Читать книгу Wind Energy Handbook - Michael Barton Graham - Страница 114

Vortex generators (VGs) are small triangular, rectangular, or similar pieces of flat, rigid sheet that act as very small half‐wings of low aspect ratio set perpendicular to the blade surface at a large angle of incidence (∼30°) to the local flow direction, as shown in Figure 3.75. Such plates generate strong leading edge (triangular delta VGs) or tip (rectangular VGs) vortices. The vortices stream over the main blade surface, stirring up the flow in the boundary layer, re‐energising the lower (inner) layers by bringing flow down from the upper (outer) layers and hence inhibiting separation. It is found that a line of small devices of this type of height about equal to the boundary layer thickness δ are very effective in inhibiting separation on the blade when placed a moderate distance upstream of the expected line of separation. VGs are easily added to the blade by fixing them via a lug at the base and are usually set to have an alternating positive and negative angle of incidence so that the rotation direction of the vortices alternates. They may be set moderately close together as in the example in the figure or farther apart up to the order of 10δ to still retain continuous effectiveness over the downstream region. In this way C_Lmax can be increased. The main drawback of such devices is that being fixed passive devices they operate continuously even when not required, and because they continue to generate vortices, they increase slightly the pre‐stall drag of the blade.

Micro VGs are much smaller versions of ordinary VGs, being of order δ/10 in height and spaced somewhat more closely. Micro VGs operate in the inner region of the boundary layer, where for a turbulent boundary layer much the strongest part of the velocity gradient normal to the blade surface exists. They can be nearly as effective as standard VGs in suppressing separation and have the advantage of generating a smaller increase in drag.

Surface air jets are inclined jets sited in a similar location as VGs would be sited but rather closer together. The jets are often fed by higher‐pressure air from near the stagnation region of the blade section or sometimes from inboard regions taking advantage of the centrifugal pressure difference between inboard locations and those farther outboard. The jets act to re‐energise the lower boundary layer through their own momentum and thus prevent stalling. They have the advantage that they can be turned off when not required. Their main disadvantage is the additional complexity, ‘plumbing’, and therefore cost required to provide each jet and the vulnerability of the jet slots.

Figure 3.75 VGs on a blade suction surface. (Flow is from right to left.)

Massless or synthetic jets are a variation of surface air jets that operate by an oscillating piston within a cavity that forces a pulsatile jet out through a small hole in the blade surface. No net mean mass flow occurs, hence their name, equal mass flows occurring into and out of the orifice. During the intake phase the flow is a sink flow that produces relatively little disturbance, whereas the outflow phase is a jet that forms a vortex ring so that the oscillatory operation of the device generates a sequence of vortex rings that can re‐energise the boundary layer. No separate intake or piping is required, but each device (orifice) must be separately actuated. They are not as yet used on wind turbine blades but may be an option for the future because they have been found effective in controlling separation in other situations. They appear to be reasonably unaffected by dirt due to the exhaust phase in each cycle.