Figure 11 - uploaded by Franz Mühle
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Comparison of the phase-averaged vorticity calculated at different downstream locations from x/D = 1.2 to x/D = 4.0 in the wake behind the two-bladed (upper row) and three-bladed rotor (lower row).
Source publication
The vortex interaction in the wake behind a two- and three-bladed model scale wind turbine is investigated. The two rotors have equal solidity, and produce similar power and thrust at the design tip speed ratio. Phase-averaged quantities of the wake flow from one to four rotor diameters behind the turbines are measured in a wind tunnel. It is found...
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Modern horizontal axis wind turbines exclusively use pitch regulation to effectively control rotational speed and power production. This is generally achieved by controlling the angle of blades during operation. Pitch bearings which allow the rotation between blades and hub are subjected to high bending moments whilst oscillating at low speeds and...
Citations
... Even further downstream, the marginally higher energy levels are found behind the blade tips of a two-bladed rotor, comparing the blue (Mühle et al, ''no winglets'') and green (Eriksen and Krogstad) curve. This is in agreement with studies on the effect of blade numbers on the time-averaged wake flow by Mühle et al 26 and Bartl et al,27 in which higher turbulent kinetic energy levels are found for a two-bladed rotor than for a three-bladed rotor from x∕D ≥ 3. ...
An experimental study of the near wake up to four rotor diameters behind a model wind turbine rotor with two different wing tip configurations is performed. A straight‐cut wing tip and a downstream‐facing winglet shape are compared on the same two‐bladed rotor operated at its design tip speed ratio. Phase‐averaged measurements of the velocity vector are synchronized with the rotor position, visualizing the downstream location of tip vortex interaction for the two blade tip configurations. The mean streamwise velocity is found not to be strongly affected by the presence of winglet tip extensions, suggesting an insignificant effect of winglets on the time‐averaged inflow conditions of a possible downstream wind turbine. An analysis of the phase‐averaged vorticity, however, reveals a significantly earlier tip vortex interaction and breakup for the wingletted rotor. In contradistinction, the tip vortices formed behind the reference configuration are assessed to be more stable and start merging into larger turbulent structures significantly further downstream. These results indicate that an optimized winglet design can not only contribute to a higher energy extraction in a rotor's tip region but also can positively affect the wake's mean kinetic energy recovery by stimulating a faster tip vortex interaction.
A numerical model is used to study the flow in the wake of a two-bladed horizontal-axis rotor. The flow topology is analyzed for three different angular velocities of the rotor. For the fastest rotor there is a region downstream where the fluid loses nearly 30% of the axial inflow velocity, whereas almost 60% is lost if this region belongs to the wake of a second-in-line rotor. The second part of this study deals with the evolution of the tip helical vortices in the presence of a turbulent inflow wind. This is found to destabilize the vortices, which subsequently perform a leapfrogging motion. As the turbulence of the inflow wind strengthens, the leapfrogging starts closer to the wind rotor. If a second-in-line rotor is located in this wake, the power it extracts from the wind depends on its location with respect to the point where the first rotor’s vortices become unstable. A second rotor located downstream of this point extracts more power than one located upstream of this point.
