About the stokes drag on a sphere (mph attached)

Dear All,

I am trying to find the drag coefficient of a sphere in a stokes flow. Since this 3D problem can be simplified into a 2D problem, I use the creeping flow module in 2D to simulate this case. Attached please find the mph file.
creeping flow BCs: As shown in the attachment, inlet velocity is 10^(-5)m/s, and outlet pressure to be 0. The two side walls are symmetric boundary condition and circle (2d section of sphere) surface velocity to be 0 (no slip).

Then I plot the pressure on the circle and I found it is proportional to cos(theta) (The result is deleted due to the limited attachement size requirement). In theoretical analysis, the form of pressure shound be 3/2*viscosity*velocity*cos(theta)/R^2. Then I check my result and I found the coefficient in front of the cos(theta) is much lower than the theoretical calculation.
And this coefficient becomes lower when I enlarge the simulation box size.

I just wonder if pressure is dependent on box size, can I get a correct result of stokes drag 6*Pi*viscosity*radius*velocity of a sphere using 2D simulation? Should I throw away 2D results and focus on 3D in order to get a correct stokes drag result?

Any help will be greatly appreciated!

Thank you!

Best,
Leo