Abstract:Abstract.Vertical Axis Wind Turbines have yet to see wide-spread use as a means of harvesting the kinetic energy of the wind. This may be due in part to the difficulty in modeling the relatively complex flow field and hence performance of these units. Additionally, similar to Horizontal Axis Wind Turbines, VAWTs are difficult to properly test in a conventional wind tunnel. Typically Reynolds numbers cannot be matched or the turbine geometry must be altered, limiting the applicability of the results. Presented … Show more
“…The results also agree with the findings of other studies that the optimal tip-speed ratio for power production decreases with increasing solidity (Miller et al. 2018 b ; Rezaeiha, Montazeri & Blocken 2018). Figure 3.Coefficient of power as a function of tip-speed ratio for the four experiments outlined in table 1.…”
Section: Experimental Methodssupporting
confidence: 92%
“…Han et al 2018). The results also agree with the findings of other studies that the optimal tip-speed ratio for power production decreases with increasing solidity (Miller et al 2018b;Rezaeiha, Montazeri & Blocken 2018).…”
Section: Field Site and Turbine Characterizationsupporting
“…The results also agree with the findings of other studies that the optimal tip-speed ratio for power production decreases with increasing solidity (Miller et al. 2018 b ; Rezaeiha, Montazeri & Blocken 2018). Figure 3.Coefficient of power as a function of tip-speed ratio for the four experiments outlined in table 1.…”
Section: Experimental Methodssupporting
confidence: 92%
“…Han et al 2018). The results also agree with the findings of other studies that the optimal tip-speed ratio for power production decreases with increasing solidity (Miller et al 2018b;Rezaeiha, Montazeri & Blocken 2018).…”
Section: Field Site and Turbine Characterizationsupporting
“…This tip-speed ratio matches the optimal tip-speed ratio for the full-scale 2-kW VAWTs at the FLOWE field site, a result shown both in field data and in laboratory experiments at full dynamic similarity [36]. Such low tip-speed ratios are characteristic of turbines with relatively high solidities (e.g., [37]), since the optimal tip-speed ratio for VAWT operation decreases with increasing solidity [38][39][40][41]. The application of the results shown in this paper to turbines with lower solidity (e.g., [42]) is discussed in more detail in Section 4.…”
This study examined three-dimensional, volumetric mean velocity fields and corresponding performance measurements for an isolated vertical-axis wind turbine (VAWT) and for co- and counter-rotating pairs of VAWTs with varying incident wind direction and turbine spacings. The purpose was to identify turbine configurations and flow mechanisms that can improve the power densities of VAWT arrays in wind farms. All experiments were conducted at a Reynolds number of R e D = 7.3 × 10 4 . In the paired arrays, performance enhancement was observed for both the upstream and downstream turbines. Increases in downstream turbine performance correlate with bluff–body accelerations around the upstream turbine, which increase the incident freestream velocity on the downstream turbine in certain positions. Decreases in downstream turbine performance are determined by its position in the upstream turbine’s wake. Changes in upstream turbine performance are related to variations in the surrounding flow field due to the presence of the downstream rotor. For the most robust array configuration studied, an average 14% increase in array performance over approximately a 50° range of wind direction was observed. Additionally, three-dimensional vortex interactions behind pairs of VAWT were observed that can replenish momentum in the wake by advection rather than turbulent diffusion. These effects and their implications for wind-farm design are discussed.
“…Free-stream velocity is measured via a pitot-static tube located upstream of the turbine model using a differential pressure transducer with a range of 3447 Pa (Validyne DP-15). This facility has been used previously to study wakes of axisymmetric bodies [12][13][14], zero-pressure gradient turbulent boundary layers [15], and more recently various wind turbine models of both vertical and horizontal axis varieties [16][17][18]. The HRTF is shown in Fig.…”
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.
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