Wind Velocity Fluctuations Research Articles

Turbulence in a coastal environment: the case of Vindeby

Abstract. The one-point and two-point power spectral densities of the wind velocity fluctuations are studied using the observations from an offshore mast at Vindeby Offshore Wind Farm, for a wide range of thermal stratifications of the atmosphere. A comparison with estimates from the FINO1 platform (North Sea) is made to identify shared spectral characteristics of turbulence between different offshore sites. The sonic anemometer measurement data at 6, 18, and 45 m a.m.s.l. (above mean sea level) are considered. These heights are lower than at the FINO1 platform, where the measurements were collected at heights between 40 and 80 m. Although the sonic anemometers are affected by transducer-flow distortion, the spectra of the along-wind velocity component are consistent with those from FINO1 when surface-layer scaling is used, for near-neutral and moderately diabatic conditions. The co-coherence of the along-wind component, estimated for vertical separations under near-neutral conditions, matches remarkably well with the results from the dataset at the FINO1 platform. These findings mark an important step toward more comprehensive coherence models for wind load calculation. The turbulence characteristics estimated from the present dataset are valuable for better understanding the structure of turbulence in the marine atmospheric boundary layer and are relevant for load estimations of offshore wind turbines. Yet, the datasets recorded at Vindeby and FINO1 cover only the lower part of the rotor of state-of-the-art offshore wind turbines. Further improvements in the characterisation of atmospheric turbulence for wind turbine design will require measurements at heights above 100 m a.m.s.l.

Evidence of Ocean Waves Signature in the Space–Time Turbulent Spectra of the Lower Marine Atmosphere Measured by a Scanning LiDAR

To achieve more accurate weather and climate forecasting, and propose efficient engineering solutions for exploiting offshore renewable energies, it is imperative to accurately describe the atmospheric turbulent flow in the offshore environment. The ocean’s dynamics raise specific challenges for the aforementioned applications, as they significantly alter the atmospheric flow through complex wind–wave interactions. These interactions are important in fairly common situations and notably in old-sea conditions, where ocean waves travel fast, under comparatively slow wind velocities. In the present study, a scanning LiDAR (sLiDAR) was deployed on the shore to study micro-scale wind–wave interactions by performing horizontal scans 18 m above the ocean, and as far as 2 km from the coast. In the proposed configuration, and in the test cases presented in old seas, the sLiDAR captures wave-induced disturbances propagating into the lower part of the marine atmospheric boundary layer. Based on measurements of high-resolution space–time maps of the Radial Wind Speed, an original two-dimensional spectral analysis of the space–time auto-correlation functions was performed. Unlike more conventional data-processing techniques, and as long as the waves travel sufficiently (∼twofold) faster than the mean wind at the measurement height, the upward transfer of motions from the waves to the wind can be clearly distinguished from the atmospheric turbulence in the wave-number–angular-frequency (k–w) turbulent spectra. These are the first space–time auto-correlation functions of the wind velocity fluctuations obtained at micro-scales above the ocean. The analyses demonstrate sLiDAR systems’ applicability in measuring k–w-dependent turbulent spectra in the coastal environment. The findings present new perspectives for the study of micro-scale wind–wave interactions.

Buffeting response of a suspension bridge based on the 2D rational function approximation model for self-excited forces

In the last thirty years, the aerodynamic nonlinearities related to the slow variation of the angle of attack produced by large-scale atmospheric turbulence and their impact on the buffeting response of long-span suspension bridges have been a hot topic in wind engineering research. Self-excited forces accounting for such an effect of turbulence have been crucial in predicting the dynamic response of bridge sectional models subjected to multi-harmonic gusts. In contrast, only few studies on full suspension bridges in realistic turbulent flows are available. To fill this gap, the 2D RFA model for self-excited forces, recently developed and experimentally validated by the authors, in this paper is incorporated into a stochastic time-variant state-space framework to assess the nonlinear buffeting response of a suspension bridge. The most important feature of this model is the modulation of the self-excited forces due to the spatio-temporal fluctuation of the angle of attack produced by low-frequency turbulence. Moreover, the nonlinear external buffeting forces are formulated to achieve a reasonable compromise between the conflicting needs of modelling both nonlinear and unsteady effects of wind velocity fluctuations. The model is applied to the Hardanger Bridge in Norway, considering different wind conditions. Indeed, the aerodynamic derivatives of this bridge deck cross-section present a strong dependence on the mean angle of attack. The results emphasise the significant impact on the buffeting response and flutter stability of considering time-variant self-excited forces, though in specific cases the classical time-invariant approach is found to provide accurate predictions of the bridge vibrations. Finally, the paper investigates the sensitivity of the response statistics to the model cut-offs used to separate the low-frequency and the high-frequency turbulence bands.

Infrasound propagation through the atmosphere with mesoscale wind velocity and temperature fluctuations

Some results on modeling and observation of infrasound propagation in the atmosphere in a presence of mesoscale and anisotropic wind velocity and temperature fluctuations are presented. The theoretical model of infrasound scattering from anisotropic wind velocity and temperature inhomogeneities of the atmosphere is developed. With this model, the appearance of the stratospheric, mesospheric, and thermospheric arrivals of the infrasound signals in the acoustic shadow zones is explained. The analytic relation between the wave field of the scattered infrasound signal and the vertical profile of the effective sound speed fluctuations is obtained. Using this relation, the vertical profiles of the fluctuations within the upper stratosphere (25–55 km) and the lower thermosphere (105–140 km) were retrieved from the waveforms and travel times of the signals recorded in the acoustic shadow zones. The theoretical frequency spectrum of the infrasound wavefield reflected from the fine-scale layered structure of the atmosphere is obtained and compared with the spectra of the observed stratospheric arrivals.

Deep learning based missing data recovery of non-stationary wind velocity

Wireless sensor technology and wireless sensor network are the developing trend of structural health monitoring (SHM) system in the future. Due to working principles, environment and other various factors, random data missing may occur easily during data transmission by mobile wireless sensors. The randomness and fluctuation of non-stationary wind velocity is strong and complex, affected by turbulence effect. Regarding random missing of non-stationary wind velocity in data transmission, a missing data recovery algorithm based on secondary decomposition of signals, convolutional neural network (CNN) and Kriging based sequence interpolation (KSI) is proposed in this paper. Before data transmission, the non-stationary wind velocity is decomposed by wavelet transform (WT) and empirical wavelet transform (EWT) to obtain a set of sub-signals with uniform frequency. Then low-frequency sub-signals are supplemented by cubic spline interpolation (CSI). Due to the randomness and complexity of high-frequency sub-signals, CNN is used to establish the recovery model to achieve preliminary recovery results. Generally, there exists lots of extreme points in high-frequency sub-signals. The recovery error may be large due to strong randomness of extreme points. Therefore, KSI based extreme value estimation is proposed for high-frequency sub-signals to estimate extreme curves, which are subsequently used to correct the preliminary recovery results. Numerical examples of field monitoring downburst data testify the superiority of the proposed algorithm. In addition, structural dynamic analysis fully verifies the effectiveness of extreme value correction.

Representation model of wind velocity fluctuations and saltation transport in aeolian sand flow

Since the stream-wise and vertical turbulent fluctuations have an important influence on the movement of sand and dust particles, the accurate quantitative estimation of natural wind fluctuations has an important role in improving the forecast level of wind erosion and dust storm. The high Reynolds number turbulence characteristics of aeolian sand flow and the complexity of sand-laden flows are the main difficulties affecting the accuracy of prediction. In this study, the characteristics of the stream-wise and vertical wind velocity fluctuations in sand-laden flows are studied by analyzing the real-time data of the wind velocity field in the atmospheric surface layer (ASL). Then, a simple empirical model representing the stream-wise and vertical wind velocity fluctuations is proposed. The model not only considers the effects of large-scale and small-scale fluctuations, but also has the advantage of predicting the stream-wise and vertical wind velocity fluctuations at any height. Verification shows the results predicted by the model are in good agreement with the observation results, and the predicted saltation sand flux is closer to the experimental results than that without considering the turbulence effect or the vertical wind velocity fluctuations. Moreover, it is a convenient model for application especially in predicting aeolian sand transport.

Study on Full-scale Measurements of Wind Field Characteristics of Coastal Complex Mountainous Terrains

Based on full-scale wind field measurements of coastal complex mountainous terrains, data of fluctuating wind velocities at the height of 30m for four sites, including mountaintop and hillsides, are obtained. The wind load characteristics of mean wind velocities, wind directions, turbulence intensities and speed-up ratios of wind velocities are comprehensively examined. Results show that the maximum mean wind velocity at the mountaintop site is 12.4 m/s. The probability density distributions of mean wind velocities for the four measurement sites can be well represented by the Weibull probability model. The predominant wind directions are around the northeast and southwest. The longitudinal, lateral and vertical turbulence intensities decrease with the increase of mean wind velocities. The turbulence intensities for the mountaintop site are as many as 0.13,0.12 and 0.089 under maximum wind velocities, respectively for the previously mentioned three directions. The speed-up ratios of wind velocities between mountaintop and hillside sites are reduced, as the wind velocities increase. However, in cases of intensive wind with mean wind velocities larger than 8 m/s, the speed-up effects of wind velocities can also appear. The maximum speed-up ratio can reach 1.17.

Study of turbulence effects on flying compact debris in tornadoes at different stages

Flying debris is an important factor influencing the destruction of buildings by tornadoes. However, research on tornado-borne debris remains limited. Thus far, the degree of this influence and methods to consider turbulence effects when predicting debris motion using the statistical information of wind velocities in tornadoes, have not been clarified. Therefore, in the present study, four methods for calculating debris motion based on the mean wind velocities and the root mean square of wind velocity fluctuations were examined. These four methods entailed (a) using only the mean wind velocities, (b) using Karimpour corrections, (c) assuming that the instantaneous flow fields of the tornado fluctuate as sinusoidal functions, and (d) assuming that the probability density functions of the instantaneous wind velocities follow Gaussian distributions. It was found that the debris concentration were sensitive to the turbulence in tornadoes. The Gaussian-distribution assumption afforded the best performances and could provide almost perfect predictions, except for the tornado-core regions.

Boundary layer wind tunnel modeling experiments on pumping ventilation through a three-story reduce-scaled building with two openings

Pumping ventilation is a special single-sided wind driven building ventilation induced by periodic vortex shedding at the building wake. This work aimed to investigate the oscillating frequency at the building façade opening and ventilation rate of pumping ventilation. The effects of opening separations on different floors of a three-story reduce-scaled building with two openings were considered using boundary-layer wind tunnel experiments. The wind velocity of the surrounding flow field and the center of the opening were measured. The ventilation rate was obtained by tracer gas method with constant and continuous injection rate. The results showed that the oscillating frequency was independent of the opening separation except on the third floor probably because of the disturbance of the rooftop shedding frequency. The oscillating frequency on the first floor was the lowest due to the resistance of the ground to the vortex shedding. Pumping ventilation indicated that its ventilation rate is greater than that of general ventilation across the single opening with the same total opening area. The promotion of ventilation rate was up to about 123% on the first floor, about 65% on the second floor and about 44% on the third floor. Expansion of opening separations could boost the ventilation rate. Meanwhile, the fluctuation of opening center wind velocity and indoor species concentration were not positively correlated with the ventilation rate of pumping ventilation. Conclusions of this research could provide some useful reference to the design of natural ventilation for buildings.

Raindrop fall velocity in turbulent flow: an observational study

Abstract. Laboratory measurements of drop fall speeds by Gunn–Kinzer under still air conditions with pressure corrections of Beard are accepted as the “gold standard”. We present measured fall speeds of 2 and 3 mm raindrops falling in turbulent flow with 2D-video disdrometer (2DVD) and simultaneous measurements of wind velocity fluctuations using a 3D-sonic anemometer. The findings based on six rain events are, (i) the mean fall speed decreases (from the Gunn–Kinzer terminal velocity) with increasing turbulent intensity, and (ii) the standard deviation increases with increase in the rms of the air velocity fluctuations. These findings are compared with other observations reported in the literature.

Simulation of fluctuating wind of high-speed vehicle–bridge united system

This study introduces a novel multivariate stochastic process simulation algorithm to generate fluctuating wind velocity time histories of the vehicle–bridge united system. The Kaimal spectrum and wind spectrum relative to a moving point were applied to the bridge and the vehicle (train) in operation, respectively. The wind time series of the vehicle–bridge united system were generated simultaneously by considering the correlation between the moving point fixed on a vehicle in operation and the static points on a bridge. This algorithm is of high computational efficiency, as it takes advantage of explicit decomposition for the coherence function matrix and frequency truncation method. Based on an error estimation, this simulation method is proven to be accurate. Numerical examples of a vehicle running on a simply supported continuous beam bridge with the speeds of 20, 40, 60 and 80 m/s were performed. The result shows that the auto-/cross-correlation functions and auto-spectrum of the simulated wind samples are in good agreement with the targets. The proposed method can facilitate the analysis of wind–vehicle–bridge aerodynamic response with high efficiency and accuracy, and avoids the discontinuity problem of wind time history of the moving point.

Numerical analysis of the influence of the near ground turbulence on the wind-sand flow under the natural wind

The near-ground turbulent flow of nature air should have an important influence on the wind-sand flow, because of the vegetation and/or micro-topography. However, due to the complexity of turbulent flows, the effects of such turbulences and dust particles are still unknown. In this paper, the large eddy simulation–discrete element method was used to simulate the wind-sand flow. The results show that the response time of the wind-sand flow to turbulence is approximately 1.6 s. In addition, the response time increases rapidly as the particle size increases. The turbulence impact on the sand transport rate first increases and then decreases as the wind velocity fluctuation intensity increases. The sand transport rate is enhanced when the turbulence intensity is less than 0.25, whereas it is weakened when the turbulence intensity exceeds 0.25. Furthermore, the smaller the particle size, the more significant the effect. The effect of turbulence is mainly concentrated at 0.1–0.2 m from the ground and the turbulence increases the height of the wind-sand flow.

Numerical study of stable stratification effects on the wind over simplified tall building models using large-eddy simulations

Atmospheric stratification is a critical factor affecting wind around buildings. However, very few studies have dealt with the atmosphere under stable stratification. In addition, the building model is typically simplified as a cubic block in previous studies concerning stable stratifications; however, the building aspect ratio (height/width) in the downtown area is often larger than 1. Furthermore, the spatial correlation of the velocities in tall buildings, which is important in determining the length scale of the flow and in evaluating pollutant dispersion, has seldom been studied. Therefore, in the present study, the flow around a simplified tall building and the flow around two groups of buildings with the plan area densities of 12.5% and 25.0%, were investigated under stable stratifications with stratified numbers K equaling to 0, 1, 2, and 3, using large eddy simulations (LES). The aspect ratio of the building models is 2. It was found that, for a single building model, the average wind acceleration above the building is directly proportional to K. Regarding the fluctuating wind velocity, the stratification effect has a weak impact. For building groups, the impact region of stratified flow usually appears when z/H < 1.2. A higher building density reduces the streamwise fluctuating wind velocity and the streamwise velocity spatial correlation.

Experimental study on flow characteristics of a large-scale open jet wind tunnel for outdoor pool fire research

An open jet wind tunnel with an exit area of 23 ​× ​9 ​m2 has been built to study the behaviors of large-scale pool fires under controlled wind conditions. A series of measurements on the mean and fluctuating wind velocities, using arrays of hot-wire probes and ultrasonic anemometers, were extensively performed in the transverse planes at the leading and trailing edges of the pool (28 ​m and 48 ​m to the exit). The entire pool was located in the potential core of the central jet flow region, and the longitudinal velocity (U) and turbulence intensity (Iu) exhibited acceptable uniform distributions within the two vertical planes. The mean turbulence intensities in the two planes were close to those in the flat open grassland and sea (7%–10%). The longitudinal velocity spectra were in good agreement with the Von Kármán spectrum at all levels, and the onset frequency for isotropy was about 1.5–3.0 ​Hz. In the central flow region, the turbulence dissipation rates (ε) were fitted well by ε ​= ​0.067U2.69Iu1.90, and the best-fit line for the turbulent diffusivity (νt) was νt ​= ​0.21U0.95Iu0.98. The detailed mean flow and turbulence characteristics provided the necessary information for the pool fire tests and numerical model verification.

Simulation of N-wave propagation in turbulent atmosphere using standard and wide-angle parabolic equations

Interest to the problem of sonic boom propagation in atmosphere is growing in recent decade due to the development of new generation of supersonic business jets. Important nonlinear wave effects occur in the last kilometer above the ground where sonic boom wave interacts with turbulence of the planetary boundary layer. Focusing and defocusing of the sonic boom on random inhomogeneities of sound speed lead to fluctuations of its peak overpressure and rise time, affecting the perceived loudness. Various one-way model equations of different complexity have been developed and actively used by the research community for analyzing wave propagation in turbulent atmosphere. The basic equation is the nonlinear parabolic equation of the Khokhlov-Zabolotskaya-Kuznetsov-type, which has limitation on diffraction angles. Improvement in accuracy of the diffraction term can be reached using wide-angle formulation. Here, the simulation results for N-wave propagation through homogeneous isotropic turbulence are compared for standard and wide-angle parabolic equations of scalar type. Turbulence wind velocity fluctuations are accounted via effective sound speed. The wide-angle model is based on split-step Pade approximation of the propagation operator. The importance of taking into account large diffraction angles for N-wave propagation in atmospheric turbulence is discussed. [Work supported by RSF-18-72-00196 and ANR-10-LABX-0060/ANR-16-IDEX-0005].

Non-stationary dynamic structural response to thunderstorm outflows

Wind loads on structures are commonly described as stationary phenomena that occur in neutral atmospheric conditions at the synoptic scale with velocity profiles in equilibrium with the atmospheric boundary layer. Nevertheless, structural systems can be also affected by thunderstorm outflows, which are non-stationary local phenomena at the mesoscale that occur in convective conditions with totally different velocity profiles with respect to synoptic winds. This paper presents a non-stationary probabilistic model that describes the wind velocity fluctuations experienced during a thunderstorm in order to estimate its effects on the dynamic structural response. The model is first calibrated on a typical thunderstorm recorded in the north-west italian coast and it is used to generate virtual time histories in a Monte Carlo simulation approach. The potential influence of the wind load non-stationary features on the peak structural response is investigated using single degree of freedom parametric analysis and statistical estimation.

Cascaded H-Bridge Inverter for Wind Driven Isolated Squirrel Cage Induction Generators

Abstract: This paper describes the simulation andharmonics analysis of cascaded five-level H-bridge invertersfed induction motor load drive for islanding mode applications.The self – excited induction generator (SEIG) fed H-bridgeMulti-Level Inverter (MLI) for wind energy conversionsystems is chosen for several islanding mode applications. Inthis paper, the SEIG (for standalone systems) fed cascaded fivelevel inverter for induction motor load applications are clearlyexplained with the help of MATLAB / SIMULINK models. Thegenerated voltage of the wind driven SEIG is mainly dependingon the wind velocity fluctuations and appropriate loadconditions. The five level cascaded inverters have interfacewith the wind driven SEIG. By choosing appropriate value ofModulation Index (MI), the variable voltage and variablefrequency of the generator can be finely controlled. Thesimulation and harmonic analysis of the proposed inverter willbe discussed, and the total harmonic distortion of statorcurrent for different modulation index at different switchingfrequencies are evaluated and the speed and torque ofinduction motor for different load torques are plotted.Index Terms: Self – Excited Induction Generator (SEIG),Variable speed Wind Turbine, Multi Level Inverter (MLI),Induction Motor (IM), Modulation Index (MI).

Effects of air-side freestream turbulence on the development of air–liquid surface waves

It is important to investigate the effects of air-side freestream turbulence on the development of air–liquid surface waves. Two turbulent-grid air flows are generated by regular and fractal grids in a wind-wave tank. Wind velocity and water-level fluctuations are measured with and without grids. The results show that regular and fractal grids can generate turbulent-grid air flows with different wind velocity fluctuations. The spectra of water-level fluctuations in the regular-type and fractal-type grid cases are consistent with those of pure wind-driven waves without a grid. Furthermore, as wind-wave properties, including the fetch law, dispersion relation, and Toba’s 3/2 power law, all hold in cases with and without grids, it is confirmed that the waves develop through the turbulence only near the free water surface, and not through the air-side freestream turbulence. This suggests that the effects of air-side freestream turbulence on wave development are negligible.

Atmospheric Boundary Layer as a Laboratory for Modeling Infrasound Propagation and Scattering in the Atmosphere

In this work, we show that large-scale processes of the long-range propagation and scattering of infrasound signals in stratospheric and thermospheric waveguides are to some extent similar to the smaller scale processes of waveguide propagation and scattering of acoustic signals in stably stratified ABL. Moreover, we note a resemblance between the zeroth order tropospheric mode and the Lamb mode that is observed for larger nuclear explosions. The results of physical modeling of the long-range propagation of infrasound signals in the atmosphere by studying the propagation of the acoustic pulses from detonation sources in the stably stratified atmospheric boundary layer (ABL) are presented. Such modeling became possible due to continuous measurement of the wind velocity and temperature profiles in the ABL by Doppler sodar and temperature profiler, respectively. The resemblance between the propagation of the infrasound signals from surface explosions (20–70 t TNT) in the “shallow” tropospheric wave guide and propagation of Lamb mode from nuclear explosions is found. Also, the propagation of infrasound signals in the stratospheric wave guide is modeled by studying the propagation of acoustic pulses in the ABL during morning hours (after sunrise), when an effective sound speed profile in the ABL is similar to that in the stratospheric wave guide. The instantaneous vertical profiles of the effective sound speed and wind velocity fluctuations in the thin layers of the stably stratified ABL located at different heights (up to 700 m) above the ground and at different distances from the source (up to 2.25 km) have been retrieved using the wave forms and travel times of the recorded arrivals of the acoustic pulses from the detonation sources. The effect of scattering of acoustic pulses from the fine-scale layered structure of the stably stratified ABL is studied to model the effect of scattering of infrasound signals from the fine-scale layered structure of the stratosphere.

The Influence of Surface Stress Fluctuation on Saltation Sand Transport Around Threshold

AbstractMultiscale wind velocity fluctuation and complex inertial response of sand saltation to wind variations give rise to intermittent and nonuniform sand transport, which greatly challenges the prediction of transport rate under wind conditions around saltation threshold. In this paper, three‐dimensional sand saltation within simulation domain of wind tunnel scale is modeled based on large eddy simulation of turbulent wind. Special attention is paid to the prediction of surface shear stress and consequently aerodynamic entrainment. When the wall stress model considering the influence of large‐scale turbulent structures was employed in case of relatively small resistance level for calculating particle aerodynamic entrainment, the model results predict sand transport below the impact threshold. The sand transport rate in this regime increases exponentially with mean wall stress, in contrast to the standard power law‐like behavior above the impact threshold. Aerodynamic entrainment and strong transport events lag behind high‐speed fluids sweep on a large scale even in wind tunnel, but the spatiotemporal variability of small‐scale sand transport is still unclear and quite complex due to transport inertia.