The effect of frequency and amplitude of the OWC (Oscillating Water Column) motion on the nonlinear reaction forces in an air duct are studied experimentally. Experimental OWC model is idealized as a simple circular cylinder with an orifice type air duct located at the middle of the top rid. Reaction forces due to forced heave oscillation are measured and analyzed. By subtracting the effect of inertia forces and restoring forces, pneumatic damping force and added spring force are deduced. The effects of the frequency and amplitude of the heave motion are discussed. Also, the effects of solidity of the duct on the reaction forces are discussed.
During last two decades various ideas have been proposed to utilize wave energy, and the system based on OWC(Oscillating Water Column) concept has turned out to be the most probable candidate for commercialization(McCormick, 1981; Masuda, 1986) because its simplicity of operational principle and easiness of construction. The key component of OWC type wave energy device is the air chamber, which converts the wave energy into kinetic energy of the oscillating airflow by the pumping action of inner free surface. The air flow is driven through a duct to a turbine generator as shown in Fig. 1. A special turbine is usually introduced to pick-up mechanical energy from kinetic energy of oscillating airflow (Hong, 2001). Efficiency of the OWC type wave energy device is determined by the shape of air chamber, the shape of the duct and the type of turbine. In order to design an optimal air chamber, the reaction force induced by the oscillating airflow into the duct should be modeled correctly. The reaction force is considered to be a damping force for a fixed OWC case, because it is in-phase with the velocity of the airflow into duct.
