Forces on a cruciform/sphere structure in uniform current

• Model experiments have been conducted in a towing tank with a cruciform structure.
• The cruciform (with center sphere) resembles a twine/knot in a net structure.
• Forces are measured individually on the different parts of the cruciform.
• The drag force on the cruciform sphere is higher than on a single sphere.

Experiments have been conducted on a cruciform structure in uniform flow. The cruciform consists of two intersecting cylinders and a center sphere and is assumed to mimick two twines (cylinders) that meet in a knot (sphere). The experiments are motivated by the need for a better understanding of drag forces acting on net based structures such as fish farms. The cruciform model was designed so that is it was possible to isolate the forces acting on the separate cylinders from the forces acting on the sphere only. It was found that the cylinders in the cruciform experienced a drag coefficient similar to established measurements of a cylinder in uniform flow (Cd≈1.0). The sphere, however, experienced a drag coefficient that was between two and three times higher (Cd in the area 1.1 to 1.6) that what previously have been found for a single sphere in uniform flow (Cd≈0.5).