大會演講

Dr. Melany L. Hunt

Dick & Dotty Hayman Professor of Mechanical Engineering

Division of Engineering and Applied Science, California Institute of Technology (Caltech), USA

Particles in Liquids: Collisions, Stresses, and the Legacy of R.A. Bagnold

Flows of particles in liquids occur in many geophysical and industrial problems, including sediment transport by wind and streams, pumping of slurries and suspensions, and landslides and debris flows. How do the particles affect the transport properties, and what is the contribution of particle collisions? An influential study that addressed these questions was experimental and analytical research by R.A. Bagnold. His 1954 paper on the rheological properties of liquid-solid suspensions continues to be widely referenced because it purportedly demonstrated a transition from a viscous regime to a grain-inertia regime involving particle collisions. Work at Caltech, however, showed that transition observed by Bagnold was a result of secondary flows and fluid turbulence rather than particle collisions. In addition to revisiting the studies by Bagnold, this presentation overviews related experiments at Caltech on the rheology of liquid-solid flows, extending from viscous-dominated to turbulent flows, along with detailed studies of immersed particle collisions in liquids.

Dr. James M. Chen

Associate Professor

Department of Mechanical and Aerospace Engineering at University at Buffalo (UB)

A Kinetic Description of Morphing Continuum and its Applications in Fluid Mechanics

The coupling between the intrinsic angular momentum and the hydrodynamic linear momentum has been known to be prominent in fluid flows involving physics across multiple length and time scales, e.g. turbulence, nonequilibrium flows and flows at micro-/nano-scale. Since the classical Navier-Stokes equations and Boltzmann’s kinetic theory are derived on the basis of monatomic gases or volumeless points, efforts to derive constitutive equations involving intrinsic rotation for fluids of polyatomic molecules have been found since the 1960s. One of the proposed continuum theories for polyatomic molecules was Morphing Continuum Theory (MCT). The theory was originally formulated under the framework of rational continuum mechanics and thermodynamic irreversible processes. The mathematically rigorous continuum mechanics presents a complete and closed set of governing equations but leaves the physical meanings unexplained. Similar to the correlation between Boltzmann's kinetic theory and the classical continuum mechanics, an advanced kinetic theory involving the Boltzmann-Curtiss (B-C) distribution function and the B-C equation will be introduced for a morphing continuum. The method of the most probable distribution method is used to derive the Boltzmann-Curtiss distribution. The corresponding Boltzmann-Curtiss equations will be demonstrated to be the MCT governing equations without any dissipation terms, i.e. the system (flows with inner structures) is in equilibrium and at the Boltzmann-Curtiss distribution. A first-order approximation to the B-C distribution will be used to further derive the B-C transport equations. The corresponding governing equations will then be compared with the MCT equations. Furthermore, a path to reduce the presented MCT equations down to the classical N-S equations will be demonstrated and discussed. Several cases, including homogeneous isotropic turbulence, supersonic turbulence over a compression ramp and flows with translational nonequilibrium, will be presented.

Dr. Keng-Lin Lee

Assistant Professor

Department of Mechanical Engineering

National Chung Hsing University

Mesoscale Stress Drop Bridges Granular Solid and Fluid Rheology

A unified rheological description of granular materials has been a longstanding goal since Coulomb's pioneering work nearly two centuries ago, driven by its fundamental importance in geophysical and industrial applications. The central challenge arises from the dual nature of granular matter: it supports loads like a solid but flows like a fluid once the applied stress exceeds a yield threshold, a transition that defies conventional continuum mechanics and statistical physics. Over the past two decades, substantial progress has been made in describing the post-yield fluid regime by extending the classical Coulomb friction law to incorporate the inertial number, which characterizes grain-inertia effects during rapid shearing, and the granular temperature, which quantifies grain velocity fluctuations. However, these models fail to capture the rich instability phenomena that emerge near yielding, such as stick-slip motion and hysteresis, leaving a unified framework spanning the pre-yield solid and post-yield fluid regimes elusive. In this work, inspired by activated dynamics in glassy materials, we propose a stress-activated description of granular rheology, in which mesoscale stress drops, arising from the elastic energy released during grain-scale avalanche events, act as a second temperature-like variable that bridges the solid and fluid regimes across the yielding transition. To test this framework, we developed a novel photoelastic double-shear apparatus that enables long-duration, simultaneous measurements of stress-drop statistics and flow rheology. Our results support the proposed stress-activated description and reveal a striking analogy between the granular solid–fluid transition and the two-temperature description of glassy systems.

Dr. Patricia Yang

Assistant Professor

Department of Power Mechanical Engineering

National Tsing Hua University

Mechanics of dust bathing in birds

Why do birds spend hours flinging themselves into dust and dirt? We show that this is a precisely tuned physical mechanism for evicting unwanted tenants. By filming 34 bird species in high-speed video, we found that wing-flapping frequency during dust bathing—ranging from 1 to 10 Hz—scales predictably with body size. Using mechanical shakers to recreate this motion, we discovered that mite removal shoots up as flapping frequency rises—until about 5 Hz, where it plateaus. Collision modeling shows that at these frequencies, sand particles slam into feathers with enough force to knock parasites loose, overpowering the mites' grip within seconds. Birds aren't just wallowing in the dirt—they're using dirt as a weapon, and evolution may have fine-tuned their flapping speed to hit the sweet spot for maximum pest control.

Keywords：flapping frequency, ectoparasite removal, eco-friendly cleaning.

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