External Flow Comparison Using CFD
China Project - Beijing 3/7/2000
Sephir Hamilton, Jeff Huang, Nobukazu Kobayashi, Dong Luo

Abstract
This document evaluates whether the study of natural ventilation in a building can be separated into two parts: indoor airflow and outdoor airflow. This decoupling allows CFD users to use necessarily coarse grids for exterior modeling and finer grids for interior modeling The two models are then coupled by pressure boundary conditions at the windows. Modeling a complete external and internal flow system with a fine enough grid for internal flow creates files that are too computationally intensive for available computers.

The results of the study compare pressure distributions around objects modeled as pure blockages (no openings) and as partial blockages with various window-to-wall ratios. Errors in pressure distribution up to 20% are realized between the pure blockage and the highest window-to-wall ratio case. Using decoupled models is not acceptable for precise quantitative flow simulation. For conceptual design, however, the separation of indoor airflow and outdoor airflow for natural ventilation design is acceptable.

Models
Two separate experiments each show similar results. One experiment uses a simple rectangular blockage, the other simulates the facade of an actual building design (China Project Design - Beijing).

The first experiment (Hamilton and Huang) models a pure rectangular blockage in 2 m/s airflow. Then it models a blockage with an open interior, walls, and exterior windows (3 cases each with different window-to-wall ratios - figure 1).

Figure 1- The blockage allows air through the windows (perpendicular to the wind). The interior partitions simulate an actual apartment's flow restriction. 15% (case 1), 11.1% (case 2), and 6.25% (case 3) window-to-wall areas were each modeled. The results from each model are compared to the 0% window area model.

Figure 1a - View of the model with the entire model-space

The second experiment (Kobayashi and Luo) models a pure blockage in the shape of the Beijing building design (China Project, 2000) in 2 m/s airflow. Then it models the same blockage with 30% window-to-wall ratio (no internal obstructions - figure 2) and with 30% window-to-wall ratio (with internal obstructions/walls - figure 3).

Figure 2 - The blockage with 30% window-to-wall area, no obstructions

Figure 3 - The blockage with 30% window-to-wall area, with internal obstructions (walls - in white)

Results
Experiment 1 (Hamilton and Huang)
The base case without the windows yielded a pressure difference between the windward and leeward sides of 4.259Pa (windward and leeward pressures equaled 5.274Pa and 1.015Pa respectively) These values were taken at the centerline. The pressure difference in case 1 (15% window-to-wall area) revealed a slightly lower pressure difference of 3.809Pa at the centerline, and 3.012Pa locally at the center of one pair of windward/leeward windows. The case 2 (11.1% window) pressure differences were 3.896Pa and 3.041Pa at the centerline and the window centerline respectively. Case 3 (6.25% window) had pressure differences of 3.863Pa at the centerline and 3.009Pa at the window centerline. Graphic and tabular results are shown below.

Table 1 - Results of pressure study for Experiment 1

Figure 4 - Pressure distribution around the pure blockage (base case)

Figure 5 - Resulting velocity vectors for case 1

Figure 6 - Pressure distribution for case 1

Figure 7 - Pressure distribution for case 2

Figure 8 - Pressure distribution for case 3

Experiment 2 (Kobayashi and Luo)
In Case-1 (without windows), the pressure difference between the front and the back of the building is approximately 20-40% higher than that of Case-2 (with windows and without inner walls). However, in Case-3 (with windows and inner walls), the pressure difference between the front and the back is almost same as that of Case-1 (without windows). The inner walls work as resistance to air flow. The results imply that windows do not make so much difference to the calculation of the pressure around buildings. The results are shown graphically and in tabular form below.

Figure 9 - Velocity distribution for case 1 (base case)

Figure 10 - Pressure distribution for case 1

Figure 11 - Velocity distribution for case 2

Figure 12 - Pressure distribution for case 2

Figure 12 - Velocity distribution for case 3

Figure 13 - Pressure distribution for case 3

Conclusion
Using decoupled models is not acceptable for precise quantitative flow simulation. For conceptual design, however, the separation of indoor airflow and outdoor airflow for natural ventilation design is acceptable.

One can successfully decouple the exterior and interior flow for a building exposed to wind to study natural ventilation for conceptual design. The process, however, produces errors in the magnitude of 10-20% for pressure which is used to re-couple the exterior and interior flows.

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