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August 5, 2007, 01:26 
How to model a turbulent flow over a cylinder?

#1 
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I want to investigate a fluid flow over a circular cylinder for Re=3900. I use kebsilon turbulent model and unsteady solver. But I could not find the approprite stream function (I found symmetry form). It doesn't show any vorticity in the wake.
Could you please help me to model any turbulent flow over a circular cylinder? 

August 5, 2007, 03:05 
Re: How to model a turbulent flow over a cylinder?

#2 
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See this exemple: http://www.fluent.com/software/sf_me...l_cylinder.htm


August 5, 2007, 05:22 
Re: How to model a turbulent flow over a cylinder?

#3 
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Dear Thanks for your reply. Yes, I tried according to that tutorial but I am confused In this tutorial 'Laminar' viscous model is considered for Re=150. Is it applicable for Re=3900? I know, at Re=3900 the flow becomes turbulent, so how can it capture a turbulent flow with that laminar model? I selected the model 'define_model_viscous_kebsilon', but it didn't demonestrate any vortex shedding. I could not understand what is my fault. Please help me if you know.
Thanking you mahbub 

August 6, 2007, 00:20 
Re: How to model a turbulent flow over a cylinder?

#4 
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Hi,
I notice in the tutorail they use periodic BC. Why is that? What if we use axis BC, will we still get the fluctutaing lift in the monitor panel? I have the problem that my lift and drag is not simulating unsteady behaviour i.e. fluctuation but only converge to a certain value. Could you pls clarify. Sham. 

August 6, 2007, 02:52 
Re: How to model a turbulent flow over a cylinder?

#5 
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you dont need to use periodic or axis. just use velocity inlet and outflow. regarding the kepsilon, it should worked well. i used it before and the results were fine. you better make the grid really fine near the wall. and make sure you use the right time step size (you can read it in the fluent tutorial).
or, you can change the rho to V (velocity ratio). For example: you Re=4000 you have a fix d (cylinder diameter) and miu (viscosity), then you can change the rho and V as long as the Re remains the same. Lets say the d=1 and miu is 1.105. your true value of rho is 1 and V is 0.04 m/s. change the ratio between rho and V. lets say, rho to V is 50:1 then rho is 1.4142 and V is 0.02828. usually, higher rho makes the shedding easily visible 

August 6, 2007, 13:43 
Re: How to model a turbulent flow over a cylinder?

#6 
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Another way you can use depends on what postprocessor you are using. If using Fluent's display, you can not change the max value of the vorticity, that is why you can not see anything. If you use Tecplot, you can export the Fluent data file into tecplot data file including the vorticity magnitude. At first, you might not see any vorticity in Tecplot. After you change the display level of the vorticity from very large to a little smaller, you will see the vorticity much clearer.
I am not very sure about the relationship between vorticity magnitude and Re. Maybe when Re increses, the vorticity magnitude should increase too. Hope this will help you! 

August 7, 2007, 08:34 
Re: How to model a turbulent flow over a cylinder?

#7 
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Dear Naomi, Haoyin and others
Thanks for your cooperations but I still couldn't display the vortex shedding in the contour of stream function or velocity magnitude. I used the mesh and followed the instruction from (Just select kebsilon turb. instead of laminar and change density to 3900 to change the Re number others are as usual) http://www.fluent.com/software/sf_me...l_cylinder.htm what's the wrong with me. Please help me. Mahbub 

August 7, 2007, 12:15 
Re: How to model a turbulent flow over a cylinder?

#8 
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Mahbub,
Actually, I simulated the flow over a cylinder couple months ago. In my understanding and my experience, you can not simulate the vortex shedding at a large Re with Kepsilon model. It is straightforward that the vortex shedding should be more significant with the increase of Re, however, it would not be the truth. I am not very sure about this. I did simulate the vortex shedding and Von Karman vortex street at Re = 140. I think that is why the tutorial you mentioned use that specific Re. You can do further research about the relationship between high Re and vortex shedding degree. 

August 21, 2007, 00:38 
Re: How to model a turbulent flow over a cylinder?

#9 
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I think you should read the following post carefully:
http://university.fluent.com/forum/v...6ef5bb5a23e1e8 

February 3, 2012, 21:29 

#10 
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cfd
Join Date: Oct 2011
Posts: 17
Rep Power: 6 
Hi,
I am modelling the turbulent flow over the cylinder by fluent(kepsilon 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 

December 24, 2012, 14:54 
tutorial

#13 
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Sasan Ghomi
Join Date: Sep 2012
Location: Iran  Tehran
Posts: 185
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Hi every body
I need a tutorial for turbulent flow over a cylinder...Can you help me? please help me..(sasanghomi@yahoo.com) thanks. 

October 29, 2014, 16:56 

#14  
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hamed mohamadian
Join Date: Oct 2014
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Quote:
In this raynold number we don't have vortex shading. Note that vortex shading hapend jaust in laminar flow,Re>40 and <300 Good luck 

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