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1. A Computational Analysis of Bubble-Structure Interaction in Near-Field Underwater Explosion

   https://doi.org/10.1115/IMECE2021-72854  

   Ma, Wentao ; Ozenkoski, Timothy ; Wang, Kevin ( November 2021 , Proceedings of ASME 2021 International Mechanical Engineering Congress and Exposition)

   Underwater explosion poses a significant threat to the structural integrity of ocean vehicles and platforms. Accurate prediction of the dynamic loads from an explosion and the resulting structural response is crucial to ensuring safety without overconservative design. When the distance between the explosive charge and the structure is relatively small (i.e., near-field explosion), the dynamics of the gaseous explosion product, i.e., the “bubble”, comes into play, rendering a multiphysics problem that features the interaction of the bubble, the surrounding liquid water, and the solid structure. The problem is highly nonlinear, as it involves shock waves, large deformation, yielding, contact, and possibly fracture. This paper investigates the two-way interaction between the cyclic expansion and collapse of an explosion bubble and the deformation of a thin-walled elastoplastic cylindrical shell in its vicinity. Intuitively, when a shock wave impinges on a thin cylindrical shell, the shell would collapse in the direction of shock propagation. However, some recent laboratory experiments have shown that under certain conditions the shell collapsed in a counter-intuitive mode in which the direction of collapse is perpendicular to that of shock propagation. In other words, the nearest point on the structural surface moved towards the explosion charge, despite being impacted by a compressive shock. This paper focuses on replicating this phenomenon through numerical simulation and elucidating the underlying mechanisms. A recently developed computational framework (“FIVER”) coupling a nonlinear finite element structural dynamics solver and a finite volume compressible fluid dynamics solver is used to complete this study. The solver utilizes an embedded boundary method to track the wetted surface of the structure (i.e. the fluid-structure interface), which is capable of handling large structural deformation and topological changes (e.g., fracture). The solver also adopts the level set method for tracking the bubble surface (i.e. the liquid-gas interface). The fluid-structure and liquid-gas interface conditions are enforced by constructing and solving one-dimensional multi-material Riemann problems, which naturally accommodates the propagation of shock waves across the interfaces. In this paper, mesh refinement study is made to examine the sensitivity of the results to various meshing parameters. The results show that the intermediate level of refinement is appropriate in terms of both the accuracy and the computation costs. Next, the deformation history of both the bubble and the structure are presented and analyzed to provide a detailed view of the counter-intuitive collapse mode mentioned above. We show that timewise, the structural collapse spans multiple cycles of bubble oscillation. Additional details about the time-histories of fluid pressure, structure displacement, and bubble size are presented to elucidate this dynamic bubble-structure interaction and the resulting structural failure. 

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2. Applying a Transformative Justice Approach to Encourage the Participation of Black and Latina Girls in Computing

   https://doi.org/10.1145/3451345  

   Erete, Sheena ; Thomas, Karla ; Nacu, Denise ; Dickinson, Jessa ; Thompson, Naomi ; Pinkard, Nichole ( December 2021 , ACM Transactions on Computing Education)

   Global protests and civil unrest in 2020 has renewed the world’s interest in addressing injustice due to structural racism and oppression toward Black and Latinx people in all aspects of society, including computing. In this article, we argue that to address and repair the harm created by institutions, policies, and practices that have systematically excluded Black and Latina girls from computer science, an intersectional, transformative justice approach must be taken. Leveraging testimonial authority, we share our past 8 years of experience designing, implementing, and studying Digital Youth Divas, a programmatic and systemic approach to encouraging middle school Black and Latina girls to participate in STEM. Specifically, we propose three principles to counter structural racism and oppression embedded in society and computing education: computing education must (1) address local histories of injustice by engaging community members; (2) counter negative stereotypes perpetuated in computer science by creating inclusive safe spaces and counter-narratives; and (3) build sustainable, computational capacity in communities. To illustrate each principle, we provide specific examples of the harm created by racist policies and systems and their effect on a specific community. We then describe our attempt to create counter structures and the subsequent outcomes for the girls, their families, and the community. This work contributes a framework for STEM and computing educators to integrate transformative justice as a method of repairing the harm that both society and the field of computing has and continues to cause Black and Latinx communities. We charge policy makers, educators, researchers, and community leaders to examine histories of oppression in their communities and to adopt holistic, transformative approaches that counter structural oppression at the individual and system level. 

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3. Parameters of fast and high-yield InAs/GaAs quantum dot semiconductor scintillator

   https://doi.org/10.1557/s43580-021-00019-y  

   Minns, A. ; Dropiewski, K. ; Yakimov, M. ; Tokranov, V. ; Hedges, M. ; Murat, P. ; Oktyabrsky, S. ( January 2021 , MRS Advances)

   null (Ed.)

   InAs quantum dots (QDs) embedded into a waveguiding GaAs semiconductor matrix may produce scintillation detectors with exceptional speed and yield, making them valuable for nuclear security, medical imaging, and high energy physics applications. In this work, we developed thick (~25um) epitaxial heterostructres with high luminescence efficiency composed of self-assembled nano-engineered InAs QDs grown by molecular beam epitaxy. The bulk GaAs acts as a stopping material for incident particles and as a waveguide when layer-transferred onto a low-index substrate. Waveguiding and self-absorption (<1cm-1) were studied using photoluminescence with scanning laser excitation and modeled with ray optics approximation and geometrical coupling of high-index waveguide to a collection fiber. Scintillating signals from alpha-particles were analyzed with an external photodiode (PD) and an integrated PD which provided an improved optical coupling. The mean charge collected by the integrated PD corresponded to 5×1e4 photoelectrons per 1 MeV of deposited energy, or ~20% of the theoretically achievable light yield. Combined with the previously measured QD scintillation time of 0.3-0.6 ns, this makes the InAs/GaAs QD heterostructures the fastest high yield scintillation material reported. 

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4. Convergence of graph Laplacian with kNN self-tuned kernels

   https://doi.org/10.1093/imaiai/iaab019  

   Cheng, Xiuyuan ; Wu, Hau-Tieng ( September 2021 , Information and Inference: A Journal of the IMA)

   Abstract Kernelized Gram matrix $W$ constructed from data points $\{x_i\}_{i=1}^N$ as $W_{ij}= k_0( \frac{ \| x_i - x_j \|^2} {\sigma ^2} ) $ is widely used in graph-based geometric data analysis and unsupervised learning. An important question is how to choose the kernel bandwidth $\sigma $, and a common practice called self-tuned kernel adaptively sets a $\sigma _i$ at each point $x_i$ by the $k$-nearest neighbor (kNN) distance. When $x_i$s are sampled from a $d$-dimensional manifold embedded in a possibly high-dimensional space, unlike with fixed-bandwidth kernels, theoretical results of graph Laplacian convergence with self-tuned kernels have been incomplete. This paper proves the convergence of graph Laplacian operator $L_N$ to manifold (weighted-)Laplacian for a new family of kNN self-tuned kernels $W^{(\alpha )}_{ij} = k_0( \frac{ \| x_i - x_j \|^2}{ \epsilon \hat{\rho }(x_i) \hat{\rho }(x_j)})/\hat{\rho }(x_i)^\alpha \hat{\rho }(x_j)^\alpha $, where $\hat{\rho }$ is the estimated bandwidth function by kNN and the limiting operator is also parametrized by $\alpha $. When $\alpha = 1$, the limiting operator is the weighted manifold Laplacian $\varDelta _p$. Specifically, we prove the point-wise convergence of $L_N f $ and convergence of the graph Dirichlet form with rates. Our analysis is based on first establishing a $C^0$ consistency for $\hat{\rho }$ which bounds the relative estimation error $|\hat{\rho } - \bar{\rho }|/\bar{\rho }$ uniformly with high probability, where $\bar{\rho } = p^{-1/d}$ and $p$ is the data density function. Our theoretical results reveal the advantage of the self-tuned kernel over the fixed-bandwidth kernel via smaller variance error in low-density regions. In the algorithm, no prior knowledge of $d$ or data density is needed. The theoretical results are supported by numerical experiments on simulated data and hand-written digit image data. 

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5. Edge reconstruction and emergent neutral modes in integer and fractional quantum Hall phases

   https://doi.org/10.1063/10.0010207  

   Khanna, Udit ; Goldstein, Moshe ; Gefen, Yuval ( May 2022 , Low Temperature Physics)

   This paper comprises a review of our recent works on fractional chiral modes that emerge due to edge reconstruction in integer and fractional quantum Hall (QH) phases. The new part added is an analysis of edge reconstruction of the ν = 2/5 phase. QH states are topological phases of matter featuring chiral gapless modes at the edge. These edge modes may propagate downstream or upstream and may support either charge or charge-neutral excitations. From topological considerations, particle-like QH states are expected to support only downstream charge modes. However the interplay between the electronic repulsion and the boundary confining potential may drive certain quantum phase transitions (called reconstructions) at the edge, which are associated to the nucleation of additional pairs of counter-propagating modes. Employing variational methods, here we study edge reconstruction in the prototypical particle-like phases at ν = 1, 1/3, and 2/5 as a function of the slope of the confining potential. Our analysis shows that subsequent renormalization of the edge modes, driven by disorder-induced tunnelling and intermode interactions, may lead to the emergence of upstream neutral modes. These predictions may be tested in suitably designed transport experiments. Our results are also consistent with previous observations of upstream neutral modes in these QH phases and could explain the absence of anyonic interference in electronic Mach-Zehnder setups.

    

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