Experimental investigation of drag and lift forces on hydroid fouled nets

Marine biofouling is growing on nylon nets used for fish farming and it is a serious problem for the industry. It affects fish health and welfare and gives larger loads to the net pen itself. The hydroid Eructra Larnyx is the most abundant type of biofouling in Norwegian waters. Some of the reasons are that hydroids are very versatile, have a quick life cycle and are reproduced easily. Elimination of the hydroids by high-pressure \textit{in situ} cleaning does not completely remove them, seeing that regenerative parts may remain. Fouling of hydroids on nets will therefore always remain an issue for fish farmers.

The calculation of forces on a fish pen is regulated by the national standard NS9415. Forces on a fouled net are accounted for by increasing the twine diameter of the net by 50%. This thesis researches the forces acting on a bio fouled net section with different solidities and angles of attack. The aim is to find connections of the variables and estimate a functional relationship of drag and lift coefficients on a net. This will give perhaps give justification to the method in use or produce a more reliable coefficient for estimating loads on a bio fouled net.

Experimental investigation of the forces is done by towing tests in the Marine Cybernetics laboratory at NTNU facilities in Trondheim. Clean and fouled twine models are made from two 1.5 mm steel rods twisted together. The artificial hydroids made of 0.32 mm multifilament fishing line is fixed in between. It replicates a 3 mm twine in the net with a hydroid length of 16 mm and a density of 1.4 hydroids/mm. The twines are configured as net panels with a solidity of 0.28, 0.237 and 0.19. They are tested for angles of attack of 0, 10, 20, 30 and 45 degrees and velocities of 0.05, 0.1, 0.2, 0.25, 0.3 and 0.35 m/s. It is of interest to look at low Reynolds number since similar experiments have not done so. The uncertainty of the measurements is based on repeated tests and calculated by student-t distribution.

Both drag and lift on nets with biofouling are amplified significantly due to the presence of hydroids. The dependency of the angle of attack and solidity is clear. Drag measurements are validated as coefficients of the clean net panel is similar to the estimation by Løland (1991). Lift on the other hand has too many negative values and large uncertainty which gives diverse results. The drag coefficients on fouled nets are defined as independent of Reynolds number for the tested range of 300 - 1100. The dependency on both solidity and angle of attack is greater for fouled nets. It appears that the deformation of the hydroids on net panels is greater for larger angles of attack. In addition, the extra projected area from the hydroids is increasing with solidity, which escalates the drag forces with increasing solidity. Since the lift coefficient on clean nets does not follow the expected trend, it is unknown if the results are due to a bias error for lift measurements or a phenomenon of the hydroids.

An update to assure technical approval for fouled nets in NS9415, is a new formula for the drag coefficient based on the solidity and angle of attack.