Wind-induced Motion Research Articles

Integrated wind load analysis and stiffness optimization of tall buildings with 3D modes

Recent trends towards constructing taller buildings with irregular geometric shapes imply that these structures are potentially more responsive to wind excitation. The wind-induced motion of modern tall buildings is generally found to involve with significant coupled lateral and torsional effects, which are attributed to the asymmetric three-dimensional (3D) mode shapes of these buildings. The 3D coupled modes also complicate the use of high frequency force balance (HFFB) technique in wind tunnel testing for predicting the wind-induced loads and effects on tall buildings. This paper firstly presents the analysis of equivalent static wind loads (ESWLs) on tall buildings with 3D modes provided that the wind tunnel derived aerodynamic wind load spectra are given. Then an integrated wind load updating analysis and optimal stiffness design technique is developed for lateral drift design of tall asymmetric buildings involving coupled lateral–torsional motions. The results of a practical 40-storey building example with significant swaying and torsional effects are presented. Not only is the technique able to produce the most cost efficient element stiffness distribution of the structure satisfying multiple serviceability wind drift design criteria, but a potential benefit of reducing the wind-induced loads can also be achieved by the stiffness design optimization method.

Cross Correlations of Modal Responses of Tall Buildings in Wind-Induced Lateral-Torsional Motion

Recent trends towards developing increasingly taller and irregularly-shaped buildings imply that these complex structures are potentially more responsive to wind excitation. Making accurate predictions of wind loads and their effects on such structures is therefore a necessary step in the design synthesis process. This paper presents a framework for dynamic analysis of the wind-induced lateral-torsional response of tall buildings with three-dimensional (3D) mode shapes. The cross correlation reflecting the statistical coupling among modal responses under spatiotemporally varying dynamic wind excitations has been investigated in detail. The effects of intermodal correlations on the lateral-torsional response of tall buildings with 3D mode shapes and closely spaced natural frequencies are elucidated and a more accurate method for quantifying intermodal cross correlations is analytically developed. Utilizing the wind tunnel derived synchronous multipressure measurements, a full-scale 60-story asymmetric building of mixed steel and concrete construction is used to illustrate the proposed framework for the coupled dynamic analysis and highlight the intermodal correlation of modal responses on the accurate prediction of coupled building acceleration.

Exchange between littoral and pelagic waters in a stratified lake due to wind-induced motions: Lake Kinneret, Israel

We used field data and three-dimensional numerical simulations to investigate the exchange processes between the lake boundaries (littoral) and lake interior (pelagic) due to wind-induced motions in Lake Kinneret, Israel. The field data showed large-scale metalimnion oscillations with amplitudes up to 10 m in response to westerly diurnal winds, the existence of a well-defined turbidity intrusion into the metalimnion of the lake and a benthic boundary layer (BBL). The observed vertical and horizontal movements of the turbidity were explained by the vertical and horizontal advection associated with basin-scale wave motions and a residual circulation set up by the basin-scale motions and the wind field. The horizontal advective transport in the metalimnion, associated with the velocities induced by the basin-scale mode-two Poincare wave, provided the primary control for the exchange between the lake boundaries and lake interior on daily time scales. Detailed comparison of simulation results with both temperature and velocity profiles revealed excellent agreement for time scales from hours to days. The validated numerical model was used to extract the residual motions that provided the secondary exchange flows, in the various regions of the lake on time scales from days to weeks. The residual motion in the surface layer consisted of a combination of an anticlockwise gyre in the western half of the lake and a weak clockwise gyre along the eastern boundary that were forced by the curl of the wind field. In the metalimnion, residual motions were predominantly forced by the basin-scale internal wave motions and an anticlockwise gyre established itself throughout the whole basin. Lastly in the BBL, residual motions consisted of an anticlockwise gyre, but the geographic distribution was patchy. Together these results provide new insight into a proper characterization of the processes underlying the flux paths of water and particles between the lake boundaries and lake interior.

Numerical Simulation of Wind-Induced Motion in Suspended Sediment Transport

In regions with broad water surfaces such as lakes, reservoirs and coastal areas, the wind stress on the flow motion generates a significant impact. The wind stress is an unsteady force which makes numerical simulation difficult. This paper presents a two-dimensional (2-D) mathematical model of the impact of wind-induced motion on suspended sediment transport at Beijing's 13-Ling Reservoir. The model uses the Diagonal Cartesian Method (DCM) with a wetting-drying dynamic boundary to trace variations in the water level. The calculation results have been tested against in situ measurements. The measurements confirm the model's accuracy and agreement with the actual situation at the reservoir. The calculations also indicate that wind stress holds the key to suspended sediment transport at Beijing's 13-Ling Reservoir, especially when westerly winds prevail.

Signaling against the Wind: Modifying Motion-Signal Structure in Response to Increased Noise

Animal signals are optimized for particular signaling environments [1-3]. While signaling, senders often choose favorable conditions that ensure reliable detection and transmission [4-8], suggesting that they are sensitive to changes in signal efficacy. Recent evidence has also shown that animals will increase the amplitude or intensity of their acoustic signals at times of increased environmental noise [9-11]. The nature of these adjustments provides important insights into sensory processing. However, only a single piece of correlative evidence for signals defined by movement suggests that visual-signal design depends on ambient motion noise [12]. Here we show experimentally for the first time that animals communicating with movement will adjust their displays when environmental motion noise increases. Surprisingly, under sustained wind conditions, the Australian lizard Amphibolurus muricatus changed the structure and increased the duration of its introductory tail flicking, rather than increasing signaling speed. The way these lizards restructure the alerting component of their movement-based aggressive display in the presence of increased motion noise highlights the challenge we face in understanding motion-detection mechanisms under natural operating conditions.

Vibration Control of a Wind-Excited Benchmark Tall Building with Complex Lateral-Torsional Modes of Vibration

This paper describes a proposed wind-excited benchmark tall building incorporating three-dimensional lateral-torsional modes of vibration, which is typical of a significant number of modern tall buildings. A series of wind tunnel pressure tests were conducted on a 1:400 scale model to determine the translational and torsional wind forces acting on the benchmark building. A finite element model was also constructed and mass, damping, and stiffness matrices were subsequently formulated as an evaluation model for numerical analysis. The evaluation model was further simplified to a state reduced-order system (ROS) using the state order reduction method. A numerical vibration control example was conducted to demonstrate the suppression of the wind-induced three-dimensional lateral-torsional motions using a bi-directional tuned mass damper (TMD) incorporating two magnetorheological (MR) dampers, one in each orthogonal direction, to act as a semi-active control system, referred to as a smart tuned mass damper (STMD). The optimal control forces generated by the MR dampers were obtained through the linear quadratic regulator (LQR) to minimize the storey accelerations. The formulation details, methodology and numerical simulation results are outlined in this paper.

Sea Ice and Current Response to the Wind: A Vector Regressional Analysis Approach

The authors describe a two-dimensional (vector) regressional model for examining the anisotropic response of ice drift and ocean current velocity (“drift velocity”) to surface wind forcing. Illustration of the method is limited to sea ice response. The principal mathematical and physical properties of the model are outlined, together with estimates of the “response matrices” and the corresponding “response ellipses” (drift velocity response to a unity wind velocity forcing). For each direction, φ, of the wind vector the method describes a corresponding “wind factor” α(φ) (relative drift speed) and “turning angle” θ(φ) (the angle between the drift velocity and wind vector). The major ellipse axis corresponds to the direction of the “effective wind” (φ = φmax) and the minor axis to the direction of the “noneffective” wind. The eigenvectors of the response matrix define wind directions that are the same as the wind-induced drift velocity directions. Depending on the water depth and offshore distance, six analytical cases are possible, ranging from rectilinear response ellipses near the coast to purely circular response ellipses in the open ocean. The model is used to examine ice drift along the western shelf of Sakhalin Island (Sea of Okhotsk). Responses derived from the vector regression (four parameter) method are less constrained and therefore more representative of wind-induced surface motions than those derived using the traditional complex transfer function (two parameter) approach.

Towers for Antarctic Telescopes

To take advantage of the exceptional seeing above the boundary layer on Antarctic sites, a high-resolution telescope must be mounted on a support tower. An open transparent tower of framework minimizes the upward temperature-disturbed airflow. A typical minimum height is 30 m. The tower platform has to be extremely stable against wind-induced rotational motions, which have to be less than fractions of an arc second, unusually small from a mechanical engineering viewpoint. In a traditional structure, structural deflections result in angular deflections of the telescope platform, which introduce tip and tilt motions in the telescope. However, a structure that is designed to deflect with parallel motion relative to the horizontal plane will undergo solely translation deflections in the telescope platform and thus will not degrade the image. The use of a parallel motion structure has been effectively demonstrated in the design of the 15-m tower for the Dutch Open Telescope (DOT) on La Palma. Special framework geometries are developed, which make it possible to construct high towers in stories having platforms with extreme stability against wind-induced tilt. These geometric solutions lead to constructions, being no more massive than a normal steel framework carrying the same load. Consequently, these lightweight towers are well suited to difficult sites as on Antarctica. A geometry with 4 stories has been worked out.

Watermass Properties of the Antarctic Slope Front: A Simple Model*

Abstract In the Antarctic, dense shelf water is formed in coastal polynyas and is differentiated from the fresher surface water by the wind-induced ice motion that displaces offshore the ice melt from production zones. Where the shelf water discharges into the deep ocean, the Antarctic Slope Front (ASF) is V shaped, separating the shelf and surface waters (referred to as “frontal” waters) from the Circumpolar Deep Water (CDW). To elucidate basic constraints on frontal properties, a minimal model of homogeneous water masses forced by offshore wind and freshwater input in a perpetual winter is considered. With the surface water stirred by—and hence aligned with—the ice cover, there is little leakage of ice or meltwater from the frontal system, so ice production and melt merely redistribute heat and salt between frontal waters. As such, the heat loss to the atmosphere needs to be supplied by entraining CDW, which then necessitates the shelf water discharge on account of the mass balance. Because of the freshwater input, the discharged shelf water may not be saltier than the CDW, which thus may descend the slope to form bottom water only because of its coldness. With both frontal waters cooled to the freezing point, dominant balances are formulated to determine their salinity and exchange rate with the ambient CDW. Although extremely crude, the model derivations are favorably compared with observations, which thus may provide physically based parameterizations for the bottom-water formation that can be incorporated into global models.

Frequency Dependence of Human Response to Wind-Induced Building Motion

There has long been debate about the necessity of considering motion frequency in the assessment of occupant comfort during wind-induced tall building motion. This paper characterizes the biodynamic human body vibration response occurring during exposures to low-frequency, constant amplitude acceleration sinusoidal motion and demonstrates a physiological frequency dependence of this motion. The findings from a series of motion simulator experiments show that, as the frequency of oscillation increases from 0.15 to 1.00 Hz, acceleration measured at the head of a human test subject is increasingly magnified. This motion magnification demonstrates that biodynamic human body vibration is frequency dependent in this frequency range. Furthermore there may also be further implications for visual perception of motion through parallax shift. Clearly, these effects need to be accounted for in developing more refined occupant comfort acceptable criteria for tall building motion.

Wind-Induced Motion Control of 76-Story Benchmark Building Using the Hybrid Damper-TLCD System

The tuned liquid column damper (TLCD) has recently been advanced for vibration control of structures subjected to wind loading. The effectiveness of the semiactive TLCD system and the hybrid viscous fluid damper-TLCD control system recently proposed by the writers is investigated for control of wind-induced motion of high-rise buildings. Simulations are performed on a 76-story building benchmark control problem subjected to wind tunnel test data and stochastic wind loads. It is shown that the semiactive TLCD control system performs comparably to a sample active tuned mass damper (ATMD) system and thus is an attractive alternative to the ATMD system. More significantly, it is shown that the hybrid viscous fluid damper-TLCD system reduces the response of the building substantially more than the semiactive TLCD system at every natural frequency of the building. Furthermore, the hybrid damper-TLCD system is robust in terms of the stiffness modeling error for control of both displacement and acceleration responses. The simulation results show that the proposed hybrid viscous fluid damper-TLCD system can perform effectively under various wind loading conditions.

Measurement of wind-induced motion of crop canopies from digital video images

We present a new measurement technique based on image correlation to capture the wind-induced motions of crop canopies. Tests were carried out on an alfalfa field submitted to natural wind. The motion is video-recorded from the side of the field and we take advantage of the natural periodicity of the crop to correct the distortion due to perspective. The computation of the canopy motions between successive images is based on Particle Image Velocimetry (PIV), and takes advantage of the natural small-scale heterogeneities of the canopy. It provides the two-dimensionnal spatio-temporal velocity field of the crop top surface with high spatial and temporal resolution. The Bi-Orthogonal Decomposition (BOD) of the velocity field then reveals large coherent propagating structures that scale with typical wavelength of wind fluctuation over canopies.

Effects of attack angle on performance of actively controlled high-rise building motion

Wind loading on high-rise buildings is complicated in nature, especially when across-wind motion caused by the vortex shedding effect is considered. Hence, wind tunnel tests serve as a reliable alternative for research on wind-induced vibration. In this paper, a four degree-of-freedom scaled (1:300) model of a high-rise building equipped with an active mass driver (AMD) system is constructed in a wind tunnel to experimentally verify effects of the wind attack angle on performance of actively controlled wind-induced motion. Using a system identification technique, aero-elasticity induced by the vortex shedding effect in the across-wind direction is appropriately modeled for control. Several controllers are determined based on the Linear Quadratic Gaussian (LQG) theorem by which a dynamic output feedback equation using acceleration feedback is formed for practical consideration. Two LQG controllers, termed as two-state gain-scheduling controllers, are designed based on nominal systems constructed from along-wind and across-wind motions, and implemented on the building model based on the attack angle. Experimental results show that performance of the controller is remarkable and robust in reducing the responses of high-rise buildings under different attack angles of wind. The best performance is achieved under smaller attack angles, but it is slightly degraded when vortex shedding is most significant, i.e., for the across-wind motion.

Sea-Ice Drift on the Northeastern Shelf of Sakhalin Island

Abstract Ice-drift velocity records from coastal radar stations, combined with data from moored current meters and coastal wind stations, are used to examine sea-ice motion off the northeastern coast of Sakhalin Island in the Sea of Okhotsk. Ice motion is shown to be governed primarily by diurnal tidal currents and wind-induced drift, which explain 92%–95% of the total ice-drift variance. Diurnal tidal motions predominate off the northern Sakhalin coast, accounting for 65%–80% of the variance, while low-frequency wind-induced motions prevail off the south-central coast, accounting for over 91% of the ice-drift variance. Maximum diurnal tidal ice-drift velocities range from 90–110 cm s−1 on the north coast to 10–15 cm s−1 on the south coast, in good agreement with the barotropic model of Kowalik and Polyakov. The presence of diurnal shelf waves accounts for the strong diurnal currents on the steeply sloping northern Sakhalin shelf, while the absence of such waves explains the weak diurnal currents on the more gently sloping south-central shelf. Using a vector regression model, the authors show that wind-induced ice-drift “response ellipses” (the current velocity response to a unity wind-velocity forcing) are consistent with a predominantly alongshore response to the wind, with wind-induced currents most pronounced off the south-central coast where water depths are relatively shallow. Time–frequency analysis of wind and ice-drift series reveals that, in winter, when sea ice is most extensive and internally cohesive, the ice response is almost entirely aligned with the alongshore component of the wind; in spring, when sea ice is broken and patchy, the ice responds to both the cross- and alongshore components of the wind.

Wind-induced motion of water in shallow-water closed basins

The motion of water induced by tangential wind stresses in a circular basin of finite depth is studied by the finite-element method. The dependences of vertical displacements of the surface of the basin and the field of horizontal wave currents on the topography of the bottom and the direction of the wind are analyzed. The transformations of wind-induced disturbances observed after termination of the wind action are investigated.

Vibration Control of the Wind-Excited 76-Story Benchmark Building by Liquid Column Vibration Absorbers

A study has been undertaken on the effectiveness of using liquid column vibration absorbers (LCVAs), in the suppression of wind-induced motion of the 76-story benchmark building. Much work has been undertaken on the behavior of the LCVA on an experimental level and its benefits have been demonstrated for wind-induced vibration control on a full-scale communications tower in Sydney, Australia, with a further installation being made on the “One Wall Center Building” Vancouver, Canada. The behavior of the LCVA has also been investigated numerically by use of computational fluid dynamics, and its potential has been illustrated in controlling a five-story building model. In this study, the LCVA adopted is composed of four identical columns of water. Initially, the performance of the LCVA is assessed without the inclusion of additional damping enhancing mechanisms. Subsequently, the same LCVA is considered with the inclusion of orifice plates, allowing a direct comparison of the two strategies. In order to address the issue of robustness, the sensitivity of the LCVA (with and without orifice) to mistuning is examined by perturbing the structural stiffness of the building by +15% and −15%, respectively. From this, an indication of the system performance under conditions of mistuning has been assessed. The performance of the adopted LCVA has also been compared to that of the sample tuned mass damper (TMD) control device. The overall response reductions of the LCVA are shown to be comparable to the TMD. Furthermore, it is shown that the LCVA has several inherent features that make it more attractive than the classical TMD.

Mitigation of Current-Driven, Vortex-Induced Vibrations of a Spar Platform via “SMART” Thrusters

The possibility of using active-control technology to mitigate long-period (100–300 s) motions of spar platforms was investigated. In particular, the technical feasibility and cost-effectiveness of using a thruster-based active-control system was examined. Only vortex-induced vibrations (VIV) of a moored spar were considered for this study, although it is believed that similar active-control systems can contend equally well with low-frequency wave-drift and wind-induced motions. VIV has been selected for this demonstration due to the considerable attention it has received for spars, because of the high cost for passive VIV-mitigation means. The investigation employed numerical simulations of the hydrodynamics around the spar in two and three dimensions. In particular, for this study, Navier-Stokes methods were used to compute the strongly nonlinear interactions between the current-flow field and the spar hull. The fluid forces (hull-integrated pressures) were then used to compute the unsteady motions of the moored spar. The effect of the motion-controlling thrusters was included as an additional external force—that also varies with time. Of the several different control strategies that were examined, it was found that the most effective one is “D-Control” wherein thruster forces are proportional to spar velocities. Using such control, spar motions can be kept below 35% of the spar diameter, whereas uncontrolled responses were computed to be as high as 80% of the spar diameter. Similar uncontrolled motions have actually been observed in current-tank model tests of spars. As part of this study, discussions were held with engineers from three different thruster companies to ascertain the number and type of thrusters that would be required to achieve desirable VIV mitigation—given the computed thruster-forces, and to provide prudent reliability. Associated costs were also very much of interest. For the spar system examined here, which is representative of those introduced into or being considered for the Gulf of Mexico, it was found that the cost of the thrusters and their electric motors is around $8 million—a cost that is significantly less than that estimated for passive VIV-control devices such as helical strakes, or discontinuous hull geometries. Furthermore, the proposed thruster system could also be employed to contend with (to mitigate) other undesirable, large-amplitude, near-resonant spar motions, e.g., low-frequency drift, for which there are no alternative remedies at present.

Numerical bridge deck studies using finite elements. Part I: flutter

Wind-induced bridge motions are numerically investigated using structural and fluid finite elements (FEs). The fluid–structure interaction solutions are based on the incompressible Navier–Stokes equations fully coupled to an elastically suspended rigid body. The unsteady fluid field equations with advection-dominated flow are approximated using the weighted residuals method of Galerkin/least squares. The bridge deck is idealized as a lumped mass, spring–dashpot system. The flow simulation around a stationary bridge deck adopts the Eulerian scheme in the entire fluid domain, whereas the simulations of flow past a moving bridge deck apply the arbitrary Lagrangian–Eulerian formulation. Predicting the flutter limit, aiming at reducing the number of physical model tests currently required, is the first step and main objective in the validation of a numerical method applied to long-span bridge decks. It is shown that the low-frequency flexural–torsional flutter motion is the result of a different aerodynamic effect than vortex-induced bridge vibrations. Despite a laminar flow assumption, the two-dimensional self-excited flow solutions, on an irregular unstructured grid of 1884 nodes, predict a flutter limit in good agreement with wind tunnel experiments. The FE model appears to be a suitable method and could be applied as a supplementary tool to wind tunnel testing for the simulation of flutter instability. It was concluded that the prediction of flutter instability for sharp edge bridge decks does not appear sensitive to turbulence and three-dimensional flow structure modelling.

Design of tall residential buildings in Singapore for wind effects

The design of high-rise building is often influenced by wind-induced motions such as accelerations and lateral deflections. Consequently, the building's structural stiffness and dynamic (vibration periods and damping) properties become important parameters in the determination of such motions. The approximate methods and empirical expressions used to quantify these parameters at the design phase tend to yield values significantly different from each other. In view of this, there is a need to examine how actual buildings in the field respond to dynamic wind loading in order to ascertain a more realistic model for the dynamic behavior of buildings. This paper describes the findings from full-scale measurements of the wind-induced response of typical high-rise buildings in Singapore, and recommends an empirical forecast model for periods of vibration of typical buildings in Singapore, an appropriate computer model for determining the periods of vibration, and appropriate expressions which relate the wind speed to accelerations in buildings based on wind tunnel force balance model test and field results.

Water wave attenuation due to opposing wind

A laboratory investigation of the attenuation of mechanically generated waves by an opposing wind has been completed. Wave attenuation was quantified by measurements of the decline in surface variance. These measurements show higher effective levels of monochromatic wave attenuation than predicted by air-side measurements: approximately an order of magnitude higher than measurements by Young & Sobey (1985) and, a factor of 3 higher than those of Donelan (1999) for waves in a JONSWAP spectrum. Furthermore, they show that theoretical estimates currently underestimate the attenuation rates by a factor of at least 3. This study has shown that the magnitude of wave attenuation rates due to opposing winds is approximately 2.5 times greater than the magnitude of wave growth rates for comparable wind forcing. At high wave steepnesses, detailed analysis suggests that air-side processes alone are not sufficient to induce the observed levels of attenuation. Rather, it appears that energy fluxes from the wave field due to the interaction between the wave-induced currents and other subsurface motions play a significant role once the mean wave steepness exceeds a critical value. A systematic relationship between the energy flux from the wave field and mean wave steepness was observed. The combination of opposing wind and wind-induced water-side motions is far more effective in attenuating waves than has previously been envisaged.