Editing Fluid dynamics (section)

===Other approximations===
There are a large number of other possible approximations to fluid dynamic problems. Some of the more commonly used are listed below.
* The ''[[Boussinesq approximation (buoyancy)|Boussinesq approximation]]'' neglects variations in density except to calculate [[buoyancy]] forces. It is often used in free [[convection]] problems where density changes are small.
* ''[[Lubrication theory]]'' and ''[[Hele–Shaw flow]]'' exploits the large [[aspect ratio]] of the domain to show that certain terms in the equations are small and so can be neglected.
* ''[[Slender-body theory]]'' is a methodology used in [[Stokes flow]] problems to estimate the force on, or flow field around, a long slender object in a viscous fluid.
* The ''[[shallow-water equations]]'' can be used to describe a layer of relatively inviscid fluid with a [[free surface]], in which surface [[slope|gradients]] are small.
* ''[[Darcy's law]]'' is used for flow in [[porous medium|porous media]], and works with variables averaged over several pore-widths.
* In rotating systems, the ''[[quasi-geostrophic equations]]'' assume an almost [[Balanced flow#Geostrophic flow|perfect balance]] between [[pressure gradient]]s and the [[Coriolis force]]. It is useful in the study of [[atmospheric dynamics]].