Jet Propulsion/Velocity diagrams
A velocity diagram for a blade row describes the velocity changes of the fluid as it passes through the row.
The blade velocity is shown in blue. The incident flow shown in green is axial and the relative velocity to the blades is shown by vector addition of the blade and fluid velocities. The blade turns the flow through an angle but leaves the axial velocity unchanged, resulting in a higher total velocity. Subtracting the blade velocity from the exit relative velocity shown in orange gives the exit actual velocity.
This blade row then causes an increase in fluid velocity and an increase in energy, hence it is a compressor rotor. For a turbine rotor the situation at the exit and entrance would be reversed and energy would be extracted from the fluid by the blade row.
For stators the blade velocity is zero, and hence no energy is added or subtracted from the fluid under ideal conditions.