You can't really isolate velocity sources like that since velocities are all combined together as the simulation progresses. This isn't a limitation of tyFlow, it's simply the nature of physics 
Experiment with the "only affect" parameters in order to not dampen downward velocities, thereby excluding your gravity. You may need 2 slow operators. One to dampen X/Y values, and then one to only dampen positive Z values.
Experiment with the "only affect" parameters in order to not dampen downward velocities, thereby excluding your gravity. You may need 2 slow operators. One to dampen X/Y values, and then one to only dampen positive Z values.