctuating 
variable components, one of which is a fluctuating velocity component. 
Shear stress: the force/unit area that results from one body sliding relative to another. For 
example, sliding a book along a table top will cause a shearing stress on both the book and the 
table top. 
Solution domain: the computational volume in which the governing equations, together with the 
boundary conditions, are solved. 
Turbulence: a type of flow that occurs when a fluid is moving quickly and/or within an 
unconfined space, characterized by a marked increase in mixing where, superimposed on the 
principle motion, there are countless irregular fluctuations. 
Viscosity: a description of thick or heavy flow in the movement of one layer of fluid over 
another. For example, a viscous fluid like maple syrup will take a long time to pour from a bottle, 
while beer, which is not as viscous, can be poured quite readily. Viscosity is usually given the 
Greek symbol “(”. Water is approximately 100 times as viscous as air, while most oils are 
approximately 1 ,000 times as viscous as water. The effects of viscosity are most easily related to 
a concept like friction. The viscosity of fluids will cause a resistance to motion, such as a drag 
that must be overcome by providing more power. If the drag caused by viscosity is small 
compared to other forces, or if it is important only in a small region like in boundary layer theory, 
then the effects of viscosity can be ignored. Such a case is called inviscid flow. It is a point of 
confusion, even for practicing aeronautical engineers, that an inviscid flow is not the flow of a 
