Horizontal axis wind turbine testing at high Reynolds numbers

Abstract

Detailed studies of modern large-scale wind turbines represent a significant challenge. The immense length scales characteristic of these machines, in combination with rotational effects, render numerical simulations and conventional wind tunnel tests unfeasible. Field experiments can give us important insight into the aerodynamics and operation, but they are always accompanied by large amounts of uncertainty, due to the changing nature of the inflow and the lack of accurate control of the test conditions. Here, a series of experiments is presented, using an alternative method that enables us to represent and study much of the physics governing the large-scale wind turbines in small-scale models. A specialized, compressed-air wind tunnel is used to achieve dynamic similarity with the field-scale, but under accurately controlled conditions of the laboratory. Power and thrust coefficients are investigated as a function of the Reynolds number up to Re D = 14 × 10 6 , at tip speed ratios representative of those typical in the field. A strong Reynolds number dependence is observed in the power coefficient, even at very high Reynolds numbers (well exceeding those occurring in most laboratory studies). We show that for an untripped rotor, the performance reaches a Reynolds number invariant state at Re c 3.5 × 10 6 , regardless of the tip speed ratio. The same model was also tested with scaled tripping devices, with a height of only 9 μm, to study the effect of transition on the rotor performance. In the tripped case, the Reynolds number dependence was eliminated for all tested cases, suggesting that the state of the boundary layer is critical for correct predictions of rotor performance.

Phase-averaged dynamics of a periodically surging wind turbine

“…The topology of the torque space spanned by 1 and will be different for full-scale systems, given the differences in full-scale turbine design and aerodynamics. A lack of Reynolds-number invariance in these results may also play a role in the precise topology of the torque space, since the chord-based Reynolds numbers encountered in these experiments (5.6 × 10 4 9.2 × 10 4 ) were much lower than the invariance criterion of ≥ 3.5 × 10 6 proposed by Miller et al 30 . Still, as long as a sufficiently reliable local linearization is possible, the procedure outlined in Section III D for computing the aerodynamic constants ℓ and should continue to give accurate predictions of timeresolved dynamics.…”

Dynamic stall at high Reynolds numbers induced by ramp-type pitching motions

“…The model is supported in an access port located 1.1 m downstream of the entrance to the test section. Further information on the facility can be found in Jiménez et al (2010), Miller et al (2018 and Miller et al (2019).…”

Solidity effects on the performance of vertical-axis wind turbines

“…In addition, any small temperature and static pressure changes during a run were measured and used to determine the true fluid properties using real-gas relationships. This facility, and the data processing and reduction techniques have been detailed in prior work using this facility (Miller et al., 2019).

Figure 1.Schematic diagram of the HRTF as viewed from above the facility.

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“…The present work more fully explores the operational space of the high-solidity VAWT for a range of values by matching and simultaneously to field-relevant conditions. To accomplish this, a specialized wind tunnel was used which operates with highly compressed air as the working fluid (as described in the earlier work of Miller et al., 2018; Miller, Kiefer, Westergaard, Hansen & Hultmark, 2019). The high static pressure inside the tunnel decreases the fluid kinematic viscosity by up to two orders of magnitude over air at atmospheric pressure.…”