Reynolds transport theorem
In differential calculus, the Reynolds transport theorem (also known as the Leibniz–Reynolds transport theorem), or in short Reynolds theorem, is a three-dimensional generalization of the Leibniz integral rule which is also known as differentiation under the integral sign. The theorem is named after Osborne Reynolds (1842–1912). It is used to recast derivatives of integrated quantities and is useful in formulating the basic equations of continuum mechanics.
Consider integrating f = f(x,t) over the time-dependent region Ω(t) that has boundary ∂Ω(t), then taking the derivative with respect to time:
If we wish to move the derivative within the integral, there are two issues: the time dependence of f, and the introduction of and removal of space from Ω due to its dynamic boundary. Reynolds transport theorem provides the necessary framework.
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 vb(x,t) is the velocity of the area element (not the flow velocity). The function f may be tensor-, vector- or scalar-valued. Note that the integral on the left hand side is a function solely of time, and so the total derivative has been used.
Form for a material element
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
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
Let J(X,t) = det F(X,t). Define
Then the integrals in the current and the reference configurations are related by
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
Converting into integrals over the reference configuration, we get
Since Ω0 is independent of time, we have
where is the material time derivative of f. The material derivative is given by
Using the identity
we then have
Using the divergence theorem and the identity (a ⊗ b) · n = (b · n)a, we have
A special case
If we take Ω to be constant with respect to time, then vb = 0 and the identity reduces to
as expected. (This simplification is not possible if the flow velocity is incorrectly used in place of the velocity of an area element.)
Interpretation and reduction to one dimension
The theorem is the higher-dimensional extension of differentiation under the integral sign and reduces to that expression in some cases. Suppose f is independent of y and z, and that Ω(t) is a unit square in the yz-plane and has x limits a(t) and b(t). Then Reynolds transport theorem reduces to
which, up to swapping x and t, is the standard expression for differentiation under the integral sign.
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