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-   -   Force_x Direction @ wing incorrect...why? (http://www.cfd-online.com/Forums/cfx/76442-force_x-direction-wing-incorrect-why.html)

 fatb0y May 25, 2010 19:32

Force_x Direction @ wing incorrect...why?

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).

 ghorrocks May 25, 2010 22:36

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?

 fatb0y May 26, 2010 01:09

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

 ghorrocks May 26, 2010 07:17

What Re number?

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