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April 22, 2011, 19:07 
Flow over 2D Cylinder, Laminar and Turbulent

#1 
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Jon
Join Date: Apr 2011
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Hi,
I am relatively new to CFD and Fluent and I am trying to determine the Drag Force and Drag Coefficient for flow over a 2D cylinder, but I am having some problems with fluent producing the correct value (based on experimental results). First I ran a case for a Reynolds Number of 1,000 and obtained a drag force of .0008 N which is close the the experimental value of .0011 N, but the drag coefficient which should be around 1 is off by a magnitude of 10. To measure drag force/coefficient I go to Report>Forces>Set X=1,Y=0 and chose the edge which represents the cylinder wall. When I run a case for a Reynolds number of 100,000 I get a drag force of 2 (should be around 11 N) and a drag coefficient of 314 (which should be around 1). Since the flow is laminar until approximately Re=400,000 for flow around a cylinder, I am using the laminar solver model. The problem also continues when I try to run a turbulent case using the kepsilon model. I created my geometry in gambit. I subtracted a .125 m diameter circle from a larger 2 m diameter circle. I created a sizing function from the edge (or cylinder) to the face. Next I meshed the face using a quad map scheme. This all seems correct to do, but I figured I'd just let you all know in case. I've tried almost everything I can think of, and even made sure my reference made sense. Any help would be greatly appreciated.  Jon 

May 29, 2011, 20:08 

#2  
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Mayank Ojha
Join Date: May 2011
Posts: 22
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Quote:
Am very much sure that at such high Re of 400,000, the flow cannot be laminar. For flow which has circulations, Ke shows delayed separation and hence gives incorrect answers. According to the literature, SST should be what you should be using. I am trying to simulate a flow over a cylinder at Re=10,000. Remember: when you are using a turbulent model, make sure your y+ value is acceptable. Regards, Mayank 

June 3, 2011, 13:48 

#3 
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Join Date: Apr 2011
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Did you try a transient simulation? Vortex problems are inherently transient and it helps a lot of times with the solution and convergence issues if you perform a transient simulation. I agree with ojha.mayank485. kw SST might be good for your case.


December 8, 2012, 05:50 
flow over two side by side cylinder... with Re100 & 200

#4 
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Rishitosh Ranjan
Join Date: Dec 2012
Location: Trichirapalli, Tamilnadu, India
Posts: 21
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hiii..
i m also new in dis CFD area... i m working on flow over two side by side cylinders... @ Re= 100 and 200.. i got good results.. but i want to find out Strouhal number for all cases... i came to know by finding shedding frequency with help of FFT, using cofficient of lift data, in fluent we can calculate Strouhal number... but i m getting helpess.. how to use FFT to find shedding frequency....????? Rishi 

December 8, 2012, 06:24 

#5 
Senior Member
Join Date: Mar 2011
Location: Germany
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Yes try with SST kw and if problem still persists think of trying Transition model.
Note: Search on youtube or in the threads on this forum, I guess "far" has uploaded some videos for the flow simulation on cylinder but i guess those simulations are 3D 

November 13, 2014, 13:13 

#6  
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Steve
Join Date: Dec 2012
Location: Minneapolis
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To use an FFT to calculate Shedding Frequency:
Place a numerical probe somewhere downstream of the cylinder where the flow looks stabilized. Measure the x and y velocity at each timestep and calculate the velocity spectra from that. In the spectra you should see a peak and that will be where your shedding frequency is. Quote:


Tags 
cylinder, drag, force, turbulent 
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