INVESTIGATION ON INFLUENCE OF POROSITY ON WIND

Transcription

1 The Seveth Asia-Pacific Coferece o Wid Egieerig, November 8-2, 29, Taipei, Taiwa INVESTIGATION ON INLUENCE O POROSITY ON WIND LOADING ON POROUS SUNSHADE ROO COVER SHEETS ON A LOW-RISE BUILDING Vu Thah Trug, Yukio Tamura 2 ad Akihito Yoshida 3 Ph.D studet, Tokyo Polytechic Uiversity, Kaagawa, Japa, d784@st.t-kougei.ac.jp 2 Professor, Tokyo Polytechic Uiversity, Kaagawa, Japa, yukio@arch.t-kougei.ac.jp 3 Associate Professor, Tokyo Polytechic Uiversity, Kaagawa, Japa, yoshida@arch.tkougei.ac.jp ABSTRACT Porous sushade roof cover sheets are used to reduce temperatures iside buildigs. This paper describes a parametric wid tuel study carried out to ivestigate the ifluece of porosity o wid loadigs. The mea, fluctuatig (rms), maximum peak ad miimum peak wid pressure distributios over the surfaces of a porous sushade roof cover sheet were measured for porosities of %, 5% ad % (ratio of area of orifices to area of porous sushade roof cover sheet). The results show that the porous sushade roof cover sheet with o porosity (%) had the highest wid loadig, ad that those with porosities of 5% ad % were most effective i reducig wid loadig. KEYWORDS: POROSITY, POROUS SUNSHADE ROO COVER SHEET, LOW-RISE BUILDING, WIND TUNNEL EXPERIMENT. Itroductio Thermal reductio is always a problem for buildig roofig systems, especially profiled steel sheet systems. It reduces requiremets for heatig ad coolig system capacities, icreases occupat comfort, ad eve elimiates codesatio o roof surfaces i cold climates. Up to ow, there are several solutios for roof isulatio such as loose laid paver system, heat isulatio roof tile ad so forth. But the cost of these solutios are still expesive. Porous sushade roof cover sheets have itroduced to be a ew solutio. This solutio has some advatages such as cheap price, quick istallatio ad applicatio for both ew ad existig roofs. The porous sushade roof cover sheets istalled o the profiled steel sheet systems reflect sulight so that they reduce the sulight to reach to the roof. igure shows some pictures of applicatio of the porous sushade roof cover sheets for a real roof of lowrise buildig. Wid loadig o this type of structure depeds o the differece betwee the pressures o the upper ad lower surfaces. It is therefore importat to be able to assess the ifluece of porosity o wid loadig o these sheets. This paper describes a ew study o wid loadig o porous sushade roof cover sheets. A few experimetal studies [H.J. Gerhardt ad C. Kramer (983) ad J.C.K. Cheug ad W.H. Melboure (988)] have bee carried out to determie wid loadigs o permeable roofig systems. 2. Experimetal set-up A model (2 mm high (H) 47 mm wide (B) 7 mm deep (D)) with porous sushade roof cover sheets was tested i a Boudary Layer Wid Tuel, 2.2 m wide by.8 m high, i Tokyo Polytechic Uiversity, Japa. The legth ad velocity scales were /5 ad

2 The Seveth Asia-Pacific Coferece o Wid Egieerig, November 8-2, 29, Taipei, Taiwa /4, respectively. Terrai category III (power law idex.2) i AIJ-RLB (24) was chose for the experimets. The turbulece itesity at height 2 mm ( m i full scale) was.26 ad the wid speed was 7m/s. Three test model cases (%, 5% ad %) were tested to determie the ifluece of porosities of porous sushade roof cover sheets for a total of 4 wid agles ( o to 36 o i o steps ad four wid agles: 45 o, 35 o, 225 o ad 35 o ). igure 2 shows mea wid speed ad turbulece itesity profiles. ixig Profiled steel sheet Hole (a) A real arragemet of porous sushade roof cover sheets o a lowrise buildig (b) Close-up view of a porous sushade roof cover sheet Porous sushade roof cover sheet igure : Pictures of applicatio of porous sushade roof cover sheet The model had sixtee porous sushade roof cover sheets, oe with 28 holes, while four porous sushade roof cover sheets had pressure taps (A, B, C ad D) (see igure 3). The porous sushade roof cover sheets of Model (porosity %) had o holes, while Models 2 (porosity 5%) ad 3 (porosity %) had holes of 2.8 mm ad 4 mm diameter, respectively. The gap betwee the porous sushade roof cover sheet ad the top of the profiled roof was mm (see igure 3)..8 Experimet Test (/5).8 Experimet Test (/5) z/zg z g = 45 m (full scale) (AIJ-RLB 24) Roof height H = m (full scale) z/zg Roof height H = m (full scale) U / z U H (a) Mea wid speed profile Turbulece itesity: I = U / U (b) Turbulece itesity profile z z z igure 2: Simulated wid coditios i wid tuel

3 The Seveth Asia-Pacific Coferece o Wid Egieerig, November 8-2, 29, Taipei, Taiwa H = 2 D C B A o θ Porous sushade roof cover sheet Hole Wid B = 47 D = 7 Buildig (a) Geometry of test model (b) Close-up view of a porous sushade roof cover sheet X 2 6 D C B A Y 2 2 θ Wid (c) Arragemet pla of porous sushade roof (d) Detail of roof sectio cover sheets igure 3: Test model (all dimesios i mm) o Hole Pressure Pressure tap Porous sushade roof cover sheet C pu C pl igure 4 shows some pictures of a test model: test model i wid tuel ad a close-up view of a porous sushade roof cover sheet. (a) Test model i wid tuel (b) Close-up view of a porous sushade roof cover sheet model igure 4: Pictures of test model

4 The Seveth Asia-Pacific Coferece o Wid Egieerig, November 8-2, 29, Taipei, Taiwa 3. Results ad discussio 3. Local wid force coefficiet The local wid force coefficiet o the sheet due to the combied effect of the upper ad lower surface pressures is C (i,t) = C pu (i,t) - C pl (i,t) () where C pu (i,t) ad C pl (i,t) are wid pressure coefficiets at measuremet tap i at time t o the upper ad lower surfaces of the sheet, respectively; ad C (i,t) is the local wid force coefficiet at measuremet tap i at time t of the sheet. The local wid force coefficiets were defied as positive i the vertically dowward directio. Distributios of mea ad rms local wid force coefficiets ( C ad C ) o sheets A, B, C ad D are show i igure 5 for porosities φ = %, 5% ad % at a wid agle of θ = 45 o. Geerally, both mea ad rms local wid force coefficiets decreased with icreasig porosity. or C, while the mea upper surface wid pressure coefficiets are always egative, the local wid force coefficiets may be egative or positive. Besides, the upper surface wid pressure coefficiets were largest at the edges but the largest values of C did ot occur at the edges. The values at there were very small. The values of C o sheet A were higher tha those o sheets B, C ad D due to coical vortices. The mea values for sheets B ad C were similar ad those for sheet D were almost zero. The values of C decreased with porosity. The largest values of C.6,.3,.2 were foud for φ = %, 5% ad %, respectively. At this wid agle, dowward local wid force coefficiets were observed o most areas of sheets A, B, C ad D. This meas that the mea lower surface wid pressure coefficiets were larger tha the mea upper surface wid pressure coefficiets. or C, the rms local wid force coefficiets o sheet A were largest at the same corer as the rms upper surface wid pressure coefficiets at the same wid agle but were smaller. The largest rms local wid force coefficiets were observed at the ceters of sheets B, C ad D. The largest values of C.5,.32 ad.26 were foud for porosities φ = %, 5% ad %, respectively, at the leadig edge of sheet A, suggestig that separatio occurred there. To clearly show the similarity betwee the upper surface wid pressure coefficiets for porosities φ = %, 5% ad %, the authors gave a simple compariso of the mea, rms, maximum peak ad miimum peak upper surface wid pressure coefficiets ( C, C pu pu, pu ad pu ) measured at each upper-upper pair o a agle-by-agle basis. igure 5 presets plots comparig the mea, rms, maximum peak ad miimum peak upper surface wid pressure coefficiets. igure 6 shows that the mea, rms, maximum peak ad miimum peak upper surface wid pressure coefficiets of porosity φ = % were highly correlated with those of porosities φ = 5% ad %, ad that the correlatio was stroger for mea ad rms upper surface pressures. Based o these observatios, it is cocluded that the upper surface pressures deped o the buildig flow field but ot o the porosity. This is cosistet with those described by [Cheug ad Melboure (988)] for wid loadig o a porous roof. The lower surface pressures icreased with icreasig porosity (results ot show here). 3.2 Pael wid force coefficiet The time histories of the upper ad lower surface pael wid pressure coefficiets o a sheet are C UP (t) ad P (t) respectively ad are calculated as follows. N ( C pu ( i, t). Ai ) C ( t) = / A (2) UP i=

5 The Seveth Asia-Pacific Coferece o Wid Egieerig, November 8-2, 29, Taipei, Taiwa N ( C pl ( i, t). Ai ) C ( t) = / A (3) LP i= where C pu (i,t) ad C pl (i,t) are the wid pressure coefficiets at poit i at time t o the upper ad lower surface of sheet, respectively; A i is the effective area o which wid pressure acts measured at poit i; N is the umber of measuremet poits o a sheet; ad A is the area of a sheet Wid (a.) Porosity φ = % (b.) Porosity φ = % Wid Wid (a.2) Porosity φ = 5% (b.2) Porosity φ = 5% Wid Wid (a.3) Porosity φ = % (b.3) Porosity φ = % (a) Mea C (b) Rms C Wid igure 5: Distributios of mea ad rms local wid force coefficiets o porous sushade roof cover sheets with differetial porosities φ at wid agle θ = 45 o

6 The Seveth Asia-Pacific Coferece o Wid Egieerig, November 8-2, 29, Taipei, Taiwa Upper surface wid pressure coefficiet (φ = 5%) Upper surface wid pressure coefficiet (φ = %) Upper surface wid pressure coefficiet (φ = 5%) Upper surface wid pressure coefficiet (φ = %) Upper surface wid pressure coefficiet (φ = %) (a.) φ = % vs. φ = 5% (b.) φ = % vs. φ = 5% Upper surface wid pressure coefficiet (φ = %) (a.2) φ = % vs. φ = % (b.2) φ = % vs. φ = % (a) Mea ad rms (b) Maximum peak ad miimum peak igure 6: Compariso of upper surface wid pressure coefficiets of porosity φ = % with porosity φ = 5% (porosity φ = %) from all wid agles. The time history of pael wid force coefficiet of a sheet was obtaied from the followig equatio C (t) = C UP (t) - P (t) (4) I here, the pael wid force coefficiet was defied positive i the vertically dowward directio. igure 7 depicts variatios of maximum peak ad miimum peak pael wid force coefficiets ( ad ) for sheets A, B, C ad D for all wid agles. At most wid agles, the pael wid force coefficiets decreased with icreasig porosity. Geerally, the absolute pael wid force coefficiets for porosity φ = % were the highest ad those for % porosity were the lowest. Upper surface wid pressure coefficiet (φ = %) or the maximum peak values, there was a strog depedece betwee ad wid agle with a rapid chage of these values for wid agles θ from o to 8 o. The variatios of Upper surface wid pressure coefficiet (φ = %)

7 The Seveth Asia-Pacific Coferece o Wid Egieerig, November 8-2, 29, Taipei, Taiwa for porosities φ = 5% ad % became more stable with icreasig wid agle. Sheet A reached its largest value of the wid agle θ that resulted i the largest values of lowest values of at a wid agle θ of about 45 o. or other sheets (B, C ad D), at a wid agle θ of about 2 o for all porosities. or the miimum peak values, the differece betwee the values of was about 9 o. All sheets had their for porosities φ = 5% ad % was small. Values of for porosity φ = % chaged quickly for wid agles θ from o to 8 o ad from 27 o to 36 o ( o ) for sheets A, C ad D ad for sheet B, respectively. Sheets A, C ad D showed their largest absolute values of at a wid agle θ of about 9 o ad wid agle θ for sheet B was 32 o for all porosities. We ca see a similar result for the lowest values of absolute values of, where a wid agle θ of about 2 o resulted i the lowest for all sheets ad all porosities. ( ) (a) Sheet A Max (%) Max (%) Max (5%) Max (%) -.4 Mi (%) -.6 Mi (5%) Mi (%) Wid agle θ (deg) ( ) Wid agle θ (deg) (b) Sheet B Max (%) Max (5%) Max (%) Mi (5%) Mi (%) Mi (%) ( ) Mi (%) -.6 Mi (5%) -.8 Mi (%) (c) Sheet C Max (%)Max (5%) Max (%) Wid agle θ (deg) ( ) Max (%).8 Max (5%).6 Max (%) Mi (%) -.6 Mi (5%) -.8 Mi (%) (d) Sheet D Wid agle θ (deg) igure 7: Variatios of maximum ad miimum peak pael wid force coefficiets ( ad ) o porous sushade roof cover sheets A, B, C ad D with differetial porosities φ for all wid agles θ

8 The Seveth Asia-Pacific Coferece o Wid Egieerig, November 8-2, 29, Taipei, Taiwa igure 8 shows variatios of largest maximum peak ad smallest miimum peak pael wid force coefficiets ad for sheets A, B, C ad D with differetial porosities φ (from all wid agles). Geerally, the absolute values of the largest ad the smallest decreased as the porosity icreased. The differeces betwee the largest for sheets A ad D ad betwee those for sheets B ad C at porosity φ = % ad % were very small. The same pheomeo occurred for values of for porosity φ = %. The largest values of for porosity φ = % ad 5% were higher tha that for porosity φ = % by up to 24% ad 7%, respectively. These values for were 22% ad 65%. (.8.6 Sheet A Sheet D ) Sheet C Sheet A.4.2 Sheet C Sheet B Sheet B Sheet D % 5% % % 5% % Porosity Porosity (a) Largest max (b) Smallest mi 4. Coclusios A ivestigatio was carried out to determie the ifluece of porosity o wid loadig o porous sushade roof cover sheets o a low-rise buildig. The pressure distributios for the porous sushade roof cover sheet were measured for several wid agles as well as for differet porosities. our porous sushade roof cover sheets had high wid loadig at wid agles from o to 8 o. Aalysis of results obtaied from the experimets showed that the pressures o the porous sushade roof cover sheets with % porosity were higher tha those for 5% ad % porosities. The porosities of the porous sushade roof cover sheets were most effective i reducig wid load o them. Ackowledgemets This study was fuded by the Miistry of Educatio, Culture, Sports, Sciece ad Techology, Japa, through the Global Ceter of Excellece Program, 28-23, which is gratefully ackowledged. The authors would also like to thak to SAWAYA Co., Ltd, Japa. Refereces igure 8: Variatios of largest ad smallest with differetial porosities φ AIJ-RLB (24), AIJ Recommedatios for Loads o Buildigs, Architectural Istitute of Japa. Cheug, J.C.K., ad Melboure, W.H. (988), Wid loadig o a porous roof, Joural of Wid Egieerig ad Idustrial Aerodyamics, 29, Gerhardt, H.J., ad Kramer, C. (983), Wid loads o wid-permeable buildig facades, Joural of Wid Egieerig ad Idustrial Aerodyamics,, -2.