Damping Report

Hi There, 

How can we report the quadratic damping calculated by MOSES using the Tanaka factor? 

Thanks, Gopi

  • Dear Gopi,

    When using the -tanaka option, MOSES utilizes the extreme geometry values of the vessel and determines the values needed to look up a Tanaka damping value. As the manual explains, the option -TANAKA defines a multiplier for the damping due to "eddy making." MOSES refers to this as "Tanaka" damping and uses the formulation outlined by Schmitke in SNAME Transactions Vol. 86, pp. 26-46, 1978.

    In case you do not want MOSES to calculate from extreme geometry, you can use the #tanaka loadgroup and provide the respective values of wetsurface, block coefficient, KG, beam, etc. Please refer to the #tanaka load group in the manual. There is also an example (vdtest.cif, vdtest.dat) in the installation directory under the runs/tests/freq path where the #tanaka option is used.

    Another option to include roll damping is the option -ROLL_DAMPING in the &describe piece command. Here you define a roll damping factor (quadratic damping factor) such that it applies a roll moment given by: roll_damp_factor * omega^2, where omega is the angular velocity. There is an example in runs/samples/how_to/roll_damp.cif in which a comparison is made between using -tanaka 1, -cs_current 1 1 1, and roll_damping with different wave steepness.

    You should also look at the b_keels.cif example in the same directory. Additionally, you can manually calculate the damping as a percentage of critical damping from the roll decay test in time domain analysis.

    Please let me know if you have further questions.

    Thanks,

    Rahul Kanotra