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-   -   Force_x Direction @ wing incorrect...why? (

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,


Just thought I would add - I am playing with domain size (going bigger).

ghorrocks May 25, 2010 22:36


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?


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...


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.


ghorrocks May 26, 2010 07:17

What Re number?

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