Objective The application of various free-flooding ships is increasing year by year, and the characteristic of these ships is the presence of both tank sloshing and connectivity between internal and external seawater. In order to study this specific phenomenon, a numerical simulation is conducted.

Method Using a CFD commercial software platform, a 3D model of a free-flooding ship section with openings on the port and starboard sides is established. The forced rolling motions under different roll periods and amplitudes are simulated in a viscous numerical water tank. Then, the relationship between the mass flow rate at the openings and the applied working conditions is studied.

Results The results show that if the rolling period is bigger, the flow amplitude ratio will also be bigger and the time difference between the mass flow and the ship’s rolling velocity will be smaller.

Conclusion The adopted method can accurately simulate the water exchange, and this water exchange will exhibit strong regularity. Based on this, further studies can be conducted on the hydrodynamic performance of free-flooding ships.