453 research outputs found
Mechanical mode engineering with orthotropic metamaterial membranes
Metamaterials are structures engineered at a small scale with respect to the
wavelength of the excitations they interact with. These structures behave as
artificial materials whose properties can be chosen by design, mocking and even
outperforming natural materials and making them the quintessential tool for
manipulation of wave systems. In this Letter we show how the acoustic
properties of a silicon nitride membrane can be affected by nanopatterning. The
degree of asymmetry in the pattern geometry induces an artificial anisotropic
elasticity, resulting in the splitting of otherwise degenerate mechanical
modes. The artificial material we introduce has a maximum Ledbetter-Migliori
anisotropy of 1.568, favorably comparing to most bulk natural crystals. With an
additional freedom in defining arbitrary asymmetry axes by pattern rotation,
our approach can be useful for fundamental investigation of material properties
as well as for devising improved sensors of light, mass or acceleration based
on micromechanical resonators
Wind-structure interaction simulations for the prediction of ovalling vibrations in silo groups
Wind-induced ovalling vibrations were observed during a storm in October 2002 on several empty silos of a closely spaced group consisting of 8 by 5 thin-walled silos in the port of Antwerp (Belgium). The purpose of the present research is to investigate if such ovalling vibrations can be predicted by means of numerical simulations. More specifically, the necessity of performing computationally demanding wind-structure interaction (WSI) simulations is assessed. For this purpose, both one-way and two-way coupled simulations are performed. Before considering the entire silo group, a single silo in crosswind is simulated. The simulation results are in reasonably good agreement with observations and WSI simulations seem to be required for a correct prediction of the observed ovalling vibrations
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Finite element investigation of tunable and non-reciprocal elastic wave metamaterials
This dissertation studies elastic wave propagation in metamaterials subjected to an externally-applied static or spatiotemporally-varying pre-strain. Elastic metamaterials are media with subwavelength structure that behave as effective materials displaying atypical effective dynamic properties that are used to directly control the propagation of macroscopic waves. One major design limitation of most metamaterial structures is that the dynamic response cannot be altered once the microstructure is manufactured. However, the ability to modify, or tune, wave propagation in the metamaterial with an external pre-strain that induces geometric nonlinearity is highly desirable for numerous applications. Acoustic and elastic metamaterials with time- and space-dependent effective material properties have also recently received significant attention as a means to induce non-reciprocal wave propagation. However, the modulation of effective material properties in space and time using mechanical deformation has been unexplored. Tunable elastic metamaterials that exhibit large effective material property changes under a varying external pre-strain are therefore strong candidates for a non-reciprocal medium.
The complex geometry present in unit cells that exhibit large geometric nonlinearity necessitates the development of a numerical technique. In this dissertation, a finite element approach is derived and implemented to study elastic wave propagation in a static pre-strained metamaterial, then generalized to include the effects of a spatiotemporally-varying pre-strain. A honeycomb structure composed of a doubly-periodic array of curved beams, known as a negative stiffness honeycomb (NSH), is analyzed as a tunable and non-reciprocal elastic metamaterial. It is shown that NSH exhibits significant tunability and a high degree of anisotropic wave behavior when a static pre-strain is imposed. This behavior can be used to guide wave energy in different directions depending on static pre-strain levels. In addition, it is shown that partial band gaps exist where only longitudinal waves propagate. The NSH therefore behaves as a meta-fluid, or pentamode metamaterial, which may be of use for applications of transformation elastodynamics such as cloaking and gradient index lens devices. A negative stiffness chain, a quasi-one-dimensional representation of NSH, is also shown as a case example of a non-reciprocal medium. It is shown in this work that this structure displays a high degree of non-reciprocity for a small amount of modulation pre-strain. The utility of the finite element approach is further demonstrated by investigating the effects of chiral geometric asymmetry to enhance the non-reciprocal behavior of elastic wave propagation in NSH.Mechanical Engineerin
Low-frequency vibrations of strongly inhomogeneous multicomponent elastic structures
The thesis deals with 1D and 2D scalar equations governing dynamic behaviour of strongly inhomogeneous layered structures. Harmonic vibrations of a composite rod and antiplane shear motions of a cylindrical body consisting of several components are studied paying particular attention to the lowest frequencies. The main focus is on a strong contrast between the parameters characterising structure components, including their sizes, material stiffness, and densities. We start with a multi-parametric analysis of the near-rigid body motions of rods and cylindrical bodies with piecewise uniform properties. The listed problems allow exact analytical solutions demonstrating that the values of all lowest eigenfrequencies tend to zero at large/small ratios of material and geometric parameters. The low-frequency behaviour is considered for so-called global and local regimes, and simple explicit conditions on the problem parameters, underlying each of the regimes, are derived. Further, we present a perturbation procedure for a more general setup based on the evaluation of the almost rigid body motions of “stronger” components assuming a high contrast of material parameters. The proposed approach is extended to structures of arbitrary shape, with variable material parameters, as well as to multi-component structures. We obtain asymptotic formulae for the lowest natural frequencies and also present illustrative examples for each of the studied problems. Many of asymptotic estimations are compared with exact solutions. The results of the thesis are applicable to a mathematical justification of shear deformation theories for multi-layered plates and shells with a strong transverse inhomogeneity
Mechanical systems in the quantum regime
Mechanical systems are ideal candidates for studying quantumbehavior of
macroscopic objects. To this end, a mechanical resonator has to be cooled to
its ground state and its position has to be measured with great accuracy.
Currently, various routes to reach these goals are being explored. In this
review, we discuss different techniques for sensitive position detection and we
give an overview of the cooling techniques that are being employed. The latter
include sideband cooling and active feedback cooling. The basic concepts that
are important when measuring on mechanical systems with high accuracy and/or at
very low temperatures, such as thermal and quantum noise, linear response
theory, and backaction, are explained. From this, the quantum limit on linear
position detection is obtained and the sensitivities that have been achieved in
recent opto and nanoelectromechanical experiments are compared to this limit.
The mechanical resonators that are used in the experiments range from
meter-sized gravitational wave detectors to nanomechanical systems that can
only be read out using mesoscopic devices such as single-electron transistors
or superconducting quantum interference devices. A special class of
nanomechanical systems are bottom-up fabricated carbon-based devices, which
have very high frequencies and yet a large zero-point motion, making them ideal
for reaching the quantum regime. The mechanics of some of the different
mechanical systems at the nanoscale is studied. We conclude this review with an
outlook of how state-of-the-art mechanical resonators can be improved to study
quantum {\it mechanics}.Comment: To appear in Phys. Re
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