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 willis January 26, 2012 08:31

Ground Effect

I am trying to model the flow around a cambered airfoil, taking ground effect into consideration.

Can anybody tell me how I would specify the right conditions for my lower boundary (the ground) in fluent, i.e a velocity and no slip condition?

Thanks very much for any help.

 truffaldino January 26, 2012 09:29

You can put no-slip on the ground, but ground must move together with airflow, if your airfoil is stationary in fluent. Or simpler solution could be to put "reflection" of your airfoil in the ground and analyse double airfoil problem.

Truffaldino

 willis January 26, 2012 10:26

Thanks Truffaldino,

I know that the ground needs to be given the same velocity as the flow, I just do not know how to do this in fluent. When defining my boundary conditions, if i select wall is there an option to specify a velocty as well?

 truffaldino January 26, 2012 11:23

Hi Willis,

I have never used fluent with no-slip on moving surface, but I think fluent has something as "moving wall" bc. Just put the speed of wall to be equal to that of air at the inlet.

But I still think putting extra "reflection airfoil" is better solution of the problem.

Truffaldino

 Martin Hegedus January 26, 2012 16:32

Out of curiosity, is the interest in race car aerodynamics, i.e. inverted cambered front wing of F1, or airplane ground effects?

 lava12005 January 26, 2012 21:05

If we consider the case whereby the airfoil is moving, the speed at the ground (yes boundary layer is there) is equal to the speed of the farfield which is basically 0.
Am I right to say that the ground boundary condition for simulation (where the airfoil is fixed) is a velocity inlet where the velocity specified is equal in both magnitude and direction at the normal inlet?

Moreover, some of the experiment in quantifying ground effect is based on the double body test. But some other perform it using a moving belt as the ground, so is the double body experiment is 'correct'? Since the velocity at the symetry plane is not necessarily equal to the movement speed of the airfoil.

 Martin Hegedus January 26, 2012 21:32

Quote:
 Moreover, some of the experiment in quantifying ground effect is based on the double body test. But some other perform it using a moving belt as the ground, so is the double body experiment is 'correct'? Since the velocity at the symetry plane is not necessarily equal to the movement speed of the airfoil.
Depends. If the airfoil is a few chords from the surface and the upper surface normal is pointing away from the ground, then symmetry is probably OK.

On the other hand, if the airfoil is close to the ground and the upper surface normal vector is pointed towards the ground, i.e. the inverted race car airfoil and the flow over the "upper" surface is moving much faster than freestream, then it may be better to use a moving ground plane.

Edit: Oh, and it depends on Reynolds number. For the race car front wing, the lower the Reynolds number, the more likely the moving ground plane will be required.

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