Editing Similitude (section)

===Fluid mechanics===
Similitude has been well documented for a large number of engineering problems and is the basis of many textbook formulas and dimensionless quantities. These formulas and quantities are easy to use without having to repeat the laborious task of dimensional analysis and formula derivation. Simplification of the formulas (by neglecting some aspects of similitude) is common, and needs to be reviewed by the engineer for each application.

Similitude can be used to predict the performance of a new design based on data from an existing, similar design. In this case, the model is the existing design. Another use of similitude and models is in validation of [[computer simulation]]s with the ultimate goal of eliminating the need for physical models altogether.

Another application of similitude is to replace the operating fluid with a different test fluid. Wind tunnels, for example, have trouble with air liquefying in certain conditions so [[helium]] is sometimes used. Other applications may operate in dangerous or expensive fluids so the testing is carried out in a more convenient substitute.

Some common applications of similitude and associated dimensionless numbers;
{|!
|'''Incompressible flow''' (see example above)
| [[Reynolds number]], [[pressure coefficient]], ([[Froude number]] and [[Weber number]] for open channel hydraulics)
|-
|'''Compressible flows'''
| [[Reynolds number]], [[Mach number]], [[Prandtl number]], [[specific heat ratio]]
|-
|'''Flow-excited vibration'''
| [[Strouhal number]]
|-
|'''Centrifugal compressors'''
| [[Reynolds number]], [[Mach number]], [[pressure coefficient]], [[velocity ratio]]
|-
|'''Boundary layer thickness'''
| [[Reynolds number]], [[Womersley number]], [[Dynamic similarity (Reynolds and Womersley numbers)|dynamic similarity]]
|-
|
|}