# Flow over 2D Cylinder, Laminar and Turbulent

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 April 22, 2011, 19:07 Flow over 2D Cylinder, Laminar and Turbulent #1 New Member   Jon Join Date: Apr 2011 Posts: 2 Rep Power: 0 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 k-epsilon 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
New Member

Mayank Ojha
Join Date: May 2011
Posts: 22
Rep Power: 6
Quote:
 Originally Posted by Tsr63 Hi, Since the flow is laminar until approximately Re=400,000 for flow around a cylinder - Jon

Am very much sure that at such high Re of 400,000, the flow cannot be laminar.
For flow which has circulations, K-e 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 Member   Join Date: Apr 2011 Posts: 30 Rep Power: 6 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. k-w SST might be good for your case.

 December 8, 2012, 05:50 flow over two side by side cylinder... with Re100 & 200 #4 New Member     Rishitosh Ranjan Join Date: Dec 2012 Location: Trichirapalli, Tamilnadu, India Posts: 18 Rep Power: 4 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 Posts: 414 Rep Power: 11 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
New Member

Steve
Join Date: Dec 2012
Location: Minneapolis
Posts: 7
Rep Power: 4
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:
 Originally Posted by rishitosh 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

 Tags cylinder, drag, force, turbulent

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