# Free Surface¶

Free surface models are simulations involving a moving free surface with a single Newtonian or non-Newtonian rheology. Common applications include filling/draining simulations, jet sprays, sloshing, vortex formation, and coating processes. Within this configuration, users must specify the fluid density, surface tension and define the constitutive relationship between fluid stress and fluid strain. As discussed in particleFields, additional phases (such as discrete bubbles and discrete solid particle) can be added to the system and one- or two-way coupled to the fluid. Secondary miscible fluids with arbitrary densities and viscosities, as discussed in Miscible Fluids, can also be added to the base fluid. Thermal fields and scalar fields can also be superimposed on the single phase fluid, as discussed in Basic Concepts and Scalar Fields.

Within free surface model, users can chose from one of four constitutive relationships:

Newtonian

Power law (with or without a yield stress)

Carreau

Custom expression

Briefly speaking, relationships (1)-(3) are familiar Newtonian and non-Newtonian rheology models. Relationship (4) represent custom expressions that may be more complex functions of strain, stress, age, species concentration, miscible fluid volume fraction, custom variables, particle concentration, and temperature. Additional details for each of these relationships are provided in Fluid Rheology

Unlike the single phase fluid, which is assumed to fill the entire interior zone of the lattice, users must specify which portions of the interior zone (if any) are initially filled with the fluid. Any initial fluid configuration can be specified using any combination of parametric geometry and/or user-imported geometry. By default, the initial condition is a parametric “Fluid Height Box”. This cuboid geometry is anchored to the main lattice domain, but the height of the box aligned with the “UP-direction” can be adjusted. The background initial condition is assumed to be empty, while the Fluid Height Box is assumed to be fluid. Ad discussed in initialFluid, an arbitrary number of initial condition regions can be added to the model.