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[FAQ]

CFX evaluates the radius coordinate r in the global coordinate system if the boundary has not been associated to any other (local) coordinate frame on the “Basic Settings” tab of the boundary condition. This is the case also if you select the option “Cyl. Vel. Components” to prescribe the inflow velocity on the “Boundary Details” tab. The definition of the rotation axis on this tab only defines the orientation of the three velocity components “Axial”, “Radial”, and “Theta”. If you access a coordinate in the expressions used to prescribe the velocity components, this axis definition has no effect on the coordinate system in which the coordinate is evaluated – it will be the global coordinate system by default. To evaluate coordinates in the local coordinate system which is defined by the centroid and normal of the inlet boundary (i.e., the system which defines the orientation of the cylindrical velocity components), you must first define a corresponding local coordinate frame, e.g., using the “Point and Normal” option (see chapter 25.2.1. Coordinate Frame: Option in the CFX-Pre User’s Guide). If you then assign this to the inflow boundary on the “Basic Settings” tab, all coordinates are evaluated in local coordinate system. To check prior to the simulation if the velocity components are prescribed correctly, you can use the plot feature on the “Plot Options” tab of the boundary condition, see chapter 14.2.6. Boundary Plot Options Tab in the CFX-Pre User’s Guide.