Below are the 2 steps needed to set-up a mass-flow-inlet with an example of 30 degrees rotation (or swirl). 1.Define the axial direction: Before setting the boundary condition, one has to ensure that the ‘axial’ direction as referred in the boundary condition is correctly setup. The axial direction of your system must be equal to the axial direction as defined by the solver. This direction can be found (and modified) under “Cell Zone Conditions -> Fluid Zone -> Reference Frame Tab -> Rotation-Axis Origin & Rotation-Axis Direction. Fluent Defaults are: 2D: X is the axial direction (i.e. X = 1, Y = 0) 3D: Z is the axial direction (i.e. X = 0, Y = 0 and Z = 1) 2.Define the rotation angle: We will assume that the radial flow direction is 0. If the angle of swirl is 30 degrees, we have an axial component Vx = V * cos(30) and a tangential component Vt = V * sin(30). Hence, the user inputs for mass-flow-inlet boundary condition should be the following: Coordinate system: Cylindrical (Radial, Tangential, Axial) Radial-Component of Flow Direction: 0 Tangential-Component of Flow Direction: sin(30) (enter 0.5) Axial-Component of Flow Direction: cos(30) (enter 0.866)