Unsteady flow effects due to fluctuating wind pressures in natural ventilation design-mean flow rates

The paper presents the results of a study into the eff ects of unsteady wind pressures on the mean flo11 rates in certain types of purpose-designed naturally ventilated buildings. The study used nondirnensional parameters and the results should therefore co\'er a wide range of conditions and should be of general applicatit111. It is concluded that unstead) elfrcts are restricted to a relatively narrow band of conditions. These conditions have been quantitied in terms of noncli111e11,ional parameters. In particular a simple procedure for calculating the mean How rates when the unsteady effects lHe large has been deriwd. In its simplest form the procedure does not require knowledge of the instantaneous wind pressures. ( 1999 Else\'i.:r Science Ltd. All rights reserved. l. Introduction I. I. Aims o/ 1rnrk The work described forms part of a larger project to determine unsteady flow effects i n re lat ion to nat ura l ventilation design . The re-emergence of natural vent i­ lation as a serious option for non-domestic buildings has led to renewed interest in design procedures e .g. [ l ] . This is part ly due to commercial necess i ty , but a lso because the use of purpose-provided openings of known geometry at known pos i t ions makes rela tively sophisticated procedures more viable [ 2 ] . However as far as is known. a l l current design proced ures employed in practice ignore unsteady flow effects . One major reason for this is the greatly i ncreased amount of information needed to take ful l account of unsteady effects . At present such data is scarce, mainly because it is difficul t and costly to obtain . Thus. the overal l a im of the project is to determine. *Tel.: +44-(0)115-951<1171: fa.x: � 44-(0)115-951-31:ilJ. E·11wil oddres.1·; dal'id.ctheridge·.a nottingham.ac.uk ( D.W. Etheridge) i n a general way, the conditions for which unsteady eAi:'cts can be ignored. There are t\\ o aspects to this problem. namely the effec ts on time-mcraged flow rates and the effects on flow pa tterns (e.g. the direction of flow through passive stacks). The present paper is concerned only with the first aspect i.e. mean (time­ averaged) flow rates . One aim is to define in a general way. by means of nondirnensional parameters. the con­ di t ions under which unsteady \\incl effects may need to be considered when calculating mean flow rates. The second aim is to provide a simple means of accounting for these effects when desirable. A l though the study is of general application. it does have l imitat ions which need to be recognised . The study is concerned pr imari ly with two-opening cases. but as explained later these do have practical signin­ cance. The s ingle-opening case is considered. but this is more of academic rather than practical interest. A fur ther l imitation i s that the unsteady flows are assumed to be generated pur e ly by uns teady surface wind pressures i .e . the effects of unsteadiness in the external veloci ty field around the open ings are neg­ lected. This is partly because the treatment of such effects is extremely difficult and partly because they wil l general ly be less important than the effects of 0360-1323 00 $ see front 111alle1· 1· 1999 Elsevier Science Ltd. All rights reser,ed. Pll: 50360-1323(9 9)0 0 0 07-4 �i·"c<r·��' --:::�"'"it"',�.,.. T�·.• . ·.-..,..�� . ..,_,. ...