[Lombo]

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!

[-mAx-]

here one example for laminar flow:
Sans titre.png

[Lombo]

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-]

Originally Posted by Lombo

and about the wall condition, do i have to use one?

I answered to this question

Originally Posted by Lombo

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]

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-]

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]

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-]

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

[Lombo]

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-]

Originally Posted by Lombo

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]

Why are you trying a steady state solution ??????? Flow over a cylinder is not a steady state problem............ It is a transient case problem.