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IDProjectCategoryView StatusLast Update
0000139OpenFOAMBugpublic2011-07-28 15:36
Reporteruser136Assigned Toadministrator  
PrioritynormalSeveritymajorReproducibilityalways
Status resolvedResolutionfixed 
Summary0000139: Wrong courant number calculation for boundary patches
DescriptionHi,

I'm not sure if this has been discussed before, or is a case of "works as designed", but here we go: For my private interFoam-variant I recognized that the calculated timestep length is often smaller than expected from the CFL-criterion. I boiled it down to the point that

$FOAM_SRC/finiteVolume/cfdTools/incompressible/CourantNo.H

(and its variants) give back wrong values if faces at inlet/outlet-patches have "high" velocities. The reason for this that mesh.surfaceInterpolation::deltaCoeffs() should return the cell-to-cell distance for a face, but for faces at boundaries it returns face-to-cell distance instead, roughly half of it. This o.k., but as a consequence the computed CFL-number is (wrongly) twice as high as expected. Thus, either boundary faces should be excluded from the CFL-computation or the "distance" should be doubled.

Bye,

Carsten

Steps To ReproduceRun the attached example with interFoam. It has a 1m x 1m x 1m grid spacing. Velocity is 1m/s. Timestep length should be adjusted to CFL=1.

Expected result: Timestep length = 1.0 s
Observed result: Timestep length = 0.5 s

TagsNo tags attached.

Activities

user136

2011-02-15 15:06

 

couranttest.zip (21,208 bytes)

henry

2011-02-15 15:24

manager   ~0000248

Thanks for the detailed report. This is a known problem and we have developed more reliable ways to evaluate the effective Courant number resolving this and other potential problems with the current approach. These new methods will be included in our next release of OpenFOAM.

user136

2011-02-15 15:33

  ~0000249

Hello Henry, is there already a scheduled date for the next version? Or a simple workaround you would suggest (a way to manipulate CourantNo.H so that boundary patches are ignored?)?.

Thanks, Carsten

Issue History

Date Modified Username Field Change
2011-02-15 15:06 user136 New Issue
2011-02-15 15:06 user136 File Added: couranttest.zip
2011-02-15 15:24 henry Note Added: 0000248
2011-02-15 15:24 henry Assigned To => henry
2011-02-15 15:24 henry Status new => acknowledged
2011-02-15 15:33 user136 Note Added: 0000249
2011-07-28 15:36 user2 Status acknowledged => resolved
2011-07-28 15:36 user2 Resolution open => fixed
2011-07-28 15:36 user2 Assigned To henry => administrator