Hiromasa Ueda

The relationship between surface-renewal and bursting motions in an open-channel flow

Surface-renewal motions in the interfacial region below a gas-liquid interface were experimentally investigated in relation to bursting motions in the wall region. To estimate the frequency of the appearance of surface-renewal eddies, mass-transport experiments with methylene-blue solution, together with velocity measurements, were done in an open-channel flow. The instantaneous concentration of methylene-blue tracer emitted from a point source positioned in the buffer layer was measured at the free surface downstream from the source by an optical probe. Instantaneous streamwise velocity was measured using a laser-Doppler velocimeter at a position in the buffer region. Frequencies of both surface-renewal and bursting events were computed from these concentration and velocity signals using a conditional-averaging method. In order to clarify whether the surface-renewal eddies actually dominate mass transfer across the gas-liquid interface, gas-absorption experiments were added. Carbon dioxide was absorbed into the water flow across the calm free surface and its mass-transfer coefficient on the liquid side was measured under the same flow conditions as used in the above mass-transport experiments. The results show that the surface-renewal motions originate in the bursting motions which vigorously occur in the buffer region. That is, the decelerated fluid which is strongly lifted towards the outer layer by bursting almost always arrives at the free surface and renews the free surface. The frequency of the surface renewal, as well as the bursting frequency, is uniquely determined by the wall variables or the outer-flow variables and the Reynolds number. Mass transfer across the gas-liquid interface is dominated by the large-scale surface-renewal eddies, and the mass-transfer coefficient on the liquid side is proportional to the square-root of the surface-renewal frequency.

Turbulence structure in stably stratified open-channel flow

The effects of stable stratification on turbulence structure have been experimentally investigated in stratified open-channel flow and a theoretical spectral-equation model has been applied to the stably stratified flow. The measurements were made in the outer layer of open-channel flow with strongly stable density gradient, where the wall effect was small. Velocity and temperature fluctuations were simultaneously measured by a laser-Doppler velocimeter and a cold-film probe. Measurements include turbulent intensities, correlation coefficients of turbulent fluxes and coherence–phase relationships. These turbulent quantities were correlated with the local gradient Richardson number and compared with the values calculated using a spectral-equation model and with other laboratory measurements. In stable conditions, turbulent motions approach wavelike motions, and negative heat and momentum transfer against the mean temperature and velocity gradient occurs in strongly stable stratification.

Photochemical formation of particulate dicarboxylic acids under long-range transport in central Japan

Abstract Long-range transport (LRT) of photochemical air pollution from the coastal area with large emission sources to the inland mountainous region occurs very frequently in the central Japan region on clear summer days. It is caused by local winds and the transport route is almost-fixed geographically. Along this route, behavior of dicarboxylic acids in the airborne aerosols was investigated in the cooperative field observation of meteorology and chemistry. Measurements were made every 3-h at inland sites. The concentrations of dicarboxylic acids as well as NO3− and total organic C increased in the daytime and decreased at night. The maximum are attained when the transported air mass was arrived at the sampling sites. These diurnal variations were similar to that of O3. In the daytime, although the phthalates (di-n-butyl and dioctyl), n-alkanes (C21C32) and pinon aldehyde were abundant in the airborne aerosols, the dicarboxylic acids (C2C10) were the most abundant species and the total concentrations attained from 30 to 50% of the total organic particulate matter. In addition, of the dicarboxylic acids, more than 70% were estimated to be produced by photochemical reactions in the daytime. Thus, it was concluded that most of the dicarboxylic acids were produced by the photochemical oxidation of anthropogenic compounds during LRT.

Numerical modeling of the nocturnal urban boundary layer

A numerical case study with a second-order turbulence closure model is proposed to study the role of urban canopy layer (UCL) for the formation of the nocturnal urban boundary layer (UBL). The turbulent diffusion coefficient was determined from an algebraic stress model. The concept of urban building surface area density is proposed to represent the UCL. Calculated results were also compared with field observation data. The height of the elevated inversion above an urban center was simulated and found to be approximately twice the average building height. The turbulent kinetic energy k, energy dissipation rate ε, and turbulence intensities 〈u2〉 and 〈w2〉 increase rapidly at the upwind edge of the urban area. The Reynolds stress 〈uw〉 displayed a nearly uniform profile inside the UBL, and the vertical sensible heat flux 〈wθ〉 had a negative value at the inversion base height. This indicates that the downward transport of sensible heat from the inversion base may play an important role in the formation of the nocturnal UBL.

An observational study of the structure of the nocturnal urban boundary layer

The formation mechanism of the nocturnal urban boundary layer (UBL), especially in the winter nighttime, was investigated based on the extensive field observations conducted during November 1984 in Sapporo, Japan. A strong, elevated inversion formed over the Sapporo urban area and the inversion base height was approximately twice the average building height. Velocity fluctuations Σu, Σw and Reynolds stress % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaaca% WG1bWaaWbaaSqabeaacaaIXaaaaGGaaOGae8hiaaIaam4DamaaCaaa% leqabaGaaGymaaaaaaaaaa!3A9C!\[\overline {u^1 w^1 } \] had nearly uniform profiles within the nocturnal UBL and decreased with height above the UBL. On the other hand, temperature fluctuations Σt, and heat fluxes % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaaca% WG1bWaaWbaaSqabeaacaaIXaaaaGGaaOGae8hiaaIaeqiUde3aaWba% aSqabeaacaaIXaaaaaaaaaa!3B56!\[\overline {u^1 \theta ^1 } \] and % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaaca% WG3bWaaWbaaSqabeaacaaIXaaaaGGaaOGae8hiaaIaeqiUde3aaWba% aSqabeaacaaIXaaaaaaaaaa!3B58!\[\overline {w^1 \theta ^1 } \] had peaks at the inversion base and small values within the nocturnal UBL. The turbulent kinetic energy budget showed that the turbulent transport term and shear generation from urban canopy elements are important in the nocturnal UBL development; the role of the buoyancy term is small. The turbulence data analysis and application of a simple advective model showed that the mechanism of UBL formation may be controlled by the downward transport of sensible heat from the elevated inversion caused by mechanically-generated turbulence.

The transport and formation of photochemical oxidants in central Japan

Abstract Major aspects of the physical and chemical transformations of the oxidant-polluted air mass resulting from high precursor emission fluxes of NO x and reactive hydrocarbons (HCs) in the Tokyo metropolitan area and their subsequent long-range transport (LRT) within the Kanto Plain and Nagano Prefecture regions of Japan are evaluated through field observations and model analysis. The field study was conducted in the Kanto District and the Nagano Prefecture in Japan in July 1983. The LRT mechanism in this region is found to involve land/sea breeze, mountain/valley winds, steady onshore winds, strong thermal low and subsidence inversions under a synoptic-scale high pressure. Detailed diagnostic analysis of the LRT event observed on 27 and 28 July 1983 is presented. The analysis is performed using a detailed Eulerian transport/chemistry model (i.e. the STEM-II model). Ozone profiles in the Tokyo metropolitan area show typical diurnal variation with maximum concentrations of ~ 60 ppb in the early afternoon. Beginning about noon local time an extended sea breeze circulation develops and transports this NO x and HC polluted air mass inland. Ozone concentrations in excess of 160 ppb occur in this air mass as it is transported into the inland mountainous regions (located ~ 150 km downwind of Tokyo) in the early evening.

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.

Meteorological conditions for long-range transport under light gradient winds

Abstract Synoptic-scale meteorological conditions causing the long-range transport of air pollution with light gradient winds were investigated by analyzing atmospheric pressures and ground level winds as well as aerological data for the period from May to September 1979. Special attention was paid to the thermal low which was generated in the daytime in the mountainous region, and the characteristics of the upper atmosphere. Long-range transport occurred under a synoptic-scale high pressure with a pressure gradient of less than 1.2 mb/100 km and with an upper wind of about half the wind speed averaged over the entire period under study. Under these conditions, local winds developed in the lower atmosphere, and the thermal low and subsidence inversion accompanied by the synoptic-scale high pressure led to the combination of the local winds into two large-scale winds blowing towards the thermal low. The synoptic-scale pressure distribution determines the location of the thermal low center and therefore also determines the horizontal extent of the long-range transport.

Estimation of vertical air motion from limited horizontal wind data—a numerical experiment

Abstract The variational calculus and direct-differencing objective analysis procedures, which are formulated to produce mass conservative wind fields from horizontal wind data, were evaluated in numerical experiments. In the initial tests, the variational calculus procedure was evaluated using a test flow field generated by a land/sea breeze numerical model. The results indicated that this procedure reproduced the complex vertical flow field quite well if a minimum measurement density level was retained. A second set of numerical experiments employed the variational calculus procedure and direct-differencing method to generate a three-dimensional wind field for the Mikawa Bay area in Japan, using horizontal surface winds as input. The weighting parameter in the variational procedure, ( α 1 α 2 )2, affected both the resulting vertical motion and the residual divergence. The vertical motion was lower for smaller values of ( α 1 α 2 )2 and there was an optimum value of ( α 1 α 2 )2 to minimize the residual divergence. The direct-differencing method yielded quite similar flow patterns to those resulting from the variational method. However, this method requires very accurate horizontal wind field data as input to obtain vertical winds of reasonable magnitude.