Breaking waves on a dynamic Hele-Shaw beach

We report the formation of quasi-steady beaches and dunes via breaking waves in our tabletop ‘Hele-Shaw’ beach experiment. Breaking waves are generated by a wave maker, and zeolite particles act as sand. The tank is narrow, just over one-particle diameter wide, creating a quasi-2D set-up. Classical breaker types are observed on a time-scale of about a second. Beach formation under breakers occurs on a longer time-scale, and is a matter of minutes for a range of mono-chromatic wave frequencies. Alternating the wave maker motion between two frequencies generally leads to beach formation but occasionally to formation of a stable dune with water on either side. Finally, the Hele-Shaw configuration explored here experimentally lends itself to multi-scale modeling of beach dynamics

Revisiting Hele-Shaw dynamics to better understand beach evolution

Wave action, particularly during storms, drives the evo lution of beaches. Beach evolution by non-linear break ing waves is poorly understood due to its three-dimensional character, the range of scales involved, and our limited understanding of particle-wave interactions. We show how a novel, three-phase extension to the classic “Hele-Shaw” laboratory experiment can be designed that creates beach morphologies with breaking waves in a quasi-two-dimensional setting. Our thin Hele-Shaw cell simplifies the inherent complexity of three-phase dynamics: all dynamics become clearly visible and measurable. We show that beaches can be created in tens of minutes by several types of breaking waves, with about one-second periods. Quasi-steady beach morphologies emerge as function of initial water depth, at-rest bed level and wave-maker frequency. These are classified mathematically and lead to beaches, berms and sand bars

Towards a Visipedia: Combining Computer Vision and Communities of Experts

Motivated by the idea of a Visipedia, where users can search and explore by image, this thesis presents tools and techniques for empowering expert communities through computer vision. The collective aim of this work is to provide a scalable foundation upon which an application like Visipedia can be built. We conduct experiments using two highly motivated communities, the birding community and the naturalist community, and report results and lessons on how to build the necessary components of a Visipedia. First, we conduct experiments analyzing the behavior of state-of-the-art computer vision classifiers on long tailed datasets. We find poor feature sharing between classes, potentially limiting the applicability of these models and emphasizing the ability to intelligently direct data collection resources. Second, we devise online crowdsourcing algorithms to make dataset collection for binary labels, multiclass labels, keypoints, and mulit-instance bounding boxes faster, cheaper, and more accurate. These methods jointly estimate labels, worker skills, and train computer vision models for these tasks. Experiments show that we can achieve significant cost savings compared to traditional data collection techniques, and that we can produce a more accurate dataset compared to traditional data collection techniques. Third, we present two fine-grained datasets, detail how they were constructed, and analyze the test accuracy of state-of-the-art methods. These datasets are then used to create applications that help users identify species in their photographs: Merlin, an app assisting users in identifying birds species, and iNaturalist, an app that assists users in identifying a broad variety of species. Finally, we present work aimed at reducing the computational burden of large scale classification with the goal of creating an application that allows users to classify tens of thousands of species in real time on their mobile device. As a whole, the lessons learned and the techniques presented in this thesis bring us closer to the realization of a Visipedia.</p

Comparison of computational codes for direct numerical simulations of turbulent Rayleigh-B\'enard convection

Computational codes for direct numerical simulations of Rayleigh-B\'enard (RB) convection are compared in terms of computational cost and quality of the solution. As a benchmark case, RB convection at $Ra=10^8$ and $Pr=1$ in a periodic domain, in cubic and cylindrical containers is considered. A dedicated second-order finite-difference code (AFID/RBflow) and a specialized fourth-order finite-volume code (Goldfish) are compared with a general purpose finite-volume approach (OpenFOAM) and a general purpose spectral-element code (Nek5000). Reassuringly, all codes provide predictions of the average heat transfer that converge to the same values. The computational costs, however, are found to differ considerably. The specialized codes AFID/RBflow and Goldfish are found to excel in efficiency, outperforming the general purpose flow solvers Nek5000 and OpenFOAM by an order of magnitude with an error on the Nusselt number $Nu$ below $5\%$. However, we find that $Nu$ alone is not sufficient to assess the quality of the numerical results: in fact, instantaneous snapshots of the temperature field from a near wall region obtained for deliberately under-resolved simulations using Nek5000 clearly indicate inadequate flow resolution even when $Nu$ is converged. Overall, dedicated special purpose codes for RB convection are found to be more efficient than general purpose codes.Comment: 12 pages, 5 figure

First-order Goedel logics

First-order Goedel logics are a family of infinite-valued logics where the sets of truth values V are closed subsets of [0, 1] containing both 0 and 1. Different such sets V in general determine different Goedel logics G_V (sets of those formulas which evaluate to 1 in every interpretation into V). It is shown that G_V is axiomatizable iff V is finite, V is uncountable with 0 isolated in V, or every neighborhood of 0 in V is uncountable. Complete axiomatizations for each of these cases are given. The r.e. prenex, negation-free, and existential fragments of all first-order Goedel logics are also characterized.Comment: 37 page

PS Foams Blown from HFC-134a/HFC-32 Blends: Foams Properties

Peer reviewed: YesNRC publication: Ye

The Lantern Vol. 13, No. 3, June 1945

• Night Shift • To John • The Challenge • My Native Land • Dear to My Heart • Plaint • Peace • Ode to a Soldier • Crossing • Alternative • Mankind\u27s Universal Disease • Chips • No Sensehttps://digitalcommons.ursinus.edu/lantern/1036/thumbnail.jp

Antigenic variation in <i>Trypanosoma brucei</i>: joining the DOTs

African trypanosomes, such as &lt;i&gt;Trypanosoma brucei&lt;/i&gt;, are protistan parasites that cause sleeping sickness. Though first described more than a century ago, trypanosomes remain a blight on the health of the human population and on the economy of sub-Saharan Africa. &lt;i&gt;T. brucei&lt;/i&gt; replicates in the bloodstream of infected mammals and traverses the blood-brain barrier to enter the central nervous system in the late, frequently fatal, stages of the disease. Because of its extracellular lifestyle, &lt;i&gt;T. brucei&lt;/i&gt; is continuously exposed to antibody challenge. To circumvent this, the parasite uses antigenic variation of a surface protein named the variant surface glycoprotein (VSG). Around 107 VSG molecules are expressed on the parasite's cell surface, creating a dense coat that prevents adaptive immunity from detecting or accessing invariant antigens. However, antibodies against the expressed VSG are generated, and periodic switches to an immunologically distinct VSG coat are necessary for parasite survival. Such switches are pre-emptive of the immune response and contribute to the pattern of trypanosome growth seen in an infected host (Figure 1): parasite numbers increase, but then drop as VSG-specific antibodies are raised by the host. Cells that have switched to another VSG coat survive this killing and seed the outgrowth of a subsequent peak of parasites, which is again decimated by anti-VSG immune killing. As a survival strategy, antigenic variation succeeds by prolonging the time that the parasite