Simultaneous advancements in high-performance computing technologies (HPC) and fluid dynamics science have set the stage for practical computational fluid dynamics (CFD) modelling of complex real-life problems including fluid-structure interaction. The presented research summarizes the capabilities of the OpenFOAM CFD toolbox to facilitate design optimization of an innovative tidal energy device. Numerical simulations were conducted for different environmental conditions and operating scenarios to characterize the flows through the device. The influence of changing the number of turbine blades as well as their submergence depth in the incident flow was tested through a series of 2D numerical simulations which consider the dynamics of the rotary component of the system. The results of the simulations were used to investigate potential strategies for design optimization, ultimately improving efficiency.