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November 15, 2012, 13:02 
Cylinder, mean value of Cd

#1 
New Member
Guido
Join Date: Dec 2009
Posts: 17
Rep Power: 7 
Hi, i'm simulating a flow past a cylinder but I've some problems.
I've used all turbulence models that Fluent offers but I'm not able to obtain a reverse flow over the cylinder before 90°. Moreover if i perform an unsteady simulation, what's the easiest way to obtain the mean value of Cd? Thanks for your answers 

November 16, 2012, 20:38 

#2 
Senior Member
Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 1,306
Rep Power: 20 
To get mean values, just activate the option "Data sampling for time statistics". Fluent will keep a running average of all variables from that timestep onwards (average x,y,z velocity, pressure, temperature, etc.). Once you've run enough timesteps to get your average you can calculate the average Cd.
Just remember that it is a running average. If you mess up or change any settings you will need to clear the stored statistics and then continue the transient simulation. 

November 17, 2012, 04:32 

#3 
New Member
Guido
Join Date: Dec 2009
Posts: 17
Rep Power: 7 
Hi, i've activated the data sampling for time statistics but...what I've to do to obtain the mean values? If i go on Report Forces, Fluent gives me the Cd value of the last time step not the mean value. So what I've to do to obtain that? Thanks for the answer


November 19, 2012, 05:05 

#4 
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Guido
Join Date: Dec 2009
Posts: 17
Rep Power: 7 
Another problem is that the pressure coefficient I have is over than 1. How is this possible?


November 19, 2012, 11:54 
Hi

#5 
New Member
Jorge Tamayo
Join Date: Jul 2012
Location: Delft, Netherlands
Posts: 16
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Another way to obtain the mean Cd is to create a drag monitor, in the same place where you create and edit your residuals. When creating the drag monitor you must enable fluent to write a file with the calculated Cd for each iteration; then you extract the .txt file with the values of Cd versus flow time and plot them using excel, you can identify where your values reach the desired behavior and simply use the AVERAGE function of excel.
There might be easier ways to do this inside fluent, but this works fine for me; so i hope this info is useful to you. 

November 21, 2012, 09:54 

#6 
New Member
Guido
Join Date: Dec 2009
Posts: 17
Rep Power: 7 
Thanks for the answer.
I have a doubt. I found this: http://www.wiley.com/college/munson/...s/cylinder.pdf In the paper a flow with Reynolds over 1000 is considered turbulent (?). Moreover they obtain a pressure coefficient over than 1 (about 1.2). Someone can explain me why? 

November 22, 2012, 09:13 

#7 
New Member

To obtain the mean value of the drag coefficient (CD), you have to write the timehistory of drag in file by choosing the option in monitor panel. This data can be used to obtain the mean values as well as to show the variation of drag with time...
The value of pressure coefficient may be greater than 1 because it is nothing but the ratio of static to dynamic pressure. Its basically the dimensionless static pressure (scaled with dynamic pressure) rather than the normalized pressure. The normalized pressure should not go above 1 but dimensionless pressure may be. I think the critical ranges of the Reynolds number (given in the paper linked above) are not correct as per standard definition of Re. You may look out the detailed categorization in the Book "Flow Around Circular Cylinders" by M.M. Zdrakovich or in the articles by C.H.K. Williamson, etc.... I hope it clears some of your confusions.... 

November 26, 2012, 05:17 

#8 
New Member
Guido
Join Date: Dec 2009
Posts: 17
Rep Power: 7 
Perfect, I've found the files of the cd history, thanks!
About Cp...what I've to do to obtain a right maximum Cp? If i reduce the operating pressure I obtain a lower Cp? I don't know if it's physically correct, I don't think so. So what to do? Really thanks for your help!!! 

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