.. meta::
    :description: Boundary conditions knowledge base.

.. _knowledgeBaseBoundaries:

:ref:`Boundary Conditions <boundariesParameters>`
**************************************************

Boundary conditions are of crucial importance in CFD as they, in addition to the initial condition, ensure that the problem is well posed, which means that the solution of the system of partial differential equations (PDEs) is unique and consistent. The numerical boundary conditions must ensure that the interface between the domain interior and exterior resembles the physical phenomena, without adversely affecting the solution stability. The following section provides guidance for specifying boundary conditions in Flow360. 

Expressions as Inputs
===================================================

For a number of boundary conditions such as :code:`NoSlipWall`, :code:`IsothermalWall` and :code:`Freestream`, a mathematical expression can be used to define the velocity field (:code:`Velocity`). Expressions define a parameter based on component-wise independent variables [x,y,z]. The result will be evaluated and returned as the value of the parameter. The allowed operators to define an expression can be found :ref:`here <UserDefinedFieldsGuidance>`.

.. csv-table::
   :file: Tables/expressionsTable.csv
   :widths: 20, 70
   :header-rows: 1
   :delim: @


.. _knowledgeBaseTurbulenceQuantities:

Specifying Turbulence Quantities
===================================================

Turbulence conditions can be specified with some boundary condition types. They can also be specified as the initial condition for the simulation. The types of turbulence quantities that can be specified depends on the turbulence model being used. Valid combinations of turbulence property specifications are listed in the following table.

.. csv-table::
   :file: Tables/validTurbulenceSpecification.csv
   :widths: 40, 70
   :header-rows: 1
   :delim: @