1,145 research outputs found
Efficient isogeometric thin shell formulations for soft biological materials
This paper presents three different constitutive approaches to model thin
rotation-free shells based on the Kirchhoff-Love hypothesis. One approach is
based on numerical integration through the shell thickness while the other two
approaches do not need any numerical integration and so they are
computationally more efficient. The formulation is designed for large
deformations and allows for geometrical and material nonlinearities, which
makes it very suitable for the modeling of soft tissues. Furthermore, six
different isotropic and anisotropic material models, which are commonly used to
model soft biological materials, are examined for the three proposed
constitutive approaches. Following an isogeometric approach, NURBS-based finite
elements are used for the discretization of the shell surface. Several
numerical examples are investigated to demonstrate the capabilities of the
formulation. Those include the contact simulation during balloon angioplasty.Comment: Typos are removed. Remark 3.4 is added. Eq. (18) in the previous
version is removed. Thus, the equations get renumbered. Example 5.5 is
updated. Minor typos in Eqs. (17), (80), (145) and (146), are corrected. They
do not affect the result
A finite membrane element formulation for surfactants
Surfactants play an important role in various physiological and biomechanical
applications. An example is the respiratory system, where pulmonary surfactants
facilitate the breathing and reduce the possibility of airway blocking by
lowering the surface tension when the lung volume decreases during exhalation.
This function is due to the dynamic surface tension of pulmonary surfactants,
which depends on the concentration of surfactants spread on the liquid layer
lining the interior surface of the airways and alveoli. Here, a finite membrane
element formulation for liquids is introduced that allows for the dynamics of
concentration-dependent surface tension, as is the particular case for
pulmonary surfactants. A straightforward approach is suggested to model the
contact line between liquid drops/menisci and planar solid substrates, which
allows the presented framework to be easily used for drop shape analysis. It is
further shown how line tension can be taken into account. Following an
isogeometric approach, NURBS-based finite elements are used for the
discretization of the membrane surface. The capabilities of the presented
computational model is demonstrated by different numerical examples - such as
the simulation of liquid films, constrained and unconstrained sessile drops,
pendant drops and liquid bridges - and the results are compared with
experimental data.Comment: Some typos are removed. Eqs. 13 and 105 are modified. Eqs. 64 and 73
are added; thus, the rest of equations is renumbered. All the numerical
experiments are repeated. The example of Sec. 6.3 is slightly modifie
A New Amplification Regime for Traveling Wave Tubes with Third Order Modal Degeneracy
Engineering of the eigenmode dispersion of slow-wave structures (SWSs) to
achieve desired modal characteristics, is an effective approach to enhance the
performance of high power traveling wave tube (TWT) amplifiers or oscillators.
We investigate here for the first time a new synchronization regime in TWTs
based on SWSs operating near a third order degeneracy condition in their
dispersion. This special three-eigenmode synchronization is associated with a
stationary inflection point (SIP) that is manifested by the coalescence of
three Floquet-Bloch eigenmodes in the SWS. We demonstrate the special features
of "cold" (without electron beam) periodic SWSs with SIP modeled as coupled
transmission lines (CTLs) and investigate resonances of SWSs of finite length.
We also show that by tuning parameters of a periodic SWS one can achieve an SIP
with nearly ideal flat dispersion relationship with zero group velocity or a
slightly slanted one with a very small (positive or negative) group velocity
leading to different operating schemes. When the SIP structure is synchronized
with the electron beam potential benefits for amplification include (i) gain
enhancement, (ii) gain-bandwidth product improvement, and (iii) higher power
efficiency, when compared to conventional Pierce-like TWTs. The proposed theory
paves the way for a new approach for potential improvements in gain, power
efficiency and gain-bandwidth product in high power microwave amplifiers.Comment: 15 pages, 11 figure
Atypical patterns of spinal segment degeneration in patients with abdominal aortic aneurysms
PURPOSE
Abdominal aortic aneurysms (AAAs) affect the vascular perfusion of the lumbar spine. The treatment of AAAs with endovascular aortic aneurysm repair (EVAR) completely occludes the direct vascular supply to the lumbar spine. We hypothesized that patients with AAA who undergo EVAR show a different pattern of spinal degeneration than individuals without AAA.
METHODS
In this retrospective institutional review board-approved study, 100 randomly selected patients with AAA who underwent EVAR with computed tomography (CT) scans between 2005 and 2017 were compared with age- and gender-matched controls without AAA. In addition, long-term follow-up CT images (> 6 months before EVAR, at the time of EVAR, and > 12 months after EVAR) of the patients were analysed to compare the progression of degeneration from before to after EVAR. Degeneration scores, lumbar levels with the most severe degeneration, and lumbar levels with progressive degeneration were analysed in all CT images. Fisher's exact test, Wilcoxon signed-rank test, and Mann-Whitney U test were performed for statistical analyses.
RESULTS
Compared with the control group (n = 94), the most severe degeneration was more commonly detected in the mid-lumbar area in the patient group (n = 100, p = 0.016), with significantly more endplate erosions being detected in the lumbar spine (p = 0.015). However, EVAR did not result in significant additional acceleration of the degenerative process in the long-term follow-up analysis (n = 51).
CONCLUSION
AAA is associated with atypical, more cranially located spinal degradation, particularly in the mid-lumbar segments; however, EVAR does not seem to additionally accelerate the degenerative process. This observation underlines the importance of disc and endplate vascularization in the pathomechanism of spinal degeneration.
LEVEL OF EVIDENCE I
Diagnostic: individual cross-sectional studies with consistently applied reference standard and blinding
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