Coupling-Induced Instability in a Ring of Thermoacoustic Oscillators

Thermoacoustic instabilities in can-annular combustors of stationary gas turbines lead to unstable Bloch modes which appear as rotating acoustic pressure waves along the turbine annulus. The multi-scale, multiphysical nature of the full problem makes a detailed analysis challenging. In this work, we derive a low-order, coupled oscillator model of an idealized can-annular combustor. The unimodal projection of the Helmholtz equation for the can acoustics is combined with the Rayleigh conductivity, which describes the aeroacoustic coupling between neighboring cans. Using a Bloch-wave ansatz, the resulting system is reduced to a single equation for the frequency spectrum. A linear stability analysis is then performed to study the perturbation of the spectrum by the can-to-can interaction. It is observed that the acoustic coupling can suppress or amplify thermoacoustic instabilities, raising the potential for instabilities in nominally stable systems.Comment: 45 pages, 10 figure

Superradiant Scattering from Nonlinear Wave-Mode Coupling

Waves scattered at a self-oscillating mode can exhibit superradiance, or net amplification of an external harmonic excitation. This exotic behavior, arising from the nonlinear coupling between the mode and the incident wave, is theoretically predicted and experimentally confirmed for the first time in this work. We propose a generic theory of nonlinear wave-mode coupling, which is derived in analogy to the temporal coupled-mode theory of [Fan et al., J. Opt. Soc. Am. A 20, 569 (2003)]. A well-reproducible aeroacoustic realization of a superradiant scatterer was used to test the theory's predictions. It is shown that the nonlinear wave-mode coupling can be exploited to quasi-passively tune the reflection and transmission coefficients of a side cavity in a waveguide. The theoretical framework used to describe this type of superradiance is applicable to non-acoustic systems and may be used to design lossless scattering devices.Comment: 5 pages, 4 figure

Certain features of planar systems

This work is concerned with features of planar dynamical systems governed by a smooth velocity field and additive white noise. By Helmholtz's theorem, the system's velocity field can be decomposed into an irrotational and a solenoidal part, defined by a scalar and a vector potential, respectively. The meaning of this decomposition, however, is generally unclear, because it yields different potentials in different coordinates, and the choice of basis may not be obvious for a given system. In contrast, the dynamics themselves are independent of the basis in which they are represented. To address this discrepancy, we first present a coordinate-independent formulation of the Helmholtz decomposition for general, noise-driven planar systems. In the second part of our investigation, we focus on noise-free, steady planar flows. For this type of system, we analytically derive conditions for ruling out closed orbits in certain regions of phase space. We demonstrate our methods on well-known examples of dynamical systems in the plane.Comment: 11 pages, 3 figure

LAS PRÁCTICAS ACADÉMICAS SOLIDARIAS EN LA UNIVERSIDAD

Es inherente a la tarea universitaria, la formación integral del Profesional tanto en lo académico como en actitudes y valores. Este último aspecto no siempre es tenido en cuenta por docentes o alumnos pero, es el que darán sentido y dirección a su futura práctica cualquiera sea el campo de aplicación. Es importante que el docente se entrene en desarrollar estrategias que vinculen la teoría con la realidad y genere proyectos comunitarios viables de tal modo que la presencia de la universidad en la comunidad resulte cada vez mas visible y esperada por los ciudadanos, ya sea, por la calidad de sus propuestas, por el nivel de responsabilidad social que expresa o porque finalmente destina espacios de su praxis a la resolución de situaciones concretas que afectan a la cada vez mas compleja vida social. . Las prácticas académicas solidarias dan lugar a un aprendizaje con sentido

Why should traceology learn from dental microwear, and vice-versa?

Dental and artifact microwear analyses have a lot in common regarding the questions they address, their developmental history and their issues. However, few paleontologists and archeologists are aware of this, and even those who are, do not take into account most of the methodological insights from the other field. In this focus article, we briefly review the main developmental steps of both methods, highlight how similar their histories are and how combining methodological developments can improve both research fields. In both cases, the traditional analyses have been strongly criticized mainly because of their subjectivity and their lack of repeatability and reproducibility. Quantitative surface texture analyses have been proposed in response, resulting in dental microwear texture analysis (DMTA) and quantitative artifact microwear analysis (QAMA). DMTA is however a more mature method than QAMA and is well supported within the paleontological community. In this paper, focused on the methodological framework of both fields, we address this topic by arguing that traceologists could borrow a lot from DMTA; this would allow QAMA to become an established method much more quickly. Dental microwear analysts can also learn from traceology, especially regarding sample preparation, experimentation and residue analysis. We hope that this focus article will stimulate more awareness, exchange and collaboration between paleontologists and archeologists, and especially between dental and artifact microwear analysts. Paleontology, archeology and the field of surface analysis as a whole would all benefit from such cooperation.info:eu-repo/semantics/publishedVersio

Back to the edge: relative coordinate system for use-wear analysis

Use-wear studies rely heavily on experiments and reference collections to infer the function of archeological artifacts. Sequential experiments, in particular, are necessary to understand how use-wear develops. Consequently, it is crucial to analyze the same location on the tool's surface during the course of an experiment. Being able to relocate the area of interest on a sample is also essential for reproducibility in use-wear studies. However, visual relocation has limited applicability and there is currently no easy and efficient alternative. Here we propose a simple protocol to create a coordinate system directly on the sample. Three ceramic beads that serve as reference markers are adhered onto the sample, either with epoxy resin or acrylic polymer. The former is easier to work with but the latter is reversible so it can be applied to archeological samples too. The microscope's software then relocates the position(s) of interest. We demonstrate the feasibility of this approach and measure its repeatability by imaging the same position on an experimental flint blade 10 times with two confocal microscopes. Our results show that the position can be relocated automatically with a horizontal positional repeatability of approximately 14% of the field of view. Quantitative surface texture measurements according to ISO 25178 vary due to this positional inaccuracy, but it is still unknown whether this variation would mask functional differences. Although still perfectible, we argue that this protocol represents an important step toward repeatability and reproducibility in experimental archeology, especially in use-wear studies.Funding Agency Romisch-Germanisches Zentralmuseum - Leibniz Research Institute for Archeology by German Federal and Rhineland Palatinate funding (Sondertatbestand "Spurenlabor")info:eu-repo/semantics/publishedVersio

A new combined approach using confocal and scanning electron microscopy to image surface modifications on quartzite

YesConfocal microscopy has been increasingly employed in the field of traceology to acquire metrological data of surface changes on a micro-scale. However, its advantages for a traditional visual inspection of use-wear are rarely highlighted. As traditional optical microscopy (OM) has proven unable to entirely fulfil the prerequisites for an ideal observation of highly reflective and irregular materials, alternative ways for providing better observation conditions must be sought. In this contribution, we explore the combination of laser scanning confocal (LSCM) and scanning electron microscopy (SEM) micro-graphs for the visual characterisation of wear on quartzite and evaluate the potential of both techniques.AHRC Fragmented Heritage project (AH/L00688X/1) at the University of Bradford, and of the MICINN-FEDER (PGC2018-093925-B-C32), the AGAUR (SGR 2017-1040) and the URV (2018PFR-URV-B2-91) projects at IPHES-URV. One of the authors (A.P.) was beneficiary of a Catalan pre-doctoral grant (2014FI B 00539), at the Rovira i Virgili University (URV), the IPHES and the Muséum national d’Histoire naturelle of Paris

AS ARMADURAS MICROLÍTICAS TRAPEZOIDAIS DA TRANSIÇÃO NEOLÍTICA DO “RIPARO GABAN” (Trento, Itália)

Apresentam-se neste artigo os dados relativos à análise das armaduras geométricas que fazem parte da categoria dos trapézios provenientes do Riparo Gaban (Trento, norte da Itália). Os materiais analisados provem de uma das sequências mais importantes concernentes o Neolítico alpino