Numerical analysis of the influence of air compressibility effects on the oscillating water column water energy converter chamber

Abstract

The most studied device used for extracting wave energy is the Oscillating Water Column (OWC). In general, numerical simulations of these cases by means of models based on Reynolds-Averaged-Navier-Stokes (RANS) equations adopt the Volume of Fluid (VoF) method to take into account the water and air whose flows are incompressible. The aim of this study is to investigate the influence of the compressibility effect on the air inside the chamber by means of FLUENT® numerical model. Both water and air flows are still considered incompressible, but, at every instant, a pressure based on an analytical equation that takes into account the turbine characteristic relation (kt) of a Wells turbine and the isentropic transformation of the air is imposed on the top boundary of the chamber. Incident waves with period of 9 s and 1 and 2 m high are imposed in a flume 10 m deep with an onshore OWC at its end (kt = 240 Pa s m-3). Results show differences between incompressible and compressible air around 12% in the air pressure amplitude inside the chamber and about 21% in the average pneumatic power.