Along-wind Turbulence Research Articles

Experimental and numerical study on buffeting force characteristics of the π -shaped bridge deck

In this study, a π-shaped main beam with typical geometric characteristic parameters was selected for conducting wind tunnel tests, and the characteristics of the buffeting force were measured. Based on the measured results, numerical expansion research was conducted using the narrowband synthetic random flow generation (NSRFG) turbulent inlet method, and a grid strategy was provided. By changing the geometric characteristic parameters of the π-shaped girder, a comparative study was conducted using proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) methods, revealing the influence of cross-sectional geometric characteristic parameters on the buffeting force characteristics and analyzing their mechanism of action. The results indicate that the inlet wind parameters of the NSRFG need to be adapted to the grid size. The grid filter size at the front end of the model should be smaller than 0.193 of the along-wind turbulence integral scale, which can then be used to solve for 80% of the turbulent kinetic energy. The smaller the aspect ratio is, the larger the buffeting force spectrum is, and the smaller the opening ratio is, the smaller the buffeting force spectrum is. The opening ratio strongly influences the buffeting lift spectrum, and the aspect ratio strongly influences the buffeting drag spectrum. The POD decomposition indicates that the geometric characteristic parameters affect the shape, strength, position, and direction of vortices at the section opening. DMD decomposition indicates that geometric feature parameters affect the frequency and growth rate of dominant modes as well as the directionality and regularity of vortex distribution.

Wind Characteristics Investigation on The Roofs of Three Adjacent High-Rise Buildings in a Coastal Area during Typhoon Meranti

This paper presents the study of the pulsating characteristics of three adjacent high-rise buildings A, B, and C under typhoon ‘Moranti’ (2016) based on the measurement of the actual top wind speed. The studied pulsating characteristics included mean wind speed and direction, turbulence intensity, gust factor, turbulence integral scale, wind speed spectrum and correlation. The relationships between each pulsating parameter and the relationship between the pulsating parameter and gust duration have been investigated. Results show that the mean wind speed and wind direction of three buildings are close. When U ≥ 10 m/s in three different sites at the same time, the turbulence intensity variation of three buildings is consistent and decreases when mean wind speed increases. Once only two locations are acquired simultaneously and the wind angle between 35° and 45°, the mean values of the along-wind and cross-wind turbulence of building A and building C are close. The along-wind turbulence of the three buildings is greater than the predicted Chinese codes for various terrains. The turbulence intensity and gust factors obtained through the analysis of the samples with the mean wind speed U ≥ 10 m/s are reasonable. The turbulence integral scales of buildings A and C are equal to the predicted values of ASCE-7 and AIJ-2004, whereas the turbulent integral scale of building B is evidently small. The gust factors of three buildings increase when the turbulence intensity increases; these two characteristics have a linear relationship. At the same time interval, building B has the maximum along-wind turbulence intensity and gust factors during the low wind speed period and building C achieves the minimum values. Building A acquires the maximum and building C obtains the minimum values in the high wind speed period. The turbulence intensity and gust factors of building B show a certain pulsation. Results show that turbulence intensity and gust factors are mainly affected by the short-term fluctuation of wind. The longitudinal wind speed spectrum of three buildings conforms well to the von Karman model. The correlation of along-wind speed depends on the wind speed, whereas the correlation of cross-wind direction is independent of wind speeds. The measured data and statistical parameters provide useful information for the wind resistance design of high-rise buildings in typhoon-prone areas.

Evaluation of mast measurements and wind tunnel terrain models to describe spatially variable wind field characteristics for long-span bridge design

The quality of the information about the wind field characteristics is crucial for accurately predicting the structural response of a long-span bridge subjected to dynamic wind loads. In this paper, in situ mast measurements and terrain model wind tunnel tests are compared with full-scale measurements of the wind field along the Hardanger Bridge girder. The aim is to investigate the performance of mast measurements and wind tunnel terrain model tests in predicting the wind field characteristics for long-span bridges in complex terrains. Wind field spatial variations and statistical distributions for the mean wind velocity and turbulence intensity are investigated. Extreme value statistics have been applied to compare the mean wind velocity recordings from two different measurement periods. Results showing terrain-induced effects on the wind directions, turbulence intensities and mean wind velocities are presented. Simultaneous spanwise wind profiles for the mean wind velocity and along-wind turbulence intensity are compared between the terrain model wind tunnel tests and the full-scale measurements, and large nonuniformities are identified. The extreme profiles of the turbulence intensities vary as much as 100% along the span, and the mean wind velocity profiles vary up to 50% along the span.

Investigations of aerodynamic effects on streamlined box girder using two-dimensional actively-controlled oncoming flow

The aerodynamic behavior of a streamlined box section model is characterized in an actively controlled wind tunnel with multiple fans and vibrating airfoils in Miyazaki University, Japan. A series of single pseudo harmonic fluctuating wind flows with discrete frequencies are generated. The aerodynamic admittance components obtained via a cross-spectral identification method for different incoming flows present obvious deviations between each other. This indicates the probable dependence of aerodynamic admittance on the characteristics of oncoming flow, which implies the limitation of conventional buffeting theory. More attention is focused on the relationship between aerodynamic forces and wind turbulence components, along- and across-wind cross-wind, in pseudo sinusoidal flow conditions. It is found that the across-wind turbulence has dominant influences on lift and pitching moment forces. With the strengthening of along-wind turbulence, the lift force increases, while the drag and pitching moment forces vary with different trends in different frequency ranges. Moreover, the contribution of along-wind turbulence to lift and pitching moment forces is considered to be negligible compared with that of the cross-wind turbulence, while for the drag force, the two turbulence components present comparative contributions.

Multivariable wind modeling in state space

Turbulence of the incoming wind field is of paramount importance to the dynamic response of wind turbines. Hence reliable stochastic models of the turbulence should be available from which time series can be generated for dynamic response and structural safety analysis. In the paper an empirical cross-spectral density function for the along-wind turbulence component over the rotor plane is taken as the starting point. The spectrum is spatially discretized in terms of a Hermitian cross-spectral density matrix for the turbulence state vector which turns out not to be positive definite. Since the succeeding state space and ARMA modeling of the turbulence rely on the positive definiteness of the cross-spectral density matrix, the problem with the non-positive definiteness of such matrices is at first addressed and suitable treatments regarding it are proposed. From the adjusted positive definite cross-spectral density matrix a frequency response matrix is constructed which determines the turbulence vector as a linear filtration of Gaussian white noise. Finally, an accurate state space modeling method is proposed which allows selection of an appropriate model order, and estimation of a state space model for the vector turbulence process incorporating its phase spectrum in one stage, and its results are compared with a conventional ARMA modeling method.

Modelling of concentration fluctuations in canopy turbulence

The knowledge of the concentration probability density function (pdf) is of importance in a number of practical applications, and a Lagrangian stochastic (LS) pdf model has been developed to predict statistics and concentration pdf generated by continuous releases of non-reactive and reactive substances in canopy generated turbulence. Turbulent dispersion is modelled using a LS model including the effects of wind shear and along-wind turbulence. The dissipation of concentration fluctuations associated with turbulence and molecular diffusivity is simulated by an Interaction by Exchange with the Conditional Mean (IECM) micromixing model. A general procedure to obtain the micromixing time scale needed in the IECM model useful in non-homogeneous conditions and for single and multiple scalar sources has been developed. An efficient algorithm based on a nested grid approach with particle splitting, merging techniques and time averaging has been used, thus allowing the calculation for cases of practical interest. The model has been tested against wind-tunnel experiments of single line and multiple line releases in a canopy layer. The approach accounted for chemical reactions in a straightforward manner with no closure assumptions, but here the validation is limited to non-reacting scalars.

Footprint characteristics of scalar concentration in the convective boundary layer

Footprint characteristics for passive scalar concentration in the convective boundary layer (CBL) are investigated. A backward Lagrangian stochastic (LS) dispersion model and a large eddy simulation (LES) model are used in the investigation. Typical characteristics of the CBL and their responses to the surface heterogeneity are resolved from the LES. Then the turbulence fields are used to drive the backward LS dispersion. To remedy the spoiled description of the turbulence near the surface, Monin-Obukhov similarity is applied to the lowest LES level and the surface for the modeling of the backward LS dispersion. Simulation results show that the footprint within approximately 1 km upwind predominates in the total contribution. But influence from farther distances also exists and is even slightly greater than that from closer locations. Surface heterogeneity may change the footprint pattern to a certain degree. A comparison to three analytical models provides a validation of the footprint simulations, which shows the possible influence of along-wind turbulence and the large eddies in the CBL, as well as the surface heterogeneity.

Low-Dimensional Representations of Inflow Turbulence and Wind Turbine Response Using Proper Orthogonal Decomposition

A demonstration of the use of Proper Orthogonal Decomposition (POD) is presented for the identification of energetic modes that characterize the spatial random field describing the inflow turbulence experienced by a wind turbine. POD techniques are efficient because a limited number of such modes can often describe the preferred turbulence spatial patterns and they can be empirically developed using data from spatial arrays of sensed input/excitation. In this study, for demonstration purposes, rather than use field data, POD modes are derived by employing the covariance matrix estimated from simulations of the spatial inflow turbulence field based on standard spectral models. The efficiency of the method in deriving reduced-order representations of the along-wind turbulence field is investigated by studying the rate of convergence (to total energy in the turbulence field) that results from the use of different numbers of POD modes, and by comparing the frequency content of reconstructed fields derived from the modes. The National Wind Technology Center’s Advanced Research Turbine (ART) is employed in the examples presented, where both inflow turbulence and turbine response are studied with low-order representations based on a limited number of inflow POD modes. Results suggest that a small number of energetic modes can recover the low-frequency energy in the inflow turbulence field as well as in the turbine response measures studied. At higher frequencies, a larger number of modes are required to accurately describe the inflow turbulence. Blade turbine response variance and extremes, however, can be approximated by a comparably smaller number of modes due to diminished influence of higher frequencies.

Stochastic Analysis of Self-Induced Vibrations

Vortex-induced vibrations of a structural element are modelled as a non-linear stochastic single-degree-of-freedom system. The deterministic part of the governing equation represents laminar flow conditions with a stationary non-zero solution corresponding to lock-in. Across-wind turbulence is included as an additive excitation and along-wind turbulence is introduced as a parametric excitation term, both assumed to be white noise processes. An approximate closed-form solution to the corresponding Fokker–Planck equation in terms of the stationary probability density of the energy is obtained. The auto spectral density of the position at a particular energy-level is approximated by the spectral density of a linear system with energy dependent damping. The spectral density is then obtained by integration of the energy conditional spectral density over all energies weighted by the probability density. The approximate theoretical expressions for the probability density of the energy and the auto spectral density of the position compare favourably with results obtained by numerical simulation.

Effectiveness of tuned liquid column dampers for vibration control of towers

The effectiveness of tuned liquid column dampers (TLCD) in controlling the wind-induced vibration of towers is studied. The nonlinear governing equation of the TLCD is linearized to obtain the stochastic response of the towers due to along-wind turbulence. Through parametric studies, the optimum parameters for maximum reduction in acceleration and displacement are presented for a wide range of towers. It is found that for any tower of practical interest, almost the same amount of reduction in acceleration can be obtained by choosing an appropriate opening ratio of the orifice in the TLCD. The same opening ratio would give almost the maximum reduction in displacement. Generally, the opening ratio needs to be varied between 0.5 and 1.0, with smaller opening ratios for shorter towers.

Gust Buffeting. I: Peak Wind Velocity and Equivalent Pressure

Although the peak wind velocity, the equivalent pressure on rigid bodies, and the dynamic alongwind response of flexible structures are component parts of the gust buffeting problem, different procedures have been developed in the technical literature with reference to each of these subjects. This fact gives rise to conflicting solutions and noticeable inconsistencies of results, especially when these procedures are applied to boundary situations. On the basis of this consideration, this paper and a companion paper propose a revised and generalized formulation of the wind buffeting problem and a unified definition of gust factor. The present paper analyzes the themes of the peak wind velocity and of the equivalent pressure solving both problems in closed form. This goal is pursued by introducing a new general expression of the power spectrum of the alongwind turbulence and by applying the equivalent wind spectrum technique. In the context of this formulation, several well-known specific procedures can be conceptually regarded as particular cases of this more general treatment.