Vortex shedding in sphere

fluidman · June 20, 2012, 04:11

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!

DonQuijote · June 20, 2012, 04:34

Hi,

What error do you have? How is your domain?? Do you have a fine mesh? What are your boundary conditions?

fluidman · June 21, 2012, 12:04

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.

DonQuijote · June 22, 2012, 07:42

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.

chris85 · June 22, 2012, 09:37

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.

fluidman · June 22, 2012, 15:51

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.

amin66 · June 23, 2012, 18:52

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

fluidman · June 24, 2012, 02:21

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.

amin66 · June 25, 2012, 13:30

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

scyllakeeper · July 3, 2016, 05:51

was anyone able to solve this problem?
i am facing a similar situation myself

Far · July 3, 2016, 12:26

this should not be problem. If you setting up problem correctly.

1. Mesh

2. Time step

3. Boundary conditions and material properties.

scyllakeeper · July 3, 2016, 15:40

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?

scyllakeeper · July 3, 2016, 15:51

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?

Far · July 3, 2016, 15:56

how did you calculate that time step?

run simulation for long time.

scyllakeeper · July 3, 2016, 16:54

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.

Far · July 4, 2016, 07:01

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.

scyllakeeper · July 5, 2016, 00:21

Alright. Will give it a shot.
Thanks! Much appreciated.

Far · July 5, 2016, 02:23

how many time steps did you use to divide one cycle time period?

scyllakeeper · July 5, 2016, 03:23

Time step size = 0.02s
Max Itr per Time step = 30

Is this what you're asking?

scyllakeeper · July 5, 2016, 03:30

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.

June 20, 2012, 04:11	Vortex shedding in sphere	#1
fluidman New Member Join Date: Jun 2012 Posts: 4 Rep Power: 14	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!

July 5, 2016, 03:23		#19
scyllakeeper New Member Pranjal Seth Join Date: Jun 2016 Posts: 17 Rep Power: 10	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 03:24. Reason: forgot to mention units

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June 20, 2012, 04:34		#2
DonQuijote Member Join Date: Jun 2012 Posts: 32 Rep Power: 14	Hi, What error do you have? How is your domain?? Do you have a fine mesh? What are your boundary conditions?

June 21, 2012, 12:04		#3
fluidman New Member Join Date: Jun 2012 Posts: 4 Rep Power: 14	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, 07:42		#4
DonQuijote Member Join Date: Jun 2012 Posts: 32 Rep Power: 14	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, 09:37		#5
chris85 New Member Chris Join Date: Nov 2011 Posts: 16 Rep Power: 15	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, 15:51		#6
fluidman New Member Join Date: Jun 2012 Posts: 4 Rep Power: 14	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, 18:52		#7
amin66 Member Amin Join Date: Mar 2012 Posts: 60 Rep Power: 14	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, 02:21		#8
fluidman New Member Join Date: Jun 2012 Posts: 4 Rep Power: 14	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, 13:30		#9
amin66 Member Amin Join Date: Mar 2012 Posts: 60 Rep Power: 14	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, 05:51		#10
scyllakeeper New Member Pranjal Seth Join Date: Jun 2016 Posts: 17 Rep Power: 10	was anyone able to solve this problem? i am facing a similar situation myself

July 3, 2016, 12:26		#11
Far Senior Member Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,558 Blog Entries: 6 Rep Power: 54	this should not be problem. If you setting up problem correctly. 1. Mesh 2. Time step 3. Boundary conditions and material properties.

July 3, 2016, 15:40		#12
scyllakeeper New Member Pranjal Seth Join Date: Jun 2016 Posts: 17 Rep Power: 10	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, 15:51		#13
scyllakeeper New Member Pranjal Seth Join Date: Jun 2016 Posts: 17 Rep Power: 10	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?

July 3, 2016, 15:56		#14
Far Senior Member Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,558 Blog Entries: 6 Rep Power: 54	how did you calculate that time step? run simulation for long time.

July 3, 2016, 16:54		#15
scyllakeeper New Member Pranjal Seth Join Date: Jun 2016 Posts: 17 Rep Power: 10	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 5, 2016, 00:21		#17
scyllakeeper New Member Pranjal Seth Join Date: Jun 2016 Posts: 17 Rep Power: 10	Alright. Will give it a shot. Thanks! Much appreciated.

July 5, 2016, 02:23		#18
Far Senior Member Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,558 Blog Entries: 6 Rep Power: 54	how many time steps did you use to divide one cycle time period?