max time step related to diffusion time

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 September 21, 2010, 15:15 max time step related to diffusion time #1 Senior Member     Daniel P. Combest Join Date: Mar 2009 Location: St. Louis, USA Posts: 612 Rep Power: 22 Hello all, I am interested in adjusting the time step for a transient solver on both the Courant number (already implemented in Openfoam) and on the time required for diffusion to be captured. Right now Im starting simple, using the icoFoam solver with species transport. Since I would like to add this to a turbulent solver later (diffusivity depends on turbulent schmidt number), I have made diffusivity a field. I want to find the average diffusion time and the minimum diffusion time based on: t_diffusion = dx^2/D I looked at the CourantNo.H file and found: surfaceScalarField SfUfbyDelta = mesh.surfaceInterpolation::deltaCoeffs()*mag(phi); CoNum = max(SfUfbyDelta/mesh.magSf()) .value()*runTime.deltaT().value(); meanCoNum = (sum(SfUfbyDelta)/sum(mesh.magSf())) .value()*runTime.deltaT().value(); velMag = max(mag(phi)/mesh.magSf()).value(); but I need to extract the delta X values, square them, and then divide them by the cell specific diffusivity. How does one do this exactly? Any help is much appreciated.

 September 23, 2010, 11:52 #2 Senior Member   Eugene de Villiers Join Date: Mar 2009 Posts: 725 Rep Power: 14 What do you mean exactly by delta X values?

September 23, 2010, 13:13
dalta x as in h, spacing, or cell-cell distance
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Daniel P. Combest
Join Date: Mar 2009
Location: St. Louis, USA
Posts: 612
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Quote:
 Originally Posted by eugene What do you mean exactly by delta X values?
dalta x as in h, spacing, or cell-cell distance. How far the species must diffuse across. If the time steps are too big then the result is incorrect.

 September 23, 2010, 18:40 #4 Senior Member   Eugene de Villiers Join Date: Mar 2009 Posts: 725 Rep Power: 14 Ok then you can get it from mesh.surfaceInterpolation::deltaCoeffs() This is a surfaceScalarField with value 1/delta - the cell-centre to cell-centre distances. (Or face centre to cell centre distance at the boundary.)

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