Boundary Conditions Flashcards
Why are boundary conditions needed?
At system boundary, constraints must be applied to flow variable
What are two requirements for boundary conditions?
- Reflect physical conditions of case being simulated
- Compatible across sets of multiple equations and flow regimes
What are the two types of boundary conditions?
Dirichlet/ Fixed Value
Neumann/ Fixed Gradient
What are ghost/dummy cells?
Additional Layer of virtual grid cells outside of simulated physical domain
What is the purpose of dummy cells?
To be dummy thic.
To simplify computation by allowing us to employ the same discretization scheme at the boundaries as inside the physical domain
What are common wall boundary conditions?
- No-slip condition
- Specified velocity
- No movement normal to wall
How are boundary conditions applied?
Through modifications to the M matrix and the source vector b.
How is the boundary mesh created?
It consists of boundary faces connected to only one cell.
The faces are grouped into patches corresponding to the different regions of the boundary (Inlet, wall, outlet etc)
What properties of convective / diffusive terms of the discretized for of NSE are required to set the boundary conditions?
Value of the flow variable at face / Surface Normal Gradient of the flow variable.
Why are dummy cells required?
When using higher order discretization schemes, dummy cells are useful because several neighboring cells are required.
What are the typical boundary conditions for walls?
- No slip condition
- Velocity at wall is specified
- Convection through the wall is zero
- Velocity gradient at wall is zero
-Velocity at wall is zero wrt to the wall.
- The pressure gradient normal to the wall is zero (Neumann condition)
- Wall boundary can move parallel to the wall but not normal
What are velocity inlets useful for?
For external incompressible subsonic flow.
What are the boundary conditions for velocity inlet?
- The pressure is not fixed but adapts to the prescribed velocity distribution
- The inflow is specified by density and the velocity components
- Static pressure at the inlet results from the calculated flow field
- The static pressure gradient normal to the inlet at the inlet is zero
Where do we use the mass flow inlet?
For internal, incompressible flows where we know the mass flow rate and want to calculate the pressure drop
What are the mass flow inlet boundary conditions?
- fixed density, fixed mass flow, and fixed direction unit vector at the inlet
- The static pressure gradient normal to the inlet at the inlet is zero
What problem does the pressure inlet solve?
Mass flow rate and velocity are unknown at the inlet and the computed velocity field must match the pressure gradient exactly to avoid divergence.
It solves the problem by specifying a total pressure at the inlet instead of the static pressure
What are the four types of pressure inlets?
Compressible / incompressible
Sonic / Supersonic
How is Incompressible flow solved using a pressure inlet?
- Total pressure boundary condition is set
- Velocity is calculated using the static pressure
- Calculated boundary field applied at the inlet
- Static pressure is updated
- Iterate
- Total pressure varies with each iteration
How does pressure inlet solve compressible flow?
- Compressible formulation of Bernoulli equation is used
- Total pressure boundary condition is set
- Velocity is calculated using the static pressure
- Calculated boundary field applied at the inlet
- Static pressure is updated
How are sonic and subsonic flows calculated differently for pressure inlet?
For subsonic flows, static pressure propagates upstream and total pressure is calculated
For supersonic flows, both total and static pressures are defined at the inlet
What are the boundary conditions for the pressure outlet?
- For subsonic flows. static outlet pressure is set as ambient pressure
- For supersonic flow, all flow variables specified by Neumann condition
How are mixed flow regimes treated in pressure outlets?
- Some flow domains like nozzles combine both sub and supersonic regimes
- Neumann condition used for static pressure at both boundaries leaving total pressure under-specified.
- Solved by moving the outlet downstream from the flow to expand to subsonic speed
When are symmetry boundary conditions used?
When the physical geometry and expected flow pattern has mirror symmetry
Is identical to free slip wall in some solvers
How can the implementation of the symmetry boundary condition be simplified?
Using dummy cells
What are two types of periodic boundaries?
Translational and rotational periodic
When are periodic boundary conditions used?
When the flow has a periodically repeating nature
What enables simple implementation of periodic boundaries?
Dummy cells. All scalar properties for these are obtained from corresponding cells
What is the periodic boundary condition?
Rotation of coordinate system
What is the pressure far field boundary condition and when is it used?
Is used to model a free stream condition at infinity for both inflow and outflow
The free stream Mach number, static conditions, and the flow direction are specified at the boundary
Only applicable when density at infinity is calculated using the ideal gas law
What is a requirement for pressure far field boundary condition?
Boundary must be placed far enough from the object of interest (> 20 half span)
What are some general guidelines for boundary conditions?
- Domain boundaries of external flow should be far away as possible
- Flow should ideally enter and exit normal to the boundary
