LIFT and DRAG???
 

Hi,
I read some of the discussions and I am really confused about the definition of Force x()@surface, Force y()@surface, Force z()@surface and normForce x()@surface.
Is the Lift of an airfoil = Force y and Drag = Force x and what is normalized force??
kindly throw light on these.

Thanks :)

Yes, lift is Force Y and drag is Force X assuming you have your model aligned with those coordinates.

If you do not understand the definition of forcenorm in the documentation then don't use it. It is only used for somewhat specialised situations so most people don't need it (I have never used it).

Thank You Glenn for the clarification, so If der is an AOA then I'll have to Use the formula for Lift like Lift = Fy*cos alpha - Fx*sin alpha, right??? my flow is in X direction.

You can model AOA by directing the flow along the axis and rotating the airfoil, or keeping the foil still and moving the air flow. The calculation of lift and drag is different between the two approaches.

Yes, But In my case, to get the angle of attack I have specified respective unit vectors for flow direction at the inlet. Now my AOA has to be 5 deg, wrt X-axis, and the perpendicular direction to the flow is Y axis. So My model is parallel to X-axis but the flow is tilted by an angle of 5 degrees. Now for this case, the Force y given in CFX will not be the lift right?? because lift is perpendicular to the flow direction. For this case to calculate Lift I'll have to use the formula I've already mentioned. M i correct??

It looks OK.

Hi everyone!

I'm studing a Naca airfoil trying to calculate the lift and drag coefficients.

Unfortunately i can't get very good results, and i think the problem is related to the definition on the drag and lift coefficients used by the utility forceCoeffs ( in controlDict). In order to understand how it works i've set ( in the controlDict file) all the parameters (rho, Aref, lref, magUInf) to 1 so i should get a Cd=2*F.
I also get the forces so I should be able to calculate manually Cd and Cl.

I think that openFoam calculate the force in x direction (drag) as the integral of P and tau on the surface of the foil. the surface should be calculated as the perimeter of the foil times its lenght. I've checked that changing the leght ( area increase) I get higher Force value, and as expected the Cd remains constant.

this is how i calculate the CD

cd = F/(0.5*rho*v^2*Aref*c)
rho= 1.2 (air 20 celsius degrees)
Aref= foil perimeter times lenght
c = cord lenght
I hope i've been clear...

at the moment I get a cd = 0.023 instead of 0.017 so the error is around 30% (solver simpleFoam)

Can anybody tell me if i'm doing right? I can't understand if i'm making a mistake calculating the coefficients or if the problem is related to something else (mesh...)

thanks a lot!
Paolo

This is the CFX forum. Try the Openfoam forum.

why Fx reduces
 

I did an airfoil setup (actually 3 airfoils comparison) in a virtual wind tunnel using cfx and I am quite happy with the lift values over speed and AoA increase (at least they make sense).

however I have issues with the drag (force X @ surface).
as Angle of attack increases the Fx decrease... this sounds strange to me as I would have expected drag force to increase with AoA.

1) is it normal to have Fx decreasing with AoA increasing at constant flow speed?

2) when Fx becomes negative does it mean that the profile stalled (even if still CFX shows Fz increase)?

Looks like simply an inaccurate simulation to me: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

not sure to follow what you mean...

here is some shots of my mesh, physics and result.


please confirm to me if I am wrong: as AoA increases, drag should also increase. correct?

In the real world yes, drag increases with AOA. When you have an inaccurate simulation which has not had the normal sensitivity checks and validation the results are random numbers and anything can happen.

So read the link I posted to give you a start on how to check the accuracy of your simulation.

I did check this out and could not find the issue. for example when I change the mesh resolution I get nearly the same results

I also tried the same simulation with Autodesk cfd and got same strange behavior so there must be something obvious that I missed out!

I can see from your mesh that it is unlikely to be fine enough to be accurate - and the blockiness of the contours of your contour plot are a give-away for a too coarse mesh.

So I am pretty sure that your mesh is way too coarse to be accurate.

I did not manage to increase the mesh around the object in ansys but did in Autodesk as attached and same result.

again, just to be sure I am clear, my problem is getting wrong tendency (ie decrease of drag with AoA increase, not the values themselves...

Still looks very coarse.

Yes, I know you are getting the wrong trend. But as I said, if you have too coarse a mesh you can get anything. Reading trends into completely bogus numbers is a path to ..... (something not very good :) ). So don't read anything into your trend going the wrong way, other than your results have a massive error in them and you need to fix it up before looking at trends.