Wall condition (no slip or sliding), fluid outlet condition

Hi all,

I am modeling a sphere moving in a moving laminar fluid domain (2D). The sphere is moved by the motion of the fluid, through boundary load on the sphere surface. I am wondering what kind of wall condition should I apply to the sphere surface, 'no slip' or 'sliding'.

Also, what kind of outlet condition should I apply to the outlet? Currently I am using 'pressure, no viscous stress', with pressure '0'. However, in the pressure plot, the pressure close to the outlet is larger than the pressure far away, which seems strange. How can the sphere move if the pressure ahead is smaller than the pressure behind?

I have attached the model and the pressure plot.

Thanks in advance for your help!