# NS Equations and convergence problems

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 November 6, 2010, 15:06 #2 Member   ganesh Join Date: Mar 2009 Posts: 40 Rep Power: 10 Dear Ramesh, From your description of the convective and viscous flux computations, it seems that you are using a simple central differencing for the inviscid part. The simple averaging of the states in fact is essentially a zero artificial dissipation method, so you are relying entirely on the physical dissipation to stabilise your code. For a reasonable Re, this might not be sufficient enough. I presume this is the cause of your divergence. Also, though not of very immediate concern, the gradient computation by averaging the centroidal gradients to the face isn't a nice option either, they can lead to odd--even decoupling which is undesirable. Please take a look at the relevant portions in the book by J. Blazek, which I believe is an excellent book from viewpoint of writing CFD codes. Hope this helps. Regards, Ganesh

 November 8, 2010, 01:28 #3 Senior Member   Hamid Zoka Join Date: Nov 2009 Posts: 212 Rep Power: 11 Dear Rameshk; I think your problem is mainly due to grid generation. this grid generation technique causes problems in regions near to pipe centerline. in other words "r" approaches zero at the vicinity of centrline and therefore some fluxes may become infinitively large and this leads in divergence. try other techniques to control the mesh shape in pipe centerline regions, such as: - Unstuctured mesh - Multi block structured mesh (it is called butterfly mesh for pipeline meshing) regards

November 8, 2010, 12:33
Thanks Hamidzoka and Ganesh
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Ramesh K
Join Date: Dec 2009
Location: Bangalore
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Quote:
 Originally Posted by Hamidzoka Dear Rameshk; I think your problem is mainly due to grid generation. this grid generation technique causes problems in regions near to pipe centerline. in other words "r" approaches zero at the vicinity of centrline and therefore some fluxes may become infinitively large and this leads in divergence. try other techniques to control the mesh shape in pipe centerline regions, such as: - Unstuctured mesh - Multi block structured mesh (it is called butterfly mesh for pipeline meshing) regards

though I aim using a cylindrical mesh My solver is in cartesian coordinates, so I have triangular prisms as volumes at the axis and quads in rest of the places.

@ Ganesh

I am adding artificial dissipation also in my code but since I have an dissipation term which is of second order I am not adding the second order dissipation term, If this second order term is added to the scheme the order of scheme gets reduced.

my doubt is even if I have physical mechanism of dissipation present in the equations do we need to add second order artificial dissipation terms.

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