Force_x Direction @ wing incorrect...why?
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?
Just thought I would add - I am playing with domain size (going bigger).
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?
What Re and Ma ranges are you modelling? Is it just a 2D aerofoil, a 3D wing or a full body?
Thanks for being gentle...
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.
What Re number?
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