• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 514
  • 137
  • 86
  • 62
  • 18
  • 14
  • 8
  • 8
  • 6
  • 6
  • 5
  • 5
  • 5
  • 5
  • 5
  • Tagged with
  • 1072
  • 709
  • 316
  • 205
  • 147
  • 74
  • 70
  • 63
  • 56
  • 55
  • 55
  • 54
  • 53
  • 52
  • 51
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
511

PARTICLE-BASED SMOOTHED PARTICLE HYDRODYNAMICS AND DISCRETE-ELEMENT MODELING OF THERMAL BARRIER COATING REMOVAL PROCESSES

Jian Zhang (11791280) 19 December 2021 (has links)
<div>Thermal barrier coatings (TBCs) made of low thermal conductivity ceramic topcoats have been extensively used in hot sections of gas turbine engines, in aircraft propulsion and power generation applications. TBC damage may occur during gas turbine operations, due to either time- and cycle-dependent degradation phenomena, external foreign object damage, and/or erosion. The damaged TBCs, therefore, need to be removed and repaired during engine maintenance cycles. Although several coating removal practices have been established which are based on the trial-and-error approach, a fundamental understanding of coating fracture mechanisms during the removal process is still limited, which hinders further development of the process.</div><div>The objective of the thesis is to develop a particle-based coating removal modeling framework, using both the smoothed particle hydrodynamics (SPH) and discrete element modeling (DEM) methods. The thesis systematically investigates the processing-property relationships in the TBC removal processes using a modeling approach, thus providing a scientific tool for process design and optimization.</div><div>To achieve the above-mentioned objective, the following research tasks are identified. First a comprehensive literature review of major coating removal techniques is presented in Chapter 2. Chapter 3 discusses an improved SPH model to simulate the high-velocity particle impact behaviors on TBCs. In Chapter 4, the abrasive water jet (AWJ) removal process is modeled using the SPH method. In Chapter 5, an SPH model of the cutting process with regular electron beam physical vapor deposition (EB-PVD) columnar grains is presented. In Chapter 6, a 3D DEM cutting model with regular EB-PVD column grains is discussed. In Chapter 7, a 2D DEM cutting model based on the realistic coating microstructure is developed. Finally, in Chapter 8, based on the particle-based coating removal modeling framework results and analytical solutions, a new fracture mechanism map is proposed, which correlates the processing parameters and coating fracture modes.</div><div>The particle-based modeling results show that: (1) for the SPH impact model, the impact hole penetration depth is mainly controlled by the vertical velocity component. (2) The SPH AWJ simulation results demonstrate that the ceramic removal rate increases with incident angle, which is consistent with the fracture mechanics-based analytic solution. (3) The SPH model with regular EB-PVD columnar grains shows that it is capable to examine the stress evolutions in the coating with columnar grain structures, which is not available if a uniform bulk coating model was used. Additional analysis reveals that the fracture of the columnar grains during the cutting process is achieved through deflection and fracture of the grains, followed by pushing against neighboring grains. (4) The 3D DEM model with regular coating columnar grains shows that, during the coating removal process, a ductile-to-brittle transition is identified which depends on the cutting depth. The transition occurs at the critical cutting depth, which is based on the Griffith fracture criterion. At small cutting depths, the ductile failure mode dominates the cutting process, leading to fine cut particles. As the cutting depth exceeds the critical cutting depth, a brittle failure mode is observed with the formation of chunk-like chips. (5) The 2D DEM model with the realistic coating microstructure shows that there are densification and fracture during the foreign object compaction process, which qualitatively agrees with the experimental observations. (6) The newly proposed coating fracture mechanism map provides guidance to predict three fracture modes, i.e., ductile brittle, and mixed ductile-brittle, as a function of processing parameters, including the cutting depth and cutting speed. The map can be used to determine the processing conditions based on required TBC removal operations: rough cut (brittle mode), semi-finish (mixed ductile-brittle mode), and finish (ductile mode).</div><div><br></div>
512

Growth in a Time of Projected Debt

McCafferty, Jacqueline Marie 26 July 2021 (has links)
No description available.
513

Interleukin-2 Receptor Alpha Nuclear Localization Impacts Vascular Smooth Muscle Cell Function and Phenotype

Dinh, Kristie Nhi 01 September 2021 (has links)
No description available.
514

Fenotypová plasticita cévních hladkosvalových buněk / Phenotypic plasticity of smooth muscle cells

Misárková, Eliška January 2015 (has links)
Vascular smooth muscle cells display a certain level of phenotype plasticity. Under specific conditions fully differentiated cells are able to undergo dedifferentiation and to restart growth and proliferation. An organ culture method is a useful technique for the analysis of dedifferentiation of vascular smooth muscle cells, because it provides an opportunity for studying the changes in cell phenotype. The aim of this study was to investigate the basic contractile characteristics in rat femoral arteries cultured for different time periods (from one to three days). In addition, the effects of fetal bovine serum (FBS), that contains various growth factors and other biological active molecules, on contractile function were studied. We also tried to attenuate cell dedifferentiation by lowering the calcium influx, because calcium is an important second messenger participating in cell growth and proliferation. To achieve this goal we used cultivation with nifedipine, a voltage-dependent calcium channel inhibitor. The cultivation without FBS slightly decreased arterial contractility, whereas the cultivation with FBS decreased arterial contractility considerably. The major change in contractility of arteries cultivated with FBS occurred approximately within 24 hours of cultivation. The cultivation with...
515

Flow-pattern-based heat transfer and pressure drop correlations for condensing refrigerants in smooth tubes

Christians, Marcel 04 July 2008 (has links)
The phase-out of ozone-depleting refrigerants, such as R-12 and R-22, according to the Montreal Protocol of 1987, has provided the incentive to increase the thermal efficiency of current heating and refrigeration systems. The purpose of this study was to increase the accuracy of the predictions of both the heat transfer and pressure drop correlations for condensing refrigerants in the Intermittent flow regime. This was done utilizing a novel method involving the temporal and spectral analysis of the light intensity of the local flow regime, as seen through a sight glass. An experimental setup was designed, built and commissioned specifically for this purpose using refrigerant R-22 and a smooth tube. It was found that the accuracy of the mean heat transfer coefficient predictions increased substantially compared to other leading correlations,particularly at low mass fluxes. In terms of the pressure drop,the predictions also increased in accuracy, and it was found that the time fraction method allows for continuous predictions over flow regime transitions when using local flow-pattern-based pressure drop models. This was previously not possible. / Dissertation (MEng (Mechanical Engineering))--University of Pretoria, 2007. / Mechanical and Aeronautical Engineering / unrestricted
516

Vascular smooth muscle: a target for treatment of aging-induced aortic stiffness

Gao, Yuan Zhao 28 October 2015 (has links)
Cardiovascular disease is the leading cause of human death worldwide. Currently, the prevalence of cardiovascular disease and health care costs associated with its onset continue to increase in both developed and developing societies. Concordant with the need to improve preventative measures is the imperative to develop more effective and efficient remedies for incident cardiovascular pathologies. Increased aortic stiffness with aging has recently emerged as an early, independent, and consistent physiological predictor of cardiovascular disease and represents an attractive target for possible therapeutic options. The success of any biomedical strategy in this regard is incumbent upon comprehension of biological processes and mechanical properties attributable to constituent components within the aortic wall. This dissertation tested the hypothesis that aging-induced changes to smooth muscle maintenance of biomechanical homeostasis within the aorta lead to undesirable increases in stiffness, correlative with increased risk of negative cardiovascular outcomes. Conventionally, mechanical studies and models have identified extracellular matrix as the primary determinant of changes in stiffness, but new research presented here shows that this may not be true. In viable ex vivo preparations of aortic tissue, roughly half of the maximal elastic modulus results from alpha-agonist activation of smooth muscle cells. Investigation of the biochemical interactions that characterize this effect revealed a link between aging and decreased expression of Src, a kinase involved in numerous signaling pathways governing cellular growth and survival, as well as defective regulation of focal adhesions between the smooth muscle cells and extracellular matrix. These findings were integrated into a model of aortic contractility and stiffness that establishes an aging-impaired regulatory complex comprising focal adhesions and non-muscle actin cytoskeleton in vascular smooth muscle cells. A better understanding of the mechanisms underlying this model may motivate the design of potential therapeutics, deliverable to previously overlooked target sites within aortic smooth muscle, and ultimately novel treatments for aging-induced cardiovascular disease. / 2017-10-27T00:00:00Z
517

Mechanical determinants of intact airway responsiveness

Harvey, Brian Christopher 28 October 2015 (has links)
Airway hyperresponsiveness (AHR) is a hallmark of asthma where constriction of airway smooth muscle (ASM) causes excessive airway narrowing. Asthmatics, unlike healthy subjects, cannot prevent or reverse this narrowing by stretching their airways with a deep inspiration (DI). Since stretching of isolated ASM causes dramatic reductions in force generation and asthmatics tend to have stiffer airways, researchers hypothesize that reduced ASM stretching during breathing and DIs results in hyperreactive airways. However, counterintuitively, excised measurement on intact airways show narrowing is minimally reversed by pressure oscillations simulating breathing and DIs. We hypothesized that AHR does not result from reduced capacity to stretch the airways; furthermore, each constituent of the airway wall experiences different strain magnitude during breathing and DIs. To test this, we used an intact airway system which controls transmural pressure (Ptm) to simulate breathing while measuring luminal diameter in response to ASM agonists. An ultrasound system and automated segmentation algorithm were implemented to quantify and compare the ability of Ptm fluctuations to reverse and prevent narrowing in larger (diameter=5.72±0.52mm) relative to smaller airways (diameter=2.92±0.29mm). We found the ability of Ptm oscillations to reverse airway narrowing was proportional to strain imposed on the airway wall. Further, tidal-like breathing Ptm oscillations (5-15cmH2O) after constriction imposed 196% more strain in smaller compared to larger airways (14.6% vs. 5.58%), resulting in 76% greater reversal of narrowing (41.2% vs. 23.4%). However, Ptm oscillations applied before and during constriction resulted in the same steady-state diameter as when Ptm oscillations were applied only after constriction. To better understand these results, we optimized an ultrasound elastography technique utilizing finite element-based image registration to estimate spatial distributions of displacements, strains, and material properties throughout an airway wall during breathing and bronchoconstriction. This required we formulate and solve an inverse elasticity problem to reconstruct the distribution of nonlinear material properties. Strains and material properties were radially and longitudinally heterogeneous, and patterns and magnitudes changed significantly after induced narrowing. Taken together, these data show AHR likely does not emerge due to reduced straining of airways prior to challenge, but remodeling that stiffens airway walls might serve to sustain constriction during an asthmatic-like attack.
518

Analysis of shear strength of rock joints with PFC2D

Lazzari, Elisa January 2013 (has links)
Joints are the main features encountered in rock and sliding of rock blocks on joints is classified as the principal source of instability in underground excavations. In this regard, joints’ peak shear strength is the controlling parameter. However, given the difficulty in estimating it, shear tests are often performed. These are often quite expensive and also time consuming and, therefore, it would be valuable if shear tests could be artificially performed using numerical models. The objective of this study is to prove the possibility to perform virtual numerical shear tests in a PCF2D environment that resemble the laboratory ones. A numerical model of a granite rock joint has been created by means of a calibration process. Both the intact rock microparameters and the smooth joint scale have been calibrated against macroparameters derived from shear tests performed in laboratory. A new parameter, the length ratio, is introduced which takes into account the effective length of the smooth joint compared to the theoretical one. The normal and shear stiffnesses, the cohesion and the tensile force ought to be scaled against the length ratio. Four simple regular joint profiles have been tested in the PFC2D environment. The analysis shows good results both from a qualitative and from a quantitative point of view. The difference in peak shear strength with respect to the one computed with Patton´s formula is in the order of 1% which indicates a good accuracy of the model. In addition, four profiles of one real rough mated joint have been tested. From the scanned surface data, a two-dimensional profile has been extracted with four different resolutions. In this case, however, interlocking of particles along the smooth joint occurs, giving rise to an unrealistic distribution of normal and shear forces. A possible explanation to the problem is discussed based on recent developments in the study of numerical shear tests with PFC2D.
519

The role of vascular smooth muscle Sirtuin-1 in aortic aneurysms

Sulser Ponce de Leon, Sandra 14 March 2022 (has links)
BACKGROUND: Sirtuin-1 (SirT1) is a NAD+-dependent deacetylase essential for maintaining the structure and function of the vasculature. Reduced SirT1 expression and activity has been correlated with the development of vascular diseases, mainly attributed to loss of SirT1’s anti-oxidant and anti-inflammatory beneficial effects. We previously found that deletion of vascular smooth muscle (VSM) SirT1 in mice is associated with increased matrix metalloproteinases (MMPs) and the subsequent development of aortic dissections or ruptures in response to the hypertensive peptide angiotensin II. Based on these previous findings, we hypothesize that loss of SirT1 activity is involved in the pathogenesis of AA. SirT1 is a stress response gene, its deacetylase activity can be impaired by excessive oxidative stress. We postulate that mutating three cysteine residues in SirT1’s catalytic domain can prevent its inactivation by oxidative insults and protect against AA and other vascular diseases. OBJECTIVES: assess the role of SirT1 in a genetic mouse model of Marfan Syndrome that develops AA; (2) Determine design and optimize an enzyme-based colorimetric ELISA to determine SirT1 activity in mouse VSM cells and aortas; (3) Produce an adeno-associated virus (AAV) expressing an oxidant-resistant triple mutant SirT1 in VSM cells that has the potential to mitigate the downstream outcomes derived from alterations in SirT1 activity, such as MMPs activation and development of AA in mgR-/- mice. METHODS: mgR-/- and littermate mgR+/+ (WT) mice aortas and VSM cells were cultured in conditioned medium and the activity of released MMPs was determined by in-gel zymography. For the development of the SirT1 activity assay, we designed a multi-step sandwich ELISA that captures a biotin- and FLAG-tagged acetylated p53 peptide, used as SirT1 deacetylase substrate. Amounts of acetylated and total p53 peptide were sequentially detected with antibodies and colorimetric substrates as index of SirT1 deacetylase activity. AAVs expressing a control or triple mutant SirT1 (3M) were produced in HEK293T cells; VSM cells were then infected with control or 3M AAV and SirT1 protein expression levels were measured by Western Blot. RESULTS: MMPs activity is increased in aortas and VSMC of mgR-/- mice; the first stage of optimization of the SirT1 activity assay successfully defined the assay conditions and experimental design, and it is ready to be optimized with mgR-/- cell and tissue samples; our novel control and SirT1 triple mutant AAVs were produced and successfully overexpressed in VSM cells. / 2024-03-14T00:00:00Z
520

Regulation of Endothelin-1 Production by a Thromboxane a<sub>2</sub> Mimetic in Rat Heart Smooth Muscle Cells

Chua, Chu Chang, Hamdy, Ronald C., Chua, Balvin H.L. 21 August 1996 (has links)
Thromboxane A2 (TXA2) and ET-1 have been known to play important roles in modulating vascular contraction and growth. The present study was undertaken to examine the effect of TXA2 on the induction of endothelin-1 (ET-1) mRNA and protein levels in smooth muscle cells derived from rat heart. U-46619, a stable TXA2 mimetic, superinduced preproET-1 mRNA in the presence of cycloheximide in these cells. This effect could be blocked by SQ-29548, a TXA2/prostaglandin H2 receptor antagonist and by actinomycin D, an RNA synthesis inhibitor. In addition, H7, a protein kinase C inhibitor, could abolish the induction. Transient transfection experiment revealed that the elevated ET-1 mRNA level after U-46619 treatment was a result of the activation of ET-1 gene activity. The elevated ET-1 message level was accompanied by increased ET-1 release into the cultured medium. These results show that the short-lived TXA2 can induce potent and long-lived ET-1. These findings support a potential role for ET-1 in the pathogenesis of coronary atherosclerosis and hypertension evoked by TXA2.

Page generated in 0.3082 seconds