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-   -   low reynolds number models in Fluent (http://www.cfd-online.com/Forums/main/63708-low-reynolds-number-models-fluent.html)

 doug April 16, 2009 16:29

low reynolds number models in Fluent

I have two questions:

1. Does the term "low Reynolds number model" mean that the model is only good for low Reynolds number flows? Or does it mean that it is a modification capable of handling both hi and low Reynolds number flows?

2. When I run fluent using periodic boundary conditions and either the Lam Bremhorst (index 1) or Launder Sharma (index 2) model at a Reynolds number of 10,000 the kinetic energy eventually goes to zero across the channel, and I get a laminar solution. Any ideas why that might be? This does not happen when I run the standard k-e model with standard wall functions or the standard k-e model with "enhanced wall treatment" with periodic boundary conditions.

Thanks.

Doug

 harishg April 16, 2009 16:47

It means that your model can be integrated upto the wall and you do not need to use any wall function. Check the value of yplus. The wall function approach requires your first grid point to be in the log layer (yplus > 30) while the low Re models requires yplus of the first grid point to be less than 5 to accurately model the near wall effects.

 doug April 16, 2009 17:19

My first wall plus value is around .01 on my current grid (100x100). I've run some more cases and find that if I coarsen the grid (50x50) I get the turbulent velocity profile as expected. However, if I refine the grid (200x200) I get the laminar solution. This is opposite of my intuition which would say that coarsening the grid would give the worse result...?

 doug April 16, 2009 17:21

Wait a minute. I take that back. The 200x200 grid is giving me a better velocity profile. I'll get back when I have better results.

Thanks for your help!

 doug April 17, 2009 15:43

Does anyone know what Fluent is using for the BCs for the low-re number models available only through the TUI? I've searched their documentation in vain to find the boundary conditions for these models. I can find them from the original literature from the models, but I'm a little wary that Fluent is fudging things near the wall...

Thanks!

 neazen April 20, 2009 10:42

About low reynolds number flow question :

Usually this term refer to specific flows such that the Reynolds number is very small so the inertia term of the Navier-Stokes equation is very small compared to the viscous forces and it is possible to neglect it. (Like for lubrification problems)
This leads to the so called (linear) Stokes equations for a viscous fluid, were the Reynolds number can be replaced by unity.
Obviously in this case the model is inappropriate to flows with high Re number.

Now the question is : does Fluent mean something else by using this term?

 asurpam August 4, 2012 14:39

hi,
Can anyone tell me how to set periodic boundary condition for low Reynolds number flow in a pipe???

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