Building Envelop and Fabric Design for Air Ventilation of High Density Cities. Area: Case Study in Hong Kong. CUHK ID1155011360 SENV 7001 Dissertation Report. Content. 1. 4. Methodology. 2. 5. Study Object. Case Study. 3. 6. Literature Review. Discussion.
Building Envelop and Fabric Design for Air Ventilation of High Density Cities
Area: Case Study in Hong Kong
CUHK ID1155011360 SENV 7001 Dissertation Report
Limitation & Further Study
On the climate prospect, in the warm humid environment as Hong Kong, the cooling strategy is more comprehensive. As for the evaporate cooling would not be applied to contribute to the thermal comfort at the pedestrian level due to the high humidity, natural ventilation would play the main role instead. As the more and more hot nights happen in Hong Kong, the cross ventilation design for the unit is required for cooling as well.
With the city development, the density of the buildings in Hong Kong is growing at the highest level. With the density grow, the blockage of wind, especially at the pedestrian level, become the most significant issue for the air ventilation in city of Hong Kong. Accordingly, improvements of building natural ventilation, especially in the high density districts in Hong Kong, in order to improve the human thermal comfort are on agenda.
A case study in Price Edward would be conducted to maximum external air ventilation on the site level, by considering the building design effect for the air ventilation at the district level at the same time, given the condition that the project is for a single building design, remains the street pattern of the district.
Definition of AVA
The concept of AVA, Air Ventilation Assessment is a comprehensive assessment system, for assessing “the wind availability and performance of a location could be experienced and enjoyed by pedestrians on ground taking into account the surrounding building and topography and the proposed development” on the site level and district level.
The Purpose for promoting Air Ventilation at pedestrian level:
Building Envelop Design for Natural Ventilation
- Climate Assessment for Natural Ventilation(20 to 25 degree Celsius, 20 %to 70% RH)
- Building envelop design for its climate types (Require Shaded veranda, Cross Ventilation, Ventilated roof space)
- Ventilation types for building design (by wind pressure difference and stack effect)
- Wind Velocity Ratio (including SVRw and LVRw)
- Retention time (Tr)
“the time for the pollutant released at street level to reach e-1(36.7%) of its original value, were computed from the history of tracer gas concentration following removal of a controlled tracer release. ”
- VASARI, FLUIENT, PLAM, PHOMENICS, TAS - AirPAK, Autodesk CFD
- Wind Tunnel, MISKAM, Digital Elevation Model(DEM), DIWIMO, KALM, STREET, MLuS-02, PROKAS
Technical Design Guideline:
China'sEcological Construction Residential Technology Assessment Manual which issued in 2011 and Green Building Technology Assessment System which issued in 2003 explicitly mentioned, ensure more than 75% of the plate construction have maintained 1.5 Pa pressure difference in summer, to avoid portioned whirlpool and dead ends, in order to make sure the effective natural ventilation in the indoor environment.
Address: No. 751, Nathan Road, Price Edward District, Kwun Ngan House, Price Edward Rd W, Kowloon City.
Lot Area: approximately 239 square meters, with the wide about 10.8 meters and length 22.2 meters.
Building Height: 48.6 m
Floor to floor height: 3.25 m
Original Accommodation Arrangement:
Fig.6 Original Ground Floor Plan
Fig.7 Floor Plan above the ground Floor for good storage
Fig. 9 Changed Standard Floor Plan (in reality)
Fig.8 Original Standard Floor Plan (on paper)
How to design the building envelop on this site, to achieve the thermal comfort at the pedestrian level?
If more buildings in the high density area could provide more building permeability, especially near the pedestrian, and the permeability space two adjacent buildings could be linked together, it may be able to achieve a breeze way without changing the street pattern which would be harder and requires more times.
How to improve the probability of wind for the flat external layout to achieve the cross ventilation?
If the redesign proposal could provide more scattered external flat Layout at the same time while making the more permeability in the building, it may be able to allow the wind to reach more facades of the units, in order to achieve more probability for the wind to penetrate the units, even achieve the cross ventilation for all units.
The street where the study project located, its street pattern is the most problematic, almost 90 degree to the annual prevailing wind direction, more than 30 degree to the annual prevailing wind direction.
Urban Climatic Map and Standards for Wind Environment – Feasibility Study
Field measurement :
Fig.14 Testo 400 Indoor Air Quality IAQ / HVAC Monitor
Field Measurement Data Analysis:
Due to the suitable temperature for natural ventilation is 20 to 25 degree Celsius , suitable relative humidity for natural ventilation is 20% to 70%, compared to the measurement data, the temperature and relative humidity inside the unit are suitable for natural ventilation in spring at Hong Kong, expected for the relative humidity in rest room is a little too high.
The wind environment is poor inside the unit.
Deep interviews of attendance:
Design consideration respond to the requirement in the interview,
A Performance Based Air Ventilation Assessment (AVA) Methodology
Fig 19 Performance Based AVA Methodology
Source: Police and technical guideline for urban planning of high-density cities – air ventilation assessment (AVA) of Hong Kong
MM5 Wind Modeling
Fig.21 Site Wind Availability Map of Hong Kong
Fig.22 Study Site Location
Fig.23 Wind Rose in 16 Direction of the study site
Table 13 Wind Probability Table
Source: Hong Kong Planning Department
CFD Wind Tunnel simulation
Vasari Wind Tunnel Tool in Project Vasari will be used in this study to resolve the meteorological wind availability data to site wind data by simulation.
But Its limitation is the accurate quantitative results are not available for output, which limited its usage to quick, initial, directional studies only
Fig.25 Vasari Wind Tunnel Tool
ECOTECT Weather Tool Analysis
Compared Fig.37 and Fig.38, we could tell the improvement result for the thermal comfort of natural ventilation contributed more than 90% in the whole multiple passive strategy which including passive solar heating, thermal mass effect, exposed mass & night-purge ventilation, natural ventilation, direct evaporated cooling, indirect evaporated cooling.
Fig 36. Passive strategy analysis
Fig.37 Multiple Passive Strategy Result Analysis
Fig.38 Natural Ventilation Result Analysis
Fig.23 Wind Rose in 16 Directions at 598m of the study site
Fig. 48 V infinity at 598m from16 directions of square 27,27
Though the annual prevailing wind at 598 meters is from the direction of 247.5 degree (East to North 22.5 degree) which the wind velocity of 8.92 meter/s.
Fig. 54 Redesign Flat Layout Function Analysis
Fig. 52 Original Flat Layout Function Analysis
Fig. 53 Detail Original Flat External Layout
Fig. 55 Detail Redesign Flat External Layout
Original Building Model:
Initial Design Proposal:
Fig. 59 Redesigned Building Model in the district
Fig.58 Original Building Model in the district
A radius with 2H of the assessment building height
Fig. 61 Assessment Area for the redesigned building
Fig. 60 Assessment Area for the original building
Remain the same building volume and street Patten
Fig. 64 Apply the original building to other buildings
Fig. 66 Apply the redesign building to other buildings
Perimeter test points:
Overall Test Points:
Fig. 67 Perimeter test points test points for SVRw
Fig. 68 Overall Test Points (64 test points) for LVRw
Data Export Accuracy Control in CFD simulation:
The Grid Size Ratio (Grid size ratio = H2/H1) should be controlled to be no bigger than 1.3, VASARI is applying the structural grids type, all the Grid Size Ratio if the grids in VASARI are 1, which meets the Grid Size Ratio requirement.
Wind Direction: N (180), Wind Speed: 4.24m/s
(simulation with 15 more wind directions in paper)
After confirming with the air ventilation assessment study, better building envelop design with more permeability in the designed building, and applied this method to the surrounding buildings at in the same district, could achieve better air ventilation for the district on the pedestrian level, given the condition that the street pattern in the district remain the same.
On the other hand, though more building permeability could encourage the wind to penetrate along the street deeper, but without changing the street pattern, it’s still hard to create breeze way by only providing more permeability for buildings.
Redesign Proposal 1:
Redesign Proposal 1:
Fig. 82 Extended Modeling Area Illustrations
(Yellow: Assessment Area, Blue: Extended Modeling Area)
CFD simulation results for Original Model and Proposal 1:
CFD simulation result for Proposal 1 and Proposal 2:
Final Building Envelop Design
In this case study, the average wind velocity at pedestrian level for Perimeter Test Points is increased about 0.9 m/s, the Overall Test Points is increased about 1.1 m/s.
SVRwis increased by about 0.17, LVRw increased by about 0.23 by the redesign, but breeze way is not able to create by applying the design to other buildings in the same district, given the street pattern remains the same.
For the design practice, building envelops and flat layout design only focus on the external design, for the air ventilation assessment and cross ventilation respectively, the internal design for the building envelops for the considering by stack effect and the detail internal flat layout with quantity technical guide for the cross ventilation would be the extended topic for the further study.