21. Connect a Cable with a PointMass

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

21.1. Tutorial overview

This tutorial covers:

  • Reviewing the pendulum tutorial
  • Creating a PointMass DObject
  • Connecting a PointMass to a cable
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Fig. 21.1 Pendulum with a PointMass

21.2. Reviewing the cable creation tutorial

  • Make a copy of the project folder made for the Simulate a Cable tutorial project input folder. The copy will be modified for use with this tutorial.

21.3. Creating a PointMass

  • Add a PointMass DObject to the simulation. This will act as the weight at the end of the cable.
A PointMass behaves the same as an ExtMass. However, an ExtMass is a feature that is added and embedded within each cable while a PointMass is an independent DObject. Multiple cables can be attached to a PointMass, so it is also useful as a connector.
  • Set density and diameter of the PointMass to 7000 kg/m3 and 1 m, respectively, to model a solid steel sphere with a diameter of 1 m.
  • Set the PointMass state with an initial position coincident with node N of the cable and zero initial velocity.
  • The PointMass $FluidLoadingMode property can be set to 0 since the PointMass will be completely submerged during the simulation. This will speed up execution speed.

21.4. Creating PointMass and cable connections

In order to have two DObjects interact, a connection must be made between them. Connections are listed under Connections in the Project Explorer. Each DObject also lists connections associated with it under the Connections tab next to the Input tab. When connecting the PointMass to the cable, the PointMass and desired cable node must be near each other to avoid stretching the cable boundary element too much, which may cause a destabilization.

  • Connect the cable to the PointMass by selecting the Connection button.

A PointMass must be the master DObject because they set the position and velocity of the boundary node of a cable. In return, the reaction forces from the cable are passed to the PointMass to complete the dynamic interaction.

There are different connection types between two DObjects. In this case, a point connection type is the only connection type between a cable and PointMass.

  • View the connection properties by selecting the Connections tab under either DObject used in the connection.
  • Enable the $DCableFollowerNodeN property by setting the property to 1. This indicates that node N of the follower cable is used for the connection.

21.5. Running the simulation and processing the results

  • Set the length of simulation to 30 seconds.
  • Execute the simulation.
  • View the results in PostPDS.