Abstract
Bidirectional ducts offer the potential for enhancing the economic efficiency of horizontal-axis tidal turbines (HATTs), thereby promoting a more widespread and efficient extraction of tidal stream energy. To better understand the performance of bidirectional ducts and bidirectional ducted HATTs (BDHATTs), a series of ducts with various designs were simulated using an actuator disc model within a validated numerical framework. Subsequently, the combinations of the representative designs and a fully modeled rotor were evaluated using a sliding mesh method. The results indicate that the flow-concentrating effect (FCE) of the duct generally decreases with increasing duct length, with the rate of decrease partially dependent on the internal resistance. However, the effect of the inner transition section's curve type is relatively limited. Furthermore, a separated vortex forms outside the duct because of the ski-jump flow near the duct leading edge, contributing to both a stronger FCE and considerable performance fluctuations for shorter ducts. Additionally, the rotor performance exhibits both low-frequency fluctuations due to the FCE variations and high-frequency fluctuations resulting from the non-uniform inside flow velocity. The rotor blades are also affected by the duct inner wall in four distinct aspects, each impacting various sections of blade tips.