Masaaki Ohba

Study on airflow characteristics inside and outside a cross-ventilation model, and ventilation flow rates using wind tunnel experiments

The internal airflow characteristics in a cross-ventilation model were investigated using split-film probes capable of measuring directional velocity components. The penetrating flow entered the inlet at steep declining angles due to the front eddy and flowed downwards to the floor. The internal turbulence spectrum of the main stream penetrating the inlet remained the same as that of the oncoming flow, but the spectra of other components were produced by the internal turbulence, whose eddy size was determined explicitly by the size of the model. The flow exiting from the outlet moved upwards due to the outside re-circulating eddy. The ventilation flow rates increased in the range of 40°⩽θ⩽60° for the incidence angle of the oncoming flow because the dynamic pressure at the inlet increased due to the change of the separated flow patterns around the model. The discharge coefficient for the inlet remained almost constant over a wide range of wind directions for the aspect ratio equivalent to 1:2.

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.

Overview of natural cross-ventilation studies and the latest simulation design tools used in building ventilation-related research

Abstract High-performance insulation, draught stripping and double glazing have effectively sealed off the fresh air routes and have made adequate natural ventilation impossible inside the modern energy-efficient home nowadays. Cross ventilation, a passive cooling method for buildings, is a major type of natural ventilation. Various study approaches have been reported; however, there is still no appropriate simulation tool that can predict the indoor thermal environment of natural ventilation. The typical building energy simulation for investigating a naturally ventilated building adopts thermal simulation and an airflow network. However, the airflow network approach for airflow estimation in building energy simulation cannot accurately predict indoor airflow by solving the pressure-flow algebraic equation, the mass balance equation and hydrostatic pressure variations. Recent advances in computer performance and computational fluid dynamics (CFD) software integrated with building energy simulation have made it possible to improve the accuracy to assess the performance of natural ventilation and also to give more realistic predictions of airflow in buildings. This chapter overviews and discusses various network airflow models integrated with CFD in the natural ventilation of buildings. Examples of results obtained with this approach are given to demonstrate the significant effects of such a coupling programme on natural ventilation prediction accuracy.

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.

Review of Cross-Ventilation Research Papers - from the Working Group for Natural Ventilation and Cross-Ventilation of the Architectural Institute of Japan

Abstract A working group for natural ventilation and cross-ventilation at the Architectural Institute of Japan (AIJ) was established in 2005 by researchers and designers with an interest in this topic. One of the tasks of the working group is to review and classify related research papers. This paper introduces the activities of the working group and presents some results of the review work. As examples of the review work, this paper concentrates on the area cross-ventilation rate and the interference coefficient concept of the cross-ventilation phenomenon that solves the problem of the remaining dynamic pressure inside a room.

Experimental study of effects of separation distance between twin high-rise tower models on gaseous diffusion behind the downwind tower model

Twin high-structures give rise to the additional complexity of a nearby structure. The separation distances between twin high-rise towers have especially large effects on the flow and the concentration fields around them. Wind tunnel experiments have shown that the primary effect of the upwind tower is to retard the flow separation on the top and sides of the downwind tower, provided the separation distance between the twin tower models is less than the building height (L/H<1.0). Experimental formulas for predicting vertical building surface concentration profiles on the downwind tower have been derived with a mean prediction error of 22%.