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Pedestrian-level wind speed enhancement with void decks in three-dimensional urban street canyons
Building and Environment 155 (2019) 399–407
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Pedestrian-level wind speed enhancement with void decks in three-dimensional urban street canyons
T
Lup Wai Chewa,∗, Leslie K. Norfordb a b
Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, 02139, USA Department of Architecture, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, 02139, USA
ARTICLE INFO
ABSTRACT
Keywords: Void ground floor Building porosity CFD simulation Wind field modification Non-uniform building height
Cities suffer from low wind speeds due to the blockage effects of urban structures. Low wind speeds inhibit a city's ability to self-ventilate and reduce thermal comfort in tropical cities. Building porosity has been shown to be an effective architectural feature to channel winds into the pedestrian level. However, previous studies have focused on building porosity in single buildings or two-dimensional urban street canyons. We extended the investigations of building porosity to three-dimensional urban street canyons with computational fluid dynamics simulations. Validated numerical models were used to conduct parametric studies on void decks (empty spaces at the ground floors). By allowing winds to flow through them, void decks can enhance pedestrian-level wind speeds by more than twofold compared to the reference case without void decks. The wind enhancement effect is the strongest at the first canyon and decreases downstream. The effectiveness of a void deck is greatly improved by increasing the void deck height. The average wind speed in the first canyon increases from 13 percent to 59 percent (of the freestream wind speed) by increasing the void deck height from 2 m to 6 m. On the other hand, the building height and canyon (height-to-width) aspect ratio have a much smaller influence on void-deck effectiveness. Non-uniform building height in an array of buildings also has a minor influence on the effectiveness. Therefore, void decks are equally effective to enhance pedestrian-level wind speeds in urban areas with tall buildings and buildings with non-uniform height.
1. Introduction Urban areas have reduced wind speeds compared to rural areas due to the blockage effects of urban structures [1–4]. Although wind speed reduction is favorable during winters or cold days in temperate climates [5,6], the opposite is true for tropical climates. Cities in the tropics rarely suffer from thermal discomfort due to wind chill [7,8]. Increasing wind speed can improve outdoor thermal comfort by increasing convective heat transfer from human bodies. For example, a field study in Singapore concluded that increasing the wind speed is the most effective method to improve thermal comfort in shaded outdoor areas [9]. Another field survey in Hong Kong found that respondents who felt warm desired stronger winds, as expected [10]. Besides thermal comfort improvement, stronger winds also help urban areas to self-ventilate and remove pollutants [11,12]. There are several strategies to increase wind speeds in urban areas. At the building scale, architectural features such as corner modifications can influence the wind field near buildings [13,14]. Roof shape and roof roughness affect the wind field by modifying the wake
∗
downstream of the buildings [15–17]. Wind catchers, commonly used to enhance indoor ventilation [18,19], have recently been shown to be applicable for outdoor wind speed enhancement [20,21]. Introducing building porosity can also enhance wind speed by reducing the blockage effects of buildings [22,23]. At the neighborhood scale, building layout plays an important role in altering the wind fields. For example, non-parallel buildings can accelerate wind flowing through reduced area between the buildings [24,25], while aligning open spaces with the prevailing wind direction helps to funnel atmospheric wind [26]. Building porosity also helps to increase wind speeds at the neighborhood scale [27]. As building porosity is an effective method to channel winds at both building scale and neighborhood scale, we will focus on building porosity for wind speed enhancement. Building porosity at the pedestrian level (1–2 m above the ground level) greatly enhances airflow [23], while podia obstruct winds [28] and should be removed or shrunk [26,29]. Xia et al. [30] conducted wind tunnel experiments and found that a lift-up building supported on cores (pillars) enhances the surrounding pedestrian-level wind by up to 11 percent. Du et al. [31] tested different configurations of lift-up
Corresponding author. 77 Massachusetts Avenue, 5-418, Cambridge, MA, 02139, USA. E-mail address: [email protected] (L.W. Chew).
https://doi.org/10.1016/j.buildenv.2019.03.058 Received 1 January 2019; Received in revised form 25 March 2019; Accepted 27 March 2019 Available online 30 March 2019 0360-1323/ Published by Elsevier Ltd.
Building and Environment 155 (2019) 399–407
L.W. Chew and L.K. Norford
Fig. 1. (a) A void deck (indicated by the dash-line box) of a public flat in Singapore. (b) A new design of a public flat with shops replacing void decks at the ground floor (indicated by the dash-line box). Photographs by the authors.
buildings and reported more than twofold increase of pedestrian-level wind speed. Subsequent studies of lift-up buildings reveal that the core height, core planar area, and building height-to-width aspect ratio can significantly affect the wind enhancement effects [14,32,33]. The above studies of lift-up buildings have only considered single buildings. Our previous study with multiple buildings has shown that voids at the ground floor can enhance pedestrian-level wind speed by up to twofold in an array of two-dimensional urban street canyons [27]. This paper extends the study to a more realistic urban setting using three-dimensional buildings with non-uniform height. Voids at the ground floor, or void decks in local terms, are a common architectural feature of public flats in Singapore. Fig. 1(a) shows an example of a flat with void decks. Void decks provide spaces for social activities and events, such as weddings and funeral wakes [34,35]. Void decks also improve walkability in a neighborhood by allowing pedestrians to walk “through” a building (underneath it) instead of going around it. Despite these benefits, new designs of public flats in Singapore have either much smaller or no void decks [36], as shown in Fig. 1(b), where the void decks have been replaced with shops. Social issues aside, “filling” the void decks could reduce pedestrian-level wind speed. This study aims to quantify the wind enhancement effects of void decks.
void deck height, Hvd, and canyon width, W (only two buildings are shown). The effective height, Heff, equals Hvd + Hb. The building geometry in the reference case (without void deck) follows that in our previous work [27], where Hb = 24 m and W = 24 m and the canyon aspect ratio is therefore 1.0. The building span-wise length (in the y direction) is fixed at 10 m for all cases. The street width between buildings is also fixed at 10 m. The incoming wind is in the positive x direction, and z represents the vertical direction. Ten simulations are conducted at different Hb and Hvd while W is fixed at 24 m. Table 1 summarizes the parameters in each case. All cases consist of fifteen canyons in the domain. Fig. 3 shows the relative sizes of the buildings and void decks in each case (the buildings are the white boxes, ignore the velocity contours for now, they will be discussed in Section 3). Case 1 is the reference case with Hb = W = 24 m and no void deck (Hvd = 0). Cases 2–6 are used for the parametric study of Hvd. Case 2 has all buildings “lifted up” by 2 m (Hvd = 2 m) while Hb is fixed at 24 m, so Heff = 26 m. Cases 3, 4, 5, and 6 have Hb fixed at 24 m but Hvd is increased to 3 m, 4 m, 5 m, and 6 m, respectively. Case 4 and Cases 7–10 are used for the parametric study of Hb, where Hb is varied from 24 m to 48 m while Hvd is fixed at 4 m. 2.1. Model setup and boundary conditions The computational fluid dynamics (CFD) model is constructed with ANSYS DesignModeler and meshed with ANSYS Meshing, both in the ANSYS Workbench package 17.2 [37]. The sizes of the CFD domains
2. Numerical models We model buildings with void decks as idealized blocks with empty spaces below. Fig. 2 shows the notations for the building height, Hb,
Table 1 The building height (Hb), void deck height (Hvd), effective canyon height (Heff), canyon width (W), and effective canyon aspect ratio (Heff/W) of the ten simulated cases.
Fig. 2. The building height, Hb, void deck height, Hvd, and canyon width, W, for (a) the reference case without void deck (i.e., Hvd = 0) and (b) the case with void deck. 400
Case
Hb (m)
Hvd (m)
Heff (m)
W (m)
Heff/W
1 2 3 4 5 6 7 8 9 10
24 24 24 24 24 24 30 36 42 48
0 2 3 4 5 6 4 4 4 4
24 26 27 28 29 30 34 40 46 52
24 24 24 24 24 24 24 24 24 24
1.00 1.08 1.13 1.17 1.21 1.25 1.42 1.67 1.92 2.17
Building and Environment 155 (2019) 399–407
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Fig. 3. Contours and vectors of normalized velocity magnitude at the middle span-wise plane (xz-plane at y = 0 m) for (a) Case 1, (b) Case 2, (c) Case 3, (d) Case 4, (e) Case 5, (f) Case 6, (g) Case 7, (h) Case 8, (i) Case 9, and (j) Case 10. Only the first canyon is shown in each case. Fig. 4. The CFD domain and boundary conditions for Case 1. The span-wise faces are transparent and the outlet is not shown. The coordinate system has x = 0 at the first building (not at the inlet) and z = 0 at the ground level (only the first seven buildings are shown). The inset shows the mesh of Case 4, where the grid resolution is the finest inside the canyons and void decks (only the first two buildings are shown).
follow the recommendations in Franke et al. [38]: the inlet is 5Heff upstream of the first building, the top surface is 6Heff from the ground, and the outlet surface is 15Heff downstream from the last building. Fig. 4 shows the CFD domain of Case 1. Note that only the first seven buildings are shown so the outlet surface is not shown. The inset shows the mesh of the first two buildings of Case 4. The grid size is W/30 inside the canyons and void decks. The mesh is coarsened above the canyons, upstream of the first building, and downstream of the last building. The maximum grid expansion ratio is 1.2 and the maximum grid size is 0.58W. The span-wise direction has 24 uniform grids. All
grids are perfectly orthogonal with zero skewness. The boundary conditions are as follows. The top surface and the span-wise faces have a symmetry boundary condition. All walls (leeward walls, windward walls, roofs, grounds) have a no-slip boundary condition. The outlet surface has a zero-gradient boundary condition. The set of atmospheric inlet boundary conditions [39] is prescribed at the inlet surface, where the inlet velocity (U), turbulence kinetic energy (tke), and turbulence dissipation rate (ε) are given in Eqs. (1)–(3):
U=
401
U
ln
z + z0 z0
(1)
Building and Environment 155 (2019) 399–407
L.W. Chew and L.K. Norford
tke =
U2 Cµ
(2)
U3 (z + z 0 )
=
(3)
Here U is the friction velocity given in Eq. (4), κ the von Karman's constant (= 0.41), z the vertical distance from the ground, z0 the surface roughness height, and Cµ a constant (= 0.09).
U =
U (z ref ) ln
(
zref + z 0 z0
)
(4)
Eqs. (1) and (4) can be simplified as a dimensionless velocity profile:
( ) ( ) z+z
ln z 0 U 0 = zref + z 0 U (z ref ) ln z 0
(5)
Fig. 5. Comparison of (a) normalized stream-wise velocity and (b) normalized turbulence kinetic energy for Case 4 in Table 1 at three mesh resolutions. The profiles are extracted at the pedestrian level (z = 1.5 m) on the middle spanwise plane (y = 0), and x/W = 0 corresponds to the windward wall of the first building (see Fig. 4 for the coordinate system).
We take z0 = 0.04 m and zref = 28 m, following our previous works for two-dimensional canyons with void decks [27]. Note that z0 = 0.04 m corresponds to open terrain in the updated DavenportWieringa classification [40]. U(zref) is a reference velocity at zref and is set at 2 m s−1, as field measurements have found that a threshold wind speed of 1.5–2 m s−1 is needed to sustain in-canyon vortices [41,42]. All simulations are conducted with the finite volume solver Open Field Operation and Manipulation (OpenFOAM) version 3.0.1 [43]. The built-in steady Reynolds-averaged Navier-Stokes (RANS) solver, "simpleFOAM," is used with the SIMPLE (Semi-Implicit Method for Pressure Linked Equations) pressure-velocity coupling and k-ε turbulence closure. The three most commonly used k-ε closure schemes are the standard k-ε, Re-Normalization Group (RNG) k-ε, and realizable k-ε. Hang et al. [44] reported that their simulation result from the standard k-ε agrees better than that from the RNG k-ε simulations. Chew et al. [27] compared the standard k-ε and realizable k-ε schemes and found that the former agreed better with their experimental results. Therefore, the standard k-ε is chosen for this study. Second order Gaussian integration with linear interpolation is used for all gradient and divergence schemes. All Laplacian schemes are based on Gaussian integration with linear interpolation and non-orthogonal correction. The standard wall function is employed to reduce computational cost by allowing a coarser mesh. The tolerance of residuals is set at 10−5 for all parameters, and iterations are continued until all residuals stop decreasing with further iterations [45]. The simulation results are postprocessed with the open-source software ParaView version 5.3.0 [46].
2.3. Model validation There is no experimental data for three-dimensional models with void decks. We follow the recommendation in Blocken [45] to divide the model validation over two sub-configurations. The first sub-configuration of three-dimensional buildings is validated with the wind tunnel experiment in Hang et al. [44] with twelve rows of nine rectangular model blocks. The second sub-configuration of two-dimensional buildings with void decks is validated with our previous water channel experiment [27] with an array of eight lift-up buildings. For the first sub-configuration, Fig. 6 compares the normalized velocity magnitude and turbulence kinetic energy (tke) profiles between our CFD results and the wind tunnel measurements. The profiles are taken along the middle street (i.e., between the sixth and the seventh
2.2. Mesh independence study Three simulations are conducted for Case 4 in Table 1. The coarse, normal, and fine mesh models have 0.36, 1.2, and 4.3 million grids, respectively. The mesh shown in Fig. 4 inset is from the normal mesh model. Fig. 5 plots the normalized profiles at the pedestrian level (z = 1.5 m) on the middle span-wise plane (y = 0). The stream-wise velocity, u, and the turbulence kinetic energy, tke, are normalized by a reference velocity, Uref, which is the freestream velocity at z = 2Heff. The horizontal distance, x, is normalized by the canyon width, W. Fig. 5(a) shows a small deviation of u/Uref between the coarse and normal mesh models. Increasing the mesh resolution from “normal” to “fine” does not change the overall profile. The tke profiles in Fig. 5(b) show negligible difference among the three mesh resolutions, except for the peaks near x/W = 0. However, this difference does not affect the tke profiles downstream (x/W > 0). Therefore, the model with the normal mesh resolution has achieved a mesh-independent solution. All subsequent simulations are conducted with the normal mesh resolution.
Fig. 6. Along-street horizontal profiles of (a) normalized velocity magnitude and (b) normalized turbulence kinetic energy at elevation z/B = 1.0, where B is the building width. B = 30 mm in the wind tunnel experiment and the reducedscale CFD, and B = 6 m in the full-scale CFD. Solid lines represent our reducedscale CFD simulation and dotted lines represent our full-scale CFD simulation. Both the experimental data and the dashed-line CFD profiles are from Hang et al. [44]. 402
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rows of buildings) at elevation z/B = 1.0, where B is the building width (30 mm in the wind tunnel experiment and the reduced-scale CFD, and 6 m in the full-scale CFD). The CFD simulation from Hang et al. [44] is also included for comparison. Both CFD results agree well with the experiments, except that the location of the peak tke is predicted upstream compared to the location of peak tke in the wind tunnel experiment. The normalized mean-square error of our CFD results are 0.015 for velocity and 0.411 for tke, both within the acceptable range suggested by Hanna and Chang [47]. In the wind tunnel experiment, the building width, B = 30 mm, was adopted as the length scale. Chew et al. [48] highlighted that reduced-scale experiments or simulations should achieve a sufficiently high Reynolds number to ensure Reynolds number independent flows. In other words, the reduced-scale experiments or simulations should reproduce the flows at full scale to be valid for applications in real built environments. Therefore, we repeat the simulation at full scale by scaling up the buildings by 200 times (where B = 6 m at full scale). The Reynolds numbers at reduced scale and full scales are 8000 and 1.7 × 106, respectively. The reduced-scale simulation (red lines) predicts the same trend as the full-scale simulation (black dotted lines), verifying that the reduced-scale experiments and CFD simulations are applicable at full scale. All subsequent simulations are conducted at full scale. For the second sub-configuration, the vertical profiles at the middle of third to sixth canyons (canyons 3–6) are provided by the water channel experiment. Fig. 7 compares the normalized stream-wise velocity profiles between our CFD result and the experiment. All buildings have Hvd of 2 cm and Hb of 12 cm (see Fig. 2 for the nomenclature) so the void decks are 0.14Heff in height. These void decks allow flows to pass through them, thereby inducing high near-ground velocities between z/H = 0 and z/H = 0.14. Canyon 3 has the largest near-ground velocity, and canyon 6 has the lowest near-ground velocity. The CFD model correctly predicts both the near-ground flows induced by the void decks and the trend of decreasing velocity at downstream canyons.
canyons), we repeat the simulations of Case 4 with five and ten canyons. The comparison among the cases with five, ten, and fifteen street canyons is outlined in Section 3.4. 3.1. Parametric study of void deck height This section discusses the parametric study by varying Hvd from 0 to 6 m while maintaining Hb at 24 m. Since our focus is on the pedestrianlevel wind speed, special attention is given to the wind distribution at z = 1.5 m. Fig. 8 plots the normalized stream-wise velocity (u/Uref) contours at the pedestrian level (xy-plane at z = 1.5 m, see Fig. 4 for the coordinate system). Overall, large u/Uref is observed near the first canyon, where the flow accelerates upon reaching the first building. Downstream of the first building, u/Uref decreases and can even become negative. Larger Hvd induces larger regions with high u/Uref. To analyze the results quantitatively, we plot the u/Uref profiles along the line that cuts through the canyons (y = 0) and the line along the street (y = Y = 10 m). The locations of y = 0 and y = Y are shown in Fig. 8(h). Fig. 9(a) plots the u/Uref profiles at y = 0. Recall that x = 0 corresponds to the upwind wall of the first building. For the profile of reference Case 1 without void decks (black line), there is no flow inside the buildings, as indicated by the white spaces in Fig. 8(a). Inside the canyons, u/Uref is negative, with canyon 1 having the largest magnitude of about 0.25. The magnitude of u/Uref decreases in the canyons downstream of canyon 1, with canyon 6 having the smallest u/Uref among all canyons. By introducing void decks with 2 m Hvd, the u/Uref profile (yellow dashed line) is continuous, since all buildings are “lifted up” by 2 m and the wind can flow underneath the buildings. This u/Uref profile shows that u/Uref is positive in most canyons. We will focus on the magnitude of u/Uref here and discuss the implication of the sign of u/Uref in the next paragraph. The magnitude of u/Uref in each canyon is relatively small at about 0.1. Increasing Hvd to 3 m significantly increases u/Uref magnitudes in the first ten canyons (blue line). Increasing Hvd to 4 m further increases u/Uref (red dashed line) and u/Uref is positive along the entire y = 0 line, except downstream of the last building. Similarly, increasing Hvd to 5 m (purple line) and 6 m (green dashed line) increases u/Uref along the whole y = 0 line. In general, Fig. 9 shows that larger Hvd induces higher u/Uref along the y = 0 line. In all cases, the first canyon has the largest magnitude of u/Uref, and the magnitude of u/Uref decreases downstream. An Hvd of 4 m or larger is required to maintain positive u/Uref along the line y = 0. We will now return to the discussion of the sign of u/Uref. Why do some regions in Fig. 9(a) observe negative u/Uref? To answer this question, we will refer to the contours and vectors of normalized velocity magnitude, Umag/Uref, on the xz-plane at y = 0. Fig. 3(a) shows that in Case 1, the flow turns downward at the pedestrian level upon reaching the first building. As the ground has a no-flux boundary condition (i.e., no wind can flow through it), the downward flow then turns span-wise into the street, which results in the accelerated flows near the side walls of the first buildings shown in Fig. 8(a). Fig. 3(a) also shows that in the first canyon, the wind at the pedestrian level is in the negative x-direction. This is induced by the downward flow along the windward wall of the second building. When this downward flow reaches the ground, part of it turns into the negative x-direction and induces negative u/Uref in the first canyon shown in Fig. 8(a). For Case 2, Fig. 3(b) shows that the void deck with Hvd = 2 m allows winds to flow through it. However, the jet that exits the first void deck loses its stream-wise momentum and turns vertically upward. In contrast, Fig. 3(c) shows that by increasing Hvd to 3 m, the jet exiting the first void deck continues to flow stream-wise and enters the second void deck. Similarly, Fig. 3(d)-(f) show that the jet exiting the first void deck enters the second void deck. In other words, Hvd = 2 m is insufficient to channel wind into the first canyon through the void deck. Increasing Hvd to 3 m increases the pedestrian-level Umag/Uref to about 0.25, while increasing Hvd to 4 m increases the pedestrian-level Umag/Uref to about 0.5 in the first canyon. Fig. 3(e) and (f) suggest that further increasing
3. Results and discussion With the validated full-scale CFD model, we conduct two parametric studies. Section 3.1 discusses the first study, which varies Hvd (Cases 2–6 in Table 1). Section 3.2 discusses the second study, which varies Hb (Case 4 and Cases 7–10 in Table 1). We then extend the study to arrays of buildings with non-uniform height, which is discussed in Section 3.3. Lastly, to verify that our results are applicable in an array with fewer than fifteen street canyons (all previous cases have fifteen street
Fig. 7. Vertical profiles of normalized stream-wise velocity at the middle lines of third to sixth canyons. The vertical distance from ground, z, is normalized by the effective canyon height, Heff = 0.14 cm, in the water channel experiment. 403
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Fig. 8. Stream-wise velocity contours at the pedestrian level (xy-plane at z = 1.5 m) for (a) Case 1, (b) Case 2, (c) Case 3, (d) Case 4, (e) Case 5, and (f) Case 6. The blocks indicate the location of buildings. For Case 1 without void decks, there is no velocity field where the buildings are located. (h) The locations of y = 0 and y = Y, where the velocity profiles are extracted for analysis.
void decks with 3 m Hvd further reduce the deceleration and maintain positive u/Uref along the street, except downstream of x/W = 28. The profiles from the cases with 4 m, 5 m, and 6 m Hvd have the same trend: larger Hvd induces higher u/Uref magnitude but the differences among these three cases are small. Overall, Hvd of 4 m is sufficient for maintaining high pedestrian-level wind speeds along the street. Increasing Hvd to 5 m and 6 m has diminishing gains in terms of wind speed enhancement. To summarize Section 3.1, the wind speed enhancement in the canyons (along the line y = 0) increases with increasing Hvd. The magnitude of u/Uref is the largest at the first canyon and decreases with downstream canyons. An Hvd of 4 m or larger is required to maintain positive u/Uref in an array with fifteen canyons. Similarly, along the street (y = Y), the magnitude of u/Uref is the largest near the first canyon and decreases downstream. An Hvd of 4 m or larger is required to maintain positive u/Uref along the street. 3.2. Parametric study of building height The previous section discusses the parametric study of Hvd while fixing Hb at 24 m. This section discusses the parametric study of Hb while maintaining Hvd at 4 m, which corresponds to Case 4 and Cases 7–10 in Table 1. Hb is varied from 24 to 48 m (with 6 m intervals), or equivalently, Heff is varied from 28 m to 52 m. The effective canyon aspect ratios range between 1.17 and 2.17. Fig. 10 plots the u/Uref contours at the pedestrian level at z = 1.5 m. Interestingly, Fig. 10(a)(e) show almost identical u/Uref contours when Hb is increased from 24 m to 48 m. As opposed to the significant changes in the u/Uref contours when Hvd is increased from 2 m to 6 m (see Fig. 8), the increase in Hb has a much weaker influence on u/Uref. Even at the largest Hb of 48 m in Case 10, the u/Uref contour is qualitatively similar to that in Case 4 with Hb of 24 m. The normalized stream-wise velocity along the line y = 0 and the line y = Y are plotted in Fig. 11. As discussed above, varying Hb has a much weaker effect on u/Uref than varying Hvd. This is confirmed by comparing Fig. 11 with Fig. 9. For example, Fig. 11(a) shows that doubling Hb from 24 m to 48 m only changes the u/Uref by about 10 percent along the line y = 0, but Fig. 9(a) shows that doubling Hvd from 3 m to 6 m increases u/Uref magnitude by more than twofold. Similarly, along the line y = Y, Fig. 11(b) shows that increasing Hb has nearly negligible effects on the u/Uref profiles. This is confirmed by the contours and vectors of Umag/Uref in Fig. 3(d) and (g)-(j), where increasing Hb does not significantly affect the strength of the jet induced into the
Fig. 9. Normalized stream-wise velocity profiles for Cases 1–6 at (a) y = 0 and (b) y = Y. See Fig. 8(h) for the locations of y = 0 and y = Y.
Hvd to 5 m and 6 m does not significantly increase the pedestrian-level Umag/Uref. Both Case 5 and Case 6 observe Umag/Uref of about 0.5 near the ground in the first canyon. To summarize, negative u/Uref indicates that the jet exiting a void deck loses momentum and can cause the pedestrian-level wind speed to reduce, as shown in Fig. 3(b). Therefore, positive u/Uref should be maintained in order to enhance pedestrianlevel wind speed. We have discussed the u/Uref profiles along the line y = 0. Next, we will refer to the u/Uref profiles along the street at the line y = Y plotted in Fig. 9(b). All cases have a similar u/Uref peak of about 0.8. The reference Case 1 without void decks observes the fastest deceleration of u/Uref. Although u/Uref in the canyons (y = 0) is negative for Case 1, u/ Uref is positive between x/W = 0 and x/W = 15 along the line y = Y. However, downstream of x/W = 15, u/Uref has become negative. This means that along a street flanked by buildings (without void decks) at both sides, the pedestrian-level wind decelerates rapidly and can become stagnant. Introducing void decks with 2 m Hvd significantly reduces the flow deceleration along the street, but Hvd of 2 m is still insufficient to sustain a positive u/Uref along the whole street. Similarly,
Fig. 10. Stream-wise velocity contours at the pedestrian level (xy-plane at z = 1.5 m) for (a) Case 4, (b) Case 7, (c) Case 8, (d) Case 9, and (e) Case 10. The blocks indicate the location of buildings.
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wind at the street next to the second building is accelerated, as shown in Fig. 13(a) and (d). Another example is Case 12 in Fig. 13(b), where flow acceleration is observed at the street next to the third building, which is taller than the second building. Nevertheless, the flow acceleration at the street due to non-uniform building height is limited to the first few canyons. The quantitative u/Uref profiles along the lines y = 0 and y = Y plotted in Fig. 14 will better illustrate this. Along the line y = 0, Fig. 14(a) shows that the u/Uref peaks depend on the height of the first building. Case 11 and Case 14 with shorter first buildings have smaller u/Uref peaks compared to the u/Uref peaks in Case 12 and Case 13 with taller first buildings. On the overall profiles, the variation of u/Uref in Fig. 14(a) is smaller than that in Fig. 9(a), but larger than that in Fig. 11(a). Therefore, the effects of non-uniform building height are less significant than the effects of varying Hvd, but more significant than the effects of varying Hb. Along the line y = Y, Fig. 14(b) shows that the u/Uref peaks also depend on the height of the first building. Case 11 and Case 14 with shorter first buildings have smaller u/Uref peaks compared to the u/Uref peaks in Case 12 and Case 13 with taller first buildings. Unlike the variation of u/Uref in Fig. 14(a) which can be significant along the whole line y = 0, the variation of u/Uref in Fig. 14(b) is significant only at the first five canyons. Downstream of canyon 5 (x/W = 10), the u/ Uref profiles show a much smaller variation compared to those upstream of canyon 5. This means that non-uniform height affects the pedestrianlevel wind speed along the street, but only at the first few canyons.
Fig. 11. Normalized stream-wise velocity profiles for Case 4 and Cases 7–10 at (a) y = 0 and (b) y = Y. See Fig. 8(h) for the locations of y = 0 and y = Y.
first void deck. This means that void decks are equally effective in channeling atmospheric winds into urban areas with tall buildings.
3.4. Number of canyons
3.3. Non-uniform building height
Our CFD domain consists of fifteen canyons. A question to ponder is whether the simulation results are applicable if there are fewer than fifteen canyons. For example, if there are only ten canyons in an urban area, can we ignore the last five canyons from the simulation results with fifteen canyons and refer to the results from the first ten canyons? In other words, do the conditions downstream of the last canyon affect the flows upstream of the last canyon? We answer this question by repeating the simulations of Case 4 with five canyons and ten canyons. The initial and boundary conditions are identical for the three cases. All three simulations have the outflow boundary located 15Heff from the last building. Fig. 15 plots the simulation results with five, ten, and fifteen canyons at lines y = 0 and y = Y. Comparing the results between the simulations with ten canyons and fifteen canyons, the u/Uref profiles are identical, except the last canyon. The last canyon in the simulation with ten canyons exhibits slightly higher u/Uref than that of the simulation with fifteen canyons. Similarly, comparing the results between the simulations with five canyons and fifteen canyons, the u/Uref profiles are identical, except the last canyon. Overall, the u/Uref profiles from the three simulations show negligible difference. Therefore, the simulation results with fifteen canyons are applicable in an array with fewer than fifteen canyons.
The previous section infers that the pedestrian-level wind speed is insensitive to Hb. All cases studied thus far have uniform Hb. However, most urban areas have non-uniform Hb. Does non-uniform Hb affect the performance of void decks to increase pedestrian-level wind speeds? To answer this question, we perform four additional simulations for arrays of buildings with non-uniform Hb, namely Cases 11–14. Fig. 12 depicts the height distribution of these cases. Case 11 has staggered Hb of 24 and 48 m, meaning that the first building has Hb of 24 m, the second building has Hb of 48 m, the third building has Hb of 24 m, and so on, as shown in Fig. 12(a). Case 12 has staggered Hb of 48 and 24 m, as shown in Fig. 12(b). For Case 13 and Case 14, each building has a random Hb of either 24 m, 30 m, 36 m, or 48 m, as shown in Fig. 12(c) and (d). All four cases in Fig. 12 have Hvd of 4 m. Fig. 13 plots the u/Uref contours at the pedestrian level at z = 1.5 m. Comparing Fig. 13(a) with Fig. 13(b), a taller first building induces higher wind speeds in the first void deck. We also see this by comparing Fig. 13(c) with Fig. 13(d), where Case 13 with a taller first building has higher wind speeds in the first void deck compared to that in Case 14 with a shorter first building. Another observation is that a taller building downstream of a shorter building can induce higher wind speeds at the street next to the taller building. For example, the second building is taller than the first building in Case 11 and Case 14. The
Fig. 12. Distribution of the building height for (a) Case 11, (b) Case 12, (c) Case 13, and (d) Case 14. See the text for the building height in each case.
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Fig. 13. Stream-wise velocity contours at the pedestrian level (xy-plane at z = 1.5 m) for (a) Case 11, (b) Case 12, (c) Case 13, and (d) Case 14. The blocks indicate the locations of buildings.
enhancement effects are expected to be lower than those in this study, which assumes no roughness in the void decks. The wind enhancement effects in our study can be considered as the upper limit of enhancement. We have considered only one wind direction, which limits the applications of this study to urban areas with prevailing wind directions aligned with the street orientation. In addition, only the aligned layout of buildings (i.e., a downstream building is directly behind the upstream building) is studied. Since the staggered layout induces larger blockage effects [49], we expect void decks to be more effective. However, this is not proven and can be included in future studies. The parametric studies on void deck height and building height are certainly not a complete study that includes all possible parameters. Many other parameters (e.g., street pattern, building density and urbanization pattern [50–53]) are not varied in our studies and could be studied in the future. In addition, real urban areas are often studied in wind tunnels to identify regions with low and high wind speeds. The potential of void decks for wind enhancement in real urban areas can be explored by conducting experiments with and without the voids, e.g., the case studies in Ng et al. [26] and Adelia et al. [54]. Our first-of-a-kind study of void decks in multiple three-dimensional buildings proves that void decks effectively enhance pedestrian-level wind speed in urban areas. This can lead to applications in real built environments. For example, trees are understood to obstruct winds and lead to higher pollutant concentrations in canyons [55,56]. Void decks can counter the obstruction effects by increasing the porosity at the pedestrian level, thereby maintaining the vegetation coverage in urban areas.
Fig. 14. Normalized stream-wise velocity profiles for Cases 11–14 at (a) y = 0 and (b) y = Y. See Fig. 8(h) for the locations of y = 0 and y = Y.
5. Conclusions We conducted CFD simulations to evaluate pedestrian-level wind speed enhancement with void decks (voids at the ground floors of buildings, see Fig. 1(a) for a photo of a building with void deck). Void decks introduce porosity at the ground level and allow wind to flow through them, thereby enhancing pedestrian-level wind speeds both in the urban street canyons and along the streets. Three observations are reported based on the CFD results:
• The void deck height significantly influences the wind enhance•
Fig. 15. Normalized stream-wise velocity profiles comparing simulations with five, ten, and fifteen canyons at (a) y = 0 and (b) y = Y. See Fig. 8(h) for the locations of y = 0 and y = Y.
•
4. Limitations and future works
ment. Taller void decks are more effective to channel wind into urban street canyons. The building height has a small influence on the wind enhancement. Doubling the building height from 24 m to 48 m causes only a 10 percent change in the overall pedestrian-level wind speeds. Non-uniform building height does not significantly influence the wind enhancement.
With the above observations, we conclude that void decks are an effective architectural feature for pedestrian-level wind speed enhancement, both in the urban street canyon and along the street. The wind enhancement effects depend strongly on the void deck height but not the building height. The wind enhancement effects are insensitive to non-uniform building height. Therefore, void decks are equally effective to enhance pedestrian-level wind speeds in urban street canyons with non-uniform height.
This study adopts an idealized building shape and assumes smooth exterior building surfaces. In a real built environment, building shapes are often complicated. The roughness elements on or near the exterior building surfaces, such as balconies and air-conditioner units, can affect the flow fields. This study does not consider the roughness in the void decks. For example, the pillars in Fig. 1(a) are not explicitly modeled. Roughness in the void decks adds resistance to the flow, thus the wind 406
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Declarations of interest
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Pedestrian-level wind speed enhancement with void decks in three-dimensional urban street canyons
T
Lup Wai Chewa,∗, Leslie K. Norfordb a b
Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, 02139, USA Department of Architecture, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, 02139, USA
ARTICLE INFO
ABSTRACT
Keywords: Void ground floor Building porosity CFD simulation Wind field modification Non-uniform building height
Cities suffer from low wind speeds due to the blockage effects of urban structures. Low wind speeds inhibit a city's ability to self-ventilate and reduce thermal comfort in tropical cities. Building porosity has been shown to be an effective architectural feature to channel winds into the pedestrian level. However, previous studies have focused on building porosity in single buildings or two-dimensional urban street canyons. We extended the investigations of building porosity to three-dimensional urban street canyons with computational fluid dynamics simulations. Validated numerical models were used to conduct parametric studies on void decks (empty spaces at the ground floors). By allowing winds to flow through them, void decks can enhance pedestrian-level wind speeds by more than twofold compared to the reference case without void decks. The wind enhancement effect is the strongest at the first canyon and decreases downstream. The effectiveness of a void deck is greatly improved by increasing the void deck height. The average wind speed in the first canyon increases from 13 percent to 59 percent (of the freestream wind speed) by increasing the void deck height from 2 m to 6 m. On the other hand, the building height and canyon (height-to-width) aspect ratio have a much smaller influence on void-deck effectiveness. Non-uniform building height in an array of buildings also has a minor influence on the effectiveness. Therefore, void decks are equally effective to enhance pedestrian-level wind speeds in urban areas with tall buildings and buildings with non-uniform height.
1. Introduction Urban areas have reduced wind speeds compared to rural areas due to the blockage effects of urban structures [1–4]. Although wind speed reduction is favorable during winters or cold days in temperate climates [5,6], the opposite is true for tropical climates. Cities in the tropics rarely suffer from thermal discomfort due to wind chill [7,8]. Increasing wind speed can improve outdoor thermal comfort by increasing convective heat transfer from human bodies. For example, a field study in Singapore concluded that increasing the wind speed is the most effective method to improve thermal comfort in shaded outdoor areas [9]. Another field survey in Hong Kong found that respondents who felt warm desired stronger winds, as expected [10]. Besides thermal comfort improvement, stronger winds also help urban areas to self-ventilate and remove pollutants [11,12]. There are several strategies to increase wind speeds in urban areas. At the building scale, architectural features such as corner modifications can influence the wind field near buildings [13,14]. Roof shape and roof roughness affect the wind field by modifying the wake
∗
downstream of the buildings [15–17]. Wind catchers, commonly used to enhance indoor ventilation [18,19], have recently been shown to be applicable for outdoor wind speed enhancement [20,21]. Introducing building porosity can also enhance wind speed by reducing the blockage effects of buildings [22,23]. At the neighborhood scale, building layout plays an important role in altering the wind fields. For example, non-parallel buildings can accelerate wind flowing through reduced area between the buildings [24,25], while aligning open spaces with the prevailing wind direction helps to funnel atmospheric wind [26]. Building porosity also helps to increase wind speeds at the neighborhood scale [27]. As building porosity is an effective method to channel winds at both building scale and neighborhood scale, we will focus on building porosity for wind speed enhancement. Building porosity at the pedestrian level (1–2 m above the ground level) greatly enhances airflow [23], while podia obstruct winds [28] and should be removed or shrunk [26,29]. Xia et al. [30] conducted wind tunnel experiments and found that a lift-up building supported on cores (pillars) enhances the surrounding pedestrian-level wind by up to 11 percent. Du et al. [31] tested different configurations of lift-up
Corresponding author. 77 Massachusetts Avenue, 5-418, Cambridge, MA, 02139, USA. E-mail address: [email protected] (L.W. Chew).
https://doi.org/10.1016/j.buildenv.2019.03.058 Received 1 January 2019; Received in revised form 25 March 2019; Accepted 27 March 2019 Available online 30 March 2019 0360-1323/ Published by Elsevier Ltd.
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Fig. 1. (a) A void deck (indicated by the dash-line box) of a public flat in Singapore. (b) A new design of a public flat with shops replacing void decks at the ground floor (indicated by the dash-line box). Photographs by the authors.
buildings and reported more than twofold increase of pedestrian-level wind speed. Subsequent studies of lift-up buildings reveal that the core height, core planar area, and building height-to-width aspect ratio can significantly affect the wind enhancement effects [14,32,33]. The above studies of lift-up buildings have only considered single buildings. Our previous study with multiple buildings has shown that voids at the ground floor can enhance pedestrian-level wind speed by up to twofold in an array of two-dimensional urban street canyons [27]. This paper extends the study to a more realistic urban setting using three-dimensional buildings with non-uniform height. Voids at the ground floor, or void decks in local terms, are a common architectural feature of public flats in Singapore. Fig. 1(a) shows an example of a flat with void decks. Void decks provide spaces for social activities and events, such as weddings and funeral wakes [34,35]. Void decks also improve walkability in a neighborhood by allowing pedestrians to walk “through” a building (underneath it) instead of going around it. Despite these benefits, new designs of public flats in Singapore have either much smaller or no void decks [36], as shown in Fig. 1(b), where the void decks have been replaced with shops. Social issues aside, “filling” the void decks could reduce pedestrian-level wind speed. This study aims to quantify the wind enhancement effects of void decks.
void deck height, Hvd, and canyon width, W (only two buildings are shown). The effective height, Heff, equals Hvd + Hb. The building geometry in the reference case (without void deck) follows that in our previous work [27], where Hb = 24 m and W = 24 m and the canyon aspect ratio is therefore 1.0. The building span-wise length (in the y direction) is fixed at 10 m for all cases. The street width between buildings is also fixed at 10 m. The incoming wind is in the positive x direction, and z represents the vertical direction. Ten simulations are conducted at different Hb and Hvd while W is fixed at 24 m. Table 1 summarizes the parameters in each case. All cases consist of fifteen canyons in the domain. Fig. 3 shows the relative sizes of the buildings and void decks in each case (the buildings are the white boxes, ignore the velocity contours for now, they will be discussed in Section 3). Case 1 is the reference case with Hb = W = 24 m and no void deck (Hvd = 0). Cases 2–6 are used for the parametric study of Hvd. Case 2 has all buildings “lifted up” by 2 m (Hvd = 2 m) while Hb is fixed at 24 m, so Heff = 26 m. Cases 3, 4, 5, and 6 have Hb fixed at 24 m but Hvd is increased to 3 m, 4 m, 5 m, and 6 m, respectively. Case 4 and Cases 7–10 are used for the parametric study of Hb, where Hb is varied from 24 m to 48 m while Hvd is fixed at 4 m. 2.1. Model setup and boundary conditions The computational fluid dynamics (CFD) model is constructed with ANSYS DesignModeler and meshed with ANSYS Meshing, both in the ANSYS Workbench package 17.2 [37]. The sizes of the CFD domains
2. Numerical models We model buildings with void decks as idealized blocks with empty spaces below. Fig. 2 shows the notations for the building height, Hb,
Table 1 The building height (Hb), void deck height (Hvd), effective canyon height (Heff), canyon width (W), and effective canyon aspect ratio (Heff/W) of the ten simulated cases.
Fig. 2. The building height, Hb, void deck height, Hvd, and canyon width, W, for (a) the reference case without void deck (i.e., Hvd = 0) and (b) the case with void deck. 400
Case
Hb (m)
Hvd (m)
Heff (m)
W (m)
Heff/W
1 2 3 4 5 6 7 8 9 10
24 24 24 24 24 24 30 36 42 48
0 2 3 4 5 6 4 4 4 4
24 26 27 28 29 30 34 40 46 52
24 24 24 24 24 24 24 24 24 24
1.00 1.08 1.13 1.17 1.21 1.25 1.42 1.67 1.92 2.17
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Fig. 3. Contours and vectors of normalized velocity magnitude at the middle span-wise plane (xz-plane at y = 0 m) for (a) Case 1, (b) Case 2, (c) Case 3, (d) Case 4, (e) Case 5, (f) Case 6, (g) Case 7, (h) Case 8, (i) Case 9, and (j) Case 10. Only the first canyon is shown in each case. Fig. 4. The CFD domain and boundary conditions for Case 1. The span-wise faces are transparent and the outlet is not shown. The coordinate system has x = 0 at the first building (not at the inlet) and z = 0 at the ground level (only the first seven buildings are shown). The inset shows the mesh of Case 4, where the grid resolution is the finest inside the canyons and void decks (only the first two buildings are shown).
follow the recommendations in Franke et al. [38]: the inlet is 5Heff upstream of the first building, the top surface is 6Heff from the ground, and the outlet surface is 15Heff downstream from the last building. Fig. 4 shows the CFD domain of Case 1. Note that only the first seven buildings are shown so the outlet surface is not shown. The inset shows the mesh of the first two buildings of Case 4. The grid size is W/30 inside the canyons and void decks. The mesh is coarsened above the canyons, upstream of the first building, and downstream of the last building. The maximum grid expansion ratio is 1.2 and the maximum grid size is 0.58W. The span-wise direction has 24 uniform grids. All
grids are perfectly orthogonal with zero skewness. The boundary conditions are as follows. The top surface and the span-wise faces have a symmetry boundary condition. All walls (leeward walls, windward walls, roofs, grounds) have a no-slip boundary condition. The outlet surface has a zero-gradient boundary condition. The set of atmospheric inlet boundary conditions [39] is prescribed at the inlet surface, where the inlet velocity (U), turbulence kinetic energy (tke), and turbulence dissipation rate (ε) are given in Eqs. (1)–(3):
U=
401
U
ln
z + z0 z0
(1)
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tke =
U2 Cµ
(2)
U3 (z + z 0 )
=
(3)
Here U is the friction velocity given in Eq. (4), κ the von Karman's constant (= 0.41), z the vertical distance from the ground, z0 the surface roughness height, and Cµ a constant (= 0.09).
U =
U (z ref ) ln
(
zref + z 0 z0
)
(4)
Eqs. (1) and (4) can be simplified as a dimensionless velocity profile:
( ) ( ) z+z
ln z 0 U 0 = zref + z 0 U (z ref ) ln z 0
(5)
Fig. 5. Comparison of (a) normalized stream-wise velocity and (b) normalized turbulence kinetic energy for Case 4 in Table 1 at three mesh resolutions. The profiles are extracted at the pedestrian level (z = 1.5 m) on the middle spanwise plane (y = 0), and x/W = 0 corresponds to the windward wall of the first building (see Fig. 4 for the coordinate system).
We take z0 = 0.04 m and zref = 28 m, following our previous works for two-dimensional canyons with void decks [27]. Note that z0 = 0.04 m corresponds to open terrain in the updated DavenportWieringa classification [40]. U(zref) is a reference velocity at zref and is set at 2 m s−1, as field measurements have found that a threshold wind speed of 1.5–2 m s−1 is needed to sustain in-canyon vortices [41,42]. All simulations are conducted with the finite volume solver Open Field Operation and Manipulation (OpenFOAM) version 3.0.1 [43]. The built-in steady Reynolds-averaged Navier-Stokes (RANS) solver, "simpleFOAM," is used with the SIMPLE (Semi-Implicit Method for Pressure Linked Equations) pressure-velocity coupling and k-ε turbulence closure. The three most commonly used k-ε closure schemes are the standard k-ε, Re-Normalization Group (RNG) k-ε, and realizable k-ε. Hang et al. [44] reported that their simulation result from the standard k-ε agrees better than that from the RNG k-ε simulations. Chew et al. [27] compared the standard k-ε and realizable k-ε schemes and found that the former agreed better with their experimental results. Therefore, the standard k-ε is chosen for this study. Second order Gaussian integration with linear interpolation is used for all gradient and divergence schemes. All Laplacian schemes are based on Gaussian integration with linear interpolation and non-orthogonal correction. The standard wall function is employed to reduce computational cost by allowing a coarser mesh. The tolerance of residuals is set at 10−5 for all parameters, and iterations are continued until all residuals stop decreasing with further iterations [45]. The simulation results are postprocessed with the open-source software ParaView version 5.3.0 [46].
2.3. Model validation There is no experimental data for three-dimensional models with void decks. We follow the recommendation in Blocken [45] to divide the model validation over two sub-configurations. The first sub-configuration of three-dimensional buildings is validated with the wind tunnel experiment in Hang et al. [44] with twelve rows of nine rectangular model blocks. The second sub-configuration of two-dimensional buildings with void decks is validated with our previous water channel experiment [27] with an array of eight lift-up buildings. For the first sub-configuration, Fig. 6 compares the normalized velocity magnitude and turbulence kinetic energy (tke) profiles between our CFD results and the wind tunnel measurements. The profiles are taken along the middle street (i.e., between the sixth and the seventh
2.2. Mesh independence study Three simulations are conducted for Case 4 in Table 1. The coarse, normal, and fine mesh models have 0.36, 1.2, and 4.3 million grids, respectively. The mesh shown in Fig. 4 inset is from the normal mesh model. Fig. 5 plots the normalized profiles at the pedestrian level (z = 1.5 m) on the middle span-wise plane (y = 0). The stream-wise velocity, u, and the turbulence kinetic energy, tke, are normalized by a reference velocity, Uref, which is the freestream velocity at z = 2Heff. The horizontal distance, x, is normalized by the canyon width, W. Fig. 5(a) shows a small deviation of u/Uref between the coarse and normal mesh models. Increasing the mesh resolution from “normal” to “fine” does not change the overall profile. The tke profiles in Fig. 5(b) show negligible difference among the three mesh resolutions, except for the peaks near x/W = 0. However, this difference does not affect the tke profiles downstream (x/W > 0). Therefore, the model with the normal mesh resolution has achieved a mesh-independent solution. All subsequent simulations are conducted with the normal mesh resolution.
Fig. 6. Along-street horizontal profiles of (a) normalized velocity magnitude and (b) normalized turbulence kinetic energy at elevation z/B = 1.0, where B is the building width. B = 30 mm in the wind tunnel experiment and the reducedscale CFD, and B = 6 m in the full-scale CFD. Solid lines represent our reducedscale CFD simulation and dotted lines represent our full-scale CFD simulation. Both the experimental data and the dashed-line CFD profiles are from Hang et al. [44]. 402
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rows of buildings) at elevation z/B = 1.0, where B is the building width (30 mm in the wind tunnel experiment and the reduced-scale CFD, and 6 m in the full-scale CFD). The CFD simulation from Hang et al. [44] is also included for comparison. Both CFD results agree well with the experiments, except that the location of the peak tke is predicted upstream compared to the location of peak tke in the wind tunnel experiment. The normalized mean-square error of our CFD results are 0.015 for velocity and 0.411 for tke, both within the acceptable range suggested by Hanna and Chang [47]. In the wind tunnel experiment, the building width, B = 30 mm, was adopted as the length scale. Chew et al. [48] highlighted that reduced-scale experiments or simulations should achieve a sufficiently high Reynolds number to ensure Reynolds number independent flows. In other words, the reduced-scale experiments or simulations should reproduce the flows at full scale to be valid for applications in real built environments. Therefore, we repeat the simulation at full scale by scaling up the buildings by 200 times (where B = 6 m at full scale). The Reynolds numbers at reduced scale and full scales are 8000 and 1.7 × 106, respectively. The reduced-scale simulation (red lines) predicts the same trend as the full-scale simulation (black dotted lines), verifying that the reduced-scale experiments and CFD simulations are applicable at full scale. All subsequent simulations are conducted at full scale. For the second sub-configuration, the vertical profiles at the middle of third to sixth canyons (canyons 3–6) are provided by the water channel experiment. Fig. 7 compares the normalized stream-wise velocity profiles between our CFD result and the experiment. All buildings have Hvd of 2 cm and Hb of 12 cm (see Fig. 2 for the nomenclature) so the void decks are 0.14Heff in height. These void decks allow flows to pass through them, thereby inducing high near-ground velocities between z/H = 0 and z/H = 0.14. Canyon 3 has the largest near-ground velocity, and canyon 6 has the lowest near-ground velocity. The CFD model correctly predicts both the near-ground flows induced by the void decks and the trend of decreasing velocity at downstream canyons.
canyons), we repeat the simulations of Case 4 with five and ten canyons. The comparison among the cases with five, ten, and fifteen street canyons is outlined in Section 3.4. 3.1. Parametric study of void deck height This section discusses the parametric study by varying Hvd from 0 to 6 m while maintaining Hb at 24 m. Since our focus is on the pedestrianlevel wind speed, special attention is given to the wind distribution at z = 1.5 m. Fig. 8 plots the normalized stream-wise velocity (u/Uref) contours at the pedestrian level (xy-plane at z = 1.5 m, see Fig. 4 for the coordinate system). Overall, large u/Uref is observed near the first canyon, where the flow accelerates upon reaching the first building. Downstream of the first building, u/Uref decreases and can even become negative. Larger Hvd induces larger regions with high u/Uref. To analyze the results quantitatively, we plot the u/Uref profiles along the line that cuts through the canyons (y = 0) and the line along the street (y = Y = 10 m). The locations of y = 0 and y = Y are shown in Fig. 8(h). Fig. 9(a) plots the u/Uref profiles at y = 0. Recall that x = 0 corresponds to the upwind wall of the first building. For the profile of reference Case 1 without void decks (black line), there is no flow inside the buildings, as indicated by the white spaces in Fig. 8(a). Inside the canyons, u/Uref is negative, with canyon 1 having the largest magnitude of about 0.25. The magnitude of u/Uref decreases in the canyons downstream of canyon 1, with canyon 6 having the smallest u/Uref among all canyons. By introducing void decks with 2 m Hvd, the u/Uref profile (yellow dashed line) is continuous, since all buildings are “lifted up” by 2 m and the wind can flow underneath the buildings. This u/Uref profile shows that u/Uref is positive in most canyons. We will focus on the magnitude of u/Uref here and discuss the implication of the sign of u/Uref in the next paragraph. The magnitude of u/Uref in each canyon is relatively small at about 0.1. Increasing Hvd to 3 m significantly increases u/Uref magnitudes in the first ten canyons (blue line). Increasing Hvd to 4 m further increases u/Uref (red dashed line) and u/Uref is positive along the entire y = 0 line, except downstream of the last building. Similarly, increasing Hvd to 5 m (purple line) and 6 m (green dashed line) increases u/Uref along the whole y = 0 line. In general, Fig. 9 shows that larger Hvd induces higher u/Uref along the y = 0 line. In all cases, the first canyon has the largest magnitude of u/Uref, and the magnitude of u/Uref decreases downstream. An Hvd of 4 m or larger is required to maintain positive u/Uref along the line y = 0. We will now return to the discussion of the sign of u/Uref. Why do some regions in Fig. 9(a) observe negative u/Uref? To answer this question, we will refer to the contours and vectors of normalized velocity magnitude, Umag/Uref, on the xz-plane at y = 0. Fig. 3(a) shows that in Case 1, the flow turns downward at the pedestrian level upon reaching the first building. As the ground has a no-flux boundary condition (i.e., no wind can flow through it), the downward flow then turns span-wise into the street, which results in the accelerated flows near the side walls of the first buildings shown in Fig. 8(a). Fig. 3(a) also shows that in the first canyon, the wind at the pedestrian level is in the negative x-direction. This is induced by the downward flow along the windward wall of the second building. When this downward flow reaches the ground, part of it turns into the negative x-direction and induces negative u/Uref in the first canyon shown in Fig. 8(a). For Case 2, Fig. 3(b) shows that the void deck with Hvd = 2 m allows winds to flow through it. However, the jet that exits the first void deck loses its stream-wise momentum and turns vertically upward. In contrast, Fig. 3(c) shows that by increasing Hvd to 3 m, the jet exiting the first void deck continues to flow stream-wise and enters the second void deck. Similarly, Fig. 3(d)-(f) show that the jet exiting the first void deck enters the second void deck. In other words, Hvd = 2 m is insufficient to channel wind into the first canyon through the void deck. Increasing Hvd to 3 m increases the pedestrian-level Umag/Uref to about 0.25, while increasing Hvd to 4 m increases the pedestrian-level Umag/Uref to about 0.5 in the first canyon. Fig. 3(e) and (f) suggest that further increasing
3. Results and discussion With the validated full-scale CFD model, we conduct two parametric studies. Section 3.1 discusses the first study, which varies Hvd (Cases 2–6 in Table 1). Section 3.2 discusses the second study, which varies Hb (Case 4 and Cases 7–10 in Table 1). We then extend the study to arrays of buildings with non-uniform height, which is discussed in Section 3.3. Lastly, to verify that our results are applicable in an array with fewer than fifteen street canyons (all previous cases have fifteen street
Fig. 7. Vertical profiles of normalized stream-wise velocity at the middle lines of third to sixth canyons. The vertical distance from ground, z, is normalized by the effective canyon height, Heff = 0.14 cm, in the water channel experiment. 403
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Fig. 8. Stream-wise velocity contours at the pedestrian level (xy-plane at z = 1.5 m) for (a) Case 1, (b) Case 2, (c) Case 3, (d) Case 4, (e) Case 5, and (f) Case 6. The blocks indicate the location of buildings. For Case 1 without void decks, there is no velocity field where the buildings are located. (h) The locations of y = 0 and y = Y, where the velocity profiles are extracted for analysis.
void decks with 3 m Hvd further reduce the deceleration and maintain positive u/Uref along the street, except downstream of x/W = 28. The profiles from the cases with 4 m, 5 m, and 6 m Hvd have the same trend: larger Hvd induces higher u/Uref magnitude but the differences among these three cases are small. Overall, Hvd of 4 m is sufficient for maintaining high pedestrian-level wind speeds along the street. Increasing Hvd to 5 m and 6 m has diminishing gains in terms of wind speed enhancement. To summarize Section 3.1, the wind speed enhancement in the canyons (along the line y = 0) increases with increasing Hvd. The magnitude of u/Uref is the largest at the first canyon and decreases with downstream canyons. An Hvd of 4 m or larger is required to maintain positive u/Uref in an array with fifteen canyons. Similarly, along the street (y = Y), the magnitude of u/Uref is the largest near the first canyon and decreases downstream. An Hvd of 4 m or larger is required to maintain positive u/Uref along the street. 3.2. Parametric study of building height The previous section discusses the parametric study of Hvd while fixing Hb at 24 m. This section discusses the parametric study of Hb while maintaining Hvd at 4 m, which corresponds to Case 4 and Cases 7–10 in Table 1. Hb is varied from 24 to 48 m (with 6 m intervals), or equivalently, Heff is varied from 28 m to 52 m. The effective canyon aspect ratios range between 1.17 and 2.17. Fig. 10 plots the u/Uref contours at the pedestrian level at z = 1.5 m. Interestingly, Fig. 10(a)(e) show almost identical u/Uref contours when Hb is increased from 24 m to 48 m. As opposed to the significant changes in the u/Uref contours when Hvd is increased from 2 m to 6 m (see Fig. 8), the increase in Hb has a much weaker influence on u/Uref. Even at the largest Hb of 48 m in Case 10, the u/Uref contour is qualitatively similar to that in Case 4 with Hb of 24 m. The normalized stream-wise velocity along the line y = 0 and the line y = Y are plotted in Fig. 11. As discussed above, varying Hb has a much weaker effect on u/Uref than varying Hvd. This is confirmed by comparing Fig. 11 with Fig. 9. For example, Fig. 11(a) shows that doubling Hb from 24 m to 48 m only changes the u/Uref by about 10 percent along the line y = 0, but Fig. 9(a) shows that doubling Hvd from 3 m to 6 m increases u/Uref magnitude by more than twofold. Similarly, along the line y = Y, Fig. 11(b) shows that increasing Hb has nearly negligible effects on the u/Uref profiles. This is confirmed by the contours and vectors of Umag/Uref in Fig. 3(d) and (g)-(j), where increasing Hb does not significantly affect the strength of the jet induced into the
Fig. 9. Normalized stream-wise velocity profiles for Cases 1–6 at (a) y = 0 and (b) y = Y. See Fig. 8(h) for the locations of y = 0 and y = Y.
Hvd to 5 m and 6 m does not significantly increase the pedestrian-level Umag/Uref. Both Case 5 and Case 6 observe Umag/Uref of about 0.5 near the ground in the first canyon. To summarize, negative u/Uref indicates that the jet exiting a void deck loses momentum and can cause the pedestrian-level wind speed to reduce, as shown in Fig. 3(b). Therefore, positive u/Uref should be maintained in order to enhance pedestrianlevel wind speed. We have discussed the u/Uref profiles along the line y = 0. Next, we will refer to the u/Uref profiles along the street at the line y = Y plotted in Fig. 9(b). All cases have a similar u/Uref peak of about 0.8. The reference Case 1 without void decks observes the fastest deceleration of u/Uref. Although u/Uref in the canyons (y = 0) is negative for Case 1, u/ Uref is positive between x/W = 0 and x/W = 15 along the line y = Y. However, downstream of x/W = 15, u/Uref has become negative. This means that along a street flanked by buildings (without void decks) at both sides, the pedestrian-level wind decelerates rapidly and can become stagnant. Introducing void decks with 2 m Hvd significantly reduces the flow deceleration along the street, but Hvd of 2 m is still insufficient to sustain a positive u/Uref along the whole street. Similarly,
Fig. 10. Stream-wise velocity contours at the pedestrian level (xy-plane at z = 1.5 m) for (a) Case 4, (b) Case 7, (c) Case 8, (d) Case 9, and (e) Case 10. The blocks indicate the location of buildings.
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wind at the street next to the second building is accelerated, as shown in Fig. 13(a) and (d). Another example is Case 12 in Fig. 13(b), where flow acceleration is observed at the street next to the third building, which is taller than the second building. Nevertheless, the flow acceleration at the street due to non-uniform building height is limited to the first few canyons. The quantitative u/Uref profiles along the lines y = 0 and y = Y plotted in Fig. 14 will better illustrate this. Along the line y = 0, Fig. 14(a) shows that the u/Uref peaks depend on the height of the first building. Case 11 and Case 14 with shorter first buildings have smaller u/Uref peaks compared to the u/Uref peaks in Case 12 and Case 13 with taller first buildings. On the overall profiles, the variation of u/Uref in Fig. 14(a) is smaller than that in Fig. 9(a), but larger than that in Fig. 11(a). Therefore, the effects of non-uniform building height are less significant than the effects of varying Hvd, but more significant than the effects of varying Hb. Along the line y = Y, Fig. 14(b) shows that the u/Uref peaks also depend on the height of the first building. Case 11 and Case 14 with shorter first buildings have smaller u/Uref peaks compared to the u/Uref peaks in Case 12 and Case 13 with taller first buildings. Unlike the variation of u/Uref in Fig. 14(a) which can be significant along the whole line y = 0, the variation of u/Uref in Fig. 14(b) is significant only at the first five canyons. Downstream of canyon 5 (x/W = 10), the u/ Uref profiles show a much smaller variation compared to those upstream of canyon 5. This means that non-uniform height affects the pedestrianlevel wind speed along the street, but only at the first few canyons.
Fig. 11. Normalized stream-wise velocity profiles for Case 4 and Cases 7–10 at (a) y = 0 and (b) y = Y. See Fig. 8(h) for the locations of y = 0 and y = Y.
first void deck. This means that void decks are equally effective in channeling atmospheric winds into urban areas with tall buildings.
3.4. Number of canyons
3.3. Non-uniform building height
Our CFD domain consists of fifteen canyons. A question to ponder is whether the simulation results are applicable if there are fewer than fifteen canyons. For example, if there are only ten canyons in an urban area, can we ignore the last five canyons from the simulation results with fifteen canyons and refer to the results from the first ten canyons? In other words, do the conditions downstream of the last canyon affect the flows upstream of the last canyon? We answer this question by repeating the simulations of Case 4 with five canyons and ten canyons. The initial and boundary conditions are identical for the three cases. All three simulations have the outflow boundary located 15Heff from the last building. Fig. 15 plots the simulation results with five, ten, and fifteen canyons at lines y = 0 and y = Y. Comparing the results between the simulations with ten canyons and fifteen canyons, the u/Uref profiles are identical, except the last canyon. The last canyon in the simulation with ten canyons exhibits slightly higher u/Uref than that of the simulation with fifteen canyons. Similarly, comparing the results between the simulations with five canyons and fifteen canyons, the u/Uref profiles are identical, except the last canyon. Overall, the u/Uref profiles from the three simulations show negligible difference. Therefore, the simulation results with fifteen canyons are applicable in an array with fewer than fifteen canyons.
The previous section infers that the pedestrian-level wind speed is insensitive to Hb. All cases studied thus far have uniform Hb. However, most urban areas have non-uniform Hb. Does non-uniform Hb affect the performance of void decks to increase pedestrian-level wind speeds? To answer this question, we perform four additional simulations for arrays of buildings with non-uniform Hb, namely Cases 11–14. Fig. 12 depicts the height distribution of these cases. Case 11 has staggered Hb of 24 and 48 m, meaning that the first building has Hb of 24 m, the second building has Hb of 48 m, the third building has Hb of 24 m, and so on, as shown in Fig. 12(a). Case 12 has staggered Hb of 48 and 24 m, as shown in Fig. 12(b). For Case 13 and Case 14, each building has a random Hb of either 24 m, 30 m, 36 m, or 48 m, as shown in Fig. 12(c) and (d). All four cases in Fig. 12 have Hvd of 4 m. Fig. 13 plots the u/Uref contours at the pedestrian level at z = 1.5 m. Comparing Fig. 13(a) with Fig. 13(b), a taller first building induces higher wind speeds in the first void deck. We also see this by comparing Fig. 13(c) with Fig. 13(d), where Case 13 with a taller first building has higher wind speeds in the first void deck compared to that in Case 14 with a shorter first building. Another observation is that a taller building downstream of a shorter building can induce higher wind speeds at the street next to the taller building. For example, the second building is taller than the first building in Case 11 and Case 14. The
Fig. 12. Distribution of the building height for (a) Case 11, (b) Case 12, (c) Case 13, and (d) Case 14. See the text for the building height in each case.
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Fig. 13. Stream-wise velocity contours at the pedestrian level (xy-plane at z = 1.5 m) for (a) Case 11, (b) Case 12, (c) Case 13, and (d) Case 14. The blocks indicate the locations of buildings.
enhancement effects are expected to be lower than those in this study, which assumes no roughness in the void decks. The wind enhancement effects in our study can be considered as the upper limit of enhancement. We have considered only one wind direction, which limits the applications of this study to urban areas with prevailing wind directions aligned with the street orientation. In addition, only the aligned layout of buildings (i.e., a downstream building is directly behind the upstream building) is studied. Since the staggered layout induces larger blockage effects [49], we expect void decks to be more effective. However, this is not proven and can be included in future studies. The parametric studies on void deck height and building height are certainly not a complete study that includes all possible parameters. Many other parameters (e.g., street pattern, building density and urbanization pattern [50–53]) are not varied in our studies and could be studied in the future. In addition, real urban areas are often studied in wind tunnels to identify regions with low and high wind speeds. The potential of void decks for wind enhancement in real urban areas can be explored by conducting experiments with and without the voids, e.g., the case studies in Ng et al. [26] and Adelia et al. [54]. Our first-of-a-kind study of void decks in multiple three-dimensional buildings proves that void decks effectively enhance pedestrian-level wind speed in urban areas. This can lead to applications in real built environments. For example, trees are understood to obstruct winds and lead to higher pollutant concentrations in canyons [55,56]. Void decks can counter the obstruction effects by increasing the porosity at the pedestrian level, thereby maintaining the vegetation coverage in urban areas.
Fig. 14. Normalized stream-wise velocity profiles for Cases 11–14 at (a) y = 0 and (b) y = Y. See Fig. 8(h) for the locations of y = 0 and y = Y.
5. Conclusions We conducted CFD simulations to evaluate pedestrian-level wind speed enhancement with void decks (voids at the ground floors of buildings, see Fig. 1(a) for a photo of a building with void deck). Void decks introduce porosity at the ground level and allow wind to flow through them, thereby enhancing pedestrian-level wind speeds both in the urban street canyons and along the streets. Three observations are reported based on the CFD results:
• The void deck height significantly influences the wind enhance•
Fig. 15. Normalized stream-wise velocity profiles comparing simulations with five, ten, and fifteen canyons at (a) y = 0 and (b) y = Y. See Fig. 8(h) for the locations of y = 0 and y = Y.
•
4. Limitations and future works
ment. Taller void decks are more effective to channel wind into urban street canyons. The building height has a small influence on the wind enhancement. Doubling the building height from 24 m to 48 m causes only a 10 percent change in the overall pedestrian-level wind speeds. Non-uniform building height does not significantly influence the wind enhancement.
With the above observations, we conclude that void decks are an effective architectural feature for pedestrian-level wind speed enhancement, both in the urban street canyon and along the street. The wind enhancement effects depend strongly on the void deck height but not the building height. The wind enhancement effects are insensitive to non-uniform building height. Therefore, void decks are equally effective to enhance pedestrian-level wind speeds in urban street canyons with non-uniform height.
This study adopts an idealized building shape and assumes smooth exterior building surfaces. In a real built environment, building shapes are often complicated. The roughness elements on or near the exterior building surfaces, such as balconies and air-conditioner units, can affect the flow fields. This study does not consider the roughness in the void decks. For example, the pillars in Fig. 1(a) are not explicitly modeled. Roughness in the void decks adds resistance to the flow, thus the wind 406
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Declarations of interest
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