[tobi1492]

Hi guys,

first of all I'm an student of mechanical engineering. I'm relative new to fluent (I startet 2 weeks ago) and I want to determine the drag and lift coefficient of an airfoil. I saw a few tutorials in how setting up an model in fluent. I got an basic understanding in how the software works and so on.
But now I'm at a point, that I'm a little bit confused on how Fluent calculates the lift coefficient.
I try to explain the problem.

I have 1 airfoil profile (S1223).

I have 2 projects, in the first project, the S1223 profile has the length of 1,28 metres. In the second project, the S1223 profile has a length of 0,53 metres.

Project 1: Length = 1,28 metres
c_lift = 4,36

Project 2: Length = 0,53 metres
c_lift = 1,72

I got the lift coefficients thereover. My expectation was, that they are nearly equal. Why aren't they equal????

if I calculate 4.36 * 0.53 / 1.28 = 1.28 the ratio of the length is equal to the ratio of the c_lifts..


So, the boundary conditions are in the same project the same. The mesh also.


I guess, the problem is the reference value panel. My area and depth (because of the 2D project) is at standard ==> 1 m² and 1m for depth.
For what are the reference values?
Some people on tutorials say you have to put the frontal area for cd into account, some people didn't make anything on the reference value panel.. I didn't find a logical response on the internet.


Can anyone help me please?

Thanks

[Zaktatir]

Reference VAlues are used only for post-processing purposes such as reference temperature for the calculation of Surface HTC or drag, lift, moment coefficient there you need the surface or the depth. In your case the depth is set to one meter and for calculating coefficients you need a surface!

[tobi1492]

ok thank you Zaktatir for your response

yeah, I recognized, that if I change any value of depth and area, I'll get an different drag and lift coefficient.

And why is my lift coefficient c_lift varying, when I change my depth?
In which case do I have to change my depth?

I thought, the only reason why I have to give him the specific areas is, that he can calculate the forces (lift and drag).

[Zaktatir]

you have to know your problem i can only comment: your lift coefficient is changing because i think that the bigger the surface the more lift i can have. Anyway your coefficient are relative to 1 m depth this may be a good rule of thumb if you are simulating two dimensional.