# 4.9.9. Buoyancy Model

The `Buoyancy Model Specification` block in solution options is where you define which buoyancy model you want to use.

    Begin Solution Options

      Begin Buoyancy Model Specification my_buoy
        Buoyancy Model = Differential
      End

Details about the Buoyancy models can be found in Conservation of Momentum. The available options for the buoyancy model are:

> - `NO_BUOYANCY`

> - `BUOYANT`

> - `DIFFERENTIAL`

> - `BOUSSINESQ`

If you use the differential or Boussinesq model you also need to provide reference properties used to calculate (and/or ).

    Begin Solution Options

      Begin Buoyancy Model Specification my_buoy
        Buoyancy Model = Boussinesq
        Buoyancy Reference Temperature = 300
      End

    Begin Solution Options

      Begin Buoyancy Model Specification my_buoy
        Buoyancy Model = Differential
        Buoyancy Reference Temperature = 300
        Buoyancy Reference Mass_Fraction O2 = 0.2
        Buoyancy Reference Mass_Fraction N2 = 0.8
        Buoyancy Reference Mass_Fraction He = 0.0
      End