# Turbulence model for flow over a cylinder at Re=10000

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 June 19, 2011, 00:10 Turbulence model for flow over a cylinder at Re=10000 #1 New Member     Mayank Ojha Join Date: May 2011 Posts: 22 Rep Power: 8 Hello, Am trying to find the Lift coeff, Drag coeff and the Strouhal number for a cylinder in a cross flow at Re=10,000 Things I tried: 1. Created a mesh and ran cases for tstep=0.001, 0.0001 & 0.00001 using KW and SST turbulence model. The y+ for the mesh was around 3.5-4. So now I created another mesh. 2. For the new mesh, the y+ value over the cylinder surface was found to be 0.62. I ran with SST and KW at tstep=0.001 which is 2% of vortex shedding time. The time step is low enough to capture the vortex shedding. Results: 1. My St number is 0.22 while the expected value from DNS and experimentsis 0.2~0.21 2. My Cd (average) was found to be 1.3 and expected value is 1.1 3. Lift coeff was found to be ~1 while expected value is 0.5 (which is where the real problem is). The value is almost double. I would like to know if there is anything else I should be doing and where am I actually missing. Thank you very much. Ojha

 June 19, 2011, 06:51 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,449 Rep Power: 104 Are you sure the lift coefficient is based on the correct reference area? Mixing up radius and diameter explains a factor of two. Now that you have got a model which is pretty close (in everything except lift), if you wish to get more accurate you should do a proper convergence study to guide you into what options you have left. Consider Richardson extrapolation, grid convergence indexes and similar techniques to really squeeze the last drip of accuracy out of it. "Computational Fluid Dynamics" by Roache is the seminal textbook in this area, but a summary of some key concepts is here: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F - and the reference to JFE is highly recommended reading.

 June 19, 2011, 12:01 #3 New Member     Mayank Ojha Join Date: May 2011 Posts: 22 Rep Power: 8 Thanks Glen. Appreciate your feedback. I had another question. I am using 5% turbulence ( i.e medium intensity) at the inlet. Should this be causing the problem ??? I realize that this problem is slightly tricky because prior to separation, the flow is laminar while after separation its completely turbulent. How about the BSL model and the non-linear RNG k-e model ? Literature say that they have been proved to be a good estimate for near wall flows and flows with rotation (which is exactly my case). But literature also say that SST should be the best (which it does not, so far). Right now am running an LES for this case. Lets see how it goes. BTW what is Richardson extrapolation and grid convergence indexes ??? Looks like the library is out of Roache's txt book.

June 19, 2011, 18:35
#4
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Glenn Horrocks
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Quote:
 I am using 5% turbulence ( i.e medium intensity) at the inlet. Should this be causing the problem ???
If that is not representative of the turbulence levels of the experiment you are comparing to then definitely, yes.

Quote:
 I realize that this problem is slightly tricky because prior to separation, the flow is laminar while after separation its completely turbulent.
In that case you might want to consider the turbulence transition model. That is the only turbulence model which can account for transition effects.

Quote:
 Right now am running an LES for this case. Lets see how it goes.
Have you checked the dissipation is correct? Made sure you are getting the correct turbulence spectrum? Defined the inlet correctly for LES? Unless you have properly set this model up for LES you are kidding yourself. You cannot just turn on the LES option and rerun it and expect to get a reasonable answer.

Quote:
 BTW what is Richardson extrapolation and grid convergence indexes ???
Looks like you need to find the textbook I referenced then! Also read the FAQ link I quoted and look them up on google. There are other ways besides the library these days.

June 20, 2011, 13:01
#5
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Mayank Ojha
Join Date: May 2011
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Quote:
 Originally Posted by ghorrocks Have you checked the dissipation is correct? Made sure you are getting the correct turbulence spectrum?
I have never run an LES. Can I have some more details/references as to how to check for dissipation and turbulent spectrum ?

Thanks Glen.

 June 20, 2011, 18:47 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,449 Rep Power: 104 LES is a whole field of CFD in itself, your library should have textbooks on the topic. Otherwise start with general CFD books (eg Anderson) as they often introduce the concepts of LES.

 June 21, 2011, 13:17 #7 Member   Paulo Alexandre Costa Rocha Join Date: Mar 2009 Posts: 71 Rep Power: 10 AFAIK a Reynolds number of 10000 is not fully turbulent for external flows. Pure RANS models are not proper. Maybe LES or a transition model can give better results. My two cents. Best regards, Paulo Rocha

 February 3, 2012, 21:49 #8 New Member   cfd Join Date: Oct 2011 Posts: 17 Rep Power: 7 Hi, I am modelling the turbulent flow over the cylinder by fluent(k-epsilon model) I have a basic question about defining the boundary conditions for the domain. The domain is rectangular. for specifying velocity inlet for domain, I have to define the turbulent intensity as well as hydraulic diameter, I want to know I should use the diameter of the cylinder as the Hydraulic diameter or the cross length of the inlet face?! Thank you

 February 4, 2012, 06:33 #9 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,329 Blog Entries: 6 Rep Power: 45 You should use transition model for this Reynolds number.

 February 4, 2013, 04:44 #10 New Member   Join Date: Jan 2013 Posts: 3 Rep Power: 6 Hi everyone, I'm running a 3D cylinder simulation in FLUENT with a Re of 100,000. I've used a Transition SST model with a time-step size of 1e-06 and 10,000 time steps. I'm trying to obtain the drag coefficient from this simulation The results I've obtained shows an oscillatory motion, with the Cd value hitting a peak of 1.21 midway before falling back to 0.726 at the end of the 10,000 time steps. Can anyone tell me why has this occurred? What do I need to do for the Cd values to be stablised? Thanks! Regards, Hee emreg likes this.

 February 4, 2013, 05:10 #11 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,449 Rep Power: 104 Your question is about Fluent, not CFX and this is the CFX forum. But the answer is the same: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

May 18, 2015, 20:11
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Emre G
Join Date: May 2011
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Quote:
 Originally Posted by hee Hi everyone, I'm running a 3D cylinder simulation in FLUENT with a Re of 100,000. I've used a Transition SST model with a time-step size of 1e-06 and 10,000 time steps. I'm trying to obtain the drag coefficient from this simulation The results I've obtained shows an oscillatory motion, with the Cd value hitting a peak of 1.21 midway before falling back to 0.726 at the end of the 10,000 time steps. Can anyone tell me why has this occurred? What do I need to do for the Cd values to be stablised? Thanks! Regards, Hee
is there a solution on this issue please?

 May 18, 2015, 21:03 #13 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,449 Rep Power: 104 My post #11 is a pretty clear response to this.

 May 19, 2015, 02:09 #14 Senior Member     Mr CFD Join Date: Jun 2012 Location: Britain Posts: 314 Rep Power: 7 Is zonal RANS a thing? I.e. define two subdomains? A laminar one and a turbulent RANS one?

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