Model Studies of Wind Effects

MODEL STUDIES OF WIND EFFECfS - A PERSPECfIYE ON THE PROBLEMS OF EXPERIMENTAL TECHNIQUE AND INSTRUMENTATION D. Surry and N. Isyumov Faculty of Engineering Science ll1e University of Western Ontario London, Ontario, Canada N6A 5B9' A!3STRACf mathcm·atidans. physiologists. and behavioural psychologists. 111is breadth makes it virtually impossible to cover all areas of Wind engineering, which comprises a large part of non wind engineering, let alone other non-aeronautical applkations. aeronautical aerodynamics, is rapidly emerging as a separate Many useful reviews of the subject area can be found in the and coherent discipline. A brief historicai review contrasts the Ii tera ture (4-8). more iIT.portant differences in the application of aerodynamics The perspective presented here attempts to be balanced; to wind engineering, as opposed to the aerospace field. The however, it is admittedly highlighted by the authors' experi strengths and weaknesses of the current methodology and ences with particular aspects of wind engineering - specifical technique of wind effects simulation are discussed, with em ly. the interaction between wind and structures, and the dis phasis on modelling of the natural wind, wind induced effects persion of plumes. Furthermore, in this paper, applications on buildings and structures, and diffusion problems. Instru are stressed, i.e. engineering tests, rather than more basic mentation in use is reviewed and areas requiring improved experiments whose techniques tend to merge with those of capabilities are indicated. other disciplines. WIND ENGINEERING - ITS EMERGENCE INTRODUCfION AS A COHERENT DISCIPLINE In the last Congress (!), a session was entitled" Broad rt is neither the aim nor a possibiJi ty to detail here the Initiatives"; however; only one paper (2) dealt exclusively history of wind engineering and the related disciplines from with the increasing applkation of aerospace facilities to prob which it emerged, although so'me fascinating historical treat lems in non-"eronautical aerodynamics. It is significant that ments are available (8-! 3). Rather, a brief summary of some at the Sixth international Congress on Instrumentation in of the high points drawn largely from these sources is pres Aerospace Simulation Facilities, a session has been dedicated ented below to add to the modern perspective. to non-aeronautical applications. ll1is increase in interest AJthough aeronautical aerodynamics became the more and activity reflects the emergence of an essentially separate sophisticated field early in the 20th century, wind engineer and coherent discipline which has only recently been chris ing can lay some claim to being the older subject. Obviously ened in North America as "Wind Engineering" and is defined wind has always influenced 'man, and his structures have by Cermak (3) as "the rational treatment of interactions shown a growing recognition of the problems induced by it - between wind and man and his engineered works on the sur first in terms of shelter and later in terms of the loads which face of the earth". This definition appears to be gaining wide structures must withstand. Naturally, simple empirical rules acceptance in North America. In contrast, the term "Indus formed the first body of knowledge and the beginnings of the trial Aerodynamics" has been widely accepted in Europe, with second; in fact, it has been suggested recently (14) that some the additional connotation of including aspects of aero structures such as Gothic cathedrals of the twelfth to four: dynamics associated with neither the wind, nor aerospace teenth centuries, embody designs which recognise and deliber applications, such as internal !lows in pipes, for example. ately account for wind loads. This paper shall essentially be iimited to the area of wind The 17th and 18th centuries heralded the beginning of engineering. the rational approach to wind loads - albiet not always cor Even within the more limited terms of wind engineering, rectly. Newton's three proportionalities between the force on the field is wide. To continue Cermak's definition: "Appli a body in a fluid and its area, the square of the speed, and cations Cof wind engineering) are not for the most part aero the l1uid density, were perfectly valid as far as th('y went. nautical in nature, but are related to wind effects on buildings, During the same era quantitative experiments to measure structures, and pedestrians; short range transport of air pol forces on bodies were also beginning by such people as lutants and local wir..d .nodifications by buildings, urban ge Mariotte, Cayley and Newton himself, using methods involving ometry and topography." The field is typified by the com devices not specifically designed for simulation (such as ob plexity of the problems and of their interdisciplinary nature. sening speeds of falling objects). The earliest specialized sim For instance, it currently involves architects, engineers of ulation method seems to have been the introduction of the many disciplines,meteorologists, fluid dynamicists, applied whirling ann by Rouse and Robinsaround1746. The first 76 - ICIASF 75 RECORD simulation in the sense of "the first avowed model experi ments" (9) is generally credited to the English engineer John Smeaton who; in 1759, reported on tests of windmill designs using the rotating arm apparatus of Fig. I. This initial test was of a structure now classified in wind engineeering but having a highly aeronautical flavour. Smeaton also introduced a formula proposed by Rouse (13) for calculating the pressure load in air (in psf), for which a structure should be designed, as being .005 Y' (Y in mph), where the prime uncertainty , then - and perhaps now - is the quantity which should be used for the velocity V. This formula corresponds to a value of Cd ~ 2. Fig. 3 Early Wind Tunnel Models Used by Irminger (after reference 12) Instrumentation consisted of a simple water manometer. From this point on, the development of the aeronautical siele of aerodynamics is well documented and rapidly led to sophis ticated facilities to simulate flight. As such. thl! design of wind tunnels concentrated on developing smooth uniform Fig. 1 Smeaton's Adaptation of the Rotating Arm for Wincl flow to represent the flight of aircraft through a still a tmo mill Tests (after reference 9) sphere. Meanwhile, the Tay Bridge collapse had initiated serious In the 18th and 19th centuries, apart from the rapid rise re-examinations of structural design for high wind loads and of the scientific method with its own motivations, the primary had prompted experiments to produce more rational formulas interests fueling the development of aerodynamics were the for load calculations, particularly for new bridges. It is desire to t1y and the design of wind sensitive structures. The worthwhile to mention that, at about the same time (in' 1889), latter consisted primarily of windmills; however, the advent of Eiffel built his tower using quite conservative but reasonable large bridges and some equally large failures - in particular the assumptions, including an allowance for wind increasing in Firth of Tay Bridge collapse in 1879 - gave wind engineering speed with height. He later tried to improve on his empirical a strong impetus at about the same time that the wind tunnel as assumptions by measurements of wind and deflection of the as a simulation facility came into being. The first wind tunnel tower, and by using the tower as a base for other aerodynamic is credited to Francis Herbert Wenham in the period 1867- measurements. His data have recently been re-examined and 1871 (10, 13). The steam injector driven wind tunnel repro discussed in detail by Davenport (11). duced in Fig. 2 was used by Phillips (9) shortly afterwards Of, particular interest are the experiments of Baker, also discussed by Davenport (11), who initially measured loads on boards of various sizes in the natural wind in the early 1880's (15) in connection with the design of the Firth of Forth Bridge. It is noteworthy that there was no lack of percep ..,- ~"A-!................. ....... .. ... ................. I" at .................................................... tion that the wind and its gustiness presented a loading prob ~( '.d __ I'i -li:= - lem not fully simulated in the wind tunnel. Baker expiained ~ ~ • 7 I k:!:;- _. b.-.£- ~ - I - @ the smaller loads on the bigger boards by the lack of simulta \. ........... _: ...................... z· .•.......... ,.... .:.. ......... _ •• _•. __ .••. ,., ••••• _-_._ ••••••• neity of the gusts. Stanton (16) compared these results to wind tunnel experiments around 1903 and continued full scale investigations in more detail. including some measure Fig. 2 Phillips Steam-injector Wind Tunnel (after ~eference 9) ment on the Tower Bridge around 1913, using instrumentation which is not only interesting in its own right, but is relevant during 1884 and 1885 to test wing sections, including effects to a later section of the paper. TIle apparatus. showninFig. 4, of camber. With the concurrent interest in aerodynamics of consisted of an ingenious mechanical system for averaging and both structures and aircraft, it is not surprising to find wind recording pressure loads across the bridge or from other tunnel testS of both areas being carried out by Irminger in separated locations. The wire over the pulleys was connected Denmark in 1893, using a small tunnel driven by the'suction via the rods to pressure sensitive diaphragms and also to a pen of a 100 ft. factory chimney (12). Figure 3 shows Irminger's recorder. Stanton correlated his time varying loads with winds early models of a wing and a house as reported by Jensen' (12). ICIASF '75 RECORD-77 but used depths con;iderably less than required (12). Never UNION FOR SUCTION PIPE theless, it was not until 1958 that the model law was formally sta ted by Jensen (20) as "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 scaie as regards the velocity profile".

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