Numerical analysis of the influence of air compressibility effects on the oscillating water column wave energy converter chamber
Rafael Adriano Alves Camargo Gonçalves1; Paulo Roberto de Freitas Teixeira1; Eric Didier2; Fernando Ramos Torres1
1 Federal University of Rio Grande; 2 Laboratório Nacional de Engenharia Civil - LNEC
doi:10.20906/CPS/CILAMCE2017-0456
Resumo
The most studied device used for extracting wave energy is the Oscillating Water Column (OWC). It consists of a chamber partly submerged in which there is an opening below the water surface. Air is trapped above the water free surface inside the chamber. Incident waves cause oscillation of the free surface inside this chamber, compress and expand the air and force it to flow through a turbine that drives an electrical generator. 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. Therefore, the inherent thermodynamic transformation inside the chamber is not considered, leading to errors in the solution. The aim of this study is to investigate the influence of the compressibility effect on the air inside the chamber. This task is very difficult because, even if some RANS models are able to consider the air flow as compressible, they demand high computational cost, due to the addiction of the energy equation and the drastically decrease in time step. To solve this difficulty, the authors proposed an alternative approach: both water and air flows are still considered incompressible, but a methodology is used for imposing, on the top boundary of the chamber, 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. The FLUENT® numerical model is employed and User Defined Functions (UDF) are used for imposing wave generation and Wells turbine effect on the air chamber top boundary. The case study consists of a flume 10 m deep, where incident waves with period of 9 s and 1 and 2 m high are imposed, with an onshore OWC at its end. The turbine characteristic relation is 240 Pa s m-3. In order to quantify the influence of the air compressibility inside the chamber, time series of free surface elevation, air pr
Palavras-chave: Numerical simulation; Wave energy; Oscillating water column; Computational Fluid Dynamics