Wind Velocity Fluctuations Research Articles

A time-localized response of wave growth process under turbulent winds

Abstract. Very short time series (with lengths of approximately 40 s or 5~7 wave periods) of wind velocity fluctuations and wave elevation were recorded simultaneously and investigated using the wavelet bispectral analysis. Rapid changes in the wave and wind spectra were detected, which were found to be intimately related to significant energy transfers through transient quadratic wind-wave and wave-wave interactions. A possible pattern of energy exchange between the wind and wave fields was further deduced. In particular, the generation and variation of the strong wave-induced perturbation velocity in the wind can be explained by the strengthening and diminishing of the associated quadratic interactions, which cannot be unveiled by linear theories. On small time scales, the wave-wave quadratic interactions were as active and effective in transferring energy as the wind-wave interactions. The results also showed that the wind turbulence was occasionally effective in transferring energy between the wind and the wave fields, so that the background turbulence in the wind cannot be completely neglected. Although these effects are all possibly significant over short times, the time-localized growth of the wave spectrum may not considerably affect the long-term process of wave development.

Dynamic response and reliability analysis of tall buildings subject to wind loading

This paper explores the wind stochastic field from a new viewpoint of stochastic Fourier spectrum (SFS). The basic random parameters of the wind stochastic field, the roughness length z 0 and the mean wind velocity at 10 m height U 10, as well as their probability density functions (PDF), are obtained. It provides opportunities to use probability density evolution method (PDEM), which had been proved to be of high accuracy and efficiency, in computing the dynamic response and reliability of tall buildings subject to the wind loading. Principals and corresponding numerical solving algorithm of the PDEM are first presented. Then, the adopted model of the wind stochastic field is described briefly. The simulation method of the fluctuating wind velocity based on the SFS is introduced. Finally, as an example of the application of the PDEM, a 20-storey frame subject to wind loading is investigated in detail. The responses, including the mean value and the standard deviation, and the reliabilities of the frame are evaluated by the PDEM. The results demonstrate that the PDEM is applicable and efficient in the dynamic response and reliability analysis of wind-excited tall building.

Evaluation Method of Train Operation Control under Gales Considering Temporal Fluctuations of Wind Velocity

This paper describes a study in which the authors investigated a method for quantitatively evaluating the safety levels involved in train control operations in gale wind conditions and the risk of a train overturning. This effort involved estimating the distribution functions of wind velocity occurrence and wind temporal fluctuations. From these functions, the authors were able to calculate the probability of a gale occurring in a particular section during train operations. This calculated probability was then compared with the relative frequency observed at the site. A risk analysis of gale conditions was then conducted for several different train operational control rules.

Remote determination of momentum-flux profiles in the lower atmospheric boundary layer

An acoustic locator—a sodar—is a unique instrument for getting the objective characteristics of the current state of the atmospheric boundary layer (ABL) owing to a combination of such properties as remoteness, mobility, resolution, and information content. This study demonstrates the capabilities of a sodar to obtain data on the second moments of the field of wind-velocity fluctuations, in particular, the profiles of momentum flux in the lower ABL, which are very important in practical applications. A corresponding method is described, and the results of its experimental verification and some examples of its application under the conditions of different ABL stratification are presented.

Dispersion of radionuclides released into a stable planetary boundary layer using a MonteCarlo model

In this paper a Monte Carlo model for describing the atmospheric dispersion ofradionuclides (represented by Lagrangian particles/neutral tracers) continuously releasedinto a stable planetary boundary layer is presented. The effect of variation in release heightand wind directional shear on plume dispersion is studied. The resultant plumeconcentration and dose rate at the ground is also calculated. The turbulent atmosphericparameters, like vertical profiles of fluctuating wind velocity components andeddy lifetime, were calculated using empirical relations for a stable atmosphere.The horizontal and vertical dispersion coefficients calculated by a numerical Lagrangianmodel are compared with the original and modified Pasquill–Gifford and Briggs empiricalσs. The comparison shows that the Monte Carlo model can successfully predict dispersion ina stable atmosphere using the empirical turbulent parameters. The predicted groundconcentration and dose rate contours indicate a significant increase in the affected areawhen wind shear is accounted for in the calculations.

The coherence function of a sound field propagating in a turbulent atmosphere

The coherence function of a sound field propagating through an atmosphere with temperature and wind velocity fluctuations is important for many practical applications involving microphone arrays. In the literature, the transverse coherence function has been studied in detail, when acoustic sensors are located in a plane perpendicular to the sound propagation path. In this paper, for the case of line‐of‐sight sound propagation, a closed‐form expression for the coherence function of a plane wave is derived for arbitrary location of acoustic sensors. Using this expression, for some limiting cases simple formulas for the coherence function are obtained which are valid for arbitrary spectra of turbulence. Furthermore, the coherence function is calculated and analyzed in detail for the Gaussian and von Kármán spectra of temperature and wind velocity fluctuations. In particular, the longitudinal coherence radius, when acoustic sensors are located along the sound propagation path, is calculated. It is also shown that contours of the coherence radius are elongated perpendicular to this path for relatively small propagation distances, and along it for large distances.

A Statistical Overview of Convection During the First CPEA Campaign

Data from the Equatorial Atmosphere Radar (EAR) were analysed during the Coupling Processes in the Equatorial Atmosphere (CPEA) campaign of April-May 2004. Statistical averages of the daily perturbations of wind velocity, temperature and humidity were examined. In the lower stratosphere, horizontal wind variances of up to 1.5 m 2 s -2 were visible above the afternoon tropospheric convection. Vertical wind variances of up to 0.03 m 2 s -2 occurred at the same time. The average variances of temperature and humidity in the lower troposphere also increased in the afternoon. Estimates of average momentum flux were made. In the lower stratosphere (18.0-20.0 km), the average magnitudes were 0.7-0.9 m 2 s -2 for both |u'w'| and |u'w'|. These fluxes were observed above the convective cells and indicate the average momentum emitted by them. Residuals of the momentum flux components showed a small westward preference and very small northward directional preference for wave propagation during CPEA. Four individual convective events and three Super Cloud Clusters (SCCs) were examined in detail. Tropospheric horizontal and vertical wind variances increased by 5 to 10 times during intense convection. Directly above this convection, in the lower stratosphere, increases in variance were also recorded. Vertical wind velocity fluctuation amplitudes in the lower stratosphere increased from 0.05-0.1 ms -1 away from convection, to be 0.1-0.4 ms -1 during convection. The amplitudes decreased to their background levels as soon as the convection had passed the EAR site. In the lower troposphere, the virtual temperature perturbations increased to 1.0-2.0°C during convection, an increase from a background value of about 0.5°C. During two of the four individual events, the amplitude of the temperature perturbations stayed enhanced for several hours after the end of convection. This contrasts with the vertical velocities in the lower troposphere, which quickly decreased following the passage of the storms. Specific humidity profiles in most cases showed large increases in the amount of water vapour below 4 km during times of convection. At certain heights and times during intense convection, the specific humidity increased by up to 50% from its background level.

Sound propagation through and scattering by internal gravity waves in a stably stratified atmosphere

A stably stratified atmosphere supports propagation of internal gravity waves (IGW). These waves result in highly anisotropic fluctuations in temperature and wind velocity that are stretched in a horizontal direction. As a result, IGW can significantly affect propagation of sound waves in nighttime boundary layers and infrasound waves in the stratosphere. In this paper, a theory of sound propagation through, and scattering by, IGW is developed. First, 3D spectra of temperature and wind velocity fluctuations due to IGW, which were recently derived in the literature for the case of large wave numbers, are generalized to account for small wave numbers. The generalized 3D spectra are then used to calculate the sound scattering cross section in an atmosphere with IGW. The dependencies of the obtained scattering cross section on the sound frequency, scattering angle, and other parameters of the problem are qualitatively different from those for the case of sound scattering by isotropic turbulence with the von Kármán spectra of temperature and wind velocity fluctuations. Furthermore, the generalized 3D spectra are used to calculate the mean sound field and the transverse coherence function of a plane sound wave propagating through IGW. The results obtained also significantly differ from those for the case of sound propagation through isotropic turbulence.

A Virtual Nozzle for Simulation of Spray Generation and Droplet Transport

The objective of this study was to develop a computer simulation model that mimics the collective behaviour of droplets in the spray cloud from a flat-fan hydraulic nozzle. The break-up region of this virtual nozzle was discretised into a mesh that ejected the droplets in radial directions . The initial size and corresponding velocity of each droplet ejected was selected at random from a two-column array of data for more than 10 000 actual droplets. That composite data file was collected from three different test nozzles of the same type and size (Spraying Systems TeeJet ® XR8002 nozzle operating at 207 kPa with tap water) with a phase/Doppler particle analyser (PDPA) traversing a plane 4 cm below the tip of each nozzle. The transport model employed an effective drag coefficient that accounted for aerodynamic drag , air entrainment and mutual interference of droplets in addition to effects of evaporation and vertical wind velocity fluctuations . Simulated targets collected the droplets at points 45 cm below the virtual nozzle to study deposition patterns. The virtual nozzle was calibrated to produce the same volumetric pattern and discharge rate as that of the test nozzles. The simulated deposition pattern from the calibrated virtual nozzle was very similar to the droplet size and number distribution measured with the PDPA 45 cm below the test nozzles. The measured and simulated mean and median diameters were of a good match.

Dynamics of wind–rail vehicle–bridge systems

An analytical model for dynamics of wind–vehicle–bridge (WVB) systems is presented in this paper in the time domain with wind, rail vehicles and bridge modeled as a coupled vibration system. The analytical model considers many special issues in a WVB system, which include fluid–solid interaction between wind and bridge, solid contact between vehicles and bridge, stochastic wind excitation on vehicles and bridge, time dependence of the system due to vehicle movement, and effect of bridge deck on vehicle wind load and vice versa. The models of wind, vehicles and bridge are presented with wind velocity fluctuations simulated using the simplified spectral representation method, with vehicles modeled as mass–spring–damper systems, and with bridge represented by a finite element model. The interactions between wind and bridge are similar to those considered in conventional buffeting analysis for long span bridges. In considering difficulties in measuring aerodynamic coefficients of moving vehicles on bridge deck, the cosine rule is adopted for the aerodynamic coefficients of moving vehicles to consider yaw angle effect, and expressions of wind forces on moving vehicles are then derived for engineering application. To include mutual effects of wind loads, aerodynamic parameters of vehicles and bridge deck are measured, respectively, using a composite section model test and a specially designed test device. The dynamic interaction between vehicle and bridge depends on both geometric and mechanical relationships between wheels of vehicles and rails on the bridge deck. The equations of motion of the coupled WVB system are derived and solved with a nonlinear iterative procedure. A cable-stayed bridge in China is finally selected as a numerical example to demonstrate dynamic interaction of the WVB system. The results show the validity of the present model as well as wind effects on the rail vehicles and the bridge.

Scattering of sound and infrasound waves by internal gravity waves in the atmosphere

Internal gravity waves (IGW) propagating in the atmosphere produce highly anisotropic fluctuations in temperature and wind velocity that are stretched in a horizontal direction. These fluctuations can significantly affect sound propagation in a night-time boundary layer and infrasound propagation in the stratosphere. In this paper, 3-D spectra of temperature and wind velocity fluctuations due to IGW, recently derived in the literature for the limiting case of large wave numbers, are generalized to account for small wave numbers. The generalized 3-D spectra are then used for development of a theory of sound propagation through and scattering by IGW. First, the sound scattering cross-section due to IGW is calculated. It is shown that the dependence of this scattering cross-section on parameters of the problem (the scattering angle, sound frequency, etc.) is qualitatively different from that for the case of sound scattering by isotropic turbulence. Second, the mean sound field and the transverse coherence function of a plane sound wave propagating in the atmosphere with IGW are calculated and compared with those for sound propagating through isotropic turbulence. [Work supported by ARO, Grant DAAD19-01-1-0640, and RFBR, Grant 03-05-04001.]

The effect of wind velocity fluctuations on drop spectrum broadening in stratocumulus clouds

In this study, a Lagrangian adiabatic parcel model and a trajectory ensemble model (TEM) are used in a sequence of experiments to investigate effects of velocity fluctuations on drop spectrum broadening in stratocumulus clouds. Using the adiabatic parcel model, it is found that even with a weakly buoyant temperature profile, if the initial updraft velocity is low enough, new drop nucleation above the cloud base region can occur via updraft acceleration accompanied by an increase in supersaturation. New nucleation can also occur via parcel recirculation as drops that have not fully evaporated during downdraft reduce the cloud base supersaturation peak in subsequent updrafts. The new nucleation produces new modes in the drop spectrum, broadening the spectrum towards smaller drops. Spectral broadening is also reproduced in the TEM, where an ensemble of 680 parcel trajectories is derived from a large eddy simulation (LES) velocity field. In the TEM, the mechanisms of in-cloud nucleation and growth/evaporation asymmetry caused by updraft acceleration and parcel recirculation promote spectral broadening predominantly in the lower and intermediate levels of the cloud, where parcels with initially low updraft velocities reside. To explicitly evaluate the effect of subgrid scale turbulent velocity fluctuations on drop spectrum evolution, such fluctuations are added to the mean LES velocity field. The velocity fluctuations are simulated using the Langevin equation, where turbulent diffusion is represented as an autoregression random process of the 1st order. The velocity fluctuations alter parcel trajectories, allowing parcels with initially low vertical velocities to be swept into higher vertical velocity eddies and to penetrate deeper into the cloud. The resulting updraft acceleration and parcel circulation lead to new drop nucleation and to broadening of the upper level drop spectra. The added subgrid turbulent velocity fluctuations also increase the overall number of parcels that exhibit new drop nucleation, bringing the fraction to half of the total ensemble under the assumption of strong turbulence. The above mechanisms help explain the existence of quite broad and bimodal stratiform cloud drop spectra, as well as the variation in stratiform drop spectra over short spatial scales. The sensitivity of the above mechanisms to the shape of the CCN spectrum and to variation in the turbulence parameters is investigated.

Time domain buffeting analysis of long suspension bridges under skew winds

This paper presents a time domain approach for predicting buffeting response of long suspension bridges under skew winds. The buffeting forces on an oblique strip of the bridge deck in the mean wind direction are derived in terms of aerodynamic coefficients measured under skew winds and equivalent fluctuating wind velocities with aerodynamic impulse functions included. The time histories of equivalent fluctuating wind velocities and then buffeting forces along the bridge deck are simulated using the spectral representation method based on the Gaussian distribution assumption. The self-excited forces on an oblique strip of the bridge deck are represented by the convolution integrals involving aerodynamic impulse functions and structural motions. The aerodynamic impulse functions of self-excited forces are derived from experimentally measured flutter derivatives under skew winds using rational function approximations. The governing equation of motion of a long suspension bridge under skew winds is established using the finite element method and solved using the Newmark numerical method. The proposed time domain approach is finally applied to the Tsing Ma suspension bridge in Hong Kong. The computed buffeting responses of the bridge under skew winds during Typhoon Sam are compared with those obtained from the frequency domain approach and the field measurement. The comparisons are found satisfactory for the bridge response in the main span.

DETERMINANTS OF LONG-DISTANCE SEED DISPERSAL BY WIND IN GRASSLANDS

Long-distance seed dispersal is an important topic in ecology, but notoriously difficult to quantify. Previous modeling approaches have failed to simulate long-distance dispersal, and it has remained unclear which mechanisms determine long-distance dispersal and what their relative importance is. We simulated wind dispersal of grassland plant seeds with four mechanistic models of increasing complexity and realism to assess which processes and which attributes of plants and their environment determine dispersal distances. We compared simulation results of the models to each other and to data from field dispersal experiments. The more complex and realistic models predicted short-distance dispersal more accurately and were the only models able to simulate long-distance dispersal. The model comparisons showed that autocorrelated turbulent fluctuations in vertical wind velocity are the key mechanism for long-distance dispersal. Seed dispersal distances are longest under high wind velocity conditions, when mechanically produced turbulent air movements are large. Under very low wind velocity conditions seeds are dispersed farther when there is more surface heating, but never as far as during strong wind events. Model sensitivity analyses showed that mean horizontal wind velocity, seed release height, and vegetation height are crucial determinants of dispersal potential and dispersal distances. Between plant species (but not within a species), seed terminal velocity is an additional important determinant of long-distance dispersal. These results imply that seed release height is the most important plant-controlled dispersal parameter for grassland plants, and that the structure of the local vegetation greatly affects dispersal distances. Thus, management plans for grasslands should take into account that changes in vegetation structure, e.g., due to eutrophication, can reduce the seed dispersal ability of wind-dispersed plant species.

Geometrically Nonlinear Buffeting Response of a Cable-Stayed Bridge

A geometrically nonlinear buffeting analysis of a cable-stayed bridge in the time domain is described. The bridge structure is modeled with three-dimensional thin-walled beam elements and three-dimensional elastic catenary cable elements. Spatially correlated wind velocity fluctuations are modeled and simulated using an algorithm for generating sample functions of a stationary, multivariate stochastic process according to its prescribed cross-spectral density matrix. Aerodynamic damping and aerodynamic stiffness are formulated based on experimentally determined flutter derivatives. The focus of this paper is on the effect of fluctuating components of the spatially correlated wind velocity on the geometrically nonlinear buffeting response for an 870 m cable-stayed bridge.

Monte Carlo simulation for the response analysis of long-span suspended cables under wind loads

This paper presents a time-domain approach for analyzing nonlinear random vibrations of long-span suspended cables under transversal wind. A consistent continuous model of the cable, fully accounting for geometrical nonlinearities inherent in cable behavior, is adopted. The effects of spatial correlation are properly included by modeling wind velocity fluctuation as a random function of time and of a single spatial variable ranging over cable span, namely as a one-variate bi-dimensional (1V-2D) random field. Within the context of a Galerkin`s discretization of the equations governing cable motion, a very efficient Monte Carlo-based technique for second-order analysis of the response is proposed. This procedure starts by generating sample functions of the generalized aerodynamic loads by using the spectral decomposition of the cross-power spectral density function of wind turbulence field. Relying on the physical meaning of both the spectral properties of wind velocity fluctuation and the mode shapes of the vibrating cable, the computational efficiency is greatly enhanced by applying a truncation procedure according to which just the first few significant loading and structural modal contributions are retained.

Coherence function and mean field of plane and spherical sound waves propagating through inhomogeneous anisotropic turbulence.

Inhomogeneity and anisotropy are intrinsic characteristics of daytime and nighttime atmospheric turbulence. For example, turbulent eddies are often stretched in the direction of the mean wind, and the turbulence statistics depends on the height above the ground. Recent studies have shown that the log-amplitude and phase fluctuations of plane and spherical sound waves are significantly affected by turbulence inhomogeneity and anisotropy. The present paper is devoted to studies of the mean sound field and the coherence functions of plane and spherical sound waves propagating through inhomogeneous anisotropic turbulence with temperature and velocity fluctuations. These statistical moments of a sound field are important in many practical applications, e.g., for source detection, ranging, and recognition. Formulas are derived for the mean sound field and coherence function of initially arbitrary waveform. Using the latter formula, we also obtained formulas for the coherence functions of plane and spherical sound waves. All these formulas coincide with those known in the literature for two limiting cases: homogeneous isotropic turbulence with temperature and wind velocity fluctuations, and inhomogeneous anisotropic turbulence with temperature fluctuations only. Using the formulas obtained, we have numerically shown that turbulence inhomogeneity significantly affects the coherence functions of plane and spherical sound waves.

A wavelet-based method to generate artificial wind fluctuation data

A method using the inverse wavelet transform is proposed to generate artificial wind velocity fluctuations. At first, in order to investigate the time-scale structure of natural wind, the wavelet transform is applied to the time history of a measured wind velocity data. Taking the results into account, the wavelet-based method is constructed such that the created time history possesses the characteristics similar to those of the natural wind data. The time histories are in particular synthesized to have a target power spectrum and intermittency similar to measured time histories. The characteristics of the time histories produced with the proposed method are discussed.

Assessing the eddy covariance technique for evaluating carbon dioxide exchange rates of ecosystems: past, present and future

AbstractThe eddy covariance technique ascertains the exchange rate of CO2 across the interface between the atmosphere and a plant canopy by measuring the covariance between fluctuations in vertical wind velocity and CO2 mixing ratio. Two decades ago, the method was employed to study CO2 exchange of agricultural crops under ideal conditions during short field campaigns. During the past decade the eddy covariance method has emerged as an important tool for evaluating fluxes of carbon dioxide between terrestrial ecosystems and the atmosphere over the course of a year, and more. At present, the method is being applied in a nearly continuous mode to study carbon dioxide and water vapor exchange at over a hundred and eighty field sites, worldwide. The objective of this review is to assess the eddy covariance method as it is being applied by the global change community on increasingly longer time scales and over less than ideal surfaces.The eddy covariance method is most accurate when the atmospheric conditions (wind, temperature, humidity, CO2) are steady, the underlying vegetation is homogeneous and it is situated on flat terrain for an extended distance upwind. When the eddy covariance method is applied over natural and complex landscapes or during atmospheric conditions that vary with time, the quantification of CO2 exchange between the biosphere and atmosphere must include measurements of atmospheric storage, flux divergence and advection.Averaging CO2 flux measurements over long periods (days to year) reduces random sampling error to relatively small values. Unfortunately, data gaps are inevitable when constructing long data records. Data gaps are generally filled with values produced from statistical and empirical models to produce daily and annual sums of CO2 exchange. Filling data gaps with empirical estimates do not introduce significant bias errors because the empirical algorithms are derived from large statistical populations. On the other hand, flux measurement errors can be biased at night when winds are light and intermittent. Nighttime bias errors tend to produce an underestimate in the measurement of ecosystem respiration.Despite the sources of errors associated with long‐term eddy flux measurements, many investigators are producing defensible estimates of annual carbon exchange. When measurements come from nearly ideal sites the error bound on the net annual exchange of CO2 is less than ±50 g C m−2 yr−1. Additional confidence in long‐term measurements is growing because investigators are producing values of net ecosystem productivity that are converging with independent values produced by measuring changes in biomass and soil carbon, as long as the biomass inventory studies are conducted over multiple years.

Evaluation of effect of micro-topography on design wind velocity

Wind flow around micro-topographic features is very complicated. It includes convergent flow, separated flow and reverse flow. Since wind directions and velocities vary greatly around such features, it is very important to estimate accurately the flow fields around them. We have utilized a triple-split probe that can measure wind directions and velocities in complicated flow fields, and investigated micro-topographic effects on design wind velocity by conducting wind tunnel tests. This paper introduces micro-topographic effects, “topographic multipliers”, for several kinds of up-slope cliffs. When no flow separation is observed, for slope gradients of 7.5° and 15°, topographic multipliers for mean wind velocities take maximum values of 1.3–1.4 at x/ H=0, but those for fluctuating wind velocities are small. When flow separation is observed, for slope gradients of 30°, 45° and 60°, topographic multipliers for mean components take maximum values of 1.3–1.4 at 0⩽ x/ H⩽1, and those for fluctuating components become very large in a separation bubble, and the influence of topography reaches x/ H=6 and z/ H=2 for each slope. No flow separation is observed for a curved surface whose mean slope gradient is 24°, and distributions of topographic multipliers resemble those for a slope gradient of 15° at x/ H⩾0. Thus, topographic multipliers for a curved surface can be predicted by its mean slope gradient.