# [Solved] Draft coefficient too Small?

Hello,

For My school I’d like to simulate flows around vehicles. We have a project of “Solar Vehicle”, and my students want to optimyze the shape. This is their firts try !!

I’ve tried to run : SimScale with an “Incompressible” simulation.

## Simulation Run => OK !

—> Air Velocity seems OK
But when I Read the drag coefficient, I have 0.001, that is clearly too small !!

Do you have an Idea Why ? Any help would be greatly appreciated !!

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Writing a problem helps solving it !

I didn’t declarated the good direction for my drag Calculation !

The wind have the direction Y.
I computed the drag coefficient in the direction X, whitch is effectively 0 !
I restarted a new simulation to compute the drag coeffcient in the direction Y…

3 Likes

Hey there!

Make sure to also input appropriate values for free stream velocity and for reference area value, as they impact Cd.

For reference, this is the formula that correlates forces (drag forces, N) to Cd:

Cd = {2*F\over (\rho*V^2*A)}

Reference area is a projection of the car area onto a plane (in a frontal view). So it would be the sum of these areas highlighted below.

Cheers

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Ahh !

I thought the reference area value was automatiquely computed !!

And I thought the air velocity was defined in the “Velocity Inlet” ??
What is the difference between Velocity Inlet and Freestream velocity ???

Thank you very much !
Guillaume

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Reference area is not automatically computed because it changes based on your application. For cars, it’s a frontal area projection. For airplane wings it would be a projection based on the bottom view. Therefore you have to specify the value that makes sense for your project.

Usually these velocities will be the same. I guess they could be different if there’s some sort of constriction in the flow domain (like venturi effect?).

3 Likes

Ok, thank you very, very much !!