1,147 research outputs found
Topology optimization using a mixed formulation: An alternative way to solve pressure load problems
Modeling Heterogeneous Materials via Two-Point Correlation Functions: II. Algorithmic Details and Applications
In the first part of this series of two papers, we proposed a theoretical
formalism that enables one to model and categorize heterogeneous materials
(media) via two-point correlation functions S2 and introduced an efficient
heterogeneous-medium (re)construction algorithm called the "lattice-point"
algorithm. Here we discuss the algorithmic details of the lattice-point
procedure and an algorithm modification using surface optimization to further
speed up the (re)construction process. The importance of the error tolerance,
which indicates to what accuracy the media are (re)constructed, is also
emphasized and discussed. We apply the algorithm to generate three-dimensional
digitized realizations of a Fontainebleau sandstone and a boron
carbide/aluminum composite from the two- dimensional tomographic images of
their slices through the materials. To ascertain whether the information
contained in S2 is sufficient to capture the salient structural features, we
compute the two-point cluster functions of the media, which are superior
signatures of the micro-structure because they incorporate the connectedness
information. We also study the reconstruction of a binary laser-speckle pattern
in two dimensions, in which the algorithm fails to reproduce the pattern
accurately. We conclude that in general reconstructions using S2 only work well
for heterogeneous materials with single-scale structures. However, two-point
information via S2 is not sufficient to accurately model multi-scale media.
Moreover, we construct realizations of hypothetical materials with desired
structural characteristics obtained by manipulating their two-point correlation
functions.Comment: 35 pages, 19 figure
Effective approach to the Nagaoka regime of the two dimensional t-J model
We argue that the t-J model and the recently proposed Ising version of this
model give the same physical picture of the Nagaoka regime for J/t << 1. In
particular, both models are shown to give compatible results for a single
Nagaoka polaron as well as for a Nagaoka bipolaron. When compared to the
standard t-J or t-Jz models, the Ising version allows for a numerical analysis
on much larger clusters by means of classical Monte Carlo simulations. Taking
the advantage of this fact, we study the low doping regime of t-J model for J/t
<< 1 and show that the ground state exhibits phase separation into hole-rich
ferromagnetic and hole-depleted antiferromagnetic regions. This picture holds
true up to a threshold concentration of holes, \delta < \delta_t ~ 0.44
\sqrt{J/t}. Analytical calculations show that \delta_t=\sqrt{J/2\pi t}.Comment: 10 pages, 10 figures, revte
Topology Optimization and 3D printing of Large Deformation Compliant Mechanisms for Straining Biological Tissues
This paper presents a synthesis approach in a density-based topology
optimization setting to design large deformation compliant mechanisms for
inducing desired strains in biological tissues. The modelling is based on
geometrical nonlinearity together with a suitably chosen hypereleastic material
model, wherein the mechanical equilibrium equations are solved using the total
Lagrangian finite element formulation. An objective based on least-square error
with respect to target strains is formulated and minimized with the given set
of constraints and the appropriate surroundings of the tissues. To circumvent
numerical instabilities arising due to large deformation in low stiffness
design regions during topology optimization, a strain-energy based
interpolation scheme is employed. The approach uses an extended robust
formulation i.e. the eroded, intermediate and dilated projections for the
design description as well as variation in tissue stiffness. Efficacy of the
synthesis approach is demonstrated by designing various compliant mechanisms
for providing different target strains in biological tissue constructs.
Optimized compliant mechanisms are 3D-printed and their performances are
recorded in a simplified experiment and compared with simulation results
obtained by a commercial software.Comment: 23 pages, 14 figure
Physical examination tests of the shoulder: a systematic review and meta-analysis of diagnostic test performance
Background: Physical examination tests of the shoulder (PETS) are clinical examination maneuvers designed to aid
the assessment of shoulder complaints. Despite more than 180 PETS described in the literature, evidence of their
validity and usefulness in diagnosing the shoulder is questioned.
Methods: This meta-analysis aims to use diagnostic odds ratio (DOR) to evaluate how much PETS shift overall
probability and to rank the test performance of single PETS in order to aid the clinicianâs choice of which tests to
use. This study adheres to the principles outlined in the Cochrane guidelines and the PRISMA statement. A fixed
effect model was used to assess the overall diagnostic validity of PETS by pooling DOR for different PETS with
similar biomechanical rationale when possible. Single PETS were assessed and ranked by DOR. Clinical performance
was assessed by sensitivity, specificity, accuracy and likelihood ratio.
Results: Six thousand nine-hundred abstracts and 202 full-text articles were assessed for eligibility; 20 articles were
eligible and data from 11 articles could be included in the meta-analysis. All PETS for SLAP (superior labral
anterior posterior) lesions pooled gave a DOR of 1.38 [1.13, 1.69]. The Supraspinatus test for any full thickness
rotator cuff tear obtained the highest DOR of 9.24 (sensitivity was 0.74, specificity 0.77). Compression-Rotation
test obtained the highest DOR (6.36) among single PETS for SLAP lesions (sensitivity 0.43, specificity 0.89) and
Hawkins test obtained the highest DOR (2.86) for impingement syndrome (sensitivity 0.58, specificity 0.67). No
single PETS showed superior clinical test performance.
Conclusions: The clinical performance of single PETS is limited. However, when the different PETS for SLAP
lesions were pooled, we found a statistical significant change in post-test probability indicating an overall
statistical validity. We suggest that clinicians choose their PETS among those with the highest pooled DOR
and to assess validity to their own specific clinical settings, review the inclusion criteria of the included
primary studies. We further propose that future studies on the validity of PETS use randomized research
designs rather than the accuracy design relying less on well-established gold standard reference tests and
efficient treatment options
Optimization of piezoelectric bimorph actuators with active damping for static and dynamic loads
Ultrawide phononic band gap for combined in-plane and out-of-plane waves
We consider two-dimensional phononic crystals formed from silicon and voids,
and present optimized unit cell designs for (1) out-of-plane, (2) in-plane and
(3) combined out-of-plane and in-plane elastic wave propagation. To feasibly
search through an excessively large design space (10e40 possible realizations)
we develop a specialized genetic algorithm and utilize it in conjunction with
the reduced Bloch mode expansion method for fast band structure calculations.
Focusing on high symmetry plain-strain square lattices, we report unit cell
designs exhibiting record values of normalized band-gap size for all three
categories. For the combined polarizations case, we reveal a design with a
normalized band-gap size exceeding 60%.Comment: 4 pages, 1 figure, submitted for journal publicatio
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