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Old   November 20, 2003, 16:59
Default 2d foil pressure forces problem
  #1
mayor
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Hi,

i have a problem with the amount of pressure forces (drag values) for a 2d airfoil (thin) the viscous forces looks correct but the pressure forces are 5 times bigger than the visoucs forces. Any ideas?

THanks
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Old   November 23, 2003, 07:56
Default Re: 2d foil pressure forces problem
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James Date
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Hi

Just a few things. My sure the Y+ value on the foil is in the correct range. Greater than 30 but less than 500. It is usually best to use the lower limit of 30 as this will ensure that the pressure forces are also resolved correctly down on the body. Make sure your boundary conditions are correctly applied and are placed sufficiently far away from you foil.

Also make sure you have enough cells placed chord-wise around the section and down in the wake.

Out of interest what section are you investigating and what type of grid are you using i.e. structured or unstructured?

Regards James
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Old   November 23, 2003, 19:57
Default Re: 2d foil pressure forces problem
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mayor
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Thanks a lot

THe Y+ value are in the correct region, i can get it adapting the mesh. The boundary i used are vel inlet and pressure outlet i havent found many difference between outflow and pressure outlet. They are situated almost 13 chords from the airfoil. I have 80 cells chordwise. I tried both structured and understructed mesh. I find for both quite big value of drag and lift For the understructured it is more important. I found that computation of pressure drag is very influenced by the aspect ratio of the cells near the airfoil. I found that i dont need many cells to get better values. Adaptation (refinement) doesn't always yield better results.

the airfoil is a naca 4314 at Re 3e6
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Old   November 24, 2003, 04:37
Default Re: 2d foil pressure forces problem
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James Date
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Hi there,

Sounds like you're doing all the right things but the solution is still giving incorrect values. The fun of CFD hay, I've had this happen on many occasions. I think for a simple 2D aerofoil a structured mesh is the best option. The aspect ratio of the cells should be kept as low if possible. Send me your mesh and solution file if you like, I'll have a look for you.

I had similar problems a few years back when trying to get accurate solutions around a NACA0012 section. I did manage it in the end!

Regards James
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Old   December 1, 2003, 04:57
Default Re: 2d foil pressure forces problem
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Charles Crosby
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Grid resolution near the leading edge could be an issue. You can get some very large gradients, and I would personally set spacing along the airfoil near the leading edge to about 1/1000 of the chord length to capture these gradients.
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