190 research outputs found
: A Reduced-Order Modeling Framework based on Machine Learning for Real-Time Applications
Digital twins have emerged as a key technology for optimizing the performance
of engineering products and systems. High-fidelity numerical simulations
constitute the backbone of engineering design, providing an accurate insight
into the performance of complex systems. However, large-scale, dynamic,
non-linear models require significant computational resources and are
prohibitive for real-time digital twin applications. To this end, reduced order
models (ROMs) are employed, to approximate the high-fidelity solutions while
accurately capturing the dominant aspects of the physical behavior. The present
work proposes a new machine learning (ML) platform for the development of ROMs,
to handle large-scale numerical problems dealing with transient nonlinear
partial differential equations. Our framework, mentioned as
, utilizes a singular value
decomposition (SVD) update methodology, to compute a linear subspace of the
multi-fidelity solutions during the simulation process,
convolutional autoencoders for nonlinear dimensionality reduction,
feed-forward neural networks to map the input parameters to
the latent spaces, and long short-term memory networks to
predict and forecast the dynamics of parametric solutions. The efficiency of
the framework is demonstrated for a 2D linear
convection-diffusion equation, the problem of fluid around a cylinder, and the
3D blood flow inside an arterial segment. The accuracy of the reconstructed
results demonstrates the robustness and assesses the efficiency of the proposed
approach.Comment: 35 pages, 22 figure
A voxelized immersed boundary (VIB) finite element method for accurate and efficient blood flow simulation
We present an efficient and accurate immersed boundary (IB) finite element
(FE) method for internal flow problems with complex geometries (e.g., blood
flow in the vascular system). In this study, we use a voxelized flow domain
(discretized with hexahedral and tetrahedral elements) instead of a box domain,
which is frequently used in IB methods. The proposed method utilizes the
well-established incremental pressure correction scheme (IPCS) FE solver, and
the boundary condition-enforced IB (BCE-IB) method to numerically solve the
transient, incompressible Navier--Stokes flow equations. We verify the accuracy
of our numerical method using the analytical solution for the Poiseuille flow
in a cylinder, and the available experimental data (laser Doppler velocimetry)
for the flow in a three-dimensional 90{\deg} angle tube bend. We further
examine the accuracy and applicability of the proposed method by considering
flow within complex geometries, such as blood flow in aneurysmal vessels and
the aorta, flow configurations that would otherwise be difficult to solve by
most IB methods. Our method offers high accuracy, as demonstrated by the
verification examples, and high applicability, as demonstrated through the
solution of blood flow within complex geometry. The proposed method is
efficient, since it is as fast as the traditional finite element method used to
solve the Navier--Stokes flow equations, with a small overhead (not more than
5) due to the numerical solution of a linear system formulated for the IB
method.Comment: arXiv admin note: substantial text overlap with arXiv:2007.0208
The link between job satisfaction and organizational commitment:differences between public and private sector employees
Employees in the public and private sectors experience different working conditions and employment relationships. Therefore, it can be assumed that their attitudes toward their job and organizations, and relationships between them, are different. The existing literature has identified the relationship between organizational commitment and job satisfaction as interesting in this context. The present field study examines the satisfaction–commitment link with respect to differences between private and public sector employees. A sample of 617 Greek employees (257 from the private sector and 360 from the public sector) completed standardized questionnaires. Results confirmed the hypothesized relationship differences: Extrinsic satisfaction and intrinsic satisfaction are more strongly related to affective commitment and normative commitment for public sector employees than for private sector ones. The results are discussed, limitations are considered, and directions for future research are proposed
Focus on the research utility of intravascular ultrasound - comparison with other invasive modalities
Intravascular ultrasound (IVUS) is an invasive modality which provides cross-sectional images of a coronary artery. In these images both the lumen and outer vessel wall can be identified and accurate estimations of their dimensions and of the plaque burden can be obtained. In addition, further processing of the IVUS backscatter signal helps in the characterization of the type of the plaque and thus it has been used to study the natural history of the atherosclerotic evolution. On the other hand its indigenous limitations do not allow IVUS to assess accurately stent struts coverage, existence of thrombus or exact site of plaque rupture and to identify some of the features associated with increased plaque vulnerability. In order this information to be obtained, other modalities such as optical coherence tomography, angioscopy, near infrared spectroscopy and intravascular magnetic resonance imaging have either been utilized or are under evaluation. The aim of this review article is to present the current utilities of IVUS in research and to discuss its advantages and disadvantages over the other imaging techniques
Morphological and Physiological Characteristics of Ruptured Plaques in Native Arteries and Neoatherosclerotic Segments: An OCT-Based and Computational Fluid Dynamics Study.
Background
Intravascular imaging has been used to assess the morphology of lesions causing an acute coronary syndrome (ACS) in native vessels (NV) and identify differences between plaques that ruptured (PR) and caused an event and those that ruptured without clinical manifestations. However, there is no data about the morphological and physiological characteristics of neoatherosclerotic plaques that ruptured (PR-NA) which constitute a common cause of stent failure.
Methods
We retrospectively analyzed data from patients admitted with an acute myocardial infarction that had optical coherence tomography (OCT) imaging of the culprit vessel before balloon pre-dilation. OCT pullbacks showing PR were segmented at every 0.4 mm. The extent of the formed cavity, lipid and calcific tissue, thrombus, and macrophages were measured, and the fibrous cap thickness (FCT) and the incidence of micro-channels and cholesterol crystals were reported. These data were used to reconstruct a representative model of the native and neoatherosclerotic lesion geometry that was processed with computational fluid dynamics (CFD) techniques to estimate the distribution of the endothelial shear stress and plaque structural stress.
Result
Eighty patients were included in the present analysis: 56 had PR in NV (PR-NV group) and 24 in NA segments (PR-NA group). The PR-NV group had a larger minimum lumen area (2.93 ± 2.03 vs. 2.00 ± 1.26 mm2, p = 0.015) but similar lesion length and area stenosis compared to PR-NA group. The mean FCT (186 ± 65 vs. 232 ± 80 μm, p = 0.009) and the lipid index was smaller (16.7 ± 13.8 vs. 25.9 ± 14.1, p = 0.008) while the of calcific index (8.3 ± 9.5 vs. 2.2 ± 1.6%, p = 0.002) and the incidence of micro-channels (41.4 vs. 12.5%, p = 0.013) was higher in the PR-NV group. Conversely, there was no difference in the incidence of cholesterol crystals, thrombus burden or the location of the rupture site between groups. CFD analysis revealed higher maximum endothelial shear stress (19.1 vs. 11.0 Pa) and lower maximum plaque structural stress (38.8 vs. 95.1 kPa) in the PR-NA compared to the PR-NV model.
Conclusion
We reported significant morphological and physiological differences between culprit ruptured plaques in native and stented segments. Further research is needed to better understand the causes of these differences and the mechanisms regulating neoatherosclerotic lesion destabilization
Bioresorbable vascular scaffold radial expansion and conformation compared to a metallic platform: Insights from in vitro expansion in a coronary artery lesion model
Aims: This study aimed to compare the acute expansion behaviour of a polymer-based bioresorbable scaffold and a second-generation metallic DES platform in a realistic coronary artery lesion model. Experimental mechanical data with conventional methods have so far shown little difference between metallic stents and currently available polymer-based bioresorbable scaffolds (BRS). Nevertheless, differences in acute results have been observed in clinical studies comparing BRS directly with metallic DES platforms. Methods and results: We examined the expansion behaviour of the bioresorbable vascular scaffold (3.0
718 mm Absorb BVS; Abbott Vascular, Santa Clara, CA, USA) and a metallic DES (3.0
718 mm XIENCE Prime; Abbott Vascular) after expansion at 37\ub0C using identical coronary artery stenosis models (in total 12 experiments were performed). Device expansion was compared during balloon inflation and after deflation using microscopy to allow assessment of plaque recoil. Minimal lumen diameter (MLD) and minimal lumen area (MLA) and stent eccentricity were quantified from optical coherence tomography (OCT) imaging at nominal diameter and after post-dilation at 18 atm. The MLA in the models with BVS deployed was 4.92\ub10.17 mm2 while in the metallic DES it was 5.40\ub10.13 mm2 (p=0.02) at nominal pressure (NP), and 5.41\ub10.20 and 6.07\ub10.25 mm2 (p=0.02), respectively, after expansion at 18 atm. Stent eccentricity index at the MLA was 0.71\ub10.02 in BVS compared to 0.81\ub10.02 in the metal stent at NP (p=0.004), and 0.73\ub10.03 compared to 0.75\ub10.02 at 18 atm (p=0.39). Conclusions: Results obtained in this in vitro lesion model were comparable to the results in randomised clinical trials comparing BVS and XIENCE stents in vivo. Such models may be useful in future BRS developments to predict their acute response in vivo in eccentric lesions
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