Achieving Good Natural Ventilation through the Use of High Performance Computer Simulations – Singapore Case Studies Po Woei Ken (powk@bsd.com.sg) Building System & Diagnostics Pte Ltd
Outlines 1. Importance of Performing Natural Ventilation CFD Simulation 2. BCA Green Mark CFD Guidelines 3. Boundary Conditions & Simulation Inputs for CFD under SG Conditions 4. Computational Size of a Typical Natural Ventilation CFD Simulation 5. Mesh Specification of NV CFD Simulation 6. Selection of Turbulence Model of NV CFD Simulation 7. Natural Ventilation CFD Simulation Methodology 8. Good Practices for Natural Ventilation Design 9. Case Studies 2
1. Importance of Performing Natural Ventilation CFD Simulation 1. Aiming towards energy and environmentally efficient buildings – to optimize building designs to draw clean fresh air from outside environment into the buildings. 2. Enhancing natural ventilation performance of a building implies: – Reducing energy consumption from mechanical ventilation – Improving human comfort within occupied spaces – Increasing thermal comfort controllability – Improving indoor air quality 3
2. BCA Green Mark CFD Requirements (GM Version 4) 1. BCA stands for (Singapore) Building Construction Authority 2. To achieve the highest Green Mark Rating (Platinum), either a wind tunnel testing or a CFD simulation must be conducted (prerequisite) 3. Final results of the CFD (micro level) simulation MUST meet the criteria set for by BCA GM Version 4.0, i.e. at least 70% of the selected typical dwelling units with good natural ventilation performance (or 0.60 m/s ), to qualify for GM Platinum Rating 4. Mathematically, this translates to the overall area weighted velocity magnitude must be at least 70% * 0.60 m/s = 0.42 m/s The process to achieve the 0.42 m/s will be explained in greater details in the next few chapters 4
3. Boundary Conditions & Simulation Inputs for CFD under SG Conditions 1. Wind dynamic in nature, its behavior is random and at times, the local meteorological data is used 2. Prevailing wind directions in Singapore (Statistical Data) 3. Constant temperature at 33 o C (Isothermal) 4. Boundary condition types include (a) Velocity Inlet, (b) Pressure Outlet, (c) Non-Slip Wall, & (d) Slip Wall 5
5. The inbound velocity is mathematically described by a log-law function (see formula below) Where: • u(z) is the wind speed magnitude expressed as a function of height ( z ) • d is the zero plane displacement • z 0 is the roughness value of the ground • z ref is the reference height of 15 m where the statistical wind speed magnitude at those four prevailing wind directions (of Singapore) are 2.0, 2.8, 2.9 and 3.2 m/s (N, S, NE and SE respectively) 6 • ψ is a stability term, usually dropped out
6. The physical representation of the roughness value of the ground is listed as follows: Terrain description [ m ] Open sea, Fetch at least 5 km 0.0002 Open flat terrain; grass, few isolated obstacles 0.03 Low crops; occasional large obstacles, x/H > 20 0.10 High crops; scattered obstacles, 15 < x/H < 20 0.25 parkland, bushes; numerous obstacles, x/H ≈ 10 0.50 Regular large obstacle coverage (suburb, forest, urban) 0.50 - 1.00 Due to the urban nature of Singapore, the roughness value most commonly used in natural ventilation CFD simulation is 1.0 [m] 7
4. Computational Size of a Typical Natural Ventilation CFD Simulation 1. The computational domain shall be big enough to include the development of interest and the surrounding buildings residing within the proximity of minimum 3 times or more of the characteristic length of the primary development 8
Non-slip Wall (for Ground and Velocity Inlet (NORTH wind) building Surface) Slip wall (for side and TOP boundary faces) Existing Surrounding Buildings Main Buildings N Pressure Outlet Boundary Condition Specification (under NORTH wind) 9
5. Mesh Specification of Natural Ventilation CFD Simulation 1. Mesh size of 0.1 m to 0.2 m within the modeling dwelling units 2. Mesh size of 0.5 m to 1.0 m around the buildings 3. Mesh size of 10 m at far field boundary with a minimum of 50 m away from the ground 10
6. Selection of Turbulence Model for Natural Ventilation CFD Simulation 1. Standard K- ε Turbulence Model 11
7. Natural Ventilation CFD Simulation Methodology (Macro & Micro Levels) A. Macro level simulation 1. Conduct CFD simulation of the main buildings, following the BCA guidelines governing the inclusion of surrounding buildings, the boundary condition and mesh specification 2. Conduct post processing – Calculating the wind pressure loads acting on ALL window openings on ALL dwelling units, at building mid height level. 3. Calculate overall average pressure differential => The Global Pressure Differential 12
The sample calculation of the individual unit pressure differential, Δ P is as follows: This example unit has 7 window openings +1.5 Pa – (-1.5 Pa) = 13
A. Macro level simulation (Continued.) The sample calculation of the average unit pressure differential under all analyzed wind directions, Δ P ave and the global pressure differential , Δ P ave global This example HDB development has 3 unit types (3RM, 4RM and 5RM) separated out in 7 tower blocks, and the unit pressure differential of all units at the mid building height is summarized in the following table. This is the Δ P ave This is the Δ P ave global 14
A. Macro level simulation (Continued.) Assuming the development has 7 unit types, with the following unit type mix ratio, then units of Type F and G are not required for micro level CFD simulation, BUT the unit pressure differential of these units of Type F and G still has to be calculated This is the end of macro level simulation 15
B. Micro level simulation 1. The 3D CAD model is modified with the incorporation of the selected (up to a maximum of 5) units, at the building mid height storey. In this example, 3 units are selected for micro level simulation: 16 CAD model incorporating 3 dwelling units
B. Micro level simulation (Continued.) An example of the calculation of the overall area weighted velocity magnitude is given as follows: Assuming for this project, there are 5 units selected for the micro level simulation, and the calculation of the area weighted velocity magnitude of the most common unit type is: 17
B. Micro level simulation (Continued.) Example (Continued.) The calculation of the area weighted velocity magnitude of the 2 nd most common unit type is: 18
B. Micro level simulation (Continued.) Example (Continued.) The summary showing the calculation of the overall area weighted velocity magnitude is: The result implies the BCA GM V4 CFD requirement is NOT met, as 19 39.5% is less than the required 70% mark (equivalent to 0.42 m/s)
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8. Good Practices for NV Design • Macro level - Tower block arrangement - Tower block orientation - Tower block spacing • Micro level - Change in unit layout design - Adjustment of window type - Adjustment of window size - Balcony glass balustrade vs. permeable railings 21
Parallel Arrangement Block on the leeward side will not receive airflow. Wind flow path is obstructed by the blocks. Wind not allowed to reach leeward areas 22
Staggered Arrangement All blocks can enjoy good airflow Wind can be channeled around the blocks to reach downstream areas. 23
Tower Block Orientation – Building primary facades to face normal to the local prevailing wind direction – In Singapore the four prevailing wind directions are: • NORTH • SOUTH • NORTH-EAST • SOUTH-EAST – Window to face the oncoming wind straight on (ideally) 24
Unit Layout and Design • Dwelling unit internal layout to promote cross ventilation 25
Adjustment of Window Type • Typical types include – Casement (100% opening) – Slider (50% effective opening for a 2 piece, 66.7% effective opening for a 3 piece etc. etc.) – Top Hung (opening size dependent on the allowed opening angle – Sliding and folding (100% opening) 26
Adjustment of Window Size • Fixed glass panel does NOT help in improving natural ventilation performance of the dwelling units • Is full width window warranted? • Height position of the window 27
Balcony Railing vs. Glass Balustrade • Glass Balustrade does NOT allow air to penetrate • Railing is permeable • The degree of permeability on railing is represented by the term: Blockage Ratio • Blockage ratio of 0 implies it is fully transparent, and air is allowed through 100% • Blockage ratio of 1 implies it is fully blocked, and no air is allowed to penetrate 28
9. Case Studies Example 1: Background: - Multi-storey warehouse type building - Floor to floor height is 9 m Important things to note: - Effect of window vertical position 29
N ( Initial Layout Design, Window @ +4 m above Floor ) 30 Velocity Vector Plot, under NORTH Wind
N ( Final Layout Design, Window @ +1 m above Floor ) 31 Velocity Vector Plot, under NORTH Wind
Example 2: Background: - High rise residential building Important things to note: - Effect of window type and size 32
(Top hung window + non full width window) 33 Velocity Contour Plot
(Casement window + non full width window) 34 Velocity Contour Plot
(Casement window + full width window) 35 Velocity Contour Plot
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