# flow instability

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 May 2, 2013, 08:45 flow instability #1 New Member   Kishore Join Date: Mar 2013 Posts: 20 Rep Power: 5 Dear all I have consider 90 degree slice model for the artery having stenosis ( like venturi). The area of restriction we fixed 85% . In such case, low Re turbulence ( shear layer instability ) takes place after the flow comes out from the throat. what kind of flow model can I use? (Note: before entering the throat the flow is laminar). We calculated the Re at throat is around 760 1. Please suggest us which turbulence model is suitable to solve this problem 2. In this condition, Can we use symmetry geometry ? Thank you Regards Kish

 May 2, 2013, 09:14 #2 Senior Member     Mr CFD Join Date: Jun 2012 Location: Britain Posts: 312 Rep Power: 7 Surely a Re of 760 is laminar and you don't need to use a turbulence model which makes things far simpler. Why do you want to force your simulation to use a turbulence model when it might not need one? If you insist on a turbulence model try the k-epsilon model to begin with, as you can use wall functions and don't need to integrate to the wall. I.e. you don't need high Y+ values if you're not interested in what happens at the walls. Your simulation seems like simple pipe flow. I wouldn't use symmetry only because your simulation sounds cheap (computationally). Exactly what do you want to get out from the CFD simulations?

May 2, 2013, 10:10
#3
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Kishore
Join Date: Mar 2013
Posts: 20
Rep Power: 5
Quote:
 Originally Posted by RicochetJ Surely a Re of 760 is laminar and you don't need to use a turbulence model which makes things far simpler. Why do you want to force your simulation to use a turbulence model when it might not need one? If you insist on a turbulence model try the k-epsilon model to begin with, as you can use wall functions and don't need to integrate to the wall. I.e. you don't need high Y+ values if you're not interested in what happens at the walls. Your simulation seems like simple pipe flow. I wouldn't use symmetry only because your simulation sounds cheap (computationally). Exactly what do you want to get out from the CFD simulations?
Thank you for the quick response

We used laminar flow and try to locate time average pressure recovery length. We could not find out. The time average pressure increases and increases. No pressure reversal takes place.

Because of this we plane to use turbulence model.

I hope you understand our simple problem.I will try to apply k-epsilon model to capture low Re turbulence situation.
Can we use SST model?

Thank you

Regards

Kish

May 2, 2013, 11:22
#4
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OJ
Join Date: Apr 2012
Location: United Kindom
Posts: 475
Rep Power: 12
From your description, the flow seems a simple laminar flow and use of a turbulence model will give you erratic results. Never use k-eps model for flows with smaller Re since this model struggles in presence of smaller turbulence kinetic energy. If you insist on using turbulence model, go for kw-SST, this should give you at least some reasonable results. But mind you, it will add diffusion and the results may suffer.

If your blood artery is symemtrical and if there is no forseeable secondary flows, symmetry is a good idea, and I would recommend a 3 deg slice, rather than 90 deg, since you won't get any benefit of having a bigger slice if the flow is axisymmetric.

Quote:
 We used laminar flow and try to locate time average pressure recovery length. We could not find out. The time average pressure increases and increases. No pressure reversal takes place. Because of this we plane to use turbulence model.
I don't understand why you'd go for a time-averaged results if the flow is not unsteady. I imagine laminar model will be difficult to converge, because there is no added diffusion like in the case of turbulence models (which add numerical diffusion). Hence for very fine grids, even the steady flow might start capturing the small perturbation and show some unsteady characteristics and will be hard to converge. While using laminar model, do not use too fine grids if you don't need to, and for this, you will need to do a mesh independent study and select the largest grid spacing that gives mesh independent solution.

Typically your pressure will start dropping shortly before the throat, it would be minimum just after the throat and then it will recover to a value and then will start dropping again at very small rate (due to wall friction of artery). So after throat you will just see increasing pressure till recovery, because it has dropped before the throat!

OJ

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