1,474 research outputs found
A case study in hexahedral mesh generation: Simulation of the human mandible
We provide a case study for the generation of pure hexahedral meshes for the numerical simulation of physiological stress scenarios of the human mandible. Due to its complex and very detailed free-form geometry, the mandible model is very demanding. This test case is used as a running example to demonstrate the applicability of a combinatorial approach for the generation of hexahedral meshes by means of successive dual cycle eliminations, which has been proposed by the second author in previous work. We report on the progress and recent advances of the cycle elimination scheme. The given input data, a surface triangulation obtained from computed tomography data, requires a substantial mesh reduction and a suitable conversion into a quadrilateral surface mesh as a first step, for which we use mesh clustering and b-matching techniques. Several strategies for improved cycle elimination orders are proposed. They lead to a significant reduction in the mesh size and a better structural quality. Based on the resulting combinatorial meshes, gradient-based optimized smoothing with the condition number of the Jacobian matrix as objective together with mesh untangling techniques yielded embeddings of a satisfactory quality. To test our hexahedral meshes for the mandible model within an FEM simulation we used the scenario of a bite on a ‘hard nut.’ Our simulation results are in good agreement with observations from biomechanical experiments
Molecular Dynamics Simulations of Detonation Instability
After making modifications to the Reactive Empirical Bond Order potential for
Molecular Dynamics (MD) of Brenner et al. in order to make the model behave in
a more conventional manner, we discover that the new model exhibits detonation
instability, a first for MD. The instability is analyzed in terms of the
accepted theory.Comment: 7 pages, 6 figures. Submitted to Phys. Rev. E Minor edits. Removed
parenthetical statement about P^\nu from conclusion
Thermal decomposition of RDX from reactive molecular dynamics
We use the recently developed reactive force field ReaxFF with molecular dynamics to study thermal induced chemistry in RDX [cyclic-[CH2N(NO2)]3] at various temperatures and densities. We find that the time evolution of the potential energy can be described reasonably well with a single exponential function from which we obtain an overall characteristic time of decomposition that increases with decreasing density and shows an Arrhenius temperature dependence. These characteristic timescales are in reasonable quantitative agreement with experimental measurements in a similar energetic material, HMX [cyclic-[CH2N(NO2)]4]. Our simulations show that the equilibrium population of CO and CO2 (as well as their time evolution) depend strongly of density: at low density almost all carbon atoms form CO molecules; as the density increases larger aggregates of carbon appear leading to a C deficient gas phase and the appearance of CO2 molecules. The equilibrium populations of N2 and H2O are more insensitive with respect to density and form in the early stages of the decomposition process with similar timescales
Fat fraction mapping using bSSFP Signal Profile Asymmetries for Robust multi-Compartment Quantification (SPARCQ)
Purpose: To develop a novel quantitative method for detection of different
tissue compartments based on bSSFP signal profile asymmetries (SPARCQ) and to
provide a validation and proof-of-concept for voxel-wise water-fat separation
and fat fraction mapping. Methods: The SPARCQ framework uses phase-cycled bSSFP
acquisitions to obtain bSSFP signal profiles. For each voxel, the profile is
decomposed into a weighted sum of simulated profiles with specific
off-resonance and relaxation time ratios. From the obtained set of weights,
voxel-wise estimations of the fractions of the different components and their
equilibrium magnetization are extracted. For the entire image volume,
component-specific quantitative maps as well as banding-artifact-free images
are generated. A SPARCQ proof-of-concept was provided for water-fat separation
and fat fraction mapping. Noise robustness was assessed using simulations. A
dedicated water-fat phantom was used to validate fat fractions estimated with
SPARCQ against gold-standard 1H MRS. Quantitative maps were obtained in knees
of six healthy volunteers, and SPARCQ repeatability was evaluated in scan
rescan experiments. Results: Simulations showed that fat fraction estimations
are accurate and robust for signal-to-noise ratios above 20. Phantom
experiments showed good agreement between SPARCQ and gold-standard (GS) fat
fractions (fF(SPARCQ) = 1.02*fF(GS) + 0.00235). In volunteers, quantitative
maps and banding-artifact-free water-fat-separated images obtained with SPARCQ
demonstrated the expected contrast between fatty and non-fatty tissues. The
coefficient of repeatability of SPARCQ fat fraction was 0.0512. Conclusion: The
SPARCQ framework was proposed as a novel quantitative mapping technique for
detecting different tissue compartments, and its potential was demonstrated for
quantitative water-fat separation.Comment: 20 pages, 7 figures, submitted to Magnetic Resonance in Medicin
Minority Stress and Leukocyte Gene Expression In Sexual Minority Men Living With Treated HIV Infection
Sexual minority (i.e., non-heterosexual) individuals experience poorer mental and physical health, accounted for in part by the additional burden of sexual minority stress occurring from being situated in a culture favoring heteronormativity. Informed by previous research, the purpose of this study was to identify the relationship between sexual minority stress and leukocyte gene expression related to inflammation, cancer, immune function, and cardiovascular function. Sexual minority men living with HIV who were on anti-retroviral medication, had viral load \u3c 200 copies/mL, and had biologically confirmed, recent methamphetamine use completed minority stress measures and submitted blood samples for RNA sequencing on leukocytes. Differential gene expression and pathway analyses were conducted comparing those with clinically elevated minority stress (n = 18) and those who did not meet the clinical cutoff (n = 20), covarying reactive urine toxicology results for very recent stimulant use. In total, 90 differentially expressed genes and 138 gene set pathways evidencing 2-directional perturbation were observed at false discovery rate (FDR) \u3c 0.10. Of these, 41 of the differentially expressed genes and 35 of the 2-directionally perturbed pathways were identified as functionally related to hypothesized mechanisms of inflammation, cancer, immune function, and cardiovascular function. The neuroactive-ligand receptor pathway (implicated in cancer development) was identified using signaling pathway impact analysis. Our results suggest several potential biological pathways for future work investigating the relationship between sexual minority stress and health
DAC-Less amplifier-less generation and transmission of QAM signals using sub-volt silicon-organic hybrid modulators
We demonstrate generation and transmission of optical signals by directly interfacing highly efficient silicon-organic hybrid (SOH) modulators to binary output ports of a field-programmable gate array. Using an SOH Mach-Zehnder modulator (MZM) and an SOH IQ modulator we generate ON-OFF- keying and binary phase-shift keying signals as well as quadrature phase-shift keying and 16-state quadrature amplitude modulation (16QAM) formats. Peak-to-peak voltages amount to only 0.27 V-pp for driving the MZM and 0.41 V-pp for the IQ modulator. Neither digital-to-analog converters nor drive amplifiers are required, and the RF energy consumption in the modulator amounts to record-low 18 fJ/bit for 16QAM signaling
Generalized Quantization Principle in Canonical Quantum Gravity and Application to Quantum Cosmology
In this paper is considered a generalized quantization principle for the
gravitational field in canonical quantum gravity, especially with respect to
quantum geometrodynamics. This assumption can be interpreted as a transfer from
the generalized uncertainty principle in quantum mechanics, which is postulated
as generalization of the Heisenberg algebra to introduce a minimal length, to a
corresponding quantization principle concerning the quantities of quantum
gravity. According to this presupposition there have to be given generalized
representations of the operators referring to the observables in the canonical
approach of a quantum description of general relativity. This also leads to
generalized constraints for the states and thus to a generalized Wheeler DeWitt
equation determining a new dynamical behaviour. As a special manifestation of
this modified canonical theory of quantum gravity there is explored quantum
cosmology. The generalized cosmological Wheeler DeWitt equation corresponding
to the application of the generalized quantization principle to the
cosmological degree of freedom is solved by using Sommerfelds polynomial
method.Comment: 11 page
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