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Wind Engineering Terminology 1 Wind Engineering Terminology Nicholas Isyumov1 1Professor Emeritus, The Boundary Layer Wind Tunnel Laboratory, University of Western Ontario, London Ontario, Canada email: [email protected] ABSTRACT: This paper provides an overview of the technical terminology of wind engineering. It is intended to promote its use and to encourage its expansion. The non-dimensional numbers, concepts and ideas which are presented in this paper reflect my interests and should not be regarded as all inclusive. There are other terms which have been given only fleeting attention and some that have not been mentioned at all. Section 2, which presents the terminology used in wind tunnel model studies is the kernel of the paper. It includes such important terms as the Jensen (Je), Reynolds (Re), Scruton (Sc), Strouhal (St), Froude (Fr) and Cauchy (Ca) numbers and discusses their roles in the scaling of wind tunnel experiments and in the interpretation of their results. Also discussed are aeroelastic model studies. Section 3 discusses new additions to our terminology, including the Davenport Chain and the Tachikawa number (Ta). Wind speed scales used to describe and categorize various types of wind storms and their effects on the natural and built environments are discussed in Section 4. Current design procedures and methods of wind tunnel model testing are based on the concept that natural wind can be described as a locally stationary, turbulent boundary layer flow. This model of natural wind has served us well in describing winds in tropical and extra-tropical cyclones. However, the concept of steady “straight line” winds does not capture the effects of severe transient wind events such as tornadoes, thunderstorms and downbursts. Existing analytical and design methods will require extension and modification in order to deal with these phenomena and new types of testing facilities will have to be developed for their study. This will requires new ideas and facilities and will lead to a growth of our technical terminology. KEY WORDS: Overview; Non-dimensional Numbers and Concepts; Similitude; Wind Tunnel Testing; Scales of Wind; Wind Loads and Responses. 1 INTRODUCTION The objectives of this paper are to identify and explain some of the terminology which is used in wind engineering research and practice. This includes non-dimensional numbers, scale and concepts, which have been named in honour of particular individuals who were instrumental in their development. It is important to mark these individual contributions for posterity and to be aware of their significance and applicability. This may avoid the “re-invention” of the wheel. Wind engineering has become a distinct discipline and its identity and visibility is enhanced by the use of specific technical terminology. In our generation we have seen the emergence of the Scruton (Sc), Jensen (Je) and Tachikawa (Ta) numbers. These are non- dimensional groupings of quantities which govern the similitude of model and prototype processes and influence performance under wind action. The Saffir-Simpson and the Fujita scales have been adopted to categorize the severity of wind storms and the Alan G. Davenport Wind Loading Chain has been introduced in order to summarize the concepts which determine wind induced loads on buildings and structures. Alan Davenport’s chain was formally recognized at ICWE 13 and is the latest addition to our terminology. Wind engineering bridges many disciplines, including structural engineering, fluid mechanics, aerodynamics, meteorology and probability theory. It is natural therefore that we have adopted relevant parts of terminologies formulated by other disciplines. Such terms as the Richardson (Ri), Rossby (Ro), Reynolds (Re), Strouhal (St), Froude (Fr), Keulegan-Carpenter (K- C) and Cauchy (Ca) numbers; the Prandtl mixing length; the Coriolis parameter; the Monin-Obukhov stability parameter; the Kolmogorov inertial sub-range of turbulence; the Beaufort and Fujita scales; and others have become part of our terminology. These terms allow us to communicate facts and ideas in a convenient and unambiguous manner. In a functional sense they provide a form of “shorthand”, much like that used by secretaries to record dictation or the proceedings of meetings. More importantly, familiarity with wind engineering terminology promotes awareness and appreciation of relevant physical laws, their applicability and the limitations which they impose. This paper provides a historical perspective of some of the wind engineering terminology and concepts and presents examples of their applicability and importance. It is not possible to cover all terms and concepts used in our discipline and emphasis is placed on terminology and concepts, which I have used and found valuable in my experience. Some are only briefly mentioned 14th International Conference on Wind Engineering – Porto Alegre, Brazil – June 21-26, 2015 2 others are discussed in detail. The included material reflects my personal experience and that of the Boundary Layer Wind Tunnel Laboratory (BLWTL) of which I have been a member since its inception 50 years ago. Emphasis throughout the paper is placed on terminology which relates to the requirements for achieving similarity of prototype wind loading data and those obtained from small scale wind tunnel model tests. Some similarity requirements are discussed in detail. Also discussed are selected non-dimensional numbers and concepts which influence the effects of wind on prototype buildings and structures and their components. The underlying objective throughout the paper is to provide a historic perspective and to encourage engineers and researchers to become familiar with the existing terminology and interested to further its development. 2 TERMINOLOGY IN WIND TUNNEL MODEL STUDIES 2.1 General Glossaries of commonly used wind engineering terminology can be found in some wind engineering texts [1] and publications [2]. Typically mentioned are various non-dimensional quantities which describe the nature of wind and its action on buildings and structures. Emphasis in this paper is on terminology used in wind tunnel model studies. The paper highlights recent additions with emphasis on the Jensen and Scruton numbers and the Alan G. Davenport Wind Loading Chain. I had the privilege of meeting both Kit Scruton and Martin Jensen and receiving first hand insights into their research and points of view. 2.2 Modeling natural wind 2.2.1 The Jensen number (Je) - Its role in geometric scaling In September 1965 I joined Alan Davenport at the Boundary Layer Wind Tunnel Laboratory (BLWTL) of the University of Western Ontario in the completion of its wind tunnel; its commissioning; and the start of experiments for commercially sponsored projects. Involvement with real engineering projects was judged essential in order to become a recognized wind tunnel testing facility. Also, commercially funded projects were needed in order to pay the bills. We interacted with Dr. Martin Jensen of the Danish Technical University in order to gain confidence in our modeling of natural wind. The success of model studies of the effects of wind on buildings and structures must be attributed to the pioneering work of Dr. Martin Jensen. His published model law [3] clearly states that “The correct model test for phenomena in the wind must be carried out in a turbulent boundary layer and the model law requires that this boundary layer be to a scale as regards the velocity profile”. Jensen’s research indicated that correct modeling is achieved by maintaining h/z0 constant in model and full scale. Here h is a characteristic dimension of a building or a structure and z0 is the roughness length of the flow. Jensen’s pioneering work was recognized by the wind engineering community [4] and the choice of the length scale of model tests in wind engineering wind tunnels has since been based on the observance of the Jensen number (Je) stated in Equation (1) or its equivalent. Je = h / z0 (1) The addition of Je to the wind engineering terminology occurred prior to ICWE 8 and a tribute to Dr. Martin Jensen appeared in its proceedings [5]. Jensen’s studies were in a turbulent boundary layer, developed naturally over a long fetch of homogeneous floor roughness. The importance of this ratio is illustrated in Figure 1. These comparisons clearly demonstrate the importance of carrying out wind tunnel model tests of buildings and structures at a geometric scale which is consistent with the length scale of the modeled natural wind. In naturally developed boundary layer flows, the modelling of zo assures a consistent simulation of other lengths of the flow, including the depth of the boundary layer and the lengths scales of the components of turbulence or the size of the gusts or “eddies”. Few existing wind engineering wind tunnels have test sections which are long enough to base the selection of the geometric scale on the matching of Je alone. Even in long test section wind tunnels, trips, spires and other flow augmentation devices are used to better match the expected full scale profiles of the mean wind speed and the salient characteristics of atmospheric turbulence [6]. Total reliance on augmentation devices becomes necessary in the shaping of wind flows in short test section wind tunnels [7]. The matching of the model and prototype winds is never exact and usually it becomes necessary to concentrate on what are judged to be the more important properties and to accept mismatches for the less important ones. Limits on mismatching atmospheric turbulence in wind tunnel model tests can be found in manuals of practice [1] and in standards.[8]. A valuable discussion of the consequences of approximations of the flow on various wind effects can be found in Reference [9]. In addition to the simulation of the “approach” wind it is also necessary to include the aerodynamic influence of immediately surrounding buildings, structures and topographic features. The simulation of this “near field” flow in complex surroundings is typically achieved by testing with a geometrically reproduced “proximity” model in place.
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