Vortex shedding in sphere
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! |
Hi,
What error do you have? How is your domain?? Do you have a fine mesh? What are your boundary conditions? |
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. :) |
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.
|
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.
|
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. |
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 :D 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 |
I have already gone through it and got no vortex shedding using the exact methodology that they have suggested. :mad:
Even the vortex shedding i obtained for a cylinder was accomplished using PISO instead of the SIMPLE that they've recommended. |
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
|
was anyone able to solve this problem?
i am facing a similar situation myself |
this should not be problem. If you setting up problem correctly.
1. Mesh 2. Time step 3. Boundary conditions and material properties. |
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? |
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? |
how did you calculate that time step?
run simulation for long time. |
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. |
1 Attachment(s)
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. http://www.cfd-online.com/Forums/att...9&d=1467626388 |
Alright. Will give it a shot.
Thanks! Much appreciated. :) |
how many time steps did you use to divide one cycle time period?
|
Time step size = 0.02s
Max Itr per Time step = 30 Is this what you're asking? |
1 Attachment(s)
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. |
All times are GMT -4. The time now is 08:01. |