Yoshihiko Akamine

Local Dynamic Similarity Model of Cross-Ventilation Part 1 - Theoretical Framework

Abstract A new model has been proposed for evaluating the discharge coefficient and flow angle at an inflow opening for cross-ventilation. This model is based on the fact that the cross-ventilation flow structure in the vicinity of an inflow opening creates dynamic similarity under the condition that the ratio of the cross-ventilation driving pressure to the dynamic pressure of cross flow at the opening is consistent. It was confirmed, from a wind tunnel experiment, that the proposed model can be applied regardless of wind direction and opening position. Change of pressure along the stream tube of a cross-ventilated flow was estimated from the results of Large Eddy Simulation, and was set as the basis of model preparation. It was found that the static pressure at the opening was exhausted by the flow‘s acceleration and by turbulent kinetic energy generation during this stage.

Local Dynamic Similarity Model of Cross-Ventilation Part 2 - Application of Local Dynamic Similarity Model

Abstract The proposed local dynamic similarity model of cross-ventilation predicted ventilation flow rates moreb accurately than the conventional orifice flow model assuming constant discharge coefficients when discharge coefficients actually decreased with change of wind direction. This model was used to develop a new method for evaluating the ventilation performance of window openings. The obstructive effect of model size on flow fields in a wind tunnel was avoided by installing the opening parallel to the wind tunnel floor. The ventilation performance for various types of inflow openings was assessed by the ventilation performance evaluation system. The discharge coefficient was expressed by an approximate expression using dimensionless room pressure PR*. A ventilation performance database was thus produced. For the field experiment in a full-sized house, it was found that about 60% of all wind data were in the range of |PR*| < 5. This reveals that the discharge coefficient decreases frequently in actual wind.

A Study on the Effects of Porosity on Discharge Coefficient in Cross-Ventilated Buildings Based on Wind Tunnel Experiments

Abstract A study was performed on the effects of porosity on discharge coefficient and airflow characteristics under the condition where uniform approaching flow directly faces to and enters the opening by using wind tunnel experiment and CFD analysis. The evaluation was performed on porosities in the range 0.4% - 64%. The results of wind tunnel experiments suggest that the discharge coefficient increases when the porosity is higher. The results of CFD analysis reveal that the contraction of airflow when it passes through the opening is correlated with discharge coefficient, and that the discharge coefficient increases when flow contraction does not occur. When porosity increases, the retardment of the streamtube ceases to occur in the region upstream of the opening, and this leads to the elimination of flow contraction, hence the increase of discharge coefficient. When we evaluated the limitation of application of the local dynamic similarity model on porosity, the effectiveness of the model was confirmed well when the porosity was 16% or lower regardless of wind direction. The validity of the model was also confirmed under the condition where airflow goes along the wall surface before reaching the opening even when the porosity was 36% or more.

Experimental Study on Predicting Wind-Driven Cross-Ventilation Flow Rates and Discharge Coefficients Based on the Local Dynamic Similarity Model

Abstract It is known that discharge coefficients vary with wind direction and opening position. The local dynamic similarity model of cross-ventilation can select discharge coefficients on this basis. This paper summarizes previous studies on various inflow opening conditions, and describes new studies on outflow openings and the evaluation of ventilation flow rates in two zones based on coupled simulation of the local dynamic similarity model and a simple network model.

A CFD Analysis of the Air Flow Characteristics at an Inflow Opening

Abstract In the present study, a numerical simulation to simulate an experiment for evaluating the cross-ventilation performance at an inflow opening by using Large Eddy Simulation (LES), the standard k-ε model, and Durbin‘s k-ε model was performed. Results showed that too much turbulent kinetic energy was produced at the leeward opening frame in the standard k-ɛ model. However, Durbin‘s k-ε model improved this defect, and reproduced the wind tunnel results fairly well, as did the LES approach. Following on from this comparison, Durbin’s k-ε model was applied to the analysis of the air flow characteristics from the viewpoint of aspect ratio, opening thickness, and whether a louver was present or not. From the results it was concluded that static pressure increase was induced by the collision of the inflowing air with the leeward opening frame. This static increase caused a decrease in the discharge coefficient. There was little influence on the cross-ventilation flow rate when the louver angle was perpendicular to the opening surface and when it was installed on the inside of the opening.

A Fundamental Study on the Air Flow Structure of Outflow Openings

Abstract A Local Dynamic Similarity Model, applicable to dynamic similarity of cross-ventilation, has been applied to outflow openings. Cross-ventilation performance at the openings on the outflow side has been evaluated, and the structure of air flows around the outflow openings has been studied by LES and wind tunnel experiments. It was found that LES reproduces the wind tunnel experiment results fairly well, such as the extensive increase of discharge coefficient in a small region where dimensionless room pressure, PR*, is low. The evaluation of the pressure field by LES revealed that the remainder of the dynamic pressure in the air flows and the change of the pressure field around the outflow openings have a strong influence on the discharge coefficient. Furthermore, by identifying the configuration of the stream tube of the ventilation air flow, it was found that the discharge coefficient is changed depending on how the air flows exit. In general, dynamic pressure, Pt, tangential to the wall surface at the outflow openings is considered to be lower than that at the inflow side. The occurrence frequency of PR* was investigated by a full-scale experiment, and it was elucidated that the region of PR* where the discharge coefficient is extensively decreased develops only very rarely.

Utilization of Cross-ventilation in High-density Urban Areas

Abstract The increased density of buildings in urban areas in Japan over the past several years has meant that less outdoor wind blows into buildings. Additionally, houses with many or large-size of openings are difficult to build due to concerns about security and privacy. In the present study, we have investigated the cross-ventilation characteristics of void, monitor roof and wind tower for the purpose of improving the cross-ventilation in detached houses built in high-density urban areas based on wind tunnel experiment and flow-network model calculation. At first, wind-pressure characteristics of a detached house in high-density urban areas were obtained by the wind tunnel experiment. From this result, we found that it is difficult to ensure adequate cross-ventilation in houses located in high-density urban areas by only openings in the wall surface. So, void, monitor roof and wind tower have been investigated as techniques of improving the cross-ventilation. Wind-pressure characteristics of each technique were examined by wind tunnel experiment. And rates of cross-ventilation flow from cases (i.e., void, monitor roof and normal roof) were calculated by flow-network model using wind-pressure coefficients from previous experiments. It can be concluded that void is a very effective technique for improving cross-ventilation in detached houses.

Development of a Simulator for Indoor Airflow Distribution in a Cross-Ventilated Building using the Local Dynamic Similarity Model

Abstract In this study, the evaluation of cross ventilation is presented based on the simultaneous analysis of inside and outside conditions and for wind directions other than 0 degrees (i.e. for wind flow that is not aimed directly or normally to the inflow opening). The first part of the paper considers a conventional CFD analysis and compares the performance of the widely used k-ε turbulence model as well as the modified Durbin k-ε turbulence model. While a CFD approach can give good results it is very labour and computationally demanding. The second part of this paper consists of a description of a simplified approach based on the Local Dynamic Similarity Model. In the simplified approach, outdoor boundary conditions are established directly at the inflow opening by means of either wind tunnel experiment or CFD analysis of external flow conditions. Results of the simplified model are compared with experimental analysis and shown to give good agreement. The method is also shown to be valid for single and multi room applications.

Cross-ventilation Utilization in Housing in Congested Urban Areas in Taiwan

abstract This research aims to investigate the current condition regarding ventilation use in congested urban areas of Taiwan. There is a remarkably high level of humidity and dense populations in many housing areas. The study targeted terraced houses, which are the most common style of housing in urban Taiwan. The current study attempted to understand the thermal environments of residential buildings as well as utilization of the cross-ventilation system. This was achieved by combining data of the current weather conditions and a questionnaire survey of those tested. Also included in the testing was a series of wind tunnel experiments conducted to examine the issues related to the ventilation flow rates of these congested areas. Following that testing, ventilation improvement techniques were proposed and studied for ways to decrease the negative environmental impact from over-use of air conditioners. The results calculated by simulation software demonstrated that through the use of our roof top designed component for air circulation, the heat removal decreased by 16% annually, compared to current condition.

Study on Utility Cross-Ventilation in Guangzhou and Shenzhen, China

abstract As rapid increase in energy consumption has raised concerns among researchers regarding the use of conventional air-conditioning systems, substantial effort has been devoted to the exploration of alternative solutions. One possible solution is a natural ventilation approach. In the present study, the authors have focused on the feasibility of cross-ventilation due to natural ventilation potential. The study centered on cases located in the Guangzhou and Shenzhen areas on the Pacific coast of China, well known for their dense population and high-energy consumption. In the current research, the analysis of weather data to investigate the feasibility of cross-ventilation in Guangzhou and Shenzhen was initially performed. A questionnaire survey was then carried out in order to grasp the residences’ actual configurations and conditions. Sequentially, based on the results obtained from the prior investigations, the existing problems were raised and solutions for cross-ventilation enhancement were tested by means of a wind tunnel experiment. It can be concluded that the presence of VOID (vertical opening common space) without an opening on the lower part of the buildings significantly improves the cross-ventilation flow rate by 1.3-2 times over the cases without VOID.