Thank you very much for your reply.

The drawing initially was as shown in Figure 1 (note that the drawing has been reversed from right to left). After executing the program, the result after the first time step is as shown in Figure 2, and the message is as shown in Figure 3 (Update-Dynamic-Mesh failed. Negative cell volume detected). There may be several other time steps before reaching the same result when reducing the viscosity of water to a level lower than that of air.