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1
Barbagallo, Alexandre. "Model reduction and closed-loop control of oscillator and noise-amplifier flows". Palaiseau, Ecole polytechnique, 2011. https://pastel.hal.science/docs/00/65/49/30/PDF/Barbagallo_PhDThesis.pdf.
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This work deals with the closed-loop control of disturbances which develop linearly in laminar and incompressible flows. The control of both oscillator and amplifier flows is assessed. We consider a LQG control strategy in which the control law is computed using a reduced-order model of the flow. This reduced-order model is obtained by a Petrov-Galerkin projection. The first part is devoted to the stabilization of an open cavity flow which behaves as an oscillator. It is shown that the unstable subspace of the flow (the unstable global modes) and the input-output behaviour between the actuator and the sensor of the stable subspace must be captured by the reduced-order model to stabilize the system. Global modes, POD modes and BPOD modes are successively evaluated as projection bases to construct a reduced-order model of the stable part of the flow. It appears that global modes are not able to reproduce the input-output behaviour of the stable part of the flow and subsequently may only stabilize the flow if the instability is very weak (close to the criticality). On the contrary, reduced-order models based on POD modes and BPOD modes efficiently extract the input-output dynamic of the stable subspace and are successful to stabilize the flow. The second part of this work is dedicated to the reduction of the disturbances' amplification on a backward facing step. The influence of the sensor's location and of the cost functional on the performance of the compensator is studied. It is shown that the truncation of the reduced-order model may lead to an unstable closed-loop system. Finally, the possibility to control a non-linear simulation using a linear compensator is evaluatedCe travail est consacré au contrôle en boucle fermée des perturbations se développant linéairement dans des écoulements laminaires et incompressibles de types oscillateurs et amplificateurs de bruit. La loi de contrôle, calculée selon la théorie du contrôle LQG, est basée sur un modèle d'ordre réduit de l'écoulement obtenu par projection de Petrov-Galerkin. La stabilisation d'un écoulement de cavité de type oscillateur est traitée dans une première partie. Il est montré que la totalité de la partie instable de l'écoulement (les modes globaux instables) ainsi que la relation entrée-sortie (action de l'actionneur sur le capteur) de la partie stable doivent être captées par le modèle réduit afin de stabiliser le système. Les modes globaux, modes POD et modes BPOD sont successivement évalués comme bases de projection pour modéliser la partie stable. Les modes globaux ne parviennent pas à reproduire le comportement entrée-sortie de la partie stable et par conséquent ne peuvent stabiliser l'écoulement que lorsque l'instabilité du système est initialement faible (nombre de Reynolds proche de la criticité). En revanche, les modes POD et plus particulièrement BPOD sont capable d'extraire la dynamique entrée-sortie stable et permettent de stabiliser efficacement l'écoulement. La seconde partie de ce travail est consacrée à la réduction de l'amplification des perturbations sur une marche descendante. L'influence de la localisation du capteur et de la fonctionnelle de coût sur la performance du compensateur est étudiée. Il est montré que la troncature du modèle réduit peut rendre le système bouclé instable. Finalement, la possibilité de contrôler une simulation non-linéaire avec un modèle linéaire est évaluée
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Salmon, Mathieu. "closed-loop control of finite amplitude perturbations : application to sub- and super-critical flow-bifurcations". Electronic Thesis or Diss., Paris, ENSAM, 2024. http://www.theses.fr/2024ENAME072.
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Les méthodes de contrôle actuelles rencontrent des difficultés pour stabiliser un champ de base soumis à des perturbations d’amplitude finie. Une frontière appelée ”edge of chaos” sépare en deux zones distinctes le portrait de phase d’un écoulement qui transitionne de façon sous-critique à la turbulence. Le bassin d’attraction turbulent rassemble les perturbations capables de faire transitionner l’écoulement, le bassin d’attraction laminaire est l’ensemble des perturbations qui sont relaminarisées. La situation précédente caractérisée par deux attracteurs locaux qui coexistent peut être rencontrée en dehors du cadre de la transition à la turbulence. Un écoulement de type cylindre à Re = 100 possède un champ de base globalement instable ainsi qu’un cycle limite stable. Deux bassins d’attraction émergent lors de la stabilisation du champ de base par un contrôleur linéaire optimisé sur les équations de Navier-Stokes linéarisées. Nous cherchons dans cette étude à augmenter la taille du bassin d’attraction du champ de base. La nouveauté de ce travail réside dans le choix de la fonctionnelle qui est optimisée avec contrôle. En effet, l’optimisation vise l’énergie d’une perturbation située sur la frontière des deux bassins d’attraction. La transition sous-critique à la turbulence est étudiée par le biais du modèle SSP de Waleffe, un modèle d’ordre réduit des équations de Navier-Stokes avec seulement quatre degrés de liberté. Les méthodes de contrôle élaborées dans ce travail sont efficaces pour induire une croissance du bassin d’attraction ”laminaire”. Dans l’écoulement de type cylindre, la robustesse d’un contrôleur initial à des perturbations d’amplitude finie est augmentée avec succès dans une direction du portrait de phaseCurrent control optimisation methods struggle to stabilize a base flow in the case of finite amplitude perturbations. A boundary called edge of chaos separates into two regions the phase space of a flow which transitions subcritically to turbulence. The turbulent basin of attraction incorporates the perturbations whose energy is sufficient to trigger transition to turbulence, the laminar basin of attraction is the set of initial perturbations which are relaminarized. Such situation with two coexisting local attractors can also be encountered in flow cases outside the scope of transition to turbulence. A cylinder flow at Re = 100 exhibits a globally unstable base flow and a stable limit-cycle. Two basins of attraction emerge from the local stabilization of the base flow by a linear controller optimized on the linearized Navier-Stokes equations. We seek in this study to increase the basin of attraction of the base flow. The novelty of this work lies in the choice of the functional to be optimised with control. Indeed, the optimisation targets the energy of a perturbation located on the boundary of the two basins of attraction. We consider subcritical transition to turbulence using the well-known SSP model of Waleffe, a reduced-order model of the Navier-Stokes equations with only four degrees of freedom. The control methods elaboratored in this work are effective to induce a growth of the ”laminar” basin of attraction. In the cylinder flow, the robustness of an initial controller to finite amplitude perturbations is increased in a chosen direction of the phase space
3
Wang, Jianhong. "Oscillatory flows round combinations of cylinders". Thesis, University of Edinburgh, 1998. http://hdl.handle.net/1842/13196.
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Comprehensive experimental and computer simulational studies have been carried out to measure the velocity fields in oscillatory flows around combinations of cylinders and also the forces acting on the cylinders in oscillatory flow. Particle Image Velocimetry (PIV) has been used to measure the instantaneous velocity flow fields. Simultaneous measurements have also been made of both inline force and lift force using a transducer installed at the end of one of the cylinders. The strengths of the vortices in the immediate vicinity of the cylinders were calculated. The numerical simulation study used a random vortex method with three overlapping meshes. Forces on the cylinders, together with the corresponding velocity vector fields, were simulated and comparisons made with the experimental results. The combinations of cylinders were (a) a square array of four cylinders with the dimensions chosen so that the array was an approximate scale model of the Heidrun Tension Leg Platform , (b) two cylinders in tandem with different distances between them, (c) a single cylinder. The Keulegan-Carpenter numbers (KC) were 5.8, 10, 13, 20, the Reynolds numbers (Re) were 2500, 4000, 5000, 6500 and 10000 and the viscous parameter (β = Re/KC) was set at 500. A comparison of the experimental results with the numerical simulations showed good agreement. For the single cylinder the main frequency component of the lift force shifted from the first harmonic, to the second, and then the third harmonic of the oscillatory flow when the KC number and oscillatory velocity increases. For the four cylinder array, in which the distance between two cylinder was greater than 2.5 times the cylinder diameter, the main frequency component of the lift force shifted from the first harmonic to the second as the KC number and oscillatory velocity increased; the third harmonic was relatively small. The strength and fundamental frequency of the force both increased faster than for the single cylinder case.
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Wybrow, M. F. "Oscillatory flows about elliptic and circular cylinders". Thesis, University of East Anglia, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.389229.
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Wijetunge, Janaka Jayasekera. "Velocity measurements in oscillatory and steady flows". Thesis, University of Cambridge, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.627385.
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Al-Asmi, Khalfan. "Vortex shedding in oscillatory flow". Thesis, University of Surrey, 1992. http://epubs.surrey.ac.uk/842864/.
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Measurements of the response to inline flow oscillations of vortex shedding from certain bluff bodies have been made. Four cylinders with fixed separation points were exposed to a mean stream with controlled sinusoidal oscillations at defined frequency ratios and amplitudes. Attention was concentrated on the highly sensitive reduced velocity regime around the inverse of twice the Strouhal number, 1/2S. Synchronisation of vortex formation was first established; then, for conditions at which synchronisation occured, threshold amplitude was measured. In order to determine the common and distinguishing features of bluff body shape the response from cylinders having 'zero' and finite afterbody were investigated. Finally, for a selected cylinder, the influence, in the presence of oscillations, of turbulence intensity (of defined scale), solid blockage and aspect ratio to the synchronous range were examined. Of particular interest were the changes that occur in the characteristic period of vortex formation and base pressure, relevant to the design and application of vortex flowmeters and self induced excitations of structures in general. The experiments were carried out in two separate blower tunnels under various oscillatory flow conditions in the Reynolds number range (0.5-5.0) X 104 and amplitudes of velocity fluctuation (+/-AU/U) of up to 0.3. It was found that amplitudes of oscillations of the order of 0.025 were sufficient to induce frequency lock-in when the reduced velocity was close to 1/2S and, provided the amplitude was sufficiently large, limited synchronisation could also be induced near the upper and lower reduced velocities of 1/S and 1/4S. Synchronisation is accompanied by enhanced vortex shedding which, in turn, can lead to increased base suction (and therefore increase in drag force). Flow visualisation revealed that the near wake vortex arrangement can vary a great deal depending on the ratio of Strouhal numbers at the forced and self-excited frequencies (N/no). It was concluded that the behaviour of the base pressure reflected this situation and that the range of synchronisation depended strongly on the stability of the prevailing mode of vortex shedding. The precise details of the flow were found to be highly dependent on body geometry and the frequency ratio N/no. The production of oscillatory flow in the wind tunnels did not proved to be an easy task. A review of the various methods that have been used in the past is presented. The advantages and disadvantages of different techniques are highlighted and details are given of a further method developed for use with the present open circuit blower tunnels of differing sizes. In the smaller tunnel, having a working section size of 0.3x0.3m, it was possible to produce sinusoidal variations of the working section flow, having peak to peak amplitude of up to 60% of the mean flow speed and frequencies up to, typically, that corresponding to the acoustic quarter-wavelength frequency determined by the tunnel length. Over the viable working range, the device is shown to produce high quality periodic flow with negligible harmonic distortion or acoustic noise difficulties.
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Dick, Jennifer Ellen. "Sediment transport in oscillatory flow". Thesis, University of Cambridge, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.235836.
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The effect of non-cohesive sediment on wave-induced bottom boundary layers was examined experimentally. Fluid velocity and sediment concentration were measured simultaneously in a sheet regime generated in a u-shaped oscillating water tunnel. A major obstacle to the understanding of fluid-grain flow has been the absence of suitable measuring devices. Thus, previous experimental investigations have been concerned primarily with low sediment concentration flows. For this study, a probe was developed to measure the instantaneous variations in sediment concentration based on the electrical conductivity of the fluid-grain mixture. Unlike earlier concentration devices, this probe is non-intrusive and is capable of measuring a wide range of sediment concentrations; from close packing within the bed to low concentration suspended load. Horizontal fluid velocities were measured simultaneously using Laser Doppler Anemometry in backscatter mode. Height and time-dependent velocity and concentration profiles were obtained for differing wave and sediment conditions. Values of the shear stress calculated from the momentum integral were found to be an order of magnitude larger than in sediment-free flows. The variation in shear stress with distance from the bed is clearly dependent on the thickness of the movable bed and also on the sediment flow regime. As expected, the eddy viscosity varied significantly during the wave cycle. The time-mean eddy viscosity decreases with height above the movable bed and at large distances from the bed, fluctuates about the mean. The sediment concentration measurements provide a comprehensive data set for sediment transport in sheet flow and near sheet flow regimes. The sediment concentration was found to be time-dependent with the amplitude and form of the temporal variation dependent on distance from the bed, wave amplitude and velocity, and sediment characteristics. With increasing wave amplitude and velocity, the number of peaks in the concentration profile increased while the magnitude of the peaks decreased. Fluid velocity and sediment concentration measurements were used to calculate rates of sediment transport which were compared with predictions from existing models.
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Stephens, Gerard Groves. "Suspension polymerisation in oscillatory flow". Thesis, University of Cambridge, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.627184.
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Tait, Nicole Lynn. "Recovery factors in zero-mean internal oscillatory flows". Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1995. http://handle.dtic.mil/100.2/ADA306233.
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Thesis (M.S. in Mechanical Engineering and M.S. in Astronautical Engineering) Naval Postgraduate School, December 1995."December 1995." Thesis advisor(s): Ashok Gopinath, Oscar Biblarz. Bibliography: p. 61. Also available online.
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Krishna, Vikas. "Numerical simulation of vortex shedding in oscillatory flows". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1995. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp05/mq25859.pdf.
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Klotsa, Katerina-Daphne. "The Dymanics of Spheres in Oscillatory Fluid Flows". Thesis, University of Nottingham, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.517843.
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Rona, Aldo. "Aerodynamic and aeroacoustic estimations of oscillatory supersonic flows". Thesis, University of Southampton, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.242729.
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Amin, Norsarahaida. "Oscillation-induced mean flows and heat transfer". Thesis, University of East Anglia, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.329339.
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Smith, Keith Buchanan. "Scale-up of oscillatory flow mixing". Thesis, University of Cambridge, 2000. https://www.repository.cam.ac.uk/handle/1810/238305.
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Oscillatory Flow Mixing is a recent development in mixing technology which has evolved over the past decade. It has a number of similarities to other mixing technologies, particularly pulsed and reciprocating plate columns, but at the laboratory scale has demonstrated a number of advantageous properties. These properties (such as control of residence time distribution, improved heat transfer and predictable mixing times) have been demonstrated at the laboratory scale for a wide range of different potential applications, but until now there has been a lack of firm understanding and research into how the technology could be scaled-up into an industrial scale process. This thesis addresses the problem of scale-up in Oscillatory Flow Mixing. It reports on a programme of experiments on geometrically scaled apparatus with the measurement of residence time distributions and flow visualisation as the principal methods of investigating the wide range of flow conditions that can be achieved by control of net flow and of oscillatory conditions. Results from these investigations are interpreted as axial dispersion coefficients and also compared with results obtained computationally using a fluid mechanics approach to simulate flow fields and the injection of inert tracers into those flow fields. Significant clarification is reported concerning the analysis of axial dispersion measurements using the diffusion model for which conflicting solutions were identified in the literature. The development of a flow visualisation technique using fluorescent dye streaklines is also reported. Using the latter technique stable manifolds in Oscillatory Flow Mixing have for the first time been experimentally observed as well as a range of other flow regimes. The study of scale-up was extended by the successful construction and investigation of an alternative reactor geometry with the potential for use in large-scale plant. From the work presented in the thesis it is concluded that Oscillatory Flow Mixing is a technology which in general lends itself readily to scaling-up from laboratory to pilot plant scale, and most probably to industrial scale. Experiments performed on small laboratory apparatus (containing less than one litre of fluid) can with confidence be used to predict mixing behaviour in much larger plant (containing hundreds of litres of fluid.)
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El, Allaoui Nazha. "Modified hydrodynamics in fragmented canopies exposed to oscillatory flows". Doctoral thesis, Universitat de Girona, 2016. http://hdl.handle.net/10803/403066.
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The general objective of this doctoral thesis has been to contribute to understand the modification of hydrodynamics in fragmented canopies. The effect of different parameters such as the canopy density, the plant height and flexibility and the architecture of the gap has been studied. Ecological implications of the results have been discussed. Results point that the fluxes of biological particles, nutrients and sediments in fragmented canopies are modified compared with those for non-fragmented canopies, impacting on their ecological function. Therefore, canopies may optimize their structural characteristics to moderate the impact of fragmentation. This thesis shows that plant density and flexibility interacts with the gap width and the degree of fragmentation to facilitate sheltering in a manner not previously predictedL'objectiu general d'aquesta tesi doctoral ha estat contribuir a entendre la modificació de la hidrodinàmica en praderies aquàtiques fragmentades. En aquesta tesi doctoral s’ha estudiat l'efecte de diferents paràmetres com ara la densitat de la praderia, l'alçada de la planta i la flexibilitat i l'arquitectura dels blancs sense vegetació. S'han discutit les conseqüències ecològiques dels resultats. Els resultats assenyalen que els fluxos de partícules biològiques, nutrients i sediments en praderies fragmentades són modificats en comparació amb els que s’obtindrien en praderies no fragmentades, fet que modifica la seva funció ecològica. Per tant, les praderies poden optimitzar les seves característiques estructurals per moderar l'impacte de la fragmentació. Aquesta tesi mostra que la densitat de plantes i flexibilitat interactua amb les dimensions del blanc i el grau de fragmentació per facilitar el refugi d'una manera no prevista anteriorment
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Choate, Eric P. Forest M. Gregory. "Small amplitude oscillatory flows of nematic liquid crystal polymers". Chapel Hill, N.C. : University of North Carolina at Chapel Hill, 2007. http://dc.lib.unc.edu/u?/etd,835.
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Thesis (Ph. D.)--University of North Carolina at Chapel Hill, 2007.Title from electronic title page (viewed Dec. 18, 2007). "... in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Mathematics." Discipline: Mathematics; Department/School: Mathematics.
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Tian, Shuai. "CFD modelling of oscillatory perturbed advection in viscous flows". Thesis, University of Birmingham, 2016. http://etheses.bham.ac.uk//id/eprint/6717/.
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Mechanical oscillation has been proven as an effective method of affecting flow behaviour and material processing of various rheologies. In this work, Computational Fluid Dynamics (CFD) is utilised to investigate the applications of mechanical oscillations in affecting flow and heat transfer in Newtonian and non-Newtonian fluids. Longitudinal oscillations are found to be generally the most effective in producing flow enhancements which exceed those generated by transverse and rotational oscillations. Transverse oscillations, unlike longitudinal or rotational oscillations, generate a vigorous swirling fluid motion and considerable radial mixing in viscous flows, thus, produce large enhancement in wall heat transfer as well as a nearly-uniform radial temperature field. A novel technique which combines transverse oscillation with a step rotation of oscillation orientation is developed and appears to create processing condition that are in great agreement with the high temperature for short time (HTST) assumption, thus, optimising the conventional continuous heat-hold-cool thermal sterilisation processing. CFD results also show that drainage of liquid film of non-Newtonian fluids and settling of spherical particle in non-Newtonian rheologies are affected by superimposed oscillations. The rate of film drainage and settling velocity depends on the vibration intensity and rheological properties.
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Forster, Graham Keith. "Instability and wave-growth within some oscillatory fluid flows". Thesis, University of St Andrews, 1996. http://hdl.handle.net/10023/14087.
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Oscillatory fluid flows arise naturally in many systems. Whether or not these systems are stable is an important question and external periodic forcing of the flow may result in rich and complicated behaviours. Here three distinct oscillatory fluid flows are examined in detail, with the stability of each being established using a range of analytical and computational methods. The first system comprises standing surface capillary-gravity waves in second-harmonic resonance subject to Faraday excitation. Using the perturbation technique of multiple scales, the amplitude equations for the system are derived. At exact resonance, and with the absence of damping, the only fixed point of the equations is found to be the origin. A computational approach reveals that the amplitudes of the two waves remain either bounded or grow to infinity depending on initial data. With the introduction of detuning and damping into the system families of fixed points now exist and some special cases are considered. The second class of flows are unbounded time-periodic flows with fixed ellipsoidal stream surfaces, and having spatially uniform but time-periodic strain rates. Using a recently developed method based on theoretical study of the Schrodinger equation with quasi-periodic potential, a computational approach is adopted which determines the stability of the flow to three-dimensional plane wave disturbances. Results for the growth rate and winding number of the disturbance clearly reveal the regions of instability. It is found that almost all these flows are highly unstable. The third class is another set of three-dimensional time-periodic flows with spatially uniform strain rates. These flows are non-axisymmetric and have sinusoidally-fluctuating rates of strain directed along the fixed coordinate axes. The same computational method is employed and it is found that instability increases along with the non-axisymmetric nature of the flow.
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Cao, Zhixin. "Non-oscillatory forward-in-time method for incompressible flows". Thesis, Loughborough University, 2018. https://dspace.lboro.ac.uk/2134/33265.
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This research extends the capabilities of Non-oscillatory Forward-in-Time (NFT) solvers operating on unstructured meshes to allow for accurate simulation of incompressible turbulent flows. This is achieved by the development of Large Eddy Simulation (LES) and Detached Eddy Simulation (DES) turbulent flow methodologies and the development of parallel option of the flow solver. The effective use of LES and DES requires a development of a subgrid-scale model. Several subgrid-scale models are implemented and studied, and their efficacy is assessed. The NFT solvers employed in this work are based on the Multidimensional Positive Definite Advection Transport Algorithm (MPDATA) that facilitates novel implicit Large Eddy Simulation (ILES) approach to treating turbulence. The flexibility and robustness of the new NFT MPDATA solver are studied and successfully validated using well established benchmarks and concentrate on a flow past a sphere. The flow statistics from the solutions are compared against the existing experimental and numerical data and fully confirm the validity of the approach. The parallel implementation of the flow solver is also documented and verified showing a substantial speedup of computations. The proposed method lays foundations for further studies and developments, especially for exploring the potential of MPDATA in the context of ILES and associated treatments of boundary conditions at solid boundaries.
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Casanellas, Vilageliu Laura. "Oscillatory pipe flow of wormlike micellar solutions". Doctoral thesis, Universitat de Barcelona, 2013. http://hdl.handle.net/10803/108992.
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Wormlike micelles are viscoelastic fluids that present an intermediate behavior between solids and ordinary liquids since they are elastic at short time scales but flow easily at large time scales. In opposition to Newtonian fluids, which have constant viscosity, these fluids usually exhibit a non-Newtonian response with a rate-dependent shear viscosity.
Wall-bounded oscillatory flows of Newtonian and complex fluids are found in many practical situations. Oscillatory pipe flows are especially important in physiology in connection with the circulatory and respiratory systems of human beings, as well as in industrial processes such as fluid pumping, secondary oil recovery or filtration, and in acoustics. Pulsating flows are of particular interest also in the rheological characterization of complex fluids.
We analyze the laminar oscillatory flow of viscoelastic fluids using the Maxwell and Oldroyd-B models. We have shown that in wall-bounded oscillatory flows of viscoelastic fluids the two characteristic lengths of the Ferry waves, the damping length and wavelength, together with the characteristic separation of the walls, define all the flow properties for fluid models with a linear shear-stress equation in unidirectional flow. In wall-bounded settings there exists the possibility that shear waves generated at different locations superpose themselves before decaying so that the shear waves interfere, giving rise to a resonant flow at well defined frequencies of driving. The theoretical predictions obtained for the laminar velocity profiles are validated by carrying out time-resolved Particle Image Velocimetry (PIV) experiments in a vertical pipe at small driving amplitudes.
The oscillatory pipe flow has been investigated in the whole range of experimentally accessible driving frequencies and amplitudes, and classified in three main flow regimes: laminar, vortical, and non-axisymmetric vortical. By ramping up and down the driving amplitude at constant frequency we have been able to characterize the transition from laminar to more complex flows, under controlled driving conditions. The first hydrodynamic instability occurs when the laminar base flow becomes unstable against the formation of axisymmetric toroidal vortices that appear distributed along the cylinder. The calculation of root-mean-square fluctuations in the vertical direction, of the vertical and radial components of the velocity (averaged in time or over the tube diameter) has allowed to determine the critical amplitude at which the instability sets in with high accuracy. In the vortical flow an abrupt increase of the fluctuations is observed, that accounts for the loss of the vertical translational symmetry and the formation of vortices in the flow. This transition exhibits hysteresis when the driving amplitude is ramped up and down, which makes us presume that the bifurcation from the laminar flow has a subcritical nature. A second hydrodynamic instability occurs when the vortical flow loses the axial symmetry. In this flow regime the vortices are heavily distorted and no longer axisymmetric.
The velocity and vorticity maps of the vortical flow measured in a meridional plane of the tube appear periodic in time, on time scales comparable to the driving period. Interestingly, the vortex formation is favored in the acceleration phases of the piston oscillation. Besides, we have uncovered a spatio-temporal dynamics on long time scales (much larger than the relaxation time of the fluid) that substantially modifies the flow organization. This slow dynamics is more effective in the bottom half of the cylinder, specially next to the driving piston. A global inspection of the vortical flow along the tube length reveals that the instability takes place earlier in the bottom part of the tube, in the vicinity of the driving piston. At increasing the driving amplitude the boundary between laminar and vortical flow progressively raises towards the top regions. And above a critical driving amplitude the entire fluid flow is vortical. The mechanism triggering the hydrodynamic instability from the laminar to the axisymmetric vortical flow is not yet clear.L'objectiu d'aquesta Tesi és estudiar el flux oscil•latori vertical en fluids micel•lars. Els fluids micel•lars són fluids complexos amb propietats viscoelàstiques, de manera que mostren un comportament intermedi entre els sòlids i els líquids: són elàstics a escales de temps curtes però flueixen a escales de temps més llargues. En contraposició als fluids Newtonians, que tenen una viscositat constant, els fluids complexos mostren un comportament no-Newtonià, amb una viscositat que depèn del ritme de deformació. El fluxos oscil•latoris de fluids Newtonians o complexos en geometries confinades són especialment importants en fisiologia, en relació amb el sistema circulatori i respiratori d'éssers humans, i també en processos industrials com el bombejat de fluids, l'extracció de petroli, i en particular són interessants en la caracterització reològica de fluids complexos. Primer estudiem el flux oscil•latori des d'una perspectiva teòrica i analitzem el flux laminar de fluids viscoelàstics utilitzant els models de Maxwell i Oldroyd-B en un tub vertical. Hem mostrat que en fluxos confinats existeix la possibilitat que les ones de cisalla generades a les diferents parets se sobreposin abans d'esmorteir-se i que eventualment donin lloc a un fenomen de ressonància. Les prediccions teòriques obtingudes pel flux laminar són validades duent a terme experiments de Velocimetria d'Imatges de Partícules (PIV) en un tub vertical, per amplituds petites del forçament oscil•latori. Quan s'incrementa l'amplitud de l'oscil•lació el flux laminar evoluciona cap a fluxos que presenten una dependència espai-temporal més complexa. Fent rampes d'amplitud creixent a una freqüència fixada hem pogut caracteritzar experimentalment la transició del flux laminar a aquests fluxos més complexos, sota condicions de forçament ben controlades. La primera inestabilitat apareix quan el flux laminar esdevé inestable amb la corresponent formació d'anells de vorticitat apilats al llarg del tub. Es manifesta una segona inestabilitat per amplituds del forçament més grans, per la qual el flux vortical perd la simetria axial. En aquest nou règim els vòrtex estan fortament distorsionats i no són axisimètrics. Fent rampes d’amplitud creixent i decreixent hem observat que aquestes dues transicions presenten histèresi, i que per tant són de caràcter subcrític.
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Ahilan, R. V. "Flow of cohesionless grains in oscillatory fluids". Thesis, University of Cambridge, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.304431.
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Rigby, Simon Geoffrey. "The electrorheological effect in oscillatory squeeze-flow". Thesis, University of Liverpool, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.284149.
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Fabiyi, Malcolm Ezekiel. "Photochemical wet oxidation using oscillatory flow mixing". Thesis, University of Cambridge, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.620986.
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Lamoureux, Alexandre. "Oscillatory flows in periodically interrupted rectangular passages in heat exchangers". Thesis, McGill University, 2006. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=99415.
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Computational and experimental studies of fluid flow and heat transfer phenomena in interrupted-surface passages are presented. The computational investigation was focused on developing and fully-developed (spatially and temporally) laminar flow and heat transfer in essentially two-dimensional regions of the following passages: (1) a straight rectangular duct with spatially periodic in-line plate inserts; and (2) staggered-plate arrays. A second-order finite-volume method was developed, validated, and used to solve these problems. Time-mean modular friction factors and Colburn factors were obtained from the domain inlet to the spatially-periodic fully-developed region for a Reynolds number range of 100 to 600, thus exploring the laminar steady and unsteady regimes. Additionally, various cyclic domains were investigated. In the experimental investigation, single hot-wire measurements were used to obtain ensemble-averaged power spectrums and Strouhal numbers in the fully-developed region of the rectangular duct with spatially periodic in-line plate inserts, for Reynolds numbers ranging from 2000 to 30000.
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Letherman, Sophie Bella. "Turbulence modelling of oscillatory flows over smooth and rough surfaces". Thesis, University of Manchester, 2000. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.488128.
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This study investigates turbulence models for application to boundary layer flows. Firstly, steady channel flow and transient pipe flows are considered. Calculations of a low-Reynolds-number k-epsilon model, a k-epsilon-S model (a strain parameter model which has not been applied to unsteady flows previously) and a Reynolds Stress Transport model are compared with experimental and DNS data. The eddy viscosity turbulence models (k-epsilon, k-epsilon-S) satisfactorily predict the mean flow parameters of steady channel flow. However the k-epsilon-S model proves superior in comparison with turbulence quantities. Near to the pipe wall, the k-epsilon-S model best captures the details of periodic pipe flow detail, whereas in the outer flow region the RSTM gives closest agreement with the experimental data. The high-Reynolds-number k-epsilon and k-l eddy viscosity turbulence models are examined in a separate study of oscillatory flows over smooth and rough beds. The computations are considered over a wider range of experimental parameters than previously investigated. The turbulence models are assessed by comparison with field measurements and laboratory data sets including a new set of experimental measurements. Both models predict the bed shear stress and velocity adequately, but the k-epsilon model emerges as the superior scheme when considering turbulence quantities. An attempt is made to quantify the uncertainty in the Reynolds shear stress and eddy viscosity experimental data. The k-epsilon model calculations more frequently lie within the experimental uncertainty bands. However this uncertainty range is wide; any improvement would require a corresponding improvement in the experimental resolution of rough bed flows.
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Fay, Sarah (Sarah C. ). "The rotor-oscillator flow : searching for coherence amidst chaos". Thesis, Massachusetts Institute of Technology, 2015. http://hdl.handle.net/1721.1/98955.
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Thesis: S.B., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2015.Cataloged from PDF version of thesis.
Includes bibliographical references (page 29).
Chaotic mixing of highly viscous fluids is common in many biological and industrial processes. This study aims to gain insight about the properties of such common processes by examining one particular case of viscous, chaotic mixing: the rotor-oscillator flow. For some couplings of the rotor motion with its oscillation, this flow has been shown to have coherent islands of fluid parcels surrounded by a sea of chaos. Through finite-time Lyapunov exponent (FTLE) analysis, a roughly optimal coupling was found. The parameters that describe this coupling are the nondimensional oscillation amplitude [epsilon] = 0.125 and frequency [lambda] = 0.4[pi]. In order to understand more about the mixing of slow-moving, highly viscous fluids, these values can and will be explored experimentally and through braid theory to further examine the regions of coherence in this generally chaotic flow.
by Sarah Fay.
S.B.
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Bhawanin, Mahesa. "Hydrodynamics and sand transport under regular and amplitude-modulated oscillatory flows". Thesis, University of Aberdeen, 2016. http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=231262.
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The majority of fundamental research on wave-driven sediment transport has been based on regular waves and regular oscillatory flows. Systematic research into isolating the effect of irregularity on oscillatory boundary layer hydrodynamics and sand transport are not presently available. This research reports on large-scale oscillatory flow tunnel experiments designed to isolate the effect of flow irregularity (specifically amplitude-modulated flows) on oscillatory flow boundary layer hydrodynamics and net sand transport rates. Two main series of experiments - one focussing on the hydrodynamics of amplitude-modulated flows over a fixed (immobile) bed and a second focussing on the sediment transport over mobile sand beds – were conducted. Detailed boundary layer velocity measurements are presented for regular and amplitude-modulated oscillatory flows over two fixed rough beds, coarse sand and gravel. The results show that amplitude-modulation of the flow has a variable effect on the hydrodynamic behaviour of the oscillatory boundary layer: the broad conclusion is that time-history effects are not significant near the bed but become more significant higher in the flow. Net transport rate measurements are presented for regular and amplitudemodulated flows for two mobile sand beds, medium and fine sand. The mobile bed experiments show that: for medium sand, net transport rates are similar for the amplitude-modulated and equivalent regular; for fine sand, net transport rates are very different for the amplitude-modulated and equivalent regular flows. The SANTOSS model-predicted sand transport rates shows that the model correctly predicts the net transport for the present experimental conditions, indicating that the major unsteady transport processes are captured in the model. The SANTOSS model is used to compare net transport rates for equivalent regular and amplitude-modulated flows across a wide range of flow conditions and two sand sizes. The results show good agreement in net transport rate between equivalent regular and amplitude-modulated flows, for conditions in which phase lag effects are weak and poor agreement when phase lags effects are strong.
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Ilori, Olusegun Mustapha. "Experimental and numerical investigations of thermal-fluid processes in oscillatory flows". Thesis, University of Leeds, 2016. http://etheses.whiterose.ac.uk/17777/.
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Heat exchangers are critical components of energy systems such as thermoacoustic engines and coolers. In this work, experimental and numerical studies were conducted on the heat transfer and acoustic pressure drop performance of heat exchangers in oscillatory flow. Experimental set-up and measurement techniques were developed to simultaneously measure parameters for the estimation of heat transfer and acoustic pressure drop. Three configurations of heat exchangers – tube-heat-exchanger, finned-heat-exchanger and plate-heat-exchanger were studied. Each configuration consists of three identical sets, arranged in series, to facilitate accurate heat transfer and acoustic pressure drop estimation. The influence of operating and geometric parameters on heat exchanger performance was investigated. The main considerations are to maximise heat transfer and minimise acoustic pressure losses from the heat exchanger, for the improvement of a system’s efficiency. The experimental results show that heat transfer performance, presented as the Nusselt number, strongly depends on drive ratio and mean pressure, especially at the low drive ratios where the gas displacement amplitudes are below, or comparable to, the heat exchanger length. The heat transfer results compared well with results from other studies. A three-dimensional model was developed in ANSYS Fluent, based on the actual experimental set-up. Experimental data was utilised for thermal, acoustic and turbulence boundary conditions and model validation. Good agreement was achieved between the predicted and experimental results. Heat transfer and pressure drop results show dependence on the drive ratio. Ogive edged T-HEX was found to minimise acoustic pressure drop by about 51% at the highest drive ratio in this study. A two-dimensional model was also developed. An appropriate edge shape is found to minimise the acoustic pressure drop and the associated minor losses without significantly affecting the heat transfer performance of the heat exchangers.
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Buchak, Peter (Peter M. ). "Flow-induced oscillation of flexible bodies". Thesis, Massachusetts Institute of Technology, 2010. http://hdl.handle.net/1721.1/64603.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mathematics, 2010.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 85-88).
We present a combined theoretical and experimental investigation of two systems in which flexible bodies are induced to oscillate by steady flows. The first system we study consists of multiple thin sheets of paper in a steady flow, clamped at the downstream end, which we call the "clapping book". Pages sequentially lift off, accumulating in a stack of paper held up by the wind. When the elasticity and weight of the pages overcome the aerodynamic force, the book claps shut; this process then repeats. We investigate this system experimentally and theoretically, using the theory of beams in high Reynolds number flow, and test our predictions of the clapping period. The second system we consider is inspired by free-reed musical instruments, which produce sound by the oscillation of reeds, thin strips of metal tuned to specific pitches. Each reed is mounted above a slot on the upstream side of a support plate, a geometry that allows a steady flow to induce finite-amplitude oscillations. We study this system experimentally and propose models, also based on the theory of elastic beams in high Reynolds number flow. The relative merits of these models is assessed by comparing their predictions with experiments.
by Peter Buchak.
Ph.D.
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Turk, Ugur. "Boiling of highly wetting liquids in oscillatory flow". Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1995. http://handle.dtic.mil/100.2/ADA306226.
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Lee-Young, James S. "Bed mechanics in combined steady and oscillatory flow". Thesis, University of Cambridge, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.276189.
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Shipman, Thomas N. "Evaluation of a novel oscillatory flow flexible chamber mixer". Access to citation, abstract and download form provided by ProQuest Information and Learning Company; downloadable PDF file 4.63 Mb., 109 p, 2006. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&res_dat=xri:pqdiss&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&rft_dat=xri:pqdiss:1435860.
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Reid, Francis John Edward School of Mathematics UNSW. "The weakly nonlinear stability of an oscillatory fluid flow". Awarded by:University of New South Wales. School of Mathematics, 2006. http://handle.unsw.edu.au/1959.4/33364.
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A weakly nonlinear stability analysis was conducted for the flow induced in an incompressible, Newtonian, viscous fluid lying between two infinite parallel plates which form a channel. The plates are oscillating synchronously in simple harmonic motion. The disturbed velocity of the flow was written in the form of a series in powers of a parameter which is a measure of the distance away from the linear theory neutral conditions. The individual terms of this series were decomposed using Floquet theory and Fourier series in time. The equations at second order and third order in were derived, and solutions for the Fourier coefficients were found using pseudospectral methods for the spatial variables. Various alternative methods of computation were applied to check the validity of the results obtained. The Landau equation for the amplitude of the disturbance was obtained, and the existence of equilibrium amplitude solutions inferred. The values of the coefficients in the Landau equation were calculated for the nondimensional channel half-widths h for the cases h = 5, 8, 10, 12, 14 and 16. It was found that equilibrium amplitude solutions exist for points in wavenumber Reynolds number space above the smooth portion of the previously determined linear stability neutral curve in all the cases examined. Similarly, Landau coefficients were calculated on a special feature of the neutral curve (called a ???finger???) for the case h = 12. Equilibrium amplitude solutions were found to exist at points inside the finger, and in a particular region outside near the top of the finger. Traces of the x-components of the disturbance velocities have been presented for a range of positions across the channel, together with the size of the equilibrium amplitude at these positions. As well, traces of the x-component of the velocity of the disturbed flow and traces of the velocity of the basic flow have been given for comparison at a particular position in the channel.
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Woolley, C. Hope. "Wall movements in oscillatory flow through an elastic tube". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp04/mq28690.pdf.
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Ho, Wai-man, i 何慧敏. "A numerical study on turbulent oscillatory plane Couette flow". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2004. http://hub.hku.hk/bib/B2977083X.
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Wright, Scott. "Well-sorted and graded sands in oscillatory sheet-flow". Thesis, University of Aberdeen, 2002. http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=191758.
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Much research effort is focused on the development of reliable empirical and numerical models for the prediction of sand transport. Confidence in these models depends on good agreement between predicted and measured transport rates for controlled conditions and, in the case of process-driven numerical models, good agreement between measured and predicted "sub-processes", such as time-dependent concentration and velocity profiles. The purpose of this project was to conduct experiments that measure these transport "sub-processes" in full-scale sinusoidal and asymmetric oscillatory sheet-flow conditions for well-sorted and graded sands. Detailed measurements have been obtained of concentrations, velocities, total and fractional transport rates and particles sizes in bed samples and in suspended and transported sands. The range and level of detail in the new concentration measurements makes it possible to interrogate concentration behaviour much more rigorously than previously possible. A new equation is presented which characterises time-dependent concentration profiles in the sheet-flow layer. The equation is based on time-dependent erosion depth and reference concentration. Analysis of the dependence of these parameters on flow and bed conditions is presented. The new velocity measurements extend "deeper" into the oscillatory boundary layer than previously possible and the results show classic features of oscillatory boundary layer flow. The product of the measured velocity and concentration data gives time-dependent sediment flux profiles. Analysis of the flux profiles reveals the detailed transport processes. The effects of unsteady behaviour and the effects of interactions between different size fractions in graded beds are evident in the sediment transport results. Unsteady effects act to reduce net transport and result in a strong offshore-directed transport in the case of fine sand. There is strong interaction between size fractions in graded beds. The mobility of the finer fractions is suppressed by the presence of coarser sands whilst the mobility of the coarser fractions is increased by the presence of finer sands.
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Jan, Darrell L. "A study of oscillatory flow through a bronchial bifurcation". Thesis, Massachusetts Institute of Technology, 1986. http://hdl.handle.net/1721.1/97293.
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Zheng, Mingzhi. "The development of a continuous oscillatory flow meso reactor". Thesis, University of Cambridge, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.612058.
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Akam, T. "Oscillatory mechanisms for controlling information flow in neural circuits". Thesis, University College London (University of London), 2012. http://discovery.ucl.ac.uk/1355099/.
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Mammalian brains generate complex, dynamic structures of oscillatory activity, in which distributed regions transiently engage in coherent oscillation, often at specific stages in behavioural or cognitive tasks. Much is now known about the dynamics underlying local circuit synchronisation and the phenomenology of where and when such activity occurs. While oscillations have been implicated in many high level processes, for most such phenomena we cannot say with confidence precisely what they are doing at an algorithmic or implementational level. This thesis presents work towards understanding the dynamics and possible function of large scale oscillatory network activity. We first address the question of how coherent oscillatory activity emerges between local networks by measuring phase response curves of an oscillating network in vitro. The network phase response curves provide mechanistic insight into inter-region synchronisation of local network oscillators. Highly simplified firing models are shown to reproduce the experimental data with remarkable accuracy. We then focus on one hypothesised computational function of network oscillations; flexibly controlling the gain of signal flow between anatomically connected networks. We investigate coding strategies and algorithmic operations that support flexible control of signal flow by oscillations, and their implementation by network dynamics. We identify two readout algorithms which selectively recover population rate coded signal with specific oscillatory modulations while ignoring other distracting inputs. By designing a spiking network model that implements one of these mechanisms, we demonstrate oscillatory control of signal flow in convergent pathways. We then investigate constraints on the structures of oscillatory activity that can be used to accurately and selectively control signal flow. Our results suggest that for inputs to be accurately distinguished from one another their oscillatory modulations must be close to orthogonal. This has implications for interpreting in vivo oscillatory activity, and may be an organising principle for the spatio-temporal structure of brain oscillations.
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SISHAH, BINIYAM BIRHAN. "Modeling the turbulent oscillatory flow over two-dimensional vortex ripples". Doctoral thesis, Università degli studi di Genova, 2022. http://hdl.handle.net/11567/1078536.
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The oscillatory turbulent flow over fixed two dimensional ripples is numerically solved
by using an appropriate turbulence closure. The aim of the contribution is that of detecting
features of the flow field which influence the ripple shape and the sediment transport.
The flow depends on three dimensionless parameters: the Reynolds number (Re), the
ripple steepness (h/L) and the ratio between the amplitude of fluid excursion close to
the sea bed and ripple wavelength (a/L). The results suggest that for increasing values
of a/L, the vortex shed on one side of the ripple crest is no longer the mirror image of
the vortex shed on the other side of the ripple during the following half oscillation cycle.
This suggests, for ripples forming in an erodible bottom, an uneven degree of sediment
entertainment from the two sides of the ripple crest. Moreover, steady recirculating cells
form. The number of cells which form per ripple length can be either one or two, depending
on the values of the parameters. The presence of steady recirculating cells is expected
to influence the equilibrium shape of the ripples. The evaluation of the plane and time
averaged velocity component shows the formation of a horizontal steady streaming, which
is directed either onshore or offshore. Such steady streaming is bound to influence the
cross-shore sediment transport.
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Paphitis, Doros. "Laboratory investigations into the threshold of movement of sand-sized sediments". Thesis, University of Southampton, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.370063.
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Waywell, M. N. "Predictions of wave and tidally induced oscillatory flows with Reynolds stress turbulence models". Thesis, University of Salford, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.308264.
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Kamsanam, Wasan. "Development of experimental techniques to investigate the heat transfer processes in oscillatory flows". Thesis, University of Leicester, 2014. http://hdl.handle.net/2381/29067.
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Heat exchangers are important components of thermoacoustic devices. In oscillatory flow conditions, the flow and temperature fields around the heat exchangers can be quite complex, and may significantly affect heat transfer behaviour. As a result, one cannot directly apply the heat transfer correlations for steady flows to the design of heat exchangers for oscillatory flows. The fundamental knowledge of heat transfer in oscillatory flows, however, is still not well-established. The aim of the current work is to investigate the influence of certain geometric parameters of heat exchangers, and of operating conditions in oscillatory flow on heat transfer performance. The heat transferred between two heat exchangers forming a couple was measured over a range of testing conditions. Three couples of finned-tube heat exchangers with different fin spacing were selected for the experiment. The main parameters considered were fin spacing, fin length, thermal penetration depth and gas displacement amplitude. Their effects on the heat exchanger performance were studied. The results are summarised and analysed in terms of heat transfer rate, Nusselt number and heat transfer effectiveness; the latter defined by the ratio of the actual heat transfer rate to the maximum possible heat transfer rate. The measurement results are compared with results from models widely used in the design of thermoacoustic heat exchangers: Time-Average Steady-Flow Equivalent (TASFE), Root Mean Square Reynolds Number (RMSRe), boundary layer conduction model and selected correlations developed by different authors. Based on the experimental data, a new correlation is established aimed at improving the reliability of oscillatory flow heat transfer predictions. The correlation is proposed for the relationship between heat transfer effectiveness, and the normalized displacement amplitude and the normalized fin spacing (the ratio of fin spacing to thermal penetration depth). The uncertainties associated with the measurement of heat transfer rate are also considered.
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Baghaei, Masoud. "Research on fluidic oscillators under incompressible and compressible flow conditions". Doctoral thesis, Universitat Politècnica de Catalunya, 2020. http://hdl.handle.net/10803/669607.
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One of the main advantages of fluidic oscillators is that they do not have moving parts, which brings high reliability whenever being used in real applications. To use these devices in real applications, it is necessary to evaluate their performance, since each application requires a particular injected fluid momentum and frequency. In this PhD., the performance of a given fluidic oscillator is evaluated at different Reynolds numbers via a 3D-computational fluid dynamics (CFD) analysis under incompressible and compressible flow conditions. In the first stage, the net momentum applied to the incoming jet is compared with the dynamic maximum stagnation pressure in the mixing chamber, to the dynamic output mass flow, to the dynamic feedback channels mass flow, to the pressure acting to both feedback channels outlets, and to the mixing chamber inlet jet oscillation angle. A perfect correlation between these parameters is obtained, therefore indicating the oscillation is triggered by the pressure momentum term applied to the jet at the feedback channels outlets. The stagnation pressure fluctuations appearing at the mixing chamber inclined walls are responsible for the pressure momentum term acting at the feedback channels outlets, thus it is proved that the oscillations are pressure-driven. In the second stage, several performance parameters were numerically evaluated as a function of different internal modifications via using 3D-CFD simulations. The evaluation is based on studying the output mass flow frequency and amplitude whenever several internal geometry parameters are modified. The geometry modifications considered were the mixing chamber inlet and outlet widths, and the mixing chamber inlet and outlet wall inclination angles. The concept behind this is, to evaluate how much the fluidic oscillator internal dimensions affect the device's main characteristics, and to analyze which parts of the oscillator produce a higher impact on the fluidic oscillator output characteristics. For the different internal modifications, evaluated, special care is taken in studying the forces required to flip the jet. The entire study is performed for three different Reynolds numbers, 8711, 16034 and 32068. Among the conclusions reached it is to be highlighted that, for a given Reynolds number, modifying the internal shape affects the oscillation frequencies and amplitudes. Any oscillator internal modification generates a much relevant effect as Reynolds number increases. Under all conditions studied, it was observed that the fluidic oscillator is pressure-driven under incompressible flow conditions as discussed in the first and second stages. In the third stage, the feedback channel effect on the oscillator output mass flow frequency and amplitude under compressible flow conditions were evaluated. In order to bring light to this point, a set of three dimensional Direct Numerical Simulations under compressible flow conditions, are introduced in the present stage, four different feedback channel lengths and two inlet fluid velocities are considered. From the results obtained, it was observed that as the inlet velocity increases, the fluidic oscillator output mass flow frequency and amplitude increase. An increase of the feedback channel length decreases the output mass flow oscillating frequency. At high feedback channel lengths, the form of the main oscillation tends to disappear, the jet inside the mixing chamber simply actuates at high frequencies, for these cases, the mass flow and pressure signals are very scattered due to the pressure waves appearing on mixing chamber converging surfaces and both feedback channels at the same time. Once the FC length exceeds a certain threshold, the oscillation stops. Under compressible conditions, the oscillations are pressure-driven as previously stated for the incompressible cases. The forces due to the pressure are much stronger than the mass flow flowing along the feedback channels.El principal avantatge dels oscil·ladors fluídics es que no te parts mòbils, i això fa que sigui més fiable en aplicacions reals. Per tal d'aplicar aquests oscil·ladors en un cas concret, es necessari avaluar el seu comportament, doncs cada cas concret necessita una freqüència i quantitat de moviment donades. En el present doctorat s'ha analitzat mitjançant 3D-CFD, una configuració de oscil·lador fluídic per diferents números de Reynolds, diferents geometries internes i considerant el fluid com incompressible i compressible. Inicialment, la quantitat de moviment aplicada al jet entrant a la cambra de barreja, es comparada amb la pressió d'estancament dinàmica a les parets convergents de la cambra de barreja, amb el cabal màssic dinàmic que surt del actuador, amb el cabal màssic dinàmic que passa per els canals de realimentació, amb la pressió dinàmica que hi ha a la sortida dels canals de realimentació i amb el angle de oscil·lació del jet a l'entrada de la cambra de barreja. Tots aquests paràmetres es va veure que estaven correlacionats i això indicava que el origen de les oscil·lacions del jet era únic i era la pressió d'estancament a les parets convergents de la cambra de barreja, provant que les oscil·lacions son dirigides per gradients de pressió. Posteriorment es va fer el mateix tipus de estudi però modificant la amplada i angle de inclinació a l'entrada de la cambra de barreja i també modificant la amplada i angle de inclinació de les parets de sortida de la cambra de barreja. Aquestes quatre modificacions de la geometria interna es van fer per tres números de Reynolds diferents, 8711, 16034 i 32068. Entre les conclusions obtingudes cal destacar que, la freqüència i amplitud de oscil·lació del jet a la sortida del actuador pot ser modificada al variar les dimensions i angles interns de la cambra de barreja. Independentment del número de Reynolds estudiat i de la modificació interna considerada, es va comprovar que les oscil·lacions estaven dirigides per els gradients de pressió existents entre les dos sortides dels conductes de realimentació. L'efecte generat per qualsevol modificació interna era sempre més rellevant a números de Reynolds alts. En la tercera fase de la tesi el fluid es va considerar com a compressible subsònic, i es va avaluar els efectes de la modificació de la longitud dels canals de realimentació, sobre la freqüència i amplitud del flux que surt del oscil·lador. Quatre diferents longituds i dos números de Mach van ser avaluats. Al augmentar la longitud del canal de realimentació, la freqüència i amplitud de la oscil·lació disminueix, la oscil·lació tendeix a ser mes caòtica, apareixen altes freqüències que fan que el jet fluctuï en lloc de oscil·lar, de fet a partir de una certa longitud les oscil·lacions desapareixen i només hi han fluctuacions. Aquestes fluctuacions apareixen abans per elevats números de Mach. Les oscil·lacions son degudes a gradients de pressió, al igual que en el cas de fluid incompressible. De fet, per fluid compressible, el cabal màssic que passa per els canals de realimentació, juga un paper menys rellevant que en el cas de fluid incompressible.
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Resendiz, Rosas Celerino. "Numerical simulation of flow separation control by oscillatory fluid injection". Diss., Texas A&M University, 2003. http://hdl.handle.net/1969.1/2461.
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In this work, numerical simulations of flow separation control are performed. The sep-aration control technique studied is called 'synthetic jet actuation'. The developed code employs a cell centered finite volume scheme which handles viscous, steady and unsteady compressible turbulent flows. The pulsating zero mass jet flow is simulated by imposing a harmonically varying transpiration boundary condition on the airfoil's surface. Turbulence is modeled with the algebraic model of Baldwin and Lomax. The application of synthetic jet actuators is based in their ability to energize the boundary layer, thereby providing signifcant increase in the lift coefficient. This has been corroborated experimentally and it is corroborated numerically in this research. The performed numerical simulation investigates the flow over a NACA0015 air-foil. For this flow Re = 9??105 and the reduced frequency and momentum coefficient are F+ = 1:1 and C?? = 0:04 respectively. The oscillatory injection takes place at 12.27% chord from the leading edge. A maximum increase in the mean lift coefficient of 93% is predicted by the code. A discrepancy of approximately 10% is observed with corresponding experimental data from the literature. The general trend is, how-ever, well captured. The discrepancy is attributed to the modeling of the injection boundary condition and to the turbulence model.A sensitivity analysis of the lift coefficient to different values of the oscillation parameters is performed. It is concluded that tangential injection, F + ?? O(1) and the utilized grid resolution around the site of injection are optimal. Streamline fields ob-tained for different angles of injection are analyzed. Flow separation and attachment as functions of the injection angle and of the velocity of injection can be observed. It is finally concluded that a reliable numerical tool has been developed which can be utilized as a support tool in the optimization of the synthetic jet operation and in the modeling of its operation.
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Ranasoma, Koruwage Indrakeerthi Mahesha. "Measurements in combined oscillatory and steady flow over rippled beds". Thesis, University of Cambridge, 1993. https://www.repository.cam.ac.uk/handle/1810/251543.
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Bechtel, Toni M. "Micro-mechanical Modeling of Brownian Spheroids in Oscillatory Shear Flow". Research Showcase @ CMU, 2018. http://repository.cmu.edu/dissertations/1144.
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We calculate the stress response, or rheology, of a micro-mechanical model suspension of rigid, Brownian spheroids in a Newtonian fluid in an oscillatory shear flow. The straining and rotation components of a linear flow affects the microstructure, or particle orientation in space and time, and thus, the suspension stress. A statistical description of the microstructure is given by an orientation probability distribution function, which quantifies the likelihood of a particle possessing a particular orientation at an instance in time. The evolution of the microstructure results from the memory of the material, advection from the flow, and rotational Brownian motion. The macroscopic stress response is calculated from ensemble averages of the stresslet weighted by the orientation distribution function. First, we calculate the linear stress response of a dilute suspension of rigid, spheroidal, self-propelled particles under a small-amplitude oscillatory shear deformation using regular perturbation theory. The particle activity leads to a direct contribution to the material stress, via self-propulsion, and an indirect contribution due to correlated tumbling events. The mechanism and strength of self-propulsion and correlation between tumbling events can be determined from the linear stress response of an active suspension. Next, we develop a framework for determining the relaxation moduli of a viscoelastic material through the combination of a memory integral expansion and a multimode-frequency oscillatory shear flow. We analytically determine the first nonlinear relaxation modulus of the model suspension through a comparison of the second normal stress difference from the microstructural stress response, calculated via regular perturbation theory, and a co-rotational memory integral expansion. The stress response of the system is reconstructed for the start-up and cessation of steady simple shear and uniaxial extension. Finally, we numerically calculate the nonlinear viscoelasticity of the model system subject to a large-amplitude oscillatory shear flow. In a sufficiently strong flow with oscillation frequency comparable to the material relaxation rate, secondary overshoots in the stress response occur. We attribute the origin of secondary overshoots to particles undergoing a Jeffery orbit during a (half) cycle of the oscillation, analogous to the case of non-Brownian spheroids in steady shear flow.
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Saraiva, Rui Miguel da Costa Neves. "The characterisation of mixing for oscillatory flow within baffled tubes". Thesis, University of Cambridge, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.368295.
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Ourmières, Yann. "Oscillatory wave induced boundary layer flow over a rippled bed". Thesis, University of Southampton, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.289685.
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Sparrow, Kathryn. "The effect of bed permeability on oscillatory boundary layer flow". Thesis, University of Aberdeen, 2013. http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=201986.
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Oscillatory boundary layer flow is found under waves in the near shore region. This region is responsible for large volumes of sediment transport and whilst a large number of studies have investigated the broader problem only a handful of studies have investigated the boundary layer flow in detail. Most of these studies have also only considered boundary layer flow over rough impermeable beds and therefore do not take into account the potential effect of the flow interaction with a permeable bed. Similar studies in other felds of environmental fluid mechanics have concluded that the presence of a permeable bed can lead to significant diff erence in the boundary layer hydrodynamics when compared to the equivalent flow over an impermeable bed. This thesis presents a series of laboratory experiments that have been conducted over an impermeable gravel bed and a permeable gravel bed to identify the differences in the boundary layer hydrodynamics with a particular reference to the differences in the horizontal velocity profile, the shear stress, turbulence and the bed friction factor. The results indicate that a difference exists in the near-bed flow between the two bed cases. The results also demonstrate that the magnitude of the differences vary with flow Reynolds number, so the more energetic the wave the larger the impact of the permeable bed. The bed shear stress and resulting friction factor was found to be 40% greater for the flows over the permeable bed for the most energetic test case whilst the friction factor for the two bed cases for the least energetic test case are similar. The results also indicate that the permeable bed introduces an unexpected asymmetry in the flow cycle. This is attributed to the high vertical velocities that have been observed. A second series of experiments have been conducted to shed light on the interaction between the flow above the bed and within the bed to help explain the results from the initial gravel-bed experiments. The second series of experiments have been conducted over a regular permeable bed that allows for velocity measurements within the pores. The results indicate that the horizontal velocity, shear stress and turbulence all display signs of being effected by the exchange of flow, or ventilation, that naturally occurs.