natural flow and free stream velocity in external transient analysis

Dear sir or madam,

I have problem with natural flow of air in external transient analysis when I use electrical conditions.

Problem is not at electrical conditions, because current path is heat up, and it is well.

But problem is at velocity about this current path. Velocity is higher with opposite direction.

How I can set gravitation..?

How I can setup SW for made law about gravitation on external area of air: air (liquid) that is hotter is slowly moved upper than cold air. Hot air is lighter than cold air.

I set in General settings/Analysis type/Gravity, but without success. Velocity is still high (more than 3 m/s) with bad direction.

Probably is problem in General settings/initial conditions / velocity parameters.

I do not know.

Thank you so much for every answer

Best regards 

Lukas