Gravity, UDF Source & Operating Density. HELP ME!

Brian

I am doing a simple 2-D sway of a rectangular box with air/water. Translation is in the x-direction and gravity acts in the -y direction. I can use my UDF for the x-momentum equation for the sway motion and specify none for the y-momentum equation. Then I turn on gravity as -9.81 in the y-direction and then I specify the operating density as the lighter phase. I run the code and get results.

If my thinking is correct I can do this another way. I can add in gravity as a UDF for the y-momentum equation instead? I made a source term for the y-momentum that is essentially rho*g. When I do this, I now specify gravity as 0 in the Operating Conditions panel for the y-direction and I still set my operating density to the lighter phase. However, my results come out slightly different. I figured they would be the same but they're not. Any suggestions?