70. Ballast tank feature

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

70.1. Tutorial overview

This tutorial covers:

  • Creating a ballasting system using the ballast tank feature
  • Simulating a submerging body

70.2. Creating the rigid body and ballast tank feature

  • In a new project create a new Rigidbody DObject called Body.

Note

  • This tutorial will model a floating body that will change it’s mass over time by flooding with ballast water and eventually submerging.
  • The ballast tank creates a volumetric cuboid of water that changes in size and mass through time.
  • Set the mass and inertia of the rigid body to be 2562.5 kg and 1000 kgm :sup:2 respectively.
  • Create a cuboid feature with an X and Z length of 1 m and a Y length of 5 m. Leave all loading flags on and leave all drag and added mass coefficients as the default values.
  • Add the cuboid feature to the Body rigid body. Leave the feature positioned at (0,0,0) m and (0,0,0) deg.

Note

  • A ballast tank feature will be created with the same dimensions and orientation as the cuboid feature.
  • Create a new ballast tank feature and add it to the Body rigid body.

Note

  • In the ballast tank feature, the $Lx and $Ly properties set the X and Y length of the tank.
  • Set the $Lx property to 1 m and the $Ly property to 5 m.

Note

  • The $MaxFillLevel property sets the Z length (or height) of the tank.
  • Set the $MaxFillLevel property to 1 m.

Note

  • The $FillSchedule property sets the fill level of the tank at the prescribed time.
  • Set the fill schedule to match the properties provided below:
// Uncategorized properties
$Lx 1
$Ly 5
$MaxFillLevel 5
$FillSchedule 0 0
$FillSchedule 20 0
$FillSchedule 40 100
$FillSchedule 60 50
$FillSchedule 80 50
$FillSchedule 100 0

Note

  • The fill schedule is set so that the simulation starts with an empty ballast tank, resulting in a floating body. At 20 seconds the tank will begin to fill and at 40 seconds the tank will be 100% full. At 40 seconds the tank will begin deballasting until it is 50% full at 60 seconds. The tank will then remain at 50% full from 60 seconds to 80 seconds. At 80 seconds the tank will deballast until it is empty at 100 seconds.
  • Leave the ballast tank feature located at (0,0,0) m and (0,0,0) deg.

Note

  • The ballast tank feature location refers to the geometric center of the tank when full. For this example it is 0.5 m in X, 2.5 m in Y, 0.5 m in Z from the bottom corner. This is similar to the cuboid feature.
  • It is often helpful to use a cuboid feature with all loading turned off and the same dimensions of the ballast tank to help visualize the location of the ballast tank. This is not necessary in this example as the ballast tank is coincident with the entire body.
  • It is important to note that the ballast feature does not calculate free surface. The water volume will only increase and decrease along the feature’s Z axis. Therefore, the user must ensure that the Z axis of the ballast tank feature is vertical even if the Rigidbody DObject Z axis is not. If the feature rotates throughout the simulation, the water will still change only in the feature Z direction, not the global Z direction.
  • The mass of the water in the ballast tank is completely independent of the mass of the rigidbody. The ballast tank will determine what the correct mass, CG, and inertia is for all the water in the tank. For the rigidbody, the user must use mass properties that represent the body when it is completely empty of ballast water. Then the ballast tank feature will add the mass of the ballast water.
  • The ballast tank uses the water density value provided in the env.ini file. It is impossible to add higher density fluid (or just change the mass without changing the CG or inertia).
  • Leave the Body rigid body state at (0,0,0) m and (0,0,0) deg.
  • Set the simulation to run for 140 seconds and run the simulation.

70.3. Visualizing and plotting the results

  • Visualize the results in PostPDS. Ensure that the tank behavior matches what you would expect.