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Notifications of Three Academic Reports on Sediment Transport and Environmental Fluid Dynamics
Ø Academic Report 1
Topic: High-resolution Numerical Simulation of Turbidity Currents
Time: October 8th (Monday), 14:00-15:00
Location: 503 room, library (舟山校区图书馆503)
Speaker: Prof. Eckart Meiburg from UC, Santa Barbara
Inviter: Prof. Zhiguo He
Abstract: Gravity and turbidity currents are very important phenomenon in geophysical environments, like estuaries and oceans. Since they are very difficult to observe in fields, high-resolution numerical simulations are becoming very popular to investigate them. As one of the leading experts in this area, Prof. Eckart Meiburg is going to share the latest advances in direct numerical simulations of gravity and turbidity currents in his speech.
Introduction of the Speaker
Eckart Meiburg is the Distinguished Professor from University of California, Santa Barbara (UCSB). He obtained his Ph.D. degree from University of Karlsruhe, Germany in 1985. After that, he did postdoctoral research at Stanford University and became an assistant professor at Brown University in 1987. Before joining in UCSB, he stayed at University of South California as an associate professor and then professor between 1990 and 2000.
Prof. Eckart Meiburg has more than over 30 years’ experience in fluid dynamics and numerical simulation. He is one of the leading scientists in high-resolution numerical simulations of gravity and turbidity currents. Most of his scientific papers were published in prestigious journals in fluid dynamics, like Journal of Fluid Mechanics, Physics of Fluids, Physical Review Fluids, and Journal of Geophysical Research. In 2010, he was invited to write the review paper, Turbidity Currents and Their Deposits, in Annual Review of Fluid Mechanics.
Prof. Eckart Meiburg is a Fellow of American Physical Society (APS) and American Society of Mechanical Engineers (ASME). He is now the chair of the Division of Fluid Dynamics, APS and the Associate Editor of Physical Review Fluids.
Ø Academic Report 2
Topic: Intermittency of Sediment Transport: From Initiation of Particle Motion to High Levels of Solid Discharges
Time: October 8th (Monday), 15:00-16:00
Location: 503 room, library (舟山校区图书馆503)
Speaker: Dr. WuRong Shih (石武融) from Lehigh University
Inviter: Prof. Niansheng Cheng
Abstract: Sediment transport has been reported as a highly nonlinear phenomenon that leads to numerous empirical formulae of bed shear stress and transport rates. These formulae are usually expressed in a power function format with exponents varying between 1.5 and 16. Such a profound variation in exponents compromises the practicality of equations to predict sediment discharges. Through a careful examination of transport physics, we identify the cause of varying exponents at different hydrodynamic conditions: the intermittency of particle movement due to turbulent flow fluctuations. This temporal irregularity in the transport record is not properly accounted by the long-term averaging method, yielding inconsistent exponent values that carry less physical meaning about the transport mechanism. We employ conditional data treatment to offset the adverse effects of the movement intermittency, and, surprisingly, derive a consistent power relation from the conditioned stress-transport data sets. The results suggest a constitutive law of sediment transport, characterized by a 1.5th power equation, which signifies the relationship between energy expenditure of turbulent flow and the corresponding sediment yields. This interpretation holds true for a wide range of transport cases, from the onset of sediment transport conditions to the fully mobilized scenarios with high levels of solid discharges.
Introduction of the Speaker
Dr. Shih received his Ph.D. degree in Civil Engineering from Lehigh University (2017). He currently works at Lehigh University as a postdoctoral research fellow to further elaborate on his Ph.D. dissertation, dealing with sediment transport problems. Affiliations include American Society of Civil Engineers (EMI: Fluid Dynamics Committee) and American Geophysical Union. He serves as reviewer of several leading journals such as Water Resources Research. Awards include Taiwan’s two-year study abroad funding and US National Science Foundation project to support his doctorate pursuit.
Ø Academic Report 3
Topic: What is the sediment transport threshold?
Time: October 8th (Monday), 16:00-17:00
Location: 503 room, library (舟山校区图书馆503)
Speaker: Prof. Thomas Pähtz from Ocean college, Zhejiang University
Abstract: Standard sediment transport formulae predict that the transport rate Q vanishes when the bottom fluid shear stress T falls below a critical value Tc. Because Q cannot vanish when entrainment of bed sediment occurs, most aeolian and fluvial geomorphologists interpret Tc as an entrainment threshold associated with an equilibrium between contact forces and forces generated by the fluid and/or particle-bed impacts (splash) acting on grains resting at the bed surface. However, this interpretation neglects that, while the predicted Q vanishes, the actual Q never truly vanishes due to turbulence and that fluvial transport formulae tend to become highly inaccurate when T is smaller than about 2Tc due to transport intermittency. We have studied this conundrum using DEM/RANS simulations of non-suspended sediment transport for a large range of Newtonian fluids driving transport, including wind, oil, and water. A few of our findings are summarized below:
1. The threshold Tc is not an entrainment threshold but the minimal fluid shear stress needed to compensate the average energy loss of particles rebounding with the bed by the fluid drag acceleration during particle trajectories (i.e., Tc is a property of mean-flow-sustained transport and thus insensitive to turbulent fluctuations).
2. The threshold Tc can be described by a universal rebound model that is consistent with measurements in wind, oil, and water streams (e.g., the Shields diagram) despite not being fitted to these measurements.
3. Entrainment controls whether the capacity state described by standard transport formulae can be reached and sustained but does not control the value of Q at capacity.
4. Standard transport formulae are only valid for continuous transport, which occurs when particle-bed impacts are effective in entraining bed material. In contrast, fluid entrainment relies on the occurrence of turbulent events and can thus only sustain intermittent transport.
