Editing
SSC/Dynamics/IsothermalSimilaritySolution
(section)
Jump to navigation
Jump to search
Warning:
You are not logged in. Your IP address will be publicly visible if you make any edits. If you
log in
or
create an account
, your edits will be attributed to your username, along with other benefits.
Anti-spam check. Do
not
fill this in!
===Proof=== Plugging the similarity solution expressions for <math>~M_r</math> and <math>~\rho</math> into the first of the three governing equations gives, <div align="center"> <table border="0" cellpadding="5" align="center"> <tr> <td align="right"> <math>~\frac{\partial}{\partial r} \biggl[ \biggl(\frac{c_s^3 t}{G}\biggr) m(\zeta) \biggr]</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~4\pi r^2 \biggl[ \biggl(\frac{c_s^2 }{4\pi G r^2}\biggr) \Rho (\zeta) \biggr] </math> </td> </tr> <tr> <td align="right"> <math>~\Rightarrow ~~~ (c_s t ) \frac{\partial}{\partial r} \biggl[ m(\zeta) \biggr]</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~\Rho (\zeta) \, .</math> </td> </tr> </table> </div> Plugging the similarity solution expressions for <math>~M_r</math>, <math>~\rho</math>, and <math>~v_r</math> into the second of the three governing equations gives, <div align="center"> <table border="0" cellpadding="5" align="center"> <tr> <td align="right"> <math>~\frac{\partial}{\partial t} \biggl[ \biggl(\frac{c_s^3 t}{G}\biggr) m(\zeta) \biggr]</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~- 4\pi r^2 \biggl[ \biggl(\frac{c_s^2 }{4\pi G r^2}\biggr) \Rho (\zeta) \biggr] \biggl[ -c_s U(\zeta)\biggr]</math> </td> </tr> <tr> <td align="right"> <math>~\Rightarrow ~~~ \frac{\partial}{\partial t} \biggl[ t m(\zeta) \biggr]</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~\Rho (\zeta) U(\zeta) </math> </td> </tr> <tr> <td align="right"> <math>~\Rightarrow ~~~ m(\zeta) + t \biggl[ \frac{\partial m(\zeta)}{\partial t} \biggr]</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~\Rho (\zeta) U(\zeta) \, .</math> </td> </tr> </table> </div> And, plugging the similarity solution expressions for <math>~M_r</math>, <math>~\rho</math>, and <math>~v_r</math> into the third of the three governing equations gives, <div align="center"> <table border="0" cellpadding="5" align="center"> <tr> <td align="right"> <math>~\frac{\partial }{\partial t} \biggl[ - c_s U(\zeta) \biggr] + \biggl[ - c_s U(\zeta) \biggr] \frac{\partial }{\partial r} \biggl[ - c_s U(\zeta) \biggr]</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~- c_s^2 \biggl[\biggl(\frac{c_s^2 }{4\pi G r^2}\biggr) \Rho (\zeta) \biggr]^{-1} \frac{\partial }{\partial r}\biggl[ \biggl(\frac{c_s^2 }{4\pi G r^2}\biggr) \Rho (\zeta) \biggr] - \frac{G}{r^2}\biggl[ \biggl(\frac{c_s^3 t}{G}\biggr) m(\zeta) \biggr] </math> </td> </tr> <tr> <td align="right"> <math>~\Rightarrow ~~~ \frac{\partial }{\partial t} \biggl[ U(\zeta) \biggr] - c_s U(\zeta) \frac{\partial }{\partial r} \biggl[ U(\zeta) \biggr]</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~\biggl[ \frac{c_s r^2}{\Rho (\zeta)} \biggr]\frac{\partial }{\partial r}\biggl[ \biggl(\frac{\Rho (\zeta)}{r^2}\biggr) \biggr] + \biggl[ \frac{c_s^2 t}{r^2} \biggr] m(\zeta) </math> </td> </tr> <tr> <td align="right"> <math>~\Rightarrow ~~~ \frac{\partial U}{\partial t} - (c_s U) \frac{\partial U}{\partial r} </math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~\frac{c_s}{\Rho} \biggl[ \biggl( \frac{\partial \Rho}{\partial r}\biggr) -\frac{2\Rho}{r} \biggr] + \biggl[ \frac{c_s^2 t}{r^2} \biggr] m(\zeta) \, .</math> </td> </tr> </table> </div> Now, from the functional dependence of <math>~m(\zeta)</math> on <math>~\Rho(\zeta)</math> and <math>~U(\zeta)</math>, we have, <div align="center"> <table border="0" cellpadding="5" align="center"> <tr> <td align="right"> <math>~\frac{\partial m}{\partial r}</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \biggl[ U + \frac{1}{\zeta} \biggr] \frac{\partial\Rho}{\partial r} + \Rho \biggl[ \frac{\partial U}{\partial r} - \frac{1}{\zeta^2} \frac{\partial \zeta}{\partial r}\biggr] </math> </td> </tr> <tr> <td align="right"> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \biggl[ U + \frac{1}{\zeta} \biggr] \frac{\partial\Rho}{\partial r} + \Rho \biggl[ \frac{\partial U}{\partial r} + \frac{1}{r \zeta} \biggr] \, , </math> </td> </tr> </table> </div> and, <div align="center"> <table border="0" cellpadding="5" align="center"> <tr> <td align="right"> <math>~\frac{\partial m}{\partial t}</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \biggl[ U + \frac{1}{\zeta} \biggr] \frac{\partial\Rho}{\partial t} + \Rho \biggl[ \frac{\partial U}{\partial t} - \frac{1}{\zeta^2} \frac{\partial \zeta}{\partial t}\biggr] </math> </td> </tr> <tr> <td align="right"> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \biggl[ U + \frac{1}{\zeta} \biggr] \frac{\partial\Rho}{\partial t} + \Rho \biggl[ \frac{\partial U}{\partial t} - \frac{1}{t \zeta} \biggr] \, . </math> </td> </tr> </table> </div> Hence, the first two governing equations become, respectively, <div align="center"> <table border="0" cellpadding="5" align="center"> <tr> <td align="right"> <math>~\Rho </math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~(r\zeta) \biggl\{ \biggl[ U + \frac{1}{\zeta} \biggr] \frac{\partial\Rho}{\partial r} + \Rho \biggl[ \frac{\partial U}{\partial r} + \frac{1}{r \zeta} \biggr] \biggr\} </math> </td> </tr> <tr> <td align="right"> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \biggl[ r\zeta U + r \biggr] \frac{\partial\Rho}{\partial r} + (r\zeta \Rho ) \frac{\partial U}{\partial r} + \Rho </math> </td> </tr> <tr> <td align="right"> <math>~\Rightarrow~~~0</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \biggl[ \zeta U + 1\biggr] \frac{\partial\Rho}{\partial r} + (\zeta \Rho ) \frac{\partial U}{\partial r} \, ; </math> </td> </tr> <tr> <td align="right"> <math>~\Rho (\zeta) U(\zeta)</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \Rho\biggl[ U + \frac{1}{\zeta}\biggr] + t \biggl\{ \biggl[ U + \frac{1}{\zeta} \biggr] \frac{\partial\Rho}{\partial t} + \Rho \biggl[ \frac{\partial U}{\partial t} - \frac{1}{t \zeta} \biggr] \biggr\} </math> </td> </tr> <tr> <td align="right"> <math>~\Rightarrow ~~~0</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ t \biggl\{ \biggl[ U + \frac{1}{\zeta} \biggr] \frac{\partial\Rho}{\partial t} + \Rho \biggl[ \frac{\partial U}{\partial t} \biggr] \biggr\} </math> </td> </tr> <tr> <td align="right"> <math>~\Rightarrow ~~~0</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \biggl[ \zeta U + 1 \biggr] \frac{\partial\Rho}{\partial t} + (\zeta \Rho) \frac{\partial U}{\partial t} \, . </math> </td> </tr> </table> </div> Now, we can use these two relations to replace derivatives of <math>~\Rho</math> with derivatives of <math>~U</math> — or ''visa versa'' — in the third governing relation. In the first case, we obtain, <div align="center"> <table border="0" cellpadding="5" align="center"> <tr> <td align="right"> <math>~ \frac{\Rho}{c_s} \biggl[\frac{\partial U}{\partial t} - (c_s U) \frac{\partial U}{\partial r} \biggr]</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~\biggl( \frac{\partial \Rho}{\partial r}\biggr) -\frac{2\Rho}{r} + \frac{\Rho^2}{r} \biggl[\zeta U + 1\biggr] </math> </td> </tr> <tr> <td align="right"> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \frac{\Rho^2(\zeta U + 1)}{r} -\frac{2\Rho}{r} - \biggl( \frac{\partial U}{\partial r}\biggr) \biggl[ \frac{\zeta \Rho}{(\zeta U + 1)} \biggr]</math> </td> </tr> <tr> <td align="right"> <math>~ \Rightarrow ~~~ \frac{1}{r} \biggl[ \Rho^2(\zeta U + 1) - 2\Rho \biggr] </math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \frac{\Rho}{c_s} \biggl[\frac{\partial U}{\partial t}\biggr] - (\Rho U) \frac{\partial U}{\partial r} + \biggl( \frac{\partial U}{\partial r}\biggr) \biggl[ \frac{\zeta \Rho}{(\zeta U + 1)} \biggr] </math> </td> </tr> <tr> <td align="right"> <math>~ \Rightarrow ~~~ \biggl[ \Rho(\zeta U + 1) - 2 \biggr] </math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \frac{r}{c_s} \biggl[\frac{\partial U}{\partial t}\biggr] + \biggl( \frac{\partial U}{\partial r}\biggr) \biggl[ \frac{r \zeta }{(\zeta U + 1)} - (rU) \biggr] </math> </td> </tr> <tr> <td align="right"> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \frac{t}{\zeta} \biggl[\frac{\partial U}{\partial t}\biggr] + r \biggl( \frac{\partial U}{\partial r}\biggr) \biggl[ \frac{\zeta - U (\zeta U + 1)}{(\zeta U + 1)} \biggr] \, . </math> </td> </tr> </table> </div> And, given that, <div align="center"> <table border="0" cellpadding="5" align="center"> <tr> <td align="right"> <math>~\frac{\partial U}{\partial t}</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~\biggl( \frac{dU}{d\zeta} \biggr) \frac{\partial \zeta}{\partial t} = \biggl( \frac{dU}{d\zeta} \biggr)\frac{c_s}{r} = \biggl( \frac{dU}{d\zeta} \biggr)\frac{\zeta}{t} \, ;</math> </td> </tr> <tr><td colspan="3" align="center">and</td></tr> <tr> <td align="right"> <math>~\frac{\partial U}{\partial r}</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \biggl( \frac{dU}{d \zeta} \biggr) \frac{\partial \zeta}{\partial r} = - \frac{c_s t}{r^2} \biggl( \frac{dU}{d \zeta} \biggr) = -\frac{\zeta^2}{c_st} \biggl( \frac{dU}{d \zeta} \biggr) \, ,</math> </td> </tr> </table> </div> we can rewrite this as an ODE of the form, <div align="center"> <table border="0" cellpadding="5" align="center"> <tr> <td align="right"> <math>~ \biggl[ \Rho(\zeta U + 1) - 2 \biggr] </math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \biggl(\frac{d U}{d\zeta}\biggr) -\zeta \biggl( \frac{d U}{d\zeta }\biggr) \biggl[ \frac{\zeta - U (\zeta U + 1)}{(\zeta U + 1)} \biggr] </math> </td> </tr> <tr> <td align="right"> <math>~\Rightarrow ~~~ [ \Rho(\zeta U + 1) - 2 ](\zeta U + 1) </math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \biggl(\frac{d U}{d\zeta}\biggr) \biggl\{(\zeta U + 1) -\zeta \biggl[ \zeta - U (\zeta U + 1) \biggr]\biggr\} </math> </td> </tr> <tr> <td align="right"> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \biggl(\frac{d U}{d\zeta}\biggr) \biggl[ \zeta^2U^2 + 2\zeta U + 1 - \zeta^2 \biggr] </math> </td> </tr> <tr> <td align="right"> <math>~ \Rightarrow ~~~ \frac{d U}{d\zeta} </math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \frac{ [\Rho(\zeta U + 1) - 2 ](\zeta U + 1)}{ [ (\zeta U + 1)^2 - \zeta^2 ] } \, .</math> </td> </tr> </table> </div> In the second case, we obtain, <div align="center"> <table border="0" cellpadding="5" align="center"> <tr> <td align="right"> <math>~\frac{c_s}{\Rho} \biggl( \frac{\partial \Rho}{\partial r}\biggr) + \frac{\zeta}{t} \biggl[ \Rho (\zeta U + 1 ) - 2\biggr] </math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \frac{\partial U}{\partial t} - (c_s U) \frac{\partial U}{\partial r} </math> </td> </tr> <tr> <td align="right"> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ -\biggl[ \frac{\zeta U +1}{\zeta \Rho} \biggr] \frac{\partial \Rho}{\partial t} + (c_s U) \biggl[ \frac{\zeta U + 1}{\zeta\Rho} \biggr] \frac{\partial \Rho}{\partial r} </math> </td> </tr> <tr> <td align="right"> <math>~\Rightarrow ~~~ \frac{\zeta}{t} \biggl[ 2- \Rho (\zeta U + 1 ) \biggr] </math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \biggl\{ \frac{c_s}{\Rho} - (c_s U) \biggl[ \frac{\zeta U + 1}{\zeta\Rho} \biggr] \biggr\}\frac{\partial \Rho}{\partial r} + \biggl[ \frac{\zeta U +1}{\zeta \Rho} \biggr] \frac{\partial \Rho}{\partial t} </math> </td> </tr> <tr> <td align="right"> <math>~\Rightarrow ~~~ \frac{\zeta^2 \Rho}{c_s t} \biggl[ 2- \Rho (\zeta U + 1 ) \biggr] </math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \biggl[ \zeta - U (\zeta U + 1 ) \biggr] \frac{\partial \Rho}{\partial r} + \frac{1}{c_s}\biggl[ \zeta U +1 \biggr] \frac{\partial \Rho}{\partial t} \, . </math> </td> </tr> </table> </div> And, given that, <div align="center"> <table border="0" cellpadding="5" align="center"> <tr> <td align="right"> <math>~\frac{\partial \Rho}{\partial t}</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~\biggl( \frac{d\Rho}{d\zeta} \biggr) \frac{\partial \zeta}{\partial t} = \biggl( \frac{d\Rho}{d\zeta} \biggr)\frac{c_s}{r} = \biggl( \frac{d\Rho}{d\zeta} \biggr)\frac{\zeta}{t} \, ;</math> </td> </tr> <tr><td colspan="3" align="center">and</td></tr> <tr> <td align="right"> <math>~\frac{\partial \Rho}{\partial r}</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \biggl( \frac{d\Rho}{d \zeta} \biggr) \frac{\partial \zeta}{\partial r} = - \frac{c_s t}{r^2} \biggl( \frac{d\Rho}{d \zeta} \biggr) = -\frac{\zeta^2}{c_st} \biggl( \frac{d\Rho}{d \zeta} \biggr) \, ,</math> </td> </tr> </table> </div> we can rewrite this as an ODE of the form, <div align="center"> <table border="0" cellpadding="5" align="center"> <tr> <td align="right"> <math>~ \frac{\zeta^2 \Rho}{c_s t} \biggl[ 2- \Rho (\zeta U + 1 ) \biggr] </math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ - \frac{\zeta^2}{c_s t} \biggl[ \zeta - U (\zeta U + 1 ) \biggr] \frac{d\Rho}{d \zeta} + \frac{\zeta}{c_s t}\biggl[ \zeta U +1 \biggr] \frac{d\Rho}{d\zeta} </math> </td> </tr> <tr> <td align="right"> <math>~\Rightarrow~~~ \zeta\Rho [ 2- \Rho (\zeta U + 1 ) ] </math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ - \zeta\biggl[ \zeta - U (\zeta U + 1 ) \biggr] \frac{d\Rho}{d \zeta} + \biggl[ \zeta U +1 \biggr] \frac{d\Rho}{d \zeta} </math> </td> </tr> <tr> <td align="right"> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \biggl\{( \zeta U +1 ) - \zeta [ \zeta - U (\zeta U + 1 ) ] \biggr\} \frac{d\Rho}{d \zeta} </math> </td> </tr> <tr> <td align="right"> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ [(\zeta U + 1)^2 - \zeta^2]\frac{d\Rho}{d \zeta} </math> </td> </tr> <tr> <td align="right"> <math>~ \Rightarrow~~~ \frac{d\Rho}{d \zeta} </math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \frac{ \zeta\Rho [ 2- \Rho (\zeta U + 1 ) ] }{[(\zeta U + 1)^2 - \zeta^2] } \, . </math> </td> </tr> </table> </div> Thus, we are able to understand the origin of the [[#CoupledODEs|pair of 1<sup>st</sup>-order ODEs, given above]], that describe the connected relationship between the two quantities, <math>~\Rho</math> and <math>~U</math>. <!-- ===Proof2=== First, note that, <div align="center"> <table border="0" cellpadding="5" align="center"> <tr> <td align="right"> <math>~\frac{\partial \zeta}{\partial t}</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~\frac{c_s}{r} = \frac{\zeta}{t} \, ;</math> </td> </tr> <tr> <td align="right"> <math>~\frac{\partial \zeta}{\partial r}</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~- \frac{c_s t}{r^2} = -\frac{\zeta^2}{c_st} \, .</math> </td> </tr> </table> </div> Next, let's take partial derivatives, with respect to both <math>~r</math> and <math>~t</math>, of the three primary physical variables, <math>~M_r</math>, <math>~\rho</math>, and <math>~v_r</math>. <div align="center"> <table border="0" cellpadding="5" align="center"> <tr> <td align="right"> <math>~\frac{\partial M_r}{\partial r}</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \biggl( \frac{c_s^3 t}{G} \biggr)\frac{dm}{d\zeta} \biggl( \frac{\partial \zeta}{\partial r} \biggr) = \biggl( \frac{c_s^3 t}{G} \biggr)\frac{dm}{d\zeta} \biggl(- \frac{\zeta^2}{c_s t} \biggr) = -\biggl( \frac{c_s^2 \zeta^2}{G} \biggr)\frac{dm}{d\zeta} \, ; </math> </td> </tr> <tr> <td align="right"> <math>~\frac{\partial M_r}{\partial t}</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ \biggl( \frac{c_s^3 t}{G} \biggr)\frac{dm}{d\zeta} \biggl( \frac{\partial \zeta}{\partial t} \biggr) + \biggl( \frac{c_s^3}{G}\biggr) m = \biggl( \frac{c_s^3 t}{G} \biggr)\frac{dm}{d\zeta} \biggl( \frac{\zeta}{t} \biggr) + \biggl( \frac{c_s^3}{G}\biggr) m = \biggl( \frac{c_s^3 }{G} \biggr) \biggl[ \zeta ~\frac{dm}{d\zeta} + m \biggr] \, ; </math> </td> </tr> <tr> <td align="right"> <math>~\frac{\partial v_r}{\partial r}</math> </td> <td align="center"> <math>~=</math> </td> <td align="left"> <math>~ -c_s \frac{dU}{d\zeta} \biggl( \frac{\partial \zeta}{\partial r} \biggr) = -c_s \frac{dU}{d\zeta} \biggl( - \frac{\zeta^2}{c_s t} \biggr) = \frac{\zeta^2}{t} \frac{dU}{d\zeta} \, ; </math> </td> </tr> </table> </div> -->
Summary:
Please note that all contributions to JETohlineWiki may be edited, altered, or removed by other contributors. If you do not want your writing to be edited mercilessly, then do not submit it here.
You are also promising us that you wrote this yourself, or copied it from a public domain or similar free resource (see
JETohlineWiki:Copyrights
for details).
Do not submit copyrighted work without permission!
Cancel
Editing help
(opens in new window)
Navigation menu
Personal tools
Not logged in
Talk
Contributions
Log in
Namespaces
Page
Discussion
English
Views
Read
Edit
View history
More
Search
Navigation
Main page
Tiled Menu
Table of Contents
Old (VisTrails) Cover
Appendices
Variables & Parameters
Key Equations
Special Functions
Permissions
Formats
References
lsuPhys
Ramblings
Uploaded Images
Originals
Recent changes
Random page
Help about MediaWiki
Tools
What links here
Related changes
Special pages
Page information