SEMINAR

DEPARTMENT OF MECHANICAL AND CIVIL ENGINEERING

The Effect of Fluctuating Forces on Particle Entrainment; Revisiting the Threshold of Motion Criterion

Panos Diplas, Professor

Environmental Hydraulics Laboratory, Civil and Environmental Engineering, Lehigh University

Abstract

The dynamic interplay between fluid forces and particle dislodgement for flows over an erodible boundary constitutes a central problem in earth surface dynamics, biological flows, and many engineering phenomena and industrial processes. For example, erosion, transportation and deposition of sediments and pollutants influence the hydrosphere, pedosphere, biosphere and atmosphere in profound ways. The global amount of sediment eroded annually over the continental surface of the earth via the action of water and wind is estimated to be around 80 billion metric tons, with 20 billion of them delivered by rivers to the oceans. This redistribution of material over the surface of the earth affects most of its physical, chemical and biological processes in ways that are exceedingly difficult to comprehend.

Determining the minimum, or critical force necessary to dislodge a particle out of its pocket, arguably constitutes one of the most fundamental and elementary problems in mechanics, regardless of the mode of movement (i.e. sliding, rolling, saltation). Yet, when it comes to flow- induced forces, identifying this critical condition has confounded scientists and engineers for several hundred years. The main culprit for this problem is the fluctuating nature of the applied fluid forces, while the resistance to particle movement remains the same. Except for the limited flow condition of extremely low particle Reynolds numbers, when the Stokes regime prevails, this is always the case, even for laminar flows, where flow separation is predominantly responsible for unsteady loadings.

The criterion presently in use for predicting particle entrainment, originally proposed by Shields in 1936, emphasizes the time-averaged boundary shear stress and therefore cannot account for the fluctuating nature of the applied hydrodynamic forces. More recent studies have emphasized the significance of the magnitude of the peak instantaneous fluid forces on particle movement. It is demonstrated here that even this latest understanding about the mechanism responsible for the initiation of motion is incapable of explaining the phenomenon, leading to the particle dislodgement paradox concept. A new criterion that was developed recently is suggested in an effort to overcome the limitations of the previous approaches and resolve this paradox. It is demonstrated here that, in addition to the magnitude, the duration of energetic near bed turbulent flow events is equally important in predicting grain removal from the bed surface. It is therefore proposed that the product of force and its duration, or impulse, is a more appropriate and universal criterion for uniquely identifying conditions suitable for particle dislodgement. Analytical formulation of the problem and experimental data are used to examine the validity of the new criterion.

Biography

Dr. Diplas received his M.S. and Ph.D. degrees in Civil Engineering from the University of Minnesota. Prior to joining Lehigh in 2013, he taught at Virginia Tech for 25 years. He has published widely in the areas environmental, fluvial and infrastructure hydraulics. His work has been recognized by a number of awards, including the NSF National Young Investigator, H. A. Einstein from ASCE, best paper published in the Journal of Hydraulic Engineering in 2012 (Hilgard Hydraulic Prize), and a Certificate of Teaching Excellence. He is currently the P.C. Rossin Professor and Chair of the CEE Department at Lehigh University.