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May 25, 2010, 19:32 |
Force_x Direction @ wing incorrect...why?
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#1 |
New Member
David Stuart
Join Date: May 2010
Location: Newcaslte Australia
Posts: 15
Rep Power: 15 |
Hi Guys/Girls,
This is my first post here... Im hip deep in CFX at the moment for a uni course and am having trouble resolving an issue I see with my CL and CD for a wing I am simulating... Basically I get an L/D far below the expected (about a quater of expected) and have been trouble shooting scince. I am quite sure that my expressions are correct, however when I used the function calculator in Post to find the force in X dir, it tells me a -ve number. This doesnt make any sense at all. Auto meshed, but the mesh doesnt look too bad (Im told). Im runnig a +ve angle of attack (AOA), 15m/s free stream (U). AOA is determined with expressions Ux=U*cos(AOA), and Uy=U*sin(AOA) All my other expressions I believe are correct, but will be influenced by the -ve Fx to give an incorrect value... I am new to CFX, but I'd argue not an idiot so Im a little puzzled. Anyone ever had this or anything like it happen before? could it be a boundary cond problem? Thanks alot, Dave [EDIT] Just thought I would add - I am playing with domain size (going bigger). Last edited by fatb0y; May 25, 2010 at 20:28. |
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May 25, 2010, 22:36 |
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#2 | ||
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,703
Rep Power: 143 |
Quote:
Firstly - look at the individual components rather than L/D ratio. Is the error in lift or drag or both? You should be able to get a pretty good lift number (except near stall) but drag is much harder. If the drag is in the wrong direction either you have discovered a perpetual motion machine or your simulation is very inaccurate. Are you sure it has adequately converged? Quote:
What Re and Ma ranges are you modelling? Is it just a 2D aerofoil, a 3D wing or a full body? |
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May 26, 2010, 01:09 |
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#3 | |
New Member
David Stuart
Join Date: May 2010
Location: Newcaslte Australia
Posts: 15
Rep Power: 15 |
Thanks for being gentle...
Quote:
I have since improved my Cl and Cd numbers by increaseing the size of my domain considerably. However the LD remains around the same value. Thanks for your reply, I have been playing with things myself I just thought I'd throw it out there at the same time this morning to see if anyone had an obvious answer - seems as though the domain size was the problem (at least the main one.) The longer domain has fixed the -ve Fx result. I understand the downfalls of the auto mesh you described - I have been taking that into account when comparing cal'd results to expected values. This accounts for my higher than expected Cd value. It is a 3D wing operating at very low subsonic airspeeds. Cheers |
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May 26, 2010, 07:17 |
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#4 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,703
Rep Power: 143 |
What Re number?
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