Carmeliet

Research in Computational Fluid Dynamics (CFD)

Jump to: Pollutant dispersion | Urban Microclimate Modeling | Flow unsteadiness | Ventilation potential | Wind tunnel modeling | Flow quality indicators | References


Time-resolved simulation of pollutant dispersion

Peter Moonen, Christof Gromke, Viktor Dorer

We performed detailed time-resolved simulations of a 1:150-scaled model of an isolated urban street canyon, with avenue-like tree planting and near-ground released traffic emissions, under perpendicular approach flow. Different crown porosities were considered. The model performance is assessed in several steps, ranging from a qualitative comparison to measured concentration profiles in the atmospheric boundary layer wind tunnel of the University of Karlsruhe, Germany, over statistical data analysis by means of scatter plots and box plots, up to the calculation of objective validation metrics. The extensive validation effort highlights and quantifies notable features and shortcomings of the model, which would otherwise remain unnoticed. This demonstrates the need for thorough model evaluation. A more detailed description of the model and its validation can be found in Moonen et al. (2013).

Instantaneous snapshot of the turbulent flow field around an urban street canyon.


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Accurate modeling of the urban micro-climate

Saba Saneinejad, Peter Moonen, Jan Carmeliet

We developed a coupled model to study the effect of evaporative cooling on the temperature conditions in an urban street canyon. The model couples three sub-models: (i) a Computational Fluid Dynamics (CFD) model, which solves heat and vapor transfer in the air, (ii) a Building Envelope Heat and Moisture (BE-HAM) transport model which solves heat and moisture transfer within the porous building walls, and (iii) a radiation model (RAD) which determines the radiative heat exchange between the surfaces. The coupled model has been successfully applied to investigate the effect of evaporation on the reduction of the surface and air temperatures in a street canyon and to assess the resulting impact on the comfort sensation in the urban street canyon. A more detailed description of the model can be found in Saneinejad et al. (2011), (2012), (2013), (2014).

Our coupled urban microclimate model combines a Computational Fluid Dynamics model (CFD), a radiation model (RAD), a heat and moisture transfer model (HAM), and a building energy simulation model (BES).


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The effect of flow unsteadiness on the mean flow pattern

Peter Moonen, Viktor Dorer, Jan Carmeliet

We investigate the effect of flow unsteadiness on the mean wind flow pattern by comparing steady RANS and unsteady LES simulations. Two different morphological configurations are considered, namely a courtyard and a cityblock. For both geometries, various ambient wind directions are considered. Our simulations reveal that the selected modelling approach can have a significant effect on (i) the mechanisms causing air exchange between the regions above and below roof level, (ii) the spatial distribution of the time-averaged exchange fluxes, and (iii) the total amount of air exchange. For some specific cases, the modelling approach even has a larger impact on the predicted air exchange rate than the ambient wind direction. A more detailed description of the methodology can be found in Moonen et al. (2011), (2012).

Flow around an isolated cityblock.


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Ventilation potential

Peter Moonen, Viktor Dorer, Jan Carmeliet

We introduce the ventilation potential (VP) as a statistical, climate-dependent measure to assess the removal of scalars, such as heat and pollutants, from courtyards or urban street canyons. The VP is obtained following a three-step approach. First, the magnitude of the flux through a horizontal surface situated at the top of the courtyard or canyon is determined by means of computational fluid dynamics (CFD) simulations for various courtyard geometries and ambient wind directions. Then, this exchange flux is normalized with the free-stream wind speed and subsequently parameterized as a function of the courtyard’s length-to-width ratio and the incidence angle of the wind flow. Finally, the combination of the parameterization with site-specific wind data yields the ventilation potential. In combination with the proposed parameterization, the ventilation potential provides a useful tool to optimize the orientation and the dimensions of a courtyard, as function of the local wind data and the desired degree of scalar removal. A more detailed description of the methodology can be found in Moonen et al. (2011).

(a) Wind rose for Schiphol, the Netherlands. Comparison between RANS (dashed) and LES (solid) of (b) the total exchange flux through the top-plane of a 1:1:2 courtyard as a function of the direction of the incident wind flow and (c) the corresponding ventilation potential as a function of the courtyard orientation.


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Wind tunnel modeling by means of CFD

Peter Moonen, Bert Blocken, Staf Roels, Jan Carmeliet

A numerical methodology to model the flow conditions in closed-circuit wind tunnels was developed. The approach consists in modeling the entire wind-tunnel in sufficient detail, except for the fan straightener section, which is replaced by a “fan boundary condition”. In this way, the computational domain does not have a flow inlet or outlet as in a conventional CFD analysis, hereby avoiding the need to specify profiles of velocity and turbulent kinetic energy along these surfaces. The approach was validated for the Jules Verne climatic wind tunnel in Nantes, France, and it was shown that the methodology can generally reproduce the wind tunnel measurements of mean velocities and turbulence intensities with an error equal to or less than 10% despite the occurrence of multiple flow separations upstream of the test section. A more detailed description of the methodology and its validation can be found in Moonen et al. (2006).

(a) Model of the geometry of the entire closed-circuit wind tunnel with a fan boundary condition. (b) Model of only the test section of the wind tunnel with an inlet and an outlet boundary condition (conventional approach).


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Wind tunnel flow quality indicators

Peter Moonen, Bert Blocken, Jan Carmeliet

A set of six new indices was developed to assess flow quality and quantify potential flow imperfections (skewness and angularity). The indicators draw attention to the regions in which the flow quality is less than required and reveal possible causes. As such, the flow quality indicators yield a tool that provides perspectives to be used in wind tunnel design, optimization and testing. A more detailed description of the methodology and its validation can be found in Moonen et al. (2007).

A component of the mean velocity vector is integrated along a horizontal line across the width of the test section to obtain a value for the corresponding index. In this way, the quality of the three dimensional flow in the test section can be shown on a two-dimensional graph.


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References

  • S. Saneinejad, P. Moonen, J. Carmeliet, Coupled CFD, radiation and porous media model for evaluating evaporative cooling and thermal comfort in a street canyon, Journal of Wind Engineering & Industrial Aerodynamics, Vol. 128, pp. 1-11, 2014. (pdf)
  • S. Saneinejad, P. Moonen, J. Carmeliet, Comparative assessment of various heat island mitigation measures, Building and Environment, Vol. 73, pp. 162-170, 2014. (pdf)
  • Y. Takano, P. Moonen, On the influence of roof shape on flow and dispersion in an urban street canyon, Journal of Wind Engineering & Industrial Aerodynamics, Vol. 123, part A, pp. 107–120, 2013. (pdf)
  • P. Moonen, C. Gromke, V. Dorer, Performance assessment of Large Eddy Simulation (LES) for modeling dispersion in an urban street canyon with tree planting, Atmospheric Environment, Vol. 75, pp. 66–76, 2013. (pdf)
  • E. Paterna, P. Moonen, V. Dorer, J. Carmeliet, Mitigation of surface reflection in PIV measurements, Measurement Science and Technology, Vol. 24(5), No 057003, 2013. (pdf)
  • P. Moonen, T. Defraeye, V. Dorer, B. Blocken, J. Carmeliet, Urban Physics: effect of the micro-climate on comfort, health and energy demand, Frontiers of Architectural Research, Vol. 1(3), pp. 197–228, 2012. (pdf)
  • P. Moonen, V. Dorer, J. Carmeliet, Effect of flow unsteadiness on the mean wind flow pattern in an idealized urban environment, Journal of Wind Engineering & Industrial Aerodynamics, Vol. 104–106, pp. 389–396, 2012. (pdf)
  • S. Saneinejad, P. Moonen, T. Defraeye, D. Derome, J. Carmeliet, Coupled CFD, radiation and porous media transport model for evaluating evaporative cooling in an urban environment, Journal of Wind Engineering & Industrial Aerodynamics, Vol. 104–106, pp. 455–463, 2012. (pdf)
  • P. Moonen, V. Dorer, J. Carmeliet, Evaluation of the ventilation potential of courtyards and urban street canyons using RANS and LES, Journal of Wind Engineering & Industrial Aerodynamics, Vol. 99(4), pp. 414-423, 2011. (pdf)
  • S. Saneinejad, P. Moonen, J. Carmeliet, Analysis of convective heat and mass transfer at the vertical walls of a street canyon, Journal of Wind Engineering & Industrial Aerodynamics, Vol. 99(4), pp. 424-433, 2011. (pdf)
  • B. Blocken, P. Moonen, T. Stathopoulos, J. Carmeliet, A numerical study on the existence of the Venturi-effect in passages between perpendicular buildings, Engineering Mechanics - ASCE, Vol. 134(12), pp. 1021-1028, 2008. (pdf)
  • P. Moonen, B. Blocken and J. Carmeliet, Indicators for the evaluation of wind tunnel test section flow quality and application to a numerical closed-circuit wind tunnel, International Journal of Wind Engineering and Industrial Aerodynamics, Vol. 95, pp. 1289-1314, 2007. (pdf)
  • P. Moonen, B. Blocken, S. Roels and J. Carmeliet, Numerical modeling of the flow conditions in a closed-circuit low-speed wind tunnel, International Journal of Wind Engineering and Industrial Aerodynamics, Vol. 94, pp. 699-723, 2006. (pdf)

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