Phase Initialization
If the study type Transient with Phase Initialization is used in the model, the level set or phase field variable is automatically initialized. For this study, two study steps are created, Phase Initialization and Time Dependent. The Phase Initialization step solves for the distance to the initial interface, Dwi. The Time Dependent step then uses the initial condition for the level set function according to the following expression:
in domains initially filled with Fluid 1 and
in domains initially filled with Fluid 2.
Correspondingly, for the phase field method the following expressions are used:
in Fluid 1 and
in Fluid 2. The initial condition for the help variable is ψ0 = 0. These expressions are based on the analytical solution of the steady state solution of Equation 7-5, Equation 7-6, and Equation 7-7 for a straight, non-moving interface.
For the initialization to work it is crucial that there are two Initial Values nodes and one Initial Interface node. One of the Initial Values nodes should use Specify phase: Fluid 1 and the other Specify phase: Fluid 2. The Initial Interface node is not default but can be added and should have all interior boundaries where the interface is initially present as selection. If the selection of the Initial Interface node is empty, the initialization fails. Note that when the value of Level set variable or Phase field variable is User defined, arbitrary user defined initial values are used. In this case, no Initial Interface node should be added, and a Time Dependent study should be used instead of Time Dependent with Phase Initialization.
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The Two-Phase Flow, Level Set and Phase Field Interfaces
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Studies and Solvers and Time Dependent with Phase Initialization in the COMSOL Multiphysics Reference Manual