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June 20, 2012, 03:11 
Vortex shedding in sphere

#1 
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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! 

June 20, 2012, 03:34 

#2 
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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? 

June 21, 2012, 11:04 

#3 
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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. 

June 22, 2012, 06:42 

#4 
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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.


June 22, 2012, 08:37 

#5 
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Chris
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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.


June 22, 2012, 14:51 

#6 
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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. 

June 23, 2012, 17:52 

#7 
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Amin
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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 

June 24, 2012, 01:21 

#8 
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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. 

June 25, 2012, 12:30 

#9 
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Amin
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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


July 3, 2016, 04:51 

#10 
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Pranjal Seth
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was anyone able to solve this problem?
i am facing a similar situation myself 

July 3, 2016, 14:40 

#12 
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Pranjal Seth
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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? 

July 3, 2016, 14:51 

#13 
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Pranjal Seth
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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=300e3; Pressure outlet=0 gauge, Noslip at sphere wall, and zero shear at domain wall. PISO solver: Skewness Correction =1; Neighbour Correction = 1; No SkewnessNeighnour 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? 

July 3, 2016, 15:54 

#15 
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Pranjal Seth
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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. 

July 4, 2016, 06:01 

#16 
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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. 

July 4, 2016, 23:21 

#17 
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Pranjal Seth
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Alright. Will give it a shot.
Thanks! Much appreciated. 

July 5, 2016, 02:23 

#19 
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Pranjal Seth
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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 

July 5, 2016, 02:30 

#20 
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Pranjal Seth
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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. 

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sphere, vortex shedding 
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