1,874 research outputs found
A neural network approach for determining spatial and geometry dependent Green's functions for thermal stress approximation in power plant header components
The trend in power generation to operate plant with a greater frequency of on/partial/off load conditions creates several concerns for the long term structural integrity of many high temperature components. The Green's function method has been used for many years to estimate the thermal stresses in components such as steam headers by attempting to solve the un-coupled thermal stress problem for a unit temperature step. Once a Green's function for a unit temperature step has been determined, realistic or actual component temperature profiles can be discretised and the time dependent stress profile reconstructed using Duhamel's theorem. Stress fluctuations can therefore be estimated and damage due to fatigue mechanisms can be quantified. A potential difficulty with this method is that Green's function approximations are determined for a single analysis point in a structure. This is because Green's functions are approximated by fitting a trial function to the results of finite element (FE) simulations. While a user can make some judgement on which point in a structure will give the âworst caseâ (or life limiting) conditions, it is foreseeable that points of interest will be dependent on the specific analysis conditions, such as the stub penetration geometry and the loading condition considered. The neural network approach described in this paper provides a means where transient thermal stress models of complex components (here taken to be steam headers) can be generated relatively quickly and used pro-actively to assess and modify plant operation. A range of header geometries have been considered to make the network applicable over an industry relevant envelope. Coefficients of determination (R2) are typically above 0.92 when reconstructed (from neural network results) unit temperature step stress profiles are compared against âtrueâ FEA results. Mean errors in the stress profiles are, for the majority of cases, less than 10%. Suggestions are also made on possible future improvements to the method through the use of additional constraints on the reconstructed stress profiles
Isotope effects and possible pairing mechanism in optimally doped cuprate superconductors
We have studied the oxygen-isotope effects on T_{c} and in-plane penetration
depth \lambda_{ab}(0) in an optimally doped 3-layer cuprate
Bi_{1.6}Pb_{0.4}Sr_{2}Ca_{2}Cu_{3}O_{10+y} (T_{c} \sim 107 K). We find a small
oxygen-isotope effect on T_{c} (\alpha_{O} = 0.019), and a substantial effect
on \lambda_{ab} (0) (\Delta \lambda_{ab} (0)/\lambda_{ab} (0) = 2.5\pm0.5%).
The present results along with the previously observed isotope effects in
single-layer and double-layer cuprates indicate that the isotope exponent
\alpha_{O} in optimally doped cuprates is small while the isotope effect on the
in-plane effective supercarrier mass is substantial and nearly independent of
the number of the CuO_{2} layers. A plausible pairing mechanism is proposed to
explain the isotope effects, high-T_{c} superconductivity and tunneling spectra
in a consistent way.Comment: 5 pages, 4 figure
Virtual Coronary Intervention: A Treatment Planning Tool Based Upon the Angiogram
Objectives: This study sought to assess the ability of a novel virtual coronary intervention (VCI) tool based on invasive angiography to predict the patient's physiological response to stenting. Background: Fractional flow reserve (FFR)-guided percutaneous coronary intervention (PCI) is associated with improved clinical and economic outcomes compared with angiographic guidance alone. Virtual (v)FFR can be calculated based upon a 3-dimensional (3D) reconstruction of the coronary anatomy from the angiogram, using computational fluid dynamics (CFD) modeling. This technology can be used to perform virtual stenting, with a predicted post-PCI FFR, and the prospect of optimized treatment planning. Methods: Patients undergoing elective PCI had pressure-wire-based FFR measurements pre- and post-PCI. A 3D reconstruction of the diseased artery was generated from the angiogram and imported into the VIRTUheart workflow, without the need for any invasive physiological measurements. VCI was performed using a radius correction tool replicating the dimensions of the stent deployed during PCI. Virtual FFR (vFFR) was calculated pre- and post-VCI, using CFD analysis. vFFR pre- and post-VCI were compared with measured (m)FFR pre- and post-PCI, respectively. Results: Fifty-four patients and 59 vessels underwent PCI. The mFFR and vFFR pre-PCI were 0.66 ± 0.14 and 0.68 ± 0.13, respectively. Pre-PCI vFFR deviated from mFFR by ±0.05 (mean Π= -0.02; SD = 0.07). The mean mFFR and vFFR post-PCI/VCI were 0.90 ± 0.05 and 0.92 ± 0.05, respectively. Post-VCI vFFR deviated from post-PCI mFFR by ±0.02 (mean Π= -0.01; SD = 0.03). Mean CFD processing time was 95 s per case. Conclusions: The authors have developed a novel VCI tool, based upon the angiogram, that predicts the physiological response to stenting with a high degree of accuracy
Restenosis in simultaneous kissing stents to treat unprotected left main stem coronary artery bifurcation disease: incidence and management
The potential for climate-driven bathymetric range shifts: sustained temperature and pressure exposures on a marine ectotherm, Palaemonetes varians
Range shifts are of great importance as a response for species facing climate change. In the light of current ocean-surface warming, many studies have focused on the capacity of marine ectotherms to shift their ranges latitudinally. Bathymetric range shifts offer an important alternative, and may be the sole option for species already at high latitudes or those within enclosed seas; yet relevant data are scant. Hydrostatic pressure (HP) and temperature have wide ranging effects on physiology, importantly acting in synergy thermodynamically, and therefore represent key environmental constraints to bathymetric migration. We present data on transcriptional regulation in a shallow-water marine crustacean (Palaemonetes varians) at atmospheric and high HP following 168-h exposures at three temperatures across the organisms' thermal scope, to establish the potential physiological limit to bathymetric migration by neritic fauna. We observe changes in gene expression indicative of cellular macromolecular damage, disturbances in metabolic pathways and a lack of acclimation after prolonged exposure to high HP. Importantly, these effects are ameliorated (less deleterious) at higher temperatures, and exacerbated at lower temperatures. These data, alongside previously published behavioural and heat-shock analyses, have important implications for our understanding of the potential for climate-driven bathymetric range shift
When is rotational angiography superior to conventional single-plane angiography for planning coronary angioplasty?
Objectives: To investigate the value of rotational coronary angiography (RoCA) in the context of percutaneous coronary intervention (PCI) planning.
Background: As a diagnostic tool, RoCA is associated with decreased patient irradiation and contrast use compared with conventional coronary angiography (CA) and provides superior appreciation of three-dimensional anatomy. However, its value in PCI remains unknown.
Methods: We studied stable coronary artery disease assessment and PCI planning by interventional cardiologists. Patients underwent either RoCA or conventional CA pre-PCI for planning. These were compared with the referral CA (all conventional) in terms of quantitative lesion assessment and operator confidence. An independent panel reanalyzed all parameters.
Results: Six operators performed 127 procedures (60 RoCA, 60 conventional CA, and 7 crossed-over) and assessed 212 lesions. RoCA was associated with a reduction in the number of lesions judged to involve a bifurcation (23 vs. 30 lesions, Pâ<â0.05) and a reduction in the assessment of vessel caliber (2.8 vs. 3.0 mm, Pâ<â0.05). RoCA improved confidence assessing lesion length (Pâ=â0.01), percentage stenosis (Pâ=â0.02), tortuosity (Pâ<â0.04), and proximity to a bifurcation (Pâ=â0.03), particularly in left coronary artery cases. X-ray dose, contrast agent volume, and procedure duration were not significantly different.
Conclusions: Compared with conventional CA, RoCA augments quantitative lesion assessment, enhances confidence in the assessment of coronary artery disease and the precise details of the proposed procedure, but does not affect X-ray dose, contrast agent volume, or procedure duration. © 2015 Wiley Periodicals, Inc
The double-kissing nano-crush for bifurcation lesions : development, bioengineering, fluid dynamics and initial clinical testing
Background
When possible, a single stent technique to treat coronary bifurcation disease is preferable. However, when two stents are required, there is scope to improve upon existing techniques. The crush technique has already been improved with the introduction of double kissing (DK) and mini-crush. We sought to refine and simplify the mini-crush technique, retaining its advantages whilst avoiding its disadvantages, by developing a DK nano-crush technique.
Methods
The DK nano-crush method allows complete lesion coverage of a bifurcation lesion without excessive metal layers. This is achieved by positioning the SB stent with minimal protrusion into the main branch (MB), implantation of the SB stent with an undeployed balloon in the MB, immediate kissing balloon inflation with formation of a minimal neocarina, stenting the MB, re-crossing the proximal part of the SB without crossing double metal layer, and final kissing. We demonstrate this technique with benchtop implantation, micro computed tomography reconstruction, computational fluid dynamics (CFD) modelling and clinically with angiographic and intravascular imaging.
Results
The DK nano-crush was practically feasible and resulted in full ostial coverage. CFD analysis demonstrated minimally disturbed blood flow. The technique was successfully utilised in nine patients with bifurcation lesions with excellent angiographic outcomes and no adverse events over twelve months.
Conclusions
The DK nano-crush technique may represent the ultimate refinement of the original âcrushâ with a number of practical and theoretical advantages. It remains to be tested against other bifurcation techniques in prospective trials
Universality and the Renormalisation Group
Several functional renormalisation group (RG) equations including Polchinski
flows and Exact RG flows are compared from a conceptual point of view and in
given truncations. Similarities and differences are highlighted with special
emphasis on stability properties. The main observations are worked out at the
example of O(N) symmetric scalar field theories where the flows, universal
critical exponents and scaling potentials are compared within a derivative
expansion. To leading order, it is established that Polchinski flows and ERG
flows - despite their inequivalent derivative expansions - have identical
universal content, if the ERG flow is amended by an adequate optimisation. The
results are also evaluated in the light of stability and minimum sensitivity
considerations. Extensions to higher order and further implications are
emphasized.Comment: 15 pages, 2 figures; paragraph after (19), figure 2, and references
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