Including gravitational force
 

Hi,
I wanted to know how cloud I include gravity effect on the flow. Consider a vertical pipe flow. I tried to do that in these ways but the results (velocity contour) are always the same and the flow is not affected;
1. Selecting buoyancy model with zero reference density (this has been done on a 2phase flow, particle transport model).
2. Adding a momentum source term for gravity (through Subdomain).

ANSYS Training Manual says:
"When buoyancy is included, a source term is added to the momentum equations based on the difference between the fluid density and a reference density".
This meas two above approaches give the same result; but why the velocity doesn't change and how could I include the gravity?

The fluid is a diesel oil with the density of 830 kg m^-3. Flow rate equals to 1.97 kg s^-1 in a pipe with 25.5 mm diameter.

Thanks for any suggestion.

1) Do not use zero reference density. Use the density of the lightest phase.
2) For gravity to have an effect you need something to be affected by gravity. This could be either density variations in the fluid, multiple phases or other things. If it is just a single phase constant density fluid then gravity will have no effect
3) Don't forget that when you activate gravity the pressure is modified to remove the hydrostatic component. So you will not see the hydrostatic head in the pressure variable.