Editing Fluid dynamics (section)

===Laminar versus turbulent flow===
[[File:Laminar-turbulent transition.jpg|thumb|The transition from laminar to turbulent flow]]
Turbulence is flow characterized by recirculation, [[Eddy (fluid dynamics)|eddies]], and apparent [[random]]ness. Flow in which turbulence is not exhibited is called [[laminar flow|laminar]]. The presence of eddies or recirculation alone does not necessarily indicate turbulent flow—these phenomena may be present in laminar flow as well. Mathematically, turbulent flow is often represented via a [[Reynolds decomposition]], in which the flow is broken down into the sum of an [[average]] component and a perturbation component.

It is believed that turbulent flows can be described well through the use of the [[Navier–Stokes equations]]. [[Direct numerical simulation]] (DNS), based on the Navier–Stokes equations, makes it possible to simulate turbulent flows at moderate Reynolds numbers. Restrictions depend on the power of the computer used and the efficiency of the solution algorithm. The results of DNS have been found to agree well with experimental data for some flows.<ref>See, for example, Schlatter et al, Phys. Fluids 21, 051702 (2009); {{doi|10.1063/1.3139294}}</ref>

Most flows of interest have Reynolds numbers much too high for DNS to be a viable option,<ref name=pope/>{{rp|344}} given the state of computational power for the next few decades. Any flight vehicle large enough to carry a human ({{mvar|L}} > 3&nbsp;m), moving faster than {{cvt|20|m/s|km/h mph}} is well beyond the limit of DNS simulation ({{mvar|Re}} = 4&nbsp;million). Transport aircraft wings (such as on an [[Airbus A300]] or [[Boeing 747]]) have Reynolds numbers of 40 million (based on the wing chord dimension). Solving these real-life flow problems requires turbulence models for the foreseeable future. [[Reynolds-averaged Navier–Stokes equations]] (RANS) combined with [[turbulence modelling]] provides a model of the effects of the turbulent flow. Such a modelling mainly provides the additional momentum transfer by the [[Reynolds stresses]], although the turbulence also enhances the [[heat transfer|heat]] and [[mass transfer]]. Another promising methodology is [[large eddy simulation]] (LES), especially in the form of [[detached eddy simulation]] (DES) — a combination of LES and RANS turbulence modelling.