Learning Forum

[xinyuan.shao]

Hi,

 

It is written in Aqwa Naut manual that Aqwa Naut is able to calculate nonlinear hydrostatic and FK force over instantaneous wetted surface at each timestep. 

I wonder how the instantaneous wetted surface is calculated. Is it relevant to the meshing of the above-water structure? I have varied the mesh density for my structure above -the-water part, however, it seems that the accuracy of the hydrostatic force is not affected even with really coarse mesh (see the second and third pictures).

Thank you!

/Xinyuan

[Shuangxing Du]

Aqwa Naut calculates the incident wave elevation at each time step, then the hull surfaces beneath the incident wave surface will be found based on the actual location and orietation of the structure, the Froude-Krylov force/moment and hydrostatic force/moment are calculated from the integration of the incident wave pressure and hydrostatic pressure over the instantaneous hull surface beneath the incident wave elevation. If a panel is partially under the incident wave surface, only the submerged part of that panel will be used in the F-K force and hydrostatic force calculation. Please read the Aqwa theory manual, Section 13.6.1 Extended Wheeler Stretching Method and 13.6.2 Second Order Correction of Linear Irregular Waves.  The mesh sizes will affect the final results depending on the complixity of the geometric property of the model.  Please check the summation of the Froude-Kryloc force and the hydrostatic force, which may provide more evident nonlinear behaviour.

[xinyuan.shao]

Hi,

Thank you for the quick reply! I still have a few doubts.

1) In the following figure, the force is the 'hydrostatic force only' output from Awqa in the vertical direction. I do not understand why the vertical hydrostatic force can be negative. Do you know the reason?

 

2)  You said, "If a panel is partially under the incident wave surface, only the submerged part of that panel will be used in the F-K force and hydrostatic force calculation." Does it mean that the nonlinear F-K and hydrostatic can be computed with satisfactory accuracy as long as the above-water mesh can describe the geometry properly? Can one single panel element be subdivided into a submerged part and an above-water part for pressure integration?

Thank you!

/Xinyuan

[Shuangxing Du]

(1) Please read the Aqwa theory manual, Section 13.6.1 Extended Wheeler Stretching Method and 13.6.2 Second Order Correction of Linear Irregular Waves. The nonlinear hydrostatic pressure is defined as -rhi*g*Z0, where Z0 is the actual vertical location of the point on the submerged hull surface in the fixed reerence axes. Z0 can be positive or negative, hence the integration of the hydrostatic pressure over the instantaneous submerged hull surface, which it is hydrostatic force, could be negative or positive.

(2) Yes, in Aqwa-Naut, the nonlinear F-K and hydrostatic can be computed with satisfactory accuracy as long as the above-water meshes can describe the geometry properly. At each time step,  any hull surface panel element is automatically subdivided into a submerged part and an above incident wave surface part for the pressure integration if that panel is water-piercing.