Atmospheric stratification involves differences in the air density caused by a positive (stable) or negative (unstable) vertical gradient of virtual potential temperature. The stability of the layer depends on the stratification and affects the atmospheric boundary layer depth and structure as well as velocity, temperature and turbulence properties.
In the first phase of the work, artificially thickened stable and unstable boundary layers were simulated in the EnFlo wind tunnel over a very rough surface, by means of spires, roughness elements and heaters. The effect of different parameters was investigated (among them, inlet temperature profile, capping inversion and surface roughness). These boundary layers were then employed as approaching flow for two idealised urban model geometries.
A regular array of rectangular blocks was considered as geometry while a pollutant tracer was released from a point source at ground level. Mean and fluctuating velocities, temperatures and concentrations were sampled, together with heat and pollutant fluxes. The analysis of the data revealed that even in case of weak stratification there are important modifications inside and above the canopy on both the urban boundary layer and the plume characteristics.
Finally, the combined effects of a stable approaching flow and local surface heating were investigated in a bi-dimensional street canyon geometry. This was an entirely novel experimental design and the results highlighted how both local and incoming stratification can significantly affect the flow and dispersion at a microscale level in a complex way that depends on the particular case of study.
This work sheds more light on the effects of stratification and encourages further work on the topic. The experimental database produced during the project is unique and of high quality. It can assist in developing, improving and validating numerical models, as well as developing parametrisations for simpler models.
Robins AG, Hayden P, Carpentieri M (2015)DIPLOS 1
University of Surrey
Stable and convective boundary layers over a very rough surface have been studied in a thermally-stratified wind tunnel. Artificial thickening by means of spires was used to accelerate the formation of a sufficiently deep boundary layer, suitable for urban-like boundary layer flow and dispersion studies. For the stable boundary layer, the methodology presented in Hancock and Hayden (2018) for low-roughness offshore surface conditions has been successfully applied to cases with higher-roughness. Different levels of stratification and roughness produced modifications in the turbulence profiles of the lower half of the boundary layer, but little or no change in the region above. Data for a stronger stability case suggested that the employed spires may not be suitable to simulate such extreme condition, though further studies are needed. The results were in reasonably good agreement with field measurements. For the convective boundary layer, great attention was given to the flow uniformity inside the test section. The selection of a non-uniform inlet temperature profile was in this case found not as determinant as for the stable boundary layer to improve the longitudinal uniformity, while the application of a calibrated capping inversion considerably improved the lateral uniformity. The non-dimensional vertical profiles of turbulent quantities and heat fluxes, did not seem to be influenced by roughness.
Scalar dispersion from ground-level sources in arrays of buildings is investigated using wind-tunnel measurements and large-eddy simulation (LES). An array of uniform-height buildings of equal dimensions and an array with an additional single tall building (wind tunnel) or a periodically repeated tall building (LES) are considered. The buildings in the array are aligned and form long streets. The sensitivity of the dispersion pattern to small changes in wind direction is demonstrated. Vertical scalar fluxes are decomposed into the advective and turbulent parts and the influences of wind direction and of the presence of the tall building on the scalar flux components are evaluated. In the uniform-height array turbulent scalar fluxes were dominant, whereas the tall building causes an increase of the magnitude of advective scalar fluxes which become the largest component. The presence of the tall building causes either an increase or a decrease to the total vertical scalar flux depending on the position of the source with respect to the tall building. The results of the simulations can be used to develop parametrizations for street canyon dispersion models and enhance their capabilities in areas with tall buildings.
Flow and pollutant dispersion models are important elements for managing air quality in urban areas, to complement and, sometimes, even substitute monitoring. Developing fast and reliable parameterisations is necessary to improve the spatial and temporal resolutions of current mathematical prediction models. Recently there has been a growing interest in the so-called "neighbourhood scale" models, that offer relatively high spatial and temporal resolutions while keeping the needed computational resources at a minimum. This paper describes experimental and numerical simulations performed to explore the interaction of flow and pollutant dispersion with local building and street geometry. The methods developed may be useful as a way for cities to improve air quality management. © 2012 Springer Science+Business Media Dordrecht.
In this paper non-neutral approaching flows were employed in a meteorological wind tunnel on a regular urban-like array of rectangular buildings. As far as stable stratication is concerned, results on the flow above and inside the canopy show a clear reduction of the Reynolds stresses and an increment of the Monin-Obukhov length up to 80%. The roughness length and displacement height were also affected, with a reduction up to 27% for the former and an increment up to 5% for the latter. A clear reduction of the turbulence within the canopy was observed. In the convective stratication cases, the friction velocity appears increased by both the effect of roughness and unstable stratication. The increased roughness causes a reduction in the surface stratication, reflected in an increase of the Monin-Obukhov length, which is double over the array compared to the approaching ow. The effect on the aerodynamic roughness length and displacement height are specular to the SBL case, an increase up to 55% of the former and a reduction of the same amount for the latter.
Wind tunnel measurements downwind of reduced scale car models have been made to study the wake regions in detail, test the usefulness of existing vehicle wake models, and draw key information needed for dispersion modelling in vehicle wakes. The experiments simulated a car moving in still air. This is achieved by (i) the experimental characterisation of the flow, turbulence and concentration fields in both the near and far wake regions, (ii) the preliminary assessment of existing wake models using the experimental database, and (iii) the comparison of previous field measurements in the wake of a real diesel car with the wind tunnel measurements. The experiments highlighted very large gradients of velocities and concentrations existing, in particular, in the near-wake. Of course, the measured fields are strongly dependent on the geometry of the modelled vehicle and a generalisation for other vehicles may prove to be difficult. The methodology applied in the present study, although improvable, could constitute a first step towards the development of mathematical parameterisations. Experimental results were also compared with the estimates from two wake models. It was found that they can adequately describe the far-wake of a vehicle in terms of velocities, but a better characterisation in terms of turbulence and pollutant dispersion is needed. Parameterised models able to predict velocity and concentrations with fine enough details at the near-wake scale do not exist.
In the present paper we have analysed experimentally (wind tunnel) and numerically (CFD) the impact of some morphological parameters on the flow within and above the urban canopy. In particular, this study is a first attempt in systematically studying the flow in and above urban canopies using simplified, yet more realistic than a simple array of cuboids, building arrays. Current mathematical models would provide the same results for the six case studies presented here (two models by three wind directions), however the measured spatially averaged profiles are quite different from each other. Results presented here highlight that the differences in the spatially averaged vertical profiles are actually significant in all six experimental/numerical cases. Besides the building height variability, other morphological features proved to be a significant factor in shaping flow and dispersion at the local to neighbourhood scale in the urban canopy and directly above: building aspect ratio (or, conversely, the street canyon aspect ratio), the angle between the street canyons and the incoming wind and local geometrical features such as, for example, the presence of much taller buildings immediately upwind of the studied area.
The work presented here is aimed at developing an indirect methodology for landfill gas emission monitoring by using an integrated approach between measurements and modelling. The proposed methodology is based on an optical measurement system, capable of quantifying concentrations of a tracer gas emitted by a waste landfill, along with a modelling system for tracer gas dispersion in the atmosphere. In the present study, this methodology has been applied, as a preliminary test, at the Case Passerini landfill site, in the Sesto Fiorentino (FI) territory. The test case allowed the evaluation of the proposed methodology, highlighting the positive aspects and the critical factors. The obtained results showed the potentiality of this approach, which can be used in order to integrate, and sometimes even to substitute, more expensive field direct measurement campaigns.
Corti A, Lombardi L, Carpentieri M, Buiatti E, Bartolacci S, Bianchi F, Linzalone N, Minichilli S, Mancuso S (2006)Valutazione di impatto sanitario del Piano di gestione rifiuti urbani della Provincia di Firenze, In: Lombardi L, Carpentieri M (eds.), Ingegneria Ambientale, QuaderniXXXIV(41)
The effects of a stably-stratified boundary layer on flow and dispersion in a bi-dimensional street canyon with unity aspect ratio have been investigated experimentally in a wind tunnel in combination with differential wall heating. Laser-Doppler anemometry together with a fast flame ionisation detector and cold-wire anemometry were employed to sample velocities, concentration, temperatures and fluxes.
A single-vortex pattern was observed in the isothermal case, preserved also when leeward wall was heated, but with a considerable increment of the vortex speed. Heating the windward wall, instead, was found to generate a counter-rotating vortex, resulting in the reduction of velocity within the canopy. The stable stratification also contributes reducing the speed, but only in the lower half of the canyon. The largest values of turbulent kinetic energy were observed above the canopy, while inside they were concentrated close to the windward wall, even when the leeward one was heated. An incoming stable stratification produced a significant and generalised turbulence reduction in all the cases. Windward heating was found to produce larger temperature increments within the canopy, while in the leeward case heat was immediately vacated above the canopy. A stable approaching flow reduced both the temperature and the heat fluxes.
A passive tracer was released from a point source located at ground level at the centre of the street canyon. The resulting plume cross-section pattern was mostly affected by the windward wall heating, which produced an increment of the pollutant concentration on the windward side by breaking the main vortex circulation. The application of an incoming stable stratification created a generalised increment of pollutant within the canopy, with concentrations twice as large. Turbulent pollutant fluxes were found significant only at roof level and close to the source. On the other hand, in the windward wall-heated case the reduction of the mean flux renders the turbulent component relevant in other locations as well.
The present work highlights the importance of boundary layer stratification and local heating, both capable of creating significant modifications in the flow and pollutant fields at microscale range.
Wind tunnel experiments have been carried out on a small-scale physical model of a municipal waste landfill (MWL) in the CRIACIV (Research Centre of Building Aerodynamics and Wind Engineering) "environmental" wind tunnel in Prato (Italy). The MWL model simulates a landfill whose surface is higher than the surrounding surface, applying a 1:200 scaling factor. Modelling an area source such as landfill is a difficult task for numerical models due to turbulence phenomena that modifies the flow near the source increasing ground level concentration (GLC). For the specific task, a new set-up of the wind tunnel has been developed, with respect to previous studies carried out on line and point sources physical models. The tracer used in the experiments was ethylene, suitable for non-buoyant plume conditions, typical for MWL emissions. A detailed result database has been obtained in terms of GLC and concentration profiles as well as flow turbulence and velocity field characterisation. (C) 2004 Elsevier Ltd. All rights reserved.
Understanding the transformation of nanoparticles emitted from vehicles is essential for developing appropriate methods for treating fine scale particle dynamics in dispersion models. This article provides an overview of significant research work relevant to modelling the dispersion of pollutants, especially nanoparticles, in the wake of vehicles. Literature on vehicle wakes and nanoparticle dispersion is reviewed, taking into account field measurements, wind tunnel experiments and mathematical approaches. Field measurements and modelling studies highlighted the very short time scales associated with nanoparticle transformations in the first stages after the emission. These transformations strongly interact with the flow and turbulence fields immediately behind the vehicle, hence the need of characterising in detail the mixing processes in the vehicle wake. Very few studies have analysed this interaction and more research is needed to build a basis for model development. A possible approach is proposed and areas of further investigation identified.
Song J, Fan S, Lin William, Mottet L, Wooward H, Davies Wykes M, Arcucci R, Xiao D, Debay J, ApSimon H, Aristodemou E, Birch David, Carpentieri Matteo, Fan F, Herzog M, Hunt G, Jones R, Pain C, Pavlidis D, Robins Alan, Short C, Linden P (2018)Natural ventilation in cities: the implications of fluid mechanics, In: Building Research & Information46(8)pp. 809-828
Taylor & Francis
Research under the Managing Air for Green Inner Cities (MAGIC) project uses measurements and modelling to investigate the connections between external and internal conditions: the impact of urban airflow on the natural ventilation of a building. The test site was chosen so that under different environmental conditions the levels of external pollutants entering the building, from either a polluted road or a relatively clean courtyard, would be significantly different. Measurements included temperature, relative humidity, local wind and solar radiation, together with levels of carbon monoxide (CO) and carbon dioxide (CO2) both inside and outside the building to assess the indoor–outdoor exchange flows. Building ventilation took place through windows on two sides, allowing for single-sided and crosswind-driven ventilation, and also stack-driven ventilation in low wind conditions. The external flow around the test site was modelled in an urban boundary layer in a wind tunnel. The wind tunnel results were incorporated in a large-eddy-simulation model, Fluidity, and the results compared with monitoring data taken both within the building and from the surrounding area. In particular, the effects of street layout and associated street canyons, of roof geometry and the wakes of nearby tall buildings were examined.
Castro IP, Xie Z-T, Fuka V, Robins Alan, Carpentieri Matteo, Hayden Paul, Hertwig D, Coceal O (2017)Measurements and computations of ﬂow in an urban street system, In: Boundary-Layer Meteorology: an international journal of physical and biological processes in the atmospheric boundary layer162(2)pp. 207-230
We present results from laboratory and computational experiments on the turbulent flow over an array of rectangular blocks modelling a typical, asymmetric urban canopy at various orientations to the approach flow. The work forms part of a larger study on dispersion within such arrays (project DIPLOS) and concentrates on the nature of the mean flow and turbulence fields within the canopy region, recognis- ing that unless the flow field is adequately represented in computational models there is no reason to expect realistic simulations of the nature of the dispersion of pollutants emitted within the canopy. Comparisons between the experimental data and those ob- tained from both large-eddy simulation (LES) and direct numerical simulation (DNS) are shown and it is concluded that careful use of LES can produce generally excellent agreement with laboratory and DNS results, lending further confidence in the use of LES for such situations. Various crucial issues are discussed and advice offered to both experimentalists and those seeking to compute canopy flows with turbulence resolving models
This study compared dispersion calculations using a street network model (SIRANE) with results from wind tunnel experiments in order to examine model performance in simulating short-range pollutant dispersion in urban areas. The comparison was performed using a range of methodologies, from simple graphical comparisons (e.g. scatter plots) to more advanced statistical analyses. A preliminary analysis focussed on the sensitivity of the model to source position, receptor location, wind direction, plume spread parameterisation and site aerodynamic parameters. Sensitivity to wind direction was shown to be by far the most significant. A more systematic approach was then adopted, analysing the behaviour of the model in response to three elements: wind direction, source position and small changes in geometry. These are three very critical aspects of fine scale urban dispersion modelling. The overall model performance, measured using the Chang and Hanna (2004) criteria can be considered as ‘good’. Detailed analysis of the results showed that ground level sources were better represented by the model than roof level sources. Performance was generally ‘good’ for wind directions that were very approximately diagonal to the street axes, while cases with wind directions almost parallel (within 20°) to street axes gave results with larger uncertainties (failing to meet the quality targets). The methodology used in this evaluation exercise, relying on systematic wind tunnel studies on a scaled model of a real neighbourhood, proved very useful for assessing strengths and weaknesses of the SIRANE model, complementing previous validation studies performed with either on-site measurements or wind tunnel measurements over idealised urban geometries.
Despite their importance for pollutant dispersion in urban areas, the special features of dispersion at street intersections are rarely taken into account by operational air quality models. Several previous studies have demonstrated the complex flow patterns that occur at street intersections, even with simple geometry. This study presents results from wind-tunnel experiments on a reduced scale model of a complex but realistic urban intersection, located in central London. Tracer concentration measurements were used to derive three-dimensional maps of the concentration field within the intersection. In combination with a previous study (Carpentieri et al., Boundary-Layer Meteorol 133:277-296, 2009) where the velocity field was measured in the same model, a methodology for the calculation of the mean tracer flux balance at the intersection was developed and applied. The calculation highlighted several limitations of current state-of-the-art canyon dispersion models, arising mainly from the complex geometry of the intersection. Despite its limitations, the proposed methodology could be further developed in order to derive, assess and implement street intersection dispersion models for complex urban areas.
Zhong J, Barlow J, Grimmond S, Best M, Bohnenstengel S, Clark P, Lean H, Christen A, Emeis S, Haeffelin M, Harman IN, Lemonsu A, Ballard S, Martilli A, Pardyjak E, Rotach MW, Boutle I, Brown A, Cai X, Carpentieri Matteo, Coceal O, Drew DR, Crawford B, Di Sabatino S, Dou J, Edwards JM, Fallmann J, Fortuniak K, Gornall J, Gronemeier T, Halios CH, Luo Z, Hertwig D, Hirano K, Holtslag AAM, Mills G, Nakayoshi M, Pain K, Schlünzen KH, Sun T, Smith S, Soulhac L, Steeneveld G-J, Theeuwes NE, Thomson D, Voogt JA, Xie Z-T, Ward HC (2017)Developing a research strategy to better understand, observe and simulate urban atmospheric processes at kilometre to sub-kilometre scales, In: Bulletin of the American Meteorological Society98(10)pp. ES261-ES264
American Meteorological Society
Several wind tunnel experiments of tracer dispersion from reduced-scale landfill models are presented in this paper. Different experimental set-ups, hot-wire anemometry, particle image velocimetry and tracer concentration measurements were used for the characterisation of flow and dispersion phenomena nearby the models. The main aim of these experiments is to build an extensive experimental data set useful for model validation purposes. To demonstrate the potentiality of the experimental data set, a validation exercise on several mathematical models was performed by means of a statistical technique. The experiments highlighted an increase in pollutant ground level concentrations immediately downwind from the landfill because of induced turbulence and mean flow deflection. This phenomenon turns out to be predominant for the dispersion process. Tests with a different set-up showed an important dependence of the dispersion phenomena from the landfill height and highlighted how complex orographic conditions downwind of the landfill do not affect significantly the dispersion behaviour. Validation exercises were useful for model calibration, improving code reliability, as well as evaluating performances. The Van Ulden model proved to give the most encouraging results.
In this experimental work both qualitative (flow visualisation) and quantitative (laser Doppler anemometry) methods were applied in a wind tunnel in order to describe the complex three-dimensional flow field in a real environment (a street canyon intersection). The main aim was an examination of the mean flow, turbulence and flow pathlines characterising a complex three-dimensional urban location. The experiments highlighted the complexity of the observed flows, particularly in the upwind region of the intersection. In this complex and realistic situation some details of the upwind flow, such as the presence of two tall towers, play an important role in defining the flow field within the intersection, particularly at roof level. This effect is likely to have a strong influence on the mass exchange mechanism between the canopy flow and the air aloft, and therefore the distribution of pollutants. This strong interaction between the flows inside and outside the urban canopy is currently neglected in most state-of-the-art local scale dispersion models.
Wind tunnel experiments were conducted to study the impact of atmospheric stratification on flow and dispersion within and over a regular array of rectangular buildings. Three stable and two convective incoming boundary layers were tested with a Richardson number ranging from -1.5 to 0.29. Dispersion measurements were carried using a fast response flame ionisation detector. The results show that the stratification effect on the plume width is significantly lower than the effect on the vertical profiles. Stable stratification did not affect the plume central axis inside the canopy, but in the unstable case the axis appeared to deviate from the neutral case direction. Above the canopy both stratification types caused an increase in the plume deflection angle compared to the neutral case. Measured mean concentrations in stable stratification were up to two times larger in the canopy compared to the neutral case, while in convective conditions they were to three times smaller. The proportionality between the vertical turbulent fluxes and the vertical mean concentration gradient was also confirmed in the stratified cases. The high-quality experimental data produced during this work may help developing new mathematical models and parametrisation for non-neutral stratified conditions, as well as validating existing and future numerical simulations.
Pollutant mass fluxes are rarely measured in the laboratory, especially their turbulent component. They play a major role in the dispersion of gases in urban areas and modern mathematical models often attempt some sort of parametrisation. An experimental technique to measure mean and turbulent fluxes in an idealised urban array was developed and applied to improve our understanding of how the fluxes are distributed in a dense street canyon network. As expected, horizontal advective scalar fluxes were found to be dominant compared with the turbulent components. This is an important result because it reduces the complexity in developing parametrisations for street network models. On the other hand, vertical mean and turbulent fluxes appear to be approximately of the same order of magnitude. Building height variability does not appear to affect the exchange process significantly, while the presence of isolated taller buildings upwind of the area of interest does. One of the most interesting results, again, is the fact that even very simple and regular geometries lead to complex advective patterns at intersections: parametrisations derived from measurements in simpler geometries are unlikely to capture the full complexity of a real urban area.
Coceal O, Xie Z-T, Robins A, Bohnenstengel SI, Boppana B, Hayden P, Goulart E, Carpentieri M, Thomas TG, Castro IP, Belcher SE (2015)DIPLOS: Dispersion of Localised Releases in a Street Network
The need to balance computational speed and simulation accuracy is a key challenge in designing atmospheric dispersion models that can be used in scenarios where near real-time hazard predictions are needed. This challenge is aggravated in cities, where models need to have some degree of building-awareness, alongside the ability to capture effects of dominant urban flow processes. We use a combination of high-resolution large-eddy simulation (LES) and wind-tunnel data of flow and dispersion in an idealised, equal-height urban canopy to highlight important dispersion processes and evaluate how these are reproduced by representatives of the most prevalent modelling approaches: (i) a Gaussian plume model, (ii) a Lagrangian stochastic model and (iii) street-network dispersion models. Concentration data from the LES, validated against the wind-tunnel data, were averaged over the volumes of streets in order to provide a high-fidelity reference suitable for evaluating the different models on the same footing. For the particular combination of forcing wind direction and source location studied here, the strongest deviations from the LES reference were associated with mean over-predictions of concentrations by approximately a factor of 2 and with a relative scatter larger than a factor of 4 of the mean, corresponding to cases where the mean plume centreline also deviated significantly from the LES. This was linked to low accuracy of the underlying flow models/parameters that resulted in a misrepresentation of pollutant channelling along streets and of the uneven plume branching observed in intersections. The agreement of model predictions with the LES (which explicitly resolves the turbulent flow and dispersion processes) greatly improved by increasing the accuracy of building-induced modifications of the driving flow field. When provided with a limited set of representative velocity parameters, the comparatively simple street-network models performed equally well or better compared to the Lagrangian model run on full 3D wind fields. The study showed that street-network models capture the dominant building-induced dispersion processes in the canopy layer through parametrisations of horizontal advection and vertical exchange processes at scales of practical interest. At the same time, computational costs and computing times associated with the network approach are ideally suited for emergency-response applications.
One of the key research topics in the computational fluid dynamics (CFD) community is to improve the convergence speed of time-stepping schemes in steady-state finite volume codes. This thesis focuses on the improvements of an implicit time stepping scheme suitable for the fast convergence of a residual which is discretised for a second order accuracy in space. The optimised implicit scheme replaces an explicit scheme implementation in the Rolls-Royce corporate CFD code Hydra. This replacement of the explicit scheme by an implicit scheme is motivated by Swanson et al. , reporting a net computational speedup of the scheme of a factor 4 − 10. The novelties presented in this work build on the implicit Runge-Kutta time stepping scheme described by Rossow  and enhanced by Swanson et al. [1, 3] and focus on two key aspects: The optimisation of Runge-Kutta coefficients and adequate implicit preconditioning. With regards to the Runge-Kutta coefficients optimisation, the flow conditions leading to slow convergence are first analysed. Based on this analysis, an optimisation procedure is proposed to find an optimal set of Runge-Kutta coefficients. With regards to the improvement of the implicit preconditioner, most notably a novel design of its viscous components is proposed. This novel design of the viscous preconditioner is shown to enhance the convergence reliability of the code on skewed meshes. Compared to the explicit implementation, the implicit code typically achieves a minimum six-fold speedup in terms of computation time on a well defined set of test cases. Compared to the latest coefficients reported in literature , the optimised Runge-Kutta coefficients lead to a speedup of 20% − 38%.