Why is tangential velocity a loss of kinetic energy in a turbine section?
We take axial flow turbines as our example here.
Tangental flow of air (flow that circulates around the engine's central axis) is imparted to the air by viscous drag between the air and the rotating fan and compressor blades. Between those fan discs are stator blades that act to straighten out the tangental flow and help convert the kinetic energy of the axial flow into pressure rise in the compressor stage. Viscous drag losses occur in the stator stages too as they perform this function. These losses dissipate some of the compression work being performed on the air by the compressor as a whole, and that amount of work then cannot contribute to thrust generation in the engine.