23. Using an SCable DObject

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

23.1. Tutorial overview

This tutorial covers:

  • SCables
  • Create and connect a PointMass to a SCable
  • Visualizing results
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Fig. 23.1 Cable terminology for (a) Cable and (b) SCable.

23.2. Introduction to SCables

Note

  • An SCable DObject is used to model a simple single element line and may be useful for representing springs or short stiff lines. An SCable should not be used if there is an expected catenary or significant distributed load in the cable, as it is represented by a single straight line.
  • An SCable has two nodes and one element, as seen in Fig. 23.1. Only a single cable segment can be applied to the SCable.
  • SCable DObjects are initialized and behave identically to Cable DObjects in PST. However, there are some differences in how the dynamics are calculated. Since SCables are intended to be short stiff lines, drag and added mass forces are not calculated. Also, torsional and bending rigidity is neglected and therefore is not used in any calculations.
  • The benefit of using SCable DObjects over Cable DObjects is an increase in simulation speed, due to the reduction in computational requirements and larger allowable integration time steps.

23.3. Creating an SCable

This tutorial will be similar to the simple pendulum tutorial, however the cable length will be much shorter.

  • Create a new project in PST. To add an SCable to the simulation, click Create within the DObject tab and select SCable as the DObject type.
  • Define the SCable initial state by selecting the SCable in the Project Explorer and clicking the State button or pressing Ctrl-K.
  • Specify node 0 at (0,0,0) m and node N at (1,0,2) m. Set the length of cable to be the exact distance between the points specified - 2.236068 m.
  • Ensure node 0 is constrained at the surface by setting $Node0Static 1.
  • Create a DCableSegment feature called RopeSegment.
  • Change the properties to the values shown below:
// Axial Rigidity
$AxialRigidityMode 0
$EA 1E8

// Fluid loading
$CDc 1.5
$CDt 0.01
$CAc 1

// Mechanical
$EI1 1
$EI2 1
$GJ 0
$Diameter 0.01
$Density 1100
$AxialDampingMode 1
$AxialReferenceDampingRatio 0.5
$BCID 0
$TCID 0
$CE 1

// Strain Limit
$ElongationLimitMode 0
  • Reference the new DCableSegment feature in the SCable input file by adding $CableSegment RopeSegment 2.236068.

23.4. Creating PointMass and SCable connections

  • Create a PointMass with a density of 7600 kg/m3 and a diameter of 0.25 m. Set $FluidLoadingMode 0, as variable hydrodynamic loading is not required for this case. Keep all other properties as their default values.
  • Set the state of the PointMass to a position of (1,0,2) m and zero initial velocity.
  • Connect the SCable to the PointMass by selecting the Connection button. Select the PointMass as the master and the SCable as the follower.

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

  • 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.

23.5. Running the simulation and processing the results

  • Set the length of simulation to 20 s
  • Execute the simulation. Notice how quickly the simulation completes.
  • View the results in PostPDS.
  • To view the tension in the SCable element, navigate to the folder the project is saved in. Open the Results folder, and then the SCable_1 folder. The element tension data is saved in the tensions.dat file. This data can be imported into a plotting tool (such as Octave) to visualize the time history of the element tension. The tension in the SCable settles out at approximately 530 N.

23.6. Comparing simulation results with the Cable DObject

  • Open up a new PST window. Add a Cable DObject to the simulation.
  • Set the state of the cable to node 0 at (0,0,0) m and node N at (1,0,2) m. Set the length of cable to be the exact distance between the points specified - 2.236068 m. Set the cable to have 3 elements.
  • Reference the DCableSegment feature in the Cable input file by adding $CableSegment RopeSegment 2.236068.
  • Ensure node 0 is constrained at the surface by setting $Node0Static 1.
  • Create a PointMass with a density of 7600 kg/m3 and a diameter of 0.25 m. Set $FluidLoadingMode 0, and keep all other properties as their default values.
  • Set the state of the PointMass to a position of (1,0,2) m and zero initial velocity.
  • Connect the Cable to the PointMass with the PointMass as the master and the Cable as the follower. Set $DCableFollowerNodeN 1.
  • Run the new simulation. Notice how the run time is significantly longer than the run time for the SCable. This is because the high frequency axial dynamics resolved by the short element lengths require numerical integration time steps two orders of magnitude smaller than what is required for the SCable.
  • View the results by navigating the cable folder within the results folder of the project.

The tension history of the cable is almost identical to that of the SCable, as seen in Fig. 23.2.

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Fig. 23.2 Comparison in element tension between SCable and Cable DObjects.