This paper presents a numerical investigation of ducted tidal turbines, employing three-dimensional Reynolds-averaged Navier-Stokes simulations. Bidirectional ducted turbines are modelled with and without apertures, referred to as ducted and open-centre turbines respectively. The turbine rotors are represented through a CFD-integrated blade element momentum model, employing realistic rotor data, capturing swirl and blade drag in addition to the extraction of linear momentum. The performances of bare, ducted, and open-centre turbines are investigated for axial flow, relating these to the flow fields exhibited. Substantial decreases in power generated by the ducted and open-centre turbines were found, relative to a bare turbine of equal total device diameter. Two further measures of performance were employed: power density, normalising the power by the rotor area, and basin efficiency, relating the power generated to the overall power removed from the flow. Moderate increases in power density can be achieved for the ducted and open-centre devices, while their basin efficiencies are of similar value to that of the bare turbine. (C) 2016 Elsevier Ltd. All rights reserved.