CAE-Boundary Conditions

Defining Boundary Conditions

You must specify information on the dependent (flow) variables at the domain boundaries

-Specify fluxes of mass, momentum, energy, etc. into the domain.

Defining boundary conditions involves:

-Identifying the location of the boundaries (e.g., inlets, walls, symmetry)

-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 types of the boundary condition available and locate the boundaries where the flow variables have known values or can be reasonably approximated

-Poorly defined boundary conditions can have a significant impact on your solution

Available Boundary Condition Types

How to Create a Boundary Condition

Right-click on the domain to insert BC’s

Inlets and Outlets

Inlets are used predominantly for regions where inflow is expected; however, inlets also support outflow as a result of velocity specified boundary conditions

Velocity specified inlets are intended for incompressible flows

-Using velocity inlets in compressible flows can lead to non-physical results

Pressure and mass flow inlets are suitable for compressible and incompressible flows

The same concept applies to outlets

Openings

Artificial walls are not erected with the opening type boundary, as both inflow and outflow are allowed

You are required to specify information that is used if the flow becomes locally inflow

Do not use opening as an excuse for a poorly placed boundary

-See the following slides for examples

Symmetry

Used to reduce computational effort in problem.

No inputs are required.

Flow field and geometry must be symmetric:

-Zero normal velocity at symmetry plane

-Zero normal gradients of all variables at symmetry plane

-Must take care to correctly define symmetry boundary locations

Can be used to model slip walls in viscous flow

Specifying Well Posed Boundary Conditions

-Consider the following case in which contain separate air and fuel supply pipes

-Three possible approaches in locating inlet boundaries:

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