Heat Convection with Air and Water

Hello everybody,

im trying to do a simulation like in the picture below.


What i expect to see is natural convetion in the Air and Fluid based on the density change in the material.

I already did some simulations with natural convection in a one phase closed cavity, which worked really well.

My assumation to solve this problem is as followed:

1. Approach
- Laminar Flow for Air (spf1)
- Heat Transfer in Fluids for Air (ht1)
- non isothermal Flow for Air(nitf1)

- Laminar Flow for Water (spf2)
- Heat Transfer in Fluids for Water (ht2)
- non isothermal Flow for Water(nitf2)


But how can i couple these two things at the transition boundary (Boundary 1)?

2. Approach
- Heat Transfer in Fluids based on the material in the region ( Temperature Distribution without convection looks well)

But how can i specify the flow if i choose this approach?
With one laminar flow for air and water the solution does not compute right, maybe a problem with the boundary conditions?
If i do it with one laminar flow physic i specify the volumeforce as :
density_air*beta_air*g*(T-Treference)
or in the water region as:
density_water*beta_water*g*(T-Treference)


Is one of the approaches right? Or do i have to do it with the multiphase flow?