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When flow reversal and breakdown into turbulence in the wake of a porous plate occurs, typically starting when the resistance coefficient K (= Δp/(½ρU²)) exceeds 4, experimental measurements show that the axial force on the body departs from the well‐known theory of Taylor (1944) for ordered flow through a porous plate. Similarly, experimental measurements of the thrust force coefficient for a rotor – for example, reported by Glauert (1926) and plotted in Figure 3.16 – show a departure from the actuator disc momentum theory C_T = 4a(1 − a). In both cases the measured forces are larger than the predictions of theory, and in both cases the point of break‐away is near the maximum predicted by the momentum theory.

Figure 3.16 Comparison of theoretical and measured values of C_T.

The thrust (or drag) coefficient for a simple, flat circular plate is given by Hoerner (1965) as 1.17 but, as demonstrated in Figure 3.16, the thrust on the rotor reaches a higher value. A major difference between the wake of the circular plate and of the rotor is that the latter contains a strong rotating component even after flow reversal in the wake has started.

It would follow from the above arguments that for high values of the axial induction factor a large part of the pressure drop across the disc is not simply associated with blade circulation, just as it is absent in the case of the circular plate. Circulation would cause a pressure drop similar to that given by the momentum theory determined by the very low axial velocity of the flow that actually permeates the disc.