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Particle resuspension by a periodically forced impinging jet

Published online by Cambridge University Press:  05 May 2017

Wen Wu*
Affiliation:
Department of Mechanical and Materials Engineering, Queen’s University, Kingston, Ontario, K7L 3N6, Canada
Giovanni Soligo
Affiliation:
Dipartimento Politecnico di Ingegneria e Architettura, Università degli Studi di Udine, Udine, 33100, Italia Institut für Strömungsmechanik und Wärmeübertragung, TU Wien, Wien, 1060, Austria
Cristian Marchioli
Affiliation:
Dipartimento Politecnico di Ingegneria e Architettura, Università degli Studi di Udine, Udine, 33100, Italia
Alfredo Soldati
Affiliation:
Dipartimento Politecnico di Ingegneria e Architettura, Università degli Studi di Udine, Udine, 33100, Italia Institut für Strömungsmechanik und Wärmeübertragung, TU Wien, Wien, 1060, Austria
Ugo Piomelli
Affiliation:
Department of Mechanical and Materials Engineering, Queen’s University, Kingston, Ontario, K7L 3N6, Canada
*
Email address for correspondence: w.wu@queensu.ca

Abstract

When hovering over sandy terrain, the rotor of helicopters generates a downward jet that induces resuspension of dust and debris. We investigate the mechanisms that govern particle resuspension in such flow using an Eulerian–Lagrangian approach based on large-eddy simulation of turbulence. The wake generated by the helicopter is modelled as a vertical impinging jet, to which a sequence of periodically forced azimuthal vortices is superposed. The resulting flow field provides a unique range of flow scales with which the particles can interact. Downstream of the impingement region, layers of negative azimuthal vorticity (secondary vortices) form on the upwash side of the primary azimuthal (large-scale) vortices. These layers then detach from the surface together with the near-wall (small-scale) vortices populating the wall-jet region. We show how the dynamics of sediments is governed by its interaction with these structures. After initial lift off from the impingement surface, particles accumulate in regions where near-wall vortices roll around the impinging azimuthal vortex, forming rib-like structures that either propel particles away from the azimuthal vortex or entrap them in the shear layer between the azimuthal and secondary vortices. We demonstrate that these trapped particles are more likely to reach the outer flow region and generate a persistent cloud of airborne particles. We also show that, in a time-averaged sense, particle resuspension and deposition fluxes balance each other near the impingement surface.

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Papers
Copyright
© 2017 Cambridge University Press 

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