## Turbine Drag

The theoretical maximum power output and drag coefficient for the CoRMaT turbine is derived by assuming that the rotors are replaced by a disc, which withdraws energy from the water passing through it. At a certain distance downstream of this disc, the water that has passed through flows with a reduced velocity. The theoretical maximum power output and drag coefficient are derived as follows: (Betz, 1966)

Velocity upstream of turbine:

Velocity at turbine rotors:

where a is the axial induction factor.
Velocity downstream of turbine:

where m is the mass of water, A is the area of the rotor and rho is the density of water.

At maximum power:

Force in axial direction:

Force at maximum power:

where CD is the drag coefficient.