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[FAQ]

The global coordinate is same as computational domain coordinate system. The body (local) coordinate is fixed to the body. The body coordinate is defined by providing the C.G values and C.G orientation (not necessary that c.g is inside the body). This is with respect to global coordinate system. During the calculation, the body coordinate changes (as the body moves), so the c.g and c.g orientation (global coordinate). Once the body is moved, all these values (c.g, c.g vel and c.g orientation) on the global coordinate will be updated in the panel. All the values in the 6DOF panel (cg, cg velocity, and cg orientation) are with respect to global coordinate system. The C.G orientations are Euler angles. The order uses the right hand rule based on the positive rotation axis. Rotation about X (Y to Z), rotation about Y (Z to X), and about Z (X to Y). The c.g. orientation is used to compute the transformation matrices. Fluent calculates angular velocity and acceleration on the body coordinate system and internally transforms to global coordinates to return to fluent (uses transformation matrices). The external moment and forces can be given on the local (body) or global coordinate system using the macro prop[SDOF_LOAD_LOCAL]. The boolean “prop[SDOF_LOAD_LOCAL]” can be used to determine whether the external forces and moments are expressed in terms of global or body coordinates. TRUE for local /body coordinates and FALSE for global coordinates. Default values is FALSE. Moment of inertia provided in the UDF should be at the c.g of the body ( i.e. on the local (body) coordinate system). The C.G and C.G orientation in the 6DOF panel fixes the body coordinate system. If both are zero initially, that means both coordinate are same at time=0.