We review the efforts made by the scientific community in more than seventy years to elucidate th... more We review the efforts made by the scientific community in more than seventy years to elucidate the behaviour of concentration fluctuations arising from localized atmospheric releases of dynamically passive and non-reactive scalars. Concentration fluctuations are relevant in many fields including the evaluation of toxicity, flammability, and odour nuisance. Characterizing concentration fluctuations requires not just the mean concentration but also at least the variance of the concentration in the location of interest. However, for most purposes the characterization of the concentration fluctuations requires knowledge of the concentration probability density function (PDF) in the point of interest and even the time evolution of the concentration. We firstly review the experimental works made both in the field and in the laboratory, and cover both point sources and line sources. Regarding modelling approaches, we cover analytical, semi-analytical, and numerical methods. For clarity of presentation we subdivide the models in two groups, models linked to a transport equation, which usually require a numerical resolution, and models mainly based on phenomenological aspects of dispersion, often providing analytical or semi-analytical relations. The former group includes: large-eddy simulations, Reynolds-averaged Navier-Stokes methods, two-particle Lagrangian stochastic models, PDF transport equation methods, and heuristic Lagrangian single-particle methods. The latter group includes: fluctuating plume models, semi-empirical models for the concentration moments, analytical models for the concentration PDF, and concentration time-series models. We close the review with a brief discussion highlighting possible useful additions to experiments and improvements to models.
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<p>... more <p>The dispersion dynamics of a contaminant released in the atmosphere is crucial for risk assessments and environmental analyses. Yet, because of the unsolved problem of turbulence, analytical solutions physically-derived are currently limited to the Gaussian models for the mean concentration field. In this work, we have obtained simple solutions for the concentration statistics of a passive scalar released from a punctual source. The main result is a novel analytical solution for the passive scalar variance, which is obtained from the contaminant transport equation. We have verified this solution against wind-tunnel data, and further adopted it in a simple stochastic model to provide closed relationships for the temporal statistics of concentration (e.g., the mean duration and occurrence of the peak events). These results may serve as a rapid and practical way to estimate the intensity and duration of the concentration fluctuations of a pollutant released in the atmosphere.</p>
Computer Methods in Applied Mechanics and Engineering, May 1, 2010
ABSTRACT In this paper we propose two error indicators aimed at estimating the space discretizati... more ABSTRACT In this paper we propose two error indicators aimed at estimating the space discretization error and the time discretization error for the unsteady Navier–Stokes equations. We define a space error indicator for evaluating the quality of the mesh and a time error indicator for evaluating the time discretization error. Moreover, we verify the reliability of the estimations through numerical experiments and we propose an effective space-time adaptive strategy for the unsteady Navier–Stokes equations. Such technique is based on two residual-based error indicators that suitably drive the mesh and the timestep-length modifications. Adaptive simulations show that the presented strategy allows to obtain accurate solutions in efficient way.
International Journal of Environment and Pollution, 2014
This study presents the performances of an inverse modelling approach aiming in identifying posit... more This study presents the performances of an inverse modelling approach aiming in identifying position and emission rate of a localised pollutant source placed within a city district. To that purpose we combine wind tunnel experiments and an urban dispersion model. Experiments are performed in an idealised urban canopy, made up of regularly spaced blocks, and provide the pollutant concentration field downwind the source within the canopy. The urban dispersion model, named SIRANE, is an operational model that simulates the main mechanisms governing the pollutant transfer within a network of streets.
This study focuses on the influence of emission conditions-velocity and temperature-on the dynami... more This study focuses on the influence of emission conditions-velocity and temperature-on the dynamics of a buoyant gas release in the atmosphere. The investigations are performed by means of wind tunnel experiments and numerical simulations. The aim is to evaluate the reliability of a Lagrangian code to simulate the dispersion of a plume produced by pollutant emissions influenced by thermal and inertial phenomena. This numerical code implements the coupling between a Lagrangian stochastic model and an integral plume rise model being able to estimate the centroid trajectory. We verified the accuracy of the plume rise model and we investigated the ability of two Lagrangian models to evaluate the plume spread by means of comparisons between experiments and numerical solutions. A quantitative study of the performances of the models through some suitable statistical indices is presented and critically discussed. This analysis shows that an additional spread has to be introduced in the Lagrangian trajectory equation in order to account the dynamical and thermal effects induced by the source conditions.
The aim of the present paper is to investigate the viability of macroscopic traffic models for mo... more The aim of the present paper is to investigate the viability of macroscopic traffic models for modeling and testing different traffic scenarios, in order to define the impact on air quality of different strategies for the reduction of traffic emissions. To this aim, we complement a well assessed traffic model on networks [13] with a strategy for estimating data needed from the model and we couple it with the urban dispersion model Sirane .
In this paper we apply to the unsteady Navier–Stokes problems some results concerning a posterior... more In this paper we apply to the unsteady Navier–Stokes problems some results concerning a posteriori error estimates and adaptive algorithms known for steady Navier–Stokes, unsteady heat and reaction-convection-diffusion equations and unsteady Stokes problems. Our target is to investigate the real viability of a fully combined space and time adaptivity for engineering problems. The comparison between our numerical simulations and the literature results demonstrates the accuracy and efficiency of this adaptive strategy.
The turbulent exchange of momentum between a two-dimensional cavity and the overlying boundary la... more The turbulent exchange of momentum between a two-dimensional cavity and the overlying boundary layer has been studied experimentally, using hot-wire anemometry and particle image velocimetry (PIV). Conditions within the boundary layer were varied by changing the width of the canyons upstream of the test canyon, whilst maintaining the square geometry of the test canyon. The results show that turbulent transfer is due to the coupling between the instabilities generated in the shear layer above the canyons and the turbulent structures in the oncoming boundary layer. As a result, there is no single, unique velocity scale that correctly characterizes all the processes involved in the turbulent exchange of momentum across the boundary layer. Similarly, there is no single velocity scale that can characterize the different properties of the turbulent flow within the canyon, which depends strongly on the way in which turbulence from the outer flow is entrained into the cavity and carried round by the mean flow. The results from this study will be useful in developing simple parametrizations for momentum exchange in the urban canopy, in situations where the street geometry consists principally of relatively long, uniform streets arranged in gridlike patterns; they are unlikely to be applicable to sparse geometries composed of isolated three-dimensional obstacles.
We derive a system of equations for the statistical moments of a passive scalar dispersed in a tu... more We derive a system of equations for the statistical moments of a passive scalar dispersed in a turbulent flow from the transport equation of the probability density function. We solve the system through a Green's function and we obtain a formally exact solution for the statistical moments of the passive scalar concentration. We use this solution to achieve an analytical relationship for the second moment of a passive scalar released from a point source. Comparison with wind-tunnel experiments shows that the relationship is valid also in a neutral turbulent boundary layer if the reflection onto the ground and an appropriate model for the mixing timescale are considered. This approach, combined with a suitable model for the distribution of the concentration, allows the statistics of the passive scalar to be obtained in the whole domain in a closed and ready-to-use form.
We review the efforts made by the scientific community in more than seventy years to elucidate th... more We review the efforts made by the scientific community in more than seventy years to elucidate the behaviour of concentration fluctuations arising from localized atmospheric releases of dynamically passive and non-reactive scalars. Concentration fluctuations are relevant in many fields including the evaluation of toxicity, flammability, and odour nuisance. Characterizing concentration fluctuations requires not just the mean concentration but also at least the variance of the concentration in the location of interest. However, for most purposes the characterization of the concentration fluctuations requires knowledge of the concentration probability density function (PDF) in the point of interest and even the time evolution of the concentration. We firstly review the experimental works made both in the field and in the laboratory, and cover both point sources and line sources. Regarding modelling approaches, we cover analytical, semi-analytical, and numerical methods. For clarity of presentation we subdivide the models in two groups, models linked to a transport equation, which usually require a numerical resolution, and models mainly based on phenomenological aspects of dispersion, often providing analytical or semi-analytical relations. The former group includes: large-eddy simulations, Reynolds-averaged Navier-Stokes methods, two-particle Lagrangian stochastic models, PDF transport equation methods, and heuristic Lagrangian single-particle methods. The latter group includes: fluctuating plume models, semi-empirical models for the concentration moments, analytical models for the concentration PDF, and concentration time-series models. We close the review with a brief discussion highlighting possible useful additions to experiments and improvements to models.
&... more &am…
<p>... more <p>The dispersion dynamics of a contaminant released in the atmosphere is crucial for risk assessments and environmental analyses. Yet, because of the unsolved problem of turbulence, analytical solutions physically-derived are currently limited to the Gaussian models for the mean concentration field. In this work, we have obtained simple solutions for the concentration statistics of a passive scalar released from a punctual source. The main result is a novel analytical solution for the passive scalar variance, which is obtained from the contaminant transport equation. We have verified this solution against wind-tunnel data, and further adopted it in a simple stochastic model to provide closed relationships for the temporal statistics of concentration (e.g., the mean duration and occurrence of the peak events). These results may serve as a rapid and practical way to estimate the intensity and duration of the concentration fluctuations of a pollutant released in the atmosphere.</p>
Computer Methods in Applied Mechanics and Engineering, May 1, 2010
ABSTRACT In this paper we propose two error indicators aimed at estimating the space discretizati... more ABSTRACT In this paper we propose two error indicators aimed at estimating the space discretization error and the time discretization error for the unsteady Navier–Stokes equations. We define a space error indicator for evaluating the quality of the mesh and a time error indicator for evaluating the time discretization error. Moreover, we verify the reliability of the estimations through numerical experiments and we propose an effective space-time adaptive strategy for the unsteady Navier–Stokes equations. Such technique is based on two residual-based error indicators that suitably drive the mesh and the timestep-length modifications. Adaptive simulations show that the presented strategy allows to obtain accurate solutions in efficient way.
International Journal of Environment and Pollution, 2014
This study presents the performances of an inverse modelling approach aiming in identifying posit... more This study presents the performances of an inverse modelling approach aiming in identifying position and emission rate of a localised pollutant source placed within a city district. To that purpose we combine wind tunnel experiments and an urban dispersion model. Experiments are performed in an idealised urban canopy, made up of regularly spaced blocks, and provide the pollutant concentration field downwind the source within the canopy. The urban dispersion model, named SIRANE, is an operational model that simulates the main mechanisms governing the pollutant transfer within a network of streets.
This study focuses on the influence of emission conditions-velocity and temperature-on the dynami... more This study focuses on the influence of emission conditions-velocity and temperature-on the dynamics of a buoyant gas release in the atmosphere. The investigations are performed by means of wind tunnel experiments and numerical simulations. The aim is to evaluate the reliability of a Lagrangian code to simulate the dispersion of a plume produced by pollutant emissions influenced by thermal and inertial phenomena. This numerical code implements the coupling between a Lagrangian stochastic model and an integral plume rise model being able to estimate the centroid trajectory. We verified the accuracy of the plume rise model and we investigated the ability of two Lagrangian models to evaluate the plume spread by means of comparisons between experiments and numerical solutions. A quantitative study of the performances of the models through some suitable statistical indices is presented and critically discussed. This analysis shows that an additional spread has to be introduced in the Lagrangian trajectory equation in order to account the dynamical and thermal effects induced by the source conditions.
The aim of the present paper is to investigate the viability of macroscopic traffic models for mo... more The aim of the present paper is to investigate the viability of macroscopic traffic models for modeling and testing different traffic scenarios, in order to define the impact on air quality of different strategies for the reduction of traffic emissions. To this aim, we complement a well assessed traffic model on networks [13] with a strategy for estimating data needed from the model and we couple it with the urban dispersion model Sirane .
In this paper we apply to the unsteady Navier–Stokes problems some results concerning a posterior... more In this paper we apply to the unsteady Navier–Stokes problems some results concerning a posteriori error estimates and adaptive algorithms known for steady Navier–Stokes, unsteady heat and reaction-convection-diffusion equations and unsteady Stokes problems. Our target is to investigate the real viability of a fully combined space and time adaptivity for engineering problems. The comparison between our numerical simulations and the literature results demonstrates the accuracy and efficiency of this adaptive strategy.
The turbulent exchange of momentum between a two-dimensional cavity and the overlying boundary la... more The turbulent exchange of momentum between a two-dimensional cavity and the overlying boundary layer has been studied experimentally, using hot-wire anemometry and particle image velocimetry (PIV). Conditions within the boundary layer were varied by changing the width of the canyons upstream of the test canyon, whilst maintaining the square geometry of the test canyon. The results show that turbulent transfer is due to the coupling between the instabilities generated in the shear layer above the canyons and the turbulent structures in the oncoming boundary layer. As a result, there is no single, unique velocity scale that correctly characterizes all the processes involved in the turbulent exchange of momentum across the boundary layer. Similarly, there is no single velocity scale that can characterize the different properties of the turbulent flow within the canyon, which depends strongly on the way in which turbulence from the outer flow is entrained into the cavity and carried round by the mean flow. The results from this study will be useful in developing simple parametrizations for momentum exchange in the urban canopy, in situations where the street geometry consists principally of relatively long, uniform streets arranged in gridlike patterns; they are unlikely to be applicable to sparse geometries composed of isolated three-dimensional obstacles.
We derive a system of equations for the statistical moments of a passive scalar dispersed in a tu... more We derive a system of equations for the statistical moments of a passive scalar dispersed in a turbulent flow from the transport equation of the probability density function. We solve the system through a Green's function and we obtain a formally exact solution for the statistical moments of the passive scalar concentration. We use this solution to achieve an analytical relationship for the second moment of a passive scalar released from a point source. Comparison with wind-tunnel experiments shows that the relationship is valid also in a neutral turbulent boundary layer if the reflection onto the ground and an appropriate model for the mixing timescale are considered. This approach, combined with a suitable model for the distribution of the concentration, allows the statistics of the passive scalar to be obtained in the whole domain in a closed and ready-to-use form.
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Papers by Massimo Marro