Advances in Wind Engineering

Held under the auspices of the International Association for Wind Engineering, 226 delegates from twenty-three countries took part in the conference. This three volume work contains about 90 papers published in full length, together with summaries and discussions on other interesting and valuable papers presented at the conference.

• Language: English
• Publisher: Elsevier Science
• Release date: Dec 2, 2012
• ISBN: 9780444599766

Book preview

Kramer

Part 1

Session 1

Wind characteristics and description

INTRODUCTION TO WIND CHARACTERISTICS AND DESCRIPTION

CHAIRMAN

J. Wieringa

Koninklijk Nederlands Meteorologisch Institut

De Bilt, Netherlands

CO–CHAIRMAN

H. Ruscheweyh

Institut für Stahlbau

Aachen Technical University

Aachen, Germany

GENERAL REPORTER

A.J. Bowen

Mechanical Engineering Department

University of Canterbury

Christchurch, New Zealand

REVIEW OF PAPERS FOR SESSION 1 : WIND CHARACTERISTICS AND DESCRIPTION

A.J. BOWEN,     Mechanical Engineering Department, University of Canterbury, Christchurch New Zealand

Publisher Summary

This chapter presents the results of a pilot study of wind velocity data gathered over 25 years at four stations in Southern Brazil. It discusses the development of data gathering and processing procedures to be used in the subsequent analysis of records from about 80 other stations throughout Brazil. The specific issues investigated are influence of storm type, influence of wind direction, and the feasibility of a scheme for regional predictions. Thunderstorms are identified by the short duration of the intense wind activity and from visual observations. The steady decrease in the annual maximum wind speeds measured throughout Japan at the weather stations may be attributed to the steady increase in ground roughness due to long-term building development rather than because of an overall decrease in the number of severe typhoons afflicting that country. Although the meteorological variation cannot be denied, the effect of ground roughness variation with time is of the same order of magnitude. The chapter describes the techniques for modelling the plume and measuring the temperature field.

It is my pleasure to introduce the first Conference Session on Wind Characteristics and Description and to present a brief review of seven submitted papers which are unable to be presented in full at this Conference.

While browsing over the proceedings of the previous four Conferences on Wind Engineering together with the present list of papers, it became apparent that the general goals of most engineering-related investigations into wind structure have not changed significantly over those sixteen years. However the growing demand for less conservative and therefore more economic structural designs has produced a strong need for improved accuracy in the prediction of wind loadings. There is therefore an urgent need for a significant improvement in the accuracy of the basic wind speed data and in the understanding of atmospheric turbulence which is available to the practising engineer. Progress in gathering more accurate models for the wind characteristics under any significant climatic event must keep pace with the outstanding improvements evident in our understanding of the distribution of pressure coefficients around a particular building shape and in the dynamic loading prediction techniques. The overall design is only as accurate as the accuracy of each of its parts, including the information used to define the design wind characteristics.

Developments in our understanding of wind characteristics for engineering purposes have been aptly categorised by Professor Hans Panofsky at the 1975 Wind Engineering Conference at Heathrow, London and these categories are still relevant today. The papers submitted to this Conference Session may also be grouped into the same categories for the purpose of this discussion.

LONG-PERIOD WIND STATISTICS

The formulation of more accurate methods of analysis and the accumulation of more accurate extreme wind speed statistics for the codified prediction of Basic Wind Speeds are still necessary. There is lately an increased interest in the extreme wind statistics of short term or local violent storms not normally identifiable in mean wind data. For example, in the occurrence of thunderstorms and their seasonal and directional dependence. Before processing, the wind speed data must be fully corrected for terrain and roughness effects and indeed, some of today’s papers address these aspects.

The first paper in this category by Riera and Nanni (1) reports the results of a pilot study of wind velocity data gathered over 25 years at four stations in Southern Brazil. The main objective was the development of data gathering and processing procedures to be used in the subsequent analysis of records from about 80 other stations throughout Brazil.

The specific issues investigated were;

a) influence of storm type (storms generated by mature pressure, systems as distinct from local intense thunderstorms),

b) influence of wind direction,

c) the feasibility of a scheme for regional predictions.

Thunderstorms were identified by the short duration of the intense wind activity and from visual observations. Winds over 13 knots were classified as an extreme wind event. The maximum 2-3 second average gust velocity was recorded for each event and corrected to standard site conditions.

The authors concluded that;

a) There was statistical evidence justifying the independent processing of major storm and local thunderstorm events.

b) For individual sectors of direction and storm type, the extreme type I (Gumbel) probability distribution presented the best fit.

c) For combined wind events, the data departed significantly from the type I distribution.

d) Regression techniques described in the paper appeared to adequately account for the influence of geographical location and wind orientation.

The second paper by Tamura and Suda (2) shows that the steady decrease in the annual maximum wind speeds measured throughout Japan at the weather stations may be attributed to the steady increase in ground roughness due to long term building development rather than due to an overall decrease in the number of severe typhoons afflicting that country. Data are presented showing that the yearly values of both the daily maximum and annual mean wind speeds have been dropping steadily since about 1960.

In contrast, the ground roughness around the weather stations is known to have increased steadily. However, as there were no available data indicating this trend in ground roughness in quantitative terms, an indirect method was developed based on building density figures. The total floor areas of all taxed buildings were obtained from the fixed property tax ledgers in the Public Offices of each administrative area and the building density estimated using an empirical relationship developed in an earlier reference. The effects of the more steady natural background roughness were also discussed.

Close correlation was found between the variations of the annual maximum wind speed and the building density averaged in Japan overall. Suitable corrections for the increase in ground roughness were applied to wind data for the whole country and for 12 chosen measurement sites. It was shown that the corrected wind speeds fitted the return period wind speed type-I distribution significantly better than the present uncorrected wind speeds in 8 cases. This left 4 sites which had significant terrain features with no improvement.

The authors conclude that although the meteorological variation cannot be denied, the effect of ground roughness variation with time is of the same order of magnitude.

STRUCTURE OF WINDS OVER SIMPLE TERRAIN

The development in understanding and the collection of more accurate wind turbulence data for quasi-static peak loads and dynamic wind load predictions under strong-wind neutrally-stable conditions are continuing. However, this is probably the only category where we are reasonably confident in the adequacy of the existing available models.

The first paper in this category by Flaga and Wrana (3) gives an assessment of the various empirical formulae describing the power spectral densities of the three turbulent velocity components that are currently encountered in the literature and in common use in engineering practice.

Their critical review resulted in the following conclusions;

a) The values of particular component spectra are dispersed, both in the range of frequencies significant to tall and slender structures (0.1 - 5 Hz) and at the low frequency range. These significant variations between model frequencies could cause errors of some hundreds of percent when estimating the dynamic response.

b) In contrast to some early spectral models and codes, the spectra must be a function of height, especially for the estimation of tall building response.

c) Vertical and lateral velocity fluctuations are often neglected in engineering practice as being insignificant, but for the higher frequencies, spectra concerning these two components are comparable in magnitude to the longitudinal velocity spectrum.

The second paper by Tieleman (4) is a review of the limited amount of data available in the literature that provide reliable information on the mean and turbulent flow in the surface layer over the ocean, with the wind engineer in mind. Mean wind profiles, velocity variances, turbulence intensities and integral length scales are discussed and compared.

Despite the difficulties associated with the acquisition of reliable wind measurements over the ocean, the results obtained from the analysis of wind data of many different sources show that the turbulence statistics in the marine surface layer is distinctly different from that over land surfaces.

a) Under strong wind conditions U > 10 m/s, the aerodynamic roughness of a fully developed sea is very small (0.1 < zo < 1 mm) despite the presence of large waves. Surface wind profiles are distorted and the velocity ratio u*/u as well as the roughness length are dependant on the magnitude of the mean wind.

b) Turbulence intensities increase with wind speed under strong wind conditions.

c) Turbulence ratios, σ/U*, do not vary with wind speed but are about 12% larger than those observed over land.

d) Along-wind integral scales are larger over the ocean than those overland with comparable aerodynamic roughness, while cross-wind scales seem to be smaller.

EFFECTS OF SPECIAL TERRAIN FEATURES

The study of wind flow over complex terrain is a rich area of research involving an impressive range of flow phenomena. This category has many applications in wind engineering such as building exposure, wind turbine siting, pollution dispersal, forestry and horticulture and also in transport.

There are now a reasonable amount of field data available for low isolated hills backed by wind tunnel measurements and some successful numerical predictions. For example, the recent international investigation of Askervein Hill led by the Canadian Atmospheric Environment Service (Toronto) has produced a vast amount of full scale and model data which is now available to the numerical modeller, the compilers of a building code and also the practising engineer.

Reasonably good prediction techniques are becoming available for the estimation of mean wind speeds over simple isolated terrain features in neutrally stable flow conditions. The paper in Session 2 later today on this topic is a good example. However, a good deal of effort is still needed in order that we may understand and predict the behaviour of the mean wind and turbulence in more complex and real situations. Most important are the effects of separation with its attendant problems of accurate wind tunnel modelling, the effects of scale involving large scale mountains and the effects of complexity with the infinite range of hill and valley systems that are encountered in real life. Non neutrally-stable conditions confuse the situation further and are, of course, responsible for a number of damaging wind conditions.

The effects of changes in surface roughness are a little better understood and already appear in some building codes but important secondary effects are still worthy of investigation.

The first paper in this category by Cermak and Edling (5) is concerned with a wind-tunnel model-study of the edge effects down-wind of a step change in surface roughness. The roughness was set on a flat surface and had a finite width across the flow. Applications were identified in city areas, forests and croplands and flows over patches of roughness on surfaces of aircraft, ships and in marine flows.

Hot wire anemometry in single yawed and crossed wire configurations was used to measure the three components of mean velocity and five components of the Reynolds stresses. Full results are available in a Colorado State University report by the same authors.

Several conclusions were drawn from the experimental results regarding the driving mechanisms of secondary flows and the three dimensionality imposed Reynolds stresses by the lateral surface-roughness discontinuity. The authors concluded that;

a) Three-dimensional effects are confined primarily to a neighbourhood along the down-wind roughness discontinuity edge with the flow over 85 percent of the roughened strip width being essentially two dimensional.

b) Within the narrow domain above the down-wind roughness discontinuity, Reynolds stresses are generated which are significantly larger than anywhere else over the roughened surface.

c) Flow perturbations developed along the roughness discontinuity diffuse into the flow in proportion to the square root of the distance downstream. This diffusion effect is much more noticeable over the smooth surface where turbulence is of lower intensity than it is over the rougher surface where turbulence levels are higher.

d) The lateral gradients in the edge region give rise to a cross-flow component onto the smoother surface which in turn leads to a mean flow downward in the boundary-layer over the roughened strip.

e) Cross-flow mean velocity components for the case studied were observed to have a maximum magnitude of about 1.5 percent of the free-stream velocity.

The second paper by Tetzlaff and Hoff (6) builds on the well established analytical Jackson and Hunt model for the prediction of wind flow over a low hill which is based on a two layer flow concept. The surface friction layer where shear stress remains predominant, is called the Inner-Layer and is driven by the pressure distribution of the Outer-Layer which is assumed to behave as potential flow. The authors build on a previous paper that developed an alternative derivation for the height of this Inner-Layer which results in significantly smaller values than those predicted by Jackson and Hunt.

However the use of this alternative Inner-Layer height value to predict velocity speed-up Δu/uRef values over the hill based on the Jackson and Hunt equation, consistently resulted in speed-up values which were too high. This prompted the authors to find an alternative expression for the calculation of velocity speed-up for use with their Inner-Layer height predictions. This method is shown to compare well with recent field test results taken during the Askervein Hill Project mentioned earlier.

WIND CHARACTERISTICS IN SPECIAL METEOROLOGICAL SITUATIONS LEADING TO VIOLENT WINDS

The violent winds that are not due to the neutrally stable, depression driven storms which are common to the middle latitudes, such as tropical cyclones, thunderstorms, tornadoes and down-slope mountain winds are still demanding a good deal of investigation. The increase in our understanding of these events must eventually be incorporated into the building codes of individual countries depending on the vagaries of their particular climate.

None of the papers I have fall into this category concerning special meteorological situations but there are some to be presented later in this Session.

EFFECTS OF BUILDINGS AND OTHER STRUCTURES ON LOCAL WIND CHARACTERISTICS

Every single case in this category will inevitably be unique and they will therefore provide an endless source of work and business for the practising wind engineer or consultant.

The only paper by Whitbread (7) identifies several wind-flow problems affecting helicopter operations on off-shore platforms in the North Sea. Recommendations by the United Kingdom Civil Aviation Authority, which have placed limits on the vertical wind speed (±0.9 m/s) during high winds (up to 25 m/s) and an ambient temperature rise (of 2° - 3°) due to gas turbine exhausts, have consequently created a demand for wind tunnel testing as the principal means of demonstrating compliance with the specified criteria.

Wind tunnel test techniques used at British Maritime Technology Ltd. and described in this paper are able to identify the presence of unacceptable vertical wind speeds over the deck together with areas of high turbulence due to separation of the wind flow from the windward edge of the helideck. Platform models to 1:100 scale are used in a modelled boundary-layer. Wind speed measurements near the helideck are made using a cross-wire anemometer set up to measure the vertical and longitudinal components of the mean wind speed and turbulence. Flow visualisation is accomplished using smoke, introduced into the flow just upstream of the model and the traces recorded on video. Worthwhile improvements to helideck designs have also been achieved using these methods.

A second wind-flow problem is that concerned with the risk of the gas turbine plume interfering with helicopter operations. Again, the techniques for modelling the plume and measuring the temperature field are described and tests are reported to have enabled detailed studies to be made at the design stage. Modelling of the gas turbine plumes requires the momentum and buoyancy of the full scale hot exhaust plume to be represented correctly in the model which is achieved using a helium-carbon dioxide gas mix.

That completes the set of 7 papers that I was asked to review. I hope that I have accomplished this task to the satisfaction of those authors who are here today.

REFERENCES

1. J. D. Riera and L. F. Nanni, Pilot Study of Extreme Wind Velocities in a Mixed Climate Considering Wind Orientation. Curso de Pós-Graduacão em Engenharia Civil, UFRGS, Av. Osvaldo Aranha, 99-90210 Porto Alegre, RS, Brazil.

2. Y. Tamura¹ and K. Suda², Correction of Annual Maximum Windspeeds Considering Yearly Variation of the Ground Roughness in Japan.¹ Tokyo Institute of Polytechnics, Atsugi, Kanagawa, Japan.² Sato Kogyo Co. Ltd., Atsugi, Kanagawa, Japan.

3. A. Flaga and B. Wrana, Analysis of Empiric Formulae of Power Spectral Densities of Three Wind Velocity Vector Components. Institute of Structure Mechanics, Technological University of Cracow, Poland.

4. H.W. Tieleman, A Survey of the Turbulence in the Marine Surface Layer. Engineering Science and Mechanics Department, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, U. S. A.

5. J. E. Cermak¹ and W. H. Edling², Three-Dimensional Turbulent Boundary-Layer Flow Along Roughness Strip.¹ Fluid Mechanics and Wind Engineering, Colorado State University, Fort Collins, Colorado, U. S. A.² Lorain County Community College, Lorain, Ohio, U. S. A.

6. G. Tetzlaff and A.M. Hoff, An Analytic Formalism to Calculate Vertical Wind Profiles in Hilly Terrain. Institut für Meteorologie und Klimatologie, Universität, Hannover, Herrenhäuser Str.2, D-3000 Hannover-21 F. R. G.

7. R.E. Whitbread, Wind-Flow Studies for Offshore Platforms, British Maritime Technology Ltd., U. K.

Extreme wind climate of the United Kingdom*

N J Cook¹ and M J Prior²,     ¹Building Research Establishment, Garston, Watford (United Kingdom); ²Meteorological Office, Bracknell (United Kingdom)

Publisher Summary

Design of safe and serviceable building structures requires assessment of the largest loads likely to be experienced in the expected lifetime of each structure. The first step in that assessment is the estimation of the extreme wind climate of the site from the available meteorological record. In temperate regions, analysis of the wind climate is conventionally performed in terms of the order statistics of annual maxima. This chapter discusses the Fisher–Tippett Type 1 (FT1) model for dynamic pressure used by several European countries. This model is more accurate than the FT1 model for wind speed used previously in the UK. It further discusses the implications for structural design practice and application of corrections to account for terrain roughness and topographic effects. For archiving purposes, the meteorological office has to use wind data as measured regardless of site exposure. There will, however, continue to be a need to express data in terms of standard conditions. A further requirement is to interpret the data for use in locations whose characteristics may be very different from those at the measurement site. Current methods used to compensate for anemometer exposure are largely subjective and, thus, often unsatisfactory.

INTRODUCTION

Design of safe and serviceable building structures requires assessment of the largest loads likely to be experienced in the expected lifetime of each structure. The first step in this assessment is the estimation of the extreme wind climate of the site from the available meteorological record. In temperate regions analysis of the wind climate is conventionally performed in terms of the order statistics of annual maxima, as reviewed by Mayne[1]. Such analyses for the United Kingdom have been repeated at intervals as the data record has increased in length and analysis methods have improved. The first comprehensive UK analysis was made in 1958 by Shellard[2], using the standard ‘Gumbel’ methodology[1]. This was repeated in 1962[3], 1965[4] and 1968[5], the last forming the basis of the ‘basic wind speed’ map in the first BRE wind loading Digest[6] and the 1970 revision of the UK Code of Practice[7]. New analyses in the early 1970s[8], again using the methods of Gumbel, resulted in further changes to the map. The refinement suggested by Lieblein[9] for the unbiased analysis of annual extremes was used by the Meteorological Office in the late 1970s, and data from about 125 meteorological stations have been re-analysed every 2–3 years since