The axis boundary type must be used as the centerline of an axisymmetric geometry (see Figure 7.16.1). It can also be used for the centerline of a cylindrical-polar quadrilateral or hexahedral grid (e.g., a grid created for a structured-grid code such as FLUENT 4). You do not need to define any boundary conditions at axis boundaries.
Calculation Procedure at Axis Boundaries
To determine the appropriate physical value for a particular variable at a point on the axis, FLUENT uses the cell value in the adjacent cell.