Assessing atmospheric stability and its impacts on rotor‐disk wind characteristics at an onshore wind farm

As the average hub height and blade diameter of new wind turbine installations continue to increase, turbines typically encounter higher wind speeds, which enable them to extract large amounts of energy, but they also face challenges due to the complex nature of wind flow and turbulence in the planetary boundary layer (PBL). Wind speed and turbulence can vary greatly across a turbine's rotor disk; this variability is partially due to whether the PBL is stable, neutral or convective. To assess the influence of stability on these wind characteristics, we utilize a unique data set including observations from two meteorological towers, a surface flux tower and high‐resolution remote‐sensing sound detection and ranging (SODAR) instrument. We compare several approaches to defining atmospheric stability to the Obukhov length (L). Typical wind farm observations only allow for the calculation of a wind shear exponent (α) or horizontal turbulence intensity (I_U) from cup anemometers, whereas SODAR gives measurements at multiple heights in the rotor disk of turbulence intensity (I) in the latitudinal (I_u), longitudinal (I_v) and vertical (I_w) directions and turbulence kinetic energy (TKE). Two methods for calculating horizontal Ifrom SODAR data are discussed. SODAR stability parameters are in high agreement with the more physically robust L,with TKE exhibiting the best agreement, and show promise for accurate characterizations of stability. Vertical profiles of wind speed and turbulence, which likely affect turbine power performance, are highly correlated with stability regime. At this wind farm, disregarding stability leads to over‐assessments of the wind resource during convective conditions and under‐assessments during stable conditions. Copyright © 2011 John Wiley & Sons, Ltd.     

Influence of atmospheric stability on wind turbine loads

Simulations of wind turbine loads for the NREL 5 MW reference wind turbine under diabatic conditions are performed. The diabatic conditions are incorporated in the input wind field in the form of wind profile and turbulence. The simulations are carried out for mean wind speeds between 3 and 16 m s ^{? 1} at the turbine hub height. The loads are quantified as the cumulative sum of the damage equivalent load for different wind speeds that are weighted according to the wind speed and stability distribution. Four sites with a different wind speed and stability distribution are used for comparison. The turbulence and wind profile from only one site is used in the load calculations, which are then weighted according to wind speed and stability distributions at different sites. It is observed that atmospheric stability influences the tower and rotor loads. The difference in the calculated tower loads using diabatic wind conditions and those obtained assuming neutral conditions only is up to 17%, whereas the difference for the rotor loads is up to 13%. The blade loads are hardly influenced by atmospheric stability, where the difference between the calculated loads using diabatic and neutral input wind conditions is up to 3% only. The wind profiles and turbulence under diabatic conditions have contrasting influences on the loads; for example, under stable conditions, loads induced by the wind profile are larger because of increased wind shear, whereas those induced by turbulence are lower because of less turbulent energy. The tower base loads are mainly influenced by diabatic turbulence, whereas the rotor loads are influenced by diabatic wind profiles. The blade loads are influenced by both, diabatic wind profile and turbulence, that leads to nullifying the contrasting influences on the loads. The importance of using a detailed boundary‐layer wind profile model is also demonstrated. The difference in the calculated blade and rotor loads is up to 6% and 8%, respectively, when only the surface‐layer wind profile model is used in comparison with those obtained using a boundary‐layer wind profile model. Finally, a comparison of the calculated loads obtained using site‐specific and International Electrotechnical Commission (IEC) wind conditions is carried out. It is observed that the IEC loads are up to 96% larger than those obtained using site‐specific wind conditions.Copyright © 2012 John Wiley & Sons, Ltd.     

Comparison of the atmospheric stability and wind profiles at two wind farm sites over a long marine fetch in the North Sea

A comparison of the atmospheric stability and wind profiles using data from meteorological masts located near two wind farm sites in the North Sea, Egmond aan Zee (up to 116 m) in the Dutch North Sea and Horns Rev (HR; up to 45 m) in the Danish North Sea, is presented. Only the measurements that represent long marine fetch are considered. It was observed that within a long marine fetch, the conditions in the North Sea are dominated by unstable [41% at Egmond aan Zee Offshore Wind Farm (OWEZ) and 33% at HR] and near‐neutral conditions (49% at OWEZ and 47% at HR), and stable conditions (10% at OWEZ and 20% at HR) occur for a limited period. The logarithmic wind profiles with the surface‐layer stability correction terms and Charnock's roughness model agree with the measurements at both sites in all unstable and near‐neutral conditions. An extended wind profile valid for the entire boundary layer is compared with the measurements. For the tall mast at Egmond aan Zee, it was found that for stable conditions, the scaling of the wind profiles with respect to boundary‐layer height is necessary, and the addition of another length scale parameter is preferred. At the lower mast at HR, the effect was not noticeable. Copyright © 2011 John Wiley & Sons, Ltd.     

Modeling stable thermal stratification and its impact on wind flow over topography

Microscale flow models used in the wind energy industry commonly assume statically neutral conditions. These models can provide reasonable wind speed predictions for statically unstable and neutral flows; however, they do not provide reliable predictions for stably stratified flows, which can represent a substantial fraction of the available energy at a given site. With the objective of improving wind speed predictions and in turn reducing uncertainty in energy production estimates, we developed a Reynolds‐Averaged Navier–Stokes (RANS)‐based model of the stable boundary layer. We then applied this model to eight prospective wind farms and compared the results with on‐site wind speed measurements classified using proxies for stability; the comparison also included results from linear and RANS wind flow models that assume neutral stratification. This validation demonstrates that a RANS‐based model of the stable boundary layer can significantly and consistently improve wind speed predictions. Copyright © 2014 John Wiley & Sons, Ltd.     

A simple analytical wind park model considering atmospheric stability

The analytical top‐down wind park model by Emeis and Frandsen 1 is enhanced by consistently making both the downward momentum flux and the momentum loss at the rough surface dependent on atmospheric stability. Specifying the surface roughness underneath the turbines in a wind farm in the model gives the opportunity to investigate principal differences between onshore and offshore wind parks, because the roughness length of the sea surface is two to three orders of magnitude lower than the roughness length of land surfaces. Implications for the necessary distance between single turbines in offshore wind farms and the distance between neighbouring wind parks are computed. It turns out from the model simulations that over smooth surfaces offshore the wind speed reduction at hub height in a wind farm is larger than over rough onshore surfaces given the same density of turbines within the park. Mean wind profiles within the park are also calculated from this model. Offshore wind farms must have a larger distance between each other in order to avoid shadowing effects of the upstream farm. Copyright © 2009 John Wiley & Sons, Ltd.     

Measurement of spectra over the Bolund hill in wind tunnel

We have determined the normal Reynolds stresses and spectra of the wind velocity over a 1:115 scale mock‐up of the Bolund hill. The experiment was run in a neutral boundary layer wind tunnel using 3‐component hot‐wire velocimetry, 2‐component particle image velocimetry, and a high‐precision traversing system. Spectra have been determined at different points along transects at 2 and 5 m height above ground level. The experiment was run for 270° wind direction and for two Reynolds numbers, and , based on the maximum height of the hill and the free wind speed at this height. Our results show how the normalized power spectral density changes over the hill. The analysis of the normalized streamwise spectrum at 2 m height, just after the escarpment, reveals that part of the energy is concentrated in the interval of normalized frequencies n_h≈0.01?0.02, which could be a signature of a weakened “flapping” phenomenon described in the literature for flows over forward facing steps. The departure of the spectra slope in the inertial subrange, from the value ?5/3, was found to be correlated with the hill geometry.     

Variable‐sized wind turbines are a possibility for wind farm optimization

The potential benefits associated with harnessing available momentum and reducing turbulence levels in a wind farm composed of wind turbines of alternating size are investigated through wind tunnel experiments. A variable size turbine array composed of 3 by 8 model wind turbines is placed in a boundary layer flow developed over both a smooth and rough surfaces under neutrally stratified thermal conditions. Cross‐wire anemometry is used to capture high resolution and simultaneous measurements of the streamwise and vertical velocity components at various locations along the central plane of the wind farm. A laser tachometer is employed to obtain the instantaneous angular velocity of various turbines. The results suggest that wind turbine size heterogeneity in a wind farm introduces distinctive flow interactions not possible in its homogeneous counterpart. In particular, reduced levels of turbulence around the wind turbine rotors may have positive effects on turbulent loading. The turbines also appear to perform quite uniformly along the entire wind farm, whereas surface roughness impacts the velocity recovery and the spectral content of the turbulent flow within the wind farm. Copyright © 2013 John Wiley & Sons, Ltd.     

Evaluation of layout and atmospheric stability effects in wind farms using large‐eddy simulation

Large‐eddy simulation (LES) has been used previously to study the effect of either configuration or atmospheric stability on the power generated by large wind farms. This is the first study to consider both stability and wind farm configuration simultaneously and methodically with LES. Two prevailing wind directions, two layouts (turbines aligned versus staggered with respect to the wind) and three stabilities (neutral and moderately unstable and stable) were evaluated. Compared with neutral conditions, unstable conditions led to reduced wake losses in one configuration, to enhanced wake losses in two and to unchanged wake losses in one configuration. Conversely, stable conditions led to increased wake losses in one, decreased wake losses in two and unchanged wake losses in one configuration. Three competing effects, namely, rates of wake recovery due to vertical mixing, horizontal spread of wakes and localized regions of acceleration caused by multiple upstream wakes, were identified as being responsible for the observed trends in wake losses. The detailed flow features responsible for these non‐linear interactions could only be resolved by the LES. Existing analytical models ignore stability and non‐linear configuration effects, which therefore need to be incorporated. Copyright © 2017 John Wiley & Sons, Ltd.     

Mesoscale modeling of coastal low‐level jets: implications for offshore wind resource estimation

Detailed and reliable spatiotemporal characterizations of turbine hub height wind fields over coastal and offshore regions are becoming imperative for the global wind energy industry. Contemporary wind resource assessment frameworks incorporate diverse multiscale prognostic models (commonly known as mesoscale models) to dynamically downscale global‐scale atmospheric fields to regional‐scale (i.e., spatial and temporal resolutions of a few kilometers and a few minutes, respectively). These high‐resolution model solutions aim at depicting the expected wind behavior (e.g., wind shear, wind veering and topographically induced flow accelerations) at a particular location. Coastal and offshore regions considered viable for wind power production are also known to possess complex atmospheric flow phenomena (including, but not limited to, coastal low‐level jets (LLJs), internal boundary layers and land breeze–sea breeze circulations). Unfortunately, the capabilities of the new‐generation mesoscale models in realistically capturing these diverse flow phenomena are not well documented in the literature. To partially fill this knowledge gap, in this paper, we have evaluated the performance of the Weather Research and Forecasting model, a state‐of‐the‐art mesoscale model, in simulating a series of coastal LLJs. Using observational data sources we explore the importance of coastal LLJs for offshore wind resource estimation along with the capacity to which they can be numerically simulated. We observe model solutions to demonstrate strong sensitivities with respect to planetary boundary layer parameterization and initialization conditions. These sensitivities are found to be responsible for variability in AEP estimates by a factor of two. Copyright © 2013 John Wiley & Sons, Ltd.     

Analysis of impact of grid‐connected wind power on small signal stability

The power system small signal stability analysis considering wind generation intermittence is studied comprehensively in this paper. The modelling of doubly‐fed induction generator (DFIG) involving the converters with application of stator flux‐oriented vector control strategy is addressed briefly. In order to reveal how the intermittent nature of wind power affects the operating behaviour of an existing power system, a probabilistic small signal stability analysis method based on Monte Carlo simulation technique is proposed to explore and exploit the impact of intermittent grid‐connected wind power on small signal stability. The IEEE New England test system is applied as benchmark to verify the proposed model and approach. Total 3 scenarios are elaborately designed to figure out the potential relationship between the small signal stability indices and the wind generation intermittence. Finally, some preliminary conclusions and comments were drawn based on the numerical simulation results. Copyright © 2010 John Wiley & Sons, Ltd.     

Turbulence modeling of atmospheric boundary layer flow over complex terrain: a comparison of models at wind tunnel and full scale

The aim of this work is to evaluate the performance of two popular k ? ? turbulence closure schemes for atmospheric boundary layer (ABL) flow over hills and valleys and to investigate the effect of using ABL‐modified model constants. The standard k ? ? and the RNG k ? ? models are used to simulate flow over the two‐dimensional analytical shapes from the RUSHIL and RUSHVAL wind tunnel experiments. Furthermore, the mean turbulent flow over the real complex terrain of Blashaval hill is simulated and the results verified with a data set of full‐scale measurements. In general, all models yield similar results. However, use of ABL‐modified constants in both models tends to decrease the predicted velocity and increase the predicted turbulent kinetic energy. Copyright © 2010 John Wiley & Sons, Ltd.     

Wind turbine power and sound in relation to atmospheric stability

Atmospheric stability cannot, with respect to modern, tall wind turbines, be viewed as a ‘small perturbation to a basic neutral state’. This can be demonstrated by comparison of measured wind velocity at the height of the rotor with the wind velocity expected in a neutral or ‘standard’ atmosphere. Atmospheric stability has a significant effect on wind shear and increases the power production substantially relative to a neutral atmosphere. This conclusion from Dutch data is corroborated by other published wind shear data from the temperate climate zone. The increase in wind shear due to atmospheric stability also has a significant effect on the sound emission, causing it to be substantially higher than predicted from near‐ground wind velocity and a neutral atmosphere, resulting in a higher noise impact on neighbouring residences. Several measures are proposed to mitigate the noise impact. To reduce noise levels, the rotational speed can be controlled with the near‐ground wind speed or sound level as the control input. To reduce the fluctuation in the sound (‘blade thumping’), it is suggested to adjust the blade pitch angle of the rotating blades continuously. To prevent stronger fluctuations at night due to the coincidence of thumps from several turbines, it is suggested to add random variations in pitch angle, mimicking the effect of large‐scale turbulent fluctuations in daytime. Copyright © 2007 John Wiley & Sons, Ltd.     

Large‐eddy simulation of stable boundary layer turbulence and estimation of associated wind turbine loads

Stochastic simulation of turbulent inflow fields commonly used in wind turbine load computations is unable to account for contrasting states of atmospheric stability. Flow fields in the stable boundary layer, for instance, have characteristics such as enhanced wind speed and directional shear; these effects can influence loads on utility‐scale wind turbines. To investigate these influences, we use large‐eddy simulation (LES) to generate an extensive database of high‐resolution ( ～ 10 m), four‐dimensional turbulent flow fields. Key atmospheric conditions (e.g., geostrophic wind) and surface conditions (e.g., aerodynamic roughness length) are systematically varied to generate a diverse range of physically realizable atmospheric stabilities. We show that turbine‐scale variables (e.g., hub height wind speed, standard deviation of the longitudinal wind speed, wind speed shear, wind directional shear and Richardson number) are strongly interrelated. Thus, we strongly advocate that these variables should not be prescribed as independent degrees of freedom in any synthetic turbulent inflow generator but rather that any turbulence generation procedure should be able to bring about realistic sets of such physically realizable sets of turbine‐scale flow variables. We demonstrate the utility of our LES‐generated database in estimation of loads on a 5‐MW wind turbine model. More importantly, we identify specific turbine‐scale flow variables that are responsible for large turbine loads—e.g., wind speed shear is found to have a greater influence on out‐of‐plane blade bending moments for the turbine studied compared with its influence on other loads such as the tower‐top yaw moment and the fore‐aft tower base moment. Overall, our study suggests that LES may be effectively used to model inflow fields, to study characteristics of flow fields under various atmospheric stability conditions and to assess turbine loads for conditions that are not typically examined in design standards. Copyright © 2013 John Wiley & Sons, Ltd.     

Characterizing the response of a wind turbine model under complex inflow conditions

A horizontal axis wind turbine model was tested in a closed‐circuit wind tunnel under various inflow conditions. Separate experiments placed the test turbine (i) in the wake of a three‐dimensional, sinusoidal hill, (ii) in the wake of another turbine and (iii) in the turbulent boundary layer, as a reference case. Simultaneous high‐frequency measurements of the turbine output voltage, rotor angular velocity along with streamwise and wall normal velocity components were collected at various locations through the turbine's miniature direct‐current (DC) generator, a high‐resolution laser tachometer and cross‐wire anemometer, respectively. Validation trials were conducted first in order to characterize the test turbine's output and response to the baseline turbulent boundary layer. Analysis was performed by comparing the cross‐wire anemometry measurements of the incoming flow with the turbine voltage output to investigate the unsteady rotor kinematics under different flow perturbations. Using spectral, auto‐correlation and cross‐correlation methods, it was found that the flow structures developing downwind of the hill leave a stronger signature on the fluctuations and spectrum of the rotor angular velocity, as compared with those flow structures filtered or deflected by placing a turbine upwind. In summary, we show that the effects on downwind turbines of complex terrain and multi‐turbine arrangements are consistent with the induced modifications by the hill or turbine on the large scale structures in the incoming flow. Copyright © 2014 John Wiley & Sons, Ltd.     

Experimental detection of laminar‐turbulent transition on a rotating wind turbine blade in the free atmosphere

This paper discusses the findings from a measurement campaign on a rotating wind turbine blade operating in the free atmosphere under realistic conditions. A total of 40 pressure sensors together with an array of 23 usable hot‐film sensors (based on constant temperature anemometry) were used to study the behavior of the boundary layer within a specific zone on the suction side of a 30 m diameter wind turbine at different operational states. A set of several hundreds of data sequences were recorded. Some of them show that under certain circumstances, the flow may be regarded as not fully turbulent. Accompanying Computational Fluid Mechanics (CFD) simulations suggest the view that a classical transition scenario according to the growth of so‐called Tollmien–Schlichting did not apply. Copyright © 2016 John Wiley & Sons, Ltd.     

Collapse analysis of wind turbine tower under the coupled effects of wind and near‐field earthquake

In this paper, the pattern of wind turbine tower collapse as a result of the coupled effects of wind and an intense, near‐field earthquake is investigated. The constitutive relation of the tower cylinder steel is simulated via a nonlinear kinematic hardening model, and the specific value of each parameter in the constitutive model is provided. A precise model of the tower structure coupled with the blade is created using a nonlinear, finite element method. This method is compared with the results from a static pushover test of a small cylindrical tower to validate the finite element modeling method in this research. Two earthquake wave sets are selected as inputs. One contains 20 near‐field velocity pulse‐like ground motion waves with various pulse periods; the other contains 20 ordinary far‐field ground motion waves. A wind turbine tower with a hub height of 60 m is selected as an example for analysis. The dynamic response of this tower as a result of the coupled effects of the two ground motion wave sets and a transient wind load is calculated using nonlinear time‐history analysis. The calculation results shows that the average horizontal displacement of the tower top as a result of the near‐field velocity pulse‐like ground motion is 33% larger than the case with far‐field ground motion. Finally, the seismic collapse vulnerability curve of this wind turbine tower is calculated. The seismic collapse capacity of the tower is evaluated, and the seismic collapse pattern of the tower is analyzed.     

An experimental and numerical study of the atmospheric stability impact on wind turbine wakes

In this paper, the impact of atmospheric stability on a wind turbine wake is studied experimentally and numerically. The experimental approach is based on full‐scale (nacelle based) pulsed lidar measurements of the wake flow field of a stall‐regulated 500 kW turbine at the DTU Wind Energy, Risø campus test site. Wake measurements are averaged within a mean wind speed bin of 1 m s^?1 and classified according to atmospheric stability using three different metrics: the Obukhov length, the Bulk–Richardson number and the Froude number. Three test cases are subsequently defined covering various atmospheric conditions. Simulations are carried out using large eddy simulation and actuator disk rotor modeling. The turbulence properties of the incoming wind are adapted to the thermal stratification using a newly developed spectral tensor model that includes buoyancy effects. Discrepancies are discussed, as basis for future model development and improvement. Finally, the impact of atmospheric stability on large‐scale and small‐scale wake flow characteristics is presently investigated. Copyright © 2015 John Wiley & Sons, Ltd.     

A novel empirical model for vertical profiles of downburst horizontal wind speed

This study proposes an empirical model for preliminary wind-resist design of downburst flow. Existing empirical models were compared with field data and found to underpredict horizontal wind speed below the height corresponding to the maximum radial velocity, due to the neglect of viscous effects and the evolution of vertical wind profiles along radial direction. To address these deficiencies, semi-empirical piecewise functions including wall shear effects in the local turbulent boundary layer and interpolation functions were proposed to improve the accuracy of existing models. The wind profile based on Coles' theory was found to agree well with field data, with the parabola interpolation function being the most desirable. Using the proposed method, the vertical profile of horizontal wind speed at different local radial locations can be predicted for wind resist design given the inlet wind speed of the downburst flow. Overall, this model improves upon existing empirical models and allows for more accurate wind-resist design.     

Impact of different forest densities on atmospheric boundary‐layer development and wind‐turbine wake

The aim of this work is to investigate the atmospheric boundary‐layer (ABL) flow and the wind turbine wake over forests with varying leaf area densities (LAD). The forest LAD profile used in this study is based on a real forest site, Ryningsnäs, located in Sweden. The reference turbine used to model the wake is a well‐documented 5‐MW turbine, which is implemented in the simulations using an actuator line model (ALM). All simulations are carried out with openFOAM using the Reynolds averaged Navier‐Stokes (RANS) approach. Twelve forest cases with leaf area index (LAI) ranging from 0.42 to 8.5 are considered. Results show that the mean velocity decreases with increasing LAI within the forest canopy, but increases with LAI above the hub height. Meanwhile, the turbulent kinetic energy (TKE) varies nonmonotonically with forest density. The TKE increases with forest density and reaches to its maximum at an average LAI of 1.70, afterwards, it decreases gradually as the density increases. It is also observed that the forest density has a clear role in the wake development and recovery. Comparisons between no‐forest and forest cases show that the forest characteristics help in damping the added turbulence from the turbine. As a consequence, the forest with the highest upstream turbulence has the shortest wake downstream of the turbine.