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"Conservation properties"
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The Politics of Heritage Management in Mali
The UNESCO World Heritage Site of Djenné, in modern day Mali, is exalted as an enduring wonder of the ancient African world by archaeologists, anthropologists, state officials, architects and travel writers. In this revealing study, the author critically examines how the politics of heritage management, conservation, and authenticity play essential roles in the construction of Djenné's past and its appropriation for contemporary purposes. Despite its great renown, the majority of local residents remain desperately poor. And while most are proud of their cultural heritage, they are often troubled by the limitations it places on their day to day living conditions. Joy argues for a more critical understanding of this paradox and urges us all to reconsider the moral and philosophical questions surrounding the ways in which we use the past in the present.
eBook
The Global Solar Dynamo
A brief summary of the various observations and constraints that underlie solar dynamo research are presented. The arguments that indicate that the solar dynamo is an alpha-omega dynamo of the Babcock-Leighton type are then shortly reviewed. The main open questions that remain are concerned with the subsurface dynamics, including why sunspots emerge at preferred latitudes as seen in the familiar butterfly wings, why the cycle is about 11 years long, and why the sunspot groups emerge tilted with respect to the equator (Joy’s law). Next, we turn to magnetic helicity, whose conservation property has been identified with the decline of large-scale magnetic fields found in direct numerical simulations at large magnetic Reynolds numbers. However, magnetic helicity fluxes through the solar surface can alleviate this problem and connect theory with observations, as will be discussed.
Journal Article
Consultation and Cultural Heritage
This pragmatic guide to consultation in cultural heritage and environmental impact management distills decades of experience by two of the leading figures in this area. Claudia Nissley and Tom King reject the idea that consulting with communities and other stakeholders is merely checking off a box on the list of tasks required to implement a project. Instead, they show government agencies and project sponsors how to find the relevant parties, to discuss the project in an open and continuous fashion, to consider alternative strategies, and to seek agreement that meets everyone's needs. The authors also provide useful guidance to community leaders and other stakeholders to represent their interests in the consultation process. Complete with practical suggestions and cases of successful (and less successful) consultation projects, Consultation and Cultural Heritage is a book that no one involved in this field should be without.
eBook
On Variational Fourier Particle Methods
In this article we describe a unifying framework for variational electromagnetic particle schemes of spectral type, and we propose a novel spectral Particle-In-Cell (PIC) scheme that preserves a discrete Hamiltonian structure. Our work is based on a new abstract variational derivation of particle schemes which builds on a de Rham complex where Low’s Lagrangian is discretized using a particle approximation of the distribution function. In this framework, which extends the recent Finite Element based Geometric Electromagnetic PIC (GEMPIC) method to a wide variety of field solvers, the discretization of the electromagnetic potentials and fields is represented by a de Rham sequence of compatible spaces, and the particle-field coupling procedure is described by approximation operators that commute with the differential operators involved in the sequence. For spectral Maxwell solvers these compatible spaces are spanned by a finite number of Fourier modes, and using
L
2
projections as commuting operators leads to the gridless Particle-In-Fourier method which involves exact Fourier coefficients and continuous convolutions in the particle-field coupling. Our new variational PIC method, which we call Fourier-GEMPIC, is obtained by using a new sequence of commuting projections which combine discrete Fourier transforms (DFT), differentiation of particle shape functions and antiderivative filtering operators in Fourier space. As the resulting particle-field coupling essentially involves pointwise evaluations of shape functions and FFT (or DFT) algorithms on a sampling grid this method resembles usual spectral PIC methods, moreover a fully discrete scheme is derived using a directional Hamiltonian splitting procedure. The corresponding time steps are then given in closed form: they preserve the Gauss laws and the discrete Poisson bracket associated with the Hamiltonian structure. These explicit steps are found to share many similarities with a standard spectral PIC method that appears as a Gauss and momentum-preserving variant of the variational method. As arbitrary filters are allowed in our framework, we also discuss aliasing errors and study a natural back-filtering procedure to mitigate the damping caused by anti-aliasing smoothing particle shapes.
Journal Article
On the Use of Different Sets of Variables for Solving Unsteady Inviscid Flows with an Implicit Discontinuous Galerkin Method
This article presents a comparison between the performance obtained by using a spatial discretization of the Euler equations based on a high-order discontinuous Galerkin (dG) method and different sets of variables. The sets of variables investigated are as follows: (1) conservative variables; (2) primitive variables based on pressure and temperature; (3) primitive variables based on the logarithms of pressure and temperature. The solution is advanced in time by using a linearly implicit high-order Rosenbrock-type scheme. The results obtained using the different sets are assessed across several canonical unsteady test cases, focusing on the accuracy, conservation properties and robustness of each discretization. In order to cover a wide range of physical flow conditions, the test-cases considered here are (1) the isentropic vortex convection, (2) the Kelvin–Helmholtz instability and (3) the Richtmyer–Meshkov instability.
Journal Article
The Discontinuous Galerkin Finite Element Method for Solving the MEG and the Combined MEG/EEG Forward Problem
In Electro- (EEG) and Magnetoencephalography (MEG), one important requirement of source reconstruction is the forward model. The continuous Galerkin finite element method (CG-FEM) has become one of the dominant approaches for solving the forward problem over the last decades. Recently, a discontinuous Galerkin FEM (DG-FEM) EEG forward approach has been proposed as an alternative to CG-FEM (Engwer et al., 2017). It was shown that DG-FEM preserves the property of
and that it can, in certain situations such as the so-called
, be superior to the standard CG-FEM approach. In this paper, we developed, implemented, and evaluated two DG-FEM approaches for the MEG forward problem, namely a conservative and a non-conservative one. The
was used as source model. The validation and evaluation work was done in statistical investigations in multi-layer homogeneous sphere models, where an analytic solution exists, and in a six-compartment realistically shaped head volume conductor model. In agreement with the theory, the conservative DG-FEM approach was found to be superior to the non-conservative DG-FEM implementation. This approach also showed convergence with increasing resolution of the hexahedral meshes. While in the EEG case, in presence of skull leakages, DG-FEM outperformed CG-FEM, in MEG, DG-FEM achieved similar numerical errors as the CG-FEM approach, i.e., skull leakages do not play a role for the MEG modality. In particular, for the finest mesh resolution of 1 mm sources with a distance of 1.59 mm from the brain-CSF surface, DG-FEM yielded mean topographical errors (relative difference measure, RDM%) of 1.5% and mean magnitude errors (MAG%) of 0.1% for the magnetic field. However, if the goal is a combined source analysis of EEG and MEG data, then it is highly desirable to employ the same forward model for both EEG and MEG data. Based on these results, we conclude that the newly presented conservative DG-FEM can at least complement and in some scenarios even outperform the established CG-FEM approaches in EEG or combined MEG/EEG source analysis scenarios, which motivates a further evaluation of DG-FEM for applications in bioelectromagnetism.
Journal Article
Preserving what is valued : museums, conservation, and First Nations
What are the \"right ways\" to preserve heritage? Are the aims and purposes of museums necessarily at odds with those of First Nations? This thoughtful book explores the concept of museum conservation in light of cultural repatriation issues, and helps readers understand the complex relationship between museums and Aboriginal peoples.
eBook