Setting the Boundary Conditions

Air enters the computational domain at the gas inlet boundary with a velocity of 5 m/s. This inlet represents a perforated plate with an open area of 4% for the air nozzles. Therefore, you specify a volume fraction of 0.04 for the air phase and 0.96 for the water phase. You specify the degassing outlet at the top of the tank as phase-permeable boundary so that only the air bubbles can escape as they rise to the top of the aeration tank. Simcenter STAR-CCM+ ensures that the total gas inflow is equal to the total gas outflow and computes the internal gas volume fraction according to continuity.

To set the boundary conditions:

  1. Select the Regions > Region > Boundaries > Degassing Outlet > Phase Conditions > Air and set Type to Phase Permeable.


    The top boundary of the aeration tank now acts as an outlet for the air phase. The water phase treats this boundary as a wall.

  2. Edit the Degassing Outlet node and set the following properties:
    Node Property Setting
    Physics Conditions > Tangential Velocity Specification Method Vector
    Physics Values > Relative Velocity Value [0.0,0.0,5.0 m/s]
    You specify a tangential wall velocity at the degassing outlet to instigate water flow in the positive z direction.
  3. To define the gas inlet boundary, edit the Gas Inlet > Phase Conditions node and set the following properties:
    Node Property Setting
    Water > Physics Values
    Velocity Magnitude Value 0.0 m/s
    Volume Fraction Value 0.96
    Air > Physics Values
    Velocity Magnitude Value 5.0 m/s
    Volume Fraction Value 0.04
  4. Save the simulation.