Crosswind Direction Research Articles

Experimental investigation on three-dimensional structures of wind wave surfaces

This study experimentally investigated the three-dimensional features of wind wave surfaces in a laboratory. Free surfaces of wind waves under five types of wind velocities at four wind fetches were reconstructed by a stereo camera system. The dispersion law of wind waves was obtained from the spatio–temporal spectrum and showed a Doppler shift owing to the drift current. The wavenumber spectrum in the crosswind and wind direction follow the same power law of k−3 in the equilibrium range. Dynamic mode decomposition (DMD) was used to investigate the three-dimensional structures of the wave surface. Two different modes—the parallel mode and the net mode—were defined to characterize the structure of the wave surface. The parallel mode represents the average wave behavior over time, whereas the net mode represents the three-dimensional characteristics of the wind waves. The ratio of the wave number in the crosswind to the wind direction decreases with increasing wind fetch and converges to approximately 0.5. The wind wave spectrum in the wavenumber space had an elliptical distribution. Using the dispersion law, the wind wave spectrum in the (ω,k) domain can be predicted by a quartic surface.

Skeleton-stabilized divergence-conforming B-spline discretizations for incompressible flow problems of high Reynolds number

We consider a stabilization method for divergence-conforming B-spline discretizations of the incompressible Navier–Stokes problem wherein jumps in high-order normal derivatives of the velocity field are penalized across interior mesh facets. We prove that this method is pressure robust, consistent, and energy stable, and we show how to select the stabilization parameter appearing in the method so that excessive numerical dissipation is avoided in both the cross-wind direction and in the diffusion-dominated regime. We examine the efficacy of the method using a suite of numerical experiments, and we find the method yields optimal L2 and H1 convergence rates for the velocity field, eliminates spurious small-scale structures that pollute Galerkin approximations, and is effective as an Implicit Large Eddy Simulation (ILES) methodology.

New Features of Bragg and Non-Polarized Radar Backscattering from Film Slicks on the Sea Surface

Suppression of radar backscattering from the sea surface has been studied in field experiments with surfactant films carried out from an Oceanographic Platform on the Black Sea and from onboard a research vessel on the Gorky Water Reservoir using an X-C-S-band two co-polarized radar instrument. Bragg and non-polarized (non-Bragg) radar backscatter components, BC and NBC, respectively, were retrieved when measuring the radar backscatter at vertical (VV-) and horizontal (HH-) polarizations. New features of microwave backscattering from the sea surface have been revealed, including a non-monotonic dependence of radar backscatter suppression (contrasts) in slicks on azimuth angle and particularities of BC contrasts on radar wave number. Namely, it is demonstrated that the backscatter contrasts achieve maximum values at azimuth angles in between the upwind and crosswind radar look directions, and BC contrasts increase with radar wave number along the wind and decrease in the crosswind directions. The suppression of BC is discussed in the frame of Bragg’s theory of microwave scattering and of a simple model of the wind wave spectrum, while the suppression of NBC is considered associated with the micro-breaking of wind waves. The obtained new features of radar contrasts can be used for the identification and characterization of marine films.

Predicting drift characteristics of life rafts: Case study of field experiments in the South China Sea

Every year, hundreds of people die at sea because of vessel and airplane accidents. As marine emergency evacuation equipment, life rafts are highly used in case of maritime accidents. Accurate predictions of drift trajectories and search and rescue (SAR) areas for life rafts at sea can enhance the efficiency and success rate of maritime SAR operations to save human lives. The objective of this study is to determine the drift characteristics of aviation and marine life rafts in the South China Sea, and develop an accurate drift prediction model. A series of maritime drift experiments were conducted in the South China Sea using the standard 6-person life raft (aviation life raft) and the standard 10-person life raft (marine life raft) to obtain 550 drift samples. This data was used to establish three drift models of life rafts: the AP98 leeway drift model, the dynamics drift model and the improved drift model. Finally, Lagrangian particle tracking method and Monte Carlo simulations were used to compare the drift models for 6-person and 10-person life raft. Results indicate that the probability of positive crosswind leeway (CWL) for 6-person and 10-person life raft is 61% and 31.4%, respectively. The jibing frequency is 2% per hour for 6-person and 4% per hour for 10-person life raft. The maximum divergence angle is 11.3° for 6-person and 20.5° for 10-person life raft, indicating that 10-person life raft is more sensitive to the wind speed in the crosswind direction. The improved drift model is more accurate than AP98 leeway drift model and dynamics drift model for predicting drift trajectories of both 6-person and 10-person life rafts. The results of this work may provide effective guidance for drift prediction of aviation life rafts and marine life rafts, which is of great significance for the maritime search and rescue (SAR) operations in the South China Sea.

The Influence of Wind Direction on the Inelastic Responses of a Base-Isolated Square Section High-Rise Building

Previous studies show that the largest wind-induced response of a square section fixed-base high-rise building occurs when the strong wind is blowing perpendicular onto a building face, and the greatest translational response is likely to occur in the crosswind direction. When it comes to a square section base-isolated high-rise building that allows the isolation system to yield under strong wind excitation, the inelastic response shows distinctive non-Gaussian characteristics under fluctuating wind excitation and mean drift phenomenon under non-zero mean wind load. These characteristics may lead to a quite different result when determining the most unfavorable wind direction. Thus, the influence of wind direction on the inelastic response of a square base-isolated high-rise building is discussed in this study based on synchronous pressure measurement. The multi-story superstructure is modeled as a linear elastic shear building, while the isolation system is represented in a bilinear hysteresis restoring force model. The peak value of the inelastic response is estimated through a moment-based Hermit model from an underlying standard Gaussian process. The results show that when the strong wind blows perpendicular onto a building face, the greatest inelastic displacement, both at the top and isolation level, occurs in the along-wind direction, which is different from the elastic response. With the change of wind direction, the largest combined inelastic displacement still occurs when the wind inclination angle is 0°, while the combined displacement in other directions is also very large, which is worthy of concern.

Flight characteristics of rod-shaped windborne debris objects in atmospheric boundary layer winds

The presented study explored the flight characteristics of rod-shaped debris objects in neutrally-stratified atmospheric boundary layer (ABL) winds. Despite the fact that rod-shaped debris objects, such as lumbers, are a common type of debris observed during extreme wind events, their three-dimensional motions had not been fully characterized and the understanding of their flight trajectories was limited to the outcome of only a few wind tunnel tests. To address the outlined research gaps, this study established a coupled simulation framework, which utilized both computational fluid dynamics (CFD) and rigid body dynamics (RBD). After validating this framework using the experimental test data, the flight trajectories of a set of rod-shaped debris objects were first evaluated in uniform winds. With placing the rod's longitudinal axis in both along-wind and cross-wind directions, the debris flight motions were characterized for various initial pitch angles. After appropriate verifications, the simulations were extended to investigate the flight of rod-shaped objects in ABL winds. The main debris flight properties, including displacement and velocity profiles, were systematically extracted over time, taking into consideration various influential factors. With the wealth of debris flight data generated through the conducted CFD-RBD simulations, predictive models were developed for assessing the kinetic energy associated with the flight of rod-shaped debris objects.

A parametric study of the mooring system design parameters to reduce wake losses in a floating wind farm

Wake effects inside a conventional fixed bottom wind farm decrease the power produced by the downwind turbines, hence decreasing the farm’s annual energy production (AEP). However, floating offshore wind turbines (FOWTs) have the ability to relocate their positions laterally through surge and sway motions. This flexibility provides a new degree of freedom (DOF) in the floating wind farm layout, which can be used to decrease the aerodynamic interactions inside the floating wind farm and hence decrease the wake losses. The lateral movement of FOWTs can be passively controlled by the mooring system design. The mooring system’s restoring characteristics allows the FOWT to only move within a specific area in the x-y plane known as the watch circle. Current state of the art mooring system designs are following oil and gas design basis where the floating platforms are not allowed to have large lateral displacements. In this work, we use full factorial design to analyse the effect of different mooring system design parameters on the ability of the floater to relocate its position. The analysis shows that each design parameter has a different way of affecting the FOWT’s response. The mooring lines’ headings control which wind directions cause the biggest displacements in the crosswind direction. The smaller the lines’ diameters the higher the displacements of the FOWT. Finally, the longer the line length the smaller the mooring system’s stiffness and hence the larger the FOWT’s displacement. The results of this study can be used as the basis for floating wind farm optimization, in which the wind turbines are allowed to passively relocate their positions according to the wind speed and wind direction.

Nocturnal jets over wind farms in complex terrain

Recent wind-energy field experiments have enhanced our understanding of stratified flows over topography by observing the flow of nocturnal jets over complex terrains in the natural environment. There is a research gap on how the intricacies of such flows could impact wind-energy production. In this study, we investigated how nocturnal jets influence the performance of two operational wind farms built over complex terrain, by combining operational data and numerical mesoscale simulations. The wind farms are similarly designed as two rows of turbines roughly aligned in the crosswind direction and separated by microscale distances. Front and back rows are located, respectively, near the leading edge and the lee of a microscale plateau with a downstream valley. Nocturnal jets occur close to midnight as the cold fast-moving air of a gravity current rushes inland after the evening transition. They produce a deeper stably-stratified layer and strong downslope winds and in some cases can cause turbines in the back row to produce twice as much power as those in the front row.

Effect of tuned mass damper in wind‐induced response of “V” plan‐shaped tall building

SummaryIn this study, a “V” plan‐shaped building with single tuned mass damper (STMD) and multiple tuned mass damper (MTMD) is analyzed in alongwind and crosswind direction considering different mass ratios and tunning ratios. The force coefficient of the “V” plan‐shaped tall building is determined by computational fluid dynamics (CFD) method to obtain the time history function of dynamic wind force. An optimum ratio has been adopted to compare the effectiveness of MTMD over STMD installed in the V‐shape building model. Power spectral density of displacement, acceleration, base shear, base moment, and torsional moment due to dynamic wind time history are determined to investigate the vibration control of MTMDs and STMDs. Finally, the optimal mass ratio of the maximum reduction in response and control effectiveness in percentage has been determined for each parameter. The present study indicates that MTMDs are more effective to mitigate wind‐induced vibration than STMDs.

2D and 2.5D Numerical Simulations for Vortex-Induced Vibration (VIV) of a 1.5:1 Rectangular Cylinder

The semi-periodic vortex-shedding phenomenon caused by flow separation at the windward corners of a rectangular cylinder would result in significant vortex-induced vibrations (VIVs). Based on the aeroelastic experiment of a rectangular cylinder with side ratio of 1.5:1, 2-dimensional (2D) and 2.5-dimensional (2.5D) numerical simulations of the VIV of a rectangular cylinder were comprehensively validated. The mechanism of VIV of the rectangular cylinder was in detail discussed in terms of vortex-induced forces, aeroelastic response, work analysis, aerodynamic damping ratio and flow visualization. The outcomes showed that the numerical results of aeroelastic displacement in the cross-wind direction and the vortex-shedding procedure around the rectangular cylinder were in general consistence with the experimental results by 2.5D numerical simulation. In both simulations, the phase difference between the lift and displacement response increased with the reduced wind speed and the vortex-induced resonance (VIR) disappeared at the phase difference of approximately 180∘. The work done by lift force shows a close relationship with vibration amplitudes at different reduced wind speeds. In 2.5D simulations, the lift force of the rectangular cylinder under different wind speeds would be affected by the presence of small-scale vortices in the turbulence flow field. Similarly, the phase difference between lift force and displacement response was not a constant with the same upstream wind speed. Aerodynamic damping identified from the VIV was mainly dependent on the reduced wind speed and negative damping ratios were revealed at the lock-in regime, which also greatly influenced the probability density function (PDF) of wind-induced displacement.

Experimental study on proximity interference induced vibration of two staggered square prisms in turbulent boundary layer flow

Slender structures placed close to each other could greatly magnify the dynamic response due to interference effects. This paper presents the results of experimental studies of the proximity interference induced vibration of two identical square prisms in a turbulent boundary layer to clarify its excitation mechanism. The upstream interfering prism was static, while the principal prism was dynamic and free to vibrate only in the crosswind direction. The divergent vibration behavior at the critical interfered location was examined in detail. Based on the time-resolved particle image velocimetry (TR-PIV) technique and unsteady pressure measurement, the vortex shedding process and corresponding pressure distribution of the dynamic principal prism were described by examining their correlation with the motion time histories. The biased gap flow between the two prisms was found to control the vibration of the dynamic principal prism. The interaction between the vortex shedding in the wake of the dynamic principal prism and the gap flow is dramatically intensified with increasing wind velocity. Such effects create a large fluctuating lift force on the dynamic principal prism at high reduced wind velocity, compared with measurement taken with two static prisms, which drives the prism to undergo a divergent crosswind response.

Influence of noise barrier and louver on ventilation and thermal performance of wet cooling towers under crosswind conditions

Aiming at the influence of noise barrier and louver on the thermal performance of wet cooling towers, a three-dimensional numerical model of wet cooling towers was established in this paper. Based on this model, the influence of the noise barrier and louver on ventilation and thermal performance was studied under crosswind conditions (crosswind velocity varies from 0 m/s to 10 m/s and crosswind direction α is 0°, 90°, 180°). To analyze the influence of noise barrier and louver on the air intake characteristics, the inlet air uniformity coefficient Cff and the effective inlet air velocity vave were defined. Simulation results manifested that the noise barrier increases the internal low wind speed area in the rain zone and the high water temperature area on the collecting basin surface. Compared with the original cooling tower, although the Cff increases, the vave and the cooling capacity contribution of the fillings zone CF decrease after installing the noise barrier, so the water temperature drop ΔTW decreases. Additionally, the louver can relieve the adverse effect of crosswind on thermal performance, and the Cff, vave, CF, and ΔTW all have an increasing trend under crosswind conditions after installing the louver. The louver can improve remarkably the thermal performance when the crosswind velocity is 6 m/s and α = 90°, and compared with the original cooling tower, the Cff, vave, CF, and ΔTW increase by 0.02, 1.33 m/s, 1.33%, 0.31 °C, respectively.

A novel two-dimensional entrainment wake model for wind turbine wakes

ABSTRACT The wake model for stand-alone turbine is an essential ingredient of wind farm wake research, which is of great concern for optimization of wind turbine layout, power generation simulation, and operation control on a wind farm. One-dimensional wake models (e.g., Jensen model and Frandsen model) based on single conservation theorem were approximately described by top-hat shape, which cannot represent the non-uniform distribution characteristics of wind turbine wake in the crosswind direction accurately and tend to underestimate the velocity deficit. Different from the above-mentioned models, entrainment wake model (EWM) can conserve both of mass and momentum within the control volume, which improves the accuracy of wake simulation to a certain extent. But the top-hat hypothesis limits the wake prediction accuracy of EWM. In this case, a novel two-dimensional EWM is proposed for the first time in this paper. Different from the advanced two-dimensional Bastankah and Porté-Agel wake model and Jensen-Gaussian wake model, the proposed model considers the influence of variable entrainment parameter on wake recovery, of which the key entrainment parameter was considered as a variable that is associated with downstream distance, ambient turbulence, and wake turbulence. The proposed Gaussian EWM has been validated by the data from Garrad Hassan (GH) wind tunnel laboratory experiments, Sexberium wind farm measurements, and the field measurements with lidar in Northwestern China. It has been concluded that the proposed model can predict the wake velocity distribution most accurately and provides the minimal power prediction error under various scenarios.

Vortex-Induced Vibration and Countermeasure of a Tubular Transmission Tower

Transmission towers with steel tubes or so-called tubular towers are widely used for ultra-high voltage (UHV) transmission because of their overall higher strength and stability, and better wind resistant capability. However, within a specific wind velocity range, tubular members with a high slenderness ratio are susceptible to joint fatigue failure under the vortex-induced vibration (VIV). To thoroughly unveil the mechanism of VIV of tubular members in UHV transmission towers, this paper first measures the acceleration and corresponding wind velocity of a tubular member under the VIV, followed by an identification of the first-order frequency. Then, a simulation framework for the tubular members subjected to the VIV is proposed. The system considered is simplified as a mass–spring–damping system with the non-linear coupling effect between the tubular member and wind field taken into account. Based on the analysis, the lock-in region and maximum displacement amplitudes in the cross-wind direction are calculated, while the simulation accuracy is verified via comparison with the on-site measured data. Meanwhile, the influence of damping ratio on the VIV is studied. Finally, a new type of radial spoiler is proposed to suppress the VIV of the tubular members. The results of the 3D simulation show that the proposed radial spoiler can effectively suppress the vortex generation, and there is no noticeable vibration after the installation of the countermeasure. In effect, it was demonstrated that the proposed countermeasure can effectively suppress the VIV of the tubular tower. The parametric analysis reveals that the distance between two adjacent spoilers has a significant impact on the control efficiency and should be carefully designed for each project.

Overturning assessment of railway vehicles under cross winds

The overturning issues in a strong wind are extremely critical to the railway vehicles, which have attracted a great deal of attention over the years. To address such problems, this paper introduces a dynamic reliability approach to evaluate the overturning risk of vehicles in crosswinds. Starting from the aerodynamic model, a novel prediction formula of unsteady crosswind forces with a consideration of the complete turbulent field effect is derived. Using the pseudo-excitation method (PEM), the power spectrum of vehicle responses is then calculated by the established vehicle model, and finally the corresponding results are used to assess the probabilistic overturning of vehicles in terms of the dynamic reliability analysis. It is found from the calculations that the time-dependent failure probability curves are related to the vehicle speed, wind speed, and crosswind direction, and the probabilistic characteristic wind curves (PCWCs) at different failure probabilities are more useful and reasonable for evaluating the overturning risk in comparison with the traditional characteristic wind curves (CWCs) in previous investigations. Furthermore, the probabilistic characteristic wind surface (PCWS) that considers the effect of crosswind direction, is developed, and it reveals that the vehicle is most vulnerable at a condition of perpendicular crosswind direction.

Wind farm layout optimization using pseudo-gradients

Abstract. This paper presents a heuristic building block for wind farm layout optimization algorithms. For each pair of wake-interacting turbines, a vector is defined. Its magnitude is proportional to the wind speed deficit of the waked turbine due to the waking turbine. Its direction is chosen from the inter-turbine, downwind, or crosswind directions. These vectors can be combined for all waking or waked turbines and averaged over the wind resource to obtain a vector, a “pseudo-gradient”, that can take the role of gradient in classical gradient-following optimization algorithms. A proof-of-concept optimization algorithm demonstrates how such vectors can be used for computationally efficient wind farm layout optimization. Results for various sites, both idealized and realistic, illustrate the types of layout generated by the proof-of-concept algorithm. These results provide a basis for a discussion of the heuristic's strong points – speed, competitive reduction in wake losses, and flexibility – and weak points – partial blindness to the objective and dependence on the starting layout. The computational speed of pseudo-gradient-based optimization is an enabler for analyses that would otherwise be computationally impractical. Pseudo-gradient-based optimization has already been used by industry in the design of large-scale (offshore) wind farms.

3D numerical study using three novel windbreak walls in natural draft dry cooling towers for performance enhancement under various crosswind conditions

Because of Natural draft dry cooling towers' (NDDCTs) extreme vulnerability to crosswind, performance enhancement of these towers is essential. Various strategies have been introduced to improve the efficiency of NDDCTs. One of the best techniques is using internal windbreak walls. Internal windbreakers' most significant problem is that they occupy much space inside the tower and block the major part of paths and openings through which air can pass. In this study, by introducing three novel internal windbreak walls named Sugarloaf-type, Swirl-type, and Impeller-type that take up less of the tower's interior portion, an attempt has been made to solve previous studies' problems. Navier-Stokes and energy equations are solved for a 3D, incompressible, and steady flow using CFD methods to investigate the flow field inside and around the tower. Temperature distribution, velocity vectors, streamlines, and thermal performance are obtained for all the models at various crosswind velocities and directions. Results show that not only did the suggested windbreakers solve the problem of occupied space, but also, they are capable of competing against the former designs under windy conditions. The asymmetric models, which are the Swirl-type and the Impeller-type, are studied in other directions, and the angle of 60° is determined to be the best direction of crosswind. Altogether, the Sugarloaf-type, symmetry windbreaker is chosen as the best model due to its independence in different crosswind directions and high-performance enhancement. The efficiency of the mentioned NDDCT is improved by 7.5% at a crosswind velocity of 10 m/s.

Study on the fluctuating wind responses of constructing bridge towers with magnetorheological elastomer variable stiffness tuned mass damper

Constructing bridge towers are high-rise and flexible structures subjected to significant wind induced vibrations. Tuned Mass Dampers (TMDs) have been widely used to reduce dynamic responses of high-rise structures under fluctuating wind loadings. By resonance with main structure, TMD can effectively suppress structural vibrations. However, the natural frequencies of bridge tower usually decrease continuously during its construction progress. The frequency shift characteristic will cause the detune of TMD, leading to a great control efficiency loss. As a novel stiffness tunable device made of magnetorheological elastomer (MRE), MRE-TMD can adjust its natural frequency in real-time to track the main structure, avoid detuning and achieve better control performance. To study the wind induced vibration control performance of MRE-TMD, this paper explores the fluctuating wind responses of constructing bridge towers in along wind and cross wind directions. The fluctuating wind loads are generated by harmonic superposition method with the along wind fluctuating wind speed spectrum and the empirical power spectrum of fluctuating lift force. By comparing among the uncontrolled, TMD controlled and MRE-TMD controlled constructing bridge towers, the simulation results show that the MRE-TMD system can effectively maintain the tuning state, and significantly reduce the wind induced vibrations during whole construction process.

Virtual element stabilization of convection-diffusion equation with shock capturing

Streamline upwind Petrov-Galerkin (SUPG) stabilization for the virtual element discretization of the convection-diffusion equation produces local oscillations in the crosswind direction. To overcome such a shortcoming, an additional shock-capturing term is added into the formulation. In this paper, we propose a nonlinear shock-capturing technique and prove the existence and stability of the discrete solution. The resulted nonlinear system of equations is solved using a simple iterative technique. Numerical experiments are conducted to show the efficiency of shock-capturing term in reducing the spurious oscillations along the crosswind direction.

Characterization of the seismic wave field radiated by a wind turbine

Due to the increased expansion of wind turbines (WTs) in the last two decades, seismologists have recently observed an impact of disturbing signals at seismic stations. Ground vibrations generated by WTs perturb the seismic background noise level at seismological monitoring sites causing the performance and functional capability of a seismological network to be limited which leads a conflict between operators of WTs and seismological institutions. With regard to a conflict settlement, the research project MISS (Minderung der Storwirkung von Windenergieanlagen auf seismologische Stationen) was initiated to investigate in detail the seismic signals emitted by a WT in order to identify and discuss methods to reduce the disturbance impact on seismic stations. A part of this research project is presented in this study and deals with movement patterns of a WT tower, foundation and the immediate subsurface as well as the amplitude decay behavior of the seismic noise with increasing distances to the WT. For this purpose, short- and long-term measurements were conducted at a single WT of the “Burgerwindpark A31 Hohe Mark” located in Heiden (NRW, Germany). First, the first four eigenfrequencies of the WT tower are identified at 0.3 Hz, 1.1 Hz, 3.25 Hz and 6.0 Hz using 1.5-h recordings of sensors that were installed in different heights on the inner wall of the tower. From the measurements, linear, elliptical and circular motion patterns could be observed at the tower. The results of the long-term measurements using 15 mobile seismic stations installed over 1 month in two circular arrays with distances of 100 and 200 m around the WT and two additional stations, one deployed on the foundation and one directly besides in the field, also show linear, elliptical and circular particle movements of the foundation and shallow underground. By correlating power spectral density (PSD) spectra and vertical displacements with the prevailing wind direction, it can be shown that at the eigenfrequencies 0.3, 3.25 and 6.0 Hz Rayleigh waves are radiated in crosswind direction from the WT. In downwind direction, the wave field is dominated by the Love wave type. In contrast, at 1.1 Hz, the dominating wave type emitted by the WT in crosswind direction is Love wave and in downwind direction mainly of the Rayleigh wave type. In order to estimate an amplitude decay relationship of the WT-induced seismic waves, a 1.5-h line-array measurement with 36 sensors installed 100–1000 m from the WT and a sensor spacing of 25 m was used to determine attenuation curves of ground motion velocities proportional to r−b, with r as the distance between sensor and WT and b as the decay parameter, for the four eigenfrequencies of the tower. The calculated b-values are increasing with frequency from 0.30 to 0.86.