Cylindrical frontwall entrance geometry optimization of an oscillating water column for utmost hydrodynamic performance

Abstract

The present research focuses on optimizing the diameter of the cylindrical frontwall entrance shape of an Oscillating Water Column (OWC). In particular, 240 experimental runs are performed for five diameter values, various PTO dampings, dimensionless wave frequencies (Kh) and wave heights. The cylindrical front wall entrance geometry is found to improve the Capture Width Ratio (CWR) of the Oscillating Water Column (OWC) under all conditions. The maximum and average CWR improvement are calculated as 45% and 25%, respectively. The simplicity of the front wall entrance modification makes this achievement even more remarkable. A negative correlation is identified between the diameter size and Kh. In particular, for relatively low, intermediate and large Kh values, optimum diameter sizes are found as 12 cm, 7 cm and 5 cm, respectively. In a specific frequency band, diameter that makes CWR maximum is also found as a function of wave height and orifice ratio. Free decay experimental tests are performed to verify the superiority of the cylindrical geometry. Experimental results indicate that cylindrical frontwall entrance geometry prevents or at least diminishes the flow separation that occurs due to sharp frontwall underlip. Consequently, the reduction in diameter size helps to alleviate the shear stresses on both sides of the front wall, thereby improving the structural integrity of the Oscillating Water Column (OWC) chamber.