Below is the complete list of seminar speakers for spring 2022. All seminars will be held over zoom.
February 1 (zoom)
Silas Alben, University of Michigan
Title: The flutter of very flexible foils and the collective locomotion of plates
Abstract: We present two separate studies in the area of fluid-structure interaction and biolocomotion. In the first, we examine the flutter of foils with very small or zero bending rigidity (i.e. membranes). We present the rich range of dynamics of these bodies, which often display sharp curvatures. Looking at a broad range of parameter space, we give scaling laws for the foils’ oscillation amplitudes, frequencies, and deformation wave numbers in both linear and nonlinear models. In the second study, we consider the propulsive properties of rectangular and rhombic lattices of flapping plates at O(10–100) Reynolds numbers. We vary five parameters: flapping amplitude, frequency (or Reynolds number), horizontal and vertical spacings between plates, and oncoming fluid stream velocity. The flows assume a variety of periodic and nonperiodic states, with and without up-down symmetry, and multiple stable self-propelled speeds can occur. At Re = 70, the maximum Froude efficiencies of time-periodic lattice flows are about twice those of an isolated plate. At lower Re, the lattices’ efficiency advantage increases.
February 8 (zoom)
Sujit Datta, Princeton University
Title: Elastic flow instabilities in 3D porous media
Abstract: Many energy, environmental, industrial, and microfluidic processes rely on the viscous flow of polymer solutions through porous media. In many cases, the macroscopic flow resistance abruptly increases above a threshold flow rate in a porous medium—but not in bulk solution. The reason why has been a puzzle for over half a century. Here, by directly visualizing the flow in a transparent three-dimensional (3D) porous medium, we demonstrate that this anomalous increase is due to the onset of an elastic instability in which the flow exhibits strong spatio-temporal fluctuations reminiscent of inertial turbulence, despite the vanishingly small Reynolds number. We find that the transition to unstable flow in each pore is continuous, arising due to the increased persistence of discrete bursts of instability above an onset flow rate; however, this onset value varies from pore to pore. Thus, unstable flow is spatially heterogeneous across the different pores of the medium, with unstable and laminar regions coexisting. Guided by these findings, we quantitatively establish that the energy dissipated by unstable pore-scale fluctuations generates the anomalous increase in flow resistance through the entire medium. Thus, by linking the onset of unstable flow at the pore scale to transport at the macroscale, our work yields generally-applicable guidelines for predicting and controlling polymer solution flows.
February 15 (zoom)
Cong Wang, California Institute of Technology
Title: Turbulent drag reduction using a dynamic free-slip boundary
Abstract: The turbulent motions in the boundary layers developed over the solid surface of the ocean liners or commercial airplanes are responsible for huge amount of energy loss (50% – 80% of the total energy expenditure). As such, turbulent boundary layer control and drag reduction has been a long-lasting task for scientists and engineers. Many promising drag reduction techniques have been developed, such as through employing a passive superhydrophobic surface or an actively oscillating solid boundary. Nevertheless, sustainable drag reduction effect in high Reynolds number turbulent flow is still challenging. Recently, we developed a novel, easy-scalable dynamic free-slip boundary method by employing an array of oscillating, wall-attached air-water interfaces, which achieves substantial drag reduction effect (up to 40% and persists over long distances). The drag reduction effect happens together with the lifting-up of shear motions and a relaminarization process in the near-wall region. The most counter intuitive observation is that a strong propulsion force (opposite to the drag force) is generated by the dynamic free-slip surfaces. These physical observations are due to the nonlinear vorticity production and accumulation process near the dynamic free-slip surface. In quiescent water, the dynamic free-slip surface induces a wall-normal, far-propagating streaming jet when under certain control conditions. We propose a drag reduction mechanism and verify it in a laminar boundary layer: through the dynamic interaction between the transverse boundary layer and the dynamic free-slip surfaces, counter-rotating streamwise vortices are created, which lifts the shear motions away from the wall. The physical discoveries demonstrate the extraordinary capability of the dynamic free-slip surface in the general area of boundary layer manipulation and flow control, which can be employed to address many engineering challenges in addition to drag reduction.
February 22 (zoom)
Irmgard Bischofberger, MIT
Title: Instabilities and flow-induced structures in lyotropic chromonic liquid crystals
Abstract: Lyotropic chromonic liquid crystals in the nematic phase are anisotropic fluids. We exploit this intrinsic anisotropy to probe growth morphology transitions that occur in the viscous-fingering instability upon the displacement of the liquid crystal by a less-viscous Newtonian liquid. In isotropic systems, this instability produces complex patterns that are characterized by repeated branching of the evolving structure, which leads to the common morphologies of fractal or dense-branching growth. In anisotropic systems, by contrast, the growth morphology changes to dendritic growth characterized by stable needle-like structures. We show that the morphology transition coincides with the onset of shear-alignment at high shear rates, where the shear forces become dominant over the elastic forces from the nematogenic potential. Below this critical shear rate, the lyotropic chromonic liquid crystal exhibits a tumbling behavior that leads to the formation of novel types of defects and flow structures, including the surprising emergence of chiral domains despite the achiral nature of the material.
March 1 (zoom)
Megan Leftwich, George Washington University
Title: The hydrodynamics of sea lion swimming
Abstract: California Sea Lions are highly maneuverable swimmers, capable of generating high thrust and agile turns. Their main propulsive surfaces, the foreflippers, feature multiple degrees of freedom, allowing their use for thrust production (through a downward, sweeping motion referred to as a “clap”), turning, stability and station holding (underwater “hovering”). To determine the two-dimensional kinematics of the California sea lion fore flipper during thrust generation, digital, high definition video is obtained using the specimen at the Smithsonian National Zoo in Washington, DC. Single camera videos are analyzed to digitize the flipper during the motions, using 10 points spanning root to tip in each frame. Digitized shapes were then fitted with an empirical function that quantitatively allows for both comparison between different claps and for extracting kinematic data. The resulting function shows a high degree of curvature (with a camber of up to 32%). Analysis of sea lion acceleration from rest shows thrust production in the range of 150-680 N and maximum flipper angular velocity (for rotation about the shoulder joint) as high as 20 rad/s. Analysis of turning maneuvers indicate extreme agility and precision of movement driven by the fore flipper surfaces. This work is being extended to three-dimensions via the addition of a second camera and a sophisticated calibration scheme to create a set of camera-intrinsic properties. Simultaneously, we have developed a robotic sea lion foreflipper to investigate the resulting fluid dynamic structures in a controlled, laboratory setting.
March 8 (zoom)
Luc Deike, Princeton University
Title: Mass transfer at the ocean-atmosphere interface: the role of wave breaking, droplets and bubbles
Abstract: Physical processes at the ocean-atmosphere interface have a large effect on climate and weather by controlling the transfer of momentum and mass. Without wave breaking, transport between the ocean and the atmosphere is through slow conduction and molecular diffusion, while wave breaking is a transitional process from laminar to turbulent flow. When waves are breaking, the surface experiences dramatic changes, with sea spray ejection in the atmosphere and air entrainment into the ocean water. In this talk I will discuss recent efforts in my group towards improving parameterizations of gas transfer and sea spray production through a multi-scale approach. We combine detailed laboratory experiments and numerical simulations on turbulent multiphase flows, including wave breaking, bubble break-up in turbulence and spray production by bubble bursting with a statistical description of breaking waves to develop a general theoretical framework. This framework aims to account for the complex nature of wave breaking and air entrainment, a two-phase turbulent process, and the very large range of scales involved in the process, from wave statistics scales of order of km, O(1m-1km), to wave breaking dynamics, O(1-10m), air bubble entrainment, bubble dynamics in turbulence and finally bubble bursting at the first surface, O(microns to mm).
March 15 (zoom)
Cecilia Huertas Cerdeira, University of Maryland
Title: Leveraging bio-inspired principles: from underwater propellers to wind energy harvesters
Abstract: The use of flexible structures and unsteady fluid phenomena significantly increases the performance of many biological mechanisms. Yet, these properties have been historically avoided in engineering due to the inherent complexity of the resulting systems. In this talk, I will show two examples of how taking inspiration from nature can lead to improved engineered designs. First, a caudal-fin-inspired propeller for autonomous underwater vehicles will be presented. Using an experimental optimization procedure, the vehicle’s gait will be optimized for different locomotive goals, achieving a propulsion system that is both efficient and maneuverable. Second, an inverted-flag wind energy harvester, inspired by the fluttering motion of leaves, will be investigated. The physical mechanisms governing the behavior of the system will be highlighted, and the effect of the flag’s angle of attack will be analyzed.
March 22 (zoom)
Judy Yang, University of Minnesota
Title: Transport of particles, bacteria, and carbon in porous media
Abstract: A wide variety of global environmental and health issues involve physical and biological interactions among fluids, particles or surfaces, and microbes at both micro- and macro-scales. For example, macro- or channel-scale sediment transport, a key process that controls coastal erosion, can vary by several orders of magnitude due to micro-scale sediment-sediment and sediment-bacteria interactions, including aggregation and biofilm formation. Other examples include soil carbon storage, responsible for the uptake of about 20% of annual anthropogenic carbon emissions, which is strongly impacted by the physical and biogeochemical interactions between clay micro-aggregates and living organisms at the micro-scale. Also, my recent research has shown that bacterial biosurfactant-induced flows at the micro- or pore-scale can cause pathogenic bacteria to spread in unsaturated porous media where air and liquid coexist in pore spaces, such as soil, plants, and lungs. In this talk, I will discuss how I use microfluidics and meso-scale experiments to understand the transport of carbon and bacterial cells in soil and other porous media. I will also briefly discuss how I use flume experiments to study the impacts of vegetation on sediment transport. Finally, I will discuss my plans to combine micro- to macro- experimental methods to understand the impacts of micro-scale flow-particle-bacteria interactions on macro-scale environmental and health issues.
March 29 (zoom)
Nicolas Roussel, Université Gustave Eiffel
Title: Gravity-driven shaping of cement-based materials
Abstract:
In the second part, we will describe the way these properties dictate the outcome of both formative and additive shaping processes. We will in parallel revisit these processing technologies under the light of the central role played by gravity-induced stresses and their competition with yield stress and its time evolution.
April 5 (zoom)
Leonardo P. Chamorro, University of Illinois at Urbana-Champaign
Title: On the Unsteady Interaction between Turbulence and Flexible Structures
Abstract:
Characterization and quantification of the different coupling between flow and flexible structures and the dominant oscillation modes remain as open problems. Applications in the environment, energy, structural design, and locomotion require a comprehensive understanding of related phenomena. Of particular interest are canopy flows containing large flexible structures arrays. Ubiquitous in natural environments with scales typically spanning five orders of magnitude, they play a crucial role in the transport of scalar and inertial particles. This talk will present insight gained from theoretical arguments and laboratory experiments under controlled conditions. Specifically, I will briefly discuss the role of various controlling parameters in the unsteady dynamics of flows, single structures, and canopies. These include flow velocity, turbulence, stiffness, aspect ratio, tip effects, layout, and submergence (in open flows). For this purpose, I will show results from particle image velocimetry (PIV), particle tracking velocimetry (PTV), and force balance, which are used to characterize the turbulence and the motion and unsteady loads on selected structures.
April 12 (zoom)
Tracy Mandel, University of New Hampshire
Title: Vortices, flumes, and plumes: Experiments and theory at the coastal free surface
Abstract: Flow interactions at coastlines play a major role in global ocean energy and nutrient budgets. However, coastal flows can be expensive, logistically challenging, and even dangerous to study in situ. In this talk, I will cover two problems that connect the water surface and subsurface dynamics in the coastal ocean, and show how we can use idealized laboratory experiments to understand the physics of these systems. First, I will discuss the first step in remotely characterizing seagrass meadows by studying the overlying water surface. Flow through a seagrass bed can generate large overturning vortex structures, which cause small perturbations in the free surface slope. Using laboratory experiments, we develop a parameterized model to reconstruct within-canopy velocity profiles solely from water surface measurements, suggesting that in some environmental flows, the subsurface hydrodynamics and geometry may be predicted by measuring the water surface behavior alone. Second, I will examine the dynamics of turbulent buoyant plumes, such as those that might emerge at the base of a marine-terminating glacier. I will discuss our recent work that teases apart the role of buoyancy in enhancing entrainment in plumes, and preliminary work that quantifies the surface expression of these buoyant plumes.
April 19 (zoom)
Douglas Kelley, University of Rochester
Title: Brain cerebrospinal fluid flow
Abstract: The brain is surrounded by water-like cerebrospinal fluid, which flows around and through brain tissue, with profound implications for human health. Occurring almost exclusively during sleep, the flow serves to remove metabolic wastes like the amyloid-beta and tau proteins whose accumulation is believed to cause Alzheimer’s disease. In unhealthy situations like stroke and cardiac arrest, however, the fluid contributes to the severe swelling that permanently damages brain tissue. My team and I study the fluid dynamics of cerebrospinal fluid in the brain, from simulation, theory, and analysis of in vivo experiments. I will talk about the characteristics of the flow, its drivers, efforts at brain-wide modeling, and potential clinical implications.
April 26 (zoom)
Jesse Ault, Brown University
Title: Diffusiophoresis in a Taylor-dispersing solute
Abstract: We consider the coupled dynamics of solutes and charged colloidal particles in a channel flow both in the presence of a background fluid flow, and with a flow driven by slip boundary conditions at the walls. First, we consider the diffusiophoresis of a suspension of charged colloidal particles in the presence of a nonuniform solute concentration, where each species experiences Taylor dispersion from spanwise velocity gradients. We describe the two-dimensional evolution of both solute and particle concentration fields in a narrow channel with background Poiseuille flow by applying the lubrication approximation along with the assumption of constant particle zeta potential. We compare the theoretical predictions with the results of numerical simulations for fixed particle zeta potential, demonstrating good agreement. Finally, we comment on three-dimensional effects, long-time dynamics, and the validity of the constant particle zeta potential assumption for our model. We perform additional simulations with a simple variable-zeta potential model that treats the particle zeta potential as a logarithmic function of solute concentration. The results show good qualitative and quantitative agreement, supporting the use of the constant particle zeta potential assumption for this system under certain circumstances. We also modify this approach to consider the case where diffusioosmosis at the channel walls drives and additional recirculation through a slip boundary condition at the channel walls.
May 3 (zoom)
Varghese Mathai, University of Massachusetts Amherst
Title: Multiphase flows for energy extraction: From active-biphasic turbulence to oscillating compliant membranes
Abstract: The interaction of deformable materials with fluid flows can result in a variety of emergent phenomena, many of them advantageous in engineering. In this talk I will present two multiphase flow systems where interfacial mechanics contribute to enhancements in thermal and mechanical energy extraction. In the first part, I will discuss flow modifications that result from the introduction of millimetric gas/vapor bubbles in thermally driven turbulent flow. Specifically, we will show how adding a small volume fraction (~ 1%) of a low-boiling liquid to a water-based thermal convection system can generate a self-sustained cycle of rising vapor bubbles and settling droplets, with a severalfold increase in heat transfer efficiency. The roles of phase change and bubble kinematics in the transport processes will be discussed. In the second part of my talk, I will discuss the fluid-structure interactions of a biologically-inspired membrane hydrofoil. We reveal the mechanisms by which the membrane’s deformability contributes to a higher power extraction as compared a rigid hydrofoil. Potential benefits of using soft materials for energy extraction in tidal and fluvial environments will be outlined.
May 10 (zoom)
Camille Duprat, Ecole Polytechnique
Title: Wetting, adhesion and aggregation of soft fibres
Abstract: There are several ways of forming macroscale elastic structures from a fibre suspension: paper sheets are obtained from the dewatering of a cellulosic fibre suspension on a forming grid, and it has been shown recently that soft viscoelastic gels are formed upon extrusion of a microfibre suspension. Furthermore, textiles, or fibrous assemblies, are commonly used to absorb liquid or filtrate droplets from an aerosol stream.
In these situations, the flexible fibres interact with liquids, in particular with liquid-air interfaces. I will illustrate some of the mechanisms at play in such fiber-liquid interactions with several model experiments. In particular, I will look at the absorption dynamics of isolated drops on a single swelling fibre, and will show that fluid is spontaneously released out of the fibre during the absorption, due to a combination of the global compression induced by swelling with the local saturation of liquid at the drop position. This local expulsion of fluid creates a new droplet at the surface of the fibre, which then interacts with the drops initially presents, inducing motions or even coalescence of the drops, and leading to complex absorption dynamics. I will then discuss the case of a drop of liquid bridging two flexible fibers, which involves a coupling between fiber elasticity and capillarity. Finally, I will present an experiment on suspensions of flexible hydrogel!
fibres:
by pulling a few fibres through an interface, we are able to extract continuous cylinders of aggregated fibres (or fibrous bundles). We characterize the conditions at which these continuous bundles are formed, and probe their structural and mechanical properties.