Mike Pettit
Ansys Employee

Hello Vishnu,

The simplest method for modelling viscous roll damping is to define a Morison Hull Drag Coefficients matrix with a non-zero RX-RX term. This value is multiplied by the square of the roll velocity to give a damping moment that is applied at the structure COG. Naturally you will need to provide an appropriate roll drag coefficient.

Alternatively, you can define Morison elements on the structure to model the viscous drag. You cannot choose which freedoms the drag forces/moments are calculated for - you will always have 3 forces and 3 moments acting at the structure COG due to the drag calculated on the Morison elements. The Morison elements can be either:

  • axisymmetric tubes, created in the geometry editor and associated with circular cross-sections, for which you define a single added mass/drag coefficient;
  • non-axisymmetric tubes, created in the geometry editor and associated with rectangular or non-uniform cross-sections, for which you define separate added mass/drag coefficients in local x and y directions;
  • discs, created in the Aqwa editor, which are point-like Morison loads with a single added mass/drag coefficient.

Using this approach the drag force on each Morison element will vary depending on the structure's position (i.e. drag drops to zero when an element leaves the water), but you still need to provide appropriate drag coefficients. Added mass coefficients should be small to avoid double-counting the structure added mass (assuming you also have diffracting panel elements in the model). For more information on this please see the discussion in https://forum.ansys.com/forums/topic/ansys-aqwa-how-to-add-insert-fins-for-damping-at-our-marine-structure/

I would certainly recommend Morison Hull Drag Coefficients as a much easier approach.

Cheers, Mike