# Street-Level Ventilation in Hypothetical Urban Areas

^{*}

^{†}

## Abstract

**:**

## 1. Introduction

## 2. Methodology

#### 2.1. Wind Tunnel Infrastructure

#### 2.2. Idealized Urban Surfaces

#### 2.3. Hot-Wire Anemometry

## 3. Results and Discussion

#### 3.1. Thickness of TBL and ISL

#### 3.2. Friction Velocity

#### 3.3. Wind Speed Profiles

#### 3.4. Turbulence Profiles

#### 3.5. Street-Level Ventilation Estimate

#### 3.6. Quadrant Analysis

#### 3.7. Frequency Spectrum

## 4. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## References

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**Figure 1.**Apparatus used in the experiments. (

**a**) Wind tunnel infrastructure in the Department of Mechanical Engineering, University of Hong Kong; (

**b**) rough surfaces in the form of ribs in cross flows; and (

**c**) schematic of the flow configuration.

**Figure 2.**Dimensionless profiles of mean wind speed $\u2329\overline{u}\u232a/{U}_{\infty}$ in wall-normal direction $z/\delta $ over ribs of aspect ratio $\mathrm{AR}$ = 1:2 (□), 1:3 (Δ), 1:4 (∇), 1:5 (⊳), 1:6 (⊲), 1:8 (⋄), 1:10 (∘) and 1:12 (+) measured in the current wind tunnel experiments. Also shown are the LES results [55] for $\mathrm{AR}$ = 1:2 (——), 1:3 (------), 1:5 (-·-·-·-·-·-) and 1:10 (⋯⋯) together with the measurements available in literature for $\mathrm{AR}$ = 1:3 (▴) [56] and $\mathrm{AR}$ = 1:1 (▴ and ▾) [57].

**Figure 3.**Dimensionless profiles of (

**a**) streamwise ${\u2329\overline{{u}^{\u2033}{u}^{\u2033}}\u232a}^{1/2}/{u}_{\tau}$ and (

**b**) vertical ${\u2329\overline{{w}^{\u2033}{w}^{\u2033}}\u232a}^{1/2}/{u}_{\tau}$ fluctuating velocities in wall-normal direction $z/\delta $ over ribs of aspect ratio $\mathrm{AR}$ = 1:2 (□), 1:3 (Δ), 1:4 (∇), 1:5 (⊳), 1:6 (⊲), 1:8 (⋄), 1:10 (∘) and 1:12 (+) measured in the current wind tunnel experiments. Also shown are the LES results [55] for $\mathrm{AR}$ = 1:2 (——), 1:3 (------), 1:5 (-·-·-·-·-·-) and 1:10 (⋯⋯) together with the measurements available in literature for $\mathrm{AR}$ = 1:3 (▴) [56] and $\mathrm{AR}$ = 1:1 (▴) [57].

**Figure 4.**Dimensionless profiles of momentum flux $\u2329\overline{{u}^{\u2033}{w}^{\u2033}}\u232a/{u}_{\tau}^{2}$ in wall-normal direction $z/\delta $ over ribs of aspect ratio $\mathrm{AR}$ = 1:2 (□), 1:3 (Δ), 1:4 (∇), 1:5 (⊳), 1:6 (⊲), 1:8 (⋄), 1:10 (∘) and 1:12 (+) measured in the current wind tunnel experiments.

**Figure 5.**Vertical fluctuating velocity scale in the roughness sublayer (RSL) ${\widehat{w}}_{\mathrm{RSL}}^{\u2033}$ plotted against the square root of drag coefficient ${C}_{d}^{1/2}$. Also shown is the linear regression y = $2.0477x-0.181$ whose correlation coefficient is ${R}^{2}$ = $0.9193$.

**Figure 6.**Shaded contours of joint probability density function (JPDF) $P\left({u}^{\u2033},{w}^{\u2033}\right)$ and contours of covariance integrand ${u}^{\u2033}{w}^{\u2033}P\left({u}^{\u2033},{w}^{\u2033}\right)$ at roof level $z/h$ = $0.26$ over street canyons of hypothetical urban areas of AR = (

**a**) 1:2 and (

**b**) 1:12.

**Figure 7.**Frequency spectra of dimensionless streamwise $\mathsf{\Phi}\left({u}^{\u2033}{u}^{\u2033}/{u}_{\tau}^{2}\right)$ and vertical $\mathsf{\Phi}\left({w}^{\u2033}{w}^{\u2033}/{u}_{\tau}^{2}\right)$ turbulence intensities at roof level $z/h$ = $0.26$ over building roof (red) and street canyon center (green) over street canyons of hypothetical urban areas of AR = (

**a**) 1:2 and (

**b**) 1:12.

**Table 1.**Configuration of the idealized urban surfaces and the flows in the wind tunnel experiments.

Types of Idealized Urban Surface | |||||||||
---|---|---|---|---|---|---|---|---|---|

A | B | C | D | E | F | G | H | ||

Rib [mm] | Size h | 19 | 19 | 19 | 19 | 19 | 19 | 19 | 19 |

Separation b | 38 | 57 | 76 | 95 | 114 | 152 | 190 | 228 | |

Size of a repeating unit l (= h + b) [mm] | 57 | 76 | 95 | 114 | 133 | 171 | 209 | 247 | |

Aspect ratio $AR$ (= $h:b$) | 1:2 | 1:3 | 1:4 | 1:5 | 1:6 | 1:8 | 1:10 | 1:12 | |

Boundary layer thickness | $\delta $ [mm] | 244 | 248 | 283 | 284 | 294 | 294 | 304 | 304 |

$\delta /h$ | $12.84$ | $13.05$ | $14.89$ | $14.95$ | $15.47$ | $15.47$ | $16.00$ | $16.00$ | |

Sampling location | ${x}_{\mathrm{sample}}$ [mm] | 3705 | 3686 | 3609 | 3648 | 3590 | 3562 | 3571 | 3619 |

${x}_{\mathrm{sample}}/h$ | 195 | 194 | 190 | 192 | 189 | 187 | 188 | 190 | |

${x}_{\mathrm{sample}}/\delta $ | $15.18$ | $14.86$ | $12.75$ | $12.85$ | $12.21$ | $12.12$ | $11.75$ | $11.90$ | |

Number of profiles in a repeating unit | 7 | 7 | 7 | 7 | 7 | 9 | 9 | 9 | |

Velocity [m s${}^{-1}$] | Free-stream ${U}_{\infty}$ | $8.0$ | $8.4$ | $8.5$ | $8.5$ | $8.5$ | $8.4$ | $9.1$ | $9.0$ |

Mean ${U}_{\mathrm{mean}}$ | $6.6$ | $6.9$ | $7.3$ | $7.3$ | $7.4$ | $7.4$ | $8.2$ | $8.2$ | |

Friction velocity | ${u}_{\tau}$ [m s${}^{-1}$] | $0.453$ | $0.516$ | $0.556$ | $0.592$ | $0.598$ | $0.598$ | $0.645$ | $0.671$ |

${u}_{\tau}/{U}_{\infty}$ | $0.057$ | $0.062$ | $0.066$ | $0.069$ | $0.070$ | $0.071$ | $0.071$ | $0.074$ | |

${u}_{\tau}/{U}_{\mathrm{mean}}$ | $0.069$ | $0.075$ | $0.077$ | $0.081$ | $0.081$ | $0.081$ | $0.079$ | $0.082$ | |

Drag coefficient f (= $2{u}_{\tau}^{2}/{U}_{\mathrm{mean}}^{2}$) [$\times {10}^{-3}$] | $9.550$ | $1.112$ | $1.176$ | $1.316$ | $1.297$ | $1.319$ | $1.252$ | $1.336$ | |

Reynolds number | ${\mathrm{Re}}_{\delta}$ (= ${U}_{\infty}\delta /\nu $) | 195,168 | 207,495 | 239,729 | 242,190 | 250,614 | 247,520 | 276,800 | 278,400 |

${\mathrm{Re}}_{\infty}$ (= ${U}_{\infty}h/\nu $) | 15,200 | 15,900 | 16,100 | 16,200 | 16,200 | 16,000 | 17,300 | 17,400 | |

${\mathrm{Re}}_{\mathrm{mean}}$ (= ${U}_{\mathrm{mean}}h/\nu $) | 12,500 | 13,100 | 13,800 | 13,900 | 14,100 | 14,000 | 15,500 | 15,600 | |

${\mathrm{Re}}_{\tau}$ (= ${u}_{\tau}h/\nu $) | 864 | 983 | 1060 | 1127 | 1138 | 1138 | 1229 | 1277 |

Quadrants | Events | ${\mathit{u}}^{\u2033}$ | ${\mathit{w}}^{\u2033}$ |
---|---|---|---|

1 | Outward interaction | + | + |

2 | Ejection | − | + |

3 | Inward interaction | − | − |

4 | Sweep | + | − |

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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**MDPI and ACS Style**

Ho, Y.-K.; Liu, C.-H. Street-Level Ventilation in Hypothetical Urban Areas. *Atmosphere* **2017**, *8*, 124.
https://doi.org/10.3390/atmos8070124

**AMA Style**

Ho Y-K, Liu C-H. Street-Level Ventilation in Hypothetical Urban Areas. *Atmosphere*. 2017; 8(7):124.
https://doi.org/10.3390/atmos8070124

**Chicago/Turabian Style**

Ho, Yat-Kiu, and Chun-Ho Liu. 2017. "Street-Level Ventilation in Hypothetical Urban Areas" *Atmosphere* 8, no. 7: 124.
https://doi.org/10.3390/atmos8070124