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-   -   Symmetric Model (http://www.cfd-online.com/Forums/fluent/41485-symmetric-model.html)

 mAx June 23, 2006 04:40

Symmetric Model

Hi, Can we compute the torque on a Symmetric Model, and concluding that the entire torque is double from Fluent Results??? (juste like the force)

 Jason June 23, 2006 07:32

Re: Symmetric Model

Yes, but only about an axis perpendicular to the symmetric plane.

Hope this helps, Jason

 mAx June 23, 2006 08:27

Re: Symmetric Model

hmmmm.... my axis is X, and my symmetry-plane is XY.

 Jason June 23, 2006 11:55

Re: Symmetric Model

Well, now we need to look into making assumptions, which are highly dependent on your geometry.

For example... I wouldn't do that on a fighter jet because there is a lot of interaction between control surfaces. So a "positive aileron" would interact with surfaces downstream and may dampen or amplify the rolling moment. Therefore causing a roll on the aircraft isn't symmetric because the positive aileron on the port side isn't creating the same moment as the negative aileron on the starboard side, due to the interactions between surfaces. Now, on a commercial airliner you may be able to get away with it because the ailerons are so far out along the wing that they don't typically interact with the tail surfaces. But you'd have to look into the model and make sure that's true. But you have to model + and - ailerons to make sure you're getting symmetric commands.

Another thing to make sure is that the airflow causing (or caused by) the device creating the moment CAN NOT interact with the symmetry plane. I was given a model and asked to look into it where they were modeling a 4 winged rocket and the moments were coming out all whacked compared to test data. Well, they were modeling it symmetrically, and in a 4 wing rocket a roll command would develop a vortical flow that flows around the entire rocket (basically the flow leaves the wing and spirals along the surface). But because of the symmetry plane, the flow would come off the wall, and instead of continuing around the body it would have to deflect away from the body when it came to the symmetry plane. So the symmetry assumption there was invalid, and because of the AoA requirements, a full model had to be run.

Hope this helps, Jason

 mAx June 23, 2006 13:11

Re: Symmetric Model

I understand what you mean, but my model is much more like an aircraft divided into the left and right side. So the calculation will be assumed only on one side. But now, I understand that, if there is a torque on x axis (symmetry plane xy), it means that there is a perturbation between the left and right side. So that means that I have to calculate the whole model.

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