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In this paper we investigate the dynamics of the inertial wave energy converter (ISWEC) device using fully-resolved computational fluid dynamics (CFD) simulations. Originally prototyped by Polytechnic University of Turin, the device consists of a floating, boat-shaped hull that is slack-moored to the sea bed. Internally, a gyroscopic power take off (PTO) unit converts the wave-induced pitch motion of the hull into electrical energy. The CFD model is based on the incompressible Navier-Stokes equations and utilizes the fictitious domain Brinkman penalization technique to couple the device physics and water wave dynamics. A numerical wave tank is used to emulate realistic sea operating conditions. A Froude scaling analysis is performed to enable two- and three-dimensional simulations for a scaled-down (1:20) ISWEC model. It is demonstrated that the scaled-down 2D model is sufficient to accurately simulate the hull's pitching motion and to predict the power generation capability of the converter. A systematic parameter study of the ISWEC is conducted, and its optimal performance in terms of power generation is determined based on the hull and gyroscope control parameters. It is demonstrated that the device achieves peak performance when the gyroscope specifications are chosen based on reactive control theory. It is shown that a proportional control of the PTO control torque is required to generate continuous gyroscope precession effects, without which the device generates no power. In an inertial reference frame, it is demonstrated that the yaw and pitch torques acting on the hull are of the same order of magnitude, informing future design investigations of the ISWEC technology. Further, an energy transfer pathway from the water waves to the hull, the hull to the gyroscope, and the gyroscope to the PTO unit is analytically described and numerically verified.