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Wave energy is a renewable source with significant amount in relation to the global demand.A good concept of a device applied to extract this type of energy is the onshore oscillating water column wave energy converter (OWC-WEC).This study shows a numerical analysis of the diameter determination of two types of turbines,Wells and Impulse,.installed in an onshore OWC device subjected to a hypothetical sea state.Commercial software FLUENT(R),which is based on RANS-VoF (Reynolds-Averaged Navier-Stokes equations and Volume of Fluid technique),is employed.A methodology that imposes air pressure on the chamber,considering the air compressibility effect,is used.The mathematical domain consists of a 10 m deep flume with a 10 m long and 10 m wide OWC chamber at its end (geometry is similar to that of the Pico's plant installed in Azores islands,Portugal).On the top of the chamber,a turbine works with air exhalation and inhalation induced by the water free surface which oscillates due to the incident wave.The hypothetical sea state,represented by a group of regular waves with periods from 6 to 12 s and heights from 1.00 to 2.00 m (each wave with an occurrence frequency),is considered to show the potential of the presented methodology.Maximum efficiency (relation between the average output and incident wave powers) of 46％ was obtained by using a Wells turbine with the diameter of 2.25 m,whereas the efficiency was 44％ by an Impulse turbine with the diameter of 1.70 m.