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MathGroup Archive 2005

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Fwd: Getting good results with NDSolve?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg62951] Fwd: Getting good results with NDSolve?
  • From: pantagruel123 at aol.com
  • Date: Fri, 9 Dec 2005 05:10:33 -0500 (EST)
  • References: <dn8g3t$bqd$1@smc.vnet.net> <43983230.6010903@metrohm.ch>
  • Sender: owner-wri-mathgroup at wolfram.com

Whoops, yes...that is what I meant...actually that is what I had in NDSolve:
 
dg/dt=norm(Grad g)*Div(Grad g/Norm (Grad g))...
 
Chris 
 
-----Original Message-----
From: dh <dh at metrohm.ch>
To: mathgroup at smc.vnet.net
Subject: [mg62951] Re: Getting good results with NDSolve?


Hi Christopher, 
something is fishy with your expression: 
 
dg/dt = Norm(Grad g) * Laplacian(Grad g / Norm (Grad g)) 
 
On the left side you have a scalar. 
On the right side you have Laplacian(Grad g ...) what is a vector. 
 
Maybe you mean Div( Grad g / Norm (Grad g))????? 
 
Daniel 
 
pantagruel123 at aol.com wrote: 
> > Hi, I'm experimenting with NDSolve, not being an expert with differential equations. I'm getting some results, but it seems like I'm getting too much precision where I don't need it, and not enough where I do. I'm wondering if playing with the options would be something worth trying. Also it takes a really long time to compute what I've been told should take seconds with a C++ routine. > > If I want to solve a PDE of three variables (t,x,y) on the region {0,1}^3, how can I tell NDSolve to have, say a resolution of N x N points for (x,y) and a resolution of M for t? I find that if I just reduce the max points for t, then NDSolve quits before gets to the end of the interval. Maybe there is a way to tell NDSolve to trash some intermediate results and just keep every Mth point? 
> > The equation i'm using is dg/dt = Norm(Grad g) * Laplacian(Grad g / Norm (Grad g)) 
> where we're looking for g(t,x,y), and the right-hand derivatives are taken only with respect to x and y. My initial value g(0,x,y) had been generated from a ListInterpolation--I'm guessing this is the best way to use a bitmap as the inital value. > > Regards, 
> > Christopher Arthur 
> >  


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