Proceedings of BS2015: 14th Conference of International Building Performance Simulation Association, Hyderabad, India, Dec. 7-9, 2015.

Transcription

1 WIND ASSESSMENT IN URBAN AREA WITH CFD TOOLS APPLICATION TO NATURAL VENTILATION POTENTIAL AND OUTDOOR PEDESTRIAN COMFORT Stéphane Sanquer, Gullaume Canot, Sachn Bandhare Meteodyn, Nantes, France ABSTRACT The paper presents a numercal methodology to assess wnd pedestran comfort and natural ventlaton n urban areas. UrbaWnd, an automatc computatonal flud dynamcs code, s mproved to model the wnd n dense urban envronments. The turbulence modellng, that s to say the dependence of turbulence length on the dstance from wall, and the model constants are calbrated n order to reproduce precsely flow separaton around buldngs walls. Numercal results match well wth the experments: separaton patterns, pressure feld on walls, and wnd speed n dense urban area. Two examples are presented at the end of the paper n order to demonstrate the advantages of the methodology for urban desgners. INTRODUCTION Background From an urban desgner pont of vew, the knowledge of the urban clmatology and especally the wnd flow around buldngs s crucal n many applcatons such as: Ar qualty and thermal behavor nsde buldngs, snce ar and heat exchange depends on wnd pressure on facades. Natural ventlaton s the method to reduce energy consumpton. Ths s all the more true snce concerns are growng on global warmng (Sanquer S., Abdesselam M., Pcgrard F., BS 0) Wnd energy producton from small wnd turbnes Wnd comfort n outdoor spaces and n open ndoor spaces exposed to wnd CFD tools dedcated to calculatng wnd flow nsde bult envronment facltates the understandng and nterpretaton of the wnd n any urban area. Wnd mappngs become useful to urban desgners to optmze the master plan: poston and orentaton of buldngs to mprove natural ventlaton, small energy producton and pedestrans wnd comfort (Janssen et al. 03, Fadl and Karadels 03, Szücz 03). In the ntal sketch desgners should gve a techncal answer n order to mprove the master plan as quckly as possble. As CFD smulatons are carred out wth a hgh level of accuracy, ths may be tme consumng for large and complex urban master plans. A x km² urban smulaton represents more than 0 mllon cells. Wth a standard computer dedcated to consultng servces, ths mples a computaton tme exceedng one day for each wnd drecton.. A short tme response s expected durng the sketch process whereas computatonal tme can be very long. In that context, a proper methodology needs to be defned to ncrease the effcency of the numercal approaches. Three axes of mprovement were hghlghted: Computaton tme reducton by mprovng the meshng process and the solver, usng a turbulence model that speeds up the convergence For natural ventlaton assessment, a macroscopc approach remans the best way to reduce the computatonal tme to obtan the Ar Change Rate (ACH). Smulate the flow through wndows and nsde any buldngs n an urban area s not common for consultng servces Informaton delvered by CFD tools s not always understandable by desgners and may not be used wthout statstcal treatment ncludng the local clmatology. The natural ventlaton of a buldng s drven by the combned forces of wnd and thermal buoyancy. However, when openngs are large enough and the ar change rate s hgh enough, the ndoor temperature s qute homogeneous and, except for mportant volumes, the natural ventlaton s manly drven by the wnd forces. Thus, for the natural ventlaton assessment n tropcal warm clmates, the thermal forces are generally assumed as neglgble compared to the wnd drven forces. The assessment of the cross-ventlaton effcency s done by consderng the wnd speed levels nsde the buldngs, and the flow-rates through the openngs or generally the ar change rate of the volume (ACH)

2 The cross-wnd flow rate depends on the sze of the openngs, on ther aerodynamc effcency, namely ther aerodynamc dscharge coeffcents, on the pressure coeffcents on the buldng envelope and on the reference wnd velocty upstream the buldng. Aynsley et al. (977) gave an analytc expresson useful for a sngle ventlated volume wth two openngs, but as soon as the number of openngs s hgher or when the nternal volume s dvded nto sub volumes, a resoluton for non-lnear system has to be used. All of these characterstcs are fundamental and depend on the external wnd pressure at the openngs. The pressure on the buldng envelope can be defned wth tables (Lddament 986, Clarke. J et al. 990, Eurocode) or usng parametrc models based on the analyss of results from wnd tunnel tests (Grosso 99, Muehlesen and Patrz 03, Castola et al. 009). These tables or correlatons are potentally napplcable when buldngs are rregular and dfferent from the smple shape or when they are surrounded by other buldngs n a complex urban area wth canyons and corners. In these confguratons, the wnd strongly fluctuates and the pressure coeffcents and wnd drven arflows evaluaton s subected to varous problems detaled hereafter. Unfortunately for desgners, the pressure value on outsde walls of buldngs cannot be assessed n urban places wth ths method for two reasons. The pressure coeffcents are hghly dependent on the buldngs shape and on the nfluence of neghborng buldngs. Moreover, the reference wnd velocty needed to convert the pressure coeffcent to pressure should not be taken near the proect but far enough and hgh enough above the urban canopy to avod the nhomogenety of wnd n urban areas. These external wnd pressures should be evaluated ether from wnd tunnel tests (Kandola, 990) or from computatonal flud dynamcs (CFD) smulatons (Ayad 999, Fahsss et al. 00, Montazer and Blocken, 03). Analytc methods such as the Aynsley method cannot be used for complex ndoor layout where pressure loss occurs. Network aerodynamc tools ncludng pressure loss dependng on the nternal porosty are necessary to assess the ar change rate n such a layout. UrbaWnd, a combned CFD-Network software dedcated to modellng the urban clmatology, provdes the mass flow rate from the computed pressure outdoor feld to assess the cross ventlaton effcency (Sanquer et al., 0). It was shown that ACH depends both on external wall porosty and ndoor porosty. The natural ventlaton potental can be reduced by 30% wth ndoor porosty. Nevertheless, wthout consderng the ndoor layout the pressure feld n a complex urban area can be analyzed n order to extract natural ventlaton potental. The am of ths work s to present a methodology that allows the quck assessment of urban master plans wthout carryng out long complex computatons, for nstance by delverng: The natural ventlaton potental based on pressure coeffcents mappngs The human comfort ndex based on wnd speeds exceedance statstcs Some valdaton cases wll be presented based on reference round robn tests. Two real examples for varous wnd exposures and clmates llustrate the practcal advantages of the approach NUMERICAL APPROACH: METHODOLOGY AND VALIDATION Wnd computaton n any complex urban area The CFD method of UrbaWnd conssts of solvng the Reynolds-Averaged Naver-Stokes equatons on an unstructured rectangular grd wth automatc refnement of the mesh near obstacles. The CFD tool delvers the tr-dmensonal mean velocty feld, the turbulence energy feld and the mean pressure for each pont n the doman. ρ u x = 0 ( ρu u ) x P + x x u u µ + x x ρu' ' + = 0 u F When the arflow s steady and the flud ncompressble, the mean equatons for the mean velocty components contan unknown quanttes. In order to close the system, a one-equaton or twoequaton model s usually assumed to solve the turbulent fluxes. The equatons resoluton s based on a fnte volume method wth a rectangular mult-bloc refned mesh. A very effcent coupled mult-grd solver s used for a fast convergence for every knd of geometry. (Ferry, 00). Boundary condtons are automatcally generated. The mean velocty profle at the computatonal doman nlet s determned by the logarthmc law n the surface layer, and by the Ekman functon above (Garratt, 99). A Blasus - type ground law s mplemented to model frctons (velocty components and turbulent knetc energy) at the surfaces (ground and buldngs). The effect of porous obstacles s modelled by ntroducng a volume drag force n the cells lyng nsde the obstacle

3 The transport equaton for the turbulent knetc energy k contans a dsspaton term ε deduced from the mxng-length theory. µ T k ρu = ε σ k Pk x k x () µ u u u = + T x x x ε The turbulence vscosty µ t s consdered as the product of a length scale wth a speed scale, whch are both characterstc scales of the turbulent fluctuatons. µ = ρ () t k / L T 3 / k and ε = C (3) µ L T The turbulent length scale L T vares lnearly wth the dstance to the nearest wall d. The dependence of L T on the dstance va a coeffcent C L was defned n order to well reproduce the flow separaton around typcal buldng façades and roofs. Cµ s usually fxed at 0.09 for grd turbulence (standard k-ε model). In the boundary layer, the Cµ constant depends on the atmospherc stablty. The model defnng the turbulence length scale by Yamada and Arrtt (Hurley, 997) suggests a dependence of Cµ on the stablty crtera n the range of Comparsons wth expermental results were made n order to calbrate the C L and Cµ values for such urban flows. The best choce to reproduce both the flow separaton n the wake and the upper vortces gve a couple of reference values for C L and Cµ. Fgure Recrculaton n the wake of a square hghrse buldng <U>/Uref X R C=0. Cµ=0.09 C=0.5 Cµ=0.09 C=0. Cµ=0.09 C=0. Cµ=0.0 C=0.5 Cµ= X/b Fgure Recrculaton n the wake of a square hghrse buldng X R s the length of the recrculaton area n the wake of a tall buldng szed as 0x0x40 m (fgure ). Velocty magntude on the wake axs was plot versus the dstance from the backward wall (fgure ). Flow separaton above the buldng roof s also hghlghted (fgure 3) and the flow separaton length X R_TOP extracted. All the results were summarzed n the table. These results were compared wth many numercal smulatons and experments of Meng and Hb (996) descrbed by Tomnaga et al.(008). X R_TOP Fgure 3 Flow separaton above the roof of a square hgh-rse buldng Table Comparson of Numercal models Reattachment length and Expermental values (b s the edge) Turbulence model Reference X R/b X R_TOP/b k-ε (Standard) Tomnaga el al..7 No separaton k-ε (Modfed) Tomnaga el al. 3 to to 0.58 Dfferental stress model Mochda et al. 4. > LES Tomnaga el al. to to 0.6 k-l (C L=0., Cµ=0.09) UrbaWnd k-l (C L=0.5, Cµ=0.09) UrbaWnd k-l (C L=0., Cµ=0.09) UrbaWnd k-l (C L=0.5, Cµ=0.0) UrbaWnd k-l (C L=0., Cµ=0.0) UrbaWnd 0.70 Experment Meng and Hb

4 Ar change rate assessment For a smple volume wth two openngs, the cross wnd flow rate was wrtten by Aynsley et al. (977): sgnature s the wnd aerodynamc nteractons wth every neghbor buldngs. Q = Cp A C Cp + A C U WIND = A a Cp U WIND (4) Where A and A are the areas of the openngs and, Cd and Cd ther dscharge coeffcents gven as functons of the openngs aerodynamc characterstcs, and U WIND s the reference wnd velocty. Hence, mass flow rate depends on three varables: the wnd velocty far upstream the buldngs U WIND, the aerodynamc area A a and the pressure dfferental. When more than two openngs are connected wth an ndoor volume, the ACH computaton needs to use an teratve process: the nternal pressure s unknown and the flow rate through openngs s solved wth a Newton-Raphson teratve method (Fhasss et al. 00). In the case of a flat wth several rooms, the free aerodynamc area of the openng k s replaced by an equvalent aerodynamc surface, takng nto account the door aerodynamc surface A door of the secondary room (Sanquer et al., 0). eq A k = / + (5) A A k door All the macroscopc methods use external pressure felds on the walls where the openngs are ftted as an nput. Before usng a CFD tool to provde pressure values, comparsons were made wth expermental data on Slsoe cube (Rchards et al., 007) resumed by Costola et al. (009). Ths s a valuable round robn test to check the ablty of CFD to capture the flow separaton around a cube. The pressure feld obtaned along the traectory wth UrbaWnd matches well the expermental results obtaned for a detached low-rse buldng (fgure 4). Further computatons were carred out for the same low-rse buldng located n a more complex urban envronment (fgure 6). Varous wnd ncdences were computed; fgure 6 shows that the natural ventlaton potental defned as the maxmum dfferental of pressure coeffcent vanshes n complex urban area. For example Cp vanshes from unty (detached, 0 ) to less than 0. (urban, 0 ). For oblque wnd ncdence (45 ), Cp vanshes from 0.7 to less than 0.. Ths analyss shows the dffcultes to extract numercal tables or analytc relatons to descrbe the nfluence of urban envronment snce the Fgure 4 Pressure on the walls of a low-rse detached buldng: comparson of dfferent wnd tunnel experments and numercal results (UrbaWnd) Fgure 5 Pressure on the walls of a low-rse buldng Influence of the urban envronment Complex Urban Wnd feld and wnd comfort analyss The computed speed-up factors have been compared to the speed-up factors obtaned from the expermental measurements. The speed up factor s normalzed by a reference wnd at 5.9 m heght. Accordng to Yoshe et al.(008) mean scalar velocty measured n the wnd tunnel usng a nondrectve thermstor anemometer s regarded as the tme averaged nstantaneous scalar velocty whereas the mean scalar velocty gven from CFD s calculated from the tme averaged velocty vector. Expermental values have been corrected to take nto account turbulent knetc energy. In fgure 7, the computed values generally agree wth expermental values (all ponts). The mean error s equal to 0.04 and the standard devaton error s equal to

5 Fgure 6 Example of CFD computaton n complex urban area: wnd speed (left) and pressure (rght) The 77 probes, located at meters heght n the fullscale experment, are represented n red n fgure 8. When error ( ) s above 0., 38 expermental values are dsplayed n fgure 8 and by red ponts n fgure 7. The dstrbuton of those ponts wth an error above 0. tends to be qute homogeneous n the flow pattern. Once several drectonal computatons are computed, one can assess mean wnd speed and turbulence ntensty thanks to mast weather data. The wnd comfort s expressed n terms of rates of wnd speed thresholds exceedance as recommended by Delpech et al. (005). If we apply for nstance the CSTB crtera, we consder the frequency of the gust speed exceeds 3.6 m/s. In ths case, the gust speed s defned as the sum of the 0 mn mean wnd speed and ts standard devaton. Accordng to CSTB crtera, the lmt of comfort s 5% exceedance for a steady poston, 0% for a walkng pedestran, and 0% for a brsk walkng. Fgure 7 Measurement Vs Computed Speed Up Factor (red ponts correspond to error > 0.) As noted by Yoshe et al.(008), CFD results often tend to underestmate the wnd speed n the wake regon of the buldng. The dfference between the scalar veloctes n the wake regon from the CFD and the expermental results s partly because the defnton of the mean scalar velocty measured by the non-drectvty thermstor anemometers was dfferent from that of CFD. Fgure 8 Computed Wnd Speed Coeffcent map n red and dfference between computed and expermental data Table Dscomfort frequency Transtory moton Recreatonal moton Short Statonary Long Statonary France F(V > 3.6) < 0% F(V > 3.6) < 0% F(V > 3.6) < 5% F(V > 3.6) < 5% Netherlands F(V > 5) < 0% F(V > 5) < 0% F(V > 5) < 5% F(V > 5) < 5% Denmark F(V > 5) < 50% F(V > 5) < 34% F(V > 5) < 5% F(V > 5) < 5% Montreal (Wnter) F(V > 4) < 5% F(V > 4) < 5% F(V > 4) < 0% F(V > 4) < 0% Montreal (Summer) F(V > 6) < 5% F(V > 6) < 5% F(V > 6) < 0% F(V > 6) < 0% Unted Kngdom F(V > ) < % F(V > 8) < 4% F(V > 5) < 6% F(V > 5) < % Notaton: F(V > 3.6) = frequency of the wnds faster than 3.6 m/s

6 EXAMPLES OF APPLICATIONS Natural ventlaton potental n tropcal humd warm clmate The frst example s located at the Reunon Island where trade wnds are moderate on the north sde of the sland. Even n urban areas the average velocty s close to m/s at 0 m above the ground and the buldngs may be naturally ventlated. Moreau and Gandermer (00) gave some recommendatons to assess the potental of natural ventlaton n tropcal warm clmates. Gudelnes are based on the pressure coeffcent dfferental Cp between upwnd and downwnd sde walls of a buldng (table 3). Table 3 Gudelnes for Natural ventlaton potental Level of natural Cp range ventlaton effcency ACH too low Cp < 0.7 ACH enough 0.7 Cp < 0.39 ACH good 0.39 Cp < 0.53 ACH very good 0.53 Cp on the locaton of the volume n the buldng (ground, mddle, top) and on the locaton of the buldng n the master plan (exposed, protected, shaded by nearby buldngs). Thermal desgn was carred out accordng to the smple methodology Batpe commonly used n such tropcal overseas terrtores and presented n a prevous BS conference (Sanquer et al. 0) A second example concerns a proect n French Guana. Close to the equator Trade wnds are weaker and the average wnd speed s below m/s at 0 meters above the ground. In ths stuaton, colored arrows were also prnted n order to hghlght the potental to help the urban desgn accordng to the table 3 crtera. Urban desgners used ths dagnostc mappng to change the layout and to fnd a compromse that avods the case of under ventlaton of volumes. The strategy conssts of ncreasng the pressure coeffcent dfferental above the lmt of 0. for all the ventlated spaces as much as possble. Some mappngs of Cp are shown for a part of the urban dstrct for the man wnd drecton. Colored arrows were over prnted on buldngs n order to optmze the openngs characterstcs. The buldngs layout may be optmzed n order to allow enough pressure on walls for each volume. Some of them, due to the orentaton of the façades and the wnd sheld of the others buldngs, could not be naturally ventlated. In these cases, mechancal ventlaton could be chosen for some specfc rooms. Fgure 0 Pressure feld mappng to assess the potental of natural ventlaton (French Guana) Wnd comfort n a reshapng area n Pars Fgure 9 Pressure feld mappng to assess the potental of natural ventlaton (La Réunon Island) Ths example s located n Pars on Segun Island. The reshapng of 0. km² area ncludes m² of terrace and 000 m² of park. The wnd comfort assessment s crtcal n those pedestran areas (overall on the rm of the sland) where the closest buldng on the opposte rver bank s located from 60 m to 00 m to Segun Island. The prevalng wnds are comng from the South to West-South-West (40 deg) representng 30 % of the wnd frequency. Several wnd acceleraton effects responsble for wnd dscomfort can be seen on fgure below: - Corner Effect on - Ventur Effect on and 3 Porosty of façades was defned accordng to the varablty of ACH and of thermal loads that depend

7 3 Prevalng Wnd Fgure Pedestran Wnd comfort assessment on Segun Island n Pars Only the East sde of the Island s sutable for long or short statonary postons. Fgure 4 Ventur effect causng strong acceleraton (red color) can be reduced wth wndbreaks Fgure Pedestran Wnd comfort (green area s sutable for statonary poston whereas yellow s not comfortable for ths actvty) Local treatments such as vegetaton or fences can prevent such strong acceleraton and enhance the wnd comfort. CONCLUSION The paper presents a numercal methodology to assess the wnd pedestran comfort and the natural ventlaton n any urban area. UrbaWnd, an automatc computatonal flud dynamcs code, was developed to model the wnd n urban envronments by optmzng the meshng and solvng processes. Accurate results computed as quckly as possble wll be useful for urban desgners. The turbulence modellng, namely the dependence of turbulence length on the dstance from wall and the Cµ value of the k-l model were calbrated n order to well reproduce the flow separaton around buldngs walls. Numercal results were compared wth well documented experments: Sloe cube as a low-rse buldng and a :: tower as hgh-rse buldngs and Ngata urban case. Comparsons were made on separaton patterns, pressure felds on walls and wnd speed on pedestran path walks. Some real examples are presented at the end of the paper n order to demonstrate the advantages of the methodology for urban desgners. REFERENCES Fgure 3 Corner effect causng strong acceleraton (red color) can be reduced wth wndbreaks Sanquer S., Abdesselam M., Pcgrard F., 0, Combned CFD-mean energy balance method to thermal comfort assessment of buldngs n a warm tropcal clmate, Buldng Smulaton Janssen W., Blocken B., Hooff T.V., 03, Use of CFD smulatons to mprove the pedestran wnd comfort around a hgh-rse buldng n a complex urban area, 3 th conference of nternatonal buldng performance smulaton assocaton, Chambéry, France, pp

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