Reply To: AQWA Eigenvalue vs Free Decay Test Resulting in Different Heave Natural Period?

[Mike Pettit]

Hi Danu,

No problem, I'm happy to help.

Just to clarify, Aqwa has two types of time domain solver:

• when the 'Analysis Type' is 'Irregular Wave Response with Slow Drift' or 'Slow Drift Only', the calculation includes drift forces, but only uses linear hydrostatic/incident/diffracted/radiated wave pressures;
• when the 'Analysis Type' is 'Irregular Wave Response' or 'Regular Wave Response', the calculation does not include drift forces, but the hydrostatic and incident wave pressures are calculated under the instantaneous water surface (diffracted/radiated pressures are still linear).

To answer your questions:

1. Ideally you should model at least some of the structure above the waterline, unless you are only going to run a Hydrodynamic Diffraction analysis. In the diffraction calculation the solver will ignore any non-diffracting panels, so they do not make a difference to the calculation time. Otherwise, in your Hydrodynamic Response analysis the solver will assume that the cut waterplane area is constant above the cone as it becomes fully submerged.
2. This is what the convolution method does for us - rather than using frequency-dependent added mass/radiation damping terms in the equation of motion, instead it calculates the acceleration impulse function matrix for the structure, then we have:

Where m is structural mass; A_inf is added mass at infinite frequency (estimated by Aqwa); X is position, X˙ is velocity, X¨ is acceleration; c is damping excluding radiation damping; K is stiffness; F is the total force; h is the acceleration impulse function, which is calculated by Aqwa from the frequency-dependent added mass/radiation damping.

If you turn off convolution, the Aqwa solver will use an RAO-based method to calculate radiation forces, then you will find that there is no damping (because you have defined no wave).

I hope this answers your questions.

Cheers, Mike