Pressures induced by regular waves on a large scale OWC

Abstract

The traditional function of coastal structures is the protection of the land from the sea. Such a function has been often carried out by breakwaters which dissipate part of the incident wave energy.Recently, the ongoing climate changes and their related issues have moved the attention toward new kinds of coastal structures which can protect the land and human activities and at the same time harvest part of the available wave energy. Such an energy source is alternative to non renewable ones like oil and gas, it is covered by the incentives of the renewable energy sector and it is subjected to an increasing interest of the international community.In such a framework, the present challenge of the coastal engineering is to design hybrid Wave Energy Coverters (WECs) which are: (i) effective in protecting the land, (ii) efficient in extracting wave energy, (iii) economically competitive with the conventional structures, (iv) structurally reliable.In comparison to other WECs, the Oscillating Water Column (OWC) breakwaters are considered among the most promising, therefore it is needed to investigate their design characteristics appropriately. Indeed, the durability of hybrid OWC breakwaters is strongly related to their response to wave action. In particular, the most critical point is the front lip, which is subjected to external wave impact and internal pressures within the chamber, at the same time.The complexity of wave-OWC interactions is studied here by means of a large scale physical model, which has been tested under regular wave conditions. The Power Take Off (PTO) characteristics are replaced here by a circular orifice, whose section has been varied. Such variations of the orifice section cause different types of response of the air inside the pneumatic chamber to external wave forcing.The response of the device under regular waves has been monitored by means of wave gauges and pressure sensors located both at the OWC and along the flume. First, the time evolution of the air pressure signal is analyzed. This is strongly affected by the air compressibility which is the key parameter in this analysis, since it controls the air-water interaction and it influences the whole OWC dynamics.The spectral analyses of the signals of pressure and free surface elevation provide insights on the relationship between water column oscillations and pressures inside and outside the chamber, in terms of both amplitudes and phase lags. It is found that the relation between inner and outer pressures is shown to be related to the resonance coefficient, which is a function of the lag between the flow across the OWC and the forcing outer pressure.