       Re: Infinite expression, NDSolve

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• Subject: [mg128406] Re: Infinite expression, NDSolve
• From: "Kevin J. McCann" <kjm at KevinMcCann.com>
• Date: Tue, 16 Oct 2012 20:11:30 -0400 (EDT)
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```Your problem is twofold, I think. One is the obvious numerical
singularity at r=0. Here you could try using r=0.00001 or some such to
get around it, but care must be taken. The second problem is the
inconsistency of the boundary conditions T[1,t]= and T[r,0]==0.

Kevin

On 10/16/2012 3:25 AM, Grasley wrote:
> Dear All,
>
> I am trying to use NDSolve to solve the heat equation for 1-D radial flow in cylindrical coordinates, with the initial condition of a spatially constant temperature, and boundary conditions of zero flux at r=0 (axisymmetry) and a constant temperature at a radius of 1.  Here is the code:
>
> NDSolve[{D[T[r, t], t] == 1/r*D[r D[T[r, t], r], r],
>    Derivative[1, 0][T][0, t] == 0, T[1, t] == 20, T[r, 0] == 0}, T, {r,
>     0, 1}, {t, 0, 10}]
>
> Power::infy: Infinite expression 1/0. encountered. >>
>
> Infinity::indet: Indeterminate expression 0. ComplexInfinity encountered. >>
>
> Power::infy: Infinite expression 1/0. encountered. >>
>
> Infinity::indet: Indeterminate expression 0. ComplexInfinity encountered. >>
>
> NDSolve::ndnum: Encountered non-numerical value for a derivative at t == 0.`. >>
>
> I've tried using nonzero (i.e. very small values) for the initial time and in place of r=0 in the boundary condition, but to no avail.  Any ideas?
>
> Thanks!
>

```

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