# Force_x Direction @ wing incorrect...why?

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 May 25, 2010, 19:32 Force_x Direction @ wing incorrect...why? #1 New Member   David Stuart Join Date: May 2010 Location: Newcaslte Australia Posts: 15 Rep Power: 9 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.

May 25, 2010, 22:36
#2
Super Moderator

Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 14,337
Rep Power: 110
Quote:
 This is my first post here...
Then I will be gentle.

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:
 Auto meshed, but the mesh doesnt look too bad (Im told).
The auto mesh will probably be OK for lift but I am very doubtful of the drag. Accurate drag needs great care in meshing.

What Re and Ma ranges are you modelling? Is it just a 2D aerofoil, a 3D wing or a full body?

May 26, 2010, 01:09
#3
New Member

David Stuart
Join Date: May 2010
Location: Newcaslte Australia
Posts: 15
Rep Power: 9
Thanks for being gentle...

Quote:
 discovered a perpetual motion
That had crossed my mind, I was going for a nobel haha

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

 May 26, 2010, 07:17 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,337 Rep Power: 110 What Re number?

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