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Clausius–Duhem inequality : ウィキペディア英語版
Clausius–Duhem inequality

The Clausius–Duhem inequality〔.〕〔.〕 is a way of expressing the second law of thermodynamics that is used in continuum mechanics. This inequality is particularly useful in determining whether the constitutive relation of a material is thermodynamically allowable.〔.〕
This inequality is a statement concerning the irreversibility of natural processes, especially when energy dissipation is involved. It was named after the German physicist Rudolf Clausius and French physicist Pierre Duhem.
== Clausius–Duhem inequality in terms of the specific entropy ==
The Clausius–Duhem inequality can be expressed in integral form as
:
\cfrac\left(\int_\Omega \rho~\eta~\text\right) \ge
\int_ \rho~\eta~(u_n - \mathbf\cdot\mathbf)~\text -
\int_ \cfrac}~\text +
\int_\Omega \cfrac~\text.

In this equation t\, is the time, \Omega\, represents a body and the integration is over the volume of the body, \partial \Omega\, represents the surface of the body, \rho\, is the mass density of the body, \eta\, is the specific entropy (entropy per unit mass), u_n\, is the normal velocity of \partial \Omega\,, \mathbf is the velocity of particles inside \Omega\,, \mathbf is the unit normal to the surface, \mathbf is the heat flux vector, s\, is an energy source per unit mass, and T\, is the absolute temperature. All the variables are functions of a material point at \mathbf at time t\,.
In differential form the Clausius–Duhem inequality can be written as
:
\rho~\dot \ge - \boldsymbol \cdot \left(\cfrac\right)
+ \cfrac

where \dot is the time derivative of \eta\, and \boldsymbol \cdot (\mathbf) is the divergence of the vector \mathbf.
(\rho~\eta)~\text \ge
-\int_ \rho~\eta~(\mathbf\cdot\mathbf)~\text -
\int_ \cfrac}~\text +
\int_\Omega \cfrac~\text.

Using the divergence theorem, we get
:
\int_\Omega \frac(\rho~\eta)~\text \ge
-\int_\Omega \boldsymbol \cdot (\rho~\eta~\mathbf)~\text -
\int_\Omega \boldsymbol \cdot \left(\cfrac\right)~\text +
\int_\Omega \cfrac~\text.

Since \Omega is arbitrary, we must have
:
\frac(\rho~\eta) \ge
-\boldsymbol \cdot (\rho~\eta~\mathbf) -
\boldsymbol \cdot \left(\cfrac\right) +
\cfrac.

Expanding out
:
\frac~\eta + \rho~\frac \ge
-\boldsymbol (\rho_\eta)\cdot\mathbf - \rho~\eta~(\boldsymbol \cdot \mathbf) -
\boldsymbol \cdot \left(\cfrac\right) +
\cfrac

or,
:
\frac~\eta + \rho~\frac \ge
-\eta~\boldsymbol \rho\cdot\mathbf - \rho~\boldsymbol \eta\cdot\mathbf -
\rho~\eta~(\boldsymbol \cdot \mathbf) -
\boldsymbol \cdot \left(\cfrac\right) +
\cfrac

or,
:
\left(\frac + \boldsymbol \rho\cdot\mathbf + \rho~\boldsymbol \cdot \mathbf\right)
~\eta +
\rho~\left(\frac + \boldsymbol \eta\cdot\mathbf\right)
\ge -\boldsymbol \cdot \left(\cfrac\right) +
\cfrac.

Now, the material time derivatives of \rho and \eta are given by
:
\dot = \frac + \boldsymbol \rho\cdot\mathbf ~;~~
\dot = \frac + \boldsymbol \eta\cdot\mathbf.

Therefore,
:
\left(\dot + \rho~\boldsymbol \cdot \mathbf\right)~\eta +
\rho~\dot
\ge -\boldsymbol \cdot \left(\cfrac\right) +
\cfrac.

From the conservation of mass \dot + \rho~\boldsymbol \cdot \mathbf = 0. Hence,
:
\cdot \left(\cfrac\right) +
\cfrac.
}

|}

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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