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Similarity Solution

Whitworth's (1981) Isothermal Free-Energy Surface
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Several authors (references given, below) have shown that when isothermal pressure gradients are important during a gas cloud's collapse, the equations governing the collapse admit a set of similarity solutions. Certain properties of these solutions can be described analytically and are instructive models for comparison with more detailed, numerical collapse calculations.

Establishing Set of Governing Equations

Accompanying chapter discussion

The lefthand side of the two equations containing time derivatives will take a different form if, alternatively, the choice is made to view the evolution from an Eulerian frame of reference. In this case the set of governing equations becomes,

Eulerian Frame

<math>~\frac{\partial M_r}{\partial r} </math>


<math>~4\pi r^2 \rho \, , </math>

<math>~\frac{\partial M_r}{\partial t} </math>


<math>~- 4\pi r^2 \rho v_r \, ,</math>

<math>~\frac{\partial v_r}{\partial t} + v_r \frac{\partial v_r}{\partial r}</math>


<math>~- c_s^2 \biggl( \frac{\partial \ln \rho}{\partial r}\biggr) - \frac{GM_r}{r^2} \, .</math>

Notice that, in place of the standard continuity equation, we will use the following equivalent statement of mass conservation:



<math>~0 </math>

<math>~\Rightarrow ~~~ 0</math>


<math>~\frac{\partial M_r}{\partial t} + v_r ~\frac{\partial M_r}{\partial r} </math>



<math>~\frac{\partial M_r}{\partial t} +4\pi r^2 \rho v_r \, .</math>


A similarity solution becomes possible for these equations when the single independent variable,

<math>~\zeta = \frac{c_s t}{r} \, ,</math>

is used to replace both <math>~r</math> and <math>~t</math>. Then, if <math>~M_r</math>, <math>~\rho</math>, and <math>~v_r</math> assume the following forms,



<math>~\biggl(\frac{c_s^2 t}{G}\biggr) m(\zeta) \, ,</math>



<math>~\biggl(\frac{c_s^2 }{4\pi G r^2}\biggr) P(\zeta) \, ,</math>



<math>~- c_s U(\zeta) \, ,</math>

the three coupled partial differential equations reduce to two coupled ordinary differential equations for the functions, <math>~P(\zeta)</math> and <math>~U(\zeta)</math>, namely,



<math>~ \frac{}{[ (\zeta U +1)^2 - \zeta^2]} \, , </math>



<math>~\frac{}{[ (\zeta U +1)^2 - \zeta^2]} \, ,</math>

and a single equation defining <math>~m(\zeta)</math>,



<math>~P \biggl[ U + \frac{1}{\zeta} \biggr] \, .</math>

See Especially

Whitworth's (1981) Isothermal Free-Energy Surface

© 2014 - 2021 by Joel E. Tohline
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Recommended citation:   Tohline, Joel E. (2021), The Structure, Stability, & Dynamics of Self-Gravitating Fluids, a (MediaWiki-based) publication,