HI. I have encounter the problem when I simulate initial moving of piston.If I set boundary conditon of velocity inlet ,like 13m/s ,I find the pressure on piston wall is so little.In My opinion,the initial moving is resulted from difference pressure on piston's faces.But setting the BC with pressure condition,the velocity contour shows supersonic velocity ,it's imposible.Some one could tell me how to set the BC with velocity and pressure. Maybe I think wrong. Like the topic.[How to set pressure BC with mass Velocity Magnitud]

I wanna get the sum force to overcome the gravity of piston to push the piston moving which is vertical.

Is Mass-flux BC ok?