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-   -   Re: Boundary condition issue (http://www.cfd-online.com/Forums/cfx/26238-re-boundary-condition-issue.html)

CFD user August 9, 2008 11:33

Re: Boundary condition issue
 
Hello,

I appreciate your insights into this interesting problem. I tried to calculate the drag and lift forces acting on a hydrofoil using CFX. Unfortunately I ended up applying an incorrect outlet boundary conditions of about 2atm pressure instead of 1atm pressure. When I computed my lift and drag values my lift was almost 1/4 of experimental one's whereas my drag value is very close to experimental value (within 20%). I can't understand why my lift value is so much off where as drag value hasn't impacted much. I will be very thankful to you for your feedback.

Thanks

Glenn Horrocks August 11, 2008 01:12

Re: Boundary condition issue
 
Hi,

I have simulated hydrofoil flows and got a lot closer than that. You should be able to get the lift quite accurately but the drag is more of a challenge.

Why did you say you applied the wrong boundary condition pressure? Either it does not matter so why mention it, or it does matter and you results are garbage and that's where your error comes from.

Glenn Horrocks

CFD USER August 11, 2008 08:22

Re: Boundary condition issue
 
Is applying 2atm pressure at outlet is a correct boundary condition.

Glenn Horrocks August 11, 2008 18:35

Re: Boundary condition issue
 
Hi,

??? I cannot tell you that, I don't know what you are trying to model.

Assuming you are trying to model a normal hydrofoil zipping along a normal water surface then no, it is not correct. You should apply a reference pressure of 1 bar and an outlet pressure of 0 bar. The ambient pressure is important so you get cavitation effects correct, if that is relevant.

Glenn Horrocks


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