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March 22, 2012, 13:21 
GGI Dynamic RPM: Speed question

#1 
Senior Member
Robert
Join Date: Sep 2010
Posts: 155
Rep Power: 7 
I am preparing a case where OpenFOAM is called from a script to run one iteration at a time. This is to extract forces and moments each timestep to calculate a new RPM and rewriting dynamicMeshDict.
Can someone elaborate on the runtime of this program, stopping and starting OpenFOAM every iteration? How much slower would it be than if I could better integrate the RPM change in the OpenFOAM software? 

March 23, 2012, 08:39 

#2 
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Gabriele
Join Date: Feb 2012
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Actually I'm interested in this problem too.
In the simple case of a turbine rotating around a fixed axis, I ask if it is possible to use a ODE solver for the equation I \dot{\omega} = M where \omega is the angular velocity (of the turbine, hence of the GGI), I is the fixed moment of inertia and M is the torque (referred to the rotation axis) that comes from "forces" library. The angular velocity in the dynamicMeshDict should be "linked" to the above equation. I'm new in OF, so I don't know if it is correct and possible. Regards 

March 23, 2012, 12:32 

#3 
Senior Member
Robert
Join Date: Sep 2010
Posts: 155
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Its definitely possible. See here: http://www.youtube.com/watch?v=wjr7NUe5hfg
I was just going to use a form of what you had stated: (torque) = (rotational inertia) * (angular acceleration) take (angular acceleration) = (change in angular velocity) / (time step) then (change in angular velocity) = (time step) * (torque from forces output) / (rotational inertia) 

March 23, 2012, 16:37 

#4 
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Gabriele
Join Date: Feb 2012
Posts: 6
Rep Power: 6 
Thanks.
I think this is the point: > (change in angular velocity) = (time step) * (torque from forces output) / (rotational inertia) Can I get a reliable result applying only this rough formula? Which is the "right" time step? I was thinking about applying also some ODE solver to get a better control of the time step, but I don't know if it is necessary. Regards 

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