Editing Fluid dynamics (section)

==={{Anchor|Steady vs unsteady flow}} Steady versus unsteady flow===<!-- [[Steady flow]] redirects here -->

[[File:HD-Rayleigh-Taylor.gif|thumb|320px|Hydrodynamics simulation of the [[Rayleigh–Taylor instability]]<ref>Shengtai Li, Hui Li "Parallel AMR Code for Compressible MHD or HD Equations" (Los Alamos National Laboratory) [http://math.lanl.gov/Research/Highlights/amrmhd.shtml] {{Webarchive|url=https://web.archive.org/web/20160303182548/http://math.lanl.gov/Research/Highlights/amrmhd.shtml|date=2016-03-03}}</ref> ]]
A flow that is not a function of time is called '''steady flow'''. Steady-state flow refers to the condition where the fluid properties at a point in the system do not change over time. Time dependent flow is known as unsteady (also called transient<ref>{{Cite web|url=https://www.cfd-online.com/Forums/main/118306-transient-state-unsteady-state.html|title=Transient state or unsteady state? -- CFD Online Discussion Forums|website=www.cfd-online.com}}</ref>). Whether a particular flow is steady or unsteady, can depend on the chosen frame of reference. For instance, laminar flow over a [[sphere]] is steady in the frame of reference that is stationary with respect to the sphere. In a frame of reference that is stationary with respect to a background flow, the flow is unsteady.

[[Turbulence|Turbulent]] flows are unsteady by definition. A turbulent flow can, however, be [[stationary process|statistically stationary]]. The random velocity field {{math|''U''(''x'', ''t'')}} is statistically stationary if all statistics are invariant under a shift in time.<ref name=pope >{{cite book|last=Pope|first=Stephen B.|title=Turbulent Flows|publisher=Cambridge University Press|year=2000|isbn=0-521-59886-9}}</ref>{{rp| 75}} This roughly means that all statistical properties are constant in time. Often, the mean [[Field (physics)|field]] is the object of interest, and this is constant too in a statistically stationary flow.

Steady flows are often more tractable than otherwise similar unsteady flows. The governing equations of a steady problem have one dimension fewer (time) than the governing equations of the same problem without taking advantage of the steadiness of the flow field.