Defining Boundary Conditions
To define a problem that results in a unique solution, you must specify information on the flow variables at boundaries.
– Specify fluxes of mass, momentum, energy, etc. into the domain.
Defining boundary conditions involves:
– Identifying the boundary locations
– Supplying information at the boundaries
The data required at a boundary depends upon the boundary condition type and the physical models employed.
You must be aware of the information that is required of the boundary condition and locate the boundaries where the information on the flow variables are known or can be reasonably approximated
– Poorly defined boundary conditions can have a significant impact on your solution
Cell Zones – Fluid
A fluid cell zone is a group of cells for which all active equations are solved.
Fluid material selection is required.
– For multiple species or multiphase
flows, the material is not shown.
Instead, the fluid zone consists of the
mixture of the phases.
Optional inputs
– Porous region
– Source terms
– Laminar region
– Fixed Values
– Radiation
Porous Media
A porous zone is a special type of fluid zone.
– Enable Porous Zone option in the Fluid panel.
– Pressure loss in flow determined via user inputs of resistance coefficients to lumped parameter model
Used to model flow through porous media and other uniformly distributed
flow resistances.
– Packed beds
– Filter papers
– Perforated plates
– Flow distributors
– Tube banks
Inputs are directional viscous and inertial resistance coefficients.
Cell Zones – Solid
-A solid zone is a group of cells for which only heat conduction problem solved. Flow equations are not solved.
-Only required input is the material name (defined in the Materials panel).
-Optional inputs allow you to set volumetric heat generation rate (heat source).
-Need to specify rotation axis if rotationally periodic boundaries adjacent to solid zone.
-Can define motion for a solid zone
Locating Boundaries – An Example
Three possible approaches in locating inlet boundaries for this example:
Upstream of manifold
-Can use uniform profile.
-Properly accounts for mixing.
-Non-premixed reaction models
-Requires more cells.
Nozzle inlet plane
-Non-premixed reaction models.
-Requires accurate inlet profile.
-Flow is still non-premixed.
3 Nozzle outlet plane
-Premixed reaction model.
-Requires accurate profile.
-Not generally recommended since inlet BCs may drive the interior solution.
General Guidelines
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