Tesis: "Unsteady computations"

1

Hellström, Fredrik. "Numerical computations of the unsteady flow in turbochargers". Doctoral thesis, KTH, Strömningsfysik, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-12742.

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Turbocharging the internal combustion (IC) engine is a common technique to increase the power density. If turbocharging is used with the downsizing technique, the fuel consumption and pollution of green house gases can be decreased. In the turbocharger, the energy of the engine exhaust gas is extracted by expanding it through the turbine which drives the compressor by a shaft. If a turbocharged IC engine is compared with a natural aspirated engine, the turbocharged engine will be smaller, lighter and will also have a better efficiency, due to less pump losses, lower inertia of the system and less friction losses. To be able to further increase the efficiency of the IC engine, the understanding of the highly unsteady flow in turbochargers must be improved, which then can be used to increase the efficiency of the turbine and the compressor. The main objective with this thesis has been to enhance the understanding of the unsteady flow in turbocharger and to assess the sensitivity of inflow conditions on the turbocharger performance. The performance and the flow field in a radial turbocharger turbine working under both non-pulsatile and pulsatile flow conditions has been assessed by using Large Eddy Simulation (LES). To assess the effects of different operation conditions on the turbine performance, different cases have been considered with different perturbations and unsteadiness of the inflow conditions. Also different rotational speeds of the turbine wheel were considered. The results show that the turbine cannot be treated as being quasi-stationary; for example,the shaft power varies for different frequencies of the pulses for the same amplitude of mass flow. The results also show that perturbations and unsteadiness that are created in the geometry upstream of the turbine have substantial effects on the performance of the turbocharger. All this can be summarized as that perturbations and unsteadiness in the inflow conditions to the turbine affect the performance. The unsteady flow field in ported shroud compressor has also been assessed by using LES for two different operational points. For an operational point near surge, the flow field in the entire compressor stage is unsteady, where the driving mechanism is an unsteadiness created in the volute. For an operational point far away from surge, the flow field in the compressor is relatively much more steady as compared with the former case. Although the stable operational point exhibits back-flow from the ported shroud channels, which implies that the flow into the compressor wheel is disturbed due to the structures that are created in the shear layer between the bulk flow and the back-flow from the ported shroud channels.
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2

Wu, Jiongyang. "Filter-based modeling of unsteady turbulent cavitating flow computations". [Gainesville, Fla.] : University of Florida, 2005. http://purl.fcla.edu/fcla/etd/UFE0011587.

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3

Hellström, Fredrik. "Numerical computations of the unsteady flow in a radial turbine". Licentiate thesis, KTH, Mechanics, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4660.

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Non-pulsatile and pulsatile flow in bent pipes and radial turbine has been assessed with numerical simulations. The flow field in a single bent pipe has been computed with different turbulence modelling approaches. A comparison with measured data shows that Implicit Large Eddy Simulation (ILES) gives the best agreement in terms of mean flow quantities. All computations with the different turbulence models qualitatively capture the so called Dean vortices. The Dean vortices are a pair of counter-rotating vortices that are created in the bend, due to inertial effects in combination with a radial pressure gradient. The pulsatile flow in a double bent pipe has also been considered. In the first bend, the Dean vortices are formed and in the second bend a swirling motion is created, which will together with the Dean vortices create a complex flow field downstream of the second bend. The strength of these structures will vary with the amplitude of the axial flow. For pulsatile flow, a phase shift between the velocity and the pressure occurs and the phase shift is not constant during the pulse depending on the balance between the different terms in the Navier- Stokes equations.

The performance of a radial turbocharger turbine working under both non-pulsatile and pulsatile flow conditions has also been investigated by using ILES. To assess the effect of pulsatile inflow conditions on the turbine performance, three different cases have been considered with different frequencies and amplitude of the mass flow pulse and different rotational speeds of the turbine wheel. The results show that the turbine cannot be treated as being quasi-stationary; for example, the shaft power varies with varying frequency of the pulses for the same amplitude of mass flow. The pulsatile flow also implies that the incidence angle of the flow into the turbine wheel varies during the pulse. For the worst case, the relative incidence angle varies from approximately −80° to +60°. A phase shift between the pressure and the mass flow at the inlet and the shaft torque also occurs. This phase shift increases with increasing frequency, which affects the accuracy of the results from 1-D models based on turbine maps measured under non-pulsatile conditions.

For a turbocharger working under internal combustion engine conditions, the flow into the turbine is pulsatile and there are also unsteady secondary flow components, depending on the geometry of the exhaust manifold situated upstream of the turbine. Therefore, the effects of different perturbations at the inflow conditions on the turbine performance have been assessed. For the different cases both turbulent fluctuations and different secondary flow structures are added to the inlet velocity. The results show that a non-disturbed inlet flow gives the best performance, while an inflow condition with a certain large scale eddy in combination with turbulence has the largest negative effect on the shaft power output.

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4

De, Rango Stan. "Implicit Navier-Stokes computations of unsteady flows using subiteration methods". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1996. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp04/MQ51537.pdf.

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5

Hellström, Fredrik. "Numerical computations of the unsteady flow in a radial turbine /". Stockholm : Mekanik, Kungliga Tekniska högskolan, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4660.

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6

Nöid, Lovisa. "CFD computations of hydropower plant intake flow using unsteady RANS". Thesis, KTH, Kraft- och värmeteknologi, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-161894.

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At the intake of hydropower plants, air-core vortex formation is known to cause severe damage. In order to study how to prevent and reduce the origin of the vortex, Vattenfall has built a scale model of the Akkats hydropower plant dam, where scale testing is possible. This thesis work consists of discerning whether Computational Fluid Dynamics (CFD) in terms of solving the Unsteady Reynolds Average Navier-Stokes equations (URANS) can be used as a complement to scale testing. For this work, the RNG k-epsilon turbulence model is chosen, and the flow field is solved with implicit time discretization using a pressure-based solver, for three different inlet flow conditions. Despite significant differences in the inflow of these three cases, the resulting flow fields are surprisingly similar. A main result is that no vortex is formed in any of the cases. The cause of this is discussed, but the number of possible answers is large. The main purpose of the report has therefore become to lay the foundation for further research. Amongst the top priorities in parameters to investigate lies the choice of turbulence model, the surface height, the pressure discretization scheme and to perform calculations on a more expensive mesh.
Virvlar som uppstår vid intaget i vattenkraftverk kan orsaka stora skador. För att kunna göra studier om hur man bäst motverkar virveln och förhindrar dess uppkomst, har Vattenfall AB byggt en småskalig modell av dammen vid Akkats vattenkraftverk. Det här arbetet behandlar frågeställningen huruvida Computational Fluid Dynamics (CFD) med lösning av ekvationerna för Unsteady Reynolds Average Navier-Stokes (URANS) kan användas som ett komplement till dessa modell-tester. I det här arbetet har turbulensmodellen RNG k−epsilon valts och flödesfältet löses för tre olika tillstånd för flödet vid inloppet, med hjälp av implicit tidsdiskretisering tillsammans med en tryckbaserad ekvationslösare. Trots betydande skillnader för inflödet för dessa tre fall är de resulterande flödesfälten överraskande lika. Ett huvudresultat är att ingen virvel formas för någon av dessa fall. Anledningen till detta har diskuterats, men antalet möjliga anledningar är många. Huvudsyftet med den här rapporten har därför blivit att lägga en grund för framtida efterforskningar på området. Några av de viktigaste parametrarna att undersöka är valet av turbulensmodell, höjden på vattenytan, tryckdiskretiserings-schema samt att genomföra beräkningar för en finare mesh.

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7

Reid, Terry Vincent. "A Computational Approach For Investigating Unsteady Turbine Heat Transfer Due To Shock Wave Impact". Diss., Virginia Tech, 1998. http://hdl.handle.net/10919/25983.

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The effects of shock wave impact on unsteady turbine heat transfer are investigated. A numerical approach is developed to simulate the flow physics present in a previously performed unsteady wind tunnel experiment. The windtunnel experiment included unheated and heated flows over a cascade of highly loaded turbine blades. After the flow over the blades was established, a single shock with a pressure ratio of 1.1 was introduced into the wind tunnel test section. A single blade was equipped with pressure transducers and heat flux microsensors. As the shock wave strikes the blade, time resolved pressure, temperature, and heat transfer data were recorded.
Ph. D.

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8

Price, Jennifer Lou. "Unsteady Measurements and Computations on an Oscillating Airfoil with Gurney Flaps". NCSU, 2001. http://www.lib.ncsu.edu/theses/available/etd-20010713-170959.

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Price, Jennifer Lou. Unsteady Measurements and Computations on an Oscillating Airfoil with Gurney Flaps. (Under the direction of Dr. Ndaona Chokani)The effect of a Gurney flap on an unsteady airfoil flow is experimentally and computationally examined. In the experiment, the details of the unsteady boundary layer events on the forward portion of the airfoil are measured. In the computation, the features of the global unsteady flow are documented and correlated with the experimental observations.The experiments were conducted in the North Carolina State University subsonic wind tunnel on an oscillating airfoil at pitch rates of 65.45 degrees/sec and 130.9 degrees/sec. The airfoil has a NACA0012 cross-section and is equipped with a 1.5% or 2.5% chord Gurney flap. The airfoil is tested at Reynolds numbers of 96,000, 169,000 and 192,000 for attached and light dynamic stall conditions. An array of surface-mounted hot-film sensors on the forward 25% chord of the airfoil is used to measure the unsteady laminar boundary layer separation, transition-to-turbulence, and turbulent reattachment. In parallel with the experiments incompressible Navier-Stokes computations are conducted for the light dynamic stall conditions on the airfoil with a 2.5%c Gurney flap at a Reynolds number of 169,000.The experimental measurements show that the effect of the Gurney flap is to move the separation, transition and reattachment forward on the airfoil. This effect is more marked during the airfoil's pitch-down than during pitch-up. The computational results verify these observations, and also show that the shedding of the dynamic stall vortex is delayed. Thus the adverse effects of dynamic stall are mitigated by the Gurney flap.

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9

Bodin, Olle. "Numerical Computations of Internal Combustion Engine related Transonic and Unsteady Flows". Licentiate thesis, Stockholm : Mekanik, Kungliga Tekniska högskolan, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-9945.

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10

Novacek, Thomas Hans. "Computations of unsteady forces and moments for a transonic rotor with jet actuation". Thesis, Massachusetts Institute of Technology, 1997. http://hdl.handle.net/1721.1/50300.

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11

Ekaterinaris, John A. "Steady and unsteady internal flow computations via the solution of the compressible navier stokes equations for low mach numbers". Diss., Georgia Institute of Technology, 1987. http://hdl.handle.net/1853/12366.

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12

Maduta, Robert [Verfasser], Cameron [Akademischer Betreuer] Tropea, Suad [Akademischer Betreuer] Jakirlić y Amsini [Akademischer Betreuer] Sadiki. "An eddy-resolving Reynolds stress model for unsteady flow computations: development and application / Robert Maduta. Betreuer: Cameron Tropea ; Suad Jakirlic ; Amsini Sadiki". Darmstadt : Universitäts- und Landesbibliothek Darmstadt, 2013. http://d-nb.info/1108094279/34.

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13

Jacquet, Clément. "Investigation par Calcul numérique de la région en « S » des courbes caractéristiques d’une turbine-pompe réversible". Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAI055.

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Les Stations de Transfert de l’Énergie par Pompage (STEP) munies de turbines-pompes réversibles de type Francis (TP) permettent de stocker et de restituer de grandes quantités d’énergie avec des rendements très élevés. Celles-ci apparaissent comme un moyen viable d’assurer la réactivité et la stabilité du réseau électrique vis-à-vis de l’augmentation croissante des sources d’énergie renouvelables intermittentes. Pour répondre aux nouveaux besoins en régulation du réseau électrique, la technologie actuelle des STEP doit être adaptée. Accroître la réactivité requiert d’optimiser les procédures de démarrage et d’arrêt des machines. Dans les quadrants turbine, turbine-frein et pompe inverse, les TP de haute chute ont des courbes caractéristiques présentant la forme d’un « S ». Cette forme particulière peut engendrer des coups de béliers lors des phases d’arrêts d’urgences, exposant les conduites à de sévères surpressions et dépressions. De plus, pour ces points de fonctionnement associés au « S » les écoulements sont fortement instationnaires et induisent des fluctuations de pression responsables de chargements dynamiques sur les parties mécaniques. Les objectifs de ce travail sont la modélisation et la compréhension des phénomènes hydrauliques complexes liés au « S ». Des simulations numériques instationnaires sont réalisées en utilisant le modèle de turbulence SAS-SST. Moins couteux que les modèles LES, ce modèle permet de résoudre d’une partie du spectre turbulent et ainsi de prendre en compte les principaux effets instationnaires. Trois configurations de turbine-pompe de même vitesse spécifique (nq=40) sont étudiées. Une seule (grande) ouverture de directrices est retenue pour chaque configuration. Les points de fonctionnement considérés couvrent une large gamme de conditions d’opération, allant du fonctionnement en régime continu (rendement élevé) jusqu’au débit nul, en passant par le point d’emballement. Les résultats des calculs sont comparés aux mesures expérimentales. La bonne corrélation entre valeurs numériques et expérimentales valide la pertinence du modèle numérique. Les analyses des performances de la machine et des fluctuations de pression permettent d’identifier les régions de l’écoulement associées aux principales instabilités. Enfin, les visualisations de l’écoulement couplées à une étude des mécanismes de dissipation de l’énergie mettent en évidence les principaux phénomènes à l’origine de la forme en « S » des courbes caractéristiques
Pumped Storage Plants (PSP) using reversible Francis pump-turbines can store large amounts of energy with high efficiency. They therefore appear as a cost-effective tool to provide stability to the energy production network against the intermittency of renewable energy sources. Nevertheless, start-up and shutdown procedures still need to be improved to increase the reactivity of the PSP. Reversible high head pump-turbines have characteristic curves that exhibit an S-Shape in the turbine, turbine-brake and reverse pump quadrants. This S-Shape may be responsible for surge transient phenomena in the case of an emergency shutdown (for large guide vane opening). Moreover, for operating point in the S-Shape region, the flow is highly unsteady and leads to a high level of pressure fluctuations and strong dynamic loadings on the mechanical parts. The objective of the current work is to perform a comprehensive study of the complex hydraulic phenomena linked with the S-Shape. Unsteady numerical computations are carried out using the turbulence model SAS-SST. Such a model can resolve part of the turbulent spectrum while maintaining affordable computational cost. It therefore offers an interesting alternative to more expensive LES computations. Three different configurations of pump-turbine with the same specific speed (nq=40) are investigated. Several operating conditions from optimal efficiency point to zero discharge condition for a given large guide vane opening are studied. Numerical results show good agreement with the experimental data. Accuracy of the numerical model is thus assessed. The investigations of the global performances of the pump-turbine and the pressure pulsations help to identify the region of the flow which are associated with the main instabilities. Finally, flow visualizations linked with the analysis of the mechanisms of energy dissipation reveal the major flow phenomena at the origin of the S-Shape

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14

Ning, Wei. "Computation of unsteady flow in turbomachinery". Thesis, Durham University, 1998. http://etheses.dur.ac.uk/4819/.

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Unsteady flow analysis has been gradually introduced in turbomachinery design systems to improve machine performance and structural integrity. A project on computation of unsteady flows in turbomachinery has been carried out. A quasi 3-D time-linearized Euler/Navier-Stokes method has been developed for unsteady flows induced by the blade oscillation and unsteady incoming wakes, hi this method, the unsteady flow is decomposed into a steady flow plus a harmonically varying unsteady perturbation. The coefficients of the linear perturbation equation are formed from steady flow solutions. A pseudo-time is introduced to make both the steady flow equation and the linear unsteady perturbation equation time-independent. The 4-stage Runge-Kutta time-marching scheme is implemented for the temporal integration and a cell-vertex scheme is used for the spatial discretization. A 1-D/2-D nonreflecting boundary condition is applied to prevent spurious reflections of outgoing waves when solving the perturbation equations. The viscosity in the unsteady Navier- Stokes perturbation equation is frozen to its steady value. The present time-linearized Euler/Navier-Stokes method has been extensively validated against other well- developed linear methods, nonlinear time-marching methods and experimental data. Based upon the time-linearized method, a novel quasi 3-D nonlinear harmonic Euler/Navier-Stokes method has been developed. In this method, the unsteady flow is divided into a time-averaged flow plus an unsteady perturbation. Time-averaging produces extra nonlinear "unsteady stress" terras in the time-averaged equations and these extra terras are evaluated from unsteady perturbations. Unsteady perturbations are obtained by solving a first order harraonic perturbation equation, while the coefficients of the perturbation equation are forraed from time-averaged solutions. A strong coupling procedure is applied to solve the time-averaged equation and the unsteady perturbation equation simultaneously in a pseudo-time domain. An approximate approach is used to linearize the pressure sensors in artificial smoothing

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15

Pishevar, Isfahani Ahmadreza. "High-order computation of unsteady-state compressible flow". Thesis, Imperial College London, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.309226.

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16

Grechy, Lorenza. "Computational studies of unsteady flow in arterio-venous fistulae". Thesis, Imperial College London, 2017. http://hdl.handle.net/10044/1/61338.

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Arterio-venous fistulae (AVF) are surgically created vascular connections between an artery and a vein in patients with End Stage Renal Disease, and are regarded as the 'gold standard' method of vascular access for patiets who require haemodialysis. However, up to 60% of AVF do not mature, and hence fail, as a result of various pathologies such as Intimal Hyperplasia (IH). Highly oscillatory flow patterns are one of the factors implicated in the development of IH, and they will be studied in this thesis. Previous studies have investigated the effect of arterial curvature on blood flow in AVF using idealised (planar) AVF configurations and non-pulsatile inflow conditions. These studies are extended here to more realistic non-planar AVF configurations with pulsatile inflow conditions. Results show that, converse to previous findings, forming an AVF by connecting a vein onto the outer curvature of an arterial bend does not, necessarily, suppresses unsteady flow in the artery. Subsequently, an optimisation process for idealised 3-dimensional geometries is introduced to identify an optimal configuration that suppress high-frequency fluctuations under steady inflow. Performance of the optimal configuration is then successfully verified with a fully pulsatile simulation. A novel medical device for maintaining AVF in the optimal shape is also proposed, and a first animal experiment is reported as a proof of principle study, which led to promising preliminary results. Finally, realistic AVF geometries are reconstructed from 60 MRI scans. Ultrasound measurements of the flow were also collected, together with details of patient outcomes, so that computational simulations could investigate the relationships between geometrical features, flow unsteadiness and AVF maturation. Results show high variability of the geometric parameters between AVF formed in the upper arm and AVF formed in the wrist. Also, some geometric parameters in upper arm AVF resulted to be correlated with flow stabilisation and AVF maturation. However results were found to be not statistically significant.

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17

Premachandran, Sarah. "Advanced computational modelling for aircraft landing gear unsteady aerodynamics". Thesis, University of Southampton, 2017. https://eprints.soton.ac.uk/418073/.

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High-fidelity turbulence modelling techniques have been applied to simulate the flow field around a simplified landing gear bay geometry. Three-dimensional Detached Eddy Simulation (DES) simulations have been performed for a cavity with the front 2/3 covered, as is representative of a nose landing gear bay. Resonant modes were observed in the shear layer, with frequencies in good agreement with the Rossiter cavity modes. The side-walls of the cavity, when compared to quasi-two-dimensional simulations with infinite span, were found to suppress the presence of acoustic modes inside the cavity, as well as causing a greater degree of breakdown in the shear layer, and changing the dominant resonant modes. The geometry was varied to incorporate a single strut, and (separately) open rear doors, to test their separate contributions to the flow field in the landing gear bay. Both were found to produce small, high-frequency vortex structures, which interacted with the cavity shear layer and caused higher resonant modes to be present than with the clean cavity. A Large Eddy Simulation (LES) methodology has been developed and improved using the in-house solver software. An improved subgrid-scale model has been implemented, and the sensitivity of the solution to a variety of settings has been tested. Using a method for efficiently simulating flat-plate turbulence by tripping the flow using a small step, realistic wall-bounded turbulence has been modelled, with mean and turbulent quantities in good agreement with the literature for flat plate boundary layer flows. The best-practice guidelines from this study were then applied to a turbulent flat plate upstream of the cavity with LES. The boundary layer turbulence structures were found to disrupt the coherence of the shear layer vortices, and lengthwise acoustic modes dominated inside the cavity in most cases. The sensitivity of this baseline simulation to several different parameters was investigated. These included the condition and thickness of the upstream boundary layer, the geometry around the lip of the cover, the turbulence modelling technique, and the spanwise length of the domain. The most significant difference was obtained by adding side walls, which was found to promote the development of shear layer resonance. Lower-mode tones were observed, with the associated pressure fluctuations being imposed and amplified inside the cavity.

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18

Boyd, David Douglas Jr. "Rotor/Fuselage Unsteady Interactional Aerodynamics: A New Computational Model". Diss., Virginia Tech, 1999. http://hdl.handle.net/10919/28591.

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A new unsteady rotor/fuselage interactional aerodynamics model has been developed. This model loosely couples a Generalized Dynamic Wake Theory (GDWT) to a Navier-Stokes solution procedure. This coupling is achieved using a newly developed unsteady pressure jump boundary condition in the Navier-Stokes model. The new unsteady pressure jump boundary condition models each rotor blade as a moving pressure jump which travels around the rotor azimuth =and is applied between two adjacent planes in a cylindrical, non-rotating grid. Comparisons are made between predictions using this new model and experiments for an isolated rotor and for a coupled rotor/fuselage configuration.
Ph. D.

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19

Unadkat, Jay. "Applications and computation of unsteady boundary layers over finite domains". Thesis, University of Manchester, 2017. https://www.research.manchester.ac.uk/portal/en/theses/applications-and-computation-of-unsteady-boundary-layers-over-finite-domains(60ce1e8b-a52b-49f7-aa74-b5d64b37d8c3).html.

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The main focus of this work was to investigate the nature of unsteady boundary-layer development over finite domains, with the behaviour of the boundary layer on a rotating sphere in an unbounded, rotating fluid used as a prototype. The sphere and its surrounding fluid are assumed to be initially rotating as a solid body, and the evolution of a boundary layer on the sphere is analysed in cases where the sphere has been smoothly slowed, or brought to a state of rotation in an opposite sense to its initial conditions. It may be seen that a characteristic property of this flow is that the boundary layer is bi-directional; over most of the streamwise domain for the flow, whether the flow is positive or negative in the streamwise coordinate direction depends on the transverse location being considered. This fact leads to challenges in the numerical evaluation of the flow field due to the parabolic nature of the boundary-layer equations. A further consideration is the implication that these regions of reversed flow cause the flow field to contain minima and maxima in the streamwise velocity component. This has been shown in a little-known study by Cowley et al. (1985) to cause the boundary layer to become susceptible to asymptotically short-scale perturbations with large frequencies. The unsteady boundary layer on a rotating sphere under these conditions is consequently shown to be extremely challenging to compute numerically. It is also found that using local approximations at the ends of the finite domain, which in the case of the sphere are the pole and equator, to investigate the two-dimensional boundary layer can cause difficulties, as in some cases there exist steady, spatial perturbations to a boundary-layer state which introduce short spatial scales. The instabilities and other features analysed in this work are framed largely in the context of the rotating sphere, but the causes of the phenomena are found to be sufficiently generic that they may be observed in other physical contexts. To demonstrate this, the shallow katabatic flow down a cooled slope is briefly investigated, and the above mathematical features are again uncovered.

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20

Gordon, David R. "Computational unsteady flow dynamics : oscillating flow about a circular cylinder". Thesis, Monterey, California. Naval Postgraduate School, 1991. http://hdl.handle.net/10945/28053.

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21

Feszty, Daniel. "Numerical simulation and analysis of high-speed unsteady spiked body flows". Thesis, University of Glasgow, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.368552.

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22

Abdo, Mohammed. "Theoretical and computational analysis of airfoils in steady and unsteady flows". Thesis, McGill University, 2004. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=84871.

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This dissertation studies three aspects of airfoil flows: (i) second-order theoretical solutions of airfoils in steady flows; (ii) unsteady solutions for oscillating flexible airfoils; and (iii) numerical analysis of airfoil flows at low Reynolds numbers.
The first part presents simple and efficient analytical solutions in closed form for the velocity and pressure distributions on airfoils of arbitrary shapes in steady flows, which are obtained using special singularities in the expression of the fluid velocity. A second-order accurate method is first developed for airfoils in inviscid incompressible flows to simultaneously solve the symmetric and anti-symmetric flow components defined by coupled boundary conditions. Then, the method is extended to take into account the viscous and compressibility effects on the pressure distribution. The resulting solutions were found to be in very good agreement with the available exact solutions (for specific airfoils), and with numerical and experimental results at various Mach and Reynolds numbers and moderate angles of attack.
The second part presents a new method of solution for the analysis of unsteady incompressible flows past oscillating rigid and flexible airfoils. The method has been successfully validated by comparison with the results obtained by Theodorsen and by Postel and Leppert for rigid airfoil and aileron oscillations in translation and rotation. The aerodynamic stiffness, damping and virtual mass contributions are specifically determined, as required in the aeroelastic studies. In all cases studied, this method led to very efficient and simple analytical solutions in closed form.
The third part presents an efficient numerical method for the incompressible flows past airfoils at low Reynolds numbers, which are of interest for micro-aircraft applications. The present analysis is based on a pseudo-time integration method using artificial compressibility to accurately solve the Navier-Stokes equations. Solutions are obtained with this method for airfoils at various incidences and very low Reynolds numbers between 400 and 6000. A detailed analysis is presented for the influence of the Reynolds number, incidence and airfoil shape on the pressure distribution, lift and drag coefficients. The flow separation is especially studied; the separation and reattachment positions are compared for various airfoil shapes, incidences and Reynolds numbers.

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23

Prosser, Daniel T. "Advanced computational techniques for unsteady aerodynamic-dynamic interactions of bluff bodies". Diss., Georgia Institute of Technology, 2015. http://hdl.handle.net/1853/53899.

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Interactions between the aerodynamics and dynamics of bluff bodies are important in many engineering applications, including suspension bridges, tall buildings, oil platforms, wind turbine towers, air drops, and construction with cranes. In the rotorcraft field, bluff bodies are commonly suspended underneath the vehicle by tethers. This approach is often the only practical way to deliver a payload in a reasonable amount of time in disaster relief efforts, search-and-rescue operations, and military operations. However, currently a fundamental understanding of the aerodynamics of these bluff bodies is lacking, and accurate dynamic simulation models for predicting the safe flying speed are not available. In order to address these shortcomings, two main advancements are presented in this thesis. The aerodynamics of several three-dimensional canonical bluff bodies are examined over a range of Reynolds numbers representative of wind-tunnel-scale to full-scale models. Numerical experiments are utilized, with a focus on uncertainty analysis and validation of the computations. Mean and unsteady forces and moments for these bluff bodies have been evaluated, and empirical models of the shear layer characteristics have been extracted to quantify the behaviors and provide predictive capability. In addition, a physics-based reduced-order simulation model has been developed for bluff bodies. The physics-based approach is necessary to ensure that the predicted behavior of new configurations is accurate, and it is made possible by the breakthroughs in three-dimensional bluff body aerodynamics presented in this thesis. The integrated aerodynamic forces and moments and dynamic behavior predicted by model are extensively validated with data from wind tunnels, flight tests, and high-fidelity computations. Furthermore, successful stability predictions for tethered loads are demonstrated. The model is applicable to the simulation of any generic bluff body configuration, is readily extensible, and has low computational cost.

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24

Lusardi, Christopher (Christopher Dean). "Characterization of unsteady loading due to impeller-diffuser interaction in centrifugal compressors". Thesis, Massachusetts Institute of Technology, 2012. http://hdl.handle.net/1721.1/72869.

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Thesis (S.M.)--Massachusetts Institute of Technology, Computation for Design and Optimization Program, 2012.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 89-90).
Time dependent simulations are used to characterize the unsteady impeller blade loading due to imipeller-diffuser interaction in centrifugal compressor stages. The capability of simulations are assessed by comparing results against unsteady pressure and velocity measurements in the vaneless space. Simulations are shown to be adequate for identifying the trends of unsteady impeller blade loading with operating and design parameters. However they are not sufficient for predicting the absolute magnitude of loading unsteadiness. Errors of up to 14% exist between absolute values of flow quantities. Evidence suggests that the k - e turbulence model used is inappropriate for centrifugal compressor flow and is the significant source of these errors. The unsteady pressure profile on the blade surface is characterized as the sum of two superimposing pressure components. The first component varies monotonically along the blade chord. The second component can be interpreted as an acoustic wave propagating upstream. Both components fluctuate at the diffuser vane passing frequency, but at a different phase angle. The unsteady loading is the sum of the fluctuation amplitude of each component minus a value that is a function of the phase relationship between the pressure component fluctuations. Simulation results for different compressor designs are compared. Differences observed are primarily attributed to the amplitude of pressure fluctuation on the pressure side of the blade and the wavelength of the pressure disturbance propagating upstream. Lower pressure side pressure fluctuations are associated with a weaker pressure non-uniformity at the diffuser inlet as a result of a lower incidence angle into the diffuser. The wavelength of the pressure disturbance propagating upstream sets the domain on the blade surface in which the phase relationship between pressure component fluctuations is favorable. A longer wavelength increases the domain over which this phase relationship is such that the amplitude of unsteadiness is reduced.
by Christopher Lusardi.
S.M.

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25

Aoussou, Jean Philippe. "An iterative pressure-correction method for the unsteady incompressible Navier-Stokes Equation". Thesis, Massachusetts Institute of Technology, 2016. http://hdl.handle.net/1721.1/104554.

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Thesis: S.M., Massachusetts Institute of Technology, Computation for Design and Optimization Program, 2016.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 53-59).
The pressure-correction projection method for the incompressible Navier-Stokes equation is approached as a preconditioned Richardson iterative method for the pressure- Schur complement equation. Typical pressure correction methods perform only one iteration and suffer from a splitting error that results in a spurious numerical boundary layer, and a limited order of convergence in time. We investigate the benefit of performing more than one iteration. We show that that not only performing more iterations attenuates the effects of the splitting error, but also that it can be more computationally efficient than reducing the time step, for the same level of accuracy. We also devise a stopping criterion that helps achieve a desired order of temporal convergence, and implement our method with multi-stage and multi-step time integration schemes. In order to further reduce the computational cost of our iterative method, we combine it with an Aitken acceleration scheme. Our theoretical results are validated and illustrated by numerical test cases for the Stokes and Navier-Stokes equations, using Implicit-Explicit Backwards Difference Formula and Runge-Kutta time integration solvers. The test cases comprises a now classical manufactured solution in the projection method literature and a modified version of a more recently proposed manufactured solution.
by Jean Philippe Aoussou.
S.M.

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26

Al-Sharif, Sharaf. "Computation of unsteady and non-equilibrium turbulent flows using Reynolds stress transport models". Thesis, University of Manchester, 2010. https://www.research.manchester.ac.uk/portal/en/theses/computation-of-unsteady-and-nonequilibrium-turbulent-flows-using-reynolds-stress-transport-models(935dbd20-b049-4b62-9e1c-eebb261675e5).html.

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In this work the predictive capability of a number of Reynolds stress transport(RST) models was first tested in a range of non-equilibrium homogeneous flows, comparisons being drawn with existing direct numerical simulation (DNS) results and physical measurements. The cases considered include both shear and normally strained flows, in some cases with a constant applied strain rate, and in others where this varied with time. Models were generally found to perform well in homogeneous shear at low shear rates, but their performance increasingly deteriorated at higher shear rates. This was attributed mainly to weaknesses in the pressure-strain rate models, leading to over-prediction of the shear stress component of the stress anisotropy tensor at high shear rates. Performance in irrotational homogeneous strains was generally good, and was more consistent over a much wider range of strain rates. In the experimental plane strain and axisymmetric contraction cases, with time-varying strain rates, there was evidence of an accelerated dissipation rate generation. Significant improvement was achieved through the use of an alternative dissipation rate generation term, Pε , in these cases, suggesting a possible route for future modelling investigation. Subsequently, the models were also tested in the inhomogeneous case of pulsating channel flow over a wide range of frequencies, the reference for these cases being the LES of Scotti and Piomelli (2001). A particularly challenging feature in this problem set was the partial laminarisation and re-transition that occurred cyclically at low and, to a lesser extent, intermediate frequencies. None of the models tested were able to reproduce correctly all of the observed flow features, and none returned consistently superior results in all the cases examined. Finally, models were tested in the case of a plane jet interacting with a rectangular dead-end enclosure. Two geometric configurations are examined, corresponding a steady regime, and an intrinsically unsteady regime in which periodic flow oscillations are experimentally observed (Mataoui et al., 2003). In the steady case generally similar flow patterns were returned by the models tested, with some differences arising in the degree of downward deflection of the impinging jet, which in turn affected the level of turbulence energy developing in the lower part of the cavity. In the unsteady case, only two of the models tested, a two-equation k-ε model and an advanced RST model, correctly returned purely periodic solutions. The other two RST models, based on linear pressure-strain rate terms, returned unsteady flow patterns that exhibited complex oscillations with significant cycle-to-cycle variations. Unfortunately, the limited availability of reliable experimental data did not allow a detailed quantitative examination of model performance.

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27

Fang, Kuan-Chieh. "Unsteady Incompressible Flow Analysis Using C-Type Grid with a Curved Branch Cut". University of Cincinnati / OhioLINK, 2000. http://rave.ohiolink.edu/etdc/view?acc_num=ucin962376293.

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28

Mai-Cao, Lan. "Meshless radial basis function method for unsteady incompressible viscous flows". University of Southern Queensland, Faculty of Engineering and Surveying, 2008. http://eprints.usq.edu.au/archive/00006227/.

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[Abstract]This thesis reports the development of new meshless schemes for solving timedependent partial differential equations (PDEs) and for the numerical simulation of some typical unsteady incompressible viscous flows.The new numerical schemes are based on the Idirect/Integrated Radial Basis Function Network (IRBFN) method which is fully meshless as no element-typemesh is required. The IRBFN method has been successfully applied to solve time-independent elliptic PDEs, some steady fluid flows and recently unsteady Navier-Stokes equations in streamfunction-vorticiy formulation using simple time integration methods (e.g. first-order backward Euler method). The main objective of the present research is to devise and implement meshless numerical schemes for unsteady problems in computational fluid dynamics where notonly the accuracy but also the efficiency and stability of the numerical schemes are of primary concerns. In addition, the effects of different parameters of theIRBFN method on the accuracy, stability and efficiency of the proposed numerical schemes are extensively studied in this research.As the first step in extending the IRBFN method to various types of timedependent PDEs, two numerical schemes combining the IRBFN method with high-order time stepping algorithms are developed for solving parabolic, hyperbolic,and advection-diffusion equations. Sensitivity analysis of the method to point density, time-step size and shape parameter are extensively performed to study the influence of these parameters to the overall accuracy of the method.A further extension of the IRBFN method for incompressible fluid flows with moving interfaces, especially for passive transport problems is accomplished in this research with a novel meshless approach in which the level set methodis coupled with the the IRBFN method for capturing moving interfaces in an ambient fluid flow without any explicit computation of the actual front location.Another contribution of this research is the development of two new meshless schemes based on the IRBFN method for the numerical simulation of unsteady incompressible viscous flows governed by the Navier-Stokes equations. In the new schemes, the splitting approach is used to deal with the momentum equation and the incompressibility constraint in a segregated manner. Numerical experiments on the new schemes in terms of accuracy and stability are performed for verification purposes.Finally, a novel meshless hybrid scheme is developed in this research to numerically simulate interfacial flows in which the motion and deformation of the interface between the two immiscible fluids are fully captured. Unlike the passive transport problems mentioned above where the influence of the moving interface on the surrounding fluid is ignored, the interfacial flows are studied here with the surface tension taken into account. As a result, a two-way interaction between the moving interface and the ambient flow is fully investigated.All numerical schemes developed in this research are verified through a wide range of transient problems including different kinds of time-dependent PDEs,typical passive transport problems and interfacial flows as well as unsteady incompressible viscous flows governed by Navier-Stokes equations.

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29

Lindquist, Dana Rae. "Computation of unsteady transonic flowfields using shock capturing and the linear perturbation Euler equations". Thesis, Massachusetts Institute of Technology, 1992. http://hdl.handle.net/1721.1/13090.

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30

Eller, David. "On an Efficient Method fo Time-Domain Computational Aeroelasticity". Doctoral thesis, KTH, Farkost och flyg, 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-584.

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The present thesis summarizes work on developing a method for unsteady aerodynamic analysis primarily for aeroelastic simulations. In contrast to widely used prediction tools based on frequency-domain representations, the current approach aims to provide a time-domain simulation capability which can be readily integrated with possibly nonlinear structural and control system models. Further, due to the potential flow model underlying the computational method, and the solution algorithm based on an efficient boundary element formulation, the computational effort for the solution is moderate, allowing time-dependent simulations of complex configurations. The computational method is applied to simulate a number of wind-tunnel experiments involving highly flexible models. Two of the experiments are utilized to verify the method and to ascertain the validity of the unsteady flow model. In the third study, simulations are used for the numerical optimization of a configuration with multiple control surfaces. Here, the flexibility of the model is exploited in order to achieve a reduction of induced drag. Comparison with experimental results shows that the numerical method attains adequate accuracy within the inherent limits of the potential flow model. Finally, rather extensive aeroelastic simulations are performed for the ASK 21 sailplane. Time-domain simulations of a pull-up maneuver and comparisons with flight test data demonstrate that, considering modeling and computational effort, excellent agreement is obtained. Furthermore, a flutter analysis is performed for the same aircraft using identified frequency-domain loads. Results are found to deviate only slightly from critical speed and frequency obtained using an industry-standard aeroelastic analysis code. Nevertheless, erratic results for control surface hinge moments indicate that the accuracy of the present method would benefit from improved control surface modeling and coupled boundary layer analysis.
QC 20100531

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31

Mello, Olympio Achilles de Faria. "An improved hybrid navier-stokes/full-potential method for computation of unsteady compressible viscous flows". Diss., Georgia Institute of Technology, 1994. http://hdl.handle.net/1853/12026.

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32

Stein, Alexander. "Computational analysis of stall and separation control in centrifugal compressors". Diss., Georgia Institute of Technology, 2000. http://hdl.handle.net/1853/11884.

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33

Ingraham, Daniel. "External Verification Analysis: A Code-Independent Approach to Verifying Unsteady Partial Differential Equation Solvers". University of Toledo / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1430491745.

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Ke, Junhao. "Direct numerical simulation of an unsteady natural convection boundary layer". Thesis, University of Sydney, 2021. https://hdl.handle.net/2123/24382.

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The present thesis considers the flow mechanics of a natural convection boundary layer (NCBL) along an isothermally heated vertical wall. Large scale direct numerical simulations are carried out to investigate the laminar stability and the turbulent mechanics of the flow. In this study, a computationally efficient temporal framework, where periodic boundary conditions are imposed in the homogeneous directions, has been used to develop a temporally evolving (instead of a spatially evolving) flow. The stability properties of the laminar temporally developing NCBL, with Prandtl number 0.71, are numerically investigated in the configuration of a temporally evolving parallel flow. By assuming the timescales of the laminar base flow and the perturbations are separate, the instantaneous linear stability of the flow is investigated by an eigenvalue approach with a quasi-steady assumption, whereby the unsteady base flow is frozen in time. Temporal responses of the discrete perturbation modes are numerically obtained by solving the two-dimensional linearised disturbance equations using a `frozen' base flow as an initial-value problem at various 〖Gr〗_δ. The resultant amplification rates of the discrete modes are compared with the quasi-steady eigenvalue analysis, and both two-dimensional and three-dimensional direct numerical simulations (DNS) of the temporally evolving flow. The amplification rate predicted by the linear theory compares well with the direct numerical simulation solutions up to a transition point. The extent of the linear regime where the perturbations linearly interact with the base flow is thus identified. The value of the transition 〖Gr〗_δ, according to the three-dimensional DNS results, is dependent on the initial perturbation amplitude. Beyond the transition point, the DNS results diverge from the linear stability predictions as nonlinear mechanisms become important. For the turbulent NCBL flows, three-dimensional direct numerical simulations (DNS) with different initial conditions were carried out to investigate the turbulent mechanics up to 〖Gr〗_δ=1.2×10^8. The turbulent NCBL is examined in two distinct regions separately: a near-wall boundary-layer-like region and an outer bulk plume-like region. In the near-wall region, a constant heat flux layer (see also in George & Capp, 1979; Ho ̈lling & Herwig, 2005) and a constant forcing layer are identified for the turbulent NCBL. In the close vicinity of the wall (y^+<5) a laminar-like sublayer has developed, and the temperature profile follows the linear relation, consistent with the studies of spatially developing flows (Tsuji & Nagano. 1988a); whereas such a linear relation cannot be observed for the velocity profile due to the extra buoyancy. Similar to earlier studies (Ng et al., 2017), this buoyancy effect is shown to asymptotically approach zero with increasing 〖Gr〗_δ. Further away from the wall (y^+>50), there is a log-law region for the mean temperature profile as reported by Tsuji & Nagano (1988a). In this region, the turbulent length scale which characterises mixing scales linearly with distance from the wall once 〖Gr〗_δ is sufficiently large. By taking the varying buoyancy into consideration with the robust mixing length model, a modified log-law for the mean velocity profile for y^+>50 is proposed. The effect of the initialization is shown to persist until relatively high 〖Gr〗_δ as a result of slow adjustment of the buoyancy (temperature) profile. Once these differences are accounted for, our two DNS cases and the spatially developing data of Tsuji & Nagano (1988a) show excellent agreement with the modified log-law. Beyond a wall-normal distance δ_i, the NCBL can be characterised as an outer bulk plume-like region. This region is found to be well described by an self-similar integral model with profile coefficients (cf. van Reeuwijk & Craske, 2015) which are 〖Gr〗_δ-independent after 〖Gr〗_δ=10^7. The entrainment coefficient for the plume-like region is investigated by decomposing contributions from shear production, buoyancy, viscosity and boundary conditions. For the turbulent NCBL, the entrainment coefficient is found to be mainly affected by the buoyancy in the flow and appears constant beyond 〖Gr〗_δ=10^7. Solution to the self-similar integral model are analytically obtained by solving ordinary differential equations (ODE) with profile coefficients empirically obtained from the DNS results. The DNS results also suggest that the wall heat transfer of the NCBL is directly related to the top-hat scales which characterise the plume-like region. The Nusselt number of the NCBL is found to follow 〖Nu〗_δ∝〖Gr〗_δ^0.373, similar to the observations by Ng et al. (2017) for a vertical NCBL in differentially heated slot and He et al. (2012) for a Rayleigh--Be ̀nard convection. This power-law correlation is higher than the empirical 1/3-power-law correlation reported for spatially developing NCBLs at lower 〖Gr〗_δ, but appears consistent with the ultimate heat transfer with a logarithmic correction suggested by Grossmann & Lohse (2011). Using empirical correlations for the wall shear stress, it is shown that the buoyancy effect in the near-wall region would become negligible, and the near-wall mechanics of the NCBL would become similar to that of a neutrally buoyant turbulent boundary layer above 〖Gr〗_δ>2×10^9 for the present study.

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35

Gonc, L. Oktay. "Computation Of External Flow Around Rotating Bodies". Phd thesis, METU, 2005. http://etd.lib.metu.edu.tr/upload/12605985/index.pdf.

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A three-dimensional, parallel, finite volume solver which uses Roe'
s upwind flux differencing scheme for spatial and Runge-Kutta explicit multistage time stepping scheme for temporal discretization on unstructured meshes is developed for the unsteady solution of external viscous flow around rotating bodies. The main aim of this study is to evaluate the aerodynamic dynamic stability derivative coefficients for rotating missile configurations. Arbitrary Lagrangian Eulerian (ALE) formulation is adapted to the solver for the simulation of the rotation of the body. Eigenvalues of the Euler equations in ALE form has been derived. Body rotation is simply performed by rotating the entire computational domain including the body of the projectile by means of rotation matrices. Spalart-Allmaras one-euqation turbulence model is implemented to the solver. The solver developed is first verified in 3-D for inviscid flow over two missile configurations. Then inviscid flow over a rotating missile is tested. Viscous flux computation algorithms and Spalarat-Allmaras turbulence model implementation are validated in 2-D by performing calculations for viscous flow over flat plate, NACA0012 airfoil and NLR 7301 airfoil with trailing edge flap. The ALE formulation is validated in 2-D on a rapidly pitching NACA0012 airfoil. Afterwards three-dimensional validation studies for viscous, laminar and turbulent flow calculations are performed on 3-D flat plate problem. At last, as a validation test case, unsteady laminar and turbulent viscous flow calculations over a spinning M910 projectile configuration are performed. Results are qualitatively in agreement with the analytical solutions, experimental measurements and previous studies for steady and unsteady flow calculations.

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36

Sundararaj, Vivekanandhan. "Computational fluid dynamic analysis of unsteady compressible flow through a single cylinder internal combustion engine /". Available to subscribers only, 2006. http://proquest.umi.com/pqdweb?did=1240704871&sid=3&Fmt=2&clientId=1509&RQT=309&VName=PQD.

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Thesis (M.S.)--Southern Illinois University Carbondale, 2006.
"Department of Mechanical Engineering and Energy Processes." Includes bibliographical references (leaves 171-174). Also available online.

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37

Linton, Daniel. "A Hybrid Computational Fluid Dynamics Method for Unsteady Simulation of the Ship-Helicopter Dynamic Interface". Thesis, University of Sydney, 2020. https://hdl.handle.net/2123/22894.

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Helicopters operating from ships are exposed to turbulent airwakes which can determine ship-helicopter operating limits. During concurrent operations rotor-rotor interactions add to the complexity of the aerodynamics. Computational fluid dynamics solvers are able to predict these aerodynamics from first principles with the aid of turbulence-resolving approaches such as detached eddy simulation. Although it is possible to create body-fitted grids to resolve the rotor blades and move them, the fuselage, and the ship relative to one another, this is a computationally expensive and labour intensive method. To avoid this expense and while accurately predicting unsteady loading, a time accurate rotor model has been coupled to a Navier-Stokes solver by introducing momentum source terms to the governing equations. A novel coupling algorithm that accounts for the effects of unsteady aerodynamics as well as the induced velocity of the wake has been developed and validated. The coupled rotor model predicts performance, thrust and torque distributions, and unsteady aerodynamic loading of isolated and interacting rotors. A time accurate wake can also be generated by the model. The method requires far fewer grid points to resolve the rotor than a body-fitted grid and grids can be generated automatically. Navier-Stokes simulation of the ship airwake is a complex task and many of the parameters of importance for such simulations have been identified in the literature. A study of grid convergence of velocity spectra and analysis of finite sample error have been performed to add to this knowledge. A method for objectively assessing the finite sample error and determining the minimum sample time required to reach a certain error has been applied to ship airwake simulations for the first time and a minimum level of grid refinement for resolved velocity spectra suggested. The ship airwake and rotor model have been combined for ship-helicopter dynamic interface simulations of single helicopter operations and concurrent helicopter operations involving five rotors. These simulations demonstrate the ability of the method to predict the aerodynamic factors that influence ship-helicopter operating limits and, to the best of our knowledge, contain more vehicles than any previously published dynamic interface simulations.

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38

Reilly, Daniel Oliver. "Inlet Distortion Effects on the Unsteady Aerodynamics of a Transonic Fan Stage". Wright State University / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=wright1482139741887976.

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39

Djayapertapa, Lesmana. "A computational method for coupled aerodynamic-structural calculations in unsteady transonic flow with active control study". Thesis, University of Bristol, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.341506.

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40

Abdulqadir, Sherwan Ahmed. "Turbulence modelling for horizontal axis wind turbine rotor blades". Thesis, University of Manchester, 2017. https://www.research.manchester.ac.uk/portal/en/theses/turbulence-modeling-for-horizontal-axis-wind-turbine-rotor-blades(2536b213-3a0c-4977-ac39-916a9fce98d2).html.

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This Thesis aims to assess the reliability of turbulence models in predicting the flow fields around the horizontal axis wind turbine (HAWT) rotor blades and also to improve our understanding of the aerodynamics of the flow field around the blades. The simulations are validated against data from the NREL/NASA Phase VI wind turbine experiments. The simulations encompass the use of fourteen turbulence models including low-and high-Reynolds-number, linear and non-linear eddy-viscosity models and Reynolds stress models. The numerical procedure is based on the finite-volume discretization of the 3D unsteady Reynolds-Averaged Navier-Stokes equations in an inertial reference frame with the sliding mesh technique to follow the motion of the rotor blades. Comparisons of power coefficient, normalised thrust, local surface pressure coefficients (CP) and the radial variation of the section average of normal force coefficients with published experimental data over a range of tip-speed ratios, lead to the identification of the turbulence models that can reliably reproduce the values of the key performance indicators. The main contributions of this study are in establishing which RANS models can produce quantitatively reliable simulations of wind turbine flows and in presenting the flow evolution over a range of operating conditions. At low (relative to the blade tip speed) wind speeds the flow over the blade surfaces remains attached and all RANS models return the correct values of key performance coefficients. At higher wind speeds there is circumferential flow separation over the downwind surface of the blade, which eventually spreads over the entire surface, Moreover, within the separation bubble the centrifugal force pumps the flow outwards, which at the higher wind speeds suppresses the formation of the classical tip vortices. More refined RANS models which do not rely on the linear effective viscosity approximation generally lead to more reliable predictions over this range of higher wind speeds. In particular the Gibson-Launder version of the Reynolds stress transport model and the high-Re versions of the Lien et al non-linear k-ε produce consistently reliable simulations over the entire range of wind speeds. By contrast some popular linear effective viscosity models, like the SST (k-ω) and the v^2-f, perform the poorest over this complex flow range. Finally all RANS models are also able to predict the dominant (lowest) frequency of the pressure fluctuations and the non-linear effective viscosity models, the Launder and Shima version of RSM and the SST are also able to return some of the higher frequencies measured.

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41

Shyam, Vikram. "3-D Unsteady Simulation of a Modern High Pressure Turbine Stage: Analysis of Heat Transfer and Flow". The Ohio State University, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=osu1258931807.

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42

Shaw, Ryan Phillip. "Application of Subjective Logic to Vortex Core Line Extraction and Tracking from Unsteady Computational Fluid Dynamics Simulations". BYU ScholarsArchive, 2012. https://scholarsarchive.byu.edu/etd/2989.

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Presented here is a novel tool to extract and track believable vortex core lines from unsteady Computational Fluid Dynamics data sets using multiple feature extraction algorithms. Existing work explored the possibility of extracting features concurrent with a running simulation using intelligent software agents, combining multiple algorithms' capabilities using subjective logic. This work modifies the steady-state approach to work with unsteady fluid dynamics and is designed to work within the Concurrent Agent-enabled Feature Extraction concept. Each agent's belief tuple is quantified using a predefined set of information. The information and functions necessary to set each component in each agent's belief tuple is given along with an explanation of the methods for setting the components. This method is applied to the analyses of flow in a lid-driven cavity and flow around a cylinder, which highlight strengths and weaknesses of the chosen algorithms and the potential for subjective logic to aid in understanding the resulting features. Feature tracking is successfully applied and is observed to have a significant impact on the opinion of the vortex core lines. In the lid-driven cavity data set, unsteady feature extraction modifications are shown to impact feature extraction results with moving vortex core lines. The Sujudi-Haimes algorithm is shown to be more believable when extracting the main vortex core lines of the cavity simulation while the Roth-Peikert algorithm succeeding in extracting the weaker vortex cores in the same simulation. Mesh type and time step is shown to have a significant effect on the method. In the curved wake of the cylinder data set, the Roth-Peikert algorithm more reliably detects vortex core lines which exist for a significant amount of time. the method was finally applied to a massive wind turbine simulation, where the importance of performing feature extraction in parallel is shown. The use of multiple extraction algorithms with subjective logic and feature tracking helps determine the expected probability that an extracted vortex core is believable. This approach may be applied to massive data sets which will greatly reduce analysis time and data size and will aid in a greater understanding of complex fluid flows.

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43

Dobes, Jiri. "Numerical algorithms for the computation of steady and unsteady compressible flow over moving geometries: application to fluid-structure interaction". Doctoral thesis, Universite Libre de Bruxelles, 2007. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/210640.

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This work deals with the development of numerical methods for compressible flow simulation with application to the interaction of fluid flows and structural bodies.

First, we develop numerical methods based on multidimensional upwind residual distribution (RD) schemes. Theoretical results for the stability and accuracy of the methods are given. Then, the RD schemes for unsteady problems are extended for computations on moving meshes. As a second approach, cell centered and vertex centered finite volume (FV) schemes are considered. The RD schemes are compared to FV schemes by means of the 1D modified equation and by the comparison of the numerical results for scalar problems and system of Euler equations. We present a number of two and three dimensional steady and unsteady test cases, illustrating properties of the numerical methods. The results are compared with the theoretical solution and experimental data.

In the second part, a numerical method for fluid-structure interaction problems is developed. The problem is divided into three distinct sub-problems: Computational Fluid Dynamics, Computational Solid Mechanics and the problem of fluid mesh movement. The problem of Computational Solid Mechanics is formulated as a system of partial differential equations for an anisotropic elastic continuum and solved by the finite element method. The mesh movement is determined using the pseudo-elastic continuum approach and solved again by the finite element method. The coupling of the problems is achieved by a simple sub-iterative approach. Capabilities of the methods are demonstrated on computations of 2D supersonic panel flutter and 3D transonic flutter of the AGARD 445.6 wing. In the first case, the results are compared with the theoretical solution and the numerical computations given in the references. In the second case the comparison with experimental data is presented.

Doctorat en Sciences de l'ingénieur
info:eu-repo/semantics/nonPublished

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44

Fernelius, Mark H. "Experimental and Computational Analysis of an Axial Turbine Driven by Pulsing Flow". BYU ScholarsArchive, 2017. https://scholarsarchive.byu.edu/etd/6548.

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Pressure gain combustion is a form of combustion that uses pressure waves to transfer energy and generate a rise in total pressure during the combustion process. Pressure gain combustion shows potential to increase the cycle efficiency of conventional gas turbine engines if used in place of the steady combustor. However, one of the challenges of integrating pressure gain combustion into a gas turbine engine is that a turbine driven by pulsing flow experiences a decrease in efficiency. The interaction of pressure pulses with a turbine was investigated to gain physical insights and to provide guidelines for designing turbines to be driven by pulsing flow. An experimental rig was built to compare steady flow with pulsing flow. Compressed air was used in place of combustion gases; pressure pulses were created by rotating a ball valve with a motor. The data showed that a turbine driven by full annular pressure pulses has a decrease in turbine efficiency and pressure ratio. The average decrease in turbine efficiency was 0.12 for 10 Hz, 0.08 for 20 Hz, and 0.04 for 40 Hz. The turbine pressure ratio, defined as the turbine exit total pressure divided by the turbine inlet total pressure, ranged from 0.55 to 0.76. The average decrease in turbine pressure ratio was 0.082 for 10 Hz, 0.053 for 20 Hz, and 0.064 for 40 Hz. The turbine temperature ratio and specific turbine work were constant. Pressure pulse amplitude, not frequency, was shown to be the main cause for the decrease in turbine efficiency. Computational fluid dynamics simulations were created and were validated with the experimental results. Simulations run at the same conditions as the experiments showed a decrease in turbine efficiency of 0.24 for 10 Hz, 0.12 for 20 Hz, and 0.05 for 40 Hz. In agreement with the experimental results, the simulations also showed that pressure pulse amplitude is the driving factor for decreased turbine efficiency and not the pulsing frequency. For a pulsing amplitude of 86.5 kPa, the efficiency difference between a 10 Hz and a 40 Hz simulation was only 0.005. A quadratic correlation between turbine efficiency and corrected pulse amplitude was presented with an R-squared value of 0.99. Incidence variation was shown to cause the change in turbine efficiency and a correlation between corrected incidence and corrected amplitude was established. The turbine geometry was then optimized for pulsing flow conditions. Based on the optimization results and observations, design recommendations were made for designing turbines for pulsing flow. The first design recommendation was to weight the design of the turbine toward the peak of the pressure pulse. The second design recommendation was to consider the range of inlet angles and reduce the camber near the leading edge of the blade. The third design recommendation was to reduce the blade turning to reduce the wake caused by pulsing flow. A new turbine design was created and tested following these design recommendations. The time-accurate validation simulation for a 10 Hz pressure pulse showed that the new turbine decreased the entropy generation by 35% and increased the efficiency by 0.04 (5.4%).

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45

Venâncio, Stênio de Sousa. "Modelo computacional para análise de transiente hidráulico em canais". Universidade de São Paulo, 2003. http://www.teses.usp.br/teses/disponiveis/18/18138/tde-16022009-191816/.

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Este trabalho representa a continuidade de estudos envolvendo a problemática dos escoamentos com superfície livre, contemplando a análise do fenômeno transiente em canais, a partir do modelo matemático unidimensional de Saint-Venant. Para tanto, é desenvolvido um modelo computacional em linguagem FORTRAN, capaz de avaliar o comportamento do escoamento não permanente. As equações hidrodinâmicas completas são discretizadas por um esquema completamente implícito de diferenças finitas e aplicadas no modelo computacional para a avaliação de dois casos. O modelo é previamente testado para um caso simples, cujos resultados são analisados viabilizando o modelo. No primeiro caso, o modelo é aplicado ao canal de alimentação da Usina Hidrelétrica Monjolinho em São Carlos-SP, para avaliar a necessidade de vertedouro quando se dá o fechamento brusco da turbina, e a ocorrência da entrada de ar na mesma quando da sua abertura repentina. No segundo caso, procurou-se avaliar o desenvolvimento do escoamento no Canal do Trabalhador, responsável pelo abastecimento da cidade de Fortaleza-CE. Com manobras de enchimento e esvaziamento do sistema, é possível determinar o tempo de antecedência de liga-desliga do sistema de recalque a partir das alturas dágua e velocidades de ocorrência, permitindo também a automação para as operações de controle. Em ambos os casos o modelo reproduziu resultados que ilustram com coerência os conceitos pré-estabelecidos, constituindo numa ferramenta útil para análise do fenômeno transiente nos escoamentos em condutos livres.
This work presents a computational model developed in FORTRAN language for the study of unsteady open-channel flows with the use of Saint-Venant one-dimensional equation. The discretization of hydrodynamic equations are presented in a completely implicit method of finite differences and applied in the model for the investigation of two cases, besides the one used previously to test the model. In the first case, the model is applied for a channel that supplies the Monjolinho hydroelectric plant in Sao Carlos SP, aiming to evaluate the need of a spillway when the turbine is closed and the flow abruptly stopped, as well as the occurrence of air entering the turbine when it is opened instantaneously. In the second case, the model simulates the development of the flow in the Trabalhador channel, responsible for the water supply in the city of Fortaleza - CE, in order to make possible the automation of operational control, based on data of flow velocity and water level. In both cases the model is presented as a useful tool for the analysis of unsteady open-channel flows, showing results and coherency with theory.

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46

Mokulys, Thomas [Verfasser]. "On the Development and Application of Accurate Numerical Models for the Computation of Steady and Unsteady Flowfields in Turbomachinery / Thomas Mokulys". Aachen : Shaker, 2007. http://d-nb.info/1163609722/34.

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Drofelnik, Jernej. "Massively parallel time- and frequency-domain Navier-Stokes Computational Fluid Dynamics analysis of wind turbine and oscillating wing unsteady flows". Thesis, University of Glasgow, 2017. http://theses.gla.ac.uk/8284/.

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Increasing interest in renewable energy sources for electricity production complying with stricter environmental policies has greatly contributed to further optimisation of existing devices and the development of novel renewable energy generation systems. The research and development of these advanced systems is tightly bound to the use of reliable design methods, which enable accurate and efficient design. Reynolds-averaged Navier-Stokes Computational Fluid Dynamics is one of the design methods that may be used to accurately analyse complex flows past current and forthcoming renewable energy fluid machinery such as wind turbines and oscillating wings for marine power generation. The use of this simulation technology offers a deeper insight into the complex flow physics of renewable energy machines than the lower-fidelity methods widely used in industry. The complex flows past these devices, which are characterised by highly unsteady and, often, predominantly periodic behaviour, can significantly affect power production and structural loads. Therefore, such flows need to be accurately predicted. The research work presented in this thesis deals with the development of a novel, accurate, scalable, massively parallel CFD research code COSA for general fluid-based renewable energy applications. The research work also demonstrates the capabilities of newly developed solvers of COSA by investigating complex three-dimensional unsteady periodic flows past oscillating wings and horizontal-axis wind turbines. Oscillating wings for the extraction of energy from an oncoming water or air stream, feature highly unsteady hydrodynamics. The flow past oscillating wings may feature dynamic stall and leading edge vortex shedding, and is significantly three-dimensional due to finite-wing effects. Detailed understanding of these phenomena is essential for maximising the power generation efficiency. Most of the knowledge on oscillating wing hydrodynamics is based on two-dimensional low-Reynolds number computational fluid dynamics studies and experimental testing. However, real installations are expected to feature Reynolds numbers of the order of 1 million and strong finite-wing-induced losses. This research investigates the impact of finite wing effects on the hydrodynamics of a realistic aspect ratio 10 oscillating wing device in a stream with Reynolds number of 1.5 million, for two high-energy extraction operating regimes. The benefits of using endplates in order to reduce finite-wing-induced losses are also analyzed. Three-dimensional time-accurate Reynolds-averaged Navier-Stokes simulations using Menter's shear stress transport turbulence model and a 30-million-cell grid are performed. Detailed comparative hydrodynamic analyses of the finite and infinite wings highlight that the power generation efficiency of the finite wing with sharp tips for the considered high energy-extraction regimes decreases by up to 20 %, whereas the maximum power drop is 15 % at most when using the endplates. Horizontal-axis wind turbines may experience strong unsteady periodic flow regimes, such as those associated with the yawed wind condition. Reynolds-averaged Navier-Stokes CFD has been demonstrated to predict horizontal-axis wind turbine unsteady flows with accuracy suitable for reliable turbine design. The major drawback of conventional Reynolds-averaged Navier-Stokes CFD is its high computational cost. A time-step-independent time-domain simulation of horizontal-axis wind turbine periodic flows requires long runtimes, as several rotor revolutions have to be simulated before the periodic state is achieved. Runtimes can be significantly reduced by using the frequency-domain harmonic balance method for solving the unsteady Reynolds-averaged Navier-Stokes equations. This research has demonstrated that this promising technology can be efficiently used for the analyses of complex three-dimensional horizontal-axis wind turbine periodic flows, and has a vast potential for rapid wind turbine design. The three-dimensional simulations of the periodic flow past the blade of the NREL 5-MW baseline horizontal-axis wind turbine in yawed wind have been selected for the demonstration of the effectiveness of the developed technology. The comparative assessment is based on thorough parametric time-domain and harmonic balance analyses. Presented results highlight that horizontal-axis wind turbine periodic flows can be computed by the harmonic balance solver about fifty times more rapidly than by the conventional time-domain analysis, with accuracy comparable to that of the time-domain solver.

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Wilson, Brandon M. "Unsteady Computational Fluid Dynamics (CFD) Validation and Uncertainty Quantification for a Confined Bank of Cylinders Using Particle Image Velocimetry (PIV)". DigitalCommons@USU, 2012. https://digitalcommons.usu.edu/etd/1197.

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Zagnoli, Daniel Anthony. "A Numerical Study of Deposition in a Full Turbine Stage Using Steady and Unsteady Methods". The Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1429796426.

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Corrêa, Valesca Alves. "Aplicação da computação simbólica na resolução de problemas de condução de calor em cilindros vazados com condições de contorno convectivas /". Guaratinguetá : [s.n.], 2007. http://hdl.handle.net/11449/106450.

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Resumo: Com a evolução dos sistemas de computação simbólica ampliou-se a capacidade de modelagem e análise de problemas provenientes de equações diferenciais. Propõe-se a resolução da equação da condução de calor em regimes permanente e transiente para uma geometria cilíndrica com condições de contorno convectivas de forma analítica e numérica utilizando o software de computação simbólica Maple. Para este propósito serão empregados para a resolução analítica, o método de separação de variáveis e para a resolução numérica, o método das diferenças finitas com o esquema Crank- Nicolson e explícito. Os resultados obtidos das resoluções analíticas e numéricas, para algumas situações avaliadas são comparadas. As vantagens computacionais da utilização do software Maple são apresentadas.
Abstract: The evolution of symbolic computation systems enlarges the capacity of modeling and analysis of problems by differential equations. The aim is the resolution of the conduction heat equation in unsteady and steady state for the cylindrical geometry with convective boundary conditions with analytical and numerical solutions using the Maple software. To this results will be used the separated variables method and finite differences to numerical solutions with Crank-Nicolson and explicit schemes. The results obtained for numerical and analytical solutions for some situations it will available and compared. The computational advantages of the Maple software are showed too.
Orientador: Luiz Roberto Carrocci
Coorientador: Marcio Abud Marcelino
Banca: Petronio Masanobu Tanisho
Banca: Rubens Alves Dias
Banca: Carlos Alberto Chaves
Banca: José Rui Camargo
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Tesis sobre el tema "Unsteady computations"

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