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-   -   shadding vortex (http://www.cfd-online.com/Forums/ansys-meshing/84829-shadding-vortex.html)

Lombo February 9, 2011 12:48

shadding vortex
 
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

http://img11.imageshack.us/img11/294...hedding.th.jpg

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!

-mAx- February 10, 2011 03:01

1 Attachment(s)
here one example for laminar flow:
Attachment 6437

Lombo February 11, 2011 06:51

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

-mAx- February 11, 2011 07:05

Quote:

Originally Posted by Lombo (Post 294483)
and about the wall condition, do i have to use one?

I answered to this question

Quote:

Originally Posted by Lombo (Post 294806)
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

Lombo February 11, 2011 11:56

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

-mAx- February 11, 2011 12:25

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)

Lombo February 11, 2011 12:51

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..

-mAx- February 14, 2011 02:31

there are formulas for k and eps.
Check online help chapter 7.2.2 "Determining Turbulence Parameters"

Lombo February 15, 2011 11:42

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....

-mAx- February 16, 2011 03:02

Quote:

Originally Posted by Lombo (Post 295359)
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)

vmlxb6 March 3, 2011 04:52

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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