Kiyoshi Uehara

Wind tunnel experiments on how thermal stratification affects flow in and above urban street canyons

The effects of atmospheric stability on flow in urban street canyons were studied using a stratified wind tunnel. We conducted experiments using a model that represented city streets with simply shaped block forms, while varying atmospheric stability across seven stages from stable (Rb=0.79) to unstable (Rb=−0.21). We used a laser Doppler anemometer (LDA) and a cold wire to measure the flow field and temperature within and above the street canyon. In addition to mean values of wind speed components and temperatures, we measured turbulence intensity, shear stress, and heat flux distribution. Our results led to the following conclusions: Cavity eddies that arose in the street canyon tended to be weak when the atmosphere was stable and strong when unstable. Stable atmospheric conditions led to a positive feedback effect in which the downward flow into the street canyon weakened due to buoyancy, which facilitated the formation of a more highly stable stratification. As a result, when stability exceeded a certain threshold (somewhere in the range of Rb=0.4–0.8), the wind speed in the street canyon dropped nearly to zero.

Seasonal variation in atmospheric aerosols concentration covering northern Kyushu, Japan and Seoul, Korea

Abstract Atmospheric aerosols were collected from October 1990 to February 1992 in northern Kyushu (Tsushima and Ogori), Japan and Seoul, Korea, simultaneously using identical sampling system and chemical analysis methods. Observed data were analyzed with meteorological data to clarify the seasonal variations in aerosol concentration and composition. Dominant ion components of aerosol were SO42− in anion and NH4+ in cation. The concentrations of ion components were higher in winter than those in summer. Equivalent concentration ratio of particulate nitrate and sulfate ( NO 3 − SO 4 2− ) showed high value in Ogori having a peak in winter season. Equivalent concentration ratio of Ca2+ and sulfate ( Ca 2+ SO 4 2− ) is highest in Seoul and shows no seasonal variation. On the contrary, Tsushima and Ogori showed low value in summer season. In summer, this area is affected by the atmospheric high-pressure system which developed at the southern Kyushu sea area and southerly monsoon wind predominate. As the result of this, supply of Ca2+ from soil should be diminished in summer. Equivalent concentration ratio of particulate Cl− and Na+ ( Cl − Na + ) in August was half compared with the ratio observed in February. Episode analysis of June 1991 and February 1992 indicated the transportation of pollutants from the outside of Japan. Air masses from the Chinese continent have been concluded to bring the greatest amount of anthropogenic pollutants to Korea and Japan.

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.

A wind tunnel for studying the effects of thermal stratification in the atmosphere

Abstract A new wind tunnel designed to study the effects of thermal stratification on flow and diffusion in the atmospheric boundary layer has been constructed. The wind tunnel features three independent temperature systems that control the ambient air temperature (from 12 to 87° C), the temperature profile (gradient up to ∂T ∂z = 25°C m −1 ), and the surface temperature (eight individual floor panels, from 7 to 112°C for each) in the test section. The wind tunnel is also equipped with a velocity profile generating cart, and surface roughness, wind orientation, variable area source, and sampling grid floor panels. Combined, these features can generate a wide range of thermal stratification and other conditions. The effects of neutral, strongly unstable and strongly stable stabilities on the flow were examined. Visualization by smoke tracer showed that in the stable case, the turbulence is damped near the ground giving a laminar-like, wavy streamline, while for the unstable case, large convective eddy motion is observed. The instantaneous w-component turbulent velocity near the ground for the stable case was drastically reduced from the neutral and unstable cases. Simulating a sea breeze configuration, a strong stable layer developed over the sea, and at the shoreline, a mixed (unstable) layer began to grow beneath the stable layer. The observation of a downward flow in the lower layer near the shore was supported by a similar flow pattern in the streamfunction calculation.

Observational study of stratospheric ozone intrusions into the lower troposphere

Abstract To clarify the dynamic behavior of stratospheric ozone intrusion processes into the lower troposphere intensive field observations were made during the middle ten days of May 1986 covering the northern Kyushu district. The data were evaluated in conjunction with spectrophotometic total ozone data. During the study the pattern of surface ozone concentration increase was similar to those for the increase in atmospheric pressure and the decrease in specific humidity. The vertical movement of locally observed ozone peaks was consistent with the movement of a northwesterly dry air zone. An ozone peak was often accompanied by a temperature inversion layer. These observational results showed two major stratospheric ozone intrusion mechanism. At first stratospheric ozone was transported into the upper troposphere due to the descending flow near the cold front and then subsided to the surface under the subsequent high pressure system.

Field and wind tunnel study of the flow and diffusion around a model cube—I. Flow measurements

Abstract Field and wind tunnel studies of the flow and diffusion around a cube were conducted. Part I of this study investigated the flow patterns behind the model. In the field study, the nondimensionalized cavity wake length (the longitudinal distance from the model center to the flow reattachment point normalized by the model height H), Lc/H, was determined using 61 small vanes arranged on the cube roof and nearby ground. Eighty three field tests were conducted and it was found that as the wind angle increased, Lc/H increased, and as the upwind turbulence intensity increased, Lc/H decreased. To isolate the most significant parameters for Lc/H, a statistical analysis was performed and it was found that wind direction had the largest positive correlation followed by Returb, while u∗/U had the largest negative correlation followed by σv/U, σu/U and σw/U. For the range of stabilities tested (−0.54 ⩽ Rif ⩽ 0.13), thermal stratification had a negligible effect on the value of Lc/H. Among these variables, the contribution of the wind direction was 62 % while that of the crosswind turbulence intensity was 12% according to a step-by-step multiple regression analysis. To clarify the results found in the field, a scale model of the cube was investigated in the wind tunnel, changing wind direction and upwind surface roughness. Similar results were noted and comparing the two studies it was found that when wind direction and upwind turbulence intensity, preferably σu/U, σv/U and u∗/U , were matched, the values of Lc/H in the field and wind tunnel agreed.

Wind tunnel observation of flow and diffusion under stable stratification

Abstract Neutral and four different levels of stable stratification (weak to strongly stable i.e. Ri δ = 0.048−0.248 ) were created in the stratified wind tunnel at NIES. The effects of stable stratification on the spread of a plume from a small, circular, ground level release were examined by flow visualization, trace gas concentration measurement, and flow field velocity and turbulence measurements. As stable stratification increased, flow field visualization showed that the turbulence was increasingly suppressed, especially near the surface, and the vertical plume spread was also inhibited with increasing stability. The normalized velocity power exponent, m , increased from 0.17 to 1.04 (for neutral to strong stability), and the u and w components of turbulent intensity were suppressed. The vertical concentration power exponent, p (from C ~ exp (−z p ) ), varied from 1.3 for the neutral case, to 3.0 for the strong stable case. The observed values of p showed reasonably good agreement with the values of p estimated from the half-empirical diffusion equation. The vertical plume spread, σ z , vs distance downwind decreased with increasing stability, appearing as logical extensions of the Pasquill-Gifford chart. However, the horizontal plume spread, σ y , vs distance at first decreased with increasing stability then began to increase as the stability increased from moderate to strongly stable, approaching the neutral case curve. This behavior is caused by the suppression of vertical motion at the stronger stable stratifications which leads to increased plume meandering and plume collapse.

Field and wind tunnel study of the flow and diffusion around a model cube—II. Nearfield and cube surface flow and concentration patterns

Part II of a study on the flow and diffusion around a cube considered the concentration and flow patterns on and around the cube. Nonbuoyant tracer gas was released from the center of the cube roof at low exhaust velocity. The concentration patterns were shown to be strongly dependent upon the flow patterns, and in particular, on the existence of reverse flow at the source location. In the field, vane measurement of the flow showed that there was reverse flow at the rooftop center when the wind angle was small. As upwind turbulence intensity increased, the wind angle at which reverse flow occurred decreased. Modeled in the wind tunnel, this configuration was investigated for five different wind angles (θ = 0–45°) and four different upwind roughnesses (very smooth to very rough), and similar tendencies were found. Reverse flow at the source position resulted in high concentrations at the leading edge of the cube for the case of the smooth upwind surface (low upwind turbulence intensity) and θ = 0°. For the very rough surface and θ = 0°, there was no reverse flow at the source location and so the maximum rooftop concentrations occurred downwind of the source. The effect of increasing upwind turbulence was a decrease in the measured concentrations C∗. In terms of concentration similarity for the downdraft phenomenon, it was found that the wind tunnel model tended to overestimate the concentration found on the roof and may therefore be applied as a conservative indicator of the prototype. However, the same similarity criteria showed that, in addition to simulating Lc/H (see Part I), the location and value of the maximum ground level concentration was in reasonably good agreement in the field and wind tunnel.

Numerical and experimental simulation of vehicle exhaust gas dispersion for complex urban roadways and their surroundings

Abstract Dispersion of automotive exhaust gas along urban roadways was studied through numerical simulation and wind tunnel experiments. Wind and concentration fields were calculated applying two and three dimensional finite difference models, using various roadway and surrounding configurations. Tracer gas dispersion experiments in a wind tunnel were implemented to validate the numerical simulations. For the various configurations, significant correlation was found between the numerical and experimental results. A case study on a real street canyon was also carried out.

Particle-Image Velocimetry Measurements of Separation and Re-attachment of Airflow over Two-Dimensional Hills with Various Slope Angles and Approach-Flow Characteristics

Airflow over two-dimensional hills was investigated in a wind tunnel using particle image velocimetry. We focus on the flow separation behaviour. A trapezoidal hill shape was used in most of the experimental runs, but the critical slope angle for flow separation was approximately the same as that established for smooth hill shapes. The re-attachment point of the separated flow became farther from the hill as the slope angle