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Old   January 16, 2003, 13:03
Default Lift and Dreag
  #1
stex
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Anybody knows how I can calculate the lift and drag cefficients of a wing profile?.
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Old   January 17, 2003, 02:09
Default Re: Lift and Dreag
  #2
Holidays
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Carry out the calculation of the forces along the two directions for your wing and calculate the following Fi = 1/2 Ci UČ with i = D and L, U = Uinfinite. To achieve a good result you will however need a fairly fine grid along the wing and its walls as well as the use of a turbulence model integrating to the wall such as SST for example I believe. Do not hesitate to carry mesh independence tests.
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Old   January 17, 2003, 07:39
Default Re: Lift and Drag
  #3
Carlo
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How can I "Carry out the calculation of the forces along the two directions for your wing " ? I have only the possibility to export pressure, temperature, and many other things but not forces!!!!

Thanks
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Old   January 17, 2003, 08:26
Default Re: Lift and Drag
  #4
Holidays
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Read the end of the out file and you wil find these forces calculated on the named patches.
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Old   January 17, 2003, 10:32
Default Re: Lift and Drag
  #5
Carlo
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It doesn't calculate the Forces......

these is the out file:

================================================== =====

Wall Force and Moment Summary ================================================== =====

Note: Pressure integrals exclude the reference pressure. To include it, set the expert parameter 'include pref in forces = t'. +-----------------------------------------------------

How can I go on?????
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Old   January 17, 2003, 16:54
Default Re: Lift and Drag
  #6
Neale
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The numbers that the solver reports just below the lines you post are the forces in the x, y and z directions. You should be able to compute lift and drag coefficents from these.

Alternatively you can set the expert parameter "output eq flows = t" (or set this in CFX-Build) and compute the forces in CFX-Post.

Neale
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Old   January 18, 2003, 05:02
Default Re: Lift and Drag
  #7
Carlo
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Hi Neale

After the lines i posted there are the components of - Pressure forces - Viscous forces I have to make the sum of these two forces?

In other hand where or how i can set the expert parameter from Build?

Thanks
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Old   January 18, 2003, 06:21
Default Re: Lift and Drag
  #8
James Date
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You might need to set >>WALL PRINTING in the command file to get the forces output. Both viscous and pressure forces are output, together with the their sum.

Regards James

P.S. I little while ago i wrote a piece of FORTRAN code to strip out these forces and calculate the lift and drag forces with respect to the inflow velocity direction, if you want a copy i can email it to you.
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Old   January 18, 2003, 13:39
Default Re: Lift and Drag
  #9
Neale
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Yes, you will have to add the two forces together. Viscous + pressure, then resolve this force vector along the two lines which define the directions for your lift and drag vectors. If your lucky your x,y,z global coordinates match these and you don't have to do a transformation.

The "expert" parameter is not for outputting equation flows is a toggle button in the solver control panel in CFX-Build. You can also explicitly add the expert parameter using the defintion file editor.

Neale.

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Old   January 18, 2003, 13:39
Default Re: Lift and Drag
  #10
Neale
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I think Carlo is talking cfx-5 rather than cfx-4.

Neale
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