User:Tohline/PGE

Principal Governing Equations

According to the eloquent discussion of the broad subject of Fluid Mechanics presented by Landau and Lifshitz (1975), the state of a moving fluid is determined by five quantities: the three components of the velocity <math>\vec{v}</math> and, for example, the pressure <math>P</math> and the density <math> \rho </math> . For our discussions of astrophysical fluid systems throughout this Hypertext Book [H_Book], we will add to this the gravitational potential <math> \Phi </math>. Accordingly, a complete system of equations of fluid dynamics should be six in number. For an ideal fluid these are:

Euler's Equation
(Momentum Conservation)

<math>\frac{D\vec{v}}{Dt} = - \frac{1}{\rho} \nabla P - \nabla \Phi</math>

Equation of Continuity
(Mass Conservation)

<math>\frac{D\vec{v}}{Dt} + \rho \nabla \cdot \vec{v} = 0</math>

Adiabatic Form of the
First Law of Thermodynamics
(Specific Entropy Conservation)

<math>\frac{D\vec{v}}{Dt} + \rho \nabla \cdot \vec{v} = 0</math>

Poisson Equation

<math>\nabla^2 \Phi = 4\pi G \rho</math>

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