Experimental study of the behaviour of a circular gravity cage in linear waves

Experiments are conducted in a flume to study the motions and the wake behind a circular gravity cage under linear waves. The gravity cage model is built with high-density polyethylene pipes and nylon nets. Wave heights and periods are measured by wave gauges both upstream and downstream. Supports are used above the flume to fix the gauges and to connect the cage model. It is found that the cage behaves differently under short and long waves. The relative motion between the floating cage and the water surface is large under short waves. It can be observed clearly how the short waves pass over the collar. On the other hand, the collar just rides on long waves without apparent relative motion. The wave height is reduced when short waves pass the cage, while the wave height reduction under long waves is too small to be measured. The effect of the cage diameter, the collar cross-section, the bottom weight and biofouling is also studied. Generally speaking, larger and heavier cages have a greater impact on waves.