Numerical modeling of micro-scale wind-induced pollutant dispersion in the built environment

Despite recent efforts directed towards the development of cleaner and more efficient energy sources, air pollution remains a major problem in many large cities worldwide, with negative consequences for human health and comfort. If the transport of pollutants by wind in urban areas can be predicted in an accurate way, remedial measures can be implemented and the exposure of people and goods to pollution can be decreased to limit these negative effects. This prediction can be achieved by experimental techniques, on-site or in wind tunnels, but also numerically, with the use of Computational Fluid Dynamics (CFD).In this thesis CFD is used to simulate wind-induced pollutant dispersion in the built environment. The accuracy of this approach in terms of pollutant concentration prediction always needs to be assessed. The reason is twofold. First, the wind flow around buildings is turbulent and cannot be solved exactly with CFD. This type of flow must therefore be approximated with so-called turbulence models. Second, various types of errors are present in the numerical solution and can affect its accuracy. The Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES) turbulence modeling approaches are the most widely used in computational wind engineering. They are compared in this thesis, and evaluated by comparison with reference wind-tunnel experiments. In the first part, several generic cases of simplified isolated buildings are considered and, in the second part, an applied case of pollutant dispersion in an actual urban area (part of downtown Montreal) is studied. In the computations, care is taken to accurately simulate three key aspects of urban pollutant dispersion: (1) the atmospheric boundary layer flow, (2) the wind flow around buildings, and (3) the dispersion process. On average, the transport of pollutants by wind can be seen as the combination of, on the one hand, the transport by the mean flow and, on the other hand, the transport by the turbulent fluctuations. This decomposition is used here to evaluate the RANS – with various turbulence models – and LES approaches. Overall, the better performance of LES in terms of flow and concentration field prediction is demonstrated. In addition, LES has the advantage to provide the time-resolved velocity and concentration fields. Given the good accuracy of LES, this approach is used to investigate the physical mechanism of pollutant dispersion for the case of a simplified isolated building. The vortical structures present in the shear layers developing from the roof and sides of the building are shown to play a crucial role in the turbulent mass transport process. LES used as a research tool also allows evaluating models employed with RANS for turbulent mass transport, which is often assumed to act as a diffusion mechanism. The results of this study show that this hypothesis is not always valid and in some cases the turbulent mass flux in the streamwise direction is directed from the low to high levels of mean concentration (counter-gradient diffusion)

Near-field Pollutant Dispersion in an Actual Urban Area: Analysis of the Mass Transport Mechanism by High-Resolution Large Eddy Simulation

Large-Eddy Simulation of near-field pollutant dispersion from stacks on the roof of a low-rise building in downtown Montreal is performed. Two wind directions are considered, with different wind-flow patterns and plume behavior. The computed mean concentration field is analyzed by means of the convective and turbulent (including subgrid-scale) mass fluxes. This decomposition provides insight into the dispersion process and allows an evaluation of common turbulent transport models used with the Reynolds-Averaged Navier–Stokes approach, such as the standard gradient-diffusion hypothesis. Despite the specific character of the flow and dispersion patterns due to the complex geometry of the urban area under study, some similarities are found with the generic case of dispersion around an isolated simple building. Moreover, the analysis of dispersion in downtown Montreal is facilitated by the physical insight gained by the study of the generic case. In this sense, the present study supports the use of generic, simplified cases to investigate and understand environmental processes as they occur in real and more complex situations. Reciprocally, the results of this applied study show the influence on the dispersion process of the rooftop structures and of the orientation of the emitting building with respect to the incoming wind flow, providing directions for further research on generic cases

CFD Simulation of Near-Field Pollutant Dispersion on a High-Resolution Grid: A Case Study by LES and RANS for a Building Group in Downtown Montreal

Turbulence modeling and validation by experiments are key issues in the simulation of micro-scale atmospheric dispersion. This study evaluates the performance of two different modeling approaches (RANS standard k-ε and LES) applied to pollutant dispersion in an actual urban environment: downtown Montreal. The focus of the study is on near-field dispersion, i.e. both on the prediction of pollutant concentrations in the surrounding streets (for pedestrian outdoor air quality) and on building surfaces (for ventilation system inlets and indoor air quality). The high-resolution CFD simulations are performed for neutral atmospheric conditions and are validated by detailed wind-tunnel experiments. A suitable resolution of the computational grid is determined by grid-sensitivity analysis. It is shown that the performance of the standard k-ε model strongly depends on the turbulent Schmidt number, whose optimum value is case-dependent and a priori unknown. In contrast, LES with the dynamic subgrid-scale model shows a better performance without requiring any parameter input to solve the dispersion equation

Healthy environments from a broad perspective : an overview of research performed at the unit Building Physics and Systems of Eindhoven University of Technology

The design and realization of a healthy indoor environment is a challenge that is investigated from different perspectives at the unit Building Physics and Systems (BPS; Faculty of Architecture, Building and Planning) of Eindhoven University of Technology. Performance requirements (for instance, with respect to air quality, thermal comfort and lighting) and performance based assessment methods are the point-of-departure, focusing at computational techniques supporting the design process. Different specific application fields such as dwellings, offices, schools, but also, operating theatres, churches, musea and multifunctional stadiums, underline the applied approach that is part of the research within the unit. In the design of healthy environments, the performance based design assessment is crucial in arriving at innovative design solutions and optimized indoor and outdoor environments. In this assessment computational support tools and experimental verification play an important role. However, assessing the right indicators in an objective way, applying the correct tools and correct application of these tools is not yet well established. Alongside, developments are still ongoing. The work performed in the unit by the different researchers relates to the research questions that can be derived from this notice. The paper gives an introduction to the Unit BPS and presents a brief overview of recent and ongoing research. An extensive list of references is provided for further reading and supports the conclusion that healthy environments can and should be addressed from a wide angle

Instantaneous transport of a passive scalar in a turbulent separated flow

The results of large-eddy simulations of flow and transient solute transport over a backward facing step and through a 180° bend are presented. The simulations are validated successfully in terms of hydrodynamics and tracer transport with experimental velocity data and measured residence time distribution curves confirming the accuracy of the method. The hydrodynamics are characterised by flow separation and subsequent recirculation in vertical and horizontal directions and the solute dispersion process is a direct response to the significant unsteadiness and turbulence in the flow. The turbulence in the system is analysed and quantified in terms of power density spectra and covariance of velocity fluctuations. The injection of an instantaneous passive tracer and its dispersion through the system is simulated. Large-eddy simulations enable the resolution of the instantaneous flow field and it is demonstrated that the instabilities of intermittent large-scale structures play a distinguished role in the solute transport. The advection and diffusion of the scalar is governed by the severe unsteadiness of the flow and this is visualised and quantified. The analysis of the scalar mass transport budget quantifies the mechanisms controlling the turbulent mixing and reveals that the mass flux is dominated by advection

Numerical modeling of micro-scale wind-induced pollutant dispersion in the built environment

Despite recent efforts directed towards the development of cleaner and more efficient energy sources, air pollution remains a major problem in many large cities worldwide, with negative consequences for human health and comfort. If the transport of pollutants by wind in urban areas can be predicted in an accurate way, remedial measures can be implemented and the exposure of people and goods to pollution can be decreased to limit these negative effects. This prediction can be achieved by experimental techniques, on-site or in wind tunnels, but also numerically, with the use of Computational Fluid Dynamics (CFD). In this thesis CFD is used to simulate wind-induced pollutant dispersion in the built environment. The accuracy of this approach in terms of pollutant concentration prediction always needs to be assessed. The reason is twofold. First, the wind flow around buildings is turbulent and cannot be solved exactly with CFD. This type of flow must therefore be approximated with so-called turbulence models. Second, various types of errors are present in the numerical solution and can affect its accuracy. The Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES) turbulence modeling approaches are the most widely used in computational wind engineering. They are compared in this thesis, and evaluated by comparison with reference wind-tunnel experiments. In the first part, several generic cases of simplified isolated buildings are considered and, in the second part, an applied case of pollutant dispersion in an actual urban area (part of downtown Montreal) is studied. In the computations, care is taken to accurately simulate three key aspects of urban pollutant dispersion: (1) the atmospheric boundary layer flow, (2) the wind flow around buildings, and (3) the dispersion process. On average, the transport of pollutants by wind can be seen as the combination of, on the one hand, the transport by the mean flow and, on the other hand, the transport by the turbulent fluctuations. This decomposition is used here to evaluate the RANS – with various turbulence models – and LES approaches. Overall, the better performance of LES in terms of flow and concentration field prediction is demonstrated. In addition, LES has the advantage to provide the time-resolved velocity and concentration fields. Given the good accuracy of LES, this approach is used to investigate the physical mechanism of pollutant dispersion for the case of a simplified isolated building. The vortical structures present in the shear layers developing from the roof and sides of the building are shown to play a crucial role in the turbulent mass transport process. LES used as a research tool also allows evaluating models employed with RANS for turbulent mass transport, which is often assumed to act as a diffusion mechanism. The results of this study show that this hypothesis is not always valid and in some cases the turbulent mass flux in the streamwise direction is directed from the low to high levels of mean concentration (counter-gradient diffusion)

CFD simulation of pollutant dispersion around isolated buildings: On the role of convective and turbulent mass fluxes in the prediction accuracy

Computational Fluid Dynamics (CFD) is increasingly used to predict wind flow and pollutant dispersion around buildings. The two most frequently used approaches are solving the Reynolds-averaged Navier–Stokes (RANS) equations and Large-Eddy Simulation (LES). In the present study, we compare the convective and turbulent mass fluxes predicted by these two approaches for two configurations of isolated buildings with distinctive features. We use this analysis to clarify the role of these two components of mass transport on the prediction accuracy of RANS and LES in terms of mean concentration. It is shown that the proper simulation of the convective fluxes is essential to predict an accurate concentration field. In addition, appropriate parameterization of the turbulent fluxes is needed with RANS models, while only the subgrid-scale effects are modeled with LES. Therefore, when the source is located outside of recirculation regions (case 1), both RANS and LES can provide accurate results. When the influence of the building is higher (case 2), RANS models predict erroneous convective fluxes and are largely outperformed by LES in terms of prediction accuracy of mean concentration. These conclusions suggest that the choice of the appropriate turbulence model depends on the configuration of the dispersion problem under study. It is also shown that for both cases LES predicts a counter-gradient mechanism of the streamwise turbulent mass transport, which is not reproduced by the gradient-diffusion hypothesis that is generally used with RANS models

CFD simulation of pollutant dispersion around isolated buildings: On the role of convective and turbulent mass fluxes in the prediction accuracy

Computational Fluid Dynamics (CFD) is increasingly used to predict wind flow and pollutant dispersion around buildings. The two most frequently used approaches are solving the Reynolds-averaged Navier-Stokes (RANS) equations and Large-Eddy Simulation (LES). In the present study, we compare the convective and turbulent mass fluxes predicted by these two approaches for two configurations of isolated buildings with distinctive features. We use this analysis to clarify the role of these two components of mass transport on the prediction accuracy of RANS and LES in terms of mean concentration. It is shown that the proper simulation of the convective fluxes is essential to predict an accurate concentration field. In addition, appropriate parameterization of the turbulent fluxes is needed with RANS models, while only the subgrid-scale effects are modeled with LES. Therefore, when the source is located outside of recirculation regions (case 1), both RANS and LES can provide accurate results. When the influence of the building is higher (case 2), RANS models predict erroneous convective fluxes and are largely outperformed by LES in terms of prediction accuracy of mean concentration. These conclusions suggest that the choice of the appropriate turbulence model depends on the configuration of the dispersion problem under study. It is also shown that for both cases LES predicts a counter-gradient mechanism of the streamwise turbulent mass transport, which is not reproduced by the gradient-diffusion hypothesis that is generally used with RANS models.status: publishe

PIV measurements of the Flow behind a 2D fence in an atmospheric boundary layer wind tunnel

High-speed stereo-PIV measurements were performed in the wake of 2.3 m wide fences (respectively 5 and 10 cm high). Average velocities and turbulent kinetic energy levels are given for a near fenceand a downstream field of view, showing the large recirculation zone of a near-2D obstacle. The results indicate that although PIV measurements can provide a good description of the turbulent flow around a wall-mounted fence, the lateral (perpendicular to the FOV) turbulence levels of the approach flow with roughness length of 0.3 mm are overestimated when compared to hot-wire measurements

Large-Eddy Simulation of pollutant dispersion around a cubical building: Analysis of the turbulent mass transport mechanism by unsteady concentration and velocity statistics

Pollutant transport due to the turbulent wind flow around buildings is a complex phenomenon which is challenging to reproduce with Computational Fluid Dynamics (CFD). In the present study we use Large-Eddy Simulation (LES) to investigate the turbulent mass transport mechanism in the case of gas dispersion around an isolated cubical building. Close agreement is found between wind tunnel measurements and the computed average and standard deviation of concentration in the wake of the building. Since the turbulent mass flux is equal to the covariance of velocity and concentration, we perform a detailed statistical analysis of these variables to gain insight into the dispersion process. In particular, the fact that turbulent mass flux in the streamwise direction is directed from the low to high levels of mean concentration (counter-gradient mechanism) is explained. The large vortical structures developing around the building are shown to play an essential role in turbulent mass transport.status: publishe