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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\rho}{dt} + \rho \nabla \cdot \vec{v} = 0</math>
Adiabatic Form of the
First Law of Thermodynamics
(Specific Entropy Conservation)
<math>\frac{d\epsilon}{dt} + P \frac{d}{dt} \biggl(\frac{1}{\rho}\biggr) = 0</math>
Poisson Equation
<math>\nabla^2 \Phi = 4\pi G \rho</math>
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