Editing Control volume (section)

{{Short description|Imaginary volume through which a substance's flow is modeled and analyzed}}
{{thermodynamics|cTopic=[[Thermodynamic system|Systems]]}}
In [[continuum mechanics]] and [[thermodynamics]], a '''control volume''' ('''CV''') is a mathematical abstraction employed in the process of creating [[mathematical model]]s of physical processes. In an [[inertial frame of reference]], it is a fictitious [[region (mathematics)|region]] of a given [[volume]] fixed in space or moving with constant [[flow velocity]] through which the ''continuuum'' (a [[continuous medium]] such as [[gas]], [[liquid]] or [[solid]]) flows. The [[closed surface]] enclosing the region is referred to as the '''control surface'''.<ref>G.J. Van Wylen and R.E. Sonntag (1985), ''Fundamentals of Classical Thermodynamics'', Section 2.1 (3rd edition), John Wiley & Sons, Inc., New York {{ISBN|0-471-82933-1}}</ref>

At [[steady state]], a control volume can be thought of as an arbitrary volume in which the [[mass]] of the continuum remains constant. As a continuum moves through the control volume, the mass entering the control volume is equal to the mass leaving the control volume. At [[steady state]], and in the absence of [[Work (thermodynamics)|work]] and [[heat transfer]], the energy within the control volume remains constant. It is analogous to the [[classical mechanics]] concept of the [[free body diagram]].