Learning Forum

[ElisaD]

Hi!

Does anyone know how to perform a decay test with Ansys AQWA? I have just read on the AQWA theory manual (Chapter 14: Extended functionalities) that:

"The damping property for a specified degree of freedom of a floating structure can be estimated using a numerical twang test record. The structure center of gravity is shifted by a distance along the specified degree of freedom from its equilibrium position in still water at time. A time history of its freely-decaying response can then be calculated by an Aqwa time domain analysis."

but I am afraid that aqwa-librium is needed, which is not available with my licese (I only have access to Hydrodynamic Diffraction and Hydrodynamic Time Response)

[pjschofi]

The decay test is a time history response analysis. It is normal to choose Irregular Wave Response With Slow Drift (Irregular Wave) and then provide an initial position to provide the offset of the particular DOF that you require the decay test for. Note that the ability to run the Aqwalibrium solver is available by selecting Stability Analysis as the response type in the Hydrodynamic Response system, but this is not required for what you are trying to achieve.

[ElisaD]

Thank you for answering, pjschofi, but still I can't understand how a decay test could be performed in the Hydrodynamic Response Module selecting the "Time Response Analysis" in "Analysis Settings".

In fact, a decay test is performed by displacing the floating body (a buoy, in our case) from its equilibrium position and then releasing it, without the presence of any wave.

So, as a first thing, if I choose "Irregular wave response with slow drift", an irregular wave spectrum is required for the analysis. Moreover, the only options that can be choosen for the "starting position" are "program controlled" or "based on geometry", so it is not possible to specify any desired position.

Could you please explain it better? Thank you very much!

[pjschofi]

Choosing Irregular wave response with slow drift does not actually require a wave spectrum to be defined, you can run that type of analysis with no environment. And while I agree that the only two options for Starting Position is initially program controlled or based upon geometry, you will find that you can insert a Starting Position for the HR system when it is connected to the HD system directly. Including this allows you to displace the floating body in one or more DOFs. A connection point associated with the body is used as the reference point for the offset.