161 research outputs found
Simulated acoustic emissions from coupled strings
We consider traveling transverse waves on two identical uniform taut strings that are elastically coupled through springs that gradually decrease their stiffness over a region of finite length. The wave system can be decomposed into two modes: an in-phase mode ( + ) that is transparent to the coupling springs, and an out-of-phase mode ( â ) that engages the coupling springs and can resonate at a particular location depending on the excitation frequency. The system exhibits linear mode conversion whereby an incoming ( + ) wave is reflected back from the resonance location both as a propagating ( + ) wave and an evanescent ( â ) wave, while both types emerge as propagating forward through the resonance location. We match a local transition layer expansion to the WKB expansion to obtain estimates of the reflection and transmission coefficients. The reflected waves may be an analog for stimulated emissions from the ear
Reflecting on Crisis: Ethics of Dis/Engagement in Migration Research
This article offers a collective âgaze from withinâ the process of migration research, on the effects the pandemic has had on our interlocutors, our research fields, and our positionalities as researchers. Drawing from our experiences of researching a field in increasing crisis, and following the methodological reflections of the article written by our colleagues in this issue, we discuss a number of dilemmas and repositionings stemming fromâand extending beyondâthe effects of the COVID-19 pandemic. Focusing on issues of positionality, ethics of (dis)engaging from the research field, and the underlying extractivist nature of Global North academia, we propose our own vision of more egalitarian and engaged research ethics and qualitative methodologies in the post-pandemic world
A Comprehensive Three-Dimensional Model of the Cochlea
The human cochlea is a remarkable device, able to discern extremely small
amplitude sound pressure waves, and discriminate between very close
frequencies. Simulation of the cochlea is computationally challenging due to
its complex geometry, intricate construction and small physical size. We have
developed, and are continuing to refine, a detailed three-dimensional
computational model based on an accurate cochlear geometry obtained from
physical measurements. In the model, the immersed boundary method is used to
calculate the fluid-structure interactions produced in response to incoming
sound waves. The model includes a detailed and realistic description of the
various elastic structures present.
In this paper, we describe the computational model and its performance on the
latest generation of shared memory servers from Hewlett Packard. Using compiler
generated threads and OpenMP directives, we have achieved a high degree of
parallelism in the executable, which has made possible several large scale
numerical simulation experiments that study the interesting features of the
cochlear system. We show several results from these simulations, reproducing
some of the basic known characteristics of cochlear mechanics.Comment: 22 pages, 5 figure
Initial/boundary-value problems of tumor growth within a host tissue
This paper concerns multiphase models of tumor growth in interaction with a
surrounding tissue, taking into account also the interplay with diffusible
nutrients feeding the cells. Models specialize in nonlinear systems of possibly
degenerate parabolic equations, which include phenomenological terms related to
specific cell functions. The paper discusses general modeling guidelines for
such terms, as well as for initial and boundary conditions, aiming at both
biological consistency and mathematical robustness of the resulting problems.
Particularly, it addresses some qualitative properties such as a priori
nonnegativity, boundedness, and uniqueness of the solutions. Existence of the
solutions is studied in the one-dimensional time-independent case.Comment: 30 pages, 5 figure
Resultant pressure distribution pattern along the basilar membrane in the spiral shaped cochlea
Cochlea is an important auditory organ in the inner ear. In most mammals, it
is coiled as a spiral. Whether this specific shape influences hearing is still
an open problem. By employing a three dimensional fluid model of the cochlea
with an idealized geometry, the influence of the spiral geometry of the cochlea
is examined. We obtain solutions of the model through a conformal
transformation in a long-wave approximation. Our results show that the net
pressure acting on the basilar membrane is not uniform along its spanwise
direction. Also, it is shown that the location of the maximum of the spanwise
pressure difference in the axial direction has a mode dependence. In the
simplest pattern, the present result is consistent with the previous theory
based on the WKB-like approximation [D. Manoussaki, Phys. Rev. Lett. 96,
088701(2006)]. In this mode, the pressure difference in the spanwise direction
is a monotonic function of the distance from the apex and the normal velocity
across the channel width is zero. Thus in the lowest order approximation, we
can neglect the existance of the Reissner's membrane in the upper channel.
However, higher responsive modes show different behavior and, thus, the real
maximum is expected to be located not exactly at the apex, but at a position
determined by the spiral geometry of the cochlea and the width of the cochlear
duct. In these modes, the spanwise normal velocities are not zero. Thus, it
indicates that one should take into account of the detailed geometry of the
cochlear duct for a more quantitative result. The present result clearly
demonstrates that not only the spiral geometry, but also the geometry of the
cochlear duct play decisive roles in distributing the wave energy.Comment: 21 pages. (to appear in J. Biol. Phys.
Contact-inhibited chemotaxis in de novo and sprouting blood-vessel growth
Blood vessels form either when dispersed endothelial cells (the cells lining
the inner walls of fully-formed blood vessels) organize into a vessel network
(vasculogenesis), or by sprouting or splitting of existing blood vessels
(angiogenesis). Although they are closely related biologically, no current
model explains both phenomena with a single biophysical mechanism. Most
computational models describe sprouting at the level of the blood vessel,
ignoring how cell behavior drives branch splitting during sprouting. We present
a cell-based, Glazier-Graner-Hogeweg-model simulation of the initial patterning
before the vascular cords form lumens, based on plausible behaviors of
endothelial cells. The endothelial cells secrete a chemoattractant, which
attracts other endothelial cells. As in the classic Keller-Segel model,
chemotaxis by itself causes cells to aggregate into isolated clusters. However,
including experimentally-observed adhesion-driven contact inhibition of
chemotaxis in the simulation causes randomly-distributed cells to organize into
networks and cell aggregates to sprout, reproducing aspects of both de novo and
sprouting blood-vessel growth. We discuss two branching instabilities
responsible for our results. Cells at the surfaces of cell clusters attempting
to migrate to the centers of the clusters produce a buckling instability. In a
model variant that eliminates the surface-normal force, a dissipative mechanism
drives sprouting, with the secreted chemical acting both as a chemoattractant
and as an inhibitor of pseudopod extension. The branching instabilities
responsible for our results, which result from contact inhibition of
chemotaxis, are both generic developmental mechanisms and interesting examples
of unusual patterning instabilities.Comment: Thoroughly revised version, now in press in PLoS Computational
Biology. 53 pages, 13 figures, 2 supporting figures, 56 supporting movies,
source code and parameters files for computer simulations provided.
Supporting information: http://www.psb.ugent.be/~romer/ploscompbiol/ Source
code: http://sourceforge.net/projects/tst
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