Wind Velocity Fluctuations Research Articles

Numerical Prediction of Wind Flow Around Irregular Models

This paper presents numerical predictions of flow around irregular-plan buildings (S-, R-, L- and U-shaped models) in high Reynolds number. The adopted computational approach and numerical models are described firstly. Then comparative analysis with the numerical and experimental data has been conducted to verify the reliability of the numerical predictions. Finally, characteristics of mean and fluctuating pressure distributions and vertical and lateral velocity profiles of the flow around the four models have been investigated and assessed thoroughly. The study shows that satisfactory results can be obtained by large eddy simulation (LES), especially when fluctuating wind velocity is considered in the inflow boundary. Distribution of mean pressure coefficients on front faces is relatively regular. Large fluctuating pressure coefficients are induced by strong vortex motion. Velocity profiles of wind flow are disturbed obviously among the four building models, especially in weak flow. The disturbed intensity decreases with increasing of the distance away from bluff body. The suggested MDS (Maximum Disturbance Scopes) away from bluff body are generally 0.25H in inflow zones, 0.4H in roof zones, 0.5H in both side zones and 3H in weak zones.

On the small-scale turbulence parameters in the atmosphere of venus and their use in the global circulation models

Vertical profiles of the turbulence parameters calculated for the planet-averaged conditions from the experimental data on the turbulent fluctuations of temperature and wind velocity are presented. Improved formulas accounting for the difference between the atmospheric gas on Venus and an ideal one, and the large difference in its thermal capacity at different altitudes, are used. The commonly used formula for the potential temperature describing the atmospheres of the Earth and Mars is inapplicable to the atmosphere of Venus. It has been shown that the opinion on the absence of turbulence in the atmosphere of Venus is based on overestimated values of the dynamic Richardson number obtained from the smoothed profiles of wind velocity, while its actual values are below unity due to the large wind velocity gradients produced by buoyancy waves. To improve the global circulation models of the atmosphere of Venus, it is necessary to use the currently available turbulence parameters calculated from experimental data.

Assessment of code recommendations through simulation of EPS wind loads along a segment of a transmission line

Part of the uncertainty in the response of transmission line (TL) towers subjected to extended pressure system (EPS), determined as prescribed by IEC 60826, is assessed in the paper by comparing the code predictions with the dynamic response of the system subjected to numerically simulated 3D-wind fields. In the evaluation process, model uncertainty was accounted for by considering various models for the power spectra of the fluctuating wind velocity components as well as for the correlation functions. The values obtained through the full dynamic analysis are quite similar to those determined by the code, resulting in estimation errors in the range between 5% and 10%. The influence on model uncertainty of the structural model of towers and conductors was studied by the authors in previous contributions, and completes the assessment advanced in the paper.

Aerosols-cloud microphysics-thermodynamics-turbulence: evaluating supersaturation in a marine stratocumulus cloud

Abstract. This work presents a unique combination of aerosol, cloud microphysical, thermodynamic and turbulence variables to characterize supersaturation fluctuations in a turbulent marine stratocumulus (SC) layer. The analysis is based on observations with the helicopter-borne measurement platform ACTOS and a detailed cloud microphysical parcel model following three different approaches: (1) From the comparison of aerosol number size distributions inside and below the SC layer, the number of activated particles is calculated as 435±87 cm−3 and compares well with the observed median droplet number concentration of Nd = 464 cm−3. Furthermore, a 50% activation diameter of Dp50≈115 nm was derived, which was linked to a critical supersaturation Scrit of 0.16% via Köhler theory. From the shape of the fraction of activated particles, we estimated a standard deviation of supersaturation fluctuations of σS' = 0.09%. (2) These estimates are compared to more direct thermodynamic observations at cloud base. Therefore, supersaturation fluctuations (S') are calculated based on highly-resolved thermodynamic data showing a standard deviation of S' ranging within 0.1%≤σS'≤0.3 %. (3) The sensitivity of the supersaturation on observed vertical wind velocity fluctuations is investigated with the help of a detailed cloud microphysical model. These results show highest fluctuations of S' with σS'=0.1% at cloud base and a decreasing σS' with increasing liquid water content and droplet number concentration. All three approaches are independent of each other and vary only within a factor of about two.

Different Self Excitation Techniques for Slip Ring Self Excited Induction Generator

paper discusses the different possible techniques to self excite the slip ring induction machine working as self excited induction generator. The generated voltage due to self excitation is initiated and sustained with constant value of power capacitors connected across the stator windings. Attention is focused on the influence of different techniques on generator voltage and output power capabilities. The generated voltage of the wind driven self - excited induction generator (SEIG) is mainly depending on the wind velocity fluctuations and load variations. By choosing the proper value of the self excitation capacitor banks achieves the reactive power requirements. In case of squirrel cage induction generator only the excitation capacitance at stator is used to achieve the reactive power requirements. But in case of slip ring induction generators with the different self excitation techniques at the rotor are used to achieve the reactive power requirements and improvement in the voltage build up. In previous literature there is no discussion about the different techniques of self excitation and its effects on the induction generator. These are the new methods proposed for the improvement in voltage build up and can also controls the slip. MATLAB/SIMULINK based work is carried out for 3.5kW machine. Proposed methods may be used for low cost variable speed wind energy conversion systems.

Wind Turbine Performance Improvements using Active Flow Control Techniques

Abstract The ability of a wind turbine to react to rapid fluctuations in wind velocity is blunted by the massive rotational inertia of the rotor assembly as a whole, as well as the mass of individual blades bearing upon pitch change mechanisms. Thus, wind turbines often operate with a less than optimal relationship to the instantaneous wind conditions. A wind turbine interacting with slow fluctuations in wind velocity may suffer a loss in potential energy extraction due to stalling of the blades. Interaction with rapid fluctuations in wind velocity can subject a wind turbine to the phenomenon of dynamic stall, which produces severe variations in the aerodynamic loads upon the blades resulting in major structural issues. Flow separation is a major contributing factor to the aerodynamic challenges associated with wind turbine operation. The ability to control or reduce the magnitude of regions of separated flow over an airfoil can play a significant role in reducing the negative effects associated with turbine operations in fluctuating wind conditions. The use of Air Jet Vortex Generators (AJVG) has been shown to provide net increases in power output on full scale turbines. In addition, AJVG's have been shown experimentally to reduce the fluctuating aerodynamic loads associated with dynamic stall. Such devices are ideal for use in rapidly fluctuating conditions, as there is potential for an active flow control technique with a rapid response time which would be more difficult to achieve with fixed Vane Vortex Generators (VVG). The current work details experiments carried out with a new type of AJVG that has proven to consume less energy compared with traditional devices. The use of such a device on full scale wind turbines may lead to greater net gains in power output, as well as reducing the magnitude of aerodynamic loads associated with dynamic stall.

Multisensors On-Site Monitoring and Characteristic Analysis of UHV Transmission Tower

It is important to clarify the variational rules between dynamic characteristics and gust response factors (GRFs) of an ultra high-voltage (UHV) transmission tower for the structural design. In this paper, the synchronous multisensors on-site monitoring has been carried out under field wind conditions, which includes field fluctuating wind velocity, acceleration responses, and base bending strains on parts of the just-built transmission tower. The dynamic characteristics of tested transmission tower were investigated by a modal identification method (MIM) and validated those results with a finite-element method (FEM). Similarly, the GRFs based on the top displacement or base dynamic bending stains are calculated and compared by means of a measurement data statistical method and some different quasistatic methods, respectively. The results have shown that the dynamic characteristics obtained from the measurement data analysis with the MIM agree with those from the FEM, which confirms that the FEM and the MIM for dynamic characteristics are consistent and correct. In turn, the GRFs form the measurement displacement analyses have some differences from those calculated with the quasistatic method, and it is necessary to further identify and verify their causations.

Scaling Of Turbulence In The Atmospheric Surface-Layer: Which Anisotropy?

This paper aims to provide an insight into the fundamental relationships between large and small scale wind velocity fluctuations within the boundary layer through careful analysis of measuring mast wind velocities. The measuring mast was in a wind farm on top of a mountain (with steep inclines of about 30°) on an island surrounded by the sea which meant the horizontal mean flow fluctuations were dominated by buoyancy forces and vertical shears at large scales (above 500m). Thus using a variety of methods including spectral, integrated spectral, integrated cospectral and multifractal analysis we were able to clearly dispel the relevance of 2D turbulence and give on the contrary some credence to the multifractal anisotropic model.

Symbolic analysis of slow solar wind data using rank order statistics

We analyze time series data of the fluctuations of slow solar wind velocity using rank order statistics. We selected a total of 18 data sets measured by the Helios spacecraft at a distance of 0.32AU from the sun in the inner heliosphere. The data sets correspond to the years 1975–1982 and cover the end of the solar activity cycle 20 to the middle of the activity cycle 21. We first apply rank order statistics to time series from known nonlinear systems and then extend the analysis to the solar wind data. We find that the underlying dynamics governing the solar wind velocity remains almost unchanged during an activity cycle. However, during a solar activity cycle the fluctuations in the slow solar wind time series increase just before the maximum of the activity cycle.

Hysteresis in a Solar Activity Cycle

We analyze in situ measurements of solar wind velocity obtained by the Advanced Composition Explorer (ACE) spacecraft during the solar activity cycle 23. We calculated a robust complexity measure, the permutation entropy (S) of solar wind time series at different phases of a solar activity cycle. The permutation entropy measure is first tested on the known dynamical data before its application to solar wind time series. It is observed that complexity of solar wind velocity fluctuations at 1 AU shows hysteresis phenomenon while following the ascending and descending phases of the activity cycle. This indicates the presence of multistability in the dynamics governing the solar wind velocity over a solar activity cycle.

Orthogonal expansion of Gaussian wind velocity field and PDEM-based vibration analysis of wind-excited structures

An effective procedure for simulation of random wind velocity field by the orthogonal expansion method is proposed in this paper. The procedure starts with decomposing the fluctuating wind velocity field into a product of a stochastic process and a random field, which represent the time property and the spatial correlation property of wind velocity fluctuations, respectively. By an innovative orthogonal expansion technology, the stochastic process for wind velocity fluctuations may be represented as a finite sum of deterministic time functions with corresponding uncorrelated random coefficients. Similarly, the random field can be expressed as a combination form with only a few random variables by the Karhunen–Loeve decomposition. This approach actually simulates the wind velocity field with stochastic functions other than methods such as spectral representation and proper orthogonal decomposition. In the second part of the paper, the probability density evolution method (PDEM) is employed to predict the stochastic dynamic response of structures subjected to wind excitations. In the PDEM, a completely uncoupled one-dimensional partial differential equation, the generalized density evolution equation, plays a central role in governing the stochastic responses of structures. The solution of this equation will give rise to instantaneous probability density function of the responses. Finally, the accuracy and effectiveness of the approach in representing the random wind velocity field and PDEM-based dynamic response of wind-excited building are investigated.

A Spectral Representation Model for Simulation of Multivariate Random Processes

A model is proposed to simulate multivariate weakly stationary Gaussian stochastic processes based on the spectral representation theorem. In this model, the amplitude, phase angle, and frequency involved in the harmonic function are random so that the generated samples are real stochastic processes. Three algorithms are then adopted to improve the simulation efficiency. A uniform cubic B-spline interpolation method is employed to fit the target factorized power spectral density function curves. A recursive algorithm for the Cholesky factorization is utilized to decompose the cross-power spectral density matrices. Some redundant cosine terms are cut off to decrease the computation quantity of superposition. Finally, an example involving simulation of turbulent wind velocity fluctuations is given to validate the capability and accuracy of the proposed model as well as the efficiency of the optimal algorithms.

Waveguide sound propagation in a turbulent atmosphere above a statistically rough surface of the ground

Waveguide sound propagation in a refractive, turbulent atmosphere above a statistically rough surface of the ground is considered. The waveguide is formed between the surface of the ground and turning points of sound waves in the downwind direction or in a nocturnal boundary layer of the atmosphere when the temperature increases with height. Using a modal decomposition of the sound field and the Chernov method, closed-form equations for the coherence function of the sound field are derived. The solution is expressed in terms of the effective turbulence spectrum, which is a linear combination of the 1-D spectra of temperature and wind velocity fluctuations, and a spectrum of the surface roughness. The coherence function can be calculated with equations obtained or by using the effective spectrum in the already existing code for the coherence function of the sound field propagating in a refractive, turbulent atmosphere above a flat ground [D. K. Wilson, V. E. Ostashev, and M. S. Lewis, Waves Rand. Comp. Media, V. 19, 369–391 (2009)]. The derived effective spectrum is also used to study the relative contributions of atmospheric turbulence and surface roughness to the coherence loss of propagating sound.

Measurement and simulation of sand saltation movement under fluctuating wind in a natural field environment

Extensive research on the near surface movement of sand particles has focused on wind tunnel experiments, theoretical analysis, and numerical simulation of sand saltation under ideal and controllable conditions. Most field observations are results on the average rate of sand transport over some hours or the whole day. However, researchers found recently that the effect of turbulent characteristics of near surface wind in real atmospheric boundary layers on the sand transport rate was obvious. The turbulent characteristics would cause a significant discrepancy between field observation and simulation of sand transport rate. In this work, a field experiment in a real-time system was designed to synchronously measure physical quantities, such as fluctuating wind velocity in the near surface region, sand transport intensity, temperature, and humidity, with the frequency of 1 Hz, at two points on a homogeneous flat sand surface located in the Minqin area, which is between the edges of the Badain Jaran Desert and the Tengger Desert. The relationship between the saltation events and some physical properties, such as the fluctuating wind velocity, temperature, and humidity, was studied. On the basis of the field observation results, a numerical model was developed to simulate sand movement under the fluctuating wind. The overall features of the experimental measurements were reproduced by simulation.

The energy and dissipation of turbulent fluctuations of wind velocity and temperature in the boundary layer of the atmosphere

The relationships between the energy of small-scale turbulence and its dissipation rate are studied based on the data of long-term high-frequency measurements of temperature and wind velocity fluctuations in urban area. It is shown that the energy of wind velocity turbulent fluctuations is linearly related to the dissipation rate ɛ. The proportionality coefficient between turbulent kinetic energy (TKE) and ɛ is dimensional and does not depend on the stratification of the atmosphere, the Richardson number, or the Monin-Obukhov scale. Measurements in different seasons show that this coefficient can be related to the mean velocity of adiabatic motions (sound speed or air temperature), which enables one to select a more universal constant, γ. A linear relationship between the temperature fluctuations variance (the characteristic of the inner energy of turbulence) and their dissipation rate is also shown. The revealed proportionality is confirmed by measurements in urban and forest conditions, as well as in the surface layer over a flat desert terrain.

풍자원 평가를 위한 건축물 주변의 유동특성

To utilize wind resources effectively around buildings in urban area, the magnitudes of wind velocity and turbulence intensity are important, which means the need of the information about the relationship between the magnitude of wind velocity and that of fluctuating wind velocity. In the paper, wind-tunnel experiments were performed to provide the information about Characteristic of Wind flow around buildings with the spanwise distance and the side ratio of buildings as variables. For a single building with the side ratios of one and two, the average velocity ratio was 1.4 and the velocity standard deviation ratio ranged from 1.4 to 2.6 at the height of 0.02m at the corner of the windward side, in which flow separation occurred. For twin buildings with the side ratios of one and two, the velocity ratio ranged from 2 to 2.5 as the spanwise distance varied at the height of 0.02m, and the velocity standard deviation ratio varied near 1.25. For twin buildings with the side ratios of one and two, the maximum velocity ratio was 1.75 at the height of 0.6m, and the maximum velocity standard deviation ratio was 2.1. It was also found from the results of CFD analysis and wind-tunnel experiments that for twin buildings with the side ratios of one and two, the difference between the velocity ratio of CFD analysis and that of wind-tunnel experiments at streamwise distances was near 0.75.

Integration of Crosswind Forces into Train Dynamic Modelling

In this paper a new method is used to calculate unsteady wind loadings acting on a railway vehicle. The method takes input data from wind tunnel testing or from computational fluid dynamics simulations (one example of each is presented in this article), for aerodynamic force and moment coefficients and combines these with fluctuating wind velocity time histories and train speed to produce wind force time histories on the train. This method is fast and efficient and this has allowed the wind forces to be applied to a vehicle dynamics simulation for a long length of track. Two typical vehicles (one passenger, one freight) have been modelled using the vehicle dynamics simulation package ‘VAMPIRE®’, which allows detailed modelling of the vehicle suspension and wheel—rail contact. The aerodynamic coefficients of the passenger train have been obtained from wind tunnel tests while those of the freight train have been obtained through fluid dynamic computations using large-eddy simulation. Wind loadings were calculated for the same vehicles for a range of average wind speeds and applied to the vehicle models using a user routine within the VAMPIRE package. Track irregularities measured by a track recording coach for a 40 km section of the main line route from London to King's Lynn were used as input to the vehicle simulations. The simulated vehicle behaviour was assessed against two key indicators for derailment; the Y/ Q ratio, which is an indicator of wheel climb derailment, and the Δ Q/ Q value, which indicates wheel unloading and therefore potential roll over. The results show that vehicle derailment by either indicator is not predicted for either vehicle for any mean wind speed up to 20 m/s (with consequent gusts up to around 30 m/s). At a higher mean wind speed of 25 m/s derailment is predicted for the passenger vehicle and the unladen freight vehicle (but not for the laden freight vehicle).

Effects of Turbulence Intensity on Dynamic Characteristics of Wind Turbine Airfoil

The aerodynamic performance of a wind turbine depends on operating conditions such as tip speed ratio and pitch settings. Generally, the wind turbine blade shapes are designed based on two-dimensional airfoil characteristics which are measured under static conditions in terms of angle of attack. However, wind turbines operated under complex condition in field environment, frequently experiencing suddenly fluctuation of wind direction and velocity. It causes fluctuations of angle of attack for the blade element. Therefore, the clarification of airfoil characteristics under dynamic conditions is important for accurate estimation of the power output and aerodynamic loads on wind turbine. In this study, wind tunnel experiments under dynamic and static conditions for different turbulence intensities are conducted for a wind turbine airfoil. As the result, the lift coefficient for dynamic condition shows the hysteresis loop when the range of angle of attack during the pitching motion includes the stall angle. The flow turbulence changes the stall characteristics of the airfoil. The size of hysteresis loop depends on the stall characteristics and the range of pitching motion.

Spline‐Interpolation‐Based FFT Approach to Fast Simulation of Multivariate Stochastic Processes

The spline‐interpolation‐based fast Fourier transform (FFT) algorithm, designated as the SFFT algorithm, is proposed in the present paper to further enhance the computational speed of simulating the multivariate stochastic processes. The proposed SFFT algorithm first introduces the spline interpolation technique to reduce the number of the Cholesky decomposition of a spectral density matrix and subsequently uses the FFT algorithm to further enhance the computational speed. In order to highlight the superiority of the SFFT algorithm, the simulations of the multivariate stationary longitudinal wind velocity fluctuations have been carried out, respectively, with resorting to the SFFT‐based and FFT‐based spectral representation SR methods, taking into consideration that the elements of cross‐power spectral density matrix are the complex values. The numerical simulation results show that though introducing the spline interpolation approximation in decomposing the cross‐power spectral density matrix, the SFFT algorithm can achieve the results without a loss of precision with reference to the FFT algorithm. In comparison with the FFT algorithm, the SFFT algorithm provides much higher computational efficiency. Likewise, the superiority of the SFFT algorithm is becoming more remarkable with the dividing number of frequency, the number of samples, and the time length of samples going up.

Aerodynamic response analysis of wind turbines

Wind energy has received increasing attention in the same way as energy crisis and environmental deterioration. The aerodynamic response of wind turbines is the major problem in wind turbine design. Blade element momentum theory was used to study the aerodynamic thrusts of the blades on the tower. Iterative solutions were used to calculate the axial flow induction factor for each cross-section of the blades. The harmonic superposition method was used to simulate the fluctuating wind velocity time histories. The wind shear equation was used to calculate the mean wind velocities at different heights. Finally, the finite element method model was used to study the aerodynamic response of wind turbines under random wind loads. Results show that the top displacement of tower is increased accordingly by increasing the rotational speed of the blades. Hence, this should be considered in wind turbine design.