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In the last two decades, the application of $$\hbox {computational fluid dynamics (CFD)}$$ to study the blood flow in intracranial aneurysms has gained popularity since hemodynamics plays a key role in the inception, growth, and rupture of these aneurysms. Although the rupture event has been the main focus of these studies, other researches have shown the importance of hemodynamics also on the initiation and growth of intracranial aneurysms. However, due to the lack of follow up examinations of a single aneurysm case, these studies are scarcer. In this work, two consecutive examinations of an unruptured lateral aneurysm followed up for 5 years were used to investigate the growth of the aneurysm by using $$\hbox {CFD}$$ . By simulating the flow in these two geometries and in the virtually reconstructed hypothetical healthy vasculature, correlations between hemodynamic parameters and the growth of the aneurysm were evaluated. The results showed that the inception of the aneurysm correlated positively with high $$\hbox {wall shear stress (WSS)}$$ and high positive $$\hbox {WSS}$$ spatial gradient, as other studies also suggest. Furthermore, and most importantly, as different biological pathways may explain the influence of hemodynamics on the growth process, the results also pointed out that the hemodynamic effects that drove the subsequent growth of the aneurysm have changed to a combination of low $$\hbox {WSS}$$ and high $$\hbox {oscillatory shear index (OSI)}$$ . Thus, even though the driving mechanisms observed during each period are distinct, they strongly agree with two main theories that currently explain aneurysm inception and growth.

Made available in DSpace on 2021-06-25T10:21:16Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-03-01 Stenting has become an important adjunctive tool for assisting coil embolization in complex-shaped intracranial aneurysms. However, as a secondary effect, stent deployment has been related to both immediate and delayed remodeling of the local vasculature. Recent studies have demonstrated that this phenomenon may assume different roles depending on the treatment stage. However, the extent of such event on the intra-aneurysmal hemodynamics is still unclear; especially when performing two-step stent-assisted coiling (SAC). Therefore, we performed computational fluid dynamics (CFD) analysis of the blood flow in four bifurcation aneurysms focusing on the stent healing period found in SAC as a two-step maneuver. Our results show that by changing the local vasculature, the intra-aneurysmal hemodynamics changes considerably. However, even though changes do occur, they were not consistent among the cases. Furthermore, by changing the local vasculature not only the shear levels change but also the shear distribution on the aneurysm surface. Additionally, a geometric analysis alone can mislead the estimation of the novel hemodynamic environment after vascular remodeling, especially in the presence of mixing streams. Therefore, although the novel local vasculature might induce an improved hemodynamic environment, it is also plausible to expect that adverse hemodynamic conditions might occur. This could pose a particularly delicate condition since the aneurysm surface remains completely exposed to the novel hemodynamic environment during the stent healing period. Finally, our study emphasizes that vascular remodeling should be considered when assessing the hemodynamics in aneurysms treated with stents, especially when evaluating the earlier stages of the treatment process. Department of Mechanical Engineering School of Engineering São Paulo State University (UNESP) Department of Mechanical Engineering Faculty of Engineering Dalhousie University Halifax Interventional Neuroradiology Hospital Israelita Albert Einstein Department of Mechanical Engineering School of Engineering São Paulo State University (UNESP)

The efficacy of integrating the lobed mixer with the core flow deswirling struts to create a single component for reducing the exhaust system length, beyond that attainable through mixer optimization alone, has been investigated. This investigation has been conducted via CFD simulations of a medium-bypass turbofan exhaust system at engine cruise representative conditions. Comparative analysis shows that integration augmented thrust output by about 0.02% while total pressure loss was increased by 3.6%. The aim of the study, to show that this new integrated design would have either minimal impact on or improve exhaust system performance, was confirmed. Comparisons of the flow fields and characteristic quantities downstream of the mixer also showed minimal impact on flow through the nozzle. The deswirling strut was offset by 0.65 Dh axially when integrated with the mixer, therefore it can be concluded that the exhaust system ducting could be reduced in length by this same measure — saving engine weight in the process.

This paper presents a computational investigation of scaled turbofan lobed mixer stiffening tabs at low-speed off-design conditions. Stiffening tabs provide rigidity to the thin lobed mixer by connecting the mixer valley to the more rigid centrebody. Evidence shows that the tabs affect the flow structures of turbofan exhaust systems at off-design core inlet swirl conditions. Observations were made downstream of the mixer in simulations that were carried out with an unstructured RANS solver and the k-ω SST turbulence model. To model off-design conditions, the core flow swirl was increased from axial flow to 10° at the moderate case and 30° at the high swirl case. The tab geometry was shown to perturb some of the less involved flow mixing structures, streamwise vortices near the lobe valley. Simulations of geometries with the tabs displayed more uniform flow throughout the common nozzle with higher thrust outputs; however, these minor improvements are negated by higher total pressure losses., European Conference on Turbomachinery Fluid Dynamics and Hermodynamics, 3-7 April, 2017, Stockholm, Sweden

This paper presents a numerical investigation of lobed mixer performance at experimentally validated low speed conditions and conditions representative of high speed engine operation. The purpose of this study was to first assess and understand how variations in bypass-to-core area ratio (AR) can affect engine performance, then isolate those effects to determine the efficacy of increasing the number of mixer lobes. The area ratio was manipulated via adjustment of the lobe crest and valley radiuses. No other geometric features were altered in any of the 5 mixers studied (12-lobe AR of 3, 2.5 and 3.5, 16-lobe AR of 3 and 18-lobe AR of 3). Results indicate that performance can be affected by area ratio. Low-speed results showed that pressure loss and thrust output were improved at lower area ratios. High speed results showed the opposite. This behavior is believed to be the result of a bypass-to-core momentum ratio difference between the two test conditions. These effects were avoided when studying the number of lobes by maintaining a constant area ratio. Results indicate that adding lobes enhanced exhaust mixing but hampered performance at low speed conditions. No appreciable performance difference was observed at high speed conditions. Fluid viscosity and associated viscous mixing losses are believed to be the parameters at fault for the reduced low-speed performance results.Copyright © 2014 by ASME and Her Majesty the Queen in right of Canada

This paper presents a detailed experimental and computational investigation of the effects of scalloping on the mixing mechanisms of a scaled 12-lobe turbofan mixer. Scalloping was achieved by eliminating approximately 70% of the lobe sidewall area. Measurements were made downstream of the mixer in a co-annular wind tunnel and the simulations were carried out using an unstructured RANS solver, Numeca FINE/Hexa, with k-ω SST model. In the core flow, the swirl angle was varied from 0° to 30°. At high swirl angles, a three-dimensional separation bubble was formed on the lobe's suction surface penetration region and resulted in the generation of a vortex at the lobe valley. The valley vortex quickly dissipated downstream. Most of the swirl was removed by the lobes, but scalloping allowed residual swirl to persist downstream of the mixer. The interaction of the swirling flow and the vortices resulted in improved mixing rates for the scalloped mixer. Inlet swirl up to 10° provided improved mixing rates, reduced pressure loss and thrust loss for both mixers. High inlet swirl resulted in improved mixing but produced higher pressure and thrust losses as compared to the zero swirl case. At high swirl, the scalloped mixer resulted in better mixing and lower pressure losses than the unscalloped mixer, but at the expense of reduced thrust., ASME Turbo Expo 2012: Turbine Technical Conference and Exposition, GT 2012, 11 June 2012 through 15 June 2012, Copenhagen

The three-dimensional piecewise parabolic finite analytic method (PPFAM3D), with vector potential and vorticity as dependent variables, was used to solve the steady three-dimensional (3-D) laminar cavity flow. Predictions were obtained for Reynolds numbers between 100 and 2000. Results for Re = 100, 400, and 1000 are compared with those available in the literature. The overall agreement is excellent. Results are also compared with the two-dimensional PPFAM cavity flow, showing the effect of three-dimensionality on the velocity predictions at the symmetry plane. The PPFAM3D is a robust numerical algorithm requiring no relaxation to produce physically meaningful and numerically converged solutions. It is a promising tool for solving a variety of flow phenomena governed by unsteady second-order partial differential convection-diffusion equations

The present contribution presents the piecewise parabolic finite analytic scheme (PPFAM3D) for the solution of the three-dimensional (3-D) convection-diffusion equation. The calculation domain is divided into cells, but unlike traditional numerical schemes, the PPFAM3D employs the analytical solution of the linearized convection-diffusion equation in each of them. The piecewise parabolic boundary conditions ensure that the coefficients of the numerical scheme are all positive and that their sum is always equal to 1, thus satisfying the Scarborough criterion. Comparison with other finite analytic schemes shows the PPFAM3D to be superior in accuracy and stability. The scheme is shown to be free form numerical diffusion and to accurately include the diffusion effect, even when in highly skewed convection situations

To meet both the qualitative and quantitative requirements of the convective diffusion process, a numerical scheme, called the piecewise parabolic finite analytic method, is established by using a piecewise parabolic function as a boundary condition. This scheme incorporates the local analytic solution of the convective diffusion equation into the numerical scheme. It is free from numerical diffusion and automatically incorporates the convective diffusion effects. In this first part, a detailed study of the piecewise parabolic finite analytic scheme is presented. Initially, the maximum and minimum principle of the convective diffusion equation is used to determine the properties of the piecewise parabolic finite analytic coefficients. Then, based upon the truncation error and properties of the coefficients, the existence, uniqueness, stability, convergence, and error estimates of the numerical solution are obtained. For nonlinear problems the existence and error estimates are also obtained.

Made available in DSpace on 2013-08-12T18:59:15Z (GMT). No. of bitstreams: 0 Previous issue date: 2009-10-01 Made available in DSpace on 2013-09-30T18:12:53Z (GMT). No. of bitstreams: 0 Previous issue date: 2009-10-01 Submitted by Vitor Silverio Rodrigues (vitorsrodrigues@reitoria.unesp.br) on 2014-05-20T13:29:26Z No. of bitstreams: 0 Made available in DSpace on 2014-05-20T13:29:26Z (GMT). No. of bitstreams: 0 Previous issue date: 2009-10-01 Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) This paper presents an immersed boundary formulation for three-dimensional incompressible flows that uses the momentum equation to calculate the Lagrangian force field indirectly imposing the no-slip condition on solid interfaces. In order to test the performance of this methodology the flow past a sphere for Reynolds numbers up to 1,000 have been calculated. Results are compared with numerical data from other authors and empirical correlations available in the literature. The agreement is found to be very good. Dalhousie Univ, Dept Mech Engn, Halifax, NS, Canada São Paulo State Univ UNESP, Dept Mech Engn, Ilha Solteira, SP, Brazil Universidade Federal de Uberlândia (UFU), Sch Mech Engn, BR-38400 Uberlandia, MG, Brazil São Paulo State Univ UNESP, Dept Mech Engn, Ilha Solteira, SP, Brazil

The contribution of flow noise to the radiated acoustic signature of CFAV Quest is of interest. Quest is the research ship used by Defence R&D Canada as a quiet platform. It is difficult to identify the flow noise component in an acoustic ranging so there is interest in predicting the flow noise as a first step towards extracting it from range measurements. Below propulsor cavitation inception speeds, machinery-induced noise dominates. While flow noise does not usually dominate in the presence of machinery-induced noise or propulsor cavitation, it is unclear what fraction of the total noise power flow noise constitutes. A direct numerical simulation for a complex ship geometry was impractical so an alternative approach was sought. An immersed boundary method was used to model the presence of the ship in the flow domain. The unsteady flow field was calculated using a finite volume method over an unstructured Cartesian grid. The flow field around Quest in straight and level flight was calculated at Reynolds numbers between 1.8×108 and 4.3×108 , corresponding to a full-scale speed range of 4 to 10 knots. Results from such flow field predictions become the hydrodynamic sources in the integrals of Lighthill’s acoustic analogy to predict the far-field acoustic signature from the flow past the hull. These far-field predictions consist of computing the propagation and radiation of the hydrodynamic sources. This assumes noise generation and its propagation are decoupled. Under certain circumstances, knowledge of the predicted flow component helps to extract it from a standard acoustic ranging.Copyright © 2007 by Government of Canada

Hydrodynamic hull noise is an important consideration for determining the detection envelope of SONAR domes mounted to surface vessels. In order to model the generation of this noise by a moving ship, a hybrid computational hydro-acoustic modeling methodology has been developed by combining the Lighthill-Curle acoustic analogy with the Numerical Wind Tunnel computational fluid dynamics code. This model has been shown in previous work to significantly over-predict the experimentally observed far field sound of Canadian Forces Auxiliary Vehicle Quest in at-sea acoustic trials. This deficiency is shown to be due in part to neglecting the Lloyd’s Mirror interference effect of the sea surface in the Lighthill-Curle equations. By utilizing a method of images solution to the acoustic analogy to simulate the Lloyd’s Mirror interference, an average sound pressure level improvement of 25 dB was obtained. This solution is compared and contrasted to a model utilizing the Lloyd’s Mirror interference of a simple source, and a normal mode sound propagation model, and is shown to be superior for transmission ranges up to 2 km.

In this second part, the piecewise parabolic finite analytic method ( PPFAM ) is used to discretize the stream function and vorticity equations. The numerical scheme thus obtained is used to simulate a laminar cavity flow and a laminar backward-facing step flow. The numerical simulation shows that the PPFAM is the least affected by numerical diffusion when compared with more traditional numerical schemes and that it can provide a stable and accurate numerical solution for real flow problems. Furthermore, compared with other numerical schemes, the PPFAM produces a stable, convergent, and more accurate solution for the cavity flow with the largest flow rate value, ∥Ψ∥ max , for Reynolds numbers ranging from 10 to 2000. When compared with available experimental and numerical results, the PPFAM produces the largest and most accurate reattachment length for the backward-facing step channel flow.

Made available in DSpace on 2014-06-11T19:23:38Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-10-11Bitstream added on 2014-06-13T19:47:44Z : No. of bitstreams: 1 barbosa_is_me_ilha.pdf: 2113993 bytes, checksum: 6a83caac772eb27b9cfa05adca637b69 (MD5) Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) O jato impingente constitui um problema clássico da mecânica dos fluidos, cuja dinâmica é fortemente dependente do número de Reynolds, do grau de confinamento e de outros parâmetros que caracterizam geometricamente o problema. No presente trabalho, jatos impingentes com diferentes níveis de confinamento foram investigados numericamente. Para tanto, utilizou-se um código computacional baseado no método dos volumes finitos, que resolve as equações de Navier-Stokes escritas em coordenadas cilíndricas bidimensionais. Com a finalidade de diminuir custos computacionais, a simetria azimutal do problema foi considerada na definição do domínio de cálculo. O disco frontal, sobre o qual incide o jato, foi modelado de duas formas distintas: a) com o auxílio do método da fronteira imersa baseado no modelo físico virtual ou b) mediante a imposição de viscosidade infinita na região por ele ocupada. A comparação entre resultados destas simulações permitiram avaliar a eficiência do método da fronteira imersa, no que diz respeito ao tempo de cálculo e à sua capacidade para representar fisicamente o problema abordado. Além disso, testes de performance do código foram também realizados utilizando-se diferentes esquemas de transporte advectivo - power law, upwind, central difference e híbrido. Simulações foram realizadas variando-se o diâmetro de ambos os discos (impingente e confinante), o afastamento entre eles e o número de Reynolds do escoamento. Para a representação de escoamentos com números de Reynolds elevados, a metodologia de simulação de grandes escalas foi implementada, com o uso da modelagem clássica de Smagorinsky e da função de amortecimento de van Driest The impingent jet is a classical problem of fluid mechanics, whose dynamics is strongly dependent on the Reynolds number, the confinement level and other parameters that characterize the geometrically problem. In this study impingent jets with different confinement levels have been numerically investigated. For this purpose, an in-house computational code based on finite volume method, which solves the Navier-Stokes equations written in two-dimensional cylindrical coordinates has been used. To save computational costs the problem has been treated as azimuthally symmetrical. The frontal disc was modeled in two different ways: a) by means of an immersed boundary method based on the virtual physical model b) by imposing infinite viscosity in the region occupied by the disc. Comparison among results has allowed to evaluate the performance of the immersed boundary method in terms of computational costs and ability to represent the physical features of the flow. Besides, tests of the code performance have also been carried out employing different advective schemes, i.e. power law, upwind, central difference, and hybrid. Simulations have been performed by varying the diameter of both disks (impingent and confining), the gap between them, and the Reynolds number. To represent turbulent flows, Large Eddy Simulation has been implemented in the code, using the classical Smagorinsky model and the damping function of van Driest