Abstract
Rotor blades can be found in many engineering applications, mainly associated with converting energy from fluids to work (or electricity). Rotor blade geometry is a key factor in the mechanical efficiency of the energy conversion process. For example, wind turbines' performance directly depends on the blade geometry and the wake flow formed behind them. We suggest to use a bioinspired blade based on the common swift wing. Common swift (Apus apus) is known to be a long-distance flyer, able to stay aloft for long periods of time by maintaining high lift and low drag. We study the near-wake flow characteristics of a freely rotating rotor with swept blades and its aerodynamic loads. These are compared with a straight-bladed rotor. The experiments were conducted in a water flume using particle image velocimetry (PIV) technique. Both blades were studied for four different flow speeds with freestream Reynolds numbers ranging from 23 000 to 41 000. Our results show that the near wake developed behind the swept-back blade was significantly different from the straight blade configuration. The near wake developed behind the swept-back blade exhibited relatively lower momentum loss and suppressed turbulent activity (mixing and production) compared with the straight blade. Comparing the aerodynamic characteristics, though the swept-back blade generated relatively less lift than the straight blade, the drag was relatively low. Thus, the swept-back blade produced two to three times higher lift-to-drag ratio than the straight blade. Based on these observations, we suggest that, with improved design optimizations, using the swept-back configuration in rotor blades (specifically used in wind turbines) can improve mechanical efficiency and reduce the energy loss during the conversion process.
Highlights
Rotor blades can be found in many engineering applications, mainly associated with converting energy from fluids to work
The utilization of different blade configurations impacts the rotor blade performance and efficiency as they control the energy transfer from the incoming flow to the blades by changing the kinetic energy stored in the flow to rotational kinetic energy of the blade, and for an actual wind turbine it would later convert to mechanical energy on the rotor shaft
We have shown that the near-wake flow characteristics of the swept rotor blade are substantially different compared with the wake developed behind the straight rotor blade
Summary
Rotor blades can be found in many engineering applications, mainly associated with converting energy from fluids to work (or electricity). In order to investigate the effect of incoming boundary layer flow on the wake development of a model wind turbine, Zhang et al [21] used stereoscopic PIV to study the near-wake flow and found significant turbulence enhancement at a distance of three rotor diameters downstream from the rotor plane. These studies characterize the near-wake region where interactions among turbinegenerated coherent structures dominate, which significantly affects the far-wake development [22]
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