Setting the Initial Conditions

You set the initial conditions for the physics continuum.

A field function sets up the initial pressure field as a linear profile: this profile is close to the expected solution. Setting the initial pressure in this way reduces the time that is required to obtain a solution. The pressure profiles at the inlet and outlet are specified to define the linear pressure field correctly.

In this case, the inlet is not modeled as a velocity inlet, since the simulation diverges when the whole-cross section is frozen, and the flow stops. A combination of pressure outlets at both inlet and outlet allows the flow to be stopped completely in the duct without violating any boundary condition

When choosing initial conditions, it is important to select a field that is bounded by the boundary conditions to avoid divergence. To satisfy this criterion, specify a linear pressure field with maximum and minimum values that are set as the boundary conditions.

The initial temperature is set to the inlet temperature. The initial velocity field is set to be close to the expected average velocity during the initial stages of the flow, that is, before the freezing commences, for faster convergence.

To set the initial conditions:

  1. Create the following field functions using the same procedure as in the previous section.

    The desired gauge inlet pressure is taken as 1.0 Pa.

    Node Title

    Target Inlet Pressure

    Dimensions

    Pressure

    Function Name

    TargetInletPressure

    Definition

    1.0

    As previously mentioned, the inlet is modeled as a pressure outlet: the dynamic pressure and turbulent stresses are added to make sure that the static inlet pressure is always 1.0 Pa.

    Node Title

    Inlet Pressure

    Dimensions

    Pressure

    Function Name

    InletPressure

    Definition

    ${TargetInletPressure} + 0.5*${Density}*pow(mag($${Velocity}), 2)

    The outlet pressure is set to 0.0 Pa.

    Node Title

    Outlet Pressure

    Dimensions

    Pressure

    Function Name

    OutletPressure

    Definition

    0.0

    The initial pressure field is then taken as a linear profile from 1.0 Pa at X = 0 m to 0.0 Pa at X = 0.3 m at the lower end of the pipe.

    Node Title

    Initial Pressure

    Dimensions

    Pressure

    Function Name

    InitialPressure

    Definition

    ${TargetInletPressure} - $${Position}[0]/0.3*(${TargetInletPressure} - ${OutletPressure})

  2. Right-click Automation > Field Functions and select Refresh to order the created field functions alphabetically.


Set the initial conditions for the solved variables in the Physics 1 continuum.
  1. Edit the Physics 1 > Initial Conditions node and set the following properties:
    Node Property Setting
    Pressure Method Field Function
    Scalar Function Initial Pressure
    Static Temperature Value 273.1 K
    Velocity Value [0.012, 0.0] m/s
    Volume Fraction Value [1.0]


  2. Save the simulation.