# A question about FDM in cylindrical coordinates.

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 November 10, 2004, 14:43 A question about FDM in cylindrical coordinates. #1 Demidov Guest   Posts: n/a Hello everybody! My question is about solving equations (for example Laplass or Poisson equations) in cylindrical coordinates using the finite difference method. Actually I dont understand which boundary conditions should I apply for this finite-difference scheme in the point r=0 (or r = "very small dr", one dr step for example). In the point r=R I use (for example) F(r,fi) = 0; In fi=0 and fi= 2pi I use F(r,0) = F(r,2pi); These are the 3 of 4 sides of the corresponding rectangualar. But which boundary condition should i write for the point r=0 (or r=dr) ? May be, F(0,fi) = F(o,fi+pi)? Or F(0,fi) = F(0, any fi)? Please, share your experience.

 November 10, 2004, 16:56 Re: A question about FDM in cylindrical coordinate #2 Junseok Kim Guest   Posts: n/a It depends on how discretize your domain, for example, cell-centered or cell-edged, etc.

 November 10, 2004, 17:17 Re: A question about FDM in cylindrical coordinate #3 Demidov Guest   Posts: n/a Sorry, can you clarify your answer? I discretize my domain using the standart 5-point Laplass operator approximation. Without coefficients it is like that: d2F/dr2 = F(r+1,fi) - 2F(r,fi) + F(r-1,fi) dF/dr = F(r+1,fi) - F(r-1,fi) F = F(r,fi) dF/dfi = F(r,fi+1) - 2*F(r,fi) + F(r,fi-1) ... and so on. So, we have 5-diagonal matrix (except for boundary conditions for fi) with F(r,fi) on the main diagonal.

 November 10, 2004, 17:27 Re: A question about FDM in cylindrical coordinate #4 John Guest   Posts: n/a See page 67 of this thesis: http://ctr.stanford.edu/thesis.pdf

 November 11, 2004, 05:33 Re: A question about FDM in cylindrical coordinate #5 Tom Guest   Posts: n/a Use dF/dR = 0 at r=0.

 November 15, 2004, 06:39 Re: A question about FDM in cylindrical coordinate #6 Salvador Guest   Posts: n/a See paper by Morinishini et al. (2004) in journal of computational physics (and references there). Remember that the centreleine in a non-physical (grid generated) singularity and special treatment is needed there.

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