From ea984e892b1cbad599f442e69a4171a0f1ef725e Mon Sep 17 00:00:00 2001 From: Michael Kuhn Date: Mon, 11 Nov 2024 10:15:58 -0700 Subject: [PATCH] update docs and README about pressure_outflow --- amr-wind/boundary_conditions/README.org | 3 ++- .../user/inputs_Boundary_conditions.rst | 25 +++++++++++++++++++ 2 files changed, 27 insertions(+), 1 deletion(-) diff --git a/amr-wind/boundary_conditions/README.org b/amr-wind/boundary_conditions/README.org index 3cf5a7e5ad..af2c03b22f 100644 --- a/amr-wind/boundary_conditions/README.org +++ b/amr-wind/boundary_conditions/README.org @@ -33,7 +33,8 @@ The boundary conditions that can be specified in the input file are: the standard outflow BC for velocity and scalars. For MAC projection and nodal-projection, it uses a Dirichlet BC for pressure. The value of pressure at the boundary is specified in the input file (default = 0). This can be - used as an inflow condition if ... + used as an inflow condition (though not recommended) via the input argument + "allow_inflow_at_pressure_outflow" associated with the desired equation. - =no_slip_wall= :: Like =mass_inflow=, this sets =BCRec= to =ext_dir= and specifies Dirichlet BC for the linear solvers. However, it differs from diff --git a/docs/sphinx/user/inputs_Boundary_conditions.rst b/docs/sphinx/user/inputs_Boundary_conditions.rst index 07dc5cfe4c..69a9ed3d36 100644 --- a/docs/sphinx/user/inputs_Boundary_conditions.rst +++ b/docs/sphinx/user/inputs_Boundary_conditions.rst @@ -87,3 +87,28 @@ The most applicable use case for this boundary condition is with the :ref:`amrwind-abl-bndry-io` 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. + +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). \ No newline at end of file