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Turbulence model / Boundary layers on a cylinder (VIV with 2DOF)

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Old   April 6, 2012, 15:15
Default Turbulence model / Boundary layers on a cylinder (VIV with 2DOF)
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Peter
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I hope someone can help me on this one.

I am modelling flow across a cylinder with Re=50000. As the flow is subcritical and has laminar boundary layers on the cylinder (Re< 1 x 10^5), can I model this flow using a turbulent solver? I'm not interested in the flow field, just how forces acting on the cylinder. I am producing simulations of VIV on a cylinder with 2DOF.
Or do I have to use a turbulent model to solve this case?

Thanks,

P
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Old   April 7, 2012, 05:05
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Are you sure, that your flow is subcritical? Re(critical) is about 10000 = 10^4 for cylindric pipe.
Without any turbulent model (or DNS, or LES) you couldn't resolve turbulent vortices right in boundary layer.
I think, you should use turbulence model.
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Old   April 7, 2012, 05:32
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Ill look into what Re the boundary layers separate. It is flow over a circular cylinder (external flow).

I have been doing a bit of reading and a few people recommend using the K Omega SST turbulence model. To use this model effectively I have read that the Y+ value should equal ~1. The mesh required for Y+ ~1 are big, and will take a long time to solve.

I have played about with various solvers (turbulent ones) and cant seem to get vortices to form. I have been using flow with the following characteristics -

Density = 1025 Kg/m3
Viscosity = 0.001008
Cylinder diameter = 0.5 m
Flow rate = 0.1 m/s

Re = 50843.254

Any advice on how I can get vortices forming?

Many thanks for that reply.
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Old   April 16, 2012, 14:23
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And how do you understand, that no vortices are forming?
Do you look vectors or pathlines? Or k,e and turbulent intencity?
I had same question too, but didn't found answer in www. In my opinion, our nodal grid for some cases are much bigger (0,09 mm versus 10^-4) than turbulent length scales (macroscales). Higher the turbulence, bigger these scales - in my flow they rose to 0,7 mm. It continues to some limit, than these scales appears to decrease...
That's only my own conclusions and calculations, any specialist's agreement/critics is needed.

You wrote, that fine mesh (y+ about 1) is not a variant. Do you use y+>30 with standard wall function? Make sure that.
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