Let’s consider this from a hydrodynamical perspective. Conventional barriers such as nets have openings allowing water to flow through more or less unhindered.

Introducing a lice skirt, the water particles are hindered from flowing through, and must pass on the sides of the barrier, or below. Waves that interact will be accompanied by diffraction and deformation of the lice skirt. Diffraction is the process where waves spread perpendicular to the direction of wave propagation. These effects will cause an increased pressure on the upstream side, and the lice skirt will be deformed. This is commonly known as hydroelasticity – a time-dependent effect, where structures deform under the influence of fluids.

The pressure-increase on the upstream side of the skirt will be transferred to the cage, and in turn lead to increase loads in the anchoring system. This is something to be aware of when planning an anchoring system at a fish farm facility.

Modeling principles

To model lice skirts in AquaSim, one can apply load formulation Lice skirt for the component type Membrane X (or Membrane).

AquaSim assumes that lice skirts always are modelled as a cylinder. Which membrane panels are upstream of waves and current, and which panels are downstream are kept track of. By this, AquaSim is able to distribute the pressure field around the lice skirt in a realistic manner. To illustrate this pressure field, let us simplify the problem and consider fluid flow around a cylinder. Upstream it will experience pressure drag force, and downstream suction drag. On the upper and lower sides, the cylinder will experience lift forces.

A consequence of AquaSim being able to keep track of what is upstream and downstream, is that each lice skirt must be modelled as its own component group. If you for example have a model of an aquaculture facility with 2 by 2 cages, you will need to model the lice skirts in 4 individual component groups. This is illustrated below.

Inside and outside definition of lice skirt

When working with models with enclosed volumes it is important to keep track of what is considered the inside and outside of the volume. This is because AquaSim facilitates defining hydrostatic- and dynamic parameters for the fluid within the enclosed volume and outside.

When considering the local coordinate system of a membrane panel, the normal vector (blue line) should point into the interior of the volume. This is indicated in the figure below.

Another useful tool to verify the orientation of the panel normals, is to apply the function Membrane side found in AquaEdit. Where the side that faces out towards the external fluid will be colored with a shade of blue, and the sides facing towards the interior will be colored with a shade of red. This is illustrated below.