FAQ

Be careful using mass proportional damping with use of multiple bodies

It seems that the mass proportional part of the damping matrix yields a different damping if the number of bodies is changed. However there is a fine agreement between the damping results from the “structure_eigenanalysis” and real time simulations. If only stiffness proportional damping is used there is also a fine agreement (regardless of the number of bodies). We are currently working on the implementation of a new method to handle the structural damping by specifying the actual damping ratio for each component.

Until this option is available we recommend two ways of obtaining the correct damping in the real time simulations by:

1. Set the mass proportional part of the damping to zero (damping_posdef 0.0 0.0 0.0 Kx Ky Kz) and only use the stiffness proportional part when tuning. However it can be impossible to adjust the high frequencies in this way.

2. If this is not possible then only use ” structure_eigenanalysis” for tuning the damping and remember to use the same number of bodies as in the real time simulations. If it is important to look into the damping on main_body level this means that it is necessary to make a set-up for all components consisting of multiple bodies which are then cantilevered (fix0) and tuned separately. Usually it is sufficient to use ”structure_eigenanalysis” on the entire structure. However the mass proportional damping has no influence with use of large number of bodies.

http://www.hawc2.dk/Support/FAQ
11 DECEMBER 2018