Acceleration of fluid caused by gravity

Hi,

I want to ask about the acceleration of fluid caused by gravity.
In order to account for gravity, I add the term 9.81 * (3-y), which means my vertical pipe is 3 meters long and the fluid is falling down inside the pipe, into my equation of the velocity field for laminar flow. However, the result (the Surface plot of velocity field) returned by FEATool is exactly the same as the result when I don't add the term into my equation of the velocity field.

Therefore, how can I account for gravitational acceleration in FEATool?

Thank you!