Wake flow in a wind farm during a diurnal cycle

Abkar, Mahdi; Sharifi, Ahmad; Porté‐Agel, Fernando

doi:10.1080/14685248.2015.1127379

Cited by 111 publications

(116 citation statements)

References 42 publications

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“…In fact, they showed that, due to the effect of the LLJ, the upstream wind turbines generate much more power at night as compared to the wind turbines deep within the wind farm. A similar scenario was also observed by Abkar et al (2016), who additionally showed that the LLJ was shifted upwards and is re-generated above the wind farm; much like the present results. To reconcile our infinite wind-farm results with these works, a solitary wind turbine was placed in the diurnal ABL simulation.…”

Section: Phenomenology Of the Low-level Jetsupporting

confidence: 76%

“…The results of the WT24 simulation are similar to the SBL simulations of Lu and Porté-Agel (2011) with a decrease in turbulence levels below the rotor-disk region and the upward shift of the LLJ, as also reported in LES of finite-sized wind farms (Abkar et al 2016) For the noWT simulation, H is smaller than L for the entire diurnal cycle (0.03 ≤ H noWT /L noWT ≤ 0.9) as expected. The upper limit of the H/L ratio is found only during the late convective period (between 1500 and 1800 h); in contrast, for the WT24 simulation, H is larger than L throughout the diurnal cycle (1.1 ≤ H WT24 /L WT24 ≤ 9).…”

Section: Evolution Of the Diurnally-varying Ablsupporting

confidence: 71%

“…The effect of wind farms on LLJs has been documented recently in Fitch et al (2013a) and Abkar et al (2016), and in both a finite-size wind farm was simulated as opposed to the infinite farm in our study. Fitch et al (2013a) found that the LLJ developed upstream of the wind farm, and is depleted by the first two rows.…”

Section: Phenomenology Of the Low-level Jetmentioning

confidence: 99%

“…This is precisely what we set out to explore through our computational experiments of diurnal flow simulations in the presence of a wind farm. Two similar experiments have been performed by Fitch et al (2013a), who used the WRF model to study the mesoscale influence of a finite-sized wind farm during the same diurnal cycle as used herein and by Abkar et al (2016) who performed a high-resolution LES of a finite-sized wind farm to study the effects of an evolving ABL on the wakes of wind turbines. While these two studies have used different simulation methodologies and have different motivations as compared to the present study, they serve as important benchmarks to validate our results.…”

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Perturbations to the Spatial and Temporal Characteristics of the Diurnally-Varying Atmospheric Boundary Layer Due to an Extensive Wind Farm

Sharma

Parlange

Calaf

2016

Boundary-Layer Meteorol

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The effect of extensive terrestrial wind farms on the spatio-temporal structure of the diurnally-evolving atmospheric boundary layer is explored. High-resolution largeeddy simulations of a realistic diurnal cycle with an embedded wind farm are performed. Simulations are forced by a constant geostrophic velocity with time-varying surface boundary conditions derived from a selected period of the CASES-99 field campaign. Through analysis of the bulk statistics of the flow as a function of height and time, it is shown that extensive wind farms shift the inertial oscillations and the associated nocturnal low-level jet vertically upwards by approximately 200 m; cause a three times stronger stratification between the surface and the rotor-disk region, and as a consequence, delay the formation and growth of the convective boundary layer (CBL) by approximately 2 h. These perturbations are shown to have a direct impact on the potential power output of an extensive wind farm with the displacement of the low-level jet causing lower power output during the night as compared to the day. The low-power regime at night is shown to persist for almost 2 h beyond the morning transition due to the reduced growth of the CBL. It is shown that the wind farm induces a deeper entrainment region with greater entrainment fluxes. Finally, it is found that the diurnally-averaged effective roughness length for wind farms is much lower than the reference value computed theoretically for neutral conditions.

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Section: Phenomenology Of the Low-level Jetsupporting

confidence: 76%

Section: Evolution Of the Diurnally-varying Ablsupporting

confidence: 71%

Section: Phenomenology Of the Low-level Jetmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Perturbations to the Spatial and Temporal Characteristics of the Diurnally-Varying Atmospheric Boundary Layer Due to an Extensive Wind Farm

Sharma

Parlange

Calaf

2016

Boundary-Layer Meteorol

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“…The latter approach corresponds to a land-sea transition and gives rise to a stable internal boundary layer, a flow regime that occurs frequently in the Baltic Sea (see, e.g., Tjernström and Smedman 1993;Smedman et al 1997;Lange et al 2004). More recently, Abkar et al (2016) investigated the wake flow in an idealised finite-size wind farm with 36 turbines over the course of a full diurnal cycle. Cortina et al (2016) and Sharma et al (2017) investigated the flow behaviour in an infinite wind-turbine array during a diurnal cycle with forcing conditions extracted from the Cooperative Atmosphere-Surface Exchange Study 1999 (CASES-99) field campaign (Poulos et al 2002).…”

Section: Introductionmentioning

confidence: 99%

Gravity Waves and Wind-Farm Efficiency in Neutral and Stable Conditions

Allaerts

Meyers

2017

Boundary-Layer Meteorol

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We use large-eddy simulations (LES) to investigate the impact of stable stratification on gravity-wave excitation and energy extraction in a large wind farm. To this end, the development of an equilibrium conventionally neutral boundary layer into a stable boundary layer over a period of 8 h is considered, using two different cooling rates. We find that turbulence decay has considerable influence on the energy extraction at the beginning of the boundary-layer transition, but afterwards, energy extraction is dominated by geometrical and jet effects induced by an inertial oscillation. It is further shown that the inertial oscillation enhances gravity-wave excitation. By comparing LES results with a simple one-dimensional model, we show that this is related to an interplay between wind-farm drag, variations in the Froude number and the dispersive effects of vertically-propagating gravity waves. We further find that the pressure gradients induced by gravity waves lead to significant upstream flow deceleration, reducing the average turbine output compared to a turbine in isolated operation. This leads us to the definition of a non-local wind-farm efficiency, next to a more standard wind-farm wake efficiency, and we show that both can be of the same order of magnitude. Finally, an energy flux analysis is performed to further elucidate the effect of gravity waves on the flow in the wind farm.

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Toward understanding the physical link between turbines and microclimate impacts from in situ measurements in a large wind farm

et al. 2016

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Recent wind farm studies have revealed elevated nighttime surface temperatures but have not validated physical mechanisms that create the observed effects. We report measurements of concurrent differences in surface wind speed, temperature, fluxes, and turbulence upwind and downwind of two turbine lines at the windward edge of a utility‐scale wind farm. On the basis of these measurements, we offer a conceptual model based on physical mechanisms of how wind farms affect their own microclimate. Periods of documented curtailment and zero‐power production of the wind farm offer useful opportunities to rigorously evaluate the microclimate impact of both stationary and operating turbines. During an 80 min nighttime wind farm curtailment, we measured abrupt and large changes in turbulent fluxes of momentum and heat leeward of the turbines. At night, wind speed decreases in the near wake when turbines are off but abruptly increases when turbine operation is resumed. Our measurements are compared with Moderate Resolution Imaging Spectroradiometer Terra and Aqua satellite measurements reporting wind farms to have higher nighttime surface temperatures. We demonstrate that turbine wakes modify surface fluxes continuously through the night, with similar magnitudes during the Terra and Aqua transit periods. Cooling occurs in the near wake and warming in the far wake when turbines are on, but cooling is negligible when turbines are off. Wind speed and surface stratification have a regulating effect of enhancing or decreasing the impact on surface microclimate due to turbine wake effects.

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Wake flow in a wind farm during a diurnal cycle

Cited by 111 publications

References 42 publications

Perturbations to the Spatial and Temporal Characteristics of the Diurnally-Varying Atmospheric Boundary Layer Due to an Extensive Wind Farm

Perturbations to the Spatial and Temporal Characteristics of the Diurnally-Varying Atmospheric Boundary Layer Due to an Extensive Wind Farm

Gravity Waves and Wind-Farm Efficiency in Neutral and Stable Conditions

Toward understanding the physical link between turbines and microclimate impacts from in situ measurements in a large wind farm

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