QA of hydrodynamic coefficients

There are two main methods, in addition to visual plot comparison, to quality assure the hydrodynamic coefficients of a SIMO-model in SIMA. These are described below.

1. Motion animation

To quality assure the hydrodynamic coefficients, SIMA can show a 3D animation of all the bodies with hydrodynamic coefficients, and how they move with a regular wave. You can tune the environment (amplitude, period and direction) and confirm the motion animation with your expectations. The animation will choose the proper coefficients based on the body type.

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To use motion animation:

  • Open (or create) a SIMO task that has at least one body with hydrodynamic coefficients.

  • Open the 3D view for the task (by right-clicking it in the navigator window, and selecting Open 3D View.

    • SIMA shows the 3D-view, with the Initial condition and Modeled result type, as seen in the dropdown-menus above the 3D view.

  • Select (from the Active condition dropdown-menu above the 3D-view) Motion animation. (It will have the value Initial before you change it.) image::motion_animation_dropdown.png[image]

    • SIMA shows the 3D-view with a (implicit) filter that makes only the following objects visible:

      • All bodies with hydrodynamic coefficients

      • The sea surface (as a regular wave)

    • SIMA automatically starts an animation that shows the movements of the bodies for the specified regular wave.

    • SIMA opens an editor window Motion Animation Parameters below the 3D-view, that displays the properties for the regular wave: Amplitude, period and direction, plus (model) time step.

  • Change the values for the wave parameters, and (visually in the 3D-animation) verify expected behavior. The 3D-animation will be updated instantly when you change the parameters.

If you close the 3D-view, or select another Active condition in the dropdown list, the window Motion Animation Parameters showing the properties for the regular wave will be closed. Because of this, the window Motion Animation Parameters does not need to be closed when you finish, as you simply close the 3D-view (or select another active condition) instead.

The motion animation is applicable for two body types: 6 DOF - time domain and 6 DOF - separated analysis.
For bodies of type 6 DOF - time somain you will need these properties:

  • First Order Wave Force Transfer Function

  • Structural mass

  • Hydrostatic stiffness data

  • Frequency dependent added mass and damping coefficients

  • Optional: Linear Damping and Quadratic Damping

For bodies of type 6 DOF - separated analysis you only need First Order Motion Transfer Functions

2. Compare (and re-calculate) retardation function

To further quality assure the hydrodynamic coefficients, you can compare the added mass and damping (that was an input to the body) with the calculated frequency dependent added mass and damping from the (calculated) retardation function. The comparison is in a 2D-plot, to see the numerical noise and rounding errors that are introduced by the calculations.

You can also discover potential bad WAMIT-input (typically for short wavelengths), and any bad time step in the retardation function calculation.

You can trigger a re-calculation of the retardation function, and re-enter the parameters for this.

2.1. Steps to trigger comparison

  • Open (or create) a SIMO task that has at least one body with the hydrodynamic coefficients.

  • In the Navigator, expand a body’s Kinetics-folder, and the sub-folder Radiation Data, and double-click either Frequency Dependent Added Mass or Frequency Dependent Damping to show the plot.

  • Click the button Compare with recalculation from retardation function

    • SIMA shows a compare-plot, and shows (in the legend) what is the current value, and what is the recalculated value.

2.2. Steps to trigger re-calculation of retardation function

  • Open (or create) a SIMO task that has at least one body with the hydrodynamic coefficients.

  • In the Navigator, expand a body’s Kinetics-folder, and the sub-folder Radiation Data, and double-click Retardation Function to show the plot.

  • Click the button Recalculate.

    • SIMA will open a wizard for entering new values (for generating retardation function).