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 pyxbird January 17, 2013 03:05

flow structure around cylinder

1 Attachment(s)
Dear all,
As you can see in the picture, velocity=45cm/s, diameter=30cm, Re=13500, RNG model is used, the grid is fine enough, the flow is steady, usually in this condition the turbulence behind the cylinder should be intensive, vortices should be seen. However from the streamlines we can see that the model performs not so good, why?

 Davahue January 18, 2013 09:24

Dear pyxbird,
I've done the same simulation: karman street vortex past a cylinder.
I did 2D with the same turbulence model. Can you specify wich is your grid spacement?
I'll recommend you to try with 2D with finner meshes and then go back 3D.
In my experience, FLOW-3D is able to catch the flow pattern past the cylinder even with other turbulence models.

Hope it helps,
Daniel

 pyxbird January 19, 2013 07:19

Quote:
 Originally Posted by Davahue (Post 402660) Dear pyxbird, I've done the same simulation: karman street vortex past a cylinder. I did 2D with the same turbulence model. Can you specify wich is your grid spacement? I'll recommend you to try with 2D with finner meshes and then go back 3D. In my experience, FLOW-3D is able to catch the flow pattern past the cylinder even with other turbulence models. Hope it helps, Daniel
Dear Daniel, thank you for your reply. In my model, the computation region is 5m long divided into 250 grids, 2m wide divided into 100 grids, 0.5m deep divided into 25 grids.
I will try your advice , if the problem exists, I will state it later, thanks.

 pyxbird February 3, 2013 01:11

2 Attachment(s)
Quote:
 Originally Posted by Davahue (Post 402660) Dear pyxbird, I've done the same simulation: karman street vortex past a cylinder. I did 2D with the same turbulence model. Can you specify wich is your grid spacement? I'll recommend you to try with 2D with finner meshes and then go back 3D. In my experience, FLOW-3D is able to catch the flow pattern past the cylinder even with other turbulence models. Hope it helps, Daniel
Dear Daniel,
I have try with finner meshes in 2D, but it doesn't work. I will show you two pictures below. Did you see the problem?

 Davahue February 4, 2013 05:58

I still think that the mesh is too coarse, anyway, have you tried incresing the velocity? What happens then?

Quote:
 Originally Posted by pyxbird (Post 405690) Dear Daniel, I have try with finner meshes in 2D, but it doesn't work. I will show you two pictures below. Did you see the problem?

 mahdi1 February 12, 2013 14:56

Quote:
 Originally Posted by pyxbird (Post 402365) Dear all, As you can see in the picture, velocity=45cm/s, diameter=30cm, Re=13500, RNG model is used, the grid is fine enough, the flow is steady, usually in this condition the turbulence behind the cylinder should be intensive, vortices should be seen. However from the streamlines we can see that the model performs not so good, why?
Hello
Could you please tell me where can I introduce Reynolds number to software?
Thanks

 jayrup February 19, 2013 00:24

Quote:
 Originally Posted by pyxbird (Post 402365) Dear all, As you can see in the picture, velocity=45cm/s, diameter=30cm, Re=13500, RNG model is used, the grid is fine enough, the flow is steady, usually in this condition the turbulence behind the cylinder should be intensive, vortices should be seen. However from the streamlines we can see that the model performs not so good, why?
Hi pyxbird

Visually RNG model may not show you the vortices at this velocity.
But if you plot Fd & Fl around the cylinder you may observer the variation.
If you want to see visually the vortex shedding try to use LES model.

Regards
Jay

 pyxbird March 7, 2013 22:01

It doesn't work.

 jayrup March 13, 2013 02:36

1 Attachment(s)
Quote:
 Originally Posted by pyxbird (Post 412420) It doesn't work.

velocity=45cm/s, diameter=30cm, Re=13500,
Either the velocity is correct or the Re.
As per the velocity Re come 135000.
As per the Re velocity comes 4.5 cm/s

Try using Second order Monotonicity preserving for Momentum advection.
Well I do get the vortices with LES.
Jay

 pyxbird March 18, 2013 07:30

Quote:
 Originally Posted by jayrup (Post 413581) velocity=45cm/s, diameter=30cm, Re=13500, Either the velocity is correct or the Re. As per the velocity Re come 135000. As per the Re velocity comes 4.5 cm/s Try using Second order Monotonicity preserving for Momentum advection. Well I do get the vortices with LES. Please see the attached image. Jay
Dear Jay,
Hi, I have tried using second order monotonicity preserving for momentum advection, but it still doesn't work. The Re is 135000, 45cm/s, 50cm in depth. And I have tried the Tlen(mixing length) for 0.07*depth=3.5, and I have also tried Tlen for 10, 50, 300. The turbulence is not intensive.
So what might be wrong with my model? Or would you please show me some parameters of your model, some parameters that I probably set uncorrectly.

 mahdi1 March 28, 2013 10:36

2 Attachment(s)
Hello

I want to simulate flow around a cylinder.
Diameter of cylinder=0.0254 m , width of channel= 0.3048 m and U=0.9 m/s
my model:
No sharp interface, RNG, wall (no-slip), uniform mesh (cell size = 0.00635 m)
2 Dimension
X : inlet (velocity), outlet (continuative), Y: Wall, Z: symmetry
second order monotonicity, GMRES

There is difference between results of flow-3D and experimental results.
Also, below message appear during running:
"there is no reference pressure"

Do you have an advice to solve it.

Thanks

 pyxbird March 28, 2013 21:42

Quote:
 Originally Posted by mahdi1 (Post 417008) Hello I want to simulate flow around a cylinder. Diameter of cylinder=0.0254 m , width of channel= 0.3048 m and U=0.9 m/s my model: No sharp interface, RNG, wall (no-slip), uniform mesh (cell size = 0.00635 m) 2 Dimension X : inlet (velocity), outlet (continuative), Y: Wall, Z: symmetry second order monotonicity, GMRES There is difference between results of flow-3D and experimental results. please see attached files. Also, below message appear during running: "there is no reference pressure" Do you have an advice to solve it. Thanks
Dear Mahdi1,
Hi, unfortunately I am not good at this part. RNG is used in your model, but I know that if LES is used, the velocity behind the cylinder will not be steady for ever so in that case you have to use time-averaged data to compare with the experiment data. By the way, where is the section?(what's the location of the velocity profile in x-direction?)
Last but not least, why can't I find any vortex in my model?

 pyxbird March 28, 2013 22:43

1 Attachment(s)
Why? Still no vortex street, I am going to give up.........

 mahdi1 March 29, 2013 07:17

Hi

velocity values are in 16D downstream of cylinder.

I think that LES doesn't use in 2 dimensional problems.

 pyxbird March 29, 2013 23:36

Quote:
 Originally Posted by mahdi1 (Post 417162) Hi velocity values are in 16D downstream of cylinder. I think that LES doesn't use in 2 dimensional problems. about your model: tell me your configuration of your model.
Cylinder D=10cm, water depth=50cm, cylinder height=25cm, inlet velocity=44.1cm/s,(developed, grid overlay is used) mesh: 2,200,000 grids ,y=80cm,x=200cm, RNG model

 mahdi1 March 30, 2013 06:23

Did you try to run your problem without Grid Overly?

 mahdi1 April 1, 2013 06:55

Hello Jay

Could you please tell me how I can plot fd (Drag Force) and fl(Lift)?
I want to calculate drag coefficient.
How can I calculate it by Flow-3D?

Thanks

 pyxbird April 1, 2013 20:36

Quote:
 Originally Posted by mahdi1 (Post 417299) Did you try to run your problem without Grid Overly?
Yes, I do. But it makes no difference.
In 2D I can simulate vortex street now. However in 3D, it is not seen yet.

 jayrup April 3, 2013 02:20

Quote:
 Originally Posted by mahdi1 (Post 417537) Hello Jay Could you please tell me how I can plot fd (Drag Force) and fl(Lift)? I want to calculate drag coefficient. How can I calculate it by Flow-3D? Thanks

To calculate the Forces on component you need to enable the output for Component in Component properties we have an output option with Pressure & Shear force output.
In your case the Lift force will be component 1 y fluid force : Pressure + shear & Drag force will be component 1 x fluid force : Pressure + shear in Analyze tab > Probe Tab > General History in Flow-3d v10.1.Don't forget to define the gravity.

Regards
Jay

 jayrup April 3, 2013 02:22

Quote:
 Originally Posted by pyxbird (Post 417110) Why? Still no vortex street, I am going to give up.........

As per the image, it seems that your geometry is not properly resolved.
Can you send me your prepin file?

Regards
Jay

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