[Anna Tian]

Hi,

I'm modeling aircraft wing by Fluent. I found a problem about drag coefficient Cd and lift coefficient Cl calculation. The sign of Cd and Cl is sometimes minus at the modeling results. But the value is quite accurate. I checked the my reference value setting. They were set reasonably. Anyone has similar problem? And how to solve this problem?

[flotus1]

When you say "sometimes", do you mean that this happens with some of the simulations you perform or does it happen several times with the same simulation?
Are you sure that the direction vector for the evaluation of the force is pointing in the right direction and that your model is oriented correctly?

[Anna Tian]

Originally Posted by flotus1

When you say "sometimes", do you mean that this happens with some of the simulations you perform or does it happen several times with the same simulation?
Are you sure that the direction vector for the evaluation of the force is pointing in the right direction and that your model is oriented correctly?

I meant this happens with some of the simulations that I perform but not to the same one. For the same one, the solution will be always either positive or negative. It won't change if I run it again.

How do I check the direction vector?

[flotus1]

Check the force vector in the monitor window.

force_vector.png

[Anna Tian]

Originally Posted by flotus1

Check the force vector in the monitor window.

Attachment 27961

My wing's leading edge is pointing at the trailing edge at the X-direction. I'm wondering what should be the direction of this force vector?

[flotus1]

A sketch might help.
Anyway, if the main flow direction is in positive x-direction, then a force vector 1/0/0 should result in a positive drag force.

You could also show the setup and results of one of your simulations with negative drag coefficient here. Maybe there is something else you are missing.

[Anna Tian]

Originally Posted by flotus1

A sketch might help.
Anyway, if the main flow direction is in positive x-direction, then a force vector 1/0/0 should result in a positive drag force.

You could also show the setup and results of one of your simulations with negative drag coefficient here. Maybe there is something else you are missing.

Ok. But for the lift vector, when the lift force direction is (0,1,0), we use (0,1,0) as the force vector direction which is the same to the real lift force direction. But when the real drag force direction is (-1,0,0) as I described before (the main flow is at positive x-direction), the force vector will be opposite? It will be (1,0,0)? That's strange.

[flotus1]

But when the real drag force direction is (-1,0,0) as I described before

Now you confused me
From the previous description of your setup, I concluded that the drag force is pointing in positive x-direction.
We definitely need a sketch.

Lets assume the main flow direction is in positive x-direction.
So to be clear a fluid element travelling with the flow would move from lower x-coordinates to higher x-coordinates.
In this case we would expect the force on an obstacle in the flow to be pointing in the same direction (positive x-direction).
A drag coefficient should have a positive sign for this setup.
The right way to achieve this is with a "force vector" in the same direction as we expect the force on the obstacle. Which means positive x-direction.