On Heavy Particle Dispersion in Turbulent Shear Flows: 3-D Analysis of the Effects of Crossing Trajectories |
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Authors: | Benoît Oesterlé |
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Institution: | (1) LEMTA, Nancy-University, CNRS, ESSTIN, 2 rue Jean Lamour, 54519 Vandoeuvre-lès-Nancy, France |
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Abstract: | In the approaches used to predict the dispersion of discrete particles moving in a turbulent flow, the effects of crossing
trajectories due to gravity (or any other external force field) are generally accounted for by modifying the integral time
scales according to the well-known analysis of Csanady (J Atmos Sci 20:201–208, 1963). Here, an alternative theoretical analysis
of the time correlation of the fluid velocity fluctuations along a particle trajectory is presented and applied in a turbulent
shear flow. The study is carried out in the frame of three-dimensional Langevin-type stochastic models, where the main unknowns
are the drift tensor components rather than the conventional integral time scales of the fluid seen by the particles. Starting
from a model for the space-time velocity covariance tensor of the turbulence under the assumption of homogeneous shear flow,
the various components of the time correlation tensor of the fluid seen are expressed in the asymptotic case of large mean
relative velocity (between the particles and the flow) compared to the particle velocity fluctuations. In order to provide
comparison with the generally used expressions arising from isotropic turbulence assumption, we examine also the conventional
integral time scales of the fluid seen in the directions parallel and perpendicular to the mean relative velocity. The most
prominent deviations from isotropic turbulence are observed when the external force field is in the direction of the mean
velocity gradient: in this case the loss of correlation in the mean flow direction is significantly lower than expected in
a uniform flow, an observation that is in qualitative agreement with the few available data. |
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Keywords: | Crossing trajectories Modelling Particle dispersion Shear flow Turbulence |
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