# Presentation: 273710

- Physical Boundary Conditions

## Boundary Conditions    

- The governing equations are solved within a finite flow domain and zone.
- A solution of the equations requires accurate specification of conditions at the boundaries of the domain and zones  Boundary Conditions.
- Distinguish between two types:
- 		Physical Boundary Conditions
- 		     at the boundary of the flow domain.
- 		Zonal Boundary Conditions
- 		     at the boundary between two zones.

- This presentation will focus on the _Physical Boundary Conditions_.

## Types of Physical Boundary Conditions    

- Physical boundary conditions can be grouped:
- _Flow Interface Boundary Conditions_
- Boundaries at which the flow crosses freely:
- Frozen
- Freestream
- Arbitrary Inflow
- Outflow
- _Wall Boundary Conditions_
- Boundaries with a solid surface (small amounts of flow may cross):
- Viscous Wall
- Inviscid Wall
- Reflection
- Bleed
- Blowing
- Moving Wall

- Boundary conditions for turbulence, chemistry, and MFD equations also.

## Characteristic Nature at Boundaries

- The *characteristic* (*acoustic wave*) nature of the flow at the boundaries provides critical information for specifying boundary conditions.
*Eigenvalues* of the flow equations at the boundaries indicate the speed of  the acoustic waves and whether they are entering (positive) or leaving (negative) the flow domain at the boundary.
- This indicates how much information should come from outside the domain (*physical*) and how much information should come from inside the domain (*numerical*).
- 		Boundary normal      is positive
- 		directed into the flow domain

- Boundary

- Eigenvalues

## Eigenvalues

- There are 5 eigenvalues  for the Navier-Stokes equations.  The first 3 are
- associated with *convective waves* while last 2 are associated with *acoustic *
*waves* (*c* is speed of sound):
- 	 > 0  indicates that a wave enters the domain
- 	and *physical information* (i.e. pressure) must
- 	be specified as part of the boundary condition.
- 	 < 0  indicates that wave exits the domain
- 	and *numerical information* (i.e. an extrapolation)
- 	must be specified as part of the boundary condition.

- Fluid speed relative to the motion of the boundary normal to the boundary.

- Positive when the flow enters the flow domain.

- Sign of 4and 5 depend on sign and magnitude of normal velocity.

## Flows and Eigenvalues

- The flow at the boundary is:
- 	Inflow when                    and so
- 	Outflow when                 and so
- 	Subsonic when
- 	Supersonic when
- Combinations are:                                                               Physical  Numerical
- 	Subsonic Inflow  	1, 2, 3, 4  > 0  and  5  < 0	4               1
- 	Subsonic Outflow	4  > 0  and  1, 2, 3, 5  < 0     	1               4
- 	Supersonic Inflow	1, 2, 3, 4, 5 > 0              	5               0
- 	Supersonic Outflow	 1, 2, 3, 4, 5 < 0 		0               5

## Subsonic Inflow

- The Subsonic Inflow condition has eigenvalues
- 	which indicates that 4 physical and 1 numerical conditions need to be imposed at the boundary.  These conditions can be imposed when using the Freestream or Arbitrary Inflow BC types at flow interfaces or the Bleed or Blowing BC types at walls.

## Subsonic Outflow

- The Subsonic Outflow condition has eigenvalues
- 	which indicates that 1 physical and 4 numerical conditions need to be imposed at the boundary.  These conditions can be imposed when using the Freestream or Outflow BC types at flow interfaces or the Bleed or Blowing BC types at walls.

## Supersonic Inflow / Frozen BC

- The Supersonic Inflow condition has all positive eigenvalues
- 	which indicates that all information should come from outside the flow domain.  The Frozen BC type is appropriate for supersonic inflow because the boundary condition simply keeps the flow conditions at the boundary unchanged. These conditions can be also imposed when using the Freestream or Arbitrary Inflow BC types at flow interfaces.

## Supersonic Outflow

- The Supersonic Outflow condition has all negative eigenvalues
- 	which indicates that all information should come from inside the flow domain.   An extrapolation of the solution to the boundary is the typical numerical method.  The Freestream or Outflow BC types are both capable of imposing the correct conditions.

## Flow BCs: Freestream

- The Freestream BC type is used at boundaries where *flow conditions are *
*expected to be fairly uniform and close to freestream conditions*.   The BC is
- flexible in that it checks whether the flow is inflow or outflow and subsonic or
- supersonic at the boundary and imposes the correct conditions.
- For supersonic inflow or supersonic outflow the previous two slides indicate the
- conditions that are imposed.
- For subsonic inflow and subsonic outflow, a characteristic-based method is used
- to determine conditions.  Assuming uniform conditions, two Riemann invariants
- R+ and R- can each be evaluated either numerically or from freestream
- conditions as appropriate and then used to compute the other flow properties.

## Flow BCs: Arbitrary Inflow

- The Arbitrary Inflow BC type is used mainly to impose a subsonic inflow
- condition, especially for internal flows (nozzle plenum).  Four physical
- conditions need to be specified while the one numerical condition is usually
- extrapolated from the interior flow solution.  The BC has the capability to:
- Hold total pressure and total temperature at the inflow.
- Specify the flow angle at the inflow.
- Hold total pressure, total temperature, and flow angles of input solution.
- Specify an inflow profile.
- Specify inflow turbulence levels.
- Specify inflow chemistry composition.
- Specify inflows with a solid-body rotation.
- Specify time-varying inflows.

## Flow BCs: Outflow

- The Outflow BC type is used mainly to impose a *subsonic outflow* condition,
- especially for internal flows.  The four numerical conditions are imposed through
- extrapolation.  The one physical condition is imposed by determining the static
- pressure at the boundary in some way.  Options include:
- Uniform Static Pressure  (constant or time-varying)
- Variable Static Pressure  (reference pressure with spatial variation over the boundary)
- Mass Flow  (for boundary and then compute the local static pressures)
- Average Mach Number  (for boundary and then compute the local static pressures)
- Compressor Face  (reflection coefficients for wave interactions with compressors)
- Non-Reflecting  (reference Mach number with non-reflecting constraints)
- In the case of *supersonic outflow*, the Outflow BC extrapolates all flow variables.

## Wall Boundary Conditions

- Wall boundary conditions involve a solid surface which may be porous.  The
- fundamental relation at a wall boundary is
- SB is the area of the cell face over which the BC is applied.
- The Bleed or Blowing BC will have             , and so,
- The Moving Wall BC will have
- For all the wall boundary conditions, the value of        is based on the presence
- of bleed or blowing.  If                 then

## Wall BCs: Viscous Wall

- The Viscous Wall BC type is used at solid boundaries for which a velocity no-
- slip condition is required.   A boundary layer forms along the wall.  The no-slip
- condition is that the tangential flow relative to the boundary is zero:
- where the two tangential vectors are perpendicular,
- Classical boundary layer theory indicates that the pressure gradient in the
- direction normal to the wall is approximately zero, this is used as a physical
- condition at the viscous wall
- Another physical condition used at the wall assumes an adiabatic condition for
- which                           or a constant temperature at the wall

## Wall BCs: Inviscid Wall / Reflection

- The Inviscid Wall BC type is used at solid boundaries for which a velocity slip
- condition is desired.   Thus the tangential velocity components      and
- must now be computed in some manner.   One approach is to simply extrapolate
- the tangential velocity components from the interior cells to the boundary.
- The Reflection BC type uses the same methods as the inviscid wall BC type;
- however, the intent of the reflection BC is to apply it at a planar boundary of
- flow symmetry.

## Wall BCs: Bleed / Blowing

- The Bleed and Blowing BC types are used at solid boundaries for which there is
- a small amount of flow across the boundary.  Bleed and blowing are often used
- in aerospace systems for flow control.    The relative normal velocity component
- is computed as
- where *A**region* is the area of the boundary over which bleed / blowing is specified.
- One model for computing the bleed mass flow is
- Which requires the specification of the bleed region porosity  and a table or
- function for the bleed sonic flow function Qsonic.

## Wall BCs: Moving Wall

- The Moving Wall boundary condtion is used at solid boundaries that have
- uniform translation or rotation.  An example of a uniform translation is an
- airplane going down a runway.   An example of a uniform rotation is the
- airplane spinning about its *x*-axis.   This BC type can be used with the viscous
- or inviscid wall BC types.

## Boundary Condition Numerical Methods

- For a cell-vertex finite-volume cell, the boundary condition is applied at the solution point on the boundary (approach of WIND).
- For a cell-centered finite-volume cell, the boundary condition is applied at the cell face on the boundary as a flux.
- Boundary conditions can be imposed *explicitly* at boundary solution points (this is the default method in WIND).
- Boundary conditions can be imposed *implicitly* by including the conditions into the implicit matrices of the numerical method for the inner flow field.  Wall BCs can be solved implicitly in WIND.
- Boundary conditions are imposed at the end of each iteration of the flow equations.

## Flow Property Extrapolations

- Most of the boundary conditions use an extrapolation of flow field properties
- to the boundary as part of imposing a numerical condition. A simple zero-order
- extrapolation of a property  (which can represent static pressure or density) is
- Where B is the boundary value and A is the value at a solution point adjacent
- to the boundary.   A first-order extrapolation has the form
- Where AA is the value at a solution point adjacent to the adjacent point to the
- boundary.  The “adjacent” direction is usually approximately normal to the
- boundary and along the grid line from the boundary.  The s are the spacings
- between the solution points.

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