Читать книгу Wind Energy Handbook - Michael Barton Graham - Страница 76

For heavily loaded turbines, when a is high, the momentum theory predicts a reversal of the flow in the wake. Such a situation cannot actually apply uniformly throughout the far wake as predicted. What happens is that the wake becomes unstable with local flow reversal and breakdown into turbulence. This increases the mixing process, which entrains air from outside the wake, re‐energising the slow moving air that has passed through the rotor.

A rotor operating at increasingly high tip speed ratios presents a decreasingly permeable disc to the flow. Eventually, when λ is high enough for the axial flow factor to be equal to one, the flow field of the disc would appear to have reached a condition like that of a normal solid disc, including the flow in the wake.

As this condition is approached, the flow through a rotor has many of the features of flow through a porous disc of low and decreasing permeability and hence a large increasing resistance to through‐flow. The air that does pass through the rotor emerges into a low‐pressure region and is moving slowly. There is insufficient kinetic energy to provide the rise in static pressure necessary to achieve the ambient atmospheric pressure that exists outside the wake and must exist in the wake far downstream. The air can only achieve this ambient pressure by gaining energy from mixing with the flow that has bypassed the rotor disc and is outside the wake. Castro (1971) has studied in detail the wake of a porous plate as the plate is made increasingly impermeable to flow. At a certain level of resistance, a counter‐rotating vortex pair (in planar 2‐D flow) or a ring vortex (in axisymmetric flow) forms downstream in the wake as a result of the instability of the wake shear. This vortex structure generates a growing region of reversed flow near the plane of symmetry or axis of the wake. As the resistance is increased further, the vortex structure and region of reversed flow moves upstream until it reaches the downstream face of the plate. Depending on the Reynolds number, but increasingly so for a high Reynolds number, the vortex structure develops further instability and the wake becomes turbulent, greatly increasing mixing with the external flow and recovery of kinetic energy. The wake of a rotor has some significant differences from that of a porous disc: in particular that the latter does not have the strong helical vortex structure present in the wake of a rotor. Nevertheless, the behaviour of the rotor wake as its resistance is increased is qualitatively very similar, although the point at which the ordered axial flow through a rotor reverses and breaks down into turbulence is not exactly the same as for a porous disc.