Turbulent shear flow behind a single row of bluff obstacles placed on plane boundary

This paper presents an experimental investigation concerned with methods of artificially thickening turbulent boundary layers on a plane surface by using a row of several bluff obstacles. The thickness of this shear layer, the velocity profile and the turbulence intensity of the shear flow behind a row of obstacles were measured and compared with those of a naturally developed turbulent boundary layer. Experiments showed that the shear layer is significantly thicker than the naturally developed turbulent boundary layer, and the velocity profile and the turbulence intensity profiles for the case of a row of cones and triangular plates with vertical angle 2α = 30° are nearly the same as those of a naturally developed turbulent boundary layer.     

An investigation of the environmental impact of an artificial thermal plume

Experimental analysis of the mean and turbulent structures of artificial thermal plumes rising in the boundary layer up into the capping stable layers is reported. Scaling parameters are used to compare the structural aspects obtained from ten experimental cases. Vertical profiles of the mean excesses (temperature and vertical velocity) and of the dissipative parameters (dissipation rate and temperature structure parameter) within the plume are given. The estimates of dissipation rate and of the horizontal width of the column are used to predict the variation of the vertical eddy diffusivity coefficient from the heat source to the top level of the plume.     

Computational Fluid Dynamics (CFD) Simulations on the Effect of Rough Surface on Atmospheric Turbulence Flow Above Hilly Terrain Shapes

The behavioral distribution of the atmospheric turbulence flow over the terrain with changes in a rough surface has become one of the most important topics of air pollution research, among such other topics as transportation and dispersion pollutants. In this study, a computational model on atmospheric turbulence flow over a terrain hill shaped with rough surface was investigated under neutral atmospheric conditions. The flow was assumed to be 2D and modeled using computational fluid dynamics (CFD) models, which were numerically solved using Reynolds-averaged Navier-Stokes equations. Rough surface conditions were modeled using a number of windbreak fences regularly spaced on the hill. The mean velocity and turbulent structures such as turbulence intensity and turbulent kinetic energy were investigated in the upwind and downwind regions over the hill, and the numerical models were validated against the wind-tunnel results to optimize the turbulence model. The computational results agreed well with the results obtained from the wind tunnel experiments. The computational results indicate that the mean velocity was observed to increase dramatically around the crest of the upwind slope of the hill. A thick internal boundary layer was observed with a fence on the crest and downwind region of the hill. The reversed flow and recirculation zone were formed in the wake region behind the hill. It was thus determined that turbulent kinetic energy decreases as the mean velocity increases.     

On the forming mechanism of the cleaning airflow of pulse-jet fabric filters

To reveal the formation mechanism of a pulse-jet airflow’s cleaning effect in a filter bag, a theoretical model is built by using the theory of the gas jet and unitary adiabatic flow according to given specifications and dimensions of the bags and resistance characteristics of the cloth and dust layer. It is about the relationship between cleaning system structure and operating parameters. The model follows the principle that the flow and kinetic energy of jet flow injected into a filter bag should be consistent with the flow of cleaning airflow in the bag and the pressure drop flowing through the filter cloth and dust layer. The purpose of the model is to achieve the peak pressure of cleaning airflow, which dominates the effect of the pulse-jet cleaning process. The cleaning system structure includes air pressure in the nozzle, structure and size of nozzle, exit velocity of nozzle, jet distance, and diameter of jet cross section. Based on the condition of the cleaning system structure and operating parameters established by using the theoretical model, Fluent software is applied to carry out a numerical simulation of the jet airflow field at the nozzle’s outlet, jet airflow field between nozzle and bag top, and cleaning airflow field in the filter bag. Experimental results are used to verify the reliability of the theoretical model. They are obtained in a pilot-scale test filter with a single bag, with length 2 m and in general full-scale dimensions of the cleaning system. The results show that when any rectification measure is not installed at the bag opening, the cross-sectional area covered by the jet gas is hardly sufficient to cover the entire area of the bag opening. A large portion of the gases injected into the filter bag will overflow reversely upward from the edge due to pressure differences between the upper area and lower area inside the bag opening. This led to a serious shortage of the cleaning airflow and ar limited increase in static pressure. When a venturi-type rectifier tube is installed at the bag opening, the jet flow is converted to funnel flow for which the cross-section velocity distribution is more uniform at the throat of the rectifier tube due to the guided effects of the upper tapered pipe. Then it is transited to stressful flow below the bag opening via rectified effects of the lower dilated pipe. The results show that the gap between the static pressure of gas in the bag and the expected value is significantly reduced. The theoretical value of the nozzle diameter is enlarged to compensate for two aspects of adverse effects of cleaning airflow and energy. This is because the flow is not a purely free-form jet from the nozzle to the entrance of the rectifier tube and because the gas suffers from local resistance while flowing through the rectifier tube. The numerical simulation and experiment show that the peak pressure of cleaning airflow in the filter bag is able to reach the expected value. The results confirm that the mechanism of the pulse-jet cleaning airflow and the calculation method of the pulse-jet cleaning system structure and operating parameters offered in this study are correct. The study results provide a scientific basis for designing the system of pulse-jet fabric filters.

Implications: Pulse-jet cleaned fabric filters are commonly used for air pollution control in many industries. Pulse-jet cleaning is widely used for this purpose as it enables frequent cleaning while the filter is operating. However, the theoretical system of the forming mechanism of the pulse-jet cleaning has not formed so far. This indicates the theoretical model plays an important role in designing effective pulse-jet cleaned fabric filters.     

Validation of ADMS against wind tunnel data of dispersion from chemical warehouse fires

Comparisons are presented of the predictions of the atmospheric dispersion modelling system (ADMS) and wind tunnel data for plume dispersion from chemical warehouse fires. The focus of the comparisons is dispersion from structurally intact buildings with open roofs and dispersion of plumes flush with the ground without obstacles, however, dispersion from building shells and doors is also considered. Both buoyancy driven and momentum driven flows are treated, although emphasis is on buoyancy driven flows as these are generally more likely to occur in warehouse fires. The study shows that the ADMS building module is able to reproduce many of the features of dispersion observed in the wind tunnel. These include a recirculating region behind the building in which material may be trapped, a main wake which brings material down towards the surface, and appropriate sensitivity to the buoyancy and momentum of the emitted material, and the location of sources on the building roof. The comparisons suggest that the ADMS building model can be used to predict dispersion from the stages of fire development studied. The precise level of agreement depends (but not in a systematic way) on the buoyancy flux parameter F_B, the momentum flux parameter F_M and the number of roof lights. There are some significant differences between the wind tunnel boundary layer and the simulated atmospheric boundary layer in ADMS which have to be considered when making wind tunnel model comparisons. These relate mainly to the near surface where the wind tunnel underestimates turbulent velocities, the boundary layer height which in the wind tunnel corresponds to an atmospheric boundary layer depth of 82.5 m (atmospheric boundary layers are frequently an order of magnitude deeper), and the boundary layer top where the ADMS boundary layer is capped by an inversion and has low turbulence levels whereas the wind tunnel boundary layer has higher levels of turbulence and no capping inversion.     

Some Factors that Affect the Deposition Rates of Sulfur Dioxide and Similar Gases on Vegetation

The deposition of sulfur dioxide on growing vegetation is affected by diverse environmental factors, many of which undergo large diurnal and spatial variations. The aerodynamic resistance to vertical transfer in the surface boundary layer can be formulated in terms of the friction velocity, height of observation, vertical heat flux, and surface roughness. Also important are the resistance in the air layer closest to the surface elements and, in dry vegetation, the average stomatal resistance of the plants. The latter variable is among the most difficult to estimate, but over many agricultural field crops like those in the midwestern U.S., a typical minimum value of average stomatal resistance to SO₂ transfer is about 0.7 s cm^-1, as is indicated by various experimental data. The deposition velocity can be estimated as the inverse of the sum of the resistances of the layers, necessarily down to where the concentrations are zero; in the surface boundary layer, any of the various resistances might be dominant. Above the surface layer, the micrometeorological relationships are known with less certainty, but reasonable approximations indicate that during unstable conditions the resistance to transfer is very small at heights of several tens of meters and during stable conditions the aerodynamic resistance is very large aloft.     

City ventilation of Hong Kong at no-wind conditions

We hypothesize that city ventilation due to both thermally-driven mountain slope flows and building surface flows is important in removing ambient airborne pollutants in the high-rise dense city Hong Kong at no-wind conditions. Both spatial and temporal urban surface temperature profiles are an important boundary condition for studying city ventilation by thermal buoyancy. Field measurements were carried out to investigate the diurnal thermal behavior of urban surfaces (mountain slopes, and building exterior walls and roofs) in Hong Kong by using the infrared thermography. The maximum urban surface temperature was measured in the early noon hours (14:00–15:00 h) and the minimum temperature was observed just before sunrise (5:00 h). The vertical surface temperature of the building exterior wall was found to increase with height at daytime and the opposite occurred at nighttime. The solar radiation and the physical properties of the various urban surfaces were found to be important factors affecting the surface thermal behaviors. The temperature difference between the measured maximum and minimum surface temperatures of the four selected exterior walls can be at the highest of 16.7 °C in the early afternoon hours (15:00 h). Based on the measured surface temperatures, the ventilation rate due to thermal buoyancy-induced wall surface flows of buildings and mountain slope winds were estimated through an integral analysis of the natural convection flow over a flat surface. At no-wind conditions, the total air change rate by the building wall flows (2–4 ACH) was found to be 2–4 times greater than that by the slope flows due to mountain surface (1 ACH) due to larger building exterior surface areas and temperature differences with surrounding air. The results provide useful insights into the ventilation of a high-rise dense city at no-wind conditions.     

Chemistry and aerosols in the marine boundary layer: 1-D modelling of the three ACE-2 Lagrangian experiments

Three Lagrangian experiments were conducted during IGAC's second aerosol characterization experiment (ACE-2) in the area between Portugal, Tenerife and Madeira in June/July 1997. During each Lagrangian experiment, a boundary layer air mass was followed for about 30 h, and the temporal evolution of its chemical and aerosol composition was documented by a series of vertical profiles and horizontal box pattern flown by the Meteorological Research Flight research aircraft Hercules C130. The wealth of observational data that has been collected during these three Lagrangian experiments is the basis for the development and testing of a one-dimensional Lagrangian boundary layer model with coupled gas, aqueous, and aerosol phase chemistry. The focus of this paper is on current model limitations and strengths. We show that the model is able to represent the dynamical and chemical evolution of the marine boundary layer, in some cases requiring adjustments of the subsidence velocity and of the surface heat fluxes. Entrainment of a layer rich in ozone and carbon monoxide from a residual continental boundary layer into the marine boundary layer as well as in-cloud oxidation of sulphur dioxide by hydrogen peroxide are simulated, and coherent results are obtained, concerning the evolution of the small, presumably sulphate–ammonia aerosol mode.     

Numerical simulation of the urban boundary layer over the complex terrain of Hong Kong

Due to the complexity of the underlying surface, urban boundary layers may exhibit very different wind-temperature field structures compared with rural areas. In this study, an urban boundary layer model with a resolution of 500 m is applied to Hong Kong, a place characterized by complex topography with high mountains and dense urban developments. Five surface land use types are considered; grass and shrub land, trees, water, old urban areas and new town developments. The urban boundary layer model is embedded into the National Center for Atmospheric Research (NCAR) Mesoscale Model, version 5 (MM5). The initial and boundary conditions are obtained from the National Centers for Environmental Prediction (NCEP)/NCAR reanalysis dataset. The modeling approach therefore takes into account both the mesoscale background field and the urban underlying surface. The model is applied to the simulation of a pollution episode in Hong Kong. Results show good agreement with meteorological data for the surface winds and temperature. The model successfully simulates the urban heat island and the occurrence of a sea–land breeze circulation, and their impact on air pollutant transport and dispersion.     

A model study for ambient air quality analysis using routine meteorological observations

In the present study, more realistic and easily adaptable input parameters have been used with a view to investigating the long-range air quality analysis for the dispersion of air pollutants emitted from an area source with a multiple box model. The model formulation has been discussed at length for the ground level sources when convective conditions prevail. The routine meteorological observations have been used for the computation of sensible surface heat flux, friction velocity and mixing depth. A radiation model provides the estimates of the sensible surface heat flux. Based on the similarity theory, an iterative procedure has been adopted for the estimation of friction velocity which provides a coupling of radiation computation and the surface layer of the planetary boundary layer through surface heat flux expression. The important parameters—wind speed and eddy diffusivity profiles—have been derived and have been used to obtain the concentration patterns as hourly averages. The procedure could be easily adopted where observed meteorological parameters may be used for studying the dispersal of pollutants from the ground level sources.     

Wind tunnel study of air entrainment into two-dimensional dense gas plumes at the Chemical Hazards Research Center

Experiments in a neutrally stable wind tunnel boundary layer were made for two-dimensional (quasi-line) sources of carbon dioxide dispersing over two types of uniformly spaced (billboard) surface roughness elements. Velocity and concentration measurements were made with each surface roughness over a wide range of source Richardson number by varying carbon dioxide release rate and wind speed. Concentration measurements were made with a FID gas analyzer using an ethane tracer in the source gas, and velocity measurements were made with independent LDV and HWA systems. For each surface roughness, this paper describes the wind tunnel boundary layer and presents alongwind and vertical concentration profiles in the gas plume. Vertical velocity and concentration profiles were measured at selected downwind distances, and the profiles were integrated to confirm the consistency of the measurements with the mass of carbon dioxide released. The data are intended for development of improved vertical turbulent entrainment correlations for use in dense gas dispersion models applied to hazardous chemical consequence analyses.     

Influence of wall roughness on the dispersion of a passive scalar in a turbulent boundary layer

Many towns and cities consist of similarly sized buildings in relatively regular arrangements with smaller scale roughness elements such as roofs, chimneys and balconies. The objective of this study is to investigate how small scale roughness elements modify the influence of the large scale organized roughness on the dispersion of a passive scalar in a turbulent boundary layer. Wind tunnel experiments were performed using a passive tracer released from a line source and concentration profiles were measured with a Flame Ionisation Detector. The measurements are compared with numerical solutions of the advection–diffusion equation.The results show that decreasing the cavity aspect ratio increases the turbulent vertical mass fluxes, and that the small scale roughness enhances these fluxes, but only in the skimming flow regime. Numerical simulations showed that outside the roughness sub-layer (RSL) the changes in surface roughness could be accounted for by a simple variation of the friction velocity, but inside the RSL the spatial variability of the flow imposed by the roughness elements has much more influence. A simple model for a spatially averaged dispersion coefficient in the RSL has been developed and is shown to agree satisfactorily with the concentrations measured in these experiments.     

Numerical calculation of flow and stack-gas concentration fluctuation around a cubical building

A numerical simulation model was developed to predict the instantaneous concentration fluctuation of a plume and applied to stack-gas diffusion around a cubical building. The flow field, including an instantaneous velocity component, was predicted using the large eddy simulation (LES) method in the developed numerical model. Then, the instantaneous concentration fluctuation was predicted using the obtained unsteady flow field. Concentration was calculated using the finite difference method, in which the LES is expanded for concentration, and the puff method, in which small volumes of the tracer gas are divided and combined according to the calculation mesh sizes. In order to avoid numerical viscous effects, a puff method and finite difference method were applied separately in the regions near and far from the stack-gas release point, respectively. Then, the flow field around a cubical building and the diffusion of stack-gas, emitted from an elevated point source at an upstream position of the building, were calculated using the model mentioned above. Numerical calculation results were compared with those obtained in wind tunnel experiments in which concentration fluctuation was measured using high-response flame ionization detectors. Although there were some discrepancies in the flow field between the calculated results and those of wind tunnel experiments, e.g., the calculated windward length of a cavity region behind the building, the calculated mean velocity and turbulent intensity showed good agreement with those of the wind tunnel experiments. Furthermore, the calculated concentration fluctuation showed good agreement with that in the wind tunnel, not only regarding the features of fluctuating concentration signals, but also statistic quantities, viz., mean concentration, fluctuation intensity and high-concentration values.     

Secondary ozone maxima in a very stable nocturnal boundary layer: observations from the Lower Fraser Valley,BC

The relationship between near-surface ozone concentration and the structure of the nocturnal boundary layer was investigated during a field campaign conducted in 1998 in the Lower Fraser Valley (LFV), British Columbia Canada. Despite the spatial and temporal variation in frequency and morphology, secondary nocturnal ozone maxima were shown to be an important feature of the diurnal ozone cycle throughout the LFV, and localised increases in ozone occasionally exceeded more than half the previous day's maximum concentration.Turbulence in the nocturnal boundary layer was shown to be weak and intermittent. Vertical profiles of Richardson number and ozone concentration indicated that the temporary turbulent coupling of the residual layer to the surface layer facilitated the transport of ozone stored aloft to the surface. Despite the overall complexity of the system, results show that seven out of the 19 ozone spikes observed at the Aldergrove site coincided with turbulence associated with the development of the down-valley wind system. A further nine spikes occurred during periods when a low-level jet was identified aloft. Significantly, ozone concentrations were shown to be highly variable in the residual layer and played an important role in determining the morphology of secondary ozone maxima at the surface. Largest increases in surface ozone concentration occurred when turbulence coincided with periods when ozone concentrations in excess of 80 ppb were observed aloft.     

Simulation of boundary layer trajectory dispersion sensitivity to soil moisture conditions: MM5 and Noah-based investigation

In this study, the sensitivity of trajectory paths to anomalous soil moisture was analyzed during three different synoptic episodes in June 2006. The MM5 and Noah land surface models were used to simulate the response of the planetary boundary layer. The HYSPLIT model was used for trajectory analysis. It was found that the response in horizontal lower-level wind field was larger at regions where vertical wind velocity changes were also large. In addition, the sensitivity to soil moisture changes was significant and localized where convective activity was well developed and synoptic effects did not dominate. A non-local effect was felt over the rest of the domain where convection was not present since the model atmosphere reacted as a whole to compensate for induced changes in vertical velocity. This finding was supported by the fact that domain averaged vertical velocities changes were of the order of 0.2 cm s^?1 or less at about 650 hPa and about 200 times smaller than modeled local vertical velocity changes. The largest change in horizontal wind field near the surface was found for weak synoptic events on June 11–12 and June 22–23 while the stronger synoptic event of June 17–18 showed smaller differences. These changes in wind field conditions impacted the transport and dispersion of pollutants. To quantify the sensitivity of air quality estimates to soil moisture uncertainty, we have used three well known measures of trajectory differences: the absolute horizontal transport deviation (AHTD), the relative horizontal transport deviation (RHTD) and the absolute vertical transport deviation (AVTD) for an ensemble of 98 trajectories departing from a region well within the computational domain. For the June 11–12 event it was found that for wet and dry soil moisture experiments, AHTD, RHTD, and AVGTD can reach values in the range 60–100 km, 10–20% and 500–900 m at 24 h run time, respectively. For the June 17–18 and June 22–23 events these values of trajectory differences were reduced more than half. These differences in behavior between time periods are largely attributed to the combined effects of synoptic forcing and the sensitivity of planetary boundary layer to soil moisture changes during well developed convection. The implication for air quality studies is that the soil moisture anomaly and related uncertainty in planetary boundary layer response needs to be incorporated in order to construct an ensemble of the most probable scenarios in which pollutants are released and transported throughout a given target region.     

An examination of boundary layer structure under the influence of the gap winds in Urumqi, China, during air pollution episode in winter

Tethered-sonde measurements of atmospheric profiles were performed at Urumuqi, capital of the Xinjiang Uyghur Autonomous Region of China, from 29 December 2008 to 14 January 2009. The data were used to examine the boundary layer structure during this severe air pollution period. Diurnal evolution of local wind flow near Urumqi was simulated using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5). Measurements from operational radiosonde data showed that a southeasterly elevated low-level jet often intruded upon Urumqi through the middle Tianshan Mountain pass to the south of the city. The tethered-sonde measurements showed that calm and northwesterly winds prevailed near the surface in Urumqi, whereas the southeasterly winds of relatively higher speed were dominant above approximately 400 m. Both temperature inversion and humidity inversion frequently occured during day and nighttime. Temperature inversion intensity could sharply rise as the stronger elevated southeasterly gale (ESEG) happened. Model simulations showed that the winds near the surface around Urumqi remained calm during nighttime and developed toward the mountains during daytime. As cool airflow in the basin confronted the southeasterly winds from the pass in the lower layer, they formed a convergence line around Urumqi city, which was not favor for dilution of air pollutants.     

A wind tunnel study of dense gas dispersion in a stable boundary layer over a rough surface

Measurements of the vertical entrainment velocity into two-dimensional dense gas plumes over fully rough surfaces were carried out as part of a co-operative research programme with wind tunnel facilities in the USA. This paper presents results obtained for stable boundary layer conditions in the EnFlo wind tunnel at the University of Surrey; a companion paper treats the neutral boundary layer case. Mean velocity and temperature, turbulent normal and shear tresses, temperature fluctuations and heat fluxes were measured and used to demonstrate that a moderately stable atmospheric boundary layer had been successfully simulated in the tunnel. Entrainment velocities, W_E, were then deduced from the streamwise development of the concentration field, non-dimensionalised with respect to the friction velocity in the undisturbed flow, u^*, and correlated with the plume Richardson number, Ri^*. Higher non-dimensional entrainment speeds, W_E/u^*, were observed for Ri^*>5 in the stable boundary layer than in the neutral boundary layer, the difference growing with increasing Richardson number. Emission velocity ratios, W₀/u^*, were however larger in the stable experiments, and exceeded one at about Ri^*=18. Entrainment in the stable boundary layer appeared therefore to be more sensitive to emission velocity ratio than in the neutral case. Entrainment behaviour for Ri^*⩽5 followed that found in the neutral boundary layer. In this regime, use of the neutral boundary layer entrainment speed correlation is unlikely to lead to the over-prediction of plume dilution rates in moderately stable boundary layers.     

An efficient Lagrangian stochastic model of vertical dispersion in the convective boundary layer

We consider the one-dimensional case of vertical dispersion in the convective boundary layer (CBL) assuming that the turbulence field is stationary and horizontally homogeneous. The dispersion process is simulated by following Lagrangian trajectories of many independent tracer particles in the turbulent flow field, leading to a prediction of the mean concentration. The particle acceleration is determined using a stochastic differential equation, assuming that the joint evolution of the particle velocity and position is a Markov process. The equation consists of a deterministic term and a random term. While the formulation is standard, attention has been focused in recent years on various ways of calculating the deterministic term using the well-mixed condition incorporating the Fokker–Planck equation. Here we propose a simple parameterisation for the deterministic acceleration term by approximating it as a quadratic function of velocity. Such a function is shown to represent well the acceleration under moderate velocity skewness conditions observed in the CBL. The coefficients in the quadratic form are determined in terms of given turbulence statistics by directly integrating the Fokker–Planck equation. An advantage of this approach is that, unlike in existing Lagrangian stochastic models for the CBL, the use of the turbulence statistics up to the fourth order can be made without assuming any predefined form for the probability distribution function (PDF) of the velocity. The main strength of the model, however, lies in its simplicity and computational efficiency. The dispersion results obtained from the new model are compared with existing laboratory data as well as with those obtained from a more complex Lagrangian model in which the deterministic acceleration term is based on a bi-Gaussian velocity PDF. The comparison shows that the new model performs well.     

Influence of trees on the dispersion of pollutants in an urban street canyon—Experimental investigation of the flow and concentration field

Flow field and concentration measurements have been performed in an idealized model of an urban street canyon with one row of trees arranged along the center axis. The model was set up in an atmospheric boundary layer wind tunnel and the approach flow was directed perpendicular to the street axis. A line source embedded in the bottom of the street was used to release tracer gas for the simulation of traffic exhaust emissions. Trees with spherical crowns were modeled and positioned inside the street canyon, varying crown diameter, crown permeability, trunk height and tree spacing. Traffic-induced turbulence was simulated by rotating belts with thin plates. Concentrations were measured at the facades of the street canyon. For small tree crowns, only little changes in concentration were measured, however, increasing crown diameters led to increasing concentrations at the leeward street canyon wall associated with a reduction of local concentrations at the windward wall. For some cases, a variation of trunk height led to a modification of the concentration pattern on the walls. Increasing the tree spacing resulted in a noticeable concentration decrease. When compared to the situation with standing (but emitting) traffic, the traffic-induced turbulence by two-way car movements always contributed to a more homogenous concentration field inside the street canyon yielding to reduced mean concentration levels.