Browsing by Author "van't Wout, Elwin"

Now showing 1 - 5 of 5
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    A fast full-wave solver for calculating ultrasound propagation in the body
    (2021) Haqshenas, S. R.; Gélat, P.; van't Wout, Elwin; Betcke, T.; Saffari, N.
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    Aerial Video Surveillance in a Megacity: A Case Study in Santiago, Chile
    (2024) van't Wout, Elwin; Molina, Ismael Aguirre
    The growth of gated communities and the omnipresence of video surveillance have changed urban life considerably in the last decades. Especially in cities governed by neoliberal public policies and widespread fear of crime, residents search for security, social status, and shared services in fortified enclaves where video cameras watch everybody who enters their premises. These developments are not restricted to private space, and affluent neighborhoods in megacities are eager to adopt a wide range of surveillance technology to protect their citizens. Eight years ago, the affluent municipality of Lo Barnechea in the metropolitan region of Santiago, Chile, installed a surveillance balloon in a residential area. The local security forces currently operate six balloons, and residents have become accustomed to the ever-present gaze of aerial video cameras. This article contrasts the presence of this technology in a megacity against different perspectives on surveillance and urbanism. A consideration of its panoptic features leaves an unsatisfactory explanation. Citizens actively participate in generating public policies and smart technology for public safety, as is characteristic of a maximum security society. The surveillance balloons have become a sustainable element in the urban landscape and an appreciated part of the social fabric of the residential neighborhoods.
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    Boundary integral formulations for acoustic modelling of high-contrast media
    (2022) van't Wout, Elwin; Haqshenas, Seyyed R.; Gelat, Pierre; Betcke, Timo; Saffari, Nader
    The boundary element method is an efficient algorithm for simulating acoustic propagation through homogeneous objects embedded in free space. The conditioning of the system matrix strongly depends on physical parameters such as density, wavespeed and frequency. In particular, high contrast in density and wavespeed across a material interface leads to an ill-conditioned discretisation matrix. Therefore, the convergence of Krylov methods to solve the linear system is slow. Here, specialised boundary integral formulations are designed for the case of acoustic scattering at high-contrast media. The eigenvalues of the resulting system matrix accumulate at two points in the complex plane that depend on the density ratio and stay away from zero. The spectral analysis of the Calderon preconditioned PMCHWT formulation yields a single accumulation point. Benchmark simulations demonstrate the computational efficiency of the high-contrast Neumann formulation for scattering at high-contrast media.
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    Geographic variability in dust and temperature in climate scaling regimes over the Last Glacial Cycle
    (2024) Acuña Reyes Nicolas Martin; van't Wout, Elwin; Lovejoy, Shaun; Lambert Fabrice; NCD Risk Factor Collaboration (NCD-RisC)
    Temperature and mineral dust records serve as valuable palaeoclimatic indicators for studying atmospheric variability across different temporal scales. In this study, we employed Haar fluctuations to analyse global spatiotemporal atmospheric variability over the Last Glacial Cycle, capturing both high- and low-frequency information within the records, regardless of uniform or non-uniform sampling. Furthermore, we utilised Haar fluctuations to compute fluctuation correlations, thereby enhancing our understanding of palaeoclimate dynamics.Our findings reveal a latitudinal dependency in the transition from macroweather to climate regimes (tau c), with polar regions experiencing shorter transitions compared to the tropics and mid-latitudes. These transitions occur at approximately 1/100th of glacial cycle length scales, suggesting a dominant forcing mechanism beyond Milankovitch cycles. Additionally, our analysis shows that polar regions have larger fluctuation amplitudes than lower latitudes as a consequence of the polar amplification effect. Furthermore, fluctuation correlations demonstrate faster synchronisation between the poles themselves compared to lower-latitude sites, achieving high correlation values within 10 kyr.Therefore, our findings suggest a consistent climate signal propagating from the poles to the Equator, representing the first empirical evidence supporting the hypothesis that the poles play a pivotal role as climate change drivers, influencing the variability in climatic transitions worldwide.
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    Towards optimal boundary integral formulations of the Poisson-Boltzmann equation for molecular electrostatics
    (2022) Search, Stefan D.; Cooper, Christopher D.; van't Wout, Elwin
    The Poisson-Boltzmann equation offers an efficient way to study electrostatics in molecular settings. Its numerical solution with the boundary element method is widely used, as the complicated molecular surface is accurately represented by the mesh, and the point charges are accounted for explicitly. In fact, there are several well-known boundary integral formulations available in the literature. This work presents a generalized expression of the boundary integral representation of the implicit solvent model, giving rise to new forms to compute the electrostatic potential. Moreover, it proposes a strategy to build efficient preconditioners for any of the resulting systems, improving the convergence of the linear solver. We perform systematic benchmarking of a set of formulations and preconditioners, focusing on the time to solution, matrix conditioning, and eigenvalue spectrum. We see that the eigenvalue clustering is a good indicator of the matrix conditioning, and show that they can be easily manipulated by scaling the preconditioner. Our results suggest that the optimal choice is problem-size dependent, where a simpler direct formulation is the fastest for small molecules, but more involved second-kind equations are better for larger problems. We also present a fast Calderon preconditioner for first-kind formulations, which shows promising behavior for future analysis. This work sets the basis towards choosing the most convenient boundary integral formulation of the Poisson-Boltzmann equation for a given problem.