Confusion abt cm & CofR position in 6dof and non-6dof scenarios

quarkz · July 16, 2022, 10:12

Hi,

I am trying to understand how cm & CofR position are defined and calculated in 6dof and non-6dof scenarios.

In non-6dof scenario, supposed the airfoil is moving up and down in the y direction using the oscillatingDisplacement function, with y = 0 being the center. If I define CofR = (0 0 0), does it mean that the moment is always calculated wrt to (0 0 0)?

Or does it follow the y = 0 reference frame of the airfoil. In other words, CofR is always moving up and down.

On the other hand, in 6dof scenario, assuming it's the same airfoil but in this case it is constrained to move up and down in the y direction, and free rotation abt the z axis, then is the cm values calculated wrt to fixed or moving reference frame (for moment.dat)?

I guess internally, when OF calculates cm to obtain angular acceleration = cm/Izz, that cm has to be wrt the airfoil reference frame, else the calculation will not be correct.

Hope someone can clarify.

quarkz · July 20, 2022, 09:31

Hi,

I wrote a similar 6dof solver and found that the force output from OF uses a fixed reference frame. Hence, if CofR = (0 0 0), even when the airfoil translate upwards, the moment outputs will be calculated based on (0 0 0). So if you want moment wrt the airfoils (0 0 0) ref point, you will need to calculate yourself.

Hope tt helps.

July 16, 2022, 10:12	Confusion abt cm & CofR position in 6dof and non-6dof scenarios	#1
quarkz Senior Member TWB Join Date: Mar 2009 Posts: 400 Rep Power: 19	Hi, I am trying to understand how cm & CofR position are defined and calculated in 6dof and non-6dof scenarios. In non-6dof scenario, supposed the airfoil is moving up and down in the y direction using the oscillatingDisplacement function, with y = 0 being the center. If I define CofR = (0 0 0), does it mean that the moment is always calculated wrt to (0 0 0)? Or does it follow the y = 0 reference frame of the airfoil. In other words, CofR is always moving up and down. On the other hand, in 6dof scenario, assuming it's the same airfoil but in this case it is constrained to move up and down in the y direction, and free rotation abt the z axis, then is the cm values calculated wrt to fixed or moving reference frame (for moment.dat)? I guess internally, when OF calculates cm to obtain angular acceleration = cm/Izz, that cm has to be wrt the airfoil reference frame, else the calculation will not be correct. Hope someone can clarify.

July 20, 2022, 09:31		#2
quarkz Senior Member TWB Join Date: Mar 2009 Posts: 400 Rep Power: 19	Hi, I wrote a similar 6dof solver and found that the force output from OF uses a fixed reference frame. Hence, if CofR = (0 0 0), even when the airfoil translate upwards, the moment outputs will be calculated based on (0 0 0). So if you want moment wrt the airfoils (0 0 0) ref point, you will need to calculate yourself. Hope tt helps.