Kelvin's circulation theorem

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In fluid mechanics, Kelvin's circulation theorem states: [1] [2]

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In a barotropic, ideal fluid with conservative body forces, the circulation around a closed curve (which encloses the same fluid elements) moving with the fluid remains constant with time.

The theorem is named after William Thomson, 1st Baron Kelvin who published it in 1869.

Stated mathematically:

where is the circulation around a material moving contour as a function of time . The differential operator is a substantial (material) derivative moving with the fluid particles. [3] Stated more simply, this theorem says that if one observes a closed contour at one instant, and follows the contour over time (by following the motion of all of its fluid elements), the circulation over the two locations of this contour remains constant.

This theorem does not hold in cases with viscous stresses, nonconservative body forces (for example the Coriolis force) or non-barotropic pressure-density relations.

Mathematical proof

The circulation around a closed material contour is defined by:

where u is the velocity vector, and ds is an element along the closed contour.

The governing equation for an inviscid fluid with a conservative body force is

where D/Dt is the convective derivative, ρ is the fluid density, p is the pressure and Φ is the potential for the body force. These are the Euler equations with a body force.

The condition of barotropicity implies that the density is a function only of the pressure, i.e. .

Taking the convective derivative of circulation gives

For the first term, we substitute from the governing equation, and then apply Stokes' theorem, thus:

The final equality arises since owing to barotropicity. We have also made use of the fact that the curl of any gradient is necessarily 0, or for any function .

For the second term, we note that evolution of the material line element is given by

Hence

The last equality is obtained by applying gradient theorem.

Since both terms are zero, we obtain the result

Poincaré–Bjerknes circulation theorem

A similar principle which conserves a quantity can be obtained for the rotating frame also, known as the Poincaré–Bjerknes theorem, named after Henri Poincaré and Vilhelm Bjerknes, who derived the invariant in 1893 [4] [5] and 1898. [6] [7] The theorem can be applied to a rotating frame which is rotating at a constant angular velocity given by the vector , for the modified circulation

Here is the position of the area of fluid. From Stokes' theorem, this is:

The vorticity of a velocity field in fluid dynamics is defined by:

Then:

See also

Notes

  1. Kundu, P and Cohen, I: Fluid Mechanics, page 130. Academic Press 2002
  2. Katz, Plotkin: Low-Speed Aerodynamics
  3. Burr, Karl P. (2003-07-07). "Proof of Kelvin's Theorem (From JNN, page 103) [Marine Hydrodynamics, Fall 2003 Lecture 7]". web.mit.edu/fluids-modules. Massachusetts Institute of Technology, Department of Ocean Engineering. Retrieved 3 June 2024.
  4. Poincaré, H. (1893). Théorie des tourbillons: Leçons professées pendant le deuxième semestre 1891-92 (Vol. 11). Gauthier-Villars. Article 158
  5. Truesdell, C. (2018). The kinematics of vorticity. Courier Dover Publications.
  6. Bjerknes, V., Rubenson, R., & Lindstedt, A. (1898). Ueber einen Hydrodynamischen Fundamentalsatz und seine Anwendung: besonders auf die Mechanik der Atmosphäre und des Weltmeeres. Kungl. Boktryckeriet. PA Norstedt & Söner.
  7. Chandrasekhar, S. (2013). Hydrodynamic and hydromagnetic stability. Courier Corporation.

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