Setting the Initial Conditions and Boundary Values

You define a mass flow inlet with 0.075 kg/s air inflow and a pressure outlet with relative pressure 0.0 Pa. You initialize the flow with velocity and turbulence parameters close to the flow field in the steady state.

To initialize the flow:
  1. To initialize the inlet velocity, select the Physics 1 > Initial Conditions > Velocity node and set Value to [0,0,20] m/s, which corresponds approximately to the inlet speed given the defined mass flow.
  2. To initialize the turbulence values, you calculate the turbulence intensity and turbulence length scale for the duct flow as functions of pipe radius and mass flow.
    1. Select the Physics 1 > Initial Conditions > Turbulence Specification and set Method to Intensity + Length Scale
    2. Right-click the Automation > Parameters node and select New > Scalar.
    3. Rename the parameter to PipeRadius and set the properties as follows:
      Parameter Value Dimensions
      PipeRadius 0.03 Length
    4. Repeat this procedure for the following parameters described in the table:
      Parameter Value Dimensions
      MassFlow 0.075 Mass/Time
      ApproxRe
      2*$MassFlow/(3.1415*$PipeRadius)/1.85e-5
      Dimensionless
      TI
      0.16*pow($ApproxRe,-1/8)
      Dimensionless
      TL
      2*0.07*$PipeRadius
      Length
    5. Select the Physics 1 > Initial Conditions > Turbulence Itensity node and set Value to $TI.
    6. Select the Turbulence Length Scale node and set Value to $TL.
    7. Select the Turbulence Velocity Scale node and set Value to 20 m/s.
  3. To set the mass flow inlet boundary:
    1. Select the Regions > Fluid > Boundaries > Inlet > Physics Values > Mass Flow Rate node and set Value to $MassFlow.
    2. Select the Inlet > Physics Conditions > Turbulence Specification node and set Method to Intensity + Length Scale
    3. Select the Physics Values > Turbulence Intensity node and set Value to $TI.
    4. Select the Turbulence Length Scale node and set Value to $TL.
  4. To set the pressure outlet boundary:
    1. Accept the default pressure value of 0.0 Pa.
    2. Select the Regions > Fluid > Boundaries > Outlet > Physics Conditions > Turbulence Specification node and set Method to Intensity + Length Scale
    3. Select the Physics Values > Turbulence Intensity node and set Value to $TI.
    4. Select the Turbulence Length Scale node and set Value to $TL.