Wind Velocity Field Research Articles

Assesment of the transverse galloping stability of a railway overhead located above a railway bridge

In this paper, galloping instability of catenary overheads located at railway bridge is assessed. A first contribution of this paper is the experimental characterization, in a wind tunnel, of the flow field above the railway platform for a cross-wind protected railway bridge. From particle image velocimetry (PIV) measurements, the averaged flow field above the railway platform is obtained and estimations of the bidimensional averaged wind velocity field are reported. Another contribution of the paper is the measurement of aerodynamic coefficients of a BC-120 contact wire cross section. From both measurements, averaged PIV flow field and aerodynamic coefficients, galloping stability is assessed using a parameter based on Den Hartog criterion. A comparison with the results obtained using the aerodynamic coefficients reported by Scanlon and Stickland is also presented and discussed.

Simulation of non-stationary wind velocity field on bridges based on Taylor series

The simulation of non-stationary wind velocity field based on the spectral representation method often requires significant computational efforts due to the summation of trigonometric functions usually involved in the simulation procedure. Some techniques which make use of FFT algorithm have been developed but most of these techniques deal with seismic ground motions. Limited effort has been devoted to the simulation of non-stationary wind velocity. Therefore, in this paper, a spectral-representation-based technique which takes advantage of FFT algorithm is proposed by combining Cholesky decomposition and Taylor series expansion. The approach consists of locating and expanding the time and frequency non separable part of the decomposed evolutionary power spectral density function by mean of Taylor series expansion to allow the application of the FFT algorithm. Samples of non-stationary wind velocity can be then generated through multiple executions of the FFT algorithm once the Taylor series expansion is successful. The present approach, which is primarily developed for the simulation of non-stationary wind velocity on long-span cable supported bridges, is very efficient since the summation of the trigonometric functions can be carried out through FFT algorithm which is well known for its higher efficiency. The approach was further improved by reformulating the simulation formulas where the order in which the summation operations are executed is imposed.

Hybrid simulation of thunderstorm outflows and wind-excited response of structures

Starting from the records detected by the monitoring network of the European Projects “Wind and Ports” and “Wind, Ports and Sea”, this paper proposes a novel strategy for simulating the wind velocity field of thunderstorm outflows. A model of the wind field along a vertical axis is first proposed. Its different ingredients and the inherent sources of randomness are then separated into five groups and a hybrid technique for the simulation of thunderstorm outflows is formulated. This technique is applied to generate artificial time-histories of the aerodynamic wind loading on three real slender vertical test structures whose dynamic response is evaluated by means of a time domain integration of the equations of motion. The results are analysed in a probabilistic frame aiming to inspect the distribution of the maximum value of the response, the role of the aerodynamic admittance, the relevance of the resonant part of the response, and the contribution of higher vibration modes in parallel with the classic analysis of the response of structures to synoptic extra-tropical cyclones. The conclusions draw some prospects on the joint calibration of the response spectrum technique and the time domain simulations.

Coupling hydrodynamics with comoving frame radiative transfer

CONTEXT: For more than two decades, stellar atmosphere codes have been used to derive the stellar and wind parameters of massive stars. Although they have become a powerful tool and sufficiently reproduce the observed spectral appearance, they can hardly be used for more than measuring parameters. One major obstacle is their inconsistency between the calculated radiation field and the wind stratification due to the usage of prescribed mass-loss rates and wind-velocity fields. AIMS: We present the concepts for a new generation of hydrodynamically consistent non-local thermodynamical equilibrium (non-LTE) stellar atmosphere models that allow for detailed studies of radiation-driven stellar winds. As a first demonstration, this new kind of model is applied to a massive O star. METHODS: Based on earlier works, the PoWR code has been extended with the option to consistently solve the hydrodynamic equation together with the statistical equations and the radiative transfer in order to obtain a hydrodynamically consistent atmosphere stratification. In these models, the whole velocity field is iteratively updated together with an adjustment of the mass-loss rate. RESULTS: The concepts for obtaining hydrodynamically consistent models using a comoving-frame radiative transfer are outlined. To provide a useful benchmark, we present a demonstration model, which was motivated to describe the well-studied O4 supergiant Zeta Pup. The obtained stellar and wind parameters are within the current range of literature values. CONCLUSIONS: For the first time, the PoWR code has been used to obtain a hydrodynamically consistent model for a massive O star. This has been achieved by a profound revision of earlier concepts used for Wolf-Rayet stars. The velocity field is shaped by various elements contributing to the radiative acceleration, especially in the outer wind. (...)

A GIS-based fire spread simulator integrating a simplified physical wildland fire model and a wind field model

ABSTRACT​​This article discusses the integration of two models, namely, the Physical Forest Fire Spread (PhFFS) and the High Definition Wind Model (HDWM), into a Geographical Information System-based interface. The resulting tool automates data acquisition, preprocesses spatial data, launches the aforementioned models and displays the corresponding results in a unique environment. Our implementation uses the Python language and Esri’s ArcPy library to extend the functionality of ArcMap 10.4. The PhFFS is a simplified 2D physical wildland fire spread model based on conservation equations, with convection and radiation as heat transfer mechanisms. It also includes some 3D effects. The HDWM arises from an asymptotic approximation of the Navier–Stokes equations, and provides a 3D wind velocity field in an air layer above the terrain surface. Both models can be run in standalone or coupled mode. Finally, the simulation of a real fire in Galicia (Spain) confirms that the tool developed is efficient and fully operational.

A critical comparison of Lagrangian methods for coherent structure detection.

We review and test twelve different approaches to the detection of finite-time coherent material structures in two-dimensional, temporally aperiodic flows. We consider both mathematical methods and diagnostic scalar fields, comparing their performance on three benchmark examples: the quasiperiodically forced Bickley jet, a two-dimensional turbulence simulation, and an observational wind velocity field from Jupiter's atmosphere. A close inspection of the results reveals that the various methods often produce very different predictions for coherent structures, once they are evaluated beyond heuristic visual assessment. As we find by passive advection of the coherent set candidates, false positives and negatives can be produced even by some of the mathematically justified methods due to the ineffectiveness of their underlying coherence principles in certain flow configurations. We summarize the inferred strengths and weaknesses of each method, and make general recommendations for minimal self-consistency requirements that any Lagrangian coherence detection technique should satisfy.

Cross-correlation and cross-power spectral density representation by complex spectral moments

A new approach to provide a complete characterization of normal multivariate stochastic vector processes is presented in this paper. Such proposed method is based on the evaluation of the complex spectral moments of the processes. These quantities are strictly related to the Mellin transform and they are the generalization of the integer-order spectral moments introduced by Vanmarcke.The knowledge of the complex spectral moments permits to obtain the power spectral densities and their cross counterpart by a complex series expansions. Moreover, with just the aid of some mathematical properties the complex fractional moments permit to obtain also the correlation and cross-correlation functions, providing a complete characterization of the multivariate stochastic vector processes.Some numerical applications are reported in order to show the capabilities of this method. In particular, the examples regard two dimensional linear oscillators forced by Gaussian white noise, the characterization of the wind velocity field, and the stochastic response analysis of vibro-impact system under Gaussian white noise.

Accurate group velocity estimation for unmanned aerial vehicle-based acoustic atmospheric tomography.

Acoustic atmospheric tomography calculates temperature and wind velocity fields in a slice or volume of atmosphere based on travel time estimates between strategically located sources and receivers. The technique discussed in this paper uses the natural acoustic signature of an unmanned aerial vehicle as it overflies an array of microphones on the ground. The sound emitted by the aircraft is recorded on-board and by the ground microphones. The group velocities of the intersecting sound rays are then derived by comparing these measurements. Tomographic inversion is used to estimate the temperature and wind fields from the group velocity measurements. This paper describes a technique for deriving travel time (and hence group velocity) with an accuracy of 0.1% using these assets. This is shown to be sufficient to obtain highly plausible tomographic inversion results that correlate well with independent SODAR measurements.

Whistler mode waves upstream of Saturn

AbstractWhistler mode waves are generated within and can propagate upstream of collisionless shocks. They are known to play a role in electron thermodynamics/acceleration and, under certain conditions, are markedly observed as wave trains preceding the shock ramp. In this paper, we take advantage of Cassini's presence at ~10 AU to explore the importance of whistler mode waves in a parameter regime typically characterized by higher Mach number (median of ~14) shocks, as well as a significantly different interplanetary magnetic field structure, compared to near Earth. We identify electromagnetic precursors preceding a small subset of bow shock crossings with properties which are consistent with whistler mode waves. We find these monochromatic, low‐frequency, and circularly polarized waves to have a typical frequency range of 0.2–0.4 Hz in the spacecraft frame. This is due to the lower ion and electron cyclotron frequencies near Saturn, between which whistler waves can develop. The waves are also observed as predominantly right handed in the spacecraft frame, the opposite sense to what is typically observed near Earth. This is attributed to the weaker Doppler shift, owing to the large angle between the solar wind velocity and magnetic field vectors at 10 AU. Our results on the low occurrence of whistler waves upstream of Saturn also underpin the predominantly supercritical bow shock of Saturn.

Nonlinear development of the ekman layer structures

The nonlinear modes of coherent structure development in the Atmospheric boundary layer are investigated. Two-scale model of Atmospheric boundary layer is used in the calculation. The velocity field splits into large-scale profile of horizontal wind velocity and three-scale velocity field. The former depends only on the vertical coordinate. The latter is connected with roll circulation and is subject to the vertical coordinate and the coordinate perpendicular to the roll direction. The influence of turbulence is parameterized by turbulent viscosity. The modification of wind profile by rolls is taken into account. Depending on the Reynolds number, different types of the hydrodynamic instabilities specific to the Atmospheric boundary layer occurred. This appears at the relative orientation of the arising geostrophic wind and roll circulation, and also at the scales and space periods of the structures. As the Reynolds number grows, the mean energy and helicity increase. Within the range of the Reynolds number between 200÷300 the dependence is close to linear, which points to the possibility of utilizing weakly nonlinear theory methods, where perturbation amplitudes increase as Re1/2. The rise of the roll asymmetry followed by remarkable growing of the extreme amplitude of a longitudinal velocity component in the direction opposite to geostrophic wind compared to the amplitudes along the lines of geostrophic wind is detected. Increase of the positive component of helicity by contrast to the negative ones is observed simultaneously. A qualitative comparison between the modeling findings and the measured characteristics of the coherent structures observed in the Atmospheric boundary layer is carried out. In July 2007, these structures were measured by acoustic sounding methods in Kalmykia, where asymmetry in the distribution of longitudinal velocity component was observed as well. An apparent pattern of roll circulation begins to reproduce in the mesoscale atmospheric model RAMS under grid size about 500 meters. Reasonably good correspondence between simulation findings and observable vortex with centers lying about 1200÷1300 meters high is received. The values of turbulent viscosity and effective Reynolds number are typical for unstable stratification conditions.

Fabrication of four-path remote outdoor wind velocity measurement system and its performance evaluation

A method is proposed for the remote measurement of the outdoor ground-surface two-dimensional (2D) vector wind velocity field averaged over a region of 10–50 m size. To this end, four-channel (4ch) sound wave transmitters and receivers were placed at the corners of a rectangular monitoring site. From the four-path travel time data, the wind velocity and direction averaged over the region were estimated under the uniform-wind-field assumption. By this method, misestimation due to the local turbulence wind field, which is encountered in conventional in situ-type anemometers, can be avoided. To achieve a satisfying speed data collection that keeps up with the rapid changes in real wind field, coded modulation signals were transmitted and received simultaneously between all the 4ch speaker/microphone pairs. Test experiments demonstrated that time variations of vector wind velocities spatially averaged over the area were successively measured with satisfying speed and accuracy.

Uncertainty quantification in the dynamics of a guyed mast subjected to wind load

A study of the stochastic dynamics of a guyed mast under wind action is reported in the present work. The simplified structural model consists of a beam-column accounting for the second order effect due to axial loads and one inclined guy which is represented by a nonlinear extensible cable. The governing system is discretized with finite elements and a reduced order model is afterwards constructed using a basis of vibration modes.Both, the load and the structural model are considered stochastic. The wind load is derived from a wind velocity field after the application of the Spectral Representation Method. Since nonlinear structures show special sensitivity to dynamic loads, the reference nominal wind velocity (suggested by the codes) is also considered a random parameter. The guy pretension is a significant parameter in the behavior of guyed structures and its variation is a relevant issue. Furthermore, the bending stiffness can vary due to the structure reinforcements. Since it is not easy to foresee their behavior, an uncertainty quantification study appears necessary.After performing Monte Carlo simulations, the outcomes are evaluated accounting for each statistical parameter (guy tension, bending stiffness and wind nominal velocity) separately. Also, parameters combinations are studied. Six illustrative cases are reported. The results are shown in different ways (probability distribution functions (PDFs), correlation, exceeding limits, etc.). The stochastic variables have different effects on the statistics of the dynamic response. The guy pretension and the nominal wind velocity are found to be the most influential random quantities. Despite the potential of adverse impact, detailed studies of guyed structures under natural actions, as wind or earthquake, are not frequent. The involved variables are usually uncertain and it can be concluded that an uncertainty quantification study leads to a more realistic approach of the dynamic behavior of the structural system, in particular regarding the sensitivity to some parameters.

Sodars and their application for investigation of the turbulent structure of the lower atmosphere

Possibilities of sodar application for investigation of the spatiotemporal dynamics of three components of wind velocity vector, longitudinal and transverse structural functions of wind velocity field, structural characteristics of temperature and wind velocity, turbulent kinetic energy dissipation rate, and outer scales of temperature and dynamic turbulence in the atmospheric boundary layer are analyzed. The original closed iterative algorithm of sodar data processing taking into account the classical and molecular absorption and the turbulent sound attenuation on the propagation path is used that allows the vertical profiles of the characteristics of temperature and wind velocity field to be reconstructed simultaneously and their interrelations to be investigated. It is demonstrated how the structure of temperature and wind turbulence is visualised in real time.

Drag force in wind tunnels: A new method

Abstract A rigid object of general shape is fixed inside a wind tunnel. The drag force exerted on it by the wind is determined by a new method based on simple basic Physics concepts, provided one has a solver, any solver, for the corresponding dynamic Navier–Stokes equation which determines the wind velocity field around the object. The method is completely general, but here we apply it to the traditional problem of a long cylinder perpendicular to the wind.

Characterization of wind velocities in the wake of a full scale wind turbine using three ground-based synchronized WindScanners

The wind energy community is in need of detailed full-field measurements in the wake of wind turbines. Here, three dimensional(3D) wind vector field measurements obtained in the near-wake region behind a full-scale test turbine are presented. Specifically, the wake of a NEG Nordtank turbine, installed at Risoe test field, has been measured from 0 to 2 diameters downstream. For this, three ground-based synchronised short-range WindScanners and a spinner lidar have been used. The 3D wind velocity field has been reconstructed in horizontal and vertical planes crossing the hub. The 10-min mean values of the three wind components reveal detailed information regarding the wake properties while propagating downwind over flat terrain. Furthermore, the wake centre is tracked from the measurements and its meander is investigated as function of yaw misalignment of the turbine. The centre-line wake deficit is calculated both in a Nacelle and Moving Frame of Reference. The results can be used in quantitative validation of numerical wake models.

Monitoring of wind effects on an instrumented low-rise building during the landfall of a severe tropical storm

Tropical cyclone-induced disasters cause significant economic losses and heavy causalities every year. Most of the casualties were caused by low-rise buildings collapsing. This study aims to investigate the characteristics of tropical cyclone-generated winds and evaluate the wind effects on a typical low-rise building under tropical cyclone conditions through field measurements. In-situ monitoring was conducted on a full-scale low-rise building during the landfall of severe tropical storm Rumbia in 2013. Both the wind velocity field around and the surface pressures on the building were recorded during the storm. Characteristics of wind velocity field near ground during the windstorm were analyzed and compared with the specifications in the load code of China. Furthermore, when the wind approached in oblique directions, the windward roof corner suffered from the worst suctions under conical vortex. It was found that the maximum shape factors under the oblique winds were significantly larger than those specified in the load code. Moreover, probability density distributions of surface pressures induced by conical vortex were found to deviate from the Gaussian distribution. The outcomes of this study are expected to be of practical and imperative use to the wind-resistant design of low-rise buildings in tropical cyclone-prone regions. Copyright © 2016 John Wiley & Sons, Ltd.

Two approaches to describing the turbulent exchange within the atmospheric surface layer

Two two-dimensional models that describe the interaction of an air flow with a heterogeneous vegetation cover within the atmospheric surface layer are presented. These models differ in the closure techniques used to obtain approximate solutions of the averaged system of the Navier−Stokes and continuity equations and also in the approaches applied to describe the interaction of the air flow with elements of vegetation. The first model is based on the one-and-a-half-order closure scheme and the interaction of an air flow with elements of vegetation is described using the viscous drag force. The second model uses the first-order closure scheme and the distribution of the wind velocity field near the boundaries between different vegetation communities is simulated by the theory of contrast structures (CSs). We also present the results of the numerical experiments to compare the developed models with respect to their ability to describe the wind-velocity field within the atmospheric surface layer above the different forms of vegetation cover heterogeneity, such as a forest belt and clear-cut areas.

Acoustic travel-time tomography of the atmospheric surface layer at the Boulder Atmospheric Observatory

Acoustic tomography of the atmospheric surface layer (ASL) is based on travel-time measurements between speakers and microphones in a spatial array, which are arranged so as to create propagation paths through the region to be sampled. Then, the temperature and wind velocity fields inside the tomographic region are reconstructed by inverting the travel times. Tomography has certain advantages over conventional point measurements, such as spatial averaging and a quadratic growth of the observations relative to the number of sensors. An array built at the Boulder Atmospheric Observatory (BAO) enables horizontal-slice tomography of the ASL at a height of 8 m above the ground, in an 80 m x 80 m region. The instrumentation and principle of operation of the BAO tomography array are explained. Inverse algorithms for reconstruction of the temperature and wind velocity fields from the travel times are reviewed. Results in numerical simulations of the BAO tomography array and reconstruction of turbulence fields in tomography experiments are presented. Acoustic tomography of the atmosphere can also be performed at other spatial scales, ranging from a size of an ultrasonic anemometer/thermometer to the height of the atmospheric boundary layer and even in the stratosphere and thermosphere.

Accuracy requirements for unmanned aerial vehicle-based acoustic atmospheric tomography

If the acoustic signature is measured onboard a propeller-driven unmanned aerial vehicle and by an array of microphones on the ground, comparison between the projected and observed signals allows the propagation delays—and hence effective sound speed values of the intervening medium—to be determined. As the ray paths along which energy travels intersect within the atmosphere at multiple locations and angles, temperature and wind velocity fields may be estimated using tomography. This paper describes signal processing strategies for determining propagation delays at sub-millisecond levels of precision; the inverse problem and a model that allows meteorological observations to be incorporated into the tomographic solution; and the algebraic iterative reconstruction technique used to overcome the ill-posed nature of the inversion. A weakly sheared daytime convective atmospheric boundary layer—synthesised through use of large eddy simulation code that utilises pseudo-spectral differencing and solves an ellipt...

Dynamical Core Model Intercomparison Project (DCMIP) tracer transport test results for CAM‐SE

The Dynamical Core Model Intercomparison Project (DCMIP) provides a set of tests and procedures designed to facilitate development and intercomparison of atmospheric dynamical cores in general circulation models (GCMs). Test category 1 examines the advective transport of passive tracers by three‐dimensional prescribed wind velocity fields, on the sphere. These tests are applied to the Spectral Element (SE) dynamical core of the Community Atmosphere Model (CAM), the default for high‐resolution simulations in the Community Earth System Model (CESM). Test case results are compared with results from the CAM‐FV (Finite Volume) and MCore models where possible. This analysis serves both to evaluate the performance of CAM‐SE's spectral‐element tracer transport routines as well as to provide a baseline for comparison with other atmospheric dynamical cores and f or future improvements to CAM‐SE itself.