Invited Speaker

Abstract: The entrainment of sediment particles represents one of the long-standing issues in water resources and environmental engineering areas. The difficulty in predicting particle entrainment resides mostly in the uncertainties of turbulent flows, particularly for the random energetic turbulent structures and corresponding fluctuations of flow velocities that impact sediment particles. Most recently, a shift from the force-dominated model to the impulse- or energy-based model has been promoted in an attempt to better account for the underlying mechanism of entrainment and, thus, attaining more accurate predictions on the threshold of motion conditions. This study contributes to such efforts by highlighting the sequences of flow velocity and hydrodynamic pressure recorded over the entrainment of particles. The advantage of using this sequence approach is twofold. First, it accounts for the duration aspect of applied forces (estimated from pressure integration), which, combined with force magnitude, yields a more versatile and consistent criterion of particle entrainment based on the impulse model. Second, it captures a time series of variations in flow velocity as turbulent structures sweep through the near-bed region and mobilize particles. This provides a more comprehensive description of fluid motion than treating it as a collection of isolated incidents occurring during the entrainment process. Specifically, the data used here are collected simultaneously through LDV and pressure transducers at a sampling rate of 250 Hz, sufficiently high to resolve turbulence characteristics that are capable of displacing sediment particles. Although the observed pressure sequences are highly related to the coincident velocity sequences, details of these records exhibit a discrepancy in the higher frequency ranges of turbulent fluctuations. Pressure response appears to lose high frequency components shown in the flow velocity variations—a main reason that velocity history alone cannot fully represent the hydrodynamic impact experienced by sediment particles. To overcome this limitation of velocity sequences, we propose to use pressure sequences as an additional means of accounting for the mechanistic aspects of entrainment. The coupled velocity and pressure sequences provide a more comprehensive perspective on the processes of particle displacement.

Keywords: Qudrant sequences, Particle entrainment, Sediment transport, Turbulence

Biography: Dr. Panayiotis 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 Civil and Envitonmental Engineering Department at Lehigh University.