386 research outputs found
Tomorrow's Metamaterials: Manipulation of Electromagnetic Waves in Space, Time and Spacetime
Metamaterials represent one of the most vibrant fields of modern science and
technology. They are generally dispersive structures in the direct and
reciprocal space and time domains. Upon this consideration, I overview here a
number of metamaterial innovations developed by colleagues and myself in the
holistic framework of space and time dispersion engineering. Moreover, I
provide some thoughts regarding the future perspectives of the area
Research in progress and other activities of the Institute for Computer Applications in Science and Engineering
This report summarizes research conducted at the Institute for Computer Applications in Science and Engineering in applied mathematics and computer science during the period April 1, 1993 through September 30, 1993. The major categories of the current ICASE research program are: (1) applied and numerical mathematics, including numerical analysis and algorithm development; (2) theoretical and computational research in fluid mechanics in selected areas of interest to LaRC, including acoustic and combustion; (3) experimental research in transition and turbulence and aerodynamics involving LaRC facilities and scientists; and (4) computer science
Summary of research in progress at ICASE
This report summarizes research conducted at the Institute for Computer Applications in Science and Engineering in applied mathematics, fluid mechanics, and computer science during the period October 1, 1992 through March 31, 1993
Impact of a heterogeneous stator on the rotor-stator interaction-noise: an analytical, experimental and numerical investigation
La présente étude vise à quantifier par une modélisation analytique, des essais et des simulations
numĂ©riques, lâimpact dâun stator hĂ©tĂ©rogĂšne sur le bruit dâinteraction rotor-stator
dans les turbomachines axiales. Le travail dĂ©veloppĂ© sâappuie sur des premiĂšres observations
sur un ventilateur axial Ă basse vitesse Ă lâĂcole Centrale de Lyon, lâĂ©tage LP3. Il
a été observé que les deux premiÚres fréquences de passage des pales (FPP) rayonnaient
Ă des niveaux Ă©levĂ©s alors quâelles devaient ĂȘtre coupĂ©es par le conduit selon le critĂšre de
Tyler & Sofrin. Une campagne expérimentale est alors réalisée sur la configuration de
ventilateur hétérogÚne qui permet la caractérisation des contenus spectral et modal. Afin
de sâassurer quâaucune distorsion dâentrĂ©e dâair nâest prĂ©sente, un Ă©cran pour le contrĂŽle
de la turbulence est utilisé. Des techniques de décomposition modale sont utilisées sur des
antennes pseudo-alĂ©atoires afin dâobtenir les modes acoustiques prĂ©dominants. Les rĂ©sultats
montrent un fort rayonnement acoustique des deux premiÚres fréquences de passage
des pales et mettent en Ă©vidence des modes dominants. La mĂȘme expĂ©rience est ensuite
simulée numériquement en utilisant la méthode de Boltzmann sur réseau. Les simulations
montrent un bon comportement de la turbomachine mais prédisent une augmentation de
pression infĂ©rieure Ă celle de lâexpĂ©rience. La comparaison entre un stator homogĂšne et
hĂ©tĂ©rogĂšne permet de quantifier directement lâimpact de lâhĂ©tĂ©rogĂ©nĂ©itĂ©. LâhĂ©tĂ©rogĂ©nĂ©itĂ©
est alors responsable dâune augmentation du niveau tonal de plus de 10 dB aux deux premiĂšres
FPP. Le contenu modal mesuré sur la configuration hétérogÚne est bien retrouvé
par les simulations numĂ©riques. En outre, lâanalyse de lâĂ©coulement dans lâespacement
inter-rotor-stator a permis de mettre en Ă©vidence lâimpact de lâhĂ©tĂ©rogĂ©nĂ©itĂ© sur le champ
potentiel. Finalement, la modélisation analytique est axée sur deux sources dominantes :
le bruit dâinteraction de sillages et le bruit dâinteraction potentielle. Les rĂ©sultats montrent
une contribution mineure de ce dernier. Les mĂȘmes modes dominants sont retrouvĂ©s
dans certaines directions de propagation en accord avec ce qui est observé expérimentalement.
En dernier lieu, une Ă©tude dâoptimisation de la position des bras support est
présentée. Une des configurations optimales montrant une forte atténuation du niveau
de bruit tonal est validée numériquement par des simulations numériques. Les résultats
montrent que lâoptimisation du positionnement angulaire des aubes structurelles permet
dâobtenir une rĂ©duction significative des niveaux aux deux premiĂšres FPP. LâĂ©tude des diffĂ©rentes
composantes (analytique, expérimentale et numérique) fournit ainsi une meilleure
comprĂ©hension des mĂ©canismes de bruit modifiĂ©s par lâhĂ©tĂ©rogĂ©nĂ©itĂ© du stator.Abstract: The present study aims to quantify by means of analytical modelling, experiments and
numerical simulations, the impact of a heterogeneous stator on the rotor-stator noise in
axial turbomachines. This study starts with the first observations on an axial low-speed
fan at Ăcole Centrale de Lyon, the LP3 stage. It has been observed that the first two blade
passing frequencies (BPF) were radiating at high levels while they were expected to be
cut-off by the duct according to Tyler & Sofrinâs criterion. An experiment is then carried
out with the heterogeneous stator configuration which makes it possible to characterize
the spectral and modal contents. To ensure that no inflow distortion is present at the
inlet, a Turbulence Control Screen is used. Modal decomposition techniques are used
with pseudo-random antennas to obtain the predominant acoustic modes. Results show
a strong acoustic radiation of the first two BPFs and evidence some dominant modes.
The same experiment is then simulated numerically using the lattice Boltzmann method.
The simulations show a good physical behaviour of the turbomachine but predict a lower
pressure-rise compared with the experiment. The comparison between homogeneous and
heterogeneous stators allows quantifying directly the impact of the heterogeneity. The
heterogeneity is responsible for a level increase of more than 10 dB at the first two BPFs.
The modal content from the numerical simulations on the heterogeneous configuration
is also in good agreement with the experiment. In addition, the analysis of the flow
in the inter-stage made it possible to highlight the impact of the heterogeneity on the
potential field. Finally, the analytical modelling is focused on two dominant sources:
wake-interaction noise and potential-interaction noise. Results put in evidence a minor
contribution of the latter despite the short rotor-stator spacing. The same dominant
modes are found in certain propagation directions in accordance with what is measured in
the experiment. Finally, an optimisation of the modified vanes angular position is carried
out. One of the optimal configurations showing a great noise attenuation is numerically
validated by the LBM. The numerical results show that the optimisation of the azimuthal
positioning of the modified vanes makes it possible to obtain a significant reduction of
the levels at the first two BPFs. Thereby, the comparison of the analytical, experimental
and numerical investigations allows achieving a better understanding of the modification
of noise mechanisms caused by the heterogeneity of the stator
3-D Metamaterials: Trends on Applied Designs, Computational Methods and Fabrication Techniques
This work was funded in part by the Predoctoral Grant FPU18/01965 and in part by the
financial support of BBVA Foundation through a project belonging to the 2021 Leonardo Grants for
Researchers and Cultural Creators, BBVA Foundation. The BBVA Foundation accepts no responsibility
for the opinions, statements, and contents included in the project and/or the results thereof, which
are entirely the responsibility of the authors.Metamaterials are artificially engineered devices that go beyond the properties of conventional
materials in nature. Metamaterials allow for the creation of negative refractive indexes;
light trapping with epsilon-near-zero compounds; bandgap selection; superconductivity phenomena;
non-Hermitian responses; and more generally, manipulation of the propagation of electromagnetic
and acoustic waves. In the past, low computational resources and the lack of proper manufacturing
techniques have limited attention towards 1-D and 2-D metamaterials. However, the true potential of
metamaterials is ultimately reached in 3-D configurations, when the degrees of freedom associated
with the propagating direction are fully exploited in design. This is expected to lead to a new era in
the field of metamaterials, from which future high-speed and low-latency communication networks
can benefit. Here, a comprehensive overview of the past, present, and future trends related to 3-D
metamaterial devices is presented, focusing on efficient computational methods, innovative designs,
and functional manufacturing techniques.Predoctoral Grant FPU18/01965BBVA Foundatio
Free and forced propagation of Bloch waves in viscoelastic beam lattices
Beam lattice materials can be characterized by a periodic microstructure realizing a geometrically regular pattern of elementary cells. Within this framework, governing the free and forced wave propagation by means of spectral design techniques and/or energy dissipation mechanisms is a major issue of theoretical interest with applications in aerospace, chemical, naval, biomedical engineering.
The first part of the Thesis addresses the free propagation of Bloch waves in non-dissipative microstructured cellular materials. Focus is on the alternative formulations suited to describe the wave propagation in the bidimensional infinite material domain, according to the classic canons of linear solid or structural mechanics. Adopting the centrosymmetric tetrachiral cell as prototypical periodic topology, the frequency dispersion spectrum is obtained by applying the Floquet-Bloch theory. The dispersion spectrum resulting from a synthetic Lagrangian beam lattice formulation is compared with its counterpart derived from different continuous models (high-fidelity first-order heterogeneous and equivalent homogenized micropolar continua). Asymptotic perturbation-based approximations and numerical spectral solutions are compared and cross-validated. Adopting the low-frequency band gaps of the dispersion spectrum as functional targets, parametric analyses are carried out to highlight the descriptive limits of the synthetic models and to explore the enlarged parameter space described by high-fidelity models. The microstructural design or tuning of the mechanical properties of the cellular microstructure is employed to successfully verify the wave filtering functionality of the tetrachiral material.
Alternatively, band gaps in the material spectrum can be opened at target center frequencies by using metamaterials with inertial resonators. Based on these motivations, in the second part of the Thesis, a general dynamic formulation is presented for determining the dispersion properties of viscoelastic metamaterials, equipped with local dissipative resonators. The linear mechanism of local resonance is realized by tuning periodic auxiliary masses, viscoelastically coupled with the beam lattice microstructure. As peculiar aspect, the viscoelastic coupling is derived by a mechanical formulation based on the Boltzmann superposition integral, whose kernel is approximated by a Prony series. Consequently, the free propagation of damped Bloch waves is governed by a linear homogeneous system of integro-differential equations of motion. Therefore, differential equations of motion with frequency-dependent coefficients are obtained by applying the bilateral Laplace transform. The corresponding complex-valued branches characterizing the dispersion spectrum are determined and parametrically analyzed. Particularly, the spectra corresponding to Taylor series approximations of the equation coefficients are investigated. The standard dynamic equations with linear viscous damping are recovered at the first order approximation. Increasing approximation orders determine non-negligible spectral effects, including the occurrence of pure damping spectral branches. Finally, the forced response to harmonic single frequency external forces in the frequency and the time domains is investigated. The response in the time domain is obtained by applying the inverse bilateral Laplace transform. The metamaterial responses to non-resonant, resonant and quasi-resonant external forces are compared and discussed from a qualitative and quantitative viewpoint
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