Paper 4
Stream Tube based Analysis of
Problems in Prediction of Cross-Ventilation Rate
Tomohiro
Kobayashi1,
Kazunobu Sagara1,
Toshio Yamanaka1,
Hisashi Kotani1,
Shogo Takeda1
and Mats Sandberg2
1Dept.
of Architectural Engineering, Osaka University, Japan
2University of Gävle, KTH Research School, Sweden
Abstract
The
airflow rate of a building ventilated by wind is usually predicted by
using the wind pressure coefficients obtained for a sealed building and
discharge coefficients based on measuring the airflow characteristics
through an opening in a sealed chamber (chamber method). This can result
in the underestimation of wind driven flow through large openings located
on opposite sides of a room. In this paper, the discharge coefficient,
based on the chamber method, and the actual condition of cross-ventilation
are calculated and compared with each other by means of stream tube
analysis. The driving pressure based on wind pressure coefficients
obtained from a sealed building are also compared with those based on
pressures inside the stream tube of the actual flow field representing a
porous rather than sealed building. A building model of dimensions 120 mm
(width)×120 mm (height)×180 mm (length) was used for the
analyses. The size of openings, expressed as the porosity (opening area divided by façade area),
was 11.6 %, 20.7 % and 46.5 %. These models were analyzed
by CFD simulation and the stream tubes caught by the opening were
determined. From the analysis the errors in discharge coefficient and wind
pressure coefficient were identified. Finally, the flow rate based on
these discharge coefficients and driving pressures were calculated and
compared. It is shown that the effect of the underestimation of the
discharge coefficients by the chamber method is significant for all cases
of porosity studied in this paper. Moreover, it is shown that the use of
wind pressure coefficients is not appropriate for the case of extremely
large openings.
Key words: cross-ventilation,
large opening, stream tube, discharge coefficient, wind pressure
coefficient.
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Contents
Paper 1
Plane-Air-Jet Corner Zone Modelling in
a Room Ventilated by an Active Chilled Beam
Paper 2
A Novel Method to Measure the
Air Entrainment Ratio of an Active Chilled Beam
Paper 3
Wind Turbulence and Multiple
Solutions for Opposing Wind and Buoyancy
Paper 4
Stream Tube based Analysis of
Problems in Prediction of Cross-Ventilation Rate
Paper 5
Development of Effective
Ventilation System for Electric Multiple Unit (EMU) Train for Mumbai
Suburban Railway
Paper 6
Integrating Active Thermal Mass
Strategies with HVAC Systems: Dynamic Thermal Modelling
Paper
7
Roof
Thermal Design for Naturally Ventilated Houses in a
Hot Humid Climate
Paper
8
Book
Review
Tropical Urban Heat Islands - Climate Buildings and Greenery
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