Ken-ichi Narita

Geometric Dependence of the Scalar Transfer Efficiency over Rough Surfaces

We performed a series of wind-tunnel experiments under neutral conditions in order to create a comprehensive database of scalar transfer coefficients for street surfaces using regular block arrays representing an urban environment. The objective is to clarify the geometric dependence of scalar transfer phenomena on rough surfaces. In addition, the datasets we have obtained are necessary to improve the modelling of scalar transfer used for computational simulations of urban environments; further, we can validate the results obtained by numerical simulations. We estimated the scalar transfer coefficients using the salinity method. The various configurations of the block arrays were designed to be similar to those used in a previous experiment to determine the total drag force acting on arrays. Our results are summarized as follows: first, the results for cubical arrays showed that the transfer coefficients for staggered and square layouts varied with the roughness packing density. The results for the staggered layout showed the possibility that the mixing effect of air can be enhanced for the mid-range values of the packing density. Secondly, the transfer coefficients for arrays with blocks of non-uniform heights were smaller than those for arrays with blocks of uniform height under conditions of low packing density; however, as the packing density increased, the opposite tendency was observed. Thirdly, the randomness of rotation angles of the blocks in the array led to increasing values of the transfer coefficients under sparse packing density conditions when compared with those for cubical arrays.

Thermal Influence of a Large Green Space on a Hot Urban Environment.

City-scale warming is becoming a serious problem in terms of human health. Urban green spaces are expected to act as a countermeasure for urban warming, and therefore better understanding of the micro-climate benefits of urban green is needed. This study quantified the thermal influence of a large green park in Tokyo, Japan on the surrounding urban area by collecting long-term measurements. Apparent variations in the temperature difference between the park and surrounding town were found at both the diurnal and seasonal scales. Advection by regional-scale wind and turbulent mixing transfers colder air from the park to urban areas in its vicinity. The extent of the parks thermal influence on the town was greater on the downwind side of the park (450 m) than on the upwind side (65 m). The extent was also greater in an area where the terrain slopes down toward the town. Even on calm nights, the extent of the thermal influence extended by the park breeze to an average of 200 m from the park boundary. The park breeze was characterized by its divergent flow in a horizontal plane, which was found to develop well in calm conditions late at night (regional scale wind <1.5 m s and after 02:00 LST). The average magnitude of the cooling effect of the park breeze was estimated at 39 Wm. This green space tempered the hot summer nights on a city block scale. These findings can help urban planners in designing a heat-adapted city.

Representative Air Temperature of Thermally Heterogeneous Urban Areas Using the Measured Pressure Gradient

A method to measure an area-averaged ground air temperature based on the hydrostatic equation is shown. The method was devised to overcome the problem of finding the most representative surface air temperature over a wide region, a problem that has seriously hindered the description of urban heat islands. The vertical pressure gradient is used and the hydrostatic equation is applied to estimate the average air temperature between two barometers, which is here called the hydrostatic temperature. The error analysis shows that the hydrostatic temperature can be estimated with a systematic error of 1.88C and a random error of 0.78C in the case in which the two barometers have a vertical separation of 228 m. The measured hydrostatic temperature agreed with the average of the directly measured temperature within 0.78C rms. For this barometer separation, the representative area of the hydrostatic temperature was experimentally found to be a 12-km-radius circle. The size of this area decreased when the vertical separation of the barometers decreased. The hydrostatic temperature is compared with the average directly measured temperature for various areas. The maximum correlation between them occurred for a circular area with a 12-km radius centered on the pressure measurements. The size of the representative area for this method is larger than that for the direct measurement of air temperature.

Study of the Airflow Structure in Cross-Ventilated Rooms based on a Full-Scale Model Experiment

Abstract Cross ventilation to reduce cooling energy is one of the most important techniques for maintaining a comfortable indoor environment in hot and mild seasons. However, at present, it is difficult to design the indoor environment under cross ventilation because there is insufficient knowledge to evaluate the effect of cross ventilation quantitatively. To develop an understanding of the flow characteristics a full-scale model experiment was performed in a large wind tunnel to examine airflow properties in a cross-ventilated space. The purpose of this paper is to clarify the airflow structure in the cross-ventilated room in relation to wind direction. The key factors determining the airflow structure in the space were found to be: the main current region, rebounding and changing flow direction, deflected flow, surface flow and circulating flow. It was observed that the main air current tends to travel in a straight line until it collides with obstacles. On collision, the flow changes direction and deflected flows are formed over and/or under the main current. When there is enough space alongside the main current region, a circulating flow is formed in each room. The room mean velocity was found to be dependent on the path of the air current. When the main current is well defined, a relatively low value of mean velocity is observed. When the main current is divided, the room containing the inflow opening has a relatively high velocity.

FIELD MEASUREMENTS ON THE COOLING EFFECT OF THE IMPERIAL PALACE AND ITS THERMAL INFLUENCE ON THE SURROUNDING BUILT UP AREA

In this paper, results are shown from micro-climatological observations performed in and around a largest green space in central Tokyo, “Imperial Palace”, during summer. Its area is 230ha, and it was once the Edo-Castle, then surrounded by a moat.In a clear calm night, the cool air flows out from the Imperial Palace to the surrounding city area gravitationally. Different from the daytime advection by prevailing wind, the turbulent mixing is very weak in nighttime seeping phenomena because of the stable atmospheric condition. In the west side, the cool air flows over the valley of moat (20m depth) and spreads out into the built-up area. In the east side, cool air front sometimes penetrates the adjacent CBD area and reach near the Tokyo Station. The wide streets have a larger effect on cool air seeping than the moat dose even though water temperature is higher than ambient air temperature.

Mixing Property and the Heat Exhaust Effect under Cross Ventilation in a Full-Scale Experimental Model

Abstract Cross ventilation is one of the most important techniques for achieving energy conservation and for maintaining a comfortable indoor environment in hot summer periods. However it is difficult to evaluate the effect of cross ventilation quantitatively and to undertake design based on a quantitative evaluation. This is because the indoor environment is uneven and changes according to the outside conditions. It is difficult to predict the ventilation rate under cross ventilation due to the variation in wind. Hence it is still difficult to estimate indoor comfort conditions given by cross ventilation. In this paper full-scale model cross ventilation experiments are described and the properties of airflow in and around the full-scale model (velocity field, pressure distribution on surface, visualized flow pattern, heat transfer coefficient distribution, discharge coefficient of openings and so on) have been examined. The mixing property and heat exhaust effect of cross ventilation are discussed. Tracer gas concentration decay measurements were made in the model to determine the spatial unevenness of mixing property, the air change index and the velocity distribution. It is shown that the different mixing properties are formed according to the airflow pattern. The heat exhaust effect, was determined by using plasterboard that was set on the floor for heat storage. The temperature of the air and surface was measured, and exhaust heat by cross ventilation was calculated. The relation between temperature reduction and the flow path was examined, and the total exhaust heat by cross ventilation was compared to the total convective heat transfer from the plasterboard. These experiments showed that ventilation rate is the most important factor in determining the amount of exhaust heat and the room mean age of air. However the airflow path has an influence in the exhaust heat and mixing property in the cross ventilated space.