text: where  is the velocity of the fluid just after interacting with the rotor. This can be written as . The radial component of the velocity will be zero; this must be true if we are to use the annular ring approach; to assume otherwise would suggest interference between annular rings at some point downstream. Since we assume that there is no change in axial velocity across the disc, . Angular momentum must be conserved in an isolated system. Thus the rotation of the wake must not die away. Thus  in the downstream section is constant. Thus Bernoulli simplifies in the downstream section:
  In other words, the Bernoulli equations up and downstream of the rotor are the same as the Bernoulli expressions in the Betz model. Therefore, we can use results such as power extraction and wake speed that were derived in the Betz model i.e.
  This allows us to calculate maximum power extraction for a system that includes a rotating wake. This can be shown to give the same value as that of the Betz model i.e. 0.59. This method involves recognising that the torque generated in the rotor is given by the following expression:
  with the necessary terms defined immediately below.