Ryozo Ooka

CFD analysis of wind climate from human scale to urban scale

Abstract The rapid growth of computational wind engineering (CWE) has led to an expansion of the research fields of wind engineering. CWE has made it possible to analyze various physical processes associated with wind climate around humans and in urban areas. This paper reviews recent achievements in CWE and its application to wind climate in scales ranging from human to urban scale.

Studies on critical Reynolds number indices for wind-tunnel experiments on flow within urban areas

Reynolds-number dependence of flow fields within a modelled urban area was studied in a wind tunnel. We measured flow around a single model building and around model city blocks at various wind speeds, and studied Reynolds number indices more appropriate than the building Reynolds number. Our results led to the following conclusions. Firstly, the flow around the models in the wind tunnel was roughly divided into three parts according to the intensities of viscous stress and Reynolds stress as follows: (1) the flow in the vicinity of the ground or the surfaces of the model, where viscous stress became dominant under certain conditions; (2) the flow detached from the surfaces of the model, where Reynolds stress was always dominant; and (3) the flow around the separation bubble at the leading edge of the building model, where the influences of both viscous stress near the wall and the Reynolds stress in the separated boundary layer were mixed.Secondly, the critical Reynolds number of the flow in the modelled urban area could be defined by using both the roughness Reynolds number Rez0 (= z0u*/ν) and the dimensionless height z+ (= zu*/ν). Reynolds-number independence could be expected for whole flow fields in the modelled urban areas as long as the critical values of Rez0 and z+ were satisfied.

CFD analysis of mesoscale climate in the Greater Tokyo area

Abstract The results of CFD analyses of mesoscale climate in the Tokyo area are presented. Here, the model for geophysical flow problems developed by Mellor and Yamada is used for turbulence closure. In the first half of the paper, the accuracy of CFD analyses is examined by comparing their results with the measured data. The topography and present land-use situation in Japan are incorporated into the predictions by using the numerical data-base provided by the National Land Agency of Japan. For comparison, a computation which does not consider the effects of the distribution of land-use conditions at the present is also carried out. In the latter part, urban climates during the 1930s and the 1990s are analysed by the CFD method developed here. By comparing the results of these analyses, the effects of urbanization on heat island circulations over the Tokyo area are investigated.

Field study on sustainable indoor climate design of a Japanese traditional folk house in cold climate area

Abstract Field measurements of the indoor climate of a typical Japanese traditional folk house located in Hokuriku district were conducted in summer and winter. The aim of the experiment was to clarify the environmental control of this folk house and evaluate the indoor thermal environment and its sustainable devices. In particular, the effect of mud walls, earthen floor or reed roof on the indoor climate is considered within this paper.

Development of a Computational Thermal Manikin Applicable in a Nonuniform Thermal Environment—Part 1: Coupled Simulation of Convection, Radiation, and Smith's Human Thermal Physiological Model for Sensible Heat Transfer from a Seated Human Body in Radiant Environment

To predict the thermal sensation of people located in a nonuniform environment, it is very important to clarify the local heat transfer in detail. In this context, the coupled simulation of convection, radiation, and the multi-element thermal physiological model established by Smith is investigated. The sensible heat transfer from the surface of a human body placed in uniform and front-back asymmetric radiant environments, with ambient air temperature of 28°C, was calculated using the coupled simulation method. According to the results, the microclimate around the human body and its thermal characteristics changed in response to the radiant conditions. However, compared with the results of the human subject experiments, which were measured under the same thermal conditions, in terms of skin temperatures, it is indicated that the simulation cannot accurately predict the skin temperature at the limbs, even in a uniform environment. Finally, measures for improving the prediction accuracy of the present coupled simulation method are suggested based on examination of the cause of the discrepancy.

Analysis of the Relationship between Changes in Meteorological Conditions and the Variation in Summer Ozone Levels over the Central Kanto Area

An increasing trend in ground-level ozone (O3) concentrations has recently been recognized in Japan, although concentrations of ozone precursors, nitrogen oxides (NOx), volatile organic compounds (VOCs) and nonmethane hydrocarbons (NMHCs) have decreased. In this paper, the relationship between meteorological factors (temperature and wind speed) and ground-level ozone concentrations in the summer over the central Kanto area of Japan was examined using both statistical analyses and numerical models. The Fifth-Generation NCAR/Penn State Mesoscale Model (MM5) and the Community Multiscale Air Quality (CMAQ) model were employed in this study. It was found that there is a close relationship between meteorological conditions and ground-level ozone concentrations over the central Kanto area. In summer, up to 84% of the long-term variation in peak ozone concentrations may be accounted for by changes in the seasonally averaged daily maximum temperature and seasonally averaged wind speed, while about 70% of the recent short-term variation in peak ozone depends on the daily maximum temperature and the daily averaged wind speed. The results of numerical simulations also indicate that urban heat island (UHI) phenomena can play an important role in the formation of high ozone concentrations in this area.

Development of a Computational Thermal Manikin Applicable in a Non-Uniform Thermal Environment—Part 2: Coupled Simulation Using Sakoi's Human Thermal Physiological Model

In order to develop a computational thermal manikin to enable the prediction of the thermal sensation of an occupant in a non-uniform environment, in a previous paper (Zhu et al. 2007) we proposed and examined a simulation method combining Smiths human thermal physiological model with convective and radiant simulation by applying the proposed method to calculate the sensible heat transfer over the body surface of an occupant located in several radiant environments with an air temperature of 28°C. However, the simulation results greatly underestimated the skin temperatures at the limbs, even in uniform conditions, due to the improper modeling of the Arteriovenous Anastomose phenomenon in Smiths model. Accordingly, a new human thermal physiological model, Sakois model (Sakoi et al. 2005a, 2006a), was developed with a three-dimensional body configuration similar to Smiths model and a thermo-regulatory mechanism by Yokoyama (1993). In this paper, Sakois model is coupled in the simulation of convection, radiation, and moisture transport to calculate the total (sensible and latent) heat transfer from a seated human body in uniform and front-back asymmetric radiant environments, which were introduced in the previous paper (Zhu et al. 2007). The comparison to the corresponding results of the subject experiments and the coupled simulation using Smiths model in terms of skin temperatures indicates that the prediction accuracy of the numerical simulation is greatly improved as a whole, especially at the limbs; however, it deteriorates around the face and body parts facing cold panels when using Sakois model.

Drivers and barriers to occupant adaptation in offices in India

Occupant window-opening behaviour in Indian offices is a nascent field. This paper relies on the thermal comfort field study data from 28 Indian offices in Hyderabad and Chennai. Occupants in naturally ventilated buildings used the windows and doors adaptively as the seasons changed and the temperature varied. We found that 50% of the windows would be opened at an indoor air temperature of 30 °C, using logistic regression. We noted some non-thermal factors possibly affecting the adaptive operation of controls as well, including: design and construction, operation and maintenance, environmental, sociocultural, attitudinal and behavioural factors. A windows potential for modifying the comfort temperature hinges on the effective handling of these hurdles. We further categorized the barriers into those in the occupants realm and beyond. Each category is further identified with the extent to which the barrier interferes with the control as an adaptive opportunity.

A Wind Tunnel Experimental Analysis of the Ventilation Characteristics of a Room with Single-Sided Opening in Uniform Flow

Abstract Wind-induced cross ventilation is one of the key technologies for saving energy while controlling the indoor environment, and seems to be a crucial element of any sustainable building policy. However, it is sometimes difficult to arrange openings on two sides for cross ventilation of a room. In buildings in built-up areas, many rooms only have openings on a single side. Since the airflow rate for a single-sided opening is much lower than that for two-sided openings, it is important to study how we can increase the airflow rate for a single-sided opening using either passive or active techniques. In this study, the characteristics of wind-induced ventilation with single-sided openings were analysed through wind tunnel experiments. In some experimental cases, a small object was attached to the opening in order to increase the airflow rate. (It was assumed that this object replicated a pivoted window whose vertical axis was in the centre of the window.) In other experimental cases, circulation flow was artificially induced in the room used in the experimental model in order to increase the airflow rate. A synergistic effect from the fresh air entering through the opening was expected with the circulation.

Progress in Numerical Modelling for Urban Thermal Environment Studies

Abstract Urbanization is progressing rapidly in many Asian cities. The process of urbanization has modified the land use from natural environment into built environment. It alters not only the surface energy balance of the urban canopy, but also brings about a great quantity of anthropogenic sources of waste heat through air-conditioning, cars, etc. In addition, the effect of urbanization on urban wind environment is likewise significant. Thus, the primary precondition is to understand how the urban environment affects the physical and climatic pattern in and around the city resulting from urban encroachments. Commonly, wind-tunnel measurements and observational campaigns enable us to understand the physical processes that take place with the morphology of urban areas. This understanding is then used to represent these processes within numerical models of different urban scales. The ever-increasing computational power together with highresolution computational fluid dynamic models has now become a useful tool to gain significant insight into detailed processes occurring within the urban context. This chapter gives an overview of the latest simulation studies for mesoscale and microscale climates, and also the assessment tools used in urban climate research. Various assessment tools are introduced and classified according to corresponding modelling scales. Next, the chapter addresses recent achievements in urban climate research for urban thermal environment studies. Examples of numerical results obtained by researchers of Japan are presented.