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Old   June 20, 2012, 03:11
Question Vortex shedding in sphere
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I'm trying to simulate vortex shedding in a sphere at Re 600, but without success. I am using PISO pressure velocity coupling.

Also, i tried providing an initial perturbation to the flow by putting in a patch with a certain velocity in some part of the flow domain, but it seems to work only in the case of a 2D cylinder and not for a sphere.

I've been stuck with this problem for 2 weeks. Please help!
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Old   June 20, 2012, 03:34
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Hi,

What error do you have? How is your domain?? Do you have a fine mesh? What are your boundary conditions?
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Old   June 21, 2012, 11:04
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hey, thanks for replying.

It just converges after a few iterations and the value of Cd and Cl becomes constant. I have a cuboidal flow domain with a very fine mesh near the surface of the sphere and a coarse mesh farther away.

The boundary conditions i'm using are velocity_inlet, pressure_outlet and symmetry.

Hope that answers your questions.
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Old   June 22, 2012, 06:42
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but if your problem converges, what is the problem you have? Otherwise, it is a good idea to make a fine mesh back to the sphere too, since fluent will solve better the sphere trail.
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Old   June 22, 2012, 08:37
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Ok havent got info on this till I look at one of my books but is vortex shedding not first observed at a higher Re number of 800. This is just off hand my thinking but havent got any literature on me to confirm.
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Old   June 22, 2012, 14:51
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For a case where vortex shedding occurs, the solution should not converge and the value of Cd and Cl should oscillate. I tried this for a cylinder and that is what happened.

Also, most sources report that vortex shedding in a sphere begins at < Re 250.
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Old   June 23, 2012, 17:52
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hi
i worked in this problem before. you can use this article:
" Simulation of Flow Past a Sphere using the Fluent Code" copy this in google and download. hehe
i couldn't reach vortex shedding at that time and i leaved that. so if you can reach the good result please inform me.
best regards
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Old   June 24, 2012, 01:21
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I have already gone through it and got no vortex shedding using the exact methodology that they have suggested.

Even the vortex shedding i obtained for a cylinder was accomplished using PISO instead of the SIMPLE that they've recommended.
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Old   June 25, 2012, 12:30
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yes PISO is more efficient for transient simulations. i will work on sphere next week again and if i reach good result then i will tell u
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Old   July 3, 2016, 04:51
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was anyone able to solve this problem?
i am facing a similar situation myself
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Old   July 3, 2016, 11:26
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this should not be problem. If you setting up problem correctly.

1. Mesh

2. Time step

3. Boundary conditions and material properties.
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Old   July 3, 2016, 14:40
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I believe I have set it up all correct.
I'm trying to simulate vortex shedding at Re 300 for a 3D sphere, enclosed within a cylindrical domain.
Been stuck for over 2 weeks.

I am not getting the oscillations for the drag and lift coefficients that should be there. It all settles to a straight line as the solution proceeds.

Any suggestions?
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Old   July 3, 2016, 14:51
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Added info:

Dia for sphere: 0.1cm
Domain: D=15cm, Lu=10cm, Ld=20cm.
Mesh is as fine as it can be.
Making use of Pressure based, transient, viscous laminar analyisis.
Density=1000, Viscousity=0.001.
Boundary Conditions: Velocity inlet=300e-3; Pressure outlet=0 gauge, No-slip at sphere wall, and zero shear at domain wall.
PISO solver: Skewness Correction =1; Neighbour Correction = 1; No Skewness-Neighnour Coupling; Least Square Cel Based gradient, Second Order pressure, QUICK momentum.
(To add: I've pretty much tried all the variations in the solver settings here.)
Default solution controls.
Time step size = 0.02
Max Itr per Time step = 30

Any advice?
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Old   July 3, 2016, 14:56
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how did you calculate that time step?

run simulation for long time.
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Old   July 3, 2016, 15:54
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I got my Time Step be making use of definition of Strouhal Number.
The Strouhal number for flow past Sphere would be between 0.16 to 0.18. (Found this in a research paper; please correct me if this is wrong.)

If my oscillations are dying as the solution proceeds, and I see no sawtooth curve, I don't think running the simulation for longer time would help.
Would still give it a try.
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Old   July 4, 2016, 06:01
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Actually in transient CFD you have three regions:

1. Initial transients due to numerical model kick starting

2. Solution settling time aka steady state region

3. True transient region

Last region comes after a very long time and which needs patience.

Here I have included an image to explain these three regions. Not made by me, taken from internet.

Attached Images
File Type: png cylinder-steady-forces.png (12.8 KB, 17 views)
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Old   July 4, 2016, 23:21
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Alright. Will give it a shot.
Thanks! Much appreciated.
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Old   July 5, 2016, 01:23
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how many time steps did you use to divide one cycle time period?
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Old   July 5, 2016, 02:23
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Time step size = 0.02s
Max Itr per Time step = 30

Is this what you're asking?

Last edited by scyllakeeper; July 5, 2016 at 02:24. Reason: forgot to mention units
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Old   July 5, 2016, 02:30
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My simulation has been running for the last 20 hours.
For now, the value for Cd is not oscillating.
Keeping the simulation running to see if any changes occur.
Attached Files
File Type: txt DragCoeffRe300Sph.txt (16.2 KB, 3 views)
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