Experimental study of current forces and deformations on a half ellipsoidal closed flexible fish cage

•Interaction between deformations and drag forces were observed.•The drag force increases for decreasing filling levels.•The increase in drag was caused by a large deformation of the bag front.•For most filling levels and velocities, this deformation was stationary.

Closed flexible fish cages are proposed as a new concept in marine aquaculture, replacing the conventional net cages in order to meet ecological challenges related to fish lice and escapes. It is important to understand the response of the cage exposed to current loads. Then more knowledge about forces and deformations on the closed flexible fish cage for different filling levels is needed. A scaled model of a closed flexible fish cage shaped like a half ellipsoid was tested in a towing-tank. Global drag forces and bag deformations were measured for four different filling levels between 70% and 100%, and steady current velocities between 0.04 m/s and 0.22 m/s in model scale, corresponding to Reynolds numbers in the range Re=3–17×104Re=3–17×104. Findings from the experiments showed that the drag force increased for decreasing filling levels. This increase was caused by a large deformation of the front of the bag affecting the drag coefficient.