Reynolds transport theorem

Reynolds transport theorem can be expressed as follows:[1][2][3]

${\displaystyle {\frac {d}{dt}}\int _{\Omega (t)}\mathbf {f} \,dV=\int _{\Omega (t)}{\frac {\partial \mathbf {f} }{\partial t}}\,dV+\int _{\partial \Omega (t)}\left(\mathbf {v} ^{b}\cdot \mathbf {n} \right)\mathbf {f} \,dA}$

in which n(x,t) is the outward-pointing unit normal vector, x is a point in the region and is the variable of integration, dV and dA are volume and surface elements at x, and v^b(x,t) is the velocity of the area element (not the flow velocity). The function f may be tensor-, vector- or scalar-valued.[4] Note that the integral on the left hand side is a function solely of time, and so the total derivative has been used.

In continuum mechanics, this theorem is often used for material elements. These are parcels of fluids or solids which no material enters or leaves. If Ω(t) is a material element then there is a velocity function v = v(x,t), and the boundary elements obey

${\displaystyle \mathbf {v} ^{b}\cdot \mathbf {n} =\mathbf {v} \cdot \mathbf {n} .}$

This condition may be substituted to obtain:[5]

${\displaystyle {\frac {d}{dt}}\left(\int _{\Omega (t)}\mathbf {f} \,dV\right)=\int _{\Omega (t)}{\frac {\partial \mathbf {f} }{\partial t}}\,dV+\int _{\partial \Omega (t)}(\mathbf {v} \cdot \mathbf {n} )\mathbf {f} \,dA.}$

Proof for a material element

Let Ω0 be reference configuration of the region Ω(t). Let the motion and the deformation gradient be given by ${\displaystyle \mathbf {x} ={\boldsymbol {\varphi }}(\mathbf {X} ,t),}$ ${\displaystyle {\boldsymbol {F}}(\mathbf {X} ,t)={\boldsymbol {\nabla }}{\boldsymbol {\varphi }}.}$ Let J(X,t) = det F(X,t). Define ${\displaystyle {\hat {\mathbf {f} }}(\mathbf {X} ,t)=\mathbf {f} ({\boldsymbol {\varphi }}(\mathbf {X} ,t),t).}$ Then the integrals in the current and the reference configurations are related by ${\displaystyle {\begin{aligned}\int _{\Omega (t)}\mathbf {f} (\mathbf {x} ,t)\,dV&=\int _{\Omega _{0}}\mathbf {f} ({\boldsymbol {\varphi }}(\mathbf {X} ,t),t)\,J(\mathbf {X} ,t)\,dV_{0}\\&=\int _{\Omega _{0}}{\hat {\mathbf {f} }}(\mathbf {X} ,t)\,J(\mathbf {X} ,t)\,dV_{0}.\end{aligned}}}$ That this derivation is for a material element is implicit in the time constancy of the reference configuration: it is constant in material coordinates. The time derivative of an integral over a volume is defined as ${\displaystyle {\frac {d}{dt}}\left(\int _{\Omega (t)}\mathbf {f} (\mathbf {x} ,t)\,dV\right)=\lim _{\Delta t\rightarrow 0}{\frac {1}{\Delta t}}\left(\int _{\Omega (t+\Delta t)}\mathbf {f} (\mathbf {x} ,t+\Delta t)\,dV-\int _{\Omega (t)}\mathbf {f} (\mathbf {x} ,t)\,dV\right).}$ Converting into integrals over the reference configuration, we get ${\displaystyle {\frac {d}{dt}}\left(\int _{\Omega (t)}\mathbf {f} (\mathbf {x} ,t)\,dV\right)=\lim _{\Delta t\rightarrow 0}{\frac {1}{\Delta t}}\left(\int _{\Omega _{0}}{\hat {\mathbf {f} }}(\mathbf {X} ,t+\Delta t)\,J(\mathbf {X} ,t+\Delta t)\,dV_{0}-\int _{\Omega _{0}}{\hat {\mathbf {f} }}(\mathbf {X} ,t)\,J(\mathbf {X} ,t)\,dV_{0}\right).}$ Since Ω0 is independent of time, we have ${\displaystyle {\begin{aligned}{\frac {d}{dt}}\left(\int _{\Omega (t)}\mathbf {f} (\mathbf {x} ,t)\,dV\right)&=\int _{\Omega _{0}}\left(\lim _{\Delta t\rightarrow 0}{\frac {{\hat {\mathbf {f} }}(\mathbf {X} ,t+\Delta t)\,J(\mathbf {X} ,t+\Delta t)-{\hat {\mathbf {f} }}(\mathbf {X} ,t)\,J(\mathbf {X} ,t)}{\Delta t}}\right)\,dV_{0}\\&=\int _{\Omega _{0}}{\frac {\partial }{\partial t}}\left({\hat {\mathbf {f} }}(\mathbf {X} ,t)\,J(\mathbf {X} ,t)\right)\,dV_{0}\\&=\int _{\Omega _{0}}\left({\frac {\partial }{\partial t}}{\big (}{\hat {\mathbf {f} }}(\mathbf {X} ,t){\big )}\,J(\mathbf {X} ,t)+{\hat {\mathbf {f} }}(\mathbf {X} ,t)\,{\frac {\partial }{\partial t}}{\big (}J(\mathbf {X} ,t){\big )}\right)\,dV_{0}.\end{aligned}}}$ The time derivative of J is given by: [6] ${\displaystyle {\begin{aligned}{\frac {\partial J(\mathbf {X} ,t)}{\partial t}}={\frac {\partial }{\partial t}}(\det {\boldsymbol {F}})&=(\det {\boldsymbol {F}})({\boldsymbol {\nabla }}\cdot \mathbf {v} )\\&=J(\mathbf {X} ,t)\,{\boldsymbol {\nabla }}\cdot \mathbf {v} {\big (}{\boldsymbol {\varphi }}(\mathbf {X} ,t),t{\big )}\\&=J(\mathbf {X} ,t)\,{\boldsymbol {\nabla }}\cdot \mathbf {v} (\mathbf {x} ,t).\end{aligned}}}$ Therefore, ${\displaystyle {\begin{aligned}{\frac {d}{dt}}\left(\int _{\Omega (t)}\mathbf {f} (\mathbf {x} ,t)\,dV\right)&=\int _{\Omega _{0}}\left({\frac {\partial }{\partial t}}\left({\hat {\mathbf {f} }}(\mathbf {X} ,t)\right)\,J(\mathbf {X} ,t)+{\hat {\mathbf {f} }}(\mathbf {X} ,t)\,J(\mathbf {X} ,t)\,{\boldsymbol {\nabla }}\cdot \mathbf {v} (\mathbf {x} ,t)\right)\,dV_{0}\\&=\int _{\Omega _{0}}\left({\frac {\partial }{\partial t}}\left({\hat {\mathbf {f} }}(\mathbf {X} ,t)\right)+{\hat {\mathbf {f} }}(\mathbf {X} ,t)\,{\boldsymbol {\nabla }}\cdot \mathbf {v} (\mathbf {x} ,t)\right)\,J(\mathbf {X} ,t)\,dV_{0}\\&=\int _{\Omega (t)}\left({\dot {\mathbf {f} }}(\mathbf {x} ,t)+\mathbf {f} (\mathbf {x} ,t)\,{\boldsymbol {\nabla }}\cdot \mathbf {v} (\mathbf {x} ,t)\right)\,dV.\end{aligned}}}$ where ${\displaystyle {\dot {\mathbf {f} }}}$ is the material time derivative of f. The material derivative is given by ${\displaystyle {\dot {\mathbf {f} }}(\mathbf {x} ,t)={\frac {\partial \mathbf {f} (\mathbf {x} ,t)}{\partial t}}+{\big (}{\boldsymbol {\nabla }}\mathbf {f} (\mathbf {x} ,t){\big )}\cdot \mathbf {v} (\mathbf {x} ,t).}$ Therefore, ${\displaystyle {\frac {d}{dt}}\left(\int _{\Omega (t)}\mathbf {f} (\mathbf {x} ,t)\,dV\right)=\int _{\Omega (t)}\left({\frac {\partial \mathbf {f} (\mathbf {x} ,t)}{\partial t}}+{\big (}{\boldsymbol {\nabla }}\mathbf {f} (\mathbf {x} ,t){\big )}\cdot \mathbf {v} (\mathbf {x} ,t)+\mathbf {f} (\mathbf {x} ,t)\,{\boldsymbol {\nabla }}\cdot \mathbf {v} (\mathbf {x} ,t)\right)\,dV,}$ or, ${\displaystyle {\frac {d}{dt}}\left(\int _{\Omega (t)}\mathbf {f} \,dV\right)=\int _{\Omega (t)}\left({\frac {\partial \mathbf {f} }{\partial t}}+{\boldsymbol {\nabla }}\mathbf {f} \cdot \mathbf {v} +\mathbf {f} \,{\boldsymbol {\nabla }}\cdot \mathbf {v} \right)\,dV.}$ Using the identity ${\displaystyle {\boldsymbol {\nabla }}\cdot (\mathbf {v} \otimes \mathbf {w} )=\mathbf {v} ({\boldsymbol {\nabla }}\cdot \mathbf {w} )+{\boldsymbol {\nabla }}\mathbf {v} \cdot \mathbf {w} ,}$ we then have ${\displaystyle {\frac {d}{dt}}\left(\int _{\Omega (t)}\mathbf {f} \,dV\right)=\int _{\Omega (t)}\left({\frac {\partial \mathbf {f} }{\partial t}}+{\boldsymbol {\nabla }}\cdot (\mathbf {f} \otimes \mathbf {v} )\right)\,dV.}$ Using the divergence theorem and the identity (a ⊗ b) · n = (b · n)a, we have ${\displaystyle {\begin{aligned}{\frac {d}{dt}}\left(\int _{\Omega (t)}\mathbf {f} \,dV\right)&=\int _{\Omega (t)}{\frac {\partial \mathbf {f} }{\partial t}}\,dV+\int _{\partial \Omega (t)}(\mathbf {f} \otimes \mathbf {v} )\cdot \mathbf {n} \,dA\\&=\int _{\Omega (t)}{\frac {\partial \mathbf {f} }{\partial t}}\,dV+\int _{\partial \Omega (t)}(\mathbf {v} \cdot \mathbf {n} )\mathbf {f} \,dA.\qquad \square \end{aligned}}}$

If we take Ω to be constant with respect to time, then v_b = 0 and the identity reduces to

${\displaystyle {\frac {d}{dt}}\int _{\Omega }f\,dV=\int _{\Omega }{\frac {\partial f}{\partial t}}\,dV.}$

as expected. (This simplification is not possible if the flow velocity is incorrectly used in place of the velocity of an area element.)

• Differentiation under the integral sign
• Leibniz integral rule

• Leal, L. G. (2007). Advanced transport phenomena: fluid mechanics and convective transport processes. Cambridge University Press. ISBN 978-0-521-84910-4.
• Marsden, J. E.; Tromba, A. (2003). Vector Calculus (5th ed.). New York: W. H. Freeman. ISBN 978-0-7167-4992-9.
• Reynolds, O. (1903). Papers on Mechanical and Physical Subjects. Vol. 3, The Sub-Mechanics of the Universe. Cambridge: Cambridge University Press.

• Osborne Reynolds, Collected Papers on Mechanical and Physical Subjects, in three volumes, published circa 1903, now fully and freely available in digital format: Volume 1, Volume 2, Volume 3,
• "Module 6 - Reynolds Transport Theorem". ME6601: Introduction to Fluid Mechanics. Georgia Tech. Archived from the original on March 27, 2008.
• http://planetmath.org/reynoldstransporttheorem