External Lids attenuate or suppress the standing waves that form in a gap between hydrodynamically interacting structures. They consist of a flat surface of panel elements, positioned at the mean water level (Z=0 in the XY plane).
Standing waves are a physical effect that is necessary to capture in our hydrodynamic analysis, but the absence of fluid viscosity in the solution means that these waves may become unrealistically large. External Lids are therefore used to model the dampening effect of fluid viscosity on the wave surface. They may also affect the radiation properties of the hydrodynamically interacting structures by removing any non-physical spikes in the radiation damping and added mass around the standing wave resonant frequency.
To illustrate the effect of an External Lid, consider the Hydrodynamic Diffraction analysis of a barge positioned next to a pier. Here are the sway-sway (Y-Y) radiation damping (red) and added mass (blue) values without an External Lid:
Here is the same analysis, now with an External Lid included, where the sharp peaks and troughs in the plots have been reduced:
In this way, External Lids are used to improve the accuracy of the Hydrodynamic Diffraction results and any linked Hydrodynamic Response calculations.
External Lids can be created as Surface Bodies in the geometry editor. However, it may be more convenient to add them directly in the Aqwa Workbench editor — for example, when a Part Transform will be applied, and the transform translates or rotates the structure out of the horizontal plane. This cannot be done with an External Lid which is modeled as geometry because the lid surface must always be positioned on the mean water surface (Z=0 in the XY plane).
To add an External Lid in the Aqwa Workbench editor:
Select a Part in the Outline tree view.
Right-click the Part and select Add > External Lid, or click the External Lid icon in the Parts toolbar.
Select the External Lid object in the Outline tree and set the properties.
An External Lid that is added in the Aqwa Workbench editor may be triangular or quadrilateral in shape. In the Details panel, use the External Lid Surface Definition to specify the points that make up the corners of the surface. Each point can be defined manually, where X and Y coordinates are entered directly or by selecting a vertex from the geometry, with an optional X or Y offset.
The defined points are automatically sorted into anti-clockwise order by the Aqwa Workbench editor. To create a triangular External Lid, set two of the points to share the same location. For more complex shapes, create multiple External Lids on the same Part.
Finally, two External Lid Hydrodynamic Properties must be defined. The first is the Lid Damping Factor, set between 0.0 and 1.0, which represents how effective the lid will be: 0.0 will result in no effect, while 1.0 will prevent any diffracted or radiated component of vertical water surface velocity under the lid. The second parameter is the Gap, which is a representative size for the lid: this is typically the distance between the two interacting vessels or the width of a moonpool. The Aqwa solver uses this value to tune the lid damping properties to the resonant frequency of waves in the gap.