# calculating Cl and Cd

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 September 27, 2009, 10:12 calculating Cl and Cd #1 New Member   s.k Join Date: Sep 2009 Posts: 7 Rep Power: 9 Hi, i am a university student and am completing a thesis using the CFD Fluent software program. Im relatively new to the program and i need a little help with a few problems i have encountered. Im trying to calculate the Coefficient of lift and Coefficient of drag values for varying angles of attack on an aerofoil. How do i get fluent to display these varying values on screen? (not just convergence graph) also, i have been told that the values of Cl and Cd which fluent gives out are not the absolute correct values. Do these values have to be altered in some way, e.g. imposed onto a vector? i would really appreciate any advice you could give me, thanks!

 October 21, 2009, 13:35 #2 New Member   Karatix Join Date: Sep 2009 Posts: 22 Rep Power: 9 Try this tutorial. It explains the meshing in Gambit and also the set up it Fluent. http://courses.cit.cornell.edu/fluent/airfoil/step1.htm For chaning the angles you have to change the velocity in the x and y direction. Enjoy

 November 24, 2009, 14:59 #3 New Member   Rich Join Date: Nov 2009 Posts: 10 Rep Power: 9 hi, i have two questions concerning the cl and cd monitor. 1) just to be sure, the last value you geton your cl/cd convergence graph is the cl and cl value for the surface you chose before you started the calculation? 2) now in case we have a 3d car (e.g. Porsche)and we want to caluclate cl over the cars spoiler if one picks x=0 y=0 and z=1, that means that the spoiler has zero attack coming from the airflow at the inlet. but in reality i think the airflow over the car isn't flowing parallel to the x axis, but its flowing angled over the cars back/rear window where it reaches the spoiler. does fluent that take into account or does one need to find out the angle of the cars back window? thanks

November 24, 2009, 18:10
#4
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Chris
Join Date: Jul 2009
Location: Ohio, USA
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Quote:
 Originally Posted by dralosdir hi, i have two questions concerning the cl and cd monitor. 1) just to be sure, the last value you geton your cl/cd convergence graph is the cl and cl value for the surface you chose before you started the calculation? 2) now in case we have a 3d car (e.g. Porsche)and we want to caluclate cl over the cars spoiler if one picks x=0 y=0 and z=1, that means that the spoiler has zero attack coming from the airflow at the inlet. but in reality i think the airflow over the car isn't flowing parallel to the x axis, but its flowing angled over the cars back/rear window where it reaches the spoiler. does fluent that take into account or does one need to find out the angle of the cars back window? thanks
1) yes

2) You set a reference force vector, not the flow direction. So if the flow was in the x direction, for example, then the lift force is in the y direction, and the reference vector for lift would be {0, 1, 0}.

 November 25, 2009, 06:48 energy eqation in flow past a sphere #5 Member   Muhammed Asmail Join Date: Nov 2009 Location: Iraq Posts: 43 Rep Power: 9 Dear Sir how are you? i have problem is flow past a sphere,3-dimension,unsteady state if the energy equation is applied in flow past a sphere,how the mean value of heat tranfer calculated on surface of the sphere at each time by using FLUENT?,please can you give me the options of FLUENT in order to calculate mean value of heat tranfer calculated on the surface of sphere at each time . i am waiting your reply quickly please,please help me many thanks to you

December 19, 2009, 07:36
#6
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Muhammed Asmail
Join Date: Nov 2009
Location: Iraq
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Quote:
 Originally Posted by Chris D 1) yes 2) You set a reference force vector, not the flow direction. So if the flow was in the x direction, for example, then the lift force is in the y direction, and the reference vector for lift would be {0, 1, 0}.
Dear Chris
the cd on cylinder appeared sin wave but on the sphere not appeared sin wave why?
many thanks