Experimental analysis of the wake of a horizontal-axis wind-turbine model

Lignarolo, L. E. M.; Ragni, Daniele; Krishnaswami, C.; Chen, Q.; Ferreira, Carlos; Bussel, G.J.W. Van

doi:10.1016/j.renene.2014.01.020

Cited by 155 publications

(127 citation statements)

References 19 publications

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“…The turbine blades and nacelle are coated with a black paint for dimming the reflection of the laser beam. More information on the wind turbine design and the characteristics of the balance are provided in a similar investigation by Lignarolo et al (2014a). The results of the thrust measurements are reported by Lignarolo et al (2014b).…”

Section: B Wind-turbine Modelmentioning

confidence: 99%

“…A comparison between a WT wake and an AD wake in the presence of leapfrogging is for this reason of particular interest. Following the experimental studies of Dobrev et al (2008), Felli et al (2011), andSherry et al (2010), Lignarolo et al (2014a) have shown how the instability of the tip-vortex helix has a major effect on the wake mixing and re-energising mechanism. Lignarolo et al (2015a) have conducted a detailed analysis of the effect on the turbulence field and wake mixing due to the presence of the leapfrogging instability.…”

mentioning

confidence: 89%

“…At the latter distance, the uniform-flow section reduces to approximately 2 Â 2 m 2 . A detailed characterization of the wind-tunnel flow can be found by Lignarolo et al (2014a). The flow is driven by a fan with an electrical engine of 500 kW and the temperature is kept constant at 20 C by a heat exchanger which provides up to 350 kW of cooling power.…”

Section: A Wind-tunnelmentioning

confidence: 99%

“…The porous disc is assembled in order to have the same diameter and drag coefficient of the WT model. The latter is the same two-bladed 60 cm diameter rotor used by Lignarolo et al (2014a). The comparison of the two wakes is conducted in the presence of an instability of the tip-vortex helical structure, the so-called leapfrogging, which causes the tip vortices to pair and roll around each other to form a single vortex structure.…”

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confidence: 99%

“…In experimental studies by Bolnot et al (2014) and Odemark and Fransson (2013), artificial disturbances were also used for triggering the tipvortex instability, as asymmetries in the rotor geometry and pulsed air jets. In the present study, the leapfrogging is triggered by an asymmetry of the blade pitch angle, as by Lignarolo et al (2014a). The use of an adapted formulation of the momentum equation in differential form allowed to compute the pressure field and stagnation enthalpy around the rotor and the porous disc directly from the velocity data in the incompressible regime (Ragni et al, 2011;2012;and van Oudheusden, 2013).…”

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confidence: 99%

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Experimental comparison of a wind-turbine and of an actuator-disc near wake

Lignarolo

Ragni

Ferreira

et al. 2016

Journal of Renewable and Sustainable Energy

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The actuator disc (AD) model is commonly used to simplify the simulation of horizontal-axis wind-turbine aerodynamics. The limitations of this approach in reproducing the wake losses in wind farm simulations have been proven by a previous research. The present study is aimed at providing an experimental analysis of the near-wake turbulent flow of a wind turbine (WT) and a porous disc, emulating the actuator disc numerical model. The general purpose is to highlight the similarities and to quantify the differences of the two models in the near-wake region, characterised by the largest discrepancies. The velocity fields in the wake of a wind turbine model and a porous disc (emulation of the actuator disc numerical model) have been measured in a wind tunnel using stereo particle image velocimetry. The study has been conducted at low turbulence intensity in order to separate the problems of the flow mixing caused by the external turbulence and the one caused by the turbulence induced directly by the AD or the WT presence. The analysis, as such, showed the intrinsic differences and similarities between the flows in the two wakes, solely due to the wake-induced flow, with no influence of external flow fluctuations. The data analysis provided the time-average three-component velocity and turbulence intensity fields, pressure fields, rotor and disc loading, vorticity fields, stagnation enthalpy distribution, and mean-flow kinetic-energy fluxes in the shear layer at the border of the wake. The properties have been compared in the wakes of the two models. Even in the absence of turbulence, the results show a good match in the thrust and energy coefficient, velocity, pressure, and enthalpy fields between wind turbine and actuator disc. However, the results show a different turbulence intensity and turbulent mixing. The results suggest the possibility to extend the use of the actuator disc model in numerical simulation until the very near wake, provided that the turbulent mixing is correctly represented.

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Section: B Wind-turbine Modelmentioning

confidence: 99%

mentioning

confidence: 89%

Section: A Wind-tunnelmentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

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Experimental comparison of a wind-turbine and of an actuator-disc near wake

Lignarolo

Ragni

Ferreira

et al. 2016

Journal of Renewable and Sustainable Energy

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Large‐eddy simulations of wind‐farm wake characteristics associated with a low‐level jet

Koo

Jin

et al. 2017

Wind Energy

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In this study, we performed a suite of flow simulations for a 12-wind-turbine array with varying inflow conditions and lateral spacings, and compared the impacts of the flow on velocity deficit and wake recovery. We imposed both laminar inflow and turbulent inflows, which contain turbulence for the Ekman layer and a low-level jet (LLJ) in the stable boundary layer. To solve the flow through the wind turbines and their wakes, we used a large-eddy simulation technique with an actuator-line method. We compared the time series for the velocity deficit at the first and rear columns to observe the temporal change in velocity deficit for the entire wind farm. The velocity deficit at the first column for LLJ inflow was similar to that for laminar inflow. However, the magnitude of velocity deficit at the rear columns for the case with LLJ inflow was 11.9% greater because of strong wake recovery, which was enhanced by the vertical flux of kinetic energy associated with the LLJ. To observe the spatial transition and characteristics of wake recovery, we performed statistical analyses of the velocity at different locations for both the laminar and LLJ inflows. These studies indicated that strong wake recovery was present, and a kurtosis analysis showed that the probability density function for the streamwise velocity followed a Gaussian distribution. In a quadrant analysis of the Reynolds stress, we found that the ejection and sweep motions for the LLJ inflow case were greater than those for the laminar inflow case. KEYWORDS actuator line method, low-level jet, velocity deficit, vertical flux of kinetic energy, wake recovery, wind farm 1 | INTRODUCTION The investigation of wind-turbine wakes is important because it is closely related to the power efficiency and duration of the entire wind farm. 1,2The wake caused by rotation of turbine blades recovers from the free-stream flow due to kinetic energy flux. If the wake does not recover fully, it enters the downstream wind turbines, which causes their power efficiency to drop drastically and fatigue failure of the blades. 3,4 The main factors that determine the wake characteristics of a wind farm are its design and the condition of the incoming wind. 5,6 Design factors such as the spacing between turbines and the alignment of the wind farm affect the interference and interaction between the wakes generated by each turbine. Moreover, the conditions of the incoming wind-such as speed, direction, and turbulence characteristics-influence both wake generation and recovery.Studies of wind-farm designs and incoming winds have been performed mainly using numerical approaches, because it is difficult to observe the detailed structure of a wake due to limitations such as low Reynolds number and low resolution of the flow. 7,8 Wake studies of various alignments and spacings in wind farms have compared the power output at each row of a wind farm. 9 These studies found that the key factor in minimizing the wake effects of turbines is a small angle of alignment between the turbines and the win...

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An experimental study on the effects of winglets on the tip vortex interaction in the near wake of a model wind turbine

et al. 2020

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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. KEYWORDStip vortex, wake, wind energy, winglets 1 2.6%. Another study by Gaunaa and Johansen 7 showed a stronger positive effect on the power coefficient for downstream-facing winglets than

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Experimental analysis of the wake of a horizontal-axis wind-turbine model

Cited by 155 publications

References 19 publications

Experimental comparison of a wind-turbine and of an actuator-disc near wake

Experimental comparison of a wind-turbine and of an actuator-disc near wake

Large‐eddy simulations of wind‐farm wake characteristics associated with a low‐level jet

An experimental study on the effects of winglets on the tip vortex interaction in the near wake of a model wind turbine

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