Determining the Interior Wall Direction in ANSYS Fluent

Boundaries in ANSYS Fluent can be broken into two groups: external boundaries and internal boundaries.  External boundaries appear on the outer boundary of meshed regions (inlets, outlets, interfaces, etc.), while internal boundaries exist within a conformal mesh (interiors, porous-jumps, fans, etc.).  Internal boundaries are not limited to only residing inside a cell zone (for example they can separate two different cell zones); instead their restriction is simply that the mesh be continuous across them.  There is one boundary type that can be used as either an external or internal boundary: walls.

While external walls are fairly self-explanatory, internal walls are called coupled walls (or two-sided walls) because they are actually formed by a pair of wall boundaries that are by default coupled together.  You most often see coupled walls separating Fluid and Solid cell zones, but they can also be used with as infinitely thin baffles with fluid on both sides.  Each coupled wall pair shows up in the boundary list as a zone and its shadow: one for each side of the wall.  Sometimes it is necessary to set different boundary conditions on either side of the coupled wall.

When the coupled wall separates two separate cell zones, it is easy to tell which side faces a certain direction because the adjacent cell zone is listed in the boundary condition window.

However, if thintwall1e coupled wall is surrounded by the same cell zone on both sides, it can be difficult to tell which direction each side is facing.  The most straight-forward way to determine this is to plot the face area in a given direction and look to see if it is positive (face normal points away from that axis) or negative (face normal points in direction of axis).

First, it is necessary to initialize the model to gain the ability to plot variables on surfaces.  Then, you can simply plot the Face Area in a primary direction that isn’t perpendicular to your baffle normal.

intwall2

The magnitude of the face area doesn’t matter (as long as it’s not 0!).  We are looking at the sign.

intwall3

The face area vector goes from the adjacent cell into the wall (i.e. it’s the opposite of the face normal).  This means that, for example, a negative X Face Area means a face normal pointing towards the +X direction.

intwall4

In the screenshot above, baffle1 (the inner-most baffle) is pointing towards the outer radius of the bend (towards -X direction), while baffle2 and baffle3 are pointing towards the inner radius of the bend (towards +X direction).

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