Section: Boundary conditions

This section controls the boundary conditions. Only non-periodic BC’s need to be defined here.

xlo.type (or ylo.type, zlo.type, xhi.type, yhi.type, zhi.type)

type: String, mandatory if non-periodic

Boundary condition type on the lo (or hi) side of the domain. Current options are: periodic, pressure_outflow, mass_inflow, mass_inflow_outflow, no_slip_wall, slip_wall, symmetric_wall and wall_model.

xlo.temperature (or ylo.temperature, etc)

type: Real, optional, default = 0.0

Specifies a temperature gradient at the wall, only activated for slip_wall and wall_model BC types.

Mass inflow boundary conditions

For the mass_inflow boundary condition type, the user can specify constant values in the input file:

xlo.type = "mass_inflow"
xlo.velocity = 1.0 1.0 0.0
xlo.temperature = 300.0

In addition to being able to specify constant values, the user has access to a variety of included complex boundary conditions (e.g., LinearProfile, PowerLawProfile, Rankine) that are activated in the input file. For example:

xlo.type = "mass_inflow"
xlo.density = 1.0
xlo.velocity = 5.0 5.0 0.0
xlo.velocity.inflow_type = LinearProfile

If the user wants to define their own boundary conditions, this is done by editing CustomScalar and CustomVelocity source and header files in the udfs folder. CustomScalar is used for scalar fields and CustomVelocity is used for velocity fields. These can then be activated in the input file as such:

xlo.type = "mass_inflow"
xlo.temperature.inflow_type = CustomScalar
CustomScalar.foo = 1.0
xlo.velocity.inflow_type = CustomVelocity
CustomVelocity.foo = 1.0

They do not both need to be defined at the same time. It is the user’s responsibility to ensure that the source files are appropriately edited for their use case. Examples of how these files can be edited are found through comparison of the other mass_inflow functions in the udfs folder.

Mass inflow-outflow boundary conditions

The mass_inflow_outflow boundary condition is designed to handle both inflow and outflow at the same boundary. For the advection schemes, it implements a Neumann type behavior at the outflow cells and a Dirichlet behavior at the inflow cells. It uses Neumann conditions for the MAC and nodal projections and enforces solvability before the projections by correcting the outflow to match with the inflow within the specified mass_inflow_outflow boundaries. It uses a Dirichlet condition for the diffusion solver.

Both the approaches mentioned above for the mass inflow condition, constant values and UDFs, can be used to specify the boundary values. The outflow values will be automatically replaced by a value from the interior cell to enforce the Neumann type behavior. See the freestream_godunov_inout test for an example that uses the TwoLayer UDF. This test involves two z-layers of the flow along opposite x-directions. The input file options are copied here:

geometry.is_periodic  =  0   1   0   # Periodic in y

# Boundary conditions
TwoLayer.bottom_vel   = -1.0 0.0 0.0
TwoLayer.top_vel      =  1.0 0.0 0.0
TwoLayer.init_perturb = 0.9
TwoLayer.z_partition  = 0.5

xlo.type = "mass_inflow_outflow"
xlo.density = 1.0
xlo.velocity.inflow_outflow_type = TwoLayer

xhi.type = "mass_inflow_outflow"
xhi.density = 1.0
xhi.velocity.inflow_outflow_type = TwoLayer

zlo.type = "slip_wall"
zhi.type = "slip_wall"

The most applicable use case for this boundary condition is with the AMR-Wind ABL Boundary I/O for flows that change directions across the vertical coordinate or with time. The work to integrate this condition with the ABL class is under progress.

Dynamic wall model (Wave model)

The Moving Surface Drag (MOSD) model developed by Ayala et al (2024) is used as the dynamic wall model. The model calculates the stress (form drag) imparted by a moving wave. The model enables wave phase-resolving physics without the use of wave-phase adapting computational grids.

wave_mosd.amplitude
**type:** Real, required, default = 0.05: Specifies the amplitude of the wave, only activated if WallFunction.wall_shear_stress_type = mosd

wave_mosd.wavenumber
**type:** Real, required, default = 4: Specifies the wave number of the wave, only activated if WallFunction.wall_shear_stress_type = mosd

wave_mosd.frequency
**type:** Real, required, default = 0.8: Specifies the frequency of the wave, only activated if WallFunction.wall_shear_stress_type = mosd

Example:

zlo.type =   "wall_model"
WallFunction.wall_shear_stress_type = mosd
wave_mosd.amplitude = 0.05
wave_mosd.wavenumber = 4
wave_mosd.frequency = 0.8

Note

This wall model is only applicable for the lower boundary zlo.type. Also, it is set for only monochromatic waves.

Currently, the dynamic wall model is only available for incflo.physics = ChannelFlow. The work to integrate this condition with the ABL class is under progress. See the channel_mosd test for an example that uses the dynamic wall model.

Pressure outflow boundary conditions

The pressure_outflow boundary condition is the most common boundary condition used when flow out of a boundary is desired. By default, this sets the pressure at the outflow plane to 0 and assumes a zero gradient for other flow quantities (e.g., velocity and scalars). Also by default, this boundary condition clips fluxes that would be advected into the domain, which is for the sake of stability.

Having a uniform pressure value at the outflow is not often physically valid for flows of interest, such as stratified ABLs and ocean waves. However, instead of changing the target pressure of the boundary condition, it is typically more useful to change the formulation of the source terms, transforming the pressure variable into the difference between the true pressure and some constant, non-uniform reference pressure profile. In the context of stratified ABLs, which typically apply gravity through the BoussinesqBuoyancy source term, this pressure modification is realized with the additional source term ABLMeanBoussinesq. In the context of ocean waves, which typically apply gravity through the GravityForcing source term, this pressure modification is realized through the input option “ICNS.use_perturb_pressure”.

Finally, the default clipping of inflow at pressure_outflow boundaries can be disabled. This is not recommended, but it is possible with the input option “allow_inflow_at_pressure_outflow”. This input argument is appended to the PDE name where it should be applied (e.g., ICNS, temperature, or tke).