Mean Wind Speed Profiles Research Articles

Analysis of Wind Power Fluctuation in Wind Turbine Wakes Using Scale-Adaptive Large Eddy Simulation

In large wind farms, the interaction of atmospheric turbulence and wind turbine wakes leads to complex vortex dynamics and energy dissipation, resulting in reduced wind velocity and subsequent loss of wind power. This study investigates the influence of vortex stretching on wind power fluctuations within wind turbine wakes using scale-adaptive large eddy simulation. The proper orthogonal decomposition method was employed to extract the most energetic contributions to the wind power spectra. Vertical profiles of mean wind speed, Reynolds stresses, and dispersive stresses were analyzed to assess energy dissipation rates. Our simulation results showed excellent agreement when compared with wind tunnel data and more advanced numerical models, such as the actuator line model and the actuator line model with hub and tower effects. This highlights the important role of coherent and energetic flow components in the spectral behavior of wind farms. The findings indicate a persistent energy cascading length scale in the wake of wind turbines, emphasizing the vertical transport of energy to turbine blades. These results complement existing literature and provide new insights into the dynamics of wind turbine wakes and their impact on wind farm performance.

Wind Speed Shear Exponent: Effects of Sea Surface Roughness in Terms of Spectral Bandwidth

Abstract This note presents results on the wind speed shear exponent demonstrating the effects of the sea surface roughness in terms of the spectral bandwidth of ocean surface waves. The wind speed shear exponent is obtained by combining the logarithmic mean wind speed profile and the power law mean wind speed profile containing the sea surface roughness as a parameter, as used by Viselli et al. (2022, “LiDAR Measurements of Wind Shear Exponents and Turbulence Intensity Offshore the Northeast United States,” ASME J. Offshore Mech. Arct. Eng., 144(4), p. 042001). In the present article, the sea surface roughness is given in terms of the spectral bandwidth as presented by Zhao and Li (2024, “Dependence of Drag Coefficient on the Spectral Width of Ocean Waves,” J. Oceanogr., 80(2), pp. 129–143). Examples of results representing realistic wave conditions are provided. The results should be useful demonstrating how the wind speed shear exponent can be estimated using the spectral bandwidth, which is a frequently used parameter to characterize wave conditions.

Development of a morphology-based wind speed model in the urban roughness sub-layer

Understanding near-surface wind variations in urban environments is essential for a wide range of applications, including urban weather forecasting and wind impact assessment. In this study, we introduced a novel model for mean wind speed profiles within the urban roughness sublayer based on theoretical analysis. To calibrate the model's coefficients, we conducted large-eddy simulations of airflow over a variety of idealized urban configurations. The resulting expression effectively captures wind speed variations across different urban morphologies. Subsequently, a regression modeling was employed to identify the relationships between building morphological parameters and these coefficients. This highlights the pivotal role of using building morphology to predict near-surface velocity profiles. The proposed model also yields significantly more accurate wind speed predictions within the urban canopy layer than traditional exponential profiles. The findings in the present study lay a more robust foundation for assessing urban wind conditions and improving urban-scale weather forecasts.

A new probability-distribution scale synthetic eddy method for large eddy simulation of wind loads

The accurate generation of inflow turbulence in large eddy simulation (LES) plays a crucial role in assessing the wind loads on buildings. To faithfully recreate turbulent flow fields, the inflow turbulence must accurately replicate vortex structures. In this study, we propose a novel method for generating turbulent inflow called the probability-distribution scale synthetic eddy method (PDSSEM). PDSSEM utilizes a random generation of eddy positions, with the length of each eddy scale determined by its probability density function (PDF). This approach ensures that PDSSEM maintains consistency in the mean wind speed profile, turbulence integral length, turbulence intensity profile, and wind speed spectra of the atmospheric boundary layer (ABL). The PDSSEM method can be more easily applied compared with multi-scale synthetic eddy method (MSSEM), making it highly versatile and adaptable. The self-sustaining characteristics of the flow field generated by PDSSEM are validated, revealing abundant vortex structures and good agreement with targeted values for turbulence flow characteristics. Furthermore, comparing the wind pressures of a tall building with the experimental data, the accuracy and feasibility of PDSSEM are comprehensively demonstrated. Thus, it indicates the great potential of the proposed method for broader computational wind engineering applications, including wind load evaluation and flow pattern investigation.

Experimental translating downbursts immersed in the atmospheric boundary layer

Thunderstorm winds are cold descending gravity currents whose impingement on the ground creates strong radial outflows with maximum wind speeds in the near-ground region. They represent one of the greatest hazards for natural and built environment as well as one of the deadliest phenomena all over the world. This study carries on the post-processing analyses of the downburst experimental campaign performed at the WindEEE Dome, at Western University in Canada, in the context of the ERC project THUNDERR. While a former study presented the interaction between downburst and atmospheric boundary layer (ABL) winds, here the focus is on the influence exerted by the thunderstorm cloud translation. This was experimentally replicated at large scale by means of impinging jet technique where the jet axis was inclined to a non-zero angle with respect to the vertical. Finally, the inclusion of background ABL wind allowed to reconstruct the complete three-dimensional and non-stationary nature of the phenomenon. The outflow radial symmetry is lost in case of inclined jet axis. This leads to an intensification of the front-wind side and weakening of the rear-wind side, where the entrainment of the counter-directed ABL wind, and consequent flow speed-up, are not as pronounced as in the vertical-axis case. The non-linearity of the complex interaction between downburst, ABL flow and cloud translation is proven and quantified. Vertical profiles of mean wind speed and turbulence intensity are discussed in relation to the mutual interaction among flow components.

Winds and eddy dynamics in the urban canopy layer over a city: A parameterization based on the mixing-layer analogy

Urban atmospheric flows are vital to the global ecology. This study characterizes urban canopy layer (UCL) dynamics and parameterizes the flows in the atmospheric surface layer (ASL) over heterogeneous urban surfaces. Large-eddy simulations (LESs) are used to transiently calculate the winds over a real, dense city. A linear function of eddy diffusivity of momentum KM is applied to the lower UCL. Analogous to its mixing-layer counterpart, the strong UCL top shear manifests an inflected mean wind speed profile which aligns well with the exponential law. The solutions to the mixing length lm and the turbulent momentum flux are analytically derived by consolidating the mixing-layer type shear and the form drag from the explicitly resolved roughness elements. The behavior of lm in the lower UCL, especially its peaked level, is captured well. Based on the balance between shear and form drag, an aerodynamic effective roof level Hae is designated where the ground effect is alleviated under shear dominance. Results reveal that a rougher urban surface generates eddies with a larger shear length scale, thus enhancing momentum transport. In-canopy turbulence mixing, which slows down wind decay, is also enhanced, resulting in stronger street-level breezes. The newly developed ASL flow model will be beneficial to urban planning by offering reliable predictions, effectuating the management of urban sustainability.

Experimental Simulation of Thunderstorm Profiles in an Atmospheric Boundary Layer Wind Tunnel

Thunderstorms have different features in comparison with synoptic events, including a typical nose-shaped mean wind speed profile and non-stationary characteristics in time intervals from 10 min to 1 h. The simulation of thunderstorms in traditional wind tunnels requires suitable devices in order to replicate their peculiar characteristics. Disregarding the non-stationary characteristics of thunderstorm outflows, this paper aims to study the possibility of adopting a passive device such as a specially designed grid in order to reproduce the nose-shaped mean wind speed profile. A widely adopted model of the mean wind velocity profile from the literature is employed as a target profile for the verification of the experimental findings. The results obtained show a good agreement between the measured and target mean wind speed profiles and an acceptable turbulence intensity level compared with full-scale and experimental measurements. The proposed device offers a practical and cost-effective solution to simulate the main characteristics of a thunderstorm event in a traditional atmospheric boundary layer wind tunnel, which could be adopted to assess the significance of thunderstorm loading on civil engineering structures and define the requirement for ad hoc specialist studies.

Investigation of spatial variability in power law index of wind profiles above the urban area around Tokyo Bay based on local objective analysis data

By utilizing meteorological local objective analysis (LA) data on a local scale, this study investigated the spatial distribution of the power law index (PLI) of the mean wind speed profile in an urban area around Tokyo Bay in Japan. First, the reliability of the LA data was verified by comparison with observational data. The PLIs of the area for the 16 wind directions at a spatial resolution of 5 km were then estimated using LA data. Under neutral atmospheric conditions, the effects of the roughness length and terrain elevation on the PLI were investigated, and the spatial distribution characteristics of the PLI were analyzed. The results showed that the LA data were consistent with the observational data, indicating that high-precision LA data produced by data assimilation can be a practical supplement to observational data over urban areas. The influence of wind direction on the PLI was evident in the urban area. Furthermore, the metropolis appeared to affect the wind profile significantly when it was within the fetch of other cities, resulting in a larger PLI in the surrounding area. Because the PLI has complex spatial heterogeneity, in addition to urban substrate roughness, changes in terrain elevation and wind direction are thought to affect the PLI significantly. Therefore, generalizing the wind profile characteristics of large-area urban regions using local on-site observational data is not recommended.

Inertial and roughness sublayer flows over real urban morphology: A comparison of wind tunnel experiment and large-eddy simulation

Turbulent flows in the inertial sublayer (ISL) and the roughness sublayer (RSL) are highly complicated by real urban surfaces with various building heights and scales. This study used wind tunnel (WT) modeling and large-eddy simulation (LES) to examine the flow structure in the ISL and RSL over a real urban district of Hong Kong. Although random vertical variations are found in the RSL, the results show that the vertical profiles of mean wind speed, streamwise and vertical fluctuating velocity together with momentum flux are comparable between WT and LES. Unlike the linear decrease of momentum flux with increasing height over idealized urban surfaces, significant vertical changes of momentum flux are found in the RSL in both WT and LES, indicating that modification of momentum flux parameterization over real urban surfaces is needed. High-speed, downward flows are observed in the RSL compared with low-speed, upward flows in ISL. Quadrant-hole analysis shows that the wakes after high-rise buildings could significantly affect Q2 and Q4 events that enlarge the turbulence motion scales. These results could help in-depth understanding of the difference in flow structure of ISL and RSL, as well as formulate the implications to CFD validation.

Field Measurements of Wind Microclimate at the Vehicle Level on a Bridge Deck under Typical Canyon Terrain

The wind field characteristics of a bridge site are the key issues for driving the safety assessment condition of mountain canyon terrain. Field measurements on a bridge deck were conducted to investigate the microclimate wind environment at the vehicle level. The measurement points were arranged in the midspan and bridge tower regions along the bridge at vehicle height above the bridge deck. The height-dependent characteristics of the microclimate wind environment at the midspan and bridge tower regions were extensively analyzed. The results show that the mean wind profiles at midspan approximately confirm the power exponential distribution. However, the mean wind profiles show non-power-exponential distribution at the tower regions. The vertical mean wind speed profiles at the midspan and bridge tower regions were then statistically fitted and suggested. The wind speed distribution at the bridge tower regions showed obvious shielding or acceleration effects. Therefore, a typical wind envelope curve across the tower region was proposed along the driving direction. For the turbulence characteristics, turbulence intensities at the tower regions are larger than those at the midspan, and integral scales at the tower regions are smaller than those at the midspan. In this study, a double logarithm third polynomial was recommended to depict the energy distribution of the turbulence microclimate wind in the frequency domain instead of the commonly used empirical spectrum functions. Gust wind speeds were also discussed, and showed that gust wind speeds are more reasonable for evaluating vehicle driving safety and comfort in long-span bridges, especially at the tower regions.

Investigation of wind characteristics of typhoon boundary layer through field experiments and CFD simulations

High-resolution observations of typhoon boundary layer above 100 m are rare as traditional wind towers are generally below 100 m, which limits the study of typhoon boundary layer and engineering applications such as wind-resistant design of tall buildings and wind turbines in typhoon-prone regions. In this study, boundary layer winds of super typhoon Lekima (2019) are observed, simulated and analyzed. Together with traditional wind tower, Doppler wind lidar is utilized for observations of typhoon boundary layer in order to obtain measured data above 100 m. Besides, Computational Fluid Dynamics (CFD) simulation based on Large Eddy Simulation (LES) method is conducted to further investigate the impact of complex terrain on the near-surface wind characteristics. The results show that the power law fits the mean wind speed profile well below 100 m. However, before and after the typhoon lands, a local reverse or low-level jet occurs in the mean wind speed profile at the height of 100–300 m, which cannot be depicted by the power law. Meanwhile, the turbulence intensity increases with height and experiences larger fluctuations. In addition, there is a significant negative correlation between the ground elevation and power exponents of the fitted mean wind speed profiles. This study provides useful information to better understand wind characteristics of the typhoon boundary layer.

Discussion: “LiDAR Measurements of Wind Shear Exponents and Turbulence Intensity Offshore the Northeast United States” (Viselli, A., Faessler, N., and Filippelli, M., 2022, ASME J. Offshore Mech. Arct. Eng., 144(4), p. 042001)

Abstract The work by Viselli et al. (Viselli, A., Faessler, N., and Filippelli, M., 2022, “LiDAR Measurements of Wind Shear Exponents and Turbulence Intensity Offshore the Northeast United States,” ASME J. Offshore Mech. Arct. Eng., 144(4), p. 042001) presents results on wind shear exponents and turbulence intensity obtained from the analysis of field data collected offshore the Northeast United States. This discussion is focused on the wind shear exponent that is obtained by combining the logarithmic mean wind speed profile and the power law mean wind speed profile, containing the sea surface roughness as a parameter. Here, the sea surface roughness is specified using three formulae given in terms of significant wave height and spectral wave steepness. The discussion points out how the sea surface roughness can be included in future applications of the authors' analysis method.

Closure scheme for stably stratified turbulence without critical Richardson number

This paper presents a new modification of the second order turbulence closure that removes the critical gradient Richardson number limitation typically found in Mellor-Yamada style models. The mean wind speed and potential temperature profiles are derived for the newly modified model in terms of similarity and structure functions depending on the gradient Richardson number. The derivation is based on a second order boundary layer approximation in neutral to very stable stratification conditions. Some recent closure assumptions for pressure-temperature and heat flux are considered. Variances and covariances of the turbulent fluctuations are also investigated with respect to the gradient Richardson number. The new model predictions are confronted with some well known models.

Multi-stage typhoon-induced wind effects on offshore wind turbines using a data-driven wind speed field model

In light of spatiotemporal variations in a typhoon wind speed field, this paper proposes a multi-stage analysis framework for typhoon-induced wind effects on offshore wind turbines. In this framework, the influence of different typhoon impact stages on the mean wind speed profile, typhoon intensities, and spectral characteristics are considered utilizing an enhanced wind speed field model. To this end, an enhanced mean wind speed profile model under marine typhoon conditions is first derived based on dropsonde measurements. This profile model modifies conventional power/logarithmic laws so as to capture accurately the effects of typhoon-induced low-level jets within the height range of 0–500 m above the sea surface. It accounts for the parameters representing typhoon characteristics such as RMW (radius of maximum winds) and boundary layer wind speed. The turbulence intensity profile is also enriched by combining the empirical turbulence ratio with the data-driven updated mean wind profile. A generalized six parameter typhoon wind spectrum is also introduced using wind field measurements during typhoon “Hagupit”. The resulting wind speed field model is calibrated concerning four impact stages of typhoon “Hagupit”, demonstrating its ability to characterize multi-stage features. Compared to existing models prescribed in codes/specifications, the enhanced multi-stage model closely matches measurements while being slightly on the conservative side. The framework proposed including the enhanced model is then utilized to analyze typhoon-induced wind effects on a 205 m offshore wind turbine. Results reflect the apparent influence of multiple impact stages of a typhoon. Moreover, the enhanced wind profile models may lead to a higher level of response at critical sections of both the turbine blades and tower than the model prescribed in the current standard.

A case application of WRF-UCM models to the simulation of urban wind speed profiles in a typhoon

In order to work out a reliable way to predict urban wind speed profiles during typhoon process, in this study, WRF-UCM coupled models are adopted to simulate an urban wind field during the landfall of a strong typhoon. Sensitivity analysis is first carried out to obtain the best typhoon track simulation, based on which the PBL scheme of Mellor-Yamada-Janjic (MYJ) and the Microphysics model of Purdue Lin (LIN) are chosen for successive simulations. A WUDAPT based Local Climate Zone (LCZ) map is drawn to replace and supplement the default underlying surface data to execute WRF-UCM coupled simulations. After examining the accuracy of the WRF-UCM simulation, the mean wind speed profiles, turbulence intensity profiles and power spectra of simulated wind fields are studied to evaluate the applicability of different UCMs to the urban typhoon simulation. Utilizing multi-layer UCM (ML-UCM) leads to a notable weakening of mean wind speed and a slight increase of turbulence intensity within lower altitudes while slab UCM (SB-UCM) or single-layer UCM (SL-UCM) shows little effect. In general, coupling suitable UCM into the WRF model can improve urban typhoon wind profiles simulation. Nevertheless, it is still hard to perfectly reproduce real typhoon wind profiles via WRF-UCM coupling models.

Large eddy simulation of the atmospheric boundary layer to investigate the Coriolis effect on wind and turbulence characteristics over different terrains

The wind veering phenomenon caused by the Coriolis force is frequently observed in the atmospheric boundary layer (ABL) and may affect the wind and turbulence characteristics of the ABL, thus influencing the wind effects on structures, especially high-rise structures. To investigate the Coriolis effect, this study firstly reproduces a realistic thousand-meter height ABL with a wind veering phenomenon by large eddy simulation. The simulated vertical profiles of mean wind speed and twist angle are validated against field observations. Three typical terrain types are considered in the simulation and the corresponding vertical profiles of wind and turbulence characteristics are presented and discussed. Furthermore, an additional group of reference cases not considering the Coriolis force is also considered for comparison to quantify the Coriolis effects on the wind and turbulence characteristics. The results show that both the Coriolis force and terrain type significantly affect the vertical profiles of mean wind speed and turbulence intensity in three orthogonal directions. The Coriolis effect on the turbulence integral length scale is more appreciable in the streamwise direction than in the other two orthogonal directions. The vertical profiles of wind speed and turbulence properties in the ABL simulated with consideration of the Coriolis force may serve as a crucial reference for the more reliable wind-resistant design of high-rise structures.

Observations of wind and turbulence structures of Super Typhoons Hato and Mangkhut over land from a 356 m high meteorological tower

Knowledge of tropical cyclone (TC, or typhoon, hurricane) boundary layer over land is of vital importance to a variety of applications such as track and intensity forecasts of TCs and design of civil structures and infrastructures. On the basis of the 356-m-high Shenzhen Meteorological Gradient Tower which is the tallest of its kind in Asia, this paper presents an observational study of wind and turbulence structure of two Super Typhoons, namely Hato in 2017 and Mangkhut in 2018. First, the mean wind speed profiles, momentum flux, and drag coefficient are analyzed, and their correlations with upwind terrain and wind speed are discussed. Then, the characteristics of wind spectra are investigated and compared with existing spectral models. Subsequently, wind speed variances and turbulent kinetic energy (TKE) are inspected, and their variations with height are examined. Last, the characteristics and vertical profiles of TKE budget are explored. The outcome of this study is expected to enhance the understanding of TC boundary layer over land, provide useful information for the design of civil structures and infrastructures, benefit the numerical simulation of landfalling TCs, and facilitate the TC hazard prevention in coastal regions.

Departure from Flux-Gradient Relation in the Planetary Boundary Layer

It is well known that when eddies are small, the eddy fluxes can be directly related to the mean vertical gradients, the so-called flux-gradient relation, but such a relation becomes weaker the larger the coherent structures. Here, we show that this relation does not hold at heights relevant for wind energy applications. The flux–gradient relation assumes that the angle (β) between the vector of vertical flux of horizontal momentum and the vector of the mean vertical gradient of horizontal velocity is zero, i.e., these vectors are aligned. Our observations do not support this assumption, either onshore or offshore. Here, we present analyses of a misalignment between these vectors from a Doppler wind lidar observations and large-eddy simulations. We also use a real-time mesoscale model output for inter-comparison with the lidar-observed vertical profiles of wind speed, wind direction, momentum fluxes, and the angle between the horizontal velocity vector and the momentum flux vector up to 500 m, both offshore and onshore. The observations show this within the height range 100–500 m, β=−18∘ offshore and β=−12∘ onshore, on average. However, the large-eddy simulations show β≈0∘ both offshore and onshore. We show that observed and mesoscale-simulated vertical profiles of mean wind speed and momentum fluxes agree well; however, the mesoscale results significantly deviate from the wind-turning observations.

Aerodynamic characteristics of trains on a viaduct with non-uniform cross-section under crosswinds by wind tunnel tests

This paper focuses on the aerodynamic characteristics of trains on a non-uniform double-track railway bridge under crosswinds through a scaled 1:40 sectional model wind tunnel test. Pressure measurements of five cross-sections of two types of trains, one with round roof and one with blunt roof, at the upstream and downstream tracks of the bridge were conducted under crosswinds with wind attack angles between −12° and 12°. The mean wind speed and turbulence intensity profiles around the windward surface of the train in the downwind and upward directions were also measured using cobra probe to obtain the boundary layer above the bridge surface. The results show that the shapes of train and bridge, as well as the wind attack angle, affect the aerodynamic characteristic of the train on the non-uniform bridge girder. The mean and fluctuating pressure coefficients are similar for all five cross-sections of the trains while the train is at the upstream track. However, when the train is at the downstream track, the extreme mean and fluctuating pressure coefficients around the windward and top surfaces of each cross-section on the train are different. At the downstream track, the mean wind speed profile and the turbulence intensity profile around the top of the train vary dramatically due to the separation flow caused by the leading edge of the bridge girder.

Observation of Typhoon Hato based on the 356-m high meteorological gradient tower at Shenzhen

Field measurements of tropical cyclone (TC) at coastal areas are of great significance for the prevention and control of TC disasters. Despite the increasing efforts on this topic in the last years, studies based on fast-response records of TC wind at multiple heights over land are still limited. To this end, a 365 ​m high meteorological observation gradient tower at Shenzhen has been constructed and put into use since 2016. It is currently the tallest mast tower in Asia and the second tallest of this kind in the world. There are 13 levels of instruments installed in a range of 10 ​m–350 ​m on the mast, to measure various weather elements at a sampling frequency utmost to 10 ​Hz. This paper presents an observational study on the wind characteristics of Typhoon Hato which made landfall with an intensity of severe typhoon at a distance of ~90 ​km from the tower on Aug. 23, 2017. The vertical profiles of mean wind speed and typical wind turbulence quantities, i.e., gust factor, turbulence intensity, integral length scale, etc., are demonstrated and compared against provisions stipulated in wind load codes. The effects of thermal stratification on these quantities are stressed.