408 research outputs found
Orthotropic rotation-free thin shell elements
A method to simulate orthotropic behaviour in thin shell finite elements is
proposed. The approach is based on the transformation of shape function
derivatives, resulting in a new orthogonal basis aligned to a specified
preferred direction for all elements. This transformation is carried out solely
in the undeformed state leaving minimal additional impact on the computational
effort expended to simulate orthotropic materials compared to isotropic,
resulting in a straightforward and highly efficient implementation. This method
is implemented for rotation-free triangular shells using the finite element
framework built on the Kirchhoff--Love theory employing subdivision surfaces.
The accuracy of this approach is demonstrated using the deformation of a
pinched hemispherical shell (with a 18{\deg} hole) standard benchmark. To
showcase the efficiency of this implementation, the wrinkling of orthotropic
sheets under shear displacement is analyzed. It is found that orthotropic
subdivision shells are able to capture the wrinkling behavior of sheets
accurately for coarse meshes without the use of an additional wrinkling model.Comment: 10 pages, 8 figure
Collapse of orthotropic spherical shells
We report on the buckling and subsequent collapse of orthotropic elastic
spherical shells under volume and pressure control. Going far beyond what is
known for isotropic shells, a rich morphological phase space with three
distinct regimes emerges upon variation of shell slenderness and degree of
orthotropy. Our extensive numerical simulations are in agreement with
experiments using fabricated polymer shells. The shell buckling pathways and
corresponding strain energy evolution are shown to depend strongly on material
orthotropy. We find surprisingly robust orthotropic structures with strong
similarities to stomatocytes and tricolpate pollen grains, suggesting that the
shape of several of Nature's collapsed shells could be understood from the
viewpoint of material orthotropy.Comment: 7 pages, 5 figure
Subdivision Shell Elements with Anisotropic Growth
A thin shell finite element approach based on Loop's subdivision surfaces is
proposed, capable of dealing with large deformations and anisotropic growth. To
this end, the Kirchhoff-Love theory of thin shells is derived and extended to
allow for arbitrary in-plane growth. The simplicity and computational
efficiency of the subdivision thin shell elements is outstanding, which is
demonstrated on a few standard loading benchmarks. With this powerful tool at
hand, we demonstrate the broad range of possible applications by numerical
solution of several growth scenarios, ranging from the uniform growth of a
sphere, to boundary instabilities induced by large anisotropic growth. Finally,
it is shown that the problem of a slowly and uniformly growing sheet confined
in a fixed hollow sphere is equivalent to the inverse process where a sheet of
fixed size is slowly crumpled in a shrinking hollow sphere in the frictionless,
quasi-static, elastic limit.Comment: 20 pages, 12 figures, 1 tabl
LOOKING BENEATH THE TIP OF THE ICEBERG: THE TWO-SIDED NATURE OF CHATBOTS AND THEIR ROLES FOR DIGITAL FEEDBACK EXCHANGE
Enterprises are forecasted to spend more on chatbots than on mobile app development by 2021. Up to today little is known on the roles chatbots play in facilitating feedback exchange. However, digitization and automation put pressure on companies to setup digital work environments that enable reskilling of employees. Therefore, a structured analysis of feedback-related chatbots for Slack was conducted. Our results propose six archetypes that reveal the roles of chatbots in facilitating feedback exchange on performance, culture and ideas. We show that chatbots do not only consist of conversational agents integrated into instant messenger but are tightly linked to complementary front-end systems such as mobile and web apps. Like the upper part of an iceberg, the conversational agent is above water and visible within the chat, whereas many user interactions of feedback-related chatbots are only possible outside of the instant messenger. Further, we extract six design principles for chatbots as digital feedback systems. We do this by analyzing chatbots and linking empirically observed design features to (meta-)requirements derived from explanatory theory on feedback, self-determination and persuasive systems. The results suggest that chatbots benefit the social environment of conversation agents and the richness of the graphical user interface of external applications
Shadowing the rotating annulus. Part I: Measuring candidate trajectory shadowing times
An intuitively necessary requirement of models used to provide forecasts of a
system's future is the existence of shadowing trajectories that are consistent
with past observations of the system: given a system-model pair, do model
trajectories exist that stay reasonably close to a sequence of observations of
the system? Techniques for finding such trajectories are well-understood in
low-dimensional systems, but there is significant interest in their application
to high-dimensional weather and climate models. We build on work by Smith et
al. [2010, Phys. Lett. A, 374, 2618-2623] and develop a method for measuring
the time that individual "candidate" trajectories of high-dimensional models
shadow observations, using a model of the thermally-driven rotating annulus in
the perfect model scenario. Models of the annulus are intermediate in
complexity between low-dimensional systems and global atmospheric models. We
demonstrate our method by measuring shadowing times against
artificially-generated observations for candidate trajectories beginning a
fixed distance from truth in one of the annulus' chaotic flow regimes. The
distribution of candidate shadowing times we calculated using our method
corresponds closely to (1) the range of times over which the trajectories
visually diverge from the observations and (2) the divergence time using a
simple metric based on the distance between model trajectory and observations.
An empirical relationship between the expected candidate shadowing times and
the initial distance from truth confirms that the method behaves reasonably as
parameters are varied.Comment: This paper was submitted to Physica D in 2010, but, after review, was
not accepted. We no longer have the time or resources to work on this topic,
but would like this record of our work to be available for others to read,
cite, and follow up. 19 pages, 9 figure
Laminar air flow reduces particle load in TKAâeven outside the LAF panel: a prospective, randomized cohort study
Purpose: Released particles are a major risk of airborne contamination during surgery. The present prospective study investigated the quantitative and qualitative particle load in the operating room (OR) depending on location, time of surgery and use of laminar air flow (LAF) system.
Methods: The particle load/m(3) was measured during the implantation of 12 total knee arthroplasties (6 x LAF, 6 x Non-LAF) by using the Met One HHPC 6 + device (Beckmann Coulter GmbH, Germany). Measurement was based on the absorption and scattering of (laser) light by particles and was performed at three different time-points [empty OR, setting up, ongoing operation) at 3 fixed measurement points [OR table (central LAF area), anaesthesia tower (marginal LAF area), surgical image amplifier (outside LAF area)].
Results: Independent of time and location, all measurements showed a significantly higher particle load in the Non-LAF group (p < 0.01). With ongoing surgical procedure both groups showed increasing particle load. While there was a major increase of fine particles (size < 1 mu m) with advancing activity in the LAF group, the Non-LAF group showed higher particle gain with increasing particle size. The lowest particle load in the LAF group was measured at the operating column, increasing with greater distance from the operating table. The Non-LAF group presented a significantly higher particle load than the LAF group at all locations.
Conclusion: The use of a LAF system significantly reduces the particle load and therefore potential bacterial contamination regardless of the time or place of measurement and therefore seems to be a useful tool for infection prevention. As LAF leads to a significant decrease of respirable particles, it appears to be a protective factor for the health of the surgical team regardless of its use in infection prevention.
Level of evidence: I
Correction of severe valgus osteoarthritis by total knee arthroplasty is associated with increased postoperative ankle symptoms
Purpose: The aim of this study was to assess the mid-term clinical outcome of the ankle joint after total knee arthroplasty (TKA) in high-grade valgus osteoarthritis.
Methods: In this caseâcontrol study, n=36 patients with a preoperative mechanical tibiofemoral angle (mTFA)â„15° who underwent TKA between December 2002 and December 2012 were included. The control group (mTFA<15°) of n=60 patients was created using case matching. Radiological [mechanical tibiofemoral angle (mTFA) and ankle joint orientation to the ground (G-AJLO)] and clinical parameters [Foot Function Index (FFI), Knee Society Score, Forgotten Joint Score,
and Range of Motion (ROM)] were analysed. The mean follow-up time was 59 months (IQR [56, 62]).
Results: The degree of correcting the mTFA by TKA signifcantly correlated with the postoperative FFI (R=0.95, p<0.05), although the knee and ankle joint lines were corrected to neutral orientations. A cut-of value of 16.5° [AUC 0.912 (0.85â0.975 95% CI), sensitivity=0.8, specifcity=0.895] was calculated, above which the odds ratio (OR) for developing ankle symptoms increased vastly [OR 34.0 (9.10â127.02 95% CI)]. ROM restrictions of the subtalar joint displayed a strong sig nifcant correlation with the FFI (R=0.74, p<0.05), demonstrating that decreased ROM of the subtalar joint was associated with aggravated outcomes of the ankle joint.
Conclusions: In this study, higher degrees of leg axis correction in TKA were associated with increased postoperative ankle symptoms. When TKA is performed in excessive valgus knee osteoarthritis, surgeons should be aware that this might trigger the onset or progression of ankle symptoms, particularly in cases of a stif subtalar joint.
Level of evidence III
Shadowing the rotating annulus. Part II: Gradient descent in the perfect model scenario
Shadowing trajectories are model trajectories consistent with a sequence of
observations of a system, given a distribution of observational noise. The
existence of such trajectories is a desirable property of any forecast model.
Gradient descent of indeterminism is a well-established technique for finding
shadowing trajectories in low-dimensional analytical systems. Here we apply it
to the thermally-driven rotating annulus, a laboratory experiment intermediate
in model complexity and physical idealisation between analytical systems and
global, comprehensive atmospheric models. We work in the perfect model scenario
using the MORALS model to generate a sequence of noisy observations in a
chaotic flow regime. We demonstrate that the gradient descent technique
recovers a pseudo-orbit of model states significantly closer to a model
trajectory than the initial sequence. Gradient-free descent is used, where the
adjoint model is set to I in the absence of a full adjoint model. The
indeterminism of the pseudo-orbit falls by two orders of magnitude during the
descent, but we find that the distance between the pseudo-orbit and the
initial, true, model trajectory reaches a minimum and then diverges from truth.
We attribute this to the use of the -adjoint, which is well suited to
noise reduction but not to finely-tuned convergence towards a model trajectory.
We find that gives optimal results, and that candidate model
trajectories begun from this pseudo-orbit shadow the observations for up to 80
s, about the length of the longest timescale of the system, and similar to
expected shadowing times based on the distance between the pseudo-orbit and the
truth. There is great potential for using this method with real laboratory
data.Comment: This paper was originally prepared for submission in 2011; but, after
Part I was not accepted, it was not submitted. It has not been peer-reviewed.
We no longer have the time or resources to work on this topic, but would like
this record of our work to be available for others to read, cite, and follow
up. 22 pages, 11 figure
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