Abstract: A tidal hydroelectric unit switches working modes frequently creating a transition condition that is easy to become the cause of operational faults. To explore the transient flow characteristics of a bidirectional tubular turbine, numerical simulations of the flows were conducted using a lattice Boltzmann method (LBM) focusing on the transients created by two types of switching: turbine mode switched to discharge mode, and discharge mode switched to pump mode. The simulations used self-defined functions to control the movement of guide vanes and blades, as well as variations in the working parameters of the turbine. Further, the internal flow characteristics were also obtained for the two transient conditions. During the flow transition from turbine to discharge mode, the flow rate was found to increases gradually with time and the calculated unit flow rate was found to be 2.88 times the initial flow on completion of the transition agreeing well with the measured values. It shows the accuracy and reliablility of the applied methodology in this research. During flow transition, apparent low pressure occurred in certain areas on the suction surface of runner blades due to flow pattern changes, which may cause cavitation erosion of the blades. The radial force on the runner was fluctuating periodically, being a potential excitation to vibrations in the shafting. Several turbulent vortices in the draft tube were observed and a spiral cavity vortex was formed due to the decrease in internal pressure.

Key words: lattice Boltzmann method, bidirectional tubular turbine, numerical simulation, working mode switching, discharging condition, transient characteristics