Where: ERF 1207

Speaker: Bill Fagan (University of Maryland College Park) - http://www.clfs.umd.edu/biology/faganlab/

Where: ERF 1207

Speaker: Naeem Masnadi (Department of Mechanical Engineering) -

Abstract: Nonlinear solitary waves are known to bifurcate from linear sinusoidal waves in deep water at the minimum phase speed of linear gravity-capillary waves (c_min=23 cm/s). This minimum happens at a finite wavenumber and the solitary waves can be viewed as modulated wavepackets with the wave envelope moving at the same speed as the wave crests. The minimum phase speed is also associated with a resonant condition; the linear response to a surface pressure distribution moving at this speed becomes unbounded. The nonlinear response offers rich and complicated dynamics and several solution regimes are found due to a delicate interplay between the effects of nonlinearity, dispersion and viscous dissipation. I will first review some theoretical background and modeling approaches to the problem of the response of a water surface to a pressure source moving at speeds close to c_min and then present our recent experimental and numerical findings.

Where: ERF 1207

Speaker: Oren Raz (University of Maryland College Park) -

Abstract: Under certain conditions, it takes less time to cool a hot system than to cool the same system initiated at a lower temperature. This phenomenon – the “Mpemba Effect”, has been observed in a variety of systems, including water, magnetic alloys and carbon nano-tube resonators. So far, no single generic mechanism to explain this counter-intuitive behavior was suggested. Using the theoretical framework of nonequilibrium thermodynamics, we construct a minimal model that describes this behavior and illustrates the fundamental principles behind it. We derive a sufficient condition for this effect in Markovian systems, and predict an inverse effect: it might take less time to heat a cold system than a warmer one.

Where: ERF 1207

Speaker: Tyrus Berry (George Mason University) - http://math.gmu.edu/~berry/

Where: ERF 1207

Speaker: Ira Schwartz (Naval Research Laboratory) - https://physics.aps.org/authors/ira_b_schwartz

Abstract: Noise in various forms is known to cause switching between states, create new meta-stable states, and form global dynamical structures. In this talk, I will review some previous work on the effects of noise in static and adaptive networks, and show how to extend the theory to heterogeneous networks with a specified degree distribution. Applications will be to epidemic spread and novel optimal network control in large populations. Specifically, we have developed new mathematical and computational techniques demonstrating that both highly connected individuals and those with a few connections should be targeted in specific proportions, using vaccination or treatment, in order to minimize mean extinction times. The optimal approach gives orders of magnitude improvement over known strategies.

Where: ERF 1207

Speaker: Balakumar Balachandran (University of Maryland College Park) - http://www.enme.umd.edu/faculty/chair

Where: ERF 1207

Speaker: Joe Hart and Rajarshi Roy (University of Maryland College Park) - http://ireap.umd.edu/faculty/roy

Where: ERF 1207

Speaker: Sean Downey (University of Maryland College Park) - https://anth.umd.edu/facultyprofile/Downey/Sean

Where: ERF 1207

Speaker: Dan Gauthier (Ohio State University) - https://physics.osu.edu/people/gauthier.51

Where: ERF 1207

Speaker: Howard Milchberg (University of Maryland College Park) - http://www.ece.umd.edu/faculty/milchberg

Abstract:

When an optical pulse propagating through a nonlinear medium exceeds a certain threshold power, it can focus itself and collapse, in theory, to a singularity. In practice, several physical mechanisms mitigate or arrest the catastrophic collapse and the pulse continues propagation as a filamentary structure. This scenario has played out in many nonlinear optics systems over decades: among them are air filamentation, relativistic self-focusing in plasmas, nonlinear generation of broadband light, laser-material processing, and unintentional (and expensive) laser damage. Recently, we showed that self-focusing collapse and collapse arrest is universally accompanied by the generation of robust topological structures: spatio-temporal optical vortices (STOVs). In contrast to the conventional orbital angular momentum vortices in, for example, Laguerre-Gaussian beams, which can be described purely by spatial coordinates, STOVs have electromagnetic phase circulation in a spatio-temporal plane that propagates with the pulse and directs the global pulse energy flow. I will describe our experiments, simulations and calculations leading to our discovery of STOVs , and discuss future possible applications.

Where: ERF 1207

Speaker: William Dorland (University of Maryland College Park) - https://umdphysics.umd.edu/people/faculty/current/item/124-bdorland.html

Abstract: The Liouville equation describing a collection of charged particles is time-reversible. In the weakly coupled limit, one can reduce this equation to a Fokker-Planck equation, which is irreversible. The problem of the fate of electromagnetic field fluctuations in a plasma in the limit of very weak irreversibility was addressed by Landau, who demonstrated that as long as there are some collisions (even if rare), and in the absence of sources, gradients, etc, typical field fluctuations are damped with an easily calculated "collisionless" damping rate -- this is Landau damping. The energy of the field fluctuations is converted to particle energy; there is irreversible heating. Landau's calculation is fine in the limit of small amplitude fluctuations, but what happens when the plasma is turbulent? I will show that in a typical nonlinear system (relevant to many physical observations), Landau damping is overwhelmed and ultimately arrested by turbulent "echoes". This finding has important implications for detailed predictions of the heating (and in some cases, for the luminosity) of some interesting astrophysical plasmas.

Where: ERF 1207

Speaker: James Yorke (University of Maryland College Park) - http://www.chaos.umd.edu/~yorke/

Where: ERF 1207

Speaker: Dmitry Doglopyat (University of Maryland College Park) - http://www.math.umd.edu/~dolgop/

Where: ERF 1207

Speaker: David Levermore (Department of Mathematics - University of Maryland) - http://www.math.umd.edu/~lvrmr/

Where: ERF 1207

Speaker: Isaac Mayorgoyz (Electrical and Computer Engineering - University of Maryland) - http://www.ece.umd.edu/faculty/mayergoyz

Where: ERF 1207

Speaker: Jeff Urbach (Department of Physics - Georgetown University) - https://physics.georgetown.edu/users/jeff-urbach

Where: ERF 1207

Speaker: Dan Lathrop (Department of Physics - University of Maryland) - http://complex.umd.edu

Helicity is a conserved quantity that arises in ideal fluid flows and ideal magnetohydrodynamic magnetic fields. I will first review the background theory of Helicity in those two cases, a famous paper by Finn and Antonsen, and another by Keith Moffatt. I will follow by covering some basic phenomenology of quantized vortices, reconnection, and Kelvin waves, and background of our visualization studies in superfluid helium. These topics lead into a discussion of what has been done, what we know, and what is predicted about Helicity dynamics. Some observations about the untangling of vortices via reconnection lead to predictions regarding the Helicity we are exploring experimentally. Some puzzles and questions about the role of invariants like the Helicity in the Gross-Pitaevskii (nonlinear Schrodinger) equation play a role in thinking about this phenomenon.

Where: ERF 1207

Speaker: Brian Hunt and Zhixin Lu (Math, UMCP and IREAP, UMCP\) -

We consider the problem of predicting a chaotic time series from a

system whose equations of motion are unknown. We use a

machine-learning technique called reservoir computing, which we find

is often able to learn the dynamics of the system that generated the

time series, in the following sense. In addition to making accurate

short-term predictions, the reservoir predictor can generate a

long-term "climate" forecast that stays close to the attractor of the

actual system. We give examples, and we discuss a preliminary theory

relating reservoir predictor performance to Lyapunov exponents and

generalized synchronization in an associated dynamical system.

Where: ERF 1207

Speaker: Elaine Oran (Mechanical Engineering, UMCP) -

As we were investigating the efficiency of fire-whirl burning on water, we observed the usual transformation of a pool fire to a fire whirl, and then suddenly, we saw the fire undergo a third transition. A blue cup appeared around the base of the fire whirl, surrounding the yellow flame, the yellow flame receded into the cup and finally disappeared. What remained was a small, rapidly spinning blue flame that burned until the fuel on the water was consumed. The blue whirl was shaped like a spinning cup, closed at the bottom near the water surface, and spreading in radius moving upwards towards the rim. Above the blue cup lip, there was a purple cone-shaped mist. The fuel initially used was n-heptane, but now it has been varied and includes crude oil, and still the blue whirl formed naturally. The height of the fire whirl on the laboratory pan was larger than a half meter, and this evolved into a blue whirl about 4–8 cm high. Occasionally the blue whirl would become “unstable” and revert to a transitional state of blue cup holding a yellow flame. When the blue whirl formed, turbulence seemed to disappear, and the flame became quiet. Videos of the experiments are used to show how this happened and discuss the evolution of the fire whirl to the blue whirl in vortex-breakdown concepts.

Where: ERF 1207

Speaker: Tom Antonsen (Department of Physics - University of Maryland) - http://www.umerc.umd.edu/faculty/antonsen

Abstract:

Physicists and engineers frequently encounter situations where calculations of the governing equations of a system of interest appear to need to be repeated many times to describe or optimize the system. It is often the case that only a particular state dependent quantity or metric needs to be determined. In this case a computational savings can be achieved if an “adjoint problem” can be found that produces the desired information without requiring multiple computations. A simple example is the design of a receiving antenna. One wishes to know and optimize the signal received as a function of the incident angle and polarization of incoming waves. It might appear that solution of Maxwell’s field equations would have to be repeated for each possible incident direction and polarization. However, due to the reciprocal property of the governing equations, the desired information is obtained by treating the antenna as a transmitter and calculating the far field radiation pattern. Thus, one computation replaces many. In this talk I will review some problems from the area of charged particle dynamics where adjoint methods have proven useful. A new example is the optimization of electron beam optics in beam sources used in microwave and millimeter wave amplifiers.

Where: ERF 1207

Speaker: Daniel Butts (Department of Biology - University of Maryland) - http://neurotheory.umd.edu/people/dbutts

A fundamental goal in brain research is to understand how electrical activity of individual neurons represents information relevant for brain function. This is most often studied in sensory systems, where neural activity can be directly related to sensory stimuli that can be experimentally controlled. However, recordings in awake animals can reveal an enormous amount to variability — that is, different responses to the same stimuli. Such variability has traditionally been characterized as noise that imposes limits on sensory processing. However, with experimental technology allowing for access to large amounts of simultaneously recorded neurons, it is becoming clear that this noise is shared and purposeful, and likely relates to a larger view of the function of sensory cortex. My lab has been developing new methods for analyzing population activity (and its dynamics) to infer what information is being represented by this variability, and how it relates to the larger functions of sensory cortex. This points to a picture where sensory processing does not occur in a vacuum, but is implicitly tied to the behavioral and motivational context of the animal.

Where: ERF 1207

Speaker: Edward Ott (University of Maryland | Department of Electrical Engineering and Department of Physics) - https://www.ece.umd.edu/faculty/ott

Ed Ott will be the recipient of the 2017 Lewis Fry Richardson Medal from The European Geosciences Union (E.G.U.). In connection with this award, he will give a lecture at the annual E.G.U. Assembly in Vienna, Austria, later this month. This Applied Dynamics Seminar will be a preview of his talk in Vienna.

Abstract: Chaos was discovered at the end of the 19th century by Poincare in his famous work on the motion of N>2 celestial bodies interacting through gravitational attraction. Although steady progress was made by mathematicians following Poincare's work, the widespread impact and development of chaos in the physical sciences is comparatively recent, i.e., approximately starting in the 1970's. This talk will review and comment on this history and will give some examples illustrating the types of questions, problems and results arising from perspectives resulting from the widespread participation of physical scientists in chaos research.

Where: ERF 1207

Speaker: James Yorke (Department of Mathematics - University of Maryland College Park) - http://www.chaos.umd.edu/~yorke/

Abstract: The most frequently studied dynamical systems are low dimensional and all the periodic orbits in a chaotic set have the same number of unstable dimensions, but this property seems to fail in high dimensional systems. In this paper, we define a property called ``multi-chaos'', in which, along with the usual properties of chaos, there is a dense set of k-dimensionally unstable periodic orbits, and this holds for more than one k. We provide examples including a piecewise linear generalized Baker map.

Where: ERF 1207

Speaker: Doug Durian (Department of Physics - University of Pennsylvania) - http://www.physics.upenn.edu/people/standing-faculty/douglas-durian

The gravity-driven flow of grains from a hole in a hopper is an iconic granular phenomenon. It’s different from a fluid in that the rate is constant also in that it can suddenly and unexpectedly clog. How does the the susceptibility to clogging decrease with increasing hole size, and is there a well-defined clogging transition above which the system never clogs? This problem is distinct from jamming due to presence of boundaries and gradients. We show how the fraction F of flow configurations that cause a clog may be deduced from the average mass discharged between clogs. We construct a simple model to account for the observation that F decays exponentially in hole width to the power of dimensionality. Thus the clogging transition is not sharp but rather is defined by observation limits similar to the glass transition. When the system is immersed in water, F barely changes. Therefore, the crucial microscopic variables are the grain positions; grain momenta play only a secondary role in destabilizing weak incipient arches. There is also a surprising effect whereby the discharge causes water to be pumped downwards, faster than the grains.

Where: ERF 1207

Speaker: Miguel Sanjuán ( Physics, Universidad Rey Juan Carlos) -

When we talk about dynamics, we do not only understand the motion of celestial bodies and solid mechanical systems, but any changes with respect to time of one or more variables. From that point of view, we can find dynamics everywhere, in any field of science. Thus, now we have a more general vision, including stock market movements and economic variables, concentration changes in chemical reactions, changes in physiological, biological and medical variables, action potentials of neurons, etc ... providing a more interdisciplinary perspective.

The various interactions between the constituent parts of a physical system and their feedback mechanisms, are a source of nonlinearity and complexity, which added to the sensitivity dependence to initial conditions which is a hallmark of chaotic behavior, constitutes a change of perspective in dynamical systems with important consequences for the understanding of science.

I will give a historical perspective of Nonlinear Dynamics, Chaos Theory and Complex Systems, including some of the different sources that have contributed to the construction of the discipline as we know it today. Among them, the three-body problem in celestial mechanics, turbulence in fluid dynamics, irreversibility and fundamentals of statistical physics and the logistic map and population dynamics in biology. Many schools of mathematics and physics have played an essential role in the historical development of the subject, including the French, Russian, Japanese and American school. The knowledge of this historical perspective allows us to understand the breadth of the discipline itself and the multiple interdisciplinary applications to various fields of science.

Where: ERF 1207

Speaker: Yoav Lahini (Engineering, Harvard University) -

From materials such as polymers and glass to properties of interfaces leading to friction and even earthquakes, many disordered systems exhibit a similar repertoire of far-from-equilibrium behaviors such as non-exponential relaxations, aging and memory effects. Yet, in spite of numerous studies of these recurring motifs, identifying the mechanisms underlying the unusual dynamics of disordered systems remains a challenge. I will describe the observation of slow relaxations, aging and memory effects - hallmarks of glassy dynamics – in two disordered mechanical systems: crumpled thin sheets and elastic foams. In particular, I’ll report the observation of a non-monotonic aging response that can last many hours. I will then describe ongoing experiments that exploit the macroscopic nature of these systems to try and uncover the underlying mechanisms. The experimental results are in good agreement with a theoretical model recently used to describe observations of monotonic aging in several glassy systems. This suggests not only a general mechanism, but also that the non-monotonic behavior we observe may be generic and that a-thermal systems can show genuine glassy behavior.

Where: ERF 1207

Speaker: Sudeshna Sinha (Indian Institute of Science Education and Research) -

We will show how spatio-temporal chaos in networks with strongly chaotic

nodal dynamics can be tamed by dynamically changing links. Specifically,

we will illustrate the results in examples ranging from neuronal networks

to disease spreading models. Further we will show how random links can

prevent blow-ups in coupled nonlinear systems suffering from unbounded

growth.

Where: ERF 1207

Speaker: Tomoo Yokoyama (Mathematics, Kyoto University of Education) -

We introduce tree representations of two dimensional flows. Applying the topological methods to an evolution of an incompressible and viscid flow around an inclined flat plate placed in a uniform flow, we can estimate when the lift-to-drag ratios of the plate are maximal and can determine transient streamline patterns between structurally

stable streamline patterns. Moreover, we state the possibilities of analyzing ocean phenomena and medical phenomena. Finally, we discuss low-dimensional dynamical systems which are theoretical backgrounds of the methods.

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Pizza will not be provided at this seminar