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Wall Shear Stress Dramatically Increases Everytime I Refine My Mesh

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Old   April 22, 2019, 06:49
Default Wall Shear Stress Dramatically Increases Everytime I Refine My Mesh
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Jonathan Kartawihardja
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Hi there,

I'm currently modelling the fluid flow from a catheter (100% saline) into a vein (100% blood) on Star CCM as part of my thesis project.

I've chosen the SST (Menter) K-Omega to model the turbulent regions as the saline interacts with the blood in the vein. I've selected the all y+ wall treatment model and kept my y+~1. I'm currently attempting to do a mesh independence study (GCI to be exact), however every time I increase my mesh by a refinement ratio of 2 (doubling the number of cells), my Max Wall Shear Stress (WSS), Average WSS and Std WSS drastically increases and results in my GCI values to very large for WSS. Also, I believe that my mesh is refined enough as i've gone up to 8,000,000 cells and still my WSS values are still increasing.

I've attached a picture of what I'm trying to model and also the WSS plots with each mesh refinement.

I'm a complete newbie when it comes to CFD and Star CCM+, so any pointers or words of advice would very much be appreciated (and the fact i don't know who else to ask).

Thank you in advance
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Old   April 22, 2019, 07:31
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Filippo Maria Denaro
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From the second diagram I suppose you restarted from the solutions of the previous grids, right? It seems you are getting towards a mesh independent solution, try to refine it further. In turbulence the wall stress requires to describe very well the velocity profile in the viscous sub-layer.

However, you should be aware of the fact that your problem would require a non-newtonian model.
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Old   April 22, 2019, 12:16
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Hi FMDenaro, that is correct i've restarted the solution from previous grids. How much further refinement (how many cells?) would you say is appropriate? as i have already gone up to 8,000,000 cells already.

Thanks.
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Old   April 22, 2019, 22:02
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Steve
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You should check the Re number in your problem.
Check velocity first..
I'm not sure you chose right physics model
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Old   April 23, 2019, 01:56
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Hi,


Is this a 2 phase problem? Then I believe this is a valid behaviour at the triple contact point (region where the meniscus touches the wall). The more you refine, the shear stresses on the wall increase.



This phenomena is predominant at large capillary numbers. I see you are using a turbulence modelling, so I suppose this should be even worse in your case.



If this is the case, try using a partial slip on the wall incase you are using a no-slip wall boundary. Choose the slip length equal to or slighty larger than the boundary cell size.


Look into the work of S. Afkhami et al. (2009), however, they dealt with capillary flows which is not similar to your flow conditions.
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