# Presentation: 019352

- Zonal Boundary Conditions

## Some Basics

- The flow domain is divided into zones and grids are generated within each zone.
- The flow equations are solved on the grid within each zone.
- Boundary conditions must be applied at the boundaries of the zones.
- Physical Boundary Conditions were applied to those zonal boundaries coincident with the flow domain boundary.
- Zonal Boundary Conditions are needed at zonal boundaries that are in contact with other zonal boundaries.
- Flow information is transferred across zonal boundaries.
- Zonal BCs can be classified as:
- Single-Zone Zonal BCs
- Multi-Zone Zonal BCs

## Single-Zone Zonal BCs

- A zonal boundary of a zone may be
- topologically connected to itself or
- other boundaries of the same zone:
- Singular Axis   A zonal boundary
- collapses to a line.
- Self-Closing   A zonal boundary
- connects to its topologically opposite
- zonal boundary.
- Self-Coupled   A zonal boundary is
- coupled to itself on some portion of the
- zonal boundary.

## Multi-Zone Zonal BCs

- A zonal boundary of a zone may be
- topologically connected to other
- boundaries of other zones.
- Connectivity defines how a zonal
- boundary is connected to other zonal
- boundaries.
- _Types of zonal connectivity_:
- 1) Abutting, Point-to-point match
- 2)  Abutting, Non point-to-point match
- 3)  Overlapping, Point-to-point match
- 4)  Overlapping, Non point-to-point match

- 1 & 3

- 2

- 4

- Outer boundary

- Interior boundary

## Zone Coupling Methods

- Information is transferred across the zonal boundaries by a couple of methods:
**Characteristic Coupling**.  This methods uses one-dimensional characteristic
- flow theory to set boundary flow field variables based on local flow direction
- and strength. These boundary variables are then transferred between zones
- using a tri-linear interpolation.
**Roe Coupling**.  This method uses Roe’s flux-difference splitting to compute
- flux cell “interface states” and the zonal boundary is considered a “cell
- interface” in the Roe scheme.   A higher-order zone coupling involves the
- transfer of solution derivatives.  This method only operates on the inviscid
- component of the flux.  The exchange of turbulence information uses the tri-
- linear interpolation.
- Both methods operate as explicit operations.  In WIND, these are done at the
- end of a cycle rather than the end of an iteration.

## Zone Coupling Issues

- Some issues regarding zone coupling:
- Avoid placing zonal boundaries in regions with large flow gradients (shocks, boundary layers, shock / boundary layer interaction).
- Best if zonal boundaries are place *normal* to the flow direction.
- Placing zonal boundaries parallel to the flow in regions of high gradients is generally not good, but often unavoidable.
- Point-to-point matching across zones is best since inviscid flux information can be directly transferred and errors are less in interpolation of viscous and turbulent information.
- Best if zones overlap by at least two grid points, especially in turbulent flows.  This improves the interpolation.  However, this may be difficult for grids with lots of zones and complicated geometry.

## Physical Models Applied at Zonal Boundaries   

- Several physical models can be applied at zonal boundaries to affect the flow
- as it crosses the zonal boundary.  Those in WIND were implemented to have
- some specific capabilities for the analysis of propulsion systems:
**Actuator Disk**.   This model imposes a rotational motion to the velocity field as the flow goes through the zonal boundary.  It is meant to simulate the addition of energy to the flow due to a rotor (compressor fan).
**Screen**.   This model imposes a loss in total pressure across the zonal boundary.
**Vortex Generators**.  This model imposes a number of small vortices that simulate the vortices produced by an array of vane-type vortex generators.

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