JP Journal of Heat and Mass Transfer
Volume 9, Issue 1, Pages 21 - 55
(February 2014)
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LES/LAGRANGIAN MODELLING FOR THE DISPERSION OF REACTIVE SPECIES BEHIND AN OBSTACLE IN A TURBULENT BOUNDARY LAYER
C. Le Ribault, J. M. Vignon and S. Simoëns
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Abstract: This paper presents a comparative study of numerical and experimental mean and fluctuating statistics of the concentration fields that are related to dispersion of reactive pollutants around schematic buildings inside a boundary layer. The passive scalar dispersion downstream of an obstacle is first studied. A hybrid Large-Eddy simulation (LES)-Lagrangian method is used for the simulations. Results are compared with data obtained from a previous experiment using PIV (Particle Image Velocimetry) and MSD (Mie Scattering Diffusion) to describe concentration and velocity fields. In this case, a passive pollutant is released from a line source flushed at the wall of a turbulent boundary layer. This source is one obstacle height downstream of the square obstacle, itself located on the wall. Qualitative and quantitative features of the velocity and scalar fields are compared with experimental results. Then, the numerical simulation introduces a second order, irreversible chemical reaction in place of passive scalar. Validation is achieved by evaluating the mixture fraction which behaves as a passive scalar.
This work discusses the predictive applicability of a hybrid LES/Lagrangian simulation to pollution situations which are at the basis of wind engineering scenarios to account for pollution in strongly inhomogeneous flows. We stress here the capabilities and limitations of hybrid LES/Lagrangian simulations for pollution with reactive species. We keep in mind that studying more complex situations or events will inevitably increase the prediction uncertainties.
The relevant features of the flow and the concentration fields around the obstacle, as the recirculation bubble, are predicted by our model.
The chemical reaction model is validated in the presence of a recirculation zone and that is of first importance as soon as we are interested in urban zones with potential depth zones where no exchanges exist with the external flow. |
Keywords and phrases: large-eddy simulation, Lagrangian stochastic, dispersion, obstacle, recirculation zone, chemical reactions. |
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