Showing papers in "Wind and Structures in 2009"

The physical simulation of thunderstorm downbursts using an impinging jet

Abstract: This paper outlines the results of a physical simulation (at a 1:700 - 1:1000 geometric scale) of a thunderstorm downburst. Three different methods are examined in order to generate the time dependent nature of a downburst: directly controlling the fans and via two different types of opening apertures. Similarities are shown to exist between each method, although the results obtained from one approach are favoured since they appear to be independent of the downdraft velocity. Significant run-to-run variations between each experiment are discovered and in general it is found beneficial to interpret the results in terms of 10 run ensemble averages. An attempt to simulate a translating downburst is also undertaken and the results are shown to compare favourably with full-scale data.

Buffeting-induced stresses in a long suspension bridge: structural health monitoring oriented stress analysis

Abstract: Structural health monitoring (SHM) systems have been recently embraced in long span cable-supported bridges, in which buffeting-induced stress monitoring is one of the tasks to ensure the safety of the bridge under strong winds. In line with this task, this paper presents a SHM-oriented finite element model (FEM) for the Tsing Ma suspension bridge in Hong Kong so that stresses/strains in important bridge components can be directly computed and compared with measured ones. A numerical procedure for buffeting induced stress analysis of the bridge based on the established FEM is then presented. Significant improvements of the present procedure are that the effects of the spatial distribution of both buffeting forces and self-excited forces on the bridge deck structure are taken into account and the local structural behaviour linked to strain/stress, which is prone to cause local damage, are estimated directly. The field measurement data including wind, acceleration and stress recorded by the wind and structural health monitoring system (WASHMS) installed on the bridge during Typhoon York are analyzed and compared with the numerical results. The results show that the proposed procedure has advantages over the typical equivalent beam finite element models.

Development of devices and methods for simulation of hurricane winds in a full-scale testing facility

Abstract: The International Hurricane Research Center (IHRC) at Florida International University (FIU) is pursuing research to better understand hurricane-induced effects on residential buildings and other structures through full-scale aerodynamic and destructive testing. The full-scale 6-fan Wall of Wind (WoW) testing apparatus, measuring 4.9 m tall by 7.3 m wide, is capable of generating hurricane-force winds. To achieve windstorm simulation capabilities it is necessary to reproduce mean and turbulence characteristics of hurricane wind flows. Without devices and methods developed to achieve target wind flows, the full-scale WoW simulations were found to be unsatisfactory. To develop such devices and methods efficiently, a small-scale (1:8) model of the WoW was built, for which simulation devices were easier and faster to install and change, and running costs were greatly reduced. The application of such devices, and the use of quasiperiodic fluctuating waveforms to run the WoW fan engines, were found to greatly influence and improve the turbulence characteristics of the 1:8 scale WoW flow. Reasonable reproductions of wind flows with specified characteristics were then achieved by applying to the full-scale WoW the devices and methods found to be effective for the 1:8 scale WoW model.

Observed tropical cyclone wind flow characteristics

Abstract: Since 1998, several institutions have deployed mobile instrumented towers to collect research-grade meteorological data from landfalling tropical cyclones. This study examines the wind flow characteristics from seven landfalling tropical cyclones using data collected from eight individual mobile tower deployments which occurred from 1998-2005. Gust factor, turbulence intensity, and integral scale statistics are inspected relative to changing surface roughness, mean wind speed and storm-relative position. Radar data, acquired from the National Weather Service (NWS) Weather Surveillance Radar - 1988 Doppler (WSR-88D) network, are examined to explore potential relationships with respect to radar reflectivity and precipitation structure (convective versus stratiform). The results indicate tropical cyclone wind flow characteristics are strongly influenced by the surrounding surface roughness (i.e., exposure) at each observation site, but some secondary storm dependencies are also documented.

Field monitoring of boundary layer wind characteristics in urban area

Abstract: This paper presents statistical analysis results of wind speed and atmospheric turbulence data measured from more than 30 anemometers installed at 15 different height levels on 325 m high Beijing Meteorological Tower and is primarily intended to provide useful information on boundary layer wind characteristics for wind-resistant design of tall buildings and high-rise structures. Profiles of mean wind speed are presented based on the field measurements and are compared with empirical models\' predictions. Relevant parameters of atmospheric boundary layer at urban terrain are determined from the measured wind speed profiles. Furthermore, wind velocity data in longitudinal, lateral and vertical directions, which were recorded from an ultrasonic anemometer during windstorms, are analyzed and discussed. Atmospheric turbulence information such as turbulence intensity, gust factor, turbulence integral length scale and power spectral densities of the three-dimensional fluctuating wind velocity are presented and used to evaluate the adequacy of existing theoretical and empirical models. The objective of this study is to investigate the profiles of mean wind speed and atmospheric turbulence characteristics over a typical urban area.

Simulated tropical cyclonic winds for low cycle fatigue loading of steel roofing

Abstract: Low rise building roofs can be subjected to large fluctuating pressures during a tropical cyclone resulting in fatigue failure of cladding. Following the damage to housing in Tropical Cyclone Tracy in Darwin, Australia, the Darwin Area Building Manual (DABM) cyclic loading test criteria, that loaded the cladding for 10000 cycles oscillating from zero to a permissible stress design pressure, and the Experimental Building Station TR440 test of 10200 load cycles which increased in steps to the permissible stress design pressure, were developed for assessing building elements susceptible to low cycle fatigue failure. Recently the ‘Low-High-Low’ (L-H-L) cyclic test for metal roofing was introduced into the Building Code of Australia (2007). Following advances in wind tunnel data acquisition and full-scale wind loading simulators, this paper presents a comparison of wind-induced cladding damage, from a “design” cyclone proposed by Jancauskas, et al. (1994), with current test criteria developed by Mahendran (1995). Wind tunnel data were used to generate the external and net pressure time histories on the roof of a low-rise building during the passage of the “design” cyclone. The peak pressures generated at the windward roof corner for a tributary area representative of a cladding fastener are underestimated by the Australian/New Zealand Wind Actions Standard. The “design” cyclone, with increasing and decreasing wind speeds combined with changes in wind direction, generated increasing then decreasing pressures in a manner similar to that specified in the L-H-L test. However, the L-H-L test underestimated the magnitude and number of large load cycles, but overestimated the number of cycles in the mid ranges. Cladding elements subjected to the L-H-L test showed greater fatigue damage than when experiencing a five hour “design” cyclone containing higher peak pressures. It is evident that the increased fatigue damage was due to the L-H-L test having a large number of load cycles cycling from zero load (R=0) in contrast to that produced during the cyclone.

Aerodynamic stabilization of central stabilizers for box girder suspension bridges

Abstract: For long-span suspension bridges with their intrinsic limit in flutter, some counter measures, for example, central stabilizers, should be adopted to improve aerodynamic stability to meet with the appropriate wind resistance requirements. The present paper introduces aerodynamic stabilization for long-span suspension bridges with box girders by using central stabilizers based on Xihoumen Bridge with the main span of 1650 m. The aerodynamic stabilization study covers experimental investigation of sectional model testing, comprehensive evaluation of three central stabilizers and theoretical analysis of stabilizing mechanism related to flutter derivatives, aerodynamic damping and degree participation.

Experimental and numerical identification of flutter derivatives for nine bridge deck sections

Abstract: This paper presents the results of a study into experimental and numerical methods for the identification of bridge deck flutter derivatives. Nine bridge deck sections were investigated in a water tunnel in order to create an empirical reference set for numerical investigations. The same sections, plus a wide range of further sections, were studied numerically using a commercially available CFD code. The experimental and numerical results were compared with respect to accuracy, sensitivity, and practical suitability. Furthermore, the relevance of the effective angle of attack, the possible assessment of non-critical vibrations, and the formulation of lateral vibrations were studied. Selected results are presented in this paper. The full set of raw data is available online to provide researchers and engineers with a comprehensive benchmarking tool.

Wind pressure measurements on a cube subjected to pulsed impinging jet flow

Abstract: A pulsed impinging jet is used to simulate the gust front of a thunderstorm downburst. This work concentrates on investigating the peak transient loading conditions on a 30 mm cubic model submerged in the simulated downburst flow. The outflow induced pressures are recorded and compared to those from boundary layer and steady wall jet flow. Given that peak winds associated with downburst events are often located in the transient frontal region, the importance of using a non-stationary modelling technique for assessing peak downburst wind loads is highlighted with comparisons.

POD-based representation of the alongwind Equivalent Static Force for long-span bridges

Abstract: This paper develops and discusses a method by which it is possible to evaluate the Equivalent Static Force (ESF) of wind in the case of long-span bridges. Attention is focused on the alongwind direction. The study herein carried out deals with the classical problems of determining the maximum effects due to the alongwind action and the corresponding ESFs. The mean value of the maximum alongwind displacement of the deck is firstly obtained both by the spectral analysis and the Gust Response Factor (GRF) technique. Successively, in order to derive the other wind-induced effects acting on the deck, the Gust Effect Factor (GEF) technique is extended to long-span bridges. By adopting the GRF technique, it is possible to define the ESF that applied on the structure produces the maximum alongwind displacement. Nevertheless the application of the ESF so obtained does not furnish the correct maximum values of other wind-induced effects acting on the deck such as bending moments or shears. Based on this observation, a new technique is proposed which allows to define an ESF able to simultaneously reproduce the maximum alongwind effects of the bridge deck. The proposed technique is based on the GEF and the POD techniques and represents a valid instrument of research for the understanding of the wind excitation mechanism.

Aerodynamic Flutter Control for Typical Girder Sections of Long-Span Cable-Supported Bridges

Abstract: Aerodynamic flutter control for long-span cable-supported bridges was investigated based on three basic girder sections, i.e. streamlined box girder section, box girder section with cantilevered slabs and two-isolated-girder section. Totally four kinds of aerodynamic flutter control measures (adding fairings, central-slotting, adding central stabilizers and adjusting the position of inspection rail) were included in this research. Their flutter control effects on different basic girder sections were evaluated by sectional model or aeroelastic model wind tunnel tests. It is found that all basic girder sections can get aerodynamically more stabled with appropriate aerodynamic flutter control measures, while the control effects are influenced by the details of control measures and girder section configurations. The control effects of the combinations of these four kinds of aerodynamic flutter control measures, such as central-slotting plus central-stabilizer, were also investigated through sectional model wind tunnel tests, summarized and compared to the flutter control effect of single measure respectively.

Wind-tunnel blockage effect on drag coefficient of circular cylinders

Abstract: This paper explains how to correctly measure the drag coefficient of a circular cylinder in wind tunnels with large blockage ratios and for the sub-critical to the super-critical flow regimes. When dealing with large blockage ratios, the drag has to be corrected for wall constraints. Different formulations for correcting blockage effect are compared for each flow regime based on drag measurements of smooth circular cylinders performed in a wind tunnel for three different blockage ratios. None of the correction model known in the literature is valid for all the flow regimes. To optimize the correction and reduce the scatter of the results, different correction models should be combined depending on the flow regime. In the sub-critical regime, the best results are obtained using Allen and Vincenti\'s formula or Maskell\'s theory with ?=0.96. In the super-critical regime, one should prefer using Glauert\'s formula with G=0.6 or the model of Modi and El-Sherbiny. The change in the formulations appears at the flow transition with a variation of the wake pattern when passing from sub-critical to super-critical flow regimes. This parameter being not considered in the known blockage corrections, these theories are not valid for all the flow regimes.

Monitoring of tall slender structures by GPS measurements

Abstract: A method is applied for the estimation of structural damage of tall slender structures using natural frequency and displacements measurements by GPS. The relationship between the variation in the global stiffness matrix (or in the stiffness of each finite element) and the change in the natural frequencies of the structure is given. In engineering practice the number of frequencies which can be derived by GPS measurement of long-period structures will be equal to one, two or three first natural frequencies. This allows us in initial studies to detect damage with frequency changes based on forward methods in which the measured frequencies are compared with the predicted analytical data. This idea, of health monitoring from possible changes to natural frequencies, or from a statement of excessive displacements is applied to the Stuttgart TV Tower.

Some aspects of the dynamic cross-wind response of tall industrial chimney

Abstract: The paper is concerned with the numerical study of the cross-wind response of the 295 m-tall six-flue industrial chimney, located in the power station of Belchatow, Poland. The response of the chimney due to turbulent wind flow is caused by the lateral turbulence component and vortex excitation with taking into account motion-induced wind forces. The cross-wind response has been estimated by means of the random vibration approach. Three power spectral density functions suggested by Kaimal, Tieleman and Solari for the evaluation of the lateral turbulence component response are taken into account. The vortex excitation response has been calculated by means of the Vickery and Basu`s model including some complements. Motion-induced wind forces acting on a vibrating chimney have been modeled as a nonlinear aerodynamic damping force. The influence of three components mentioned above on the total cross-wind response of the chimney has been investigated. Moreover, the influence of damping ratios, evaluated by Multi-mode Random Decrement Technique, and number of mode shapes of the chimney have been examined. Computer programmes have been developed to obtain responses of the chimney. The numerical results and their comparison are presented.

Spanwise coherent structure of wind turbulence and induced pressure on rectangular cylinders

Abstract: Studying the spatial distribution in coherent fields such as turbulence and turbulence-induced force is important to model and evaluate turbulence-induced forces and response of structures in the turbulent flows. Turbulence field-based coherence function is commonly used for the spatial distribution characteristic of the turbulence-induced forces in the frequency domain so far. This paper will focus to study spectral coherent structure of the turbulence and induced forces in not only the frequency domain using conventional Fourier transform-based coherence, but also temporo-spectral coherence one in the time-frequency plane thanks to wavelet transform-based coherence for better understanding of the turbulence and force coherences and their spatial distributions. Effects of spanwise separations, bluff body flow, flow conditions and Karman vortex on coherent structures of the turbulence and induced pressure, comparison between turbulence and pressure coherences as well as intermittency of the coherent structure in the time-frequency plane will be investigated here. Some new findings are that not only the force coherence is higher than the turbulence coherence, the coherences of turbulence and forces depend on the spanwise separation as previous studies, but also the coherent structures of turbulence and forces relate to the ongoing turbulence flow and bluff body flow, moreover, intermittency in the time domain and low spectral band is considered as the nature of the coherent structure. Simultaneous measurements of the surface pressure and turbulence have been carried out on some typical rectangular cylinders with slenderness ratios B/D=1 (without and with splitter plate) and B/D=5 under the artificial turbulent flows in the wind tunnel.

Numerical and experimental investigation of control performance of active mass damper system to high-rise building in use

Abstract: This paper numerically and experimentally investigates the control performance of the active mass damper (AMD) systems in a 26-story high-rise building in use. This is the first full-scale application of the AMD system for suppressing the wind-induced vibration of a building structure in Korea. In addition, the AMD system was installed on top of the building already in use, which may be the world\'s first implementation case. In order to simultaneously mitigate the transverse-torsional coupled vibration of the building, two AMD systems were applied. Moreover, the H-infinity control algorithm has been developed to utilize the maximum capacity of the AMD system. From the results of numerical simulation using the wind load obtained from the wind tunnel tests, it was found that the maximum acceleration responses of the building were reduced significantly. Moreover, the control performance of the installed AMD system was examined by carrying out the free and forced vibration tests. The acceleration responses on top of the building in the controlled case measured under strong wind loads were compared with those in the uncontrolled case numerically simulated by using the wind load deduced from the measured data and a structural model of the building. It is demonstrated that the AMD system shows good control performance in reducing the building accelerations.

CFD based simulations of flutter characteristics of ideal thin plates with and without central slot

Abstract: In this paper, the airflow around an ideal thin plate (hereafter referred to as ITP) with various ratios of central slot is simulated by using the finite-difference-method (FDM)-based Arbitrary-Lagrangian-Eulerian descriptions for the rigid oscillating body. The numerical procedure employs the second-order projection scheme to decouple the governing equations, and the multigrid algorithm with three levels to improve the computational efficiency in evaluating of the pressure equation. The present CFD method is validated through comparing the computed flutter derivatives of the ITP without slot to Theodorsen analytical solutions. Then, the unsteady aerodynamics of the ITP with and without central slot is investigated. It is found that even a smaller ratio of central slot of the ITP has notable effects on pressure distributions of the downstream section, and the pressure distributions on the downstream section will further be significantly affected by the slot ratio and the reduced wind speeds. Continuous increase of with the increase of central slot may be the key feature of the slotted ITP. Finally, flutter analyses based on the flutter derivatives of the slotted ITP are performed, and moreover, flutter instabilities of a scaled sectional model of a twin-deck bridge with various ratios of deck slot are investigated. The results confirm that the central slot is effective to improve bridge flutter stabilities, and that the flutter critical wind speeds increase with the increase of slot ratio.

Analysis of light-frame, low-rise buildings under simulated lateral wind loads.

Abstract: The Monte Carlo procedure was used to simulate wind load effects on a light-frame low-rise structure of irregular shape and a main wind force resisting system. Two analytical models were studied: rigid-beam and rigid-plate models. The models assumed that roof diaphragms were rigid beam or rigid plate and shear walls controlled system behavior and failure. The parameters defining wall stiffness, including imperfections, were random and included wall stiffness, wall capacity and yield displacements. The effect of openings was included in the simulation via a set of discrete multipliers with uniform distribution. One and two-story buildings were analyzed and the models can be expanded into multiple-floor structures provided that the assumptions made in this paper are not violated.

The damping efficiency of vortex-induced vibration bytuned-mass damper of a tower-supported steel stack

Abstract: Many tower-supported steel stacks have been constructed in Japan, primarily for economic reasons. However the dynamic behavior of these stacks under a strong wind is not well known and the wind load design standard for this type of a stack has not yet been formulated. In light of this situation, we carried out wind response observation of an operating tower-supported steel stack with and without a tuned-mass damper. The observation revealed the performance of the tuned-mass damper installed on the stack in order to control the wind-induced vibration. Based on the observed data, we performed a wind tunnel test of a specimen of the stack. In this paper we report the results of the wind tunnel test and some comparisons with the results of observation. Our findings are as follows: 1) the tuned-mass damper installed on the specimen in the wind tunnel test worked as well as the one on the observed stack, 2) the amplitude of the vortex-induced vibration of the specimen corresponded approximately to that of the observed stack, and 3) correlation between Scruton number and reduced amplitude, y/d, (y is amplitude, d is diameter) was confirmed by both the wind tunnel test and the observed results.

Wind flow around rectangular obstacles with aspect ratio

Abstract: It has long been studied about the flow around bluff bodies, but the effect of aspect ratio on the sharp-edged bodies in thick turbulent boundary layers is still argued. The author investigates the flow characteristics around a series of rectangular bodies (40d x 80w x 80h, 80d x 80w x80h and 160dx80wx 80h in mm) placed in a deep turbulent boundary layer. The study is aiming to identify the extant Reynolds number independence of the rectangular bodies and furthermore understand the surface pressure distribution around the bodies such as the suction pressure in the leading edge, when the shape of bodies is changed, responsible for producing extreme suction pressures around the bluff bodies. The experiments are carried out at three different Reynolds numbers, based on the velocity U at the body height h, of 24,000, 46,000 and 67,000, and large enough that the mean boundary layer flow is effectively Reynolds number independent. The experiment includes wind tunnel work with the velocity and surface pressure measurements. The results show that the generation of the deep turbulent boundary layer in the wind tunnel and the surface pressure around the bodies were all independent of Reynolds number and the longitudinal length, but highly dependent of the transverse width.

Fundamental restrictions for the closed-loop control of wind-loaded, slender bridges

Abstract: Techniques for stabilising slender bridges under wind loads are presented in this article. A mathematically consistent description of the acting aerodynamic forces is essential when investigating these ideas. Against this background, motion-induced aerodynamic forces are characterised using a linear time-invariant transfer element in terms of rational functions. With the help of these functions, the aeroelastic system can be described in the form of a linear, time-invariant state-space model. It is shown that the divergence wind speed constitutes an upper bound for the application of the selected mechanical actuators. Even active control with full state feedback cannot overcome this limitation. The results are derived and explained with methods of control theory.

Effects of frequency ratio on bridge aerodynamics determined by free-decay sectional model tests

Abstract: A series of wind tunnel free-decay sectional model dynamic tests were conducted to examine the effects of torsional-to-vertical natural frequency ratio of 2DOF bridge dynamic systems on the aerodynamic and dynamic properties of bridge decks. The natural frequency ratios tested were around 2.2:1 and 1.2:1 respectively, with the fundamental vertical natural frequency of the system held constant for all the tests. Three 2.9 m long twin-deck bridge sectional models, with a zero, 16% (intermediate gap) and 35% (large gap) gap-to-width ratio, respectively, were tested to determine whether the effects of frequency ratio are dependent on bridge deck cross-section shapes. The results of wind tunnel tests suggest that for the model with a zero gap-width, a model to approximate a thin flat plate, the flutter derivatives, and consequently the aerodynamic forces, are relatively independent of the torsional-to-vertical frequency ratio for a relatively large range of reduced wind velocities, while for the models with an intermediate gap-width (around 16%) and a large gap-width (around 35%), some of the flutter derivatives, and therefore the aerodynamic forces, are evidently dependent on the frequency ratio for most of the tested reduced velocities. A comparison of the modal damping ratios also suggests that the torsional damping ratio is much more sensitive to the frequency ratio, especially for the two models with nonzero gap (16% and 35% gap-width). The test results clearly show that the effects of the frequency ratio on the flutter derivatives and the aerodynamic forces were dependent on the aerodynamic cross-section shape of the bridge deck.

Finite element analysis of 2D turbulent flows using the logarithmic form of the κ-ε model

Abstract: The logarithmic form for turbulent flow analysis guarantees the positivity of the turbulence variables as k and ? of the k-? model by using the natural logarithm of these variables. In the present study, the logarithmic form is incorporated into the finite element solution procedure for the unsteady turbulent flow analysis. A backward facing step flow using the standard k-? model and a flow around a 2D square cylinder using the modified k-? model (the Kato-Launder model) are simulated. These results show that the logarithmic form effectively keeps adequate balance of turbulence variables and makes the analysis stable during transient or unsteady processes.

Response of double hinged articulated tower platforms to wind forces

Abstract: Articulated tower platforms due to its compliant nature are more susceptible to the dynamic effects of wind than conventional fixed platforms. Dynamic response analysis of a double hinged articulated tower excited by low frequency wind forces with random waves is presented in this paper. The exposed super structure of the platform, housing the drilling and production facilities is subjected to mean and fluctuating wind loads, while the submerged portion is acted upon by wind driven waves. The fluctuating component of the wind velocity is modeled by Emil Simiu`s spectrum, while the sea state is characterized by Pierson-Moskowitz spectrum. Nonlinearities in the system due to drag force, added mass, variable submergence and instantaneous tower orientation are considered in the analysis. To account for these nonlinearities, an implicit time integration scheme (Newmark`s-) has been employed which solves the equation of motion in an iterative fashion and response time histories are obtained. The power spectra obtained from random response time histories show the significance of low frequency responses.

Influence of tilt and surface roughness on the outflow wind field of an impinging jet

Abstract: A physical and numerical steady flow impinging jet has been used to simulate the bulk characteristics of a downburst-like wind field. The influence of downdraft tilt and surface roughness on the ensuing wall jet flow has been investigated. It was found that a simulated downdraft impinging the surface at a non-normal angle has the potential for causing larger structural loads than the normal impingement case. It was also found that for the current impinging jet simulations, surface roughness played a minor role in determining the storm maximum wind structure, but this influence increased as the wall jet diverged. However, through comparison with previous research it was found that the influence of surface roughness is Reynolds number dependent and therefore may differ from that reported herein for full-scale downburst cases. Using the current experimental results an empirical model has been developed for laboratory-scale impinging jet velocity structure that includes the influence of both jet tilt and surface roughness.

Determination of Urban Surface Aerodynamic Characteristics Using Marquardt Method

Abstract: Ning Zhang; School of Atmospheric Sciences, Nanjing University, Nanjing 210093, China, State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China Weimei Jiang; School of Atmospheric Sciences, Nanjing University, Nanjing 210093, China Zhiqiu Gao; State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China Fei Hu; State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China Zhen Peng; School of Atmospheric Sciences, Nanjing University, Nanjing 210093, China

Tunable compression of wind tunnel data

Abstract: Synchronous wind-induced pressures, measured in wind-tunnel tests on model buildings instrumented with hundreds of pressure taps, are an invaluable resource for designing safe buildings efficiently. They enable a much more detailed, accurate representation of the forces and moments that drive engineering design than conventional tables and graphs do. However, the very large volumes of data that such tests typically generate pose a challenge to their widespread use in practice. This paper explains how a wavelet representation for the time series of pressure measurements acquired at each tap can be used to compress the data drastically while preserving those features that are most influential for design, and also how it enables incremental data transmission, adaptable to the accuracy needs of each particular application. The loss incurred in such compression is tunable and known. Compression rates as high as 90% induce distortions that are statistically indistinguishable from the intrinsic variability of wind-tunnel testing, which we gauge based on an unusually large collection of replicated tests done under the same wind-tunnel conditions.

Large-eddy simulation and wind tunnel study of flow over an up-hill slope in a complex terrain

Abstract: This study examines the accuracy of large-eddy simulation (LES) to simulate the flow around a large irregular sloping complex terrain. Typically, real built up environments are surrounded by complex terrain geometries with many features. The complex terrain surrounding The Hong Kong University of Science and Technology campus was modelled and the flow over an uphill slope was simulated. The simulated results, including mean velocity profiles and turbulence intensities, were compared with the flow characteristics measured in a wind tunnel model test. Given the size of the domain and the corresponding constraints on the resolution of the simulation, the mean velocity components within the boundary layer flow, especially in the stream-wise direction were found to be reasonably well replicated by the LES. The turbulence intensity values were found to differ from the wind tunnel results in the building recirculation zones, mostly due to the constraints placed on spatial and temporal resolutions. Based on the validated mean velocity profile results, the flow-structure interactions around these buildings and the surrounding terrain were examined.

Reliability based analysis of torsional divergence of long span suspension bridges.

Abstract: A systematic reliability evaluation approach for torsional divergence analysis of long span suspension bridges is proposed, consisting of the first order reliability method and a simplified torsional divergence analysis method. The proposed method was implemented in the deterministic torsional divergence analysis program SIMTDB through a new strategy involving interfacing the proposed method with SIMTDB via a freely available MATLAB software tool (FERUM). A numerical example involving a detailed computational model of a long span suspension bridge with a main span of 888 m is presented to demonstrate the applicability and merits of the proposed method and the associated software strategy. Finally, the most influential random variables on the reliability of long span suspension bridges against torsional divergence failure are identified by a sensitivity analysis.

Effect of blockage on the drag of a triangular cylinder

Abstract: A method is presented to estimate the form drag and the base pressure on a triangular cylinder in the presence of blockage effect. The Strouhal number, which is found to increase with the flow constriction experimentally by Ramamurthy & Ng (1973), may be decoupled from the blockage effect when re-defined by using the velocity at flow separation and a theoretical wake width. By incorporating this wake width into the momentum equation by Maskell (1963) for the confined flow, a relationship between the form drag and the base pressure is derived. Independently, the experimental data of surface pressure from Ramamurthy & Lee (1973) are found to be independent of the blockage effect when expressed in terms of a modified pressure coefficient involving the pressure at separation. Using the potential flow model by Parkinson & Jandali (1970) and its subsequent development in Yeung & Parkinson (2000) for the unconfined flow, a linear relation between the pressure at separation and the form drag is formulated. By solving the two equations simultaneously with a specified blockage ratio and an apex angle of the triangular cylinder, the predictions of the drag and the base pressure are in reasonable agreement with experimental data. A new theoretical relationship for the Strouhal number, pressure drag coefficient and base pressure proposed in this study allows the confinement effect to be appropriately taken into consideration. The present approach may be extended to three-dimensional bluff bodies.