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Old   March 23, 2005, 22:35
Default Negative lift coefficient.
  #1
Sham
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Hello all,

I am modelling a cylinder resting on seabed(wall) now. I got Cl value of around -5 whereas it shoulod be positive(moving away from wall). Can someone tell me where have I done wrong?

Sham.
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Old   March 24, 2005, 02:28
Default Re: Negative lift coefficient.
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Fluent user
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Did you try to have a look at the pressure variations and velocity variations. Try to run the problem for standard cases of flow past a cylinder . You can find standard values online for the results obtained. You can even try like an approximate analytical solution and verify the results with the obtained answer. First make sure you have simulated the geometry correctly.

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Old   March 24, 2005, 08:15
Default Re: Negative lift coefficient.
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Jason
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If you have gravity on, then you have a hydrostatic pressure. Check that your reference pressure is the same as the hydrostatic pressure that your cylinder will see. Fluent uses this reference pressure as the pressure on the opposite side of your cylinder (since you don't have any flow field there, and a force is a deltaP). If you have your reference pressure set at 1atm, but your hydrostatic pressure is 2atm, that could over power your lift force caused by the local flow field.

Hope this helps, and goodluck, Jason
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Old   March 24, 2005, 21:30
Default Re: Negative lift coefficient.
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Sham
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Hi,

I did not activate the gravity and I set the reference pressure to zero. This is how I get the negative Cl. When I look at the pressure contour, it shows that the pressure on the top part of the cylinder is less than the bottom part. This should give a positive Cl.

Should I activate the gravity. If so, what should I put the reference P as and I am modelling cylinder on seabed and take into account only 5m height from sea bottom instead of the whole depth. Hope you can help me. Its urgent.

Sham.
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Old   March 25, 2005, 00:08
Default Re: Negative lift coefficient.
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Saad
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Is your y-axis defined positive upwards or positive downwards. This may affect the sign of C_l.

Saad.
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Old   March 25, 2005, 00:18
Default Re: Negative lift coefficient.
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Sham
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my y axis is positive upward. I put the origin in the middle of my model(it is a 2d rectangular model) and the cylinder is at the bottom of the 'box'. Will this affect the results??

Sham.
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