Takashi Kurabuchi

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.

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.

Numerical Study of Cross-Ventilation Using Two-Equation RANS

Abstract Cross-ventilation is a mechanism using the pressure difference between the outdoor environment and indoor space to provide an energy-saving method for ventilation design. Since the ventilating flow in the vicinity of the opening is highly turbulent and unsteady, the ideal numerical method to resolve the structure of the ventilating flow is by using a time-dependent approach such as large eddy simulation (LES). However, LES requires large computing resources and there are also some uncertainties associated with the discretisation of time scales and length scales of turbulence. Therefore, an alternative has been sought. This study compared the flow simulations computed by the standard k-ε, RNG k-ε, standard k-ω and SST k-ω models as well as LES and all the results were compared with experimental measurement data. The main findings concluded that the SST k-ω model was able to depict the flow features satisfactorily and that the calculation of flow rate was also accurate under various wind directions.

Study on the Numerical Predictive Accuracy of Wind Pressure Distribution and Air Flow Characteristics—Part 1 Optimization of Turbulence Models for Practical Use; Part 2 Prediction Accuracy of Wind Pressure Distribution of Various Shaped Buildings

Abstract To evaluate wind pressure distribution on a building by using CFD (computational fluid dynamics), it has been general practice to use k-ϵ models. However, it is known that the use of the standard k-ϵ model has disadvantages such as overestimation of wind pressure coefficient and turbulent kinetic energy on the windward surface where wind impinges on the building. To overcome these problems, various modifications of the k-ϵ model have been proposed. In the present study, a number of modified k-ϵ models and a k-ω model were applied for the estimation of wind pressure distribution on a parallelepiped shaped building. The characteristics of each of these turbulence models were confirmed using a wind tunnel model. The results suggest that a modified k-ϵ model incorporating Durbins limiter (model parameter α=0.5) showed satisfactory results for the estimation of wind pressure distribution. In the overall evaluation, the modified k-ϵ models (incorporating Durbins limiter (α=0.65), RNG model (renormalization group theory) and Quadratic model provided good results. Part one of the study was performed on an object of extremely simple shape, and questions may arise if this is applied on an actual building. In this respect, Part 2 of the paper covers a similar evaluation on a complicated shaped object. For this case, it was found that a RNG model provides high reproduction accuracy just as in the case of the object with simple shape. Also, a problem with the model incorporating Durbins limiter (α=0.65) was found when considering the object with a complicated shape. Consequently, a modified model incorporating Durbins limiter with a higher value for a shows better results when compared to the RNG model.

Applying the Local Dynamic Similarity Model and CFD for the Study of Cross-Ventilation

Abstract The Local Dynamic Similarity Model (LDSM) is a ventilation model for predicting the discharge coefficient and the inflow angle at the opening of a cross-ventilated building. This model requires a dynamic pressure generated by the wind velocity component tangential to the opening in addition to wind pressure. Also, total pressure, wind pressure, static pressure, room pressure and inflow velocity components are needed for model validation. Under cross-ventilation, it is rather difficult to measure these parameters, especially the total pressure and the velocity components at the opening, as the inflow angle is not known a priori. Therefore, an alternative was sought. This study applied a CFD method to determine the required parameters as a way of using the local dynamic similarity model. The CFD method had been validated with experimental results before the CFD data was used for LDSM. Good agreement was obtained between CFD and LDSM. Consequently the LDSM was also verified by CFD and it was viable to combine LDSM and CFD for the study of cross-ventilation.