[ICEM] Convergence problem due to meshing

raunakjung · March 19, 2017, 08:47

I am using fluent for my simulations. I have created hexahedral mesh in ICEM .My geometry consists of 2100000 elements.

I have following quality reports:
Determinant 3*3*3 : All elements more than 0.4 ; 0.2 for 4 elements
Quality 120 elements less than 2
Min angle 540 elements having less than 9
Eriksson skewness 112 elements less than 0.1
Because I am having difficulty in convergence. I would some like advice whether this is the cause of it. The worst elements aren't in region of interest of geometry.

my numerical settings are ke model with wall functions , SIMPLE , discretizations both first and second order.

Thank You

bluebase · March 20, 2017, 05:19

Hello raunak,

You could check the residual field to see where areas of high numeric errors are. (I don't recall how to do this in fluent).

Some more generel comments: a minumum angle of 9 degree is very small for hexas. This should be improved. I try to improve every blocking to have a eriksson skewness of above 0.3. This limits numerical diffusion and avoid divergent simulations. With reduced skewness you will also have better orthogonality (determinant).

Another reason for convergence issues you should have in mind are strong changes in element size. With other words hugh elements next to very small elements.

Moreover, when you simulate problems stationary which do not converge, this can also mean, that you problem is not completely stationary and you might have to switch to transient methods.

With regards,
Sebastian

March 19, 2017, 08:47	Convergence problem due to meshing	#1
raunakjung Senior Member raunak jung pandey Join Date: Jun 2016 Posts: 102 Rep Power: 10	I am using fluent for my simulations. I have created hexahedral mesh in ICEM .My geometry consists of 2100000 elements. I have following quality reports: Determinant 333 : All elements more than 0.4 ; 0.2 for 4 elements Quality 120 elements less than 2 Min angle 540 elements having less than 9 Eriksson skewness 112 elements less than 0.1 Because I am having difficulty in convergence. I would some like advice whether this is the cause of it. The worst elements aren't in region of interest of geometry. my numerical settings are ke model with wall functions , SIMPLE , discretizations both first and second order. Thank You

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March 20, 2017, 05:19		#2
bluebase Senior Member Sebastian Engel Join Date: Jun 2011 Location: Germany Posts: 567 Rep Power: 21	Hello raunak, You could check the residual field to see where areas of high numeric errors are. (I don't recall how to do this in fluent). Some more generel comments: a minumum angle of 9 degree is very small for hexas. This should be improved. I try to improve every blocking to have a eriksson skewness of above 0.3. This limits numerical diffusion and avoid divergent simulations. With reduced skewness you will also have better orthogonality (determinant). Another reason for convergence issues you should have in mind are strong changes in element size. With other words hugh elements next to very small elements. Moreover, when you simulate problems stationary which do not converge, this can also mean, that you problem is not completely stationary and you might have to switch to transient methods. With regards, Sebastian