# Forces report

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 August 22, 2006, 12:36 Forces report #1 Ramine Guest   Posts: n/a Hi all, I'm wondering on the values the forces report fluent gives me... I'm modelling a foil and when I ask for the lift and drag, it gives me opposite values for the forces on the top and bottom of the foil. For example, I get 1020 N for the top (this one seem correct as it points upwards)of the foil and -401 N for the bottom (this one seems to be pointing the wrong way). Fluent then adds up these values and gives me 619N. The value still points the correct way but I don't beleive that the botom of the foil sucks down when the angle of attack is 6°... Can someone explain me how can this be possible? What am I doing wrong? Cheers, Ramine

 August 22, 2006, 15:52 Re: Forces report #2 Jason Guest   Posts: n/a Forces are an integration of pressure difference. On a wall, the flow field defines one side of the pressure, and the reference pressure defines the other side of the wall. Your reference pressure must be higher than the average pressure on the lower surface, which is what's causing it to push "down" (also why your upper surface has such a large "up" force). You're not actually doing anything wrong. Remember, your lift is essentially an integration of the difference of pressure between the upper and lower surfaces. If you're using your pressure distribution for structural analysis, you're going to have to give them information for internal pressure. Hope this helps, and good luck, Jason

 August 23, 2006, 08:15 Re: Forces report #3 Ramine Guest   Posts: n/a Thanks Jason for the clear explaination. But I still don't understand how I can get for example opposite values for say the pressure and viscous drag (X values) for the same part of the foil (i.e. the bottom)... In one case, I have for example -55N X pressure force and +8N X viscous force on the bottom of the foil. This would mean that the viscous force actually reduces the total drag of the foil... I don't have any recirculation so that is not possible. I'm doing trials on a 2D case for the moment in order to try to understand the problem but I still get those weird results. There must be some physics I'm missing or/and something else... Cheers, Ramine

 August 23, 2006, 08:50 Re: Ooops... #4 Ramine Guest   Posts: n/a I think I know why I had these negative values but I'm too ashamed to tell why! Sorry, I'll try to invest in a working brain for Xmas... Ramine

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