The arteriovenous fistula (AVF) is characterized by enhanced blood flow and is the most widely used vascular access for chronic haemodialysis (Sivanesan et al., 1998). A large proportion of the AVF late failures are related to local haemodynamics (Sivanesan et al., 1999a).As in AVF, blood flow dynamics plays an important role in growth, rupture, and surgical treatment of aneurysm. Several techniques have been used to study the flow patterns in simplified models of vascular anastomose and aneurysm. In the present investigation, Computational Fluid Dynamics (CFD) is used to analyze the flow patterns in AVF and aneurysm through the velocity waveform obtained from experimental surgeries in dogs (Galego et al., 2000), as well as intra-operative blood flow recordings of patients with radiocephalic AVF (Sivanesan et al., 1999b) and physiological pulses (Aires, 1991), respectively. The flow patterns in AVF for dog and patient surgeries data are qualitatively similar. Perturbation, recirculation and separation zones appeared during cardiac cycle, and these were intensified in the diastole phase for the AVF and aneurysm models. The values of wall shear stress presented in this investigation of AVF and aneurysm models oscillated in the range that can both cause damage to endothelial cells and develop atherosclerosis.
Bessa, Kleiber Lima de Redução de arrasto por adição de polímeros em escoamento pulsátil laminar e turbulento em leitos arteriais caudais de ratos normotensos e hipertensos e tubos rígidos / K.L. de Bessa.
Finned tubes exist in diverse geometric configurations, usually the convective heat transfer coefficient is unknown and approximated as that for a geometry similar to the actual one. This paper presents a numerical investigation about the convective heat transfer in a horizontal finned tube. Ten geometric configurations were considered, which differed on the distance between fins, and two materials were analysed for the fins: aluminium and carbon steel. Eleven different values were considered for the temperature difference between the base of the tube and air. Therefore, a total of one hundred and ten different conditions were studied, for each material. The flow regime was laminar. The analysis showed that approximating the convective heat transfer of the finned tube as that for a tube without fins, for which correlations are available in the literature, can lead to significant errors. The maximum difference verified was for the spacing between fins equal to 2 mm, for which the convective heat transfer coefficient was about seven times lower than that for the tube without fins. For spacings equal to 6 mm and 8 mm, associated to the maximum heat transfer rate, the convective heat transfer coefficient for the tube without fins was about 30% to 50% higher than that for the finned tube. The common assumption of uniform fins surface temperature was also evaluated for the two considered materials the fins were made of. The results showed that for the steel fins the approximation leads to an error of about 10%, while for aluminium it is only about 3%. Results that allow a better understanding of the physical phenomena related to the occurrence of the optimum spacing between fins, which maximize the heat transfer, are also presented and analysed.
Drag is one of the main energy-dissipating phenomena in engineering applications. Drag-reduction mechanisms have been studied to reduce this cost. Superhydrophobic surfaces (SHS) have high water repellency and have been studied as an alternative mechanism for reducing drag. The high level of repellency is due to the hierarchical structures in the micro- and nano-scales, making these surfaces able to trap air layers that impose the condition of slipping. The present work investigated the phenomenon of drag reduction on surfaces made of Sylgard® 184 elastomer and modified by low-pressure plasma treatments. Atmospheres with 40% Argon and 60% Acetylene, and 20% Argon and 80% Acetylene were used, varying the treatment times from 10 to 15 min of exposure to Acetylene. The surface, morphological and chemical modifications were confirmed by XPS and AFM analyses, showing the impression of a rough structure on the nanometric scale with deposition of chemical elements from the gas plasma. Furthermore, the obtained SHS showed lower resistance to flow, tested by the imposition of flow in channels.
"Due to the great risk of contamination by leaking in underground fuel storage tanks (UST) of gas stations all over the world, the establishment of effective monitoring methods in this environment is extremely necessary. Among UST monitoring methods the tightness test is one of the most effective ones in identifying leaks, it can be done in two different ways, either wet part test or dry part test. But while both of the tests are permitted, they show a great difference in rigorousness, when it comes to approving or not a tank. This study envisions to deeply explore the causes of the difference of rigorousness between both tests, and discover ways in which simulations can approach the real situation. The research allowed us to identify not only the cause of such difference in rigor, but also to establish a constant that approximates the theory to the real situation."
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