# Liftdrag calculation

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 July 17, 2005, 17:36 I am a new user and I am doing #1 New Member   marco Join Date: Mar 2009 Location: Italy Posts: 2 Rep Power: 0 I am a new user and I am doing some bench tests on a 0012 naca airfoil 2D simulation. Could you better explain me how LiftDrag postprocessing utility calculate drag and lift coefficient ? More in detail: the coefficient are referred to "body axis" or "wind axis" (like is usual in aeronautic field)? The program has an output of 1 value for drag and 3 values for lift: I expected only 2 values for lift (Cl_y and Cl_z) where Cl_z is =0 for 2D case. How openFoam calculate the reference area ? I' ve the airfoil aligned with xyz geometric axis and I change the angle of attack changing the U-components values. Thank you

 July 18, 2005, 05:40 There is no concept of body ax #2 Super Moderator   Mattijs Janssens Join Date: Mar 2009 Posts: 1,416 Rep Power: 16 There is no concept of body axis or wind axis in liftDrag. reference inlet velocity: average of all fixedValue boundaries. Per wall patch -------------- - reference area: area of patch in direction of inlet velocity. Hopefully this is the frontal area. - reference length: length/width of patch in direction of inlet velocity.

 July 18, 2005, 17:54 OK, so if I well understand dr #3 New Member   marco Join Date: Mar 2009 Location: Italy Posts: 2 Rep Power: 0 OK, so if I well understand drag coefficient is on X direction. But which is the meanings of the 3 lift coefficient value ?

 September 23, 2005, 09:54 CIAO MARCO TI HO MANDATO UN em #4 Member   Muzio Grilli Join Date: Mar 2009 Posts: 36 Rep Power: 8 CIAO MARCO TI HO MANDATO UN email anch'io sto facendo la stessa cosa peṛho dei problemi con le condizioni al contorn e quelle iniziali mi puoi dare una mano?

 December 12, 2005, 07:22 Hi everybody. I've just beg #5 Member   Rosario Russo Join Date: Mar 2009 Location: Trieste, Italy Posts: 56 Rep Power: 8 Hi everybody. I've just begun to use OpenFoam and I'm doing bench tests similar to Marco's ones. I have the same doubts about the meaning of the three components of lift... Moreover in my cases (even in simpler laminar cases without any turbulence model) I always get 0 for the drag coefficient and also the stresses at the walls are 0, even if velocity gradients at the walls are not 0. I tried to use both simpleFoam and icoFoam solvers. Thank you for any suggestion. Ros.

 June 19, 2006, 01:20 Hello I also have the same q #6 Member   Marcus Hammar Join Date: Mar 2009 Posts: 33 Rep Power: 8 Hello I also have the same question about three components of lift? I have heard somewhere that the "liftDrag" utility is not to be trusted. He told me to write my own code instead. Anyone know anything about the reliability in liftDrag? To write my own code I need to know the syntax for a few things I guess(I know programming but not the C++ syntax): 1. I need to find somekind of index for the face I want to calculate for? 2. Then I need the pressure and skin friction for this face? 3. I maybe need the length/area of each cell so I can integrate? Thank you

 June 19, 2006, 05:21 Hello, I suggest you to try #7 Member   Rosario Russo Join Date: Mar 2009 Location: Trieste, Italy Posts: 56 Rep Power: 8 Hello, I suggest you to try writing your own code, so that you have full control on what you're actually computing. What I did is to start from the liftdrag utility modifying it according to my needs. All the basic steps are in there and the code is pretty clear. (Look also at the library OpenFOAM-1.2/src/postProcessing/incompressible/liftDrag). In my opinion if you really want to exploit OF you have to learn a little of c++ and to write sometime few lines of code. Otherwise it is better to use other tools. Ros.

 June 19, 2006, 05:26 Hi Ok, thats sounds like o #8 Member   Marcus Hammar Join Date: Mar 2009 Posts: 33 Rep Power: 8 Hi Ok, thats sounds like o good way to start! I agree with your opinion. Lets begin some writing codes... Thanx

 June 19, 2006, 05:29 Wrong order of words in the la #9 Member   Marcus Hammar Join Date: Mar 2009 Posts: 33 Rep Power: 8 Wrong order of words in the last sentence... I am too tired, I blame it on the WC in football. /M

 March 21, 2008, 05:32 Hi to everybody, i am calc #10 New Member   Fabian Korn Join Date: Mar 2009 Location: Heilbronn, Germany Posts: 21 Rep Power: 8 Hi to everybody, i am calculatian a flow around a cylinder with Re is between 100 and 500. When i run the liftDrag tool the results are completly wrong, i get a drag coeff. of around -22 and also my reference length and Area are a little bit wrong. Reference length is 4,26 instead of 5. The liftDrag tool runs at 2D cases with a wide mesh correctly. Thanks for helping me

 March 6, 2009, 10:51 Hi friends I am new user to r #11 Member   Join Date: Mar 2009 Posts: 39 Rep Power: 8 Hi friends I am new user to rhoSonicFoam. I am solving inviscid Compressible flow past an airfoil. I want to calculate Cd-Cl value but it shows err that "No valid model for viscous stress calculation." So, can you suggest me what step should I follow? Thanks...

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