Unreasonable CD

Zohardv · March 29, 2012, 16:30

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
ZoharDV

dklastiwka · March 29, 2012, 18:45

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.

Zohardv · March 30, 2012, 05:24

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?

dklastiwka · March 31, 2012, 16:48

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.

LuckyTran · March 31, 2012, 16:53

Quote:

Originally Posted by dklastiwka

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.

Zohardv · April 1, 2012, 02:16

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!

March 29, 2012, 16:30	Unreasonable CD	#1
Zohardv New Member Zohar Join Date: Mar 2012 Posts: 3 Rep Power: 14	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 ZoharDV

March 29, 2012, 18:45		#2
dklastiwka New Member Derek Lastiwka Join Date: Feb 2010 Location: Calgary, Alberta, Canada Posts: 26 Rep Power: 16	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. __________________ Derek Lastiwka Mechanical & Fluids Engineer dlastiwka@graymattereng.com Gray Matter Engineering Inc.

March 31, 2012, 16:48		#4
dklastiwka New Member Derek Lastiwka Join Date: Feb 2010 Location: Calgary, Alberta, Canada Posts: 26 Rep Power: 16	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. __________________ Derek Lastiwka Mechanical & Fluids Engineer dlastiwka@graymattereng.com Gray Matter Engineering Inc.

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March 30, 2012, 05:24		#3
Zohardv New Member Zohar Join Date: Mar 2012 Posts: 3 Rep Power: 14	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?

April 1, 2012, 02:16		#6
Zohardv New Member Zohar Join Date: Mar 2012 Posts: 3 Rep Power: 14	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!