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Accueil > Evénements Scientifiques > Conf’luences > Conf’luences 2012 > Conf’luence S. Balachandar : Recent results on lift and drag forces on particles in near wall shear flows and what it means to incipient motion, resuspension and bedload transport.

Conf’luence S. Balachandar : Recent results on lift and drag forces on particles in near wall shear flows and what it means to incipient motion, resuspension and bedload transport.

27 août 2012

S. Balachandar, Department of Mechanical & Aerospace Engineering, Department of Civil & Coastal Engineering (affiliate)
University of Florida, Gainesville.

31 août 2012 à 10h30
Amphithéâtre Nougaro

Résumé :
The key mechanism that dictates the rate of particle transport and resuspension is the
effective hydrodynamic force that either rolls the particle along the bed or resuspends
it into the bulk. Much of the existing bedload transport and resuspension models are
empirically driven. Given the stochastic nature of the driving force arising from the
turbulent flow and the stochastic nature of the resisting force arising from the random
distribution of geometric configurations in which the particles are sitting on the bed,
the need for empirical closures becomes clear. An essential building block to our
understanding and physics-based modeling of particle motion and resuspension
is to consider the problem of forces on a particle in a turbulent boundary layer
on a rough bed.

There are three elementary contributions to lift force on a particle moving close to a boundary :
wall-induced lift force on a translating particle, shear-induced lift force due to ambient shear,
rotation-induced lift force due to particle rotation. These contributions and their cross-coupling
are well understood in the low Reynolds number regime. Here we extend our understanding for
the case of finite Reynolds number. Through sequence of controlled numerical simulations we
develop accurate drag and lift expression for the case of a particle in arbitrary motion in a wall-bounded
turbulent boundary layer. At first we consider the case of a flat boundary. We then consider
direct numerical simulations of a particle on a rough boundary and extent the smooth wall
correlations to the practical case of a particle in an uneven bottom boundary.

The simulation results are compared with available experimental results at higher Reynolds numbers and with
asymptotic theories at low Reynolds numbers. The drag and lift force expressions obtained for a
particle sitting on a wall in a logarithmic turbulent boundary layer are used to obtain the
Shields’ diagram, where the simulation results allow us to explore the role of drag force, lift force and
the hydrodynamic torque.

Recently the importance of critical impulse, as opposed to critical shear stress or critical force
on the particle, has been proposed. We extend this argument further to advance the concept of a critical
energy barrier for particle motion. Based on this we obtain the bedload transport rate as a function
of shield’s parameter naturally from the simulation results.