Tidal turbines represent a new frontier for extracting energy from a huge potential of renewable source such as the tides. Despite the technology being mature enough, new solutions aimed at improving the machines efficiency with reduced environmental impact and installation costs are currently under investigation. A novel tidal turbine, characterized by being self-balanced and not requiring structures and foundations, was recently proposed. The machine geometry presents an open-center, which affects the flow field and improves the machine performance in terms of extracted power and power coefficient. At the best of authors’ knowledge, despite being open-center turbines produced and installed, an evaluation of the fluid-dynamics effects of the open-center on machine performance is still missing. The main objective of the present work is to fill this gap and to contribute to have a deeper insight in the flow-field in the wake region and to investigate its effects on machine performance. The analysis is carried out by means of a CFD tool, for three different machine geometries: the open-center machine, which is modeled in single-rotor and double-rotors configurations, and the traditional wind-like machine. A comparison with literature experimental data is also included for validation purpose. For each analyzed case, the flow analysis is carried out for the optimal tip-speed ratio. A free stream velocity of 3 m/s is selected for simulations and a scaled model of the turbine having the diameter of about 1m is considered. The results show that the open-center configurations allow the achievement of higher performance in terms of power coefficient and extracted power, in comparison to the traditional wind-like turbines. In particular, for the new double-rotor prototype, the estimated Cp value is about 0.43, with improvements of about 15% with respect to traditional configurations. Moreover, the corresponding increase in the extracted power, which for 1m diameter prototype increases from 5.2 kW to 6 kW, is achieved with a lighter machine. The weight/power ratio, in fact, reduces from about 2 kg/kW for the standard machine configuration to 1.6 kg/kW for the new double-rotor prototype. The improvements in machine performance are related to the fluid dynamics characteristics of the central opening. This, in fact, causes a suction effect, which contributes to reduce the energy losses due to radial and tangential kinetic energy displaced out from the rotor disk that for a standard machine amount to about 36% of the inlet kinetic energy. Only part of these energy losses is recovered as mechanical power to the turbine, while the remaining part increases the flow kinetic energy lost in the open-center region. The amount of energy recovered as mechanical power at the turbine increases considerably in the double-rotor configuration, where, besides the suction effect of the open center, the external counter-rotating rotor gives the major contribution to the recovery of the kinetic energy lost in tangential and radial components of the inner rotor, which is then reduced down to 12.5%.