A Green's function approach to predict nonlinear thermoacoustic instabilities in combustors

The prediction of thermoacoustic instabilities is fundamental for combustion systems such as domestic burners and industrial gas turbine engines. High-amplitude pressure oscillations cause thermal and mechanical stress to the equipment, leading to premature wear or even critical damage. In this paper we present a new approach to produce nonlinear (i.e. amplitude-dependent) stability maps of a combustion system as a function of various parameters. Our approach is based on the tailored Green’s function of the combustion system, which we calculate analytically. To this end, we assume that the combustor is one-dimensional, and we describe its boundary conditions through reflection coefficients. The heat release is modelled by a generalised law. This includes a direct-feedback term in addition to the usual time-lag term; moreover, its parameters (time lag, coupling coefficients) depend on the oscillation amplitude. The model provides new insight into the physical mechanism of the feedback between heat release rate and acoustic perturbations. It predicts the key nonlinear features of the thermoacoustic feedback, such as limit cycles, bistability and hysteresis. It also explains the frequency shift in the acoustic modes

Analysis of the interaction of thermoacoustic modes with a Green's function approach:

In this paper, we will present a fast prediction tool based on a one-dimensional Green's function approach that can be used to bypass numerically expensive computational fluid dynamics simulations. The Green's function approach has the advantage of providing a clear picture of the physics behind the generation and evolution of combustion instabilities. In addition, the method allows us to perform a modal analysis; single acoustic modes can be treated in isolation or in combination with other modes. In this article, we will investigate the role of higher-order modes in determining the stability of the system. We will initially produce the stability maps for the first and second mode separately. Then the time history of the perturbation will be computed, where both the modes are present. The flame will be modelled by a generic Flame Describing Function, i.e. by an amplitude-dependent Flame Transfer Function. The time-history calculations show the evolution of the two modes resulting from an initial perturbation; both transient and limit-cycle oscillations are revealed. Our study represents a first step towards the modelling of nonlinearity and non-normality in combustion processes

Chapter Interpretazione dei progetti delle fortezze nel Codice Ashb.361 di Francesco di Giorgio Martini

The 43rd UID conference, held in Genova, takes up the theme of ‘Dialogues’ as practice and debate on many fundamental topics in our social life, especially in these complex and not yet resolved times. The city of Genova offers the opportunity to ponder on the value of comparison and on the possibilities for the community, naturally focused on the aspects that concern us, as professors, researchers, disseminators of knowledge, or on all the possibile meanings of the discipline of representation and its dialogue with ‘others’, which we have broadly catalogued in three macro areas: History, Semiotics, Science / Technology. Therefore, “dialogue” as a profitable exchange based on a common language, without which it is impossible to comprehend and understand one another; and the graphic sign that connotes the conference is the precise transcription of this concept: the title ‘translated’ into signs, derived from the visual alphabet designed for the visual identity of the UID since 2017. There are many topics which refer to three macro sessions: - Witnessing (signs and history) - Communicating (signs and semiotics) - Experimenting (signs and sciences) Thanks to the different points of view, an exceptional resource of our disciplinary area, we want to try to outline the prevailing theoretical-operational synergies, the collaborative lines of an instrumental nature, the recent updates of the repertoires of images that attest and nourish the relations among representation, history, semiotics, sciences

Steady magnetic-field generation via surface-plasma-wave excitation

International audienceThe possibility of inducing a magnetic field via surface plasma-wave excitation is investigated with a simple nonrelativistic hydrodynamic model. A static magnetic field is predicted at the plasma surface, scaling with the square of the surface-wave field amplitude, and the influence of the electron plasma density is studied. In the case of resonant surface-wave excitation by laser this result can be applied to low intensities such that the electron quiver velocity in the field of the surface wave is less than its thermal velocity

The Church of St. George in the Kyrenia Castle in the North of the Island of Cyprus: Bringing out the Shape of Architecture

The contribution focuses on the digital documentation of St. George’s church, dating to the Early Byzantine Period, close to the Roman fortress of Kyrenia in the north of the island of Cyprus. In medieval times the Kyrenia castle became a focal point in the defense of Cyprus, being increasingly fortified, first in the Crusader period, then by the Venetians and the Ottomans, when it has incorporated within its defensive structures the church of St George of which today only the central dome covering is perceived from the outside. A digital survey campaign was carried out, integrating the morphological data coming from laser scanners with the texture of photogrammetry, so to be able to study the church and its architectural features; the textured 3D model was used both as a three-dimensional space fruition system through digital platforms, and as an informative base necessary for the graphic restitution of technical architectural drawings to build-up cognitive analyzes. The adoption of a three-dimensional model is useful to understand the distributional complexity of the architectural volumes that hide the Church of St George. It is beneficial for the visitor in the context of virtual museums, and appropriate to obtain traditional 2d drawings as a base to analyze the construction techniques for a stratigraphic-evolutionary study of the building that, together with a careful analysis of historical sources, allows to understand the development phases of the church in relationship with the enlargement of the castle's defensive structures.info:eu-repo/semantics/publishedVersio