Forced heaving motion of a floating air-filled bag

A proposed wave energy converter related to that described by Farley (2011) consists of a pressurised axi-symmretic air-filled bag, ballasted to float at about half depth. The bag contracts and expands under the action of wave-induced heaving motion, pumping air into and out of a secondary, rigid, chamber which acts as an air spring.
This paper describes experiments and numerical modelling aimed at understanding the behaviour of this device. Rather than test it in waves, in this initial investigation we chose to replace the power take-off with an oscillating air pump. Forcing air into and out of the bag periodically caused it to heave and radiate waves when floating in water initially at rest. Measurements of the bag’s response are compared with the predictions of a linear frequency-domain radiation theory, which uses a finite difference approach to model the harmonic deformations of the bag. Agreement is promising on the whole, providing some insights into the likely performance of the device when operating as a wave energy converter.