Center for Fluid Mechanics

Past Seminars

  • Virtual

    Fluids Seminar: Paolo Luzzatto-Fegiz, UC Santa Barbara

    Location: via Zoom
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    Center for Fluid Mechanics Seminar Series

    Title: Two problems in wall-bounded flow: fluid energy extraction in wind farms, and surfactant effects in superhydrophobic drag reduction

    Abstract: In this talk, we consider two fluid problems directly linked to decarbonization efforts. In the first part, we investigate fundamental limits to the performance of large wind farms. Since wind turbines are often deployed in arrays of hundreds of units, wake interactions can lead to drastic losses in power output. Remarkably, while the theoretical “Betz” maximum has long been established for the output of a single turbine, no corresponding theory appears to exist for a generic, large-scale energy extraction system. We develop a model for an array of energy-extracting devices of arbitrary design and layout, first focusing on the fully-developed regime, which is relevant for large wind farms. We validate our model against data from field measurements, experiments and simulations. By defining a suitable ideal limit, we establish an upper bound on the performance of a large wind farm. This is an order of magnitude larger than the output of existing arrays, thus supporting the notion that large performance improvements may be possible.

    In the second part of this talk, we examine flow past superhydrophobic surfaces (SHS). These coatings have long promised large drag reductions; however, experiments have provided inconsistent results, with many textures yielding little or no benefit. By performing surfactant-laden simulations and unsteadily-driven experiments, we demonstrate that surfactant-induced Marangoni stresses can be to blame. We find that extremely low surfactant concentrations, unavoidable in practice, can drastically increase drag, at least in laminar flows. To obtain accurate drag predictions on SHS, one must therefore solve the mass, momentum, bulk surfactant and interfacial surfactant conservation equations, which is not feasible in most applications. To address this issue, we propose a theory that captures how the near-surface dynamics depend on the seven dimensionless groups for surfactant. We validate our theory extensively in 2D, and describe progress toward 3D and turbulent models. Our theory significantly improves predictions relative to a surfactant-free one, which can otherwise overestimate drag reduction by several orders of magnitude.

    Bio:
     Paolo Luzzatto-Fegiz graduated with a BEng in Aerospace Engineering from the University of Southampton, where he received the Royal Aeronautical Society Prize for highest first-class degree and the Graham Prize for best experimental project in the School of Engineering Sciences. After a summer working with the ATLAS Magnet Team at CERN, he completed an MSc in Applied Mathematics at Imperial College, and an MS and PhD in Aerospace Engineering at Cornell University. His doctoral work received the Acrivos Award of the American Physical Society for outstanding dissertation in Fluid Dynamics at a U.S. university. He was awarded a Devonshire Postdoctoral Scholarship from the Woods Hole Oceanographic Institution, as well as a Junior Research Fellowship from Churchill College, Cambridge. He is currently an Assistant Professor in Mechanical Engineering at UCSB, where he has received the Northrop Grumman Teaching Award and a Gallery of Fluid Motion Award from APS-DFD. He co-invented a salinity sensor for oceanography that has been adopted by 20 institutions, led the first microgravity experiment from NSF CBET in 2018, and is presently co-developing a new experiment on photo-active surfactants for the International Space Station.
  • Title: Synthetic swimmers: microorganism swimming without microorganisms

    Abstract: The effect of non Newtonian liquid rheology on the swimming performance of microorganisms is still poorly understood, despite numerous recent studies. In our effort to clarify some aspects of this problem, we have developed a series of magnetic synthetic swimmers that self-propel immersed in a fluid by emulating the swimming strategy of flagellated microorganisms. With these devices, it is possible to control some aspects of the motion with the objective to isolate specific effects. In this talk, recent results on the effects of shear-thinning viscosity and viscoelasticity on the motion of helical swimmers will be presented and discussed. Also, a number of other new uses of the synthetic swimmers will be presented including swimming across gradients, swimming in sand, interactions and rheometry.

  • Virtual

    Fluid Mechanics Webinar Series, featuring Baylor Fox-Kemper

    Location: via Zoom
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    The Journal of Fluid Mechanics invites you to the Fluid Mechanics Webinar Series. Organizers: Leeds Institute for Fluid Dynamics , the Department of Applied Mathematics and Theoretical Physics at the University of Cambridge, the UK Fluids Network , and the Journal of Fluid Mechanics .

    Brown University’s Baylor Fox-Kemper, Associate Professor of Earth, Environmental and Planetary Sciences, will present a talk: “Affronting Ocean Models: Submesoscale Interactions between Fronts, Instabilities, and Waves” 

    New attendees can register for the fluid mechanics webinar series herePlease note that registration closes at 12:00 eastern time on the Thursday prior to the webinar. Zoom links and passwords will be sent prior to the event.

    Abstract: Ocean fronts - sharp horizontal gradients in temperature, salinity, and density - are a key feature of the upper ocean that affect the transport of pollutants and the nature of near surface flows. I will highlight some of the recent modeling and theoretical work our group and collaborators have taken on to understand how fronts, frontal instabilities and turbulence, and surface waves interact. Traditional geophysical boundary layer theory neglects horizontal variations, and so is unable to capture frontal dynamics. Some consequences of these features found in large scale modeling and observations of oil, plastics and biological tracer dispersion; boundary layers; fluid energy cycling and dissipation statistics; and finally climate sensitivity will be elucidated.

  • Virtual

    Fluids Seminar presents Ajay Harishankar Kumar, Brown University

    Location: Via Zoom Cost: Free
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    Fluids at Brown and Fluids and
    Thermal Sciences Joint Seminar Series

    Tuesday, May 12, 2020
    3:30 PM
    Via Zoom

    Ajay Harishankar Kumar, Brown University

    Title: Taylor-Aris Dispersion of Elongated Rods

    Abstract : Particles transported in fluid flows, such as cells, polymers or nanorods, are rarely spherical in nature. In this study, we numerically and theoretically investigate the dispersion of an initial concentration of elongated rods in 2D pressure-driven shear flow. The rods translate due to diffusion and advection, and rotate due to rotational diffusion as well as their classical Jeffery’s orbit in shear flow. When rotational diffusion dominates, we approach the classical Taylor Dispersion result for the longitudinal spreading rate by using an orientationally averaged translational diffusivity for the rods. However, in the high shear limit, the rods tend to align with the flow and ultimately disperse more as a direct consequence of their anisotropic diffusivities. The relative importance of the shear-induced orbit and rotational diffusivity can be represented by a rotational Peclet number, and allows us to bridge these two regimes.

  • Virtual

    Fluids Seminar presents Aakash Sane, Brown University

    Location: Via Zoom Cost: Free
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    Fluids at Brown and Fluids and
    Thermal Sciences Joint Seminar Series

    Tuesday, May 12, 2020
    3:00 PM
    Via Zoom

    Aakash Sane, Brown University

    Title: Predictability of coastal ocean model ROMS-OSOM: Narragansett Bay

    Abstract : In this talk I will describe the predictability studies on a coastal model we have developed for Narragansett Bay. I will define predictability and its importance in forecasting in ocean modeling. Metrics from information theory have been used to find the predictability time scales on ensemble simulations. Predictability time scale enhances readily estimable timescales such as the freshwater/ saline water flushing timescale. The predictability of the model is around 10-20 days, varying by perturbation parameters and season. Internal variability is low when compared to forced variability for the current resolution suggesting modest chaos at this resolution. Freshwater flushing time scale and total exchange flow was calculated for the coastal model. The freshwater flushing time scale was found to be ~20 days and varies with the choice of the estuary boundary. The predictability time scales and flushing time scales reveal important dynamics of the tracers involved and elucidate their role in driving the estuary.

  • Virtual

    Fluids Seminar presents Zhong Zheng

    Location: Via Zoom Cost: Free
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    Fluids at Brown and Fluids and
    Thermal Sciences Joint Seminar Series

    Tuesday, May 5, 2020
    3:00 PM
    Via Zoom

    Zhong Zheng

    Title: Reduced-order Transport Models forEnergy and the Environment

    Abstract : In this talk, I will discuss several reduced-order transport modeling studies motivated by energy and environmental processes: (i) Inspired by CO 2 geological storage, we study fluid (CO 2 ) injection into a confined porous reservoir initially saturated with another fluid (brine), and characterize the time evolution of the fluid-fluid (CO 2 -brine) interface. Because of the effect of confinement, we identify a transition from an early-time self-similar solution to three branches of late-time self-similar solutions for the interface shape. (ii) Inspired by shale gas recovery, we study the fluid-driven cracks in an elastic matrix and characterize the evolution of the crack shape; we also study the elasticity-driven backflow process following fluid injection, and obtain a simple scaling law to derive a universal crack shape and for the backflow rate of the fracking fluids. (iii) I will also introduce our fundamental study on the viscous fingering instability, which is related to enhanced oil recovery, and report a series of time-dependent strategies for the stabilization of the viscous fingering instability at fluid-fluid (e.g., water-oil, gas-oil) interfaces. I will close the talk by discussing ideas for future exploration and collaboration.

  • Virtual

    Fluids Seminar presents Fernando Vereda, University of Granada, Spain

    Location: Via Zoom Cost: Free
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    Fluids at Brown and Fluids and
    Thermal Sciences Joint Seminar Series

    Tuesday, April 28, 2020
    3:30 PM
    Via Zoom

    Fernando Vereda University of Granada, Spain

    Title: Shear-induced migration of a suspension under planar confinement

    Abstract : The oscillatory flow of a suspension of neutrally buoyant, non-Brownian spherical particles in a rectangular channel at low Reynolds number is studied through experiments and numerical simulations. Particles, which are practically confined to a plane, migrate to regions of lower shear rate. Prior experimental and numerical work in oscillating Poiseuille flows has demonstrated the importance of the strain amplitude on shear-induced migration. In this talk, we present results for the early development of the suspension, including the dependence of the steady state configuration of the system and the dynamics of the shear-induced migration on particle concentration and strain amplitude. Our measurements are compared to simulations using the Force Coupling Method (FCM) for monodispersed spherical particles in a channel. Observations are in agreement with those previously reported for more conventional 3D geometries. Concretely, for larger concentrations the dynamics of the migration is faster and the onset of irreversibility is observed at smaller strain amplitudes. This geometry allows for the optimal visualization of the particles and thus shows great promise for the study of shear-induced migration.

  • Fluids at Brown and Fluids and
    Thermal Sciences Joint Seminar Series

    Tuesday, April 28, 2020
    3:00 PM
    Via Zoom

    Giuseppe Pucci, Institute of Physics of Rennes, France

    Title: Capillary surfers: Self-propelling particles at an oscillating fluid interface

    Abstract : In the present work, we explore the dynamics of millimetric bodies trapped at the air-water interface of an oscillating bath. The relative vertical motion of the body and the free surface leads to the generation of propagating capillary waves. We demonstrate that when the rotational symmetry of an individual particle is broken, the particle can steadily self-propel along the interface. Such self-propelled particles interact with one another through their mutual capillary wavefield and resultant fluid flows, and exhibit a rich set of collective modes characterized by a discrete number of equilibrium spacings for a given set of experimental parameters. Our results open the door to further investigations of this novel active system at the fluid interface. Ongoing work and future directions will be discussed.

  • Virtual

    Fluids Seminar presents Federico Hernandez-Sanchez, UNAM

    Location: Via Zoom Cost: Free
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    Fluids at Brown and Fluids and
    Thermal Sciences Joint Seminar Series

    Tuesday, April 21, 2020
    3:00 PM

    Via Zoom

    Federico Hernandezaniel, National Autonomous University of Mexico

    Title: Ultra-high speed visualization of a flash-boiling jet and a Synthetic jets induced levitation.

    AbstractWe visualized the atomization of liquid jets due to flash-boiling with video recordings up to five million frames per second. Such temporal resolution allowed us to capture the details of the bubble expansion mechanism for the first time. We documented that there is an abrupt transition from a laminar to a fully external flashing jet while reducing the ambient pressure. The experiments revealed that the spray spreads in all directions and bubble expansion speeds achieve up to 140m/s. Also, the ejected droplets achieve speeds much larger than the jet velocity and drop sizes orders of magnitude smaller than the diameter of the nozzle. Furthermore, hole growth speeds measured on the bubble’s film in combination with Taylor–Culick predictions suggest that the smallest droplet sizes are on the hundreds of nanometer or submicron range, which contravenes the general belief that flash- boiling atomization results in uniform drop sizes.

     

    In a previous report, it was argued that Acoustic Levitation at low frequencies results as a consequence of sound radiation. However, our most recent study suggests that this type of Acoustic Levitation occurs due to an air-flow driven by the undulating displacement of the speaker. Measuring the vortex velocity, we estimated the momentum flow in terms of the Reynolds and the Strouhal numbers. Surprisingly, the scaling demonstrated to be valid for an extensive range. These results remain preliminary.

  • Virtual

    CANCELED: Fluids Seminar presents Jihai Dong, Brown University

    Location: Via Zoom Cost: Free
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    Fluids at Brown and Fluids and
    Thermal Sciences Joint Seminar Series

    Tuesday, April 14, 2020
    3:00 PM
    Via Zoom

    Jihai Dong, Brown University

    Title: The Seasonality of Submesoscale Energy Production, Content, and Cascade

    Abstract : Submesoscale processes in the upper ocean vary seasonally, in tight correspondence with mixed layer thickness variability. Based on a global high‐resolution MITgcm simulation, seasonal evaluation of strong vorticity and spectral analysis of the kinetic energy in the Kuroshio Extension System show the strongest submesoscales occur in March, implying a lag of about a month behind mixed layer thickness maximum in February. An analysis of spectral energy sources and transfers indicates that the seasonality of the submesoscale energy content is a result of the competition between the conversion of available potential energy into submesoscale kinetic energy via a buoyancy production/vertical buoyancy flux associated with mixed layer instability and nonlinear energy transfers to other scales associated with an energy cascade. The buoyancy production is seasonally in phase with the mixed layer depth, but the transfers of energy across scales makes energizing the reservoir of submesoscale kinetic energy lag behind by a month.

  • Virtual

    Fluids Seminar presents Daniel Harris, Brown University

    Location: Via Zoom Cost: Free
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    Fluids at Brown and Fluids and
    Thermal Sciences Joint Seminar Series

    Tuesday, April 7, 2020
    3:00 PM
    Via Zoom

    Daniel Harris, Brown University

    Title: Hydrodynamic mechanisms for particle aggregation at fluid interfaces

    Abstract : Understanding the forces on small bodies at fluid interfaces has significant relevance to a range of natural and artificial systems. In this talk, I will discuss two recent investigations of fluid-mediated attraction mechanisms of non-Brownian particles, at free surfaces and within density stratified fluids.

     

    In the first part, I will present direct measurements of the attractive force between centimetric disks floating at an air-water interface. It is well known that objects at a fluid interface may interact due to the mutual deformation they induce on the free surface, however few direct measurements of such forces have been reported. In the present work, we characterize how the attraction force depends on the disk radius, mass, and relative spacing. The measured forces are rationalized with scaling arguments and compared directly to numerical predictions.

     In the second part, I will describe a novel attractive mechanism by which particles at isopycnals within a density stratified fluid may self-assemble and form large aggregates without need for short-range binding effects (adhesion). This phenomenon arises through a subtle interplay of effects involving solute diffusion, impermeable boundaries, and the geometry of the aggregate. Control experiments with two particles isolate the individual dynamics, which are quantitatively predicted through numerical integration of the underlying equations of motion.

     Ongoing and future work in these areas will also be discussed.

  • Virtual Fluids Seminar

    Location: webinar
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    “Bubbles and drinks”
    Roberto Zenit, Professor of Engineering
    Brown University

    Most people find bubbly drinks to be attractive and refreshing. With the excuse of trying to answer why, we explore the physics involved in this particular kind of two-phase, mass-transfer-driven flows. Discussion and analysis of the processes of bubble formation, ascension, accumulation and bursting are presented. Links to other relevant flow phenomena are presented in each case.

    https://brown.zoom.us/j/520948930

    (BYO coffee and cookies)

  • Fluids Seminar presents Marco Ellero, Basque Center for Applied Mathematics

    Location: 170 Hope Street Room: 108 Cost: Free
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    Fluids at Brown and Fluids and
    Thermal Sciences Joint Seminar Series

    Wednesday, November 20, 2019
    9:30 AM
    170 Hope St, Room 108

    Marco Ellero , Basque Center for Applied Mathematics

    Title: Modeling complex suspensions with particle methods

    Abstract : The modelling and simulation of “simple” suspensions of Brownian and non-Brownian particles dispersed in Newtonian media has been studied extensively in the last decades and several rheological responses can now be reproduced numerically and understood.

    Much less investigated is the case of “complex” suspensions of particles interacting with non-Newtonian media. In this talk, I will present recent advances in this field using particle-based models such as Smoothed Particle Hydrodynamics or its stochastic version, Smoothed Dissipative Particle Dynamics.

    As an application of the present simulation framework, two particulate systems will be considered and their rheology discussed in relation to experimental findings:

    1) shear-thinning of a non-colloidal suspension interacting with a ‘nominally-Newtonian’ fluid.

    2) shear-thickening of a non-colloidal suspension interacting with a highly elastic polymeric matrix.

  • Fluids Seminar presents Jie Feng, University of Illinois

    Location: Barus and Holley Room: 190 Cost: Free
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    Fluids at Brown and Fluids and
    Thermal Sciences Joint Seminar Series

    Tuesday, November 12, 2019
    3:00 PM
    Barus & Holley, Room 190

    Jie Feng , University of Illinois

    Title: Manipulating Soft Materials with Interfacial Dynamics: from Bubble Bursting to Nanoparticle Denaturation

    Abstract : Interfaces between two distinct phases are ubiquitous in nature and many engineering processes. From the fundamental studies in physics, materials and biology, to applications in various fields, interfaces control many aspects of the thermodynamics and dynamics of multi-phase systems. Not surprisingly, questions about the dynamics of interfaces, e.g., their flow and response to forces, occur widely in colloid science, fluid mechanics and other areas of science and engineering. Therefore, understanding various interfacial dynamics remains a canonical problem with strong intellectual interest and broad industrial impacts.

    In this talk, we will describe two distinct problems where we investigate the interfacial dynamics of structured complex fluids, and the understandings can be extended to soft materials engineering for applications in the environmental and health science. First, we will present the study of bubble bursting at a compound air-oil-water interface. We document the hitherto unreported formation and dispersal of submicrometer oil droplets into the water column. Surprisingly, the droplet size is selected by the physicochemical interactions rather than by hydrodynamic effects. The implications of the dispersal mechanism for oil-spill remediation and multi-functional nanoemulsion formation will also be demonstrated. Second, we will discuss the evolution of polymeric nanoparticle attachment at an air-liquid interface over time scales from 100 millisecond to a few seconds. We document three distinct stages in the nanoparticle adsorption. In addition to an early stage of free diffusion and a later one with steric adsorption barriers, we find a hitherto unrealized region where the interfacial energy changes due to surface “denaturation” or restructuring of the nanoparticles at the interface. We adopt a quantitative model to calculate the diffusion coefficient, adsorption rate and barrier, and extent of nanoparticle hydrophobic core exposure at different stages. Our findings offer new insights for the interfacial behavior of nanoparticles, as well as the application of their controlled release at the interface.

  • Fluids Seminar presents Yong Lin Kong, University Utah

    Location: Barus and Holley Room: 190 Cost: Free
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    Fluids at Brown and Fluids and
    Thermal Sciences Joint Seminar Series

    Tuesday, October 8, 2019
    3:00 PM
    Barus & Holley, Room 190

    Yong Lin Kong , University of Utah

    Title: Multiscale additive manufacturing of fractional electonics and biomedical devices

    Abstract : My research group focuses on the development of 3D printing technologies to create multifunctional structures and devices that cannot be fabricated with conventional fabrication methods. We seek to advance the scientific understanding of the assembly and processing of functional nanomaterials to functionalize a wide range of constructs. We develop a multi-scale, multi-material additive manufacturing approach that is fundamentally free from the constraint of the conventional two-dimensional, top-down fabrication methodologies to achieve seamless integration of diverse classes of materials. The freeform fabrication approach could overcome the geometrical, mechanical and material dichotomies between conventional manufacturing technologies and a broad range of three-dimensional systems. As an example, I will first highlight the development of 3D printed quantum-dots light-emitting diode, which extended the reach of 3D printing and demonstrated that active electronic materials and devices can be entirely 3D printed. In the second part of the talk, I will highlight the latest development of a 3D printed gastric resident electronics system, which leverages the significant space and immune-tolerant environment available within the gastrointestinal tract to circumvent the potential complications associated with surgically placed medical implants. Ultimately, we strive to address unmet clinical needs by creating tailorable three-dimensional free-form biomedical devices with 3D printing technologies.

  • Fluids at Brown and Fluids and
    Thermal Sciences Joint Seminar Series

    Wednesday, April 24, 2019
    4:00 PM
    170 Hope St, Room 108

    Nobuyuki Fujisawa , Niigata University

    Title: Pipeline Break Mechanism Nuclear Power Plant by Flow Accelerated Corrosion

    Abstract : Pipeline break mechanism in Mihama Nuclear Power Plant (NPP) is known as one of the wall thinning accidents in Japan in 2004, which is caused by flow accelerated corrosion (FAC). FAC is a corrosion phenomenon of carbon steel pipeline caused by flow turbulence. The Mihama pipeline consisted of an elbow and orifice, while the flow was highly swirling in the upstream. The water tunnel experiment by the authors showed that the mass transfer coefficient was locally increased behind the orifice to a value several times larger than that of the straight pipe. This phenomenon was found to be caused by the high intensity swirling flow through the elbow, which generated a spiral motion downstream of the elbow and sustained a longer distance than expected from an elbow flow without swirl. This type of non-axisymmetrical flow triggered strongly biased flow at the orifice, and resulted in non-axisymmetric pipe-wall thinning downstream of the orifice leading to pipe-break accident in the NPP.

  • How to make a splash: a multi-scale framework for understanding high speed drop impact

    Radu Cimpeanu, Ph.D.
    University of Oxford

    ABSTRACT:

    The rich structures arising from the impingement dynamics of water drops onto solid substrates at high velocities will be discussed over a range of length- and timescales. Current methodologies in the aircraft industry for estimating water collection are based on particle trajectory calculations and empirical extensions thereof in order to approximate the complex fluid-structure interactions. The presented approach incorporates the detailed fluid dynamical processes often ignored in this setting, such as the drop interaction with the surrounding air flow, drop deformation, rupture and coalescence, as well as the motion of the ejected microdrops in the system. One-to-one comparisons are performed with experimental data available in the pre-impact stage, while the early stages of the impact itself are validated using an extension of the asymptotic analysis machinery provided by Wagner theory. The main body of results is created using parameters relevant to flight conditions with droplet sizes in the range of tens to several hundreds of microns impacting at speeds of up to 100 m/s.

  • Turbulence and flow structure in Extended Wind Farms

    Charles Meneveau
    Johns Hopkins University

     ABSTRACT:
    In this presentation we discuss several properties of turbulence and the mean flow structure in the wind turbine array boundary layer (WTABL). This particular type of shear flow develops when the atmospheric boundary layer interacts with an array of large wind turbines. Based on such understanding, we aim to develop reduced order, analytically tractable models. These are important engineering tools for wind energy, both for design and control purposes. We will focus on two fluid mechanical themes relevant to wind farm design and control. The first topic deals with spectral characteristics of the fluctuations in power generated by an array of wind turbines in a wind farm. We show that modeling of the spatio-temporal structure of canonical turbulent boundary layers coupled with variants of the Kraichnan’s random sweeping hypothesis can be used to develop analytical predictions of the frequency spectrum of power fluctuations of wind farms. In the second part we describe a simple (deterministic) dynamic wake model, its use for wind farm control, and its extension to the case of yawed wind turbines. The work to be presented arose from collaborations with Juliaan Bossuyt, Johan Meyers, Richard Stevens, Tony Martinez, Michael Wilczek, Carl Shapiro and Dennice Gayme. We are grateful for National Science Foundation support.

  • Tyler Van Buren, Ph.D., Research Scholar at Princeton, will present a talk: “Improving Performance of Unsteady Propulsors through Biological Inspiration”.

    Abstract: Many species of aquatic life have evolved to swim fast and efficiently over long distances. Some of these swimmers, such as dolphin and tuna, use propulsion methods where the principal thrust comes from oscillating a propulsive airfoil-like surface, such as a fluke or caudal fin. Using these biological systems as inspiration, we hope to develop propulsion systems with better maneuverability, efficiency, and speed.
    Here we will explore the physical mechanisms that govern the performance—especially swimming speed and efficiency—of unsteady propulsive techniques inspired by biology. We will see that scaling laws developed from basic principles can be used to model the performance of these types of propulsors. Moreover, we can use this model as a guide to achieve superior function, for example, by modifying the aerodynamic shape of the propulsors. Through a better understanding of aquatic swimmers, we can extend the performance of human-made propulsors beyond the limits of biology.

    Bio: Dr. Van Buren is a Research Scholar in the Mechanical and Aerospace Engineering Department at Princeton University working under Prof. Alexander Smits. He received his Ph.D. from Rensselaer Polytechnic Institute (RPI) under the advisement of Prof. Michael Amitay. Currently, his research interests are in unsteady aerodynamics/hydrodynamics; bio-inspired propulsion; turbulent structure and stability; and flow control devices/strategies. Learn more at www.vanburenlabs.com .

  • Droplets walking in a circular corral: dynamics and statistics 

     Matthew Durey
    Massachusetts Institute of Technology

     ABSTRACT:
    A droplet may ‘walk’ on the surface of a vertically vibrating fluid bath, propelled by the Faraday waves generated from all previous impacts. This hydrodynamic pilot-wave system exhibits many features that were previously thought to be exclusive to the quantum realm, such as tunneling, emergent statistics, and quantized droplet dynamics. In this talk, we present a theoretical investigation into the dynamics of a droplet confined to a circular corral. Starting from first-principles, we derive a discrete-time iterative map for the evolution of the droplet’s position and the interaction of the wave field with the submerged topography. We study the cavity modes for the fluid system and rationalize the effect of the forcing frequency on the nature of the Faraday instability. By analyzing the existence and stability of circular orbits, we elucidate the orbital quantization in the limit of high vibrational forcing. Furthermore, we see the emergence of more exotic periodic orbits, including lemniscates, trefoils and straight-line oscillations. We then explore the chaotic ‘intermittent-regime’, in which the system continually switches between these unstable eigenstates, yielding the emergence of wavelike statistics. Finally, we relate the droplet’s statistical distribution to the mean Faraday wave field. 

  • Active Matter Invasion of a Viscous Fluid and a No-Flow Theorem

    Saverio Spagnolie,  Associate Professor
    University Of Wisconsin-Madison

     Abstract:

    Suspensions of active particles in fluids exhibit incredibly rich behavior, from organization on length scales much longer than the individual particle size to mixing flows and negative viscosities. We will discuss the dynamics of hydrodynamically interacting motile and non-motile stress-generating swimmers or particles as they invade a surrounding viscous fluid, modeled by coupled equations for particle motions and viscous fluid flow. Depending on the nature of their self-propulsion, colonies of swimmers can either exhibit a dramatic splay, or instead a cascade of transverse concentration instabilities as the group moves into the bulk. A stability analysis of concentrated distributions of particles matches the results of our full numerical simulations, and provides some exciting connections to classical hydrodynamic instabilities in seemingly unrelated inertial flows. Along the way we will prove a very surprising “no-flow theorem”: particle distributions initially isotropic in orientation lose isotropy immediately but in such a way that results in no fluid flow *anywhere* and *at any time*.

     

    Bio:
    Saverio Spagnolie is an associate professor in mathematics at the University of Wisconsin-Madison, with a courtesy appointment in chemical and biological engineering. Using classical methods of applied mathematics and the development of novel numerical methods, he studies problems in biological propulsion, soft matter, sedimentation, and complex fluids. Before arriving in Madison, Saverio received a Ph.D. in mathematics at the Courant Institute of Mathematical Sciences, then held postdoctoral positions in the Mechanical/Aerospace Engineering department at UCSD and in the School of Engineering at Brown University.

  • Dynamics of buoyant particles and air bubbles in turbulent flows

    Varghese Mathai, Postdoctoral researcher, Brown University


    Abstract:

    Particle suspensions in turbulent flows occur widely in nature and industry. In most situations, the particles have a density that is different from the carrier fluid density, which can affect their motion in multiphase flow settings. In this talk, I will discuss the use of Lagrangian particle-tracking experiments to study the dynamics of light (buoyant) particles in turbulent flows. In the first part, we examine the applicability of small Stokes number bubbles as tracers of turbulent acceleration. We reveal how gravity can cause the accelerations of even tiny bubbles to deviate from that of the fluid. In the second part, we examine the role of gravity on buoyant spherical particles of finite size (particle size large compared to the dissipative scales of turbulence). For spheres, buoyancy produces interesting variability in 3D translational dynamics. In addition, we reveal the role of a largely ignored control parameter: the particle’s moment of inertia. Using experiments and direct numerical simulations, we demonstrate that the moment of inertia can be tuned to trigger distinctly different wake-induced motions for isotropic bodies including spheres and two-dimensional cylinders. These help draw analogies to some of the motions previously observed for anisotropic objects such as falling cards and paper.

    Bio:

    Varghese Mathai obtained a PhD in 2017 from University of Twente, the Netherlands. His research interests are in high Reynolds number dispersed two-phase and particle-laden flows, and membrane aerodynamics. He was recipient of the 2017 Da Vinci award (Europe) for top-5 PhD theses in fluid mechanics, and the 2018 European COST Prize for best research in flowing matter. At Brown he works with Kenny Breuer, on bio-inspired membrane flows and energy harvesting.

     

     

  • CANCELED: Fluids at Brown presents Christin Murphy, Naval Undersea Warfare Center

    Location: Barus and Holley Room: 190 Cost: Free
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    Whiskers as Hydrodynamic Sensors: Structures, Signals, and Sensitivity

     Dr. Christin Murphy
     Naval Undersea Warfare Center

    ABSTRACT:

    Seals have the most highly specialized vibrissae (whiskers) of all mammals. They can utilize their vibrissal system to detect and track underwater hydrodynamic disturbances generated by swimming prey with fine discrimination of features such as size, shape, and movement direction. Seals’ hydrodynamic detection abilities are unparalleled among animal systems and surpass that of any current sensor technology. As the vibrissae move through the water, self-excited vibrations are induced, and the features of these vibrations encode information about the disturbance source. This seminar will discuss research done to characterize the seal’s vibrissal system using laser vibrometry and high-speed videography to examine the fluid interactions of the vibrissae, and CT scanning to investigate their unique morphology. These efforts aim to improve the understanding of this sophisticated biological system and advance bio-inspired sensor research.

     

    Bio:

    Dr. Christin Murphy is a research biologist at the Naval Undersea Warfare Center in Newport, RI. Her research investigates the hydrodynamic detection abilities of marine mammals. Dr. Murphy received her Ph.D. in biological oceanography from University of South Florida’s College of Marine Science. She is a Fulbright scholar and a National Science Foundation Graduate Research fellow, and received the Navy’s Top Scientists of the year Emergent Investigator award in 2017.  Dr. Murphy applies her background in marine biology, neuroscience, and sensory biology to her work at the Navy to investigate sensory biology of marine organisms and advance bio-inspired sensor research.

  • Transition from turbulent to coherent flows in confined three-dimensional active fluids

    Kun-Ta Wu, Ph.D.,

    Worcester Polytechnic Institute

    ABSTRACT:  Transport of fluid through a pipe is essential for the operation of macroscale machines and microfluidic devices. Conventional fluids only flow in response to external pressure. We demonstrate that an active isotropic fluid, composed of microtubules and molecular motors, autonomously flows through meter-long three- dimensional channels. We establish control over the magnitude, velocity profile, and direction of the self-organized flows and correlate these to the structure of the extensile microtubule bundles. The inherently three-dimensional transition from bulk-turbulent to confined-coherent flows occurs concomitantly with a transition in the bundle orientational order near the surface and is controlled by a scale- invariant criterion related to the channel profile. The nonequilibrium transition of confined isotropic active fluids can be used to engineer self-organized soft machines.

  • Condensate, fluctuations and symmetries — a tale of 2D turbulence

    Anna Frishman, Postdoctoral Fellow

    Princeton Center for Theoretical Science (PCTS)

     

    ABSTRACT:  Earths jet streams, Jupiters Great Red Spot and its zonal winds are all examples of persistent large scale flows, whose dynamics is to a good approximation two-dimensional.  These flows are also highly turbulent, and the interaction between the turbulence and these coherent structures remains poorly understood.  Apart from its geophysical relevance, 2D turbulence is a rich and beautiful fundamental system — where turbulence takes a counter-intuitive role. Indeed, in 2D, energy is transferred to progressively larger scales, which can terminate in the self-organization of the turbulence into a large scale coherent structure, a so called condensate, on top of small scale fluctuations. I will describe a recent theoretical framework in which the profile of this coherent mean flow can be obtained, along with the mean momentum flux of the fluctuations. I will explain how and when the relation between the two can be deduced from dimensional analysis and symmetry considerations, and how it can be derived. Finally, I will show that, to leading order, the velocity two-point correlation function solves a scale invariant advection equation. The solution determines the average energy of the fluctuations, but does not contribute at this order to the momentum flux, due to parity + time reversal symmetry.  Using analytic expressions for the solutions, matched to data from extensive numerical simulations, it is then possible to determine the main characteristics of the average energy. This is the first-ever self-consistent theory of turbulence-flow interaction. 

    Bio:  Anna Frishman did her PhD at the Weizmann Institute of Science with Prof. Grisha Falkovich, working on Lagrangian aspects of turbulence. She is currently a postdoctoral fellow at the Princeton Center for Theoretical Science (PCTS). Her research interests include out-of-equilbrium statistical physics, fluid dynamics, and, at the interface, turbulence. The subjects of her recent projects include 2D turbulence, stochastic force inference for over-damped systems, and bubble breakup dynamics.  

  • How Elastic Flow Instabilities Can Induce Motion in Flexible Solid Structures

     

    Jonathan P. Rothstein

    Department Mechanical and Industrial Engineering

    University of Massachusetts Amherst

     

    ABSTRACT:

     

    When a flexible object such as an elastic sheet or cylinder is placed in a flow of a Newtonian fluid, the shedding of separated vortices at high Reynolds number can drive the motion of the structure. This phenomenon is known as Vortex-Induced Vibration (VIV) and has been studied extensively for Newtonian fluids. If the same flexible object is placed in non-Newtonian flows, however, the structure’s response is still unknown. Unlike Newtonian fluids, the flow of viscoelastic fluids can become unstable at infinitesimal Reynolds numbers due to a purely elastic flow instability. In this talk, I will investigate the fluid structure interaction between a wormlike micelle solution at high Weissenberg number and a flexible elastic sheet and flexible circular cylinder in cross flow.  Elastic flow instabilities have been observed for wormlike micelle solutions in a number of flows including flow into a contraction and flow past a circular cylinder.  Here we will present a detailed study of the unstable flow past a cylinder for a series of wormlike micelle solutions whose rheology we have fully characterized. Next we will show that a similar elastic flow instabilities can occur in the vicinity of a thin flexible polymer sheet.  We will show that the time varying fluid forces exerted on the flexible sheet can grow large enough to cause a structural motion which can in turn feed back into the flow to modify the flow instability.  We will show the same interactions can occur for flexible and flexibly mounted circular cylinders.  The static and time varying displacement of the flexible sheets and cylinders, including their oscillation frequency and amplitude, will be presented for varying geometries, for varying fluid flow rates, and for varying fluid compositions and properties.  In addition, measurements of flow induced birefringence will be presented in order to quantify the time variation of the flow field and the state of stress in the fluid.

     

    Bio - Jonathan Rothstein is a Professor in the Mechanical and Industrial Engineering Department at the University of Massachusetts Amherst where he has been since 2001.  He received his B.Eng. from The Cooper Union in 1996, his M.S. from Harvard University in 1998 and his Ph.D. from MIT in 2001.  His research interests include experimental fluid mechanics, micro fluidics, multiphase flows, non-Newtonian flows, rheology, drag reduction, superhydrophobicity and fabrication of micro- and nano-patterned materials.  He has won a number of prestigious awards including an NSF CAREER Award, an ONR Young Investigator Award and the Arthur B. Metzner Early Career Award from the Society of Rheology.

  • Local and global perspectives on time series analysis of observed climate change

    Sandra Chapman, Physics Dept., University of Warwick, UK. CSP, Dept. of Astronomy, Boston University, UK

    ABSTRACT:
    Estimates of how our climate is changing are needed both locally and globally. For local adaptation this requires quantifying the geographical patterns in changes at specific quantiles or thresholds in distributions of variables such as daily surface temperature and precipitation. This talk will first focus on these local changes and on a model independent method [1] to transform daily observations into patterns of local climate change. This method estimates how fast different quantiles across the distributions are changing. This determines which quantiles and geographical locations show the greatest change and also those at which any change is highly uncertain. For daily temperature changes in the distribution itself can yield robust results [2]. For fatter-tailed distributions such as precipitation we can focus on quantities that characterize the changes in time of the likelihood above a threshold and of the relative amount of precipitation in those days [3]. The fundamental timescales of anthropogenic climate change limit the identification of societally relevant aspects of changes but nevertheless it is possible to extract, solely from observations, some confident quantified assessments of change at certain thresholds and locations. We demonstrate this approach using E-OBS gridded data timeseries [4] from specific locations across Europe over the last 60 years.

  • Scalar excursions in large-eddy simulations
    Georgios Matheou
    Department of Mechanical Engineering, University of Connecticut
    ABSTRACT:
    The range of values of scalar fields in turbulent flows is bounded by their boundary values, for passive scalars, and by a combination of boundary values, reaction rates, phase changes, etc., for active scalars. In practice, as a result of numerical artifacts, this fundamental constraint is often violated with scalars exhibiting unphysical excursions. Passive-scalar excursions, a form of numerical model error, are studied in large-eddy simulations (LES) of a shear flow, and methods for diagnosis and assessment of the problem are examined. The analysis of scalar-excursion statistics provides support of the central hypothesis of the study that unphysical scalar excursions in LES result from dispersive errors of the convection-term discretization where the subgrid-scale model (SGS) provides insufficient dissipation to produce a sufficiently smooth scalar field. In the LES runs three parameters are varied: the discretization of the convection terms, the SGS model, and grid resolution. Two types of excursion diagnostics are studied: global excursions, which violate the boundary values of the scalar transport equation; and local excursions, which violate local scalar bounds. Global excursions are analyzed by considering the minimum and maximum in the entire computational domain and the volume of fluid with scalar values exceeding an excursion threshold. Local excursions are primarily used to obtain unphysical scalar-excursion information in the mixed fluid. As expected, unphysical scalar-excursion statistics strongly depend on the SGS model and model parameters. The excursions are significantly reduced when the characteristic SGS scale is set to double the grid spacing in runs with the stretched-vortex model, following similar trends to momentum-transport model error.

  • Experimental Reaseaches on Interfacial Flow : 1. Double emulsion droplet under high electric field; 2. Tree-inspired pump and actuator
    Jinkee Lee, PhD
    School of Mechanical Engineering, Sungkyunkwan University,
    Suwon, Gyeonggi-do 16419, South Korea
    ABSTRACT:
    In this presentation, I want to show the research results focus on (1) emulsion under high electric field and (2) development of tree mimicking mechanical devices. (1) We investigate numerically, theoretically, and experimentally how EHD deformation and breakup of double emulsion droplet can occur under DC electric fields. Based on comprehensive experiments, we observe four different breakup modes for double emulsion droplet depending on various physical constraints, i.e. viscosity, conductivity, permittivity, and volume fraction between the core and shell fluid. The breakup modes are classified such as a unidirectional breakup mode, two different bidirectional breakup modes, and tip-streaming continuous breakup mode. We obtained phase diagram to depict the different breakup modes which can contribute to control the emulsion droplet shape. Furthermore, we employed a theoretical study to predict the core droplet migration inside the shell. (2) An artificial leaf mimicking structure using hydrogel, which has a nanoporous structure is fabricated. The cryogel method is used to develop a hierarchy structure on the nano- and microscale in the hydrogel media that is similar to the mesophyll cells and veins of a leaf, respectively. The suction pressure of the artificial leaf is affected by several variables (e.g., pore size, wettability of the structure, nano particle modification). Finally, by decreasing the pore size and increasing the wettability, the maximum negative pressure of the artificial leaf, 7.9 kPa is obtained. Also, We have developed a hygromorphic metallic oxide monolayer film capable of actuation by electrochemically producing superhydrophilic free-standing nano-capillary forest of titanium oxide with an anatase crystal structure of high aspect ratio (~80) nano capillaries. This hygromorphic metallic monolayer is activated by the generation of forces from the spreading and capillary-driven imbibing into nano gaps during hydration and evaporation. This system possesses a great stability and repeatability for long time usage and has a high bending energy density of ~1250 kJ/m3. The results suggest that these hygromorphic structures could possibly exhibit high energy densities and therefore potentially play important roles for external stimuli-responsive materials that are efficient energy converters and actuators.
    Biography:
    Prof. Jinkee Lee received B.S. and M.S. degrees in Mechanical Engineering from Korea Advanced Institute of Science and Technology (KAIST), Korea in 1997 and 1999, respectively, and Ph.D. degree from Brown University in 2008, where he held the prestigious Simon Ostrach Fellowship. Following his graduate studies, he was a Postdoctoral Research Fellow at jointly in School of Engineering and Applied Science and Department of Organismic and Evolutionary Biology in Harvard University from 2008 to 2009, then moved back to Brown University as an Assistant Professor (Research) in School of Engineering from 2009 to 2011. In 2012, he joined Sungkyunkwan University (SKKU), where he is currently Associate Professor and Director of Multiscale Fluid Mechanics Laboratory in School of Mechanical Engineering.
    His research interests is the Interfacial Flow & Transport Phenomena and their Applications falls under the area of Mechanical Engineering, Chemical Engineering, Material Science, Physics and Micro-/Nano-Technologies. He has published 48 peer-reviewed journal articles. He was a recipient of the SKKU Teaching Award 2016 awarded by President of SKKU, which is chosen by evaluating level of contribution, innovation for education and passions for teaching.

  • New laser-imaging technology elucidates form, function, and ecological impact of deep sea, giant larvacean mucus houses
    Dr. Kakani Katija
    Research and Development, Monterey Bay Aquarium Research Institute,
    Moss Landing, CA, USA
    ABSTRACT:
    The midwater region of the ocean (below the euphotic zone and above the benthos) is one of the largest ecosystems on our planet, yet remains one of the least explored. Little-known marine organisms that inhabit midwater have developed life strategies that contribute to their evolutionary success, and may inspire engineering solutions for societally relevant challenges. Although significant advances in underwater vehicle technologies have improved access to midwater, small-scale, in situ fluid mechanics measurement methods that seek to quantify the interactions that midwater organisms have with their physical environment are lacking. Here we present DeepPIV, an instrumentation package affixed to a remotely operated vehicle that quantifies fluid motions from the surface of the ocean down to 4000 m depths. Utilizing ambient suspended particulate, fluid-structure interactions are evaluated on a range of marine organisms in midwater (and the benthos). Initial science targets include larvaceans, biological equivalents of flapping flexible foils that create mucus houses to filter food. Little is known about the structure of these mucus houses and the function they play in selectively filtering particles, and these dynamics can serve as particle-mucus models for human health. Using DeepPIV, we reveal the complex structures and flows generated within larvacean mucus houses, and elucidate how these structures function.

  • A touch of non-linearity at intermediate Reynolds numbers: where spheres “think” collectively and swim together.
    Daphne Klotsa
    University of North Carolina at Chapel Hill
    ABSTRACT:
    From crawling cells to orca whales, swimming in nature occurs at different scales. The study of swimming across length scales can shed light onto the biological functions of natural swimmers or inspire the design of artificial swimmers with applications ranging from targeted drug delivery to deep-water explorations. In this talk, I will present experiments and simulations of how oscillating spheres, universally simple geometric objects, can utilize non-linearities to demonstrate complex pattern formation in a granular system, or different swimming behaviors in a spherobot (robot made out of spheres) when placed in a fluid at intermediate Reynolds numbers. I will talk about how a simple swimmer transitions from rest to motility and then switches direction as a function of the Reynolds number.

  • Locomotion in Generalized Newtonian Fluids: Living Organisms & Active Colloids
    Dr. David Gagnon
    Georgetown University
    ABSTRACT:

    Biofilm formation, mammalian reproduction, and infection typically occur in environments where surrounding fluids comprise suspensions of polymers. These polymeric suspensions possess non-Newtonian rheological properties, such as rate-dependent viscosity and viscoelasticity, and present numerous experimental and modeling challenges. Using well-studied polymeric fluids, we aim to systematically investigate the effects of generalized Newtonian (rate-dependent) fluids on locomotion.
    In this talk, I will discuss the effect of rate-dependent viscosity on (i) the swimming behavior of the nematode Caenorhabditis elegans and (ii) the rheology of active kinesin-driven microtubule suspensions. First, we investigate the swimming behavior of the low Reynolds number swimmer C. elegans using tracking methods and flow velocity measurements. With knowledge of the local flow behavior, we then address the important question of whether rate-dependent viscosity modifies the nematode’s cost of swimming. We find the cost of swimming in shear-thinning fluids is less than or equal to the cost of swimming in Newtonian fluids of the same zero-shear viscosity; furthermore, the cost of swimming in shear-thinning fluids scales with a fluid’s effective or average viscosity and can be predicted using rheological properties and simple swimming kinematics.
    Second, we explore the rheology and dynamics of an active suspension of microtubules and kinesin motors in a dilute polymeric suspension using a confocal rheometer, which provides both rheological measurements and fluorescent imaging of microscale dynamics. We find the activity of microtubules enhances both the zero-shear viscosity and the shear-thinning behavior of the suspension. Using velocimetry techniques, we examine local mesoscale flow dynamics for insight into the underlying mechanisms responsible for this macroscale rheological behavior.

  • Hydrodynamic interactions in non Newtonian liquids
    Roberto Zenit
    Universidad Nacional Autonoma de Mexico
    ABSTRACT:
    The understanding of hydrodynamic forces around particles, drops, or bubbles moving in Newtonian liquids is modestly mature. It is possible to obtain predictions of the attractive–repulsive interaction for moving ensembles of dispersed particulate objects. There is a certain intuition of what the effects of viscous, inertial, and surface tension forces should be. When the liquid is non-Newtonian, this intuition is gone. In this talk, we summarize recent efforts at gaining fundamental understanding of hydrodynamic interactions in non-Newtonian liquids. Due to the complexity of the problem, most investigations rely on experimental observations. However, computations of non-Newtonian fluid flow have made increasingly significant contributions to our understanding of particle, drop, and bubble interactions. We address the case of gravity-driven flows: rise or sedimentation of single spheroidal objects, pairs, and dispersions. We identify the effects of two main rheological attributes—viscoelasticity and shear-dependent viscosity—on the interaction and potential aggregation of particles, drops, and bubbles. We end by highlighting the open questions in the subject and by suggesting possible future directions.
    Biography: Roberto Zenit received his Ph.D. from the Mechanical Engineering Department at Caltech in 1998. After a postdoctoral period at Cornell University, he moved to Mexico City in 2000 to become a faculty member at the Universidad Nacional Autónoma de México (UNAM). He has been there ever since. He is now a Full Professor of Mechanical Engineering and a researcher at the Instituto de Investigaciones en Materiales, both at UNAM. His area of expertise is fluid mechanics; he has worked in a wide variety of subjects including multiphase and granular flows, biological flows, rheology, and more recently, the fluid mechanics of art history.

  • Change in stripes for cholesteric shells via anchoring in moderation

    Lisa Tran

    University of Pennsylvania.

    Chirality, ubiquitous in complex biological systems, can be controlled and quantified in synthetic materials such as cholesteric liquid crystal (CLC) systems. In this talk, I will present my recent study of spherical shells of CLC under weak anchoring conditions. Anchoring transitions are induced at the inner and outer boundaries by changing the surfactant concentration in the surrounding water phase. The shell confinement leads to new states and associated surface structures: a state where large stripes on the shell can be filled with smaller, perpendicular substripes, and a focal conic domain (FCD) state, where thin stripes wrap into at least two, topologically required, double spirals. Focusing on the latter state, we use a Landau–de Gennes model of the CLC to simulate its detailed configurations as a function of anchoring strength. By abruptly changing the topological constraints on the shell, we can study the interconversion between director defects and pitch defects, a phenomenon usually restricted by the complexity of the cholesteric phase. I will then touch upon preliminary work where the water-cholesteric interface is used as a self-assembly blueprint for surface active nanoparticles.