1,235 research outputs found
Analytical solutions for Bloch waves in resonant phononic crystals: deep-subwavelength energy splitting and mode steering between topologically protected interfacial and edge states
No full textWe derive analytical solutions based on singular Green's functions, which enable efficient computations of scattering simulations or Floquet-Bloch dispersion relations for waves propagating through an elastic plate, whose surface is patterned by periodic arrays of elastic beams. Our methodology is versatile and allows us to solve a range of problems regarding arrangements of multiple beams per primitive cell, over Bragg to deep-subwavelength scales; we cross-verify against finite element numerical simulations to gain further confidence in our approach, which relies upon the hypothesis of Euler-Bernoulli beam theory considerably simplifying continuity conditions such that each beam can be replaced by point forces and moments applied to the neutral plane of the plate. The representations of Green's functions by Fourier series or Fourier transforms readily follows, yielding rapid and accurate analytical schemes. The accuracy and flexibility of our solutions are demonstrated by engineering topologically non-trivial states, from primitive cells with broken spatial symmetries, following the phononic analogue of the Quantum Valley Hall Effect. Topologically protected states are produced and coexist along: interfaces between adjoining chiral-mirrored bulk media, and edges between one such chiral bulk and the surrounding bare elastic plate, allowing topological circuits to be designed with robust waveguiding. Our topologically protected interfacial states correspond to zero-line modes, and our topological edgestates are produced in accordance with the bulk-edge correspondence. These topologically non-trivial states exist within near flexural resonances of the constituent beams of the phononic crystal and hence can be tuned into a deep-subwavelength regime
Compound viscous thread with electrostatic and electrokinetic effects
No full text07.1.14 KB. OK to add published version to spiral, 12 months embargo has elapsed. CUP polic
Van der Waals Force Assisted Heat Transfer
No full textPhonons (collective atomic vibrations in solids) are more effective in transporting heat than photons. This is the reason why the conduction mode of heat transport in nonmetals (mediated by phonons) is dominant compared to the radiation mode of heat transport (mediated by photons). However, since phonons are unable to traverse a vacuum gap (unlike photons), it is commonly believed that two bodies separated by a gap cannot exchange heat via phonons. Recently, a mechanism was proposed [J. B. Pendry, K. Sasihithlu, and R. V. Craster, Phys. Rev. B 94, 075414 (2016)] by which phonons can transport heat across a vacuum gap – through the Van der Waals interaction between two bodies with gap less than the wavelength of light. Such heat transfer mechanisms are highly relevant for heating (and cooling) of nanostructures; the heating of the flying heads in magnetic storage disks is a case in point. Here, the theoretical derivation for modelling phonon transmission is revisited and extended to the case of two bodies made of different materials separated by a vacuum gap. Magnitudes of phonon transmission, and hence the heat transfer, for commonly used materials in the micro- and nano-electromechanical industry are calculated and compared with the calculation of conduction heat transfer through air for small gaps as well as the heat transfer calculation due to photon exchange
Viscoplastic flow over an inclined surface
No full textWe review viscoplastic flow over inclined surfaces, focusing on constant-flux extrusions from small vents and the slumping of a fixed volume of material. Lubrication theory is used for shallow and slow flows to reduce the governing equations to a nonlinear diffusion-type equation for the local fluid depth; this model is used as the basis for exploration of the problem. Theory is compared to experiments. A number of complications and additional physical effects are discussed that enrich real situations
Constraint Handling in RV-GOMEA
No full textThe Real-Valued Gene-pool Optimal Mixing Evolutionary Algorithm (RV-GOMEA) is a state-of-the-art algorithm for single-objective, real-valued optimization. As many practical applications are inherently constrained, evolutionary algorithms are equipped with constraint handling techniques to allow optimizing constrained problems. The approach currently in use with RV-GOMEA prioritizes solution feasibility over the objective value in all cases, pressuring the algorithm to find feasible solutions. However, this can be inefficient if the constrained optimum is located at the constraint boundary, as search is discouraged from exploring the search space close to infeasible solutions.In this thesis, several well-known constraint handling techniques from literature are adapted for use with RV-GOMEA and evaluated on different benchmark problems, identifying the strengths and limitations of the various techniques. Furthermore, the inefficiency of the current technique is investigated in detail. Based on the insights gained, modifications to the existing techniques are proposed, leading to promising preliminary results.Computer Science | Artificial Intelligenc
Viscoplastic flow over an inclined surface (Erratum)
No full textWe review viscoplastic flow over inclined surfaces, focusing on constant-flux extrusions from small vents and the slumping of a fixed volume of material. Lubrication theory is used for shallow and slow flows to reduce the governing equations to a non-linear diffusion-type equation for the local fluid depth; this model is used as the basis for exploration of the problem. Theory is compared to experiments. A number of complications and additional physical effects are discussed that enrich real situations. © 2006 Elsevier B.V. All rights reserved
Platonic Localisation: One Ring to Bind Them
No full textIn this paper, we present an asymptotic model describing localised flexural
vibrations along a structured ring containing point masses or spring-mass
resonators in an elastic plate. The values for the required masses and
stiffnesses of resonators are derived in a closed analytical form. It is shown
that spring-mass resonators can be tuned to produce a "negative inertia" input,
which is used to enhance localisation of waveforms around the structured ring.
Theoretical findings are accompanied by numerical simulations, which show
exponentially localised and leaky modes for different frequency regimes
Viscoplastic dam breaks and the Bostwick consistometer
No full textWe present a theoretical and experimental analysis of the dam break of a viscoplastic fluid in a horizontal channel. A shallow, slow fluid model based on the Herschel-Bulkley constitutive law allows one to characterize the early and late stages of the flow, the final state and the dependence on yield stress and nonlinear viscosity. A particular diagnostic is advanced (time ratios based on the length of time required for the fluid to slump certain distances from the broken dam) that may assist an experimentalist to unravel those dependences. Experiments are conducted with cornsyrup, and aqueous suspensions of xanthan gum, kaolin, carbopol, cornstarch and apple puree. Cornsyrup xanthan gum and kaolin show fair quantitative agreement with theory. Carbopol compares less favourably, due primarily to inertial effects which are missing from the theory. The results for cornstarch confirm that it is shear thickening, but its detailed rheology remains unknown (and unexplored). Apple puree also appears to compare well with theory, although repeating the dam break in a roughened channel leads to substantially different results, suggesting that fluid separation can induce effective wall slip (a problem that also probably plagues the Bostwick device). Finally, theory is compared with Bostwick tests with fruit puree, with limited success
Serial RV wall stress measurements:association with right ventricular function in repaired Tetralogy of Fallot patients
No full textBackground: Optimal timing of pulmonary valve replacement (PVR) in Tetralogy of Fallot (TOF) patients remains challenging. Ventricular wall stress is considered to be an early marker of right ventricular (RV) dysfunction.Objectives: To investigate the association of RV wall stresses and their change over time with functional parameters in TOF patients.Methods: Ten TOF patients after surgical repair with moderate/severe pulmonary regurgitation were included. At two timepoints (median follow-up time 7.2 years), patient-specific computational biventricular models for wall stress assessment were created using CMR short-axis cine images and echocardiography-based RV pressures. RV ejection fraction (RVEF), NT-proBNP and cardiopulmonary exercise tests were used as outcome measures reflecting RV function. Associations between regional RV diastolic wall stress and RV function were investigated using linear mixed models.Results: Increased wall stress correlated with lower RV mass (rrm = −0.70, p = 0.017) and lower RV mass-to-volume (rrm = −0.80, p = 0.003) using repeated measures. Wall stress decreased significantly over time, especially in patients with a stable RVEF (p < 0.001). Higher wall stress was independently associated with lower RVEF, adjusted for left ventricular ejection fraction, RV end-diastolic volume and time since initial surgery (decrease of 1.27% RVEF per kPa increase in wall stress, p = 0.029) using repeated measurements. No association was found between wall stress, NT-proBNP, and exercise capacity.Conclusions: Using a computational method to calculate wall stress locally in geometrically complex ventricles, we demonstrated that lower wall stress might be important to maintain ventricular function. RV wall stress assessment can be used in serial follow-up, and is potentially an early marker of impending RV dysfunction
On a class of three-phase checkerboards with unusual effective properties
No full textInternational audienceWe examine the band spectrum, and associated Floquet-Bloch eigensolutions, arising in a class of three-phase periodic checkerboards. On a periodic cell [-1, 1[2, the refractive index, n, is defined by n(2) = 1 + g(1)(x(1)) + g(2)(x(2)) with g(i)(x(i)) = r(2) for 0 -1 the lowest frequency branch goes through origin with linear behaviour, which leads to effective properties encountered in most periodic structures. However, the case whereby r(2) = -1 is very unusual, as the frequency A behaves like near the origin, where k is the wavenumber. Finally, when r(2) < -1, the lowest branch does not pass through the origin and a zero-frequency band gap opens up. In the last two cases, effective medium theory breaks down even in the quasi-static limit, while the high-frequency homogenization [R.V. Craster, J. Kaplunov, A.V. Pichugin, High-frequency homogenization for periodic media, Proc. R. Soc. Lond. Ser. A 466 (2010) 2341-2362] neatly captures the detailed features of band diagrams
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