# RE: Reference Pressure dilemma

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 August 27, 2008, 09:02 RE: Reference Pressure dilemma #1 CFD USER Guest   Posts: n/a Hello, I am trying to calculate Lift and drag forces over a body In the first case I left reference pressure as zero and outlet pressure at 1atm relative static my lift value was about 1642N In the second case I changed my reference pressure to 1atm and outlet pressure to 0static pressure averaged over whole outlet. Now I am getting a lift of about 750N. I am really confused about this difference and I will really appreicate your insights. Thanks, CFDuser.

 August 28, 2008, 08:39 Re: RE: Reference Pressure dilemma #2 CFD USER Guest   Posts: n/a Please guys any help. I will appreciate your time. Thanks

 August 28, 2008, 19:31 Re: RE: Reference Pressure dilemma #3 Glenn Horrocks Guest   Posts: n/a Hi, Sounds like you have not got a converged solution, mesh not fine enough, or boundaries too close to the foil. Probably one of those issues is causing the problem. Glenn

 August 29, 2008, 11:19 Re: RE: Reference Pressure dilemma #4 CFD USER Guest   Posts: n/a Hello Glenn, Thanks a lot for your reply. I was getting a convergence level of about 10-5 with a constant lift and drag force values after like 200 iterations. Hence I believed that I have a converged solution. I haven't did any grid dependence check or wall location issues. But do you think those factors will have such a large influence on my force values. Thanks a lot, I will appreciate your time.

 August 31, 2008, 21:56 Re: RE: Reference Pressure dilemma #5 Glenn Horrocks Guest   Posts: n/a I would not mention them unless they can cause problems. Check mesh refinement and proximity of the external boundaries. Glenn Horrocks

 September 2, 2008, 16:04 Re: RE: Reference Pressure dilemma #6 CFD USER Guest   Posts: n/a Hello Glenn, Thanks lot for your response. I doubled my distance to outer walls and still my results are same. I am not sure if it makes any difference but the body that I am simulating is a under-water unit which is placed directly below the boat hull. Hence in my simulation I artificially created a slip wall which is attached to the hydrofoil section for which I am measuring the lift force. Does having this slip wall attached to the hydrofoil section will have any influence on my lift force computations based on my reference pressure. Over all these simulations my drag value remained constant. Thanks a lot, I will appreicate your time,

 September 2, 2008, 16:17 Re: RE: Reference Pressure dilemma #7 CycLone Guest   Posts: n/a Are you integrating the force over the entire surface (top and bottom) or just one side? If you are not including the entire surface, you need to include the effect of the reference pressure on the other side. This would explain the difference in your results. The reference pressure is just an offset to help minimize round-off error. It should have no effect on your solution except for the contribution from round-off. Round off can occur if your reference pressure is zero and the computer is computing small differences in pressure at large pressure levels (ie. the difference between 101,000 [Pa] and 101,000.1 [Pa]). -CycLone

 September 3, 2008, 07:51 Re: RE: Reference Pressure dilemma #8 CFD USER Guest   Posts: n/a I was calcuating the force on one side and I am using CFX function F_x and F_z to calculate lift and drag forces. But now it makes sense why my lift forces are dependent on reference pressure. Thanks a lot Cyclone.