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{{Expert-subject|Physics|date=November 2008}}

In [[fluid dynamics]], the '''Kirchhoff equations''', named after [[Gustav Kirchhoff]], describe the motion of a [[rigid body]] in an [[ideal fluid]].

: <math>
\begin{align}
{d\over{dt}} {{\partial T}\over{\partial \vec \omega}}
& = {{\partial T}\over{\partial \vec \omega}} \times \vec \omega + {{\partial
T}\over{\partial \vec v}} \times \vec v + \vec Q_h + \vec Q, \\[10pt]
{d\over{dt}} {{\partial T}\over{\partial \vec v}} 
& = {{\partial T}\over{\partial \vec v}} \times \vec \omega + \vec F_h + \vec F, \\[10pt]
T &  = {1 \over 2} \left( \vec \omega^T \tilde I \vec \omega + m v^2 \right) \\[10pt]
\vec Q_h & =-\int p \vec x \times \hat n \, d\sigma, \\[10pt]
\vec F_h & =-\int p \hat n \, d\sigma
\end{align}
</math>

where <math>\vec \omega</math> and <math>\vec v</math> are the angular and linear velocity vectors at the point <math>\vec x</math>, respectively; <math>\tilde I</math> is the moment of inertia tensor, <math>m</math> is the body's mass; <math>\hat n</math> is
a unit normal to the surface of the body at the point <math>\vec x</math>;
<math>p</math> is a pressure at this point; <math>\vec Q_h</math> and <math>\vec F_h</math> are the hydrodynamic
torque and force acting on the body, respectively;
<math>\vec Q</math> and <math>\vec F</math> likewise denote all other torques and forces acting on the
body. The integration is performed over the fluid-exposed portion of the
body's surface.

If the body is completely submerged body in an infinitely large volume of irrotational, incompressible, inviscid fluid, that is at rest at infinity, then the vectors <math>\vec Q_h</math> and <math>\vec F_h</math> can be found via explicit integration, and the dynamics of the body is described by the [[Gustav Kirchhoff|Kirchhoff]] – [[Clebsch]] equations:

: <math>
{d\over{dt}}
{{\partial L}\over{\partial \vec \omega}} = {{\partial L}\over{\partial \vec \omega}} \times \vec \omega + {{\partial L}\over{\partial \vec v}} \times \vec v, \quad  {d\over{dt}}
{{\partial L}\over{\partial \vec v}} = {{\partial L}\over{\partial \vec v}} \times \vec \omega,
</math>

: <math>
L(\vec \omega, \vec v) = {1 \over 2} (A \vec \omega,\vec \omega) + (B \vec \omega,\vec v) + {1 \over 2} (C \vec v,\vec v) + (\vec k,\vec \omega) + (\vec l,\vec v). 
</math>

Their first integrals read

: <math>
J_0 = \left({{\partial L}\over{\partial \vec \omega}}, \vec \omega \right) + \left({{\partial L}\over{\partial \vec v}}, \vec v \right) - L, \quad
J_1 = \left({{\partial L}\over{\partial \vec \omega}},{{\partial L}\over{\partial \vec v}}\right), \quad J_2 = \left({{\partial L}\over{\partial \vec v}},{{\partial L}\over{\partial \vec v}}\right)
 </math>.

Further integration produces explicit expressions for position and velocities. 

== References == 
* Kirchhoff G. R. ''Vorlesungen ueber Mathematische Physik, Mechanik''. Lecture 19. Leipzig: Teubner. 1877. 
* Lamb, H., ''Hydrodynamics''. Sixth Edition Cambridge (UK): Cambridge University Press. 1932.

[[Category:Mechanics]]
[[Category:Classical mechanics]]
[[Category:Rigid bodies]]

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