Down­draughts and ed­dies in city streets

The Borneo Post - - NATURE - By Alan Rogers colum­nists@the­bor­neo­

THROUGH­OUT the world, cities have been grow­ing at an alarm­ing rate.

As they grow, they have caused changes in cli­matic con­di­tions. Nearly ev­ery city has sky­scrapers, with many a city claim­ing to have con­structed the tallest build­ing in their nation or even the world.

This up­ward growth in build­ing-height is to max­imise floor space in cen­tral city ar­eas, where high land-rent val­ues ex­ist.

Grad­u­ally, the pro­lif­er­a­tion of high rise build­ings has pre­sented ar­chi­tects, engi­neers, plan­ners and cli­ma­tol­o­gists with ev­er­more com­plex prob­lems.

The in­ter­ac­tion be­tween tall build­ings and the sur­round­ing at­mos­phere has been viewed in terms of wind load­ing and thus engi­neers have thought long and hard to find so­lu­tions to the enor­mous wind pres­sures ex­erted on build­ings at high lev­els.

As wind speeds in­crease with al­ti­tude, tall build­ings en­counter in­creas­ingly strong winds up to height of be­tween 500 and 1,000 me­tres.

At­ten­tion has be­come in­creas­ingly fo­cussed on the com­fort and safety of pedes­tri­ans in prox­im­ity to tower and slab blocks.

Clearly cities built in dif­fer­ent cli­matic zones in the world re­quire dif­fer­ent ap­proaches. In mid­dle and high lat­i­tudes, shel­ter from the wind is im­por­tant whereas in the sub­trop­ics and trop­ics, max­i­mum ven­ti­la­tion is a pre­req­ui­site.

Our phys­i­o­log­i­cal re­sponse to the at­mos­phere is com­pli­cated by the types of cloth­ing we wear, the na­ture of our ac­tiv­ity and our re­spec­tive age.

The most re­cent re­search into the mi­cro­cli­mate of city cen­tres has fo­cussed on the wind en­vi­ron­ment for pedes­tri­ans, in or­der to make it com­fort­able for them. Beau­fort wind scale

We can, with­out know­ing it, de­ter­mine the force of a wind on the Beau­fort scale. This is an in­ter­na­tion­ally ac­cepted wind scale, in­vented by the Bri­tish Ad­mi­ral Beau­fort in 1805. Pri­mar­ily de­signed for mariners, it is of­ten used on ship­ping weather fore­casts even to­day.

Above Force 3 (five me­tres per se­cond wind speed), peo­ple feel un­com­fort­able for many ac­tiv­i­ties. We are all fa­mil­iar with the dis­com­fort of ex­ces­sively gusty and strong winds around the base of tall build­ings. One needs to per­se­vere in try­ing to open an um­brella in the city streets on windy days un­less one is aware of the build­ing’s aero­dy­nam­ics.

Windy con­di­tions can present real haz­ards to pedes­tri­ans. The pres­sure ex­erted by the wind in­creases with the square of the wind speed and a dou­bling of the wind speed around the corner of a build­ing causes a four­fold in­crease in pres­sure upon pedes­tri­ans.

El­derly peo­ple, the in­firm, and young chil­dren find it hard to with­stand sud­den changes in wind di­rec­tion and the re­sul­tant in­creases of pres­sure on their bod­ies and may be blown over.

Lo­cal shop­keep­ers whose busi­nesses have been lo­cated in low level shop lots for many a year may be af­fected by high rise de­vel­op­ments near them, usu­ally re­lated to in­ner city re­gen­er­a­tion projects, in the form of apart­ment or of­fice tower blocks. The swirling winds may drive cus­tomers away.

Are such mi­cro­cli­mates the un­avoid­able prod­uct of ur­ban­i­sa­tion?

In a city’s sub­urbs or in small towns gen­er­ally build­ings are al­most uni­form in height, thus keep­ing the street-level en­vi­ron­ment pedes­trian safe – apart from road traf­fic.

As soon as build­ings rise up to 25 to 30 me­tres, the wind speeds in­crease.

Faster wind speeds are partly de­flected around the top of a high rise build­ing, and the rest be­ing de­flected down­wards as down­draughts along the face of the build­ing, caus­ing ed­dies to de­velop at pave­ment level. This can be seen by the swirls of lit­ter in streets.

With an open­ing or pas­sage­way un­der a tall build­ing, winds are sucked through as ‘through­flow’. Ac­tu­ally, plant life is also af­fected as shrubs and trees in or­na­men­tal flower beds bend much the same as they do on wind ex­posed coastal ar­eas or on moun­tains.

Even in Sin­ga­pore, I have no­ticed such par­tially mis­shapen plants. Sadly, de­vel­op­ers’ pri­or­i­ties over­look the man in the street, for who cares about ur­ban ‘whirl­winds’? Many ur­ban de­vel­op­ments hap­pen with­out any con­sul­ta­tion with me­te­o­rol­o­gists and aero­dy­namic spe­cial­ists, re­ly­ing solely on the de­vel­oper’s purse and net re­turn and an ar­chi­tect’s imag­i­na­tive de­sign.

Look­ing at high rise apart­ment build­ings around the world, I fre­quently won­der why ‘in­fin­ity’ swim­ming pools are lo­cated on the roof of such tow­er­ing blocks and why out­door bas­ket­ball and ten­nis courts are lo­cated at the foot of th­ese build­ings for in both ar­eas wind speeds are strong­est un­less sports fa­cil­i­ties are lo­cated on the lee­side.

Although wind tun­nel ex­per­i­ments for fu­ture tall build­ing de­sign are ex­pen­sive to run, they do save de­vel­op­ers much money in cor­rect­ing their de­signs and avoid­ing ex­pen­sive al­ter­ations in hind­sight after a build­ing has been con­structed.

Re­in­forced ver­ti­cal glass wind­break screens and over­head canopies are costly ad­di­tions and work well in de­flect­ing the de­scend­ing wind shears from tall build­ings.

Equally, a tall square build­ing should never face the pre­vail­ing wind for fear of winds be­ing de­flected around its right an­gled edges, thus af­fect­ing pedes­tri­ans. Cer­tainly, this as­pect of ur­ban mi­cro­cli­ma­tol­ogy is worth ex­plor­ing to avoid wind ed­dies, with dust and lit­ter swirling around our streets but, more­over to keep vul­ner­a­ble cit­i­zens safe.

Whilst vis­it­ing, on windy days, cen­tral city ar­eas in Lon­don, Sin­ga­pore, and Kuala Lumpur, I have felt the tall build­ings’ down­draughts and seen ed­dies of dust.

For­tu­nately, in Kuching, Miri and Kota Kin­a­balu, be­cause of the rel­a­tive spac­ing of high rise build­ings, such whirl­winds do not oc­cur.

May ur­ban plan­ners, ar­chi­tects, and lo­cal coun­cils not be tempted to build high in the sky or such build­ings close to each other. Please, do bear in mind the knowl­edge of lo­cal mi­cro­cli­matic and aero­dy­namic ex­perts, for the ben­e­fit of daily pedes­tri­ans and nearby shop­keep­ers, restau­ra­teurs and our cof­fee shop own­ers.

The forces of na­ture can be over­come and in the very tech­no­log­i­cal 21st cen­tury we should adapt th­ese tech­no­log­i­cal achieve­ments and ad­vances to the ben­e­fit of mankind, now, and for the fu­ture. We need to think be­yond the present.

Photo shows tower blocks in Sin­ga­pore.

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