Study on change mechanism of hydrodynamic force on circular pier with excitation frequency near free surface under earthquake

Many studies show that the hydrodynamic force on the unit height of the deep-water pier near free surface under earthquakes changes with excitation frequency, but no investigation has been conducted to uncover the change mechanism behind this phenomenon. Based on radiation wave theory, the hydrodynamic force is decomposed into the radiation wave force and the inertial force for the first time. It's found that the added mass coefficient of radiation wave force changes with excitation frequency, which is responsible for the change of hydrodynamic force with excitation frequency. When excitation frequency is smaller, the increment of the radiation wave force is larger than the decrease of the inertial force, resulting the increase of hydrodynamic force when getting closer to the free surface; when excitation frequency is larger, the increment of radiation wave force is ignorable and the decrease of inertial force is larger, resulting the decrease of hydrodynamic force when getting closer to the free surface. Further investigation in flow field reveals that when excitation frequency is smaller, the flow field near free surface is dominated by wave flow field; when excitation frequency is larger, the flow field is dominated by oscillating flow. • Hydrodynamic force on cylinder near free surface under earthquake is composed of radiation wave force and inertial force. • Change of radiation wave force is the main reason why hydrodynamic force changes with excitation frequency. • The larger excitation frequency, the smaller radiation wave force, ultimately the smaller hydrodynamic force, vice versa. • When excitation frequency is larger, flow filed is dominated by oscillating filed; otherwise it is dominated by wave field.