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Old   March 29, 2012, 15:30
Default Unreasonable CD
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Zohar
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Hello all.

I'm trying to model a flow around a Catamaran.
My setup is based on and existing tutorial published any Ansys which simulates Heave and Pitch motion.

I've made my own geometry using Solidworks and used DesignModeler to import it and to build the control volume for my case.

In fluent, I use k-epsilon SST model. I also apply VOF and Wave BC multiphase models, as well as 6Dof dynamic mesh.
The simulation is set to transient.
The 6Dof UDF limits the model's motion about X axis (along the flow) and it's rotation around X axis.

For this simulation I use air and water, Velocity Inlet and Pressure Outlet are set for boundary conditions and the solution method is set to Green-gauss Cell based. Reference values are calculated from the ship's Hull.

The simulation results in a very unreasonable Drag Coefficient - 2.5e3 and I'm afraid it renders the entire case unreliable.

Can anyone suggest what is wrong with my setup?

Thanks
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Old   March 29, 2012, 17:45
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Derek Lastiwka
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What are you using for your reference variables that are used to calculate Cd. I am assuming that you are letting fluent calculate the Cd and you are not doing it yourself.
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Old   March 30, 2012, 04:24
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Hi Derek.
Do you mean Reference Value? If so, it is taken from a named selection called "hull" and consist of the boat's hull.

I let Fluent calculate the Drag Coefficient using Drag monitor, is it wrong? Is there a better way?
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Old   March 31, 2012, 15:48
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Derek Lastiwka
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Not sure which version of fluent you are using but I don't think it will matter to much. In the upper drop down menu in Fluent 14.0 there should be "report". If you click on that there should be "reference values". Select it and a Reference Values windows should open. All the values in here are used by fluent to calculate non dimensional properties like Cd. If these are not correct or do not represent your model correctly the Cd will be incorrect.

I do not like selecting from a zone because Fluent does not always choose the correct values.
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Old   March 31, 2012, 15:53
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Quote:
Originally Posted by dklastiwka View Post
Not sure which version of fluent you are using but I don't think it will matter to much. In the upper drop down menu in Fluent 14.0 there should be "report". If you click on that there should be "reference values". Select it and a Reference Values windows should open. All the values in here are used by fluent to calculate non dimensional properties like Cd. If these are not correct or do not represent your model correctly the Cd will be incorrect.

I do not like selecting from a zone because Fluent does not always choose the correct values.
Zohard, this is a common mistake.
Do as Derek explained and it will work. Do not simply use compute from (does anyone even use this?). Input them in manually.
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Old   April 1, 2012, 01:16
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Got it...

Apparently, my reference values were way off - 998.2 kg/m^3 as density of air and 1 m^2 as projected area instead of 0.03 m^2,
so I guess this caused a huge error.

When I input those values manually everything goes as plan (CD=0.3) .

Thank you guys!
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