[badoumba]

Hello guys,

It seems that for car simulation, reference area for the model is the frontal projection area. For aircrafts, people seem to use top-view projection area.

What is the rule? By the way why this area isn't the total area of the model?

Thanks for you clarifications!

[KateEisenhower]

Hi,

this reference area is only used for the calculation of the relevant dimensionless coefficients.

In automotive your prime interest is in drag, so you use the frontal projection area since it is most relevant for the overall drag.

In aircraft historically you are primarily interested in lift, so you use the vertical projection area.

Please note that you can't compare the coefficients for aircraft and automotive by any means.

Best regards,

Kate

[badoumba]

Understood Kate.

Does it mean that with the vertical projection that I am using my lift coefficient will be correct but not the drag one? Do have have to launch 2 separate simulations for better accuracy?

Thanks

[KateEisenhower]

Sorry,

Maybe my post was missleading. Here is an example:


Suppose you have two wings in different wind tunnels:

Wing1 Wing2
profile 1 2
span 1 m 2 m
chord length 1 m 1 m
Lift 100 N 150 N
Re number is the same

Now you want to know if profile 1 or 2 is better. Based on the Lift forces you'd say Wing two is better. But now we look at the lift coefficient:

The lift coefficient for an airfoil is:

c_L = (2*L)/(rho*u^2*A)

with

L = Lift
rho = air density = 1.3 kg/m^3
u = velocity = 50 m/s
A = wing area

Our new table looks like that:

Wing1 Wing2
profile 1 2
span 1 m 2 m
chord length 1 m 1 m
Lift 100 N 150 N
c_L 0.61 0.46

When you compare the lift coefficients now, you can see that Wing1 is better in reality. Wing2 only looked better when you compare only the absolute forces.

[badoumba]

Thanks Kate.

Found this simple but clear page
https://www.grc.nasa.gov/WWW/K-12/airplane/sized.html

As I need to compare lift and drag to extract the best ratio in my project, I understand from this page that I have to choose the same reference for both - so the total area (which is actually not the total but half of it, hence the vertical projection).

If I am interested in drag, we should logically choose the total area as reference for an angle of attack of 0 (since drag is mainly caused by frictions), and front area projection at an angle of attack close to the stall (since drag is due to resistance to airflow). If we have an object with a large length/width/height ratio, there might be a huge difference of value between these 2 references. This confuses me a bit. If we switch from one to the other, the drag coeff will have completely different values. Should we do a kind of interpolation while exploring a range of attack angles ?

Thx

[KateEisenhower]

Originally Posted by badoumba

Thanks Kate.

Found this simple but clear page
https://www.grc.nasa.gov/WWW/K-12/airplane/sized.html

As I need to compare lift and drag to extract the best ratio in my project, I understand from this page that I have to choose the same reference for both - so the total area (which is actually not the total but half of it, hence the vertical projection).

Hi,
Yes, normally the same reference area is taken for both lift and drag. But I think your conclusion is wrong. Even if you have different reference areas, the AoA of the best ratio should stay the same.

If I am interested in drag, we should logically choose the total area as reference for an angle of attack of 0 (since drag is mainly caused by frictions), and front area projection at an angle of attack close to the stall (since drag is due to resistance to airflow). If we have an object with a large length/width/height ratio, there might be a huge difference of value between these 2 references. This confuses me a bit. If we switch from one to the other, the drag coeff will have completely different values. Should we do a kind of interpolation while exploring a range of attack angles ?

Thx

Keep in mind that usually the reference area is taken to be constant even when examining different AoAs. This whole reference area thing is only for comparison purpose.
I think it would help you to measure the absolute values and then try to calculate the coefficients. You will realize quickly, if you want to compare your data to other tests with similar obstacles, you'll have to choose the same reference area.

Best regards, I hope this is some help,

Kate

[badoumba]

Thank you Kate. Things get clearer indeed!

What still confuses me is that in my mind, area is the basis for L or D integration. How can we obtain a real value of L or D with an (almost) arbitrary choice of area? Unless area is only used for coefficients.

Anyone knows where I can see the maths behind in OF? That would probably answer all my questions.

Thanks