Analysis

Now that you are introduced to the different parameters and what they mean, we should run an analysis and then have a look at key result parameters in AquaView.

A load condition is prepared, go the Export and the tab Normal. A wave amplitude of 1.4 meters and -period of 4 seconds is applied. Also, current along x-direction is used.

Since we are working with Membrane X, remember to check that Membrane normals are verified is activated. This option ‘lock’ the orientation of the membrane normals. If this is not selected, you risk that AquaSim automatically change the orientation.

Export the model and start the analysis. This analysis will take 5-8 minutes to complete, depending on your computer capabilities. We named the analysis liceskirt_.

Post processing

Load the result-file, the one that ends with .avz. If you have not run your own analysis, you may open the liceskirt_01.avz that is associated with this tutorial.

Impermeable nets, such as lice skirts are in AquaView viewed as a membrane but with a darker rendering.

When you apply Lice skirt in your model, some additional results are available in AquaView. They are found under Result > Impermeable net. Here you can view different forces that is relevant for impermeable nets. Select Result > Impermeable net > Relative pressure [mH2O]. This is the hydrodynamic pressure difference that acts around the skirt due to waves and current. Meaning the pressure on the outside surface minus the pressure on the inside surface. This option can therefore indicate how strongly the lice skirt is being pushed by fluid flow.

From the results, it is observed on the upstream side a region of high negative pressure (blue region). But further down in the water column, the pressure increases (green region). The blue region may be due to stagnation of the flow, then it starts to accelerate and try du “escape” underneath the skirt leading the fluid to be accelerated.

The largest relative pressure is seen on the sides, in the region where the bridles are attached. This is where the flow accelerations are highest. On the downstream side the flow separates and forms a wake. In this region negative (suction) pressure is created.

Comparison models with and without lice skirt

One interesting point is to investigate the effect the lice skirt has on response of the aquaculture cage and force distribution in the anchor lines. In this section we shall present and illustrate the effect lice skirt has.

Load the AquaSim model Net.amodel. This model is identical to LiceSkirt.amodel, only that the load formulation of the lice skirt is changed from Lice skirt to Regular net. This will result in a conventional net that is permeable.

Select Export and the tab Normal. If you compare the two models, you may see that the load conditions are identical. With a wave amplitude of 1.4 meters, period 4 seconds and current along x-axis 0.5m/s.

Export the model and run an analysis. We named the analysis net_, and will result in an avz-file that is named net_01.avz. When the analysis is finished, open this file.

You may have the liceskirt_01.avz and net_01.avz open parallel to compare more easily.

Response and deformations

The two models are presented side-by-side in the figure below.

You can see that introducing a lice skirt significantly changes the response of the cage. With a regular net (to right in the figure above), the flow is almost free to pass through the net. This results in moderate deformations of the cage; it remains close to circular.

When the lice skirt is introduced (to left in the figure above), it acts as a solid barrier. The effect of pressure drag upstream, and suction drag downstream, result in much larger deformations. The deformations are also asymmetric compared to when only having a permeable net.

You may also see that the response of the floating collar is somewhat slower and more rigid in the case with the lice skirt. This is due to the trapped water inside the lice skirt – but also the water on the outside – that must be accelerated in order for the floating collar to displace.

Forces in the floating collar and anchor system

Comparing von Mises stress in the floating collar, one can clearly see in the figure below that the introduction of a lice skirt has increased the load on this construction part.