Learning Forum

[jaevin10]

Hi,

How is the equivalent volumetric displacement calculated in ANSYS AQWA? Or rather, what influences the calculated value? I already checked the normal faces of my geometry to be consistently facing outward so that the actual value is positive but my equivalent value is still off.

[Mike Pettit]

The equivalent volumetric displacement is the total defined mass of the structure (from Point Masses and/or Line Bodies) divided by the water density. How much volumetric displacement are you expecting for your structure?
Cheers, Mike

[jaevin10]

Ah I see, thanks for the info.
I was expecting a value for the equivalent close to the actual volumetric displacement, but numerically its not possible since the total mass is lighter compared to the water density. So I have to adjust the submerged depth of my structure. I was simulating a single floater with the assumption that the submerged part is 50% of its height.
Does the value of actual and equivalent volumetric displacement have to be in range with each other?

[Mike Pettit]

Sorry - another one where the website has not sent me a notification of your reply If the actual and equivalent displacements are not the same, then the structure is not in hydrostatic equilibrium for the Hydrodynamic Diffraction calculation. This may not be a problem, but you should be aware that this may affect the RAOs, and the added mass and radiation damping terms may not be the same as for the structure in its freely-floating position.
You can use the Aqwa solver to find the freely-floating position without having to guess the submerged depth:
On the Project Schematic, add a Hydrodynamic Response stability analysis to the workflow.
In the Aqwa Workbench editor, set the Surface Type for all of the surface bodies to be Non-Diffracting.
Add a Connection Point to the structure at [0, 0, 0].
Solve the Hydrodynamic Diffraction analysis - setting the surfaces as non-diffracting means that the diffraction calculation takes no time to run.
Solve the Hydrodynamic Response stability analysis (with no moorings/environment defined).
Plot the stability analysis Structure Position results for Z, RX, RY with the Connection Point selected as the result Reference Point, where the Final Value for each freedom is the difference between the initial position and the equilibrium (freely-floating) position.
Go back into the Geometry editor and move the structure by these Z, RX, RY values, so that the geometric position is now the freely-floating position.
Re-import the geometry, put the Surface Types back to Program Controlled, and re-solve the hydrostatics to check that the actual and equivalent displacements are now matching.
I hope this is useful!
Cheers, Mike

[jaevin10]

Thanks for the info. Very informative.
I did hand calculations to determine the cut surface plane, but your method seems more efficient and straight-forward. Will try to explore more on this after I'm done with my project.