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

peatmac · April 6, 2012, 16:15

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

villager · April 7, 2012, 06:05

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.

peatmac · April 7, 2012, 06:32

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.

villager · April 16, 2012, 15:23

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.

April 6, 2012, 16:15	Turbulence model / Boundary layers on a cylinder (VIV with 2DOF)	#1
peatmac New Member Peter Join Date: Jan 2012 Posts: 29 Rep Power: 14	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

April 7, 2012, 06:05	...	#2
villager Member vlg Join Date: Jul 2011 Location: My home :) Posts: 81 Rep Power: 18	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.

April 16, 2012, 15:23	turbulence...	#4
villager Member vlg Join Date: Jul 2011 Location: My home :) Posts: 81 Rep Power: 18	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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April 7, 2012, 06:32		#3
peatmac New Member Peter Join Date: Jan 2012 Posts: 29 Rep Power: 14	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.