Yasushi Ogawa

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.

Aircraft survey of the secondary photochemical pollutants covering the Tokyo metropolitan area

To investigate the photochemical smog formation mechanism over the Kanto Plain, a series of aircraft measurements was conducted from 8 to 12 August 1978 covering the Tokyo Metropolitan area. Various flight patterns were designed to provide appropriate data for determining both the horizontal and vertical pollutant profile. It was found that there is a relationship between the pollutant concentration and the local weather pattern, in particular, the sea-land breeze. 9 August was typical of a sea-land breeze day. In the morning, the sea breeze front formed near the shore, gradually moving inland. The concentration pattern showed two peak regions with the sea breeze front as the dividing line. Using trajectory analysis, it was found that the high concentration zone in the land breeze area is caused by pollutants trapped overnight, and the high concentration zone in the sea breeze region is caused by fresh pollutants. On 12 August 1978, high photochemical smog was observed in the southern part of the Kanto Plain (south of the Tokyo Metropolitan area) caused by just such a complex wind structure. The easterly wind replaced the morning land breeze (northerly wind) during the afternoon. This resulted in a low O3 concentration zone where the clean easterly wind had penetrated into the northern Kanto region. South of this clean zone, a high O3 concentration zone stretched from Tokyo to the Yokohama area. These results show the importance of the long-time transportation of polluted air masses and the complex wind field structure in understanding the behaviour of photochemical smog in this area.

A field investigation of the flow and diffusion around a model cube

Abstract A field experiment was conducted to investigate the downdraft phenomenon behind a simple cube in flat terrain. The effects of wind direction on the mean wind velocity, turbulent intensity, power spectra, cavity wake length, and rooftop and wake region concentrations were measured by eight ultrasonic anemometers, a three-dimensional network of wind vanes, and SF6 tracer gas experiments. As the wind direction from the plane perpendicular to the upwind face of the model increased, the wake region shifted causing shifts in both the velocity deficit region and increased turbulent intensity region. Because of the flow/building interaction, the normalized u and v component peak frequencies behind the cube (at Y H = 0 ) increased but began to recover to their upwind values as X H increased. As θ, the angle between the wind direction and the normal to the upwind face, increased, the power spectra for those observation points which moved into the wake region shifted to smaller λm. At θ = 0°, the cavity wake length, L c H , was approximately 1.7 and the velocity deficit region extended past X H = 6.5 . As θ increased, the maximum normalized ground level concentration increased.

Three-dimensional behaviour of photochemical pollutants covering the Tokyo metropolitan area

Abstract The spatial distribution and transport process of photochemical pollutants covering the Tokyo Metropolitan Area in Japan were investigated from 31 July to 2 August 1979. In the experiment, the vertical profiles of pollutants were observed using four instrumented aircraft. This paper mainly considers the transport process of the polluted air mass using three-dimensional trajectory analysis in which the wind field was determined by objective analysis techniques from pilot-balloon observation data. In the Tokyo Metropolitan Area the sea-land breeze circulation is an important factor in photochemical oxidant formation inland when the geostrophic wind is weak. The night-time radiation inversion which persists into the early morning prevents the dispersion of primary pollutants emitted from the big coastal industrial zones around Tokyo Bay. These pollutants are then advected by the land breeze to the Sagami Bay area which acts as storage tank. Conversion of such pollutants results in a high O3 air mass inland with the penetration of the sea breeze. The sea breeze layer is thermally stable and inhibits vertical mixing of NOx. On the other hand, at the front of the sea breeze zone, a highly turbulent area transports the NOx to 1000–1500 m above mean sea level. Polluted air masses, with O3 concentration exceeding 100 ppb, were observed at 500–1000 m on both 31 July and 1 August 1979. The maximum O3 concentration on the 2nd day exceeded that on the 1st day. These high O3 air masses contained aged pollutant and were entrained into the mixing layer as the depth of the mixing layer increased. This accelerated the formation rate of secondary pollutants. These early morning, high O3 concentrations, and the storage capacity of the Sagami Bay area, are important factors in the time scale of air pollution phenomena in this region.

Surface roughness and thermal stratification effects on the flow behind a two-dimensional fence—I. Field study

Abstract A small-scale field study was conducted to investigate the flow around a two-dimensional fence which was placed perpendicular to the oncoming flow. Characteristics of the oncoming flow and flow behind the fence were measured and, in addition, the flow behind the model was visualized with small wind vanes. The turbulent Reynolds number ( Re turb = UH K , where U is the mean velocity at model height H and K is the turbulent diffusivity) and flux Richardson number at the model height are chosen as the characteristic nondimensional numbers for the oncoming flow. These nondimensional numbers, derived from the normalized momentum and energy equations, are useful and convenient when comparing the field and wind tunnel data for similarity. The cavity wake length L c (chosen as flow characteristic) normalized by the model height H , L c H , was found to increase as Re turb increases. The effects of thermal stratification are unclear for the range of weak stratification tested. For such cases, the effect of the (weak) stratification is probably felt indirectly through its effect on the turbulence. These results will later serve as input and comparative data for a related wind tunnel study (Ogawa and Diosey, 1980) which will investigate the similarity criteria required for laboratory simulation of such phenomena.

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.

Observation of lake breeze penetration and subsequent development of the thermal internal boundary layer for the Nanticoke II Shoreline Diffusion Experiment

The turbulent structure of the lake breeze penetration and subsequent development of the thermal internal boundary layer (TIBL) was observed using a kytoon-mounted ultrasonic anemometer-thermometer. The lake breeze penetrated with an upward rolling motion associated with the upward flow near the lake breeze front. After the lake breeze front passed, the behaviors of the velocity and temperature at the top of the lake breeze layer were similar to those found in convective boundary layers (CBL). Comparing Σgq/θ*, Σu/w* and Σw/w* between the present observation of TIBL development after the passage of the lake breeze front and CBL data from the literature, the Σθ/θ* values showed reasonable agreement; however, Σu/w* and Σw/W* had smaller values in the TIBL than in the CBL at higher altitudes. This is due to the differences in the mean velocity profiles. While the CBL has a uniform velocity profile, the TIBL has a peak at lower elevation due to the lake breeze penetration; the velocity then decreases with height.

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.

Surface roughness and thermal stratification effects on the flow behind a two-dimensional fence—II. A wind tunnel study and similarity considerations

A wind tunnel study of the flow behind a two-dimensional model fence placed perpendicular to the wind was conducted in the NIES Atmospheric Diffusion Simulation Wind Tunnel. Two inflow conditions were studied for their effects on the nondimensionalized cavity wake length, LcH: surface roughness (Returb ≃ 15 to 62) and thermal stratification (Ri ≃ −0.20 to +0.23). It was found that as Returb increases, LcH increases. In the unstable region, LcH decreases only slightly as Ri decreases (from 13.0 in neutral conditions to ∼12.5 for strong unstable); however, in the stable region, LcH decreases drastically as the stability increases (to ∼7.0 for strongly stable). Similar tendencies were found in a related field study (Ogawa and Diosey, 1980) and a comparison of both sets of data in order to determine proper similarity criteria showed that: 1. (1) as a practical choice, the equality of turbulent intensities may be the appropriate similarity criterion for this type of flow; 2. (2) when using over-sized roughness (roughness large relative to model height) to achieve similar flow behind the model, better agreement for the oncoming mean velocity profile was found with the inclusion of a zero-plane displacement factor assumed equal to the roughness element height; and 3. (3) using over-sized roughness for the purpose of simulating the large u-component eddy size found in the atmosphere can lead to an over-sized w-component scale of eddies in the wind tunnel.