Analysis of the Geometric Constraints Employed in Constructal Design for Oscillating Water Column Devices Submitted to the Wave Spectrum through a Numerical Approach

Abstract

This paper aims a numerical investigation about the fluid dynamic behavior of an oscillating water column (OWC) wave energy converter (WEC) into electrical energy. Constructal design is employed to perform a geometric evaluation of an OWC WEC submitted a Pierson-Moskowitz wave spectrum. The objective function is to maximize the energy conversion. The hydropneumatic chamber volume (VHC) and the total OWC volume (VT) are adopted as geometric constraints. In the first stage, the values are constant during the maximization process. However, in a second stage they are changed according to the constraint variation (CV). One of the goals is to analyze the influence of the choice of this geometric constraints value on the OWC performance in relation to the wave spectrum. For this purpose, are considered three different scenarios: 1) VHyd is equal to the minimum incident wavelength (λmin), that is relative to the maximum frequency of the wave spectrum times the significant wave height (HS); 2) VHyd is equal to the peak incident wavelength (λpeak), that is relative to peak frequency of the wave spectrum times the significant wave height (HS); and 3)VHyd is equal to the maximum incident wavelength (λmax), that is relative to minimum frequency of the wave spectrum times the significant wave height (HS). To do so, constructal design is employed varying the degree of freedom (DOF) H1/L (ratio between the height and length of OWC chamber), while the others DOF’s H2/l (ratio between height and length of chimney) and H3 (lip submergence), are kept fixed. It is employed a Pierson-Moskowitz wave spectrum with significant period (TS) equal to 7.5 s and significant wave height (HS) equal to 1.5 m. For the numerical solution it is used the computational fluid dynamic (CFD) code, based on the finite volume method (FVM). The multiphase volume of fluid (VOF) model is applied to tackle with the water-air interaction. The computational domain is represented by the OWC WEC coupled with the wave tank. The results showed that when CV = 2.25 for λmax and (H1/L)O = 0.2152 the highest average for power was obtained, nearly 18,000 W. While for λmin and (H1/L)O = 0.2193 it was smaller than 1,000 W. Besides, it was obtained a theoretical recommendation about the geometric constraints employed for the constructal design application, aiming the maximization of the OWC energy conversion from the incident wave spectrum.