Determination of damping coefficient experimentally and mathematical vibration modelling of OWC surface fluctuations Anıl Çelik * , Abdüsselam Altunkaynak Istanbul Technical University Faculty of Civil Engineering, Hydraulics and Water Resource Engineering Division, Maslak, 34469, Istanbul, Turkey article info Article history: Received 23 January 2019 Received in revised form 12 September 2019 Accepted 21 September 2019 Available online 24 September 2019 Keywords: Oscillating Water Column Wave energy Physical experimental model Mathematical vibration model Free surface fluctuations Damping coefficient abstract Water surface fluctuations inside the chamber of an Oscillating water column (OWC) type wave energy converter (WEC) are very important since they are the conveying processes in conversion of wave energy to electricity. In this study, a mathematical vibration model is developed to estimate the water surface average fluctuations in the chamber and the related phase angles. Resistive forces against the motion of the water column in the chamber are represented by introduced damping coefficient in the equations and determined experimentally by a novel way that is not present in the literature. A particular relative opening height of the chamber is revealed that provides minimum damping which in turn maximizes the highest average chamber water surface fluctuation value regardless of the incident wave parameters. A mathematical vibration model is developed to simulate the water surface fluctuations inside the chamber under different wave conditions and chamber opening heights. Physical experiments were performed to validate the mathematical vibration model results. It is observed that a good agreement exists between the physical experimental data and the mathematical vibration model results. © 2019 Elsevier Ltd. All rights reserved. 1. Introduction Increasing global energy demand is being supplied widely by fossil fuels which have negative effects on the environment, yet, is still cost effective. Humanity must scale up and diversify the forms of renewable energy sources to compete with fossil fuels by heading towards to untapped resources. For this reason, in a world covered with oceans more than 71%, it is not surprising that ocean renewable energy sources have had great interest. Among the ocean energy sources, almost untapped wave energy has been trying to find its way to help humanity for their great challenge. At this stage, efforts for exploiting the wave energy ended up more than one thousand patents throughout the world [1]. The Oscil- lating Water Column (OWC), which has the simplest working principle among others, is one type of many wave energy converter systems. Its accessibility, adaptability and easy to construct features with having no moving parts and power transmission cables under water (land fixed OWC) have attracted the researchers, engineers and investors. Furthermore, for cost sharing purposes, OWC device can be integrated into breakwaters to absorb a part of the incident wave energy. Accordingly, OWCs have been one of the most studied wave energy converter types which led them to the prototype stage of deployment. OWC technology takes advantage of the oscillating dynamic pressure under an incident wave that acts on a water column inside a partially submerged hollow chamber through a seaward opening. The oscillatory motion of the water column forces the trapped air above it to exit the chamber from a narrow duct at the back or top of the system [2]. Pioneering efforts to understand the hydrody- namic characteristics of the OWC device started with theoretical studies that were performed by Refs. [3e6] with others. While first investigations assumed a uniform pressure distribution over the chamber water surface [7], carried out studies that allowed simu- lating non-uniform pressure distributions over the water column surface area. By applying linear wave theory to study the hydro- dynamics of a two-dimensional OWC device [8], found optimal pneumatic damping to obtain the highest efficiency with both linear and non-linear power take-off systems considering air compressibility effects [9]. developed a theoretical model using potential theory to investigate the hydrodynamic efficiency of an OWC device interacting with incident waves. The submergence of front wall and the width of the chamber were found important for the conversion efficiency. [10] examined the hydrodynamic performance of an OWC type * Corresponding author. E-mail addresses: celikani@itu.edu.tr (A. Çelik), altunkay@itu.edu.tr (A. Altunkaynak). Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene https://doi.org/10.1016/j.renene.2019.09.104 0960-1481/© 2019 Elsevier Ltd. All rights reserved. Renewable Energy 147 (2020) 1909e1920