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Old   February 9, 2011, 12:48
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Hi!
i'm a cfd newbie and i'm trying to solve a tipical cfd problem: catching shedding vortex (shown in the figure below) generated by a steady flow around a circle (2D) with Fluent



i'm trying to solve it with a k-eps model (unsteady analisys). for the mesh i use a boundary layer around the circle and a triangular growing size mesh all over, (more thick in the tail) is this correct?
and about the wall condition, do i have to use one?

thank you very much!
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Old   February 10, 2011, 03:01
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here one example for laminar flow:
Sans titre.png
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Old   February 11, 2011, 06:51
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thanks, but is not i was looking for....

now i undertstand that i can obtain the phenomenou by chosing a different viscous turbolence model according to the reynolds number...

is it correct?

thanks
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Old   February 11, 2011, 07:05
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Quote:
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and about the wall condition, do i have to use one?
I answered to this question

Quote:
Originally Posted by Lombo View Post
now i undertstand that i can obtain the phenomenou by chosing a different viscous turbolence model according to the reynolds number...
is it correct?
No the phenomenon doesn't depend on the turbulence model, but on the Reynolds number
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Old   February 11, 2011, 11:56
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yes! thank you very much!

i resolved the case with k-eps model, and now i'm trying to change their values (k and eps) to understand how they affect the phenomenou...

but i don't really understand...

moreover i would know how to choose this parameters
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Old   February 11, 2011, 12:25
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k and eps are fixed (you impose a specific turbulent flowfield given by Re >> check litterature)
So you are not supposed to change their values as you are not supposed to change the inlet velocity (for a given Re)
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Old   February 11, 2011, 12:51
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i use fluent and with this software i have to specify the velocity at the inlet, k and eps (how choose them?)... i calculate the reynolds manually..
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Old   February 14, 2011, 02:31
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there are formulas for k and eps.
Check online help chapter 7.2.2 "Determining Turbulence Parameters"
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Old   February 15, 2011, 11:42
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what is the different between the steady and unsteady command?

i tried to solve a simple 2-d problem of a beam section:

with the steady solution a k-eps viscous model sometimes i cath the oscillation of the lift coefficient, but with the unsteady k-eps i don't reach the same results...

i don't understand why this happens, because in general the flow should be unsteady....
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Old   February 16, 2011, 03:02
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with the steady solution a k-eps viscous model sometimes i cath the oscillation of the lift coefficient, but with the unsteady k-eps i don't reach the same results...
If you get oscillations with a steady solver, then the oscillations are due to the convergence. Understand you solution is moving untill it reaches its converged value (if your model has a steady solution)
In unsteady solver, the oscillations are unsteady oscillations with respect of time (and not iterations as in steady solver)
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Old   March 3, 2011, 04:52
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Why are you trying a steady state solution ??????? Flow over a cylinder is not a steady state problem............ It is a transient case problem.
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