41. Multi-Mesh RigidBody

The identification tag for this tutorial is PDS-ABE. Pregenerated input files for this tutorial are found in the folder named PDS-ABE in the provided tutorial input files.

41.1. Tutorial overview

This tutorial covers:

  • Adding multiple load and mesh features to a rigid body
  • Referencing global, rigid body, and feature frames
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Fig. 41.1 Simple raft with multiple load features

41.2. Setting a up simulation with a rigid body

  • Create a new RigidBody and name it raft.
  • By design, the raft will be 50 % submerged in calm conditions due to the buoyancy of the pontoons. Set $Mass 3e4 and set the moments of inertia arbitrarily to 1e4.
  • To help keep the raft stable, set $CGPosition 0 0 1.

41.3. Creating multiple rigid body features

Note

  • An abstract diagram showing feature frames (f1 and f2) relative to the RigidBody frame (rb) which is relative to the global frame (O) can be seen in Fig. 41.2.
  • Every feature associated with a RigidBody has this type of reference frame hierarchy.
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Fig. 41.2 Reference frame layout

Note

  • Three different sized cylinder load features are used for the pontoon floats, the struts between them, and the A-frame booms.
  • Add a cylinder feature to the library, name it pontoon, and set the diameter and length to 2 and 10, respectively.
  • Add a cylinder feature to the library, name it strut, and set the diameter and length to 0.2 and 6, respectively.
  • Add a cylinder feature to the library, name it frame, and set the diameter and length to 0.5 and 6, respectively.
  • Each cylinder must be added to the rigid body. The position and orientation of the feature frame is specified relative to the rigid body frame. To simplify setup, the rigid body frame is located between the two horizontal pontoons.
  • Position the features as shown below:

$Cylinder pontoon 0 4 0 0 90 0
$Cylinder pontoon 0 -4 0 0 90 0

$Cylinder strut 4 0 0 0 90 90
$Cylinder strut -4 0 0 0 90 90

$Cylinder frame -4 1.5 -3 0 45 45
$Cylinder frame -4 -1.5 -3 0 45 -45

Note

  • Note rotation angles to position the booms in the A-frame at the rear end of the raft.

41.4. Running the simulation

  • Run the short simulation.
  • View the results in PostPDS. Right click on the RigidBody DObject and select Display Local Origin. This will display the position of the RigidBody local coordinate frame origin.

Note

  • The large pontoon floats provide buoyancy forces to keep the raft afloat. Incorporating the A-frame geometry is necessary to ensure wind forces will be accounted for. While the struts in between the pontoons are small, they may contribute significant load from drag forces as well.

41.5. Observing changes in cylinder feature properties

  • Shift the CG position aft to better represent the distribution of mass from the A-frame. Observe the change in steady position.
  • Increase the resolution of the cylinder meshes and observe the influence in calm conditions.
  • Create additional simple custom mesh features such as a box for a barge deck and add it to the raft.