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Levold, Pål. "Viscous Flow Around Finite Lenght Circular Cylinder." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for marin teknikk, 2012. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-18641.
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Viscous flow around circular cylinders is a classical research topic in fluid dynamics with a vast amount of practical applications in the field of offshore marine technology. In the flow around cylinders of finite length, complex wake behaviours and coherent structures occur even at relatively low Reynolds numbers. An understanding of the nature and dynamics behind such behaviour could form a basis for improved designs and innovative solutions for offshore and subsea constructions.In the present study, flow around long finite cylinders at Re = 100 is investigated numerically using the incompressible Navier-Stokes Equations solver MGLET. To study the isolated flow near the free end, a cylinder with aspect ratio L/D = 50 is chosen. The flow over the free end gives rise to a wake consisting of two vortex shedding cells with different shedding frequencies; one small near the free end and one larger in the central region of the span. It is found that each vortex shed in the end cell bends horizontally and connects with the upstream vortex shed from the opposite side of the cylinder. The horizontal vortex shedding is found to give rise to a pair of trailing vortices in the time averaged flow.When a vortex is shed with a large phase difference between the two cells, the vortex is split and connects with other surrounding vortices. This phenomena is commonly referred to as vortex dislocations and occurs with the beat frequency, i.e. the difference between the two vortex shedding frequencies. It is found that this frequency can be detected in time histories of $u$ in the wake at the spanwise centre.A second configuration, consisting of a wall mounted cylinder with aspect ratio L/D = 25 is simulated in order to study the effect of introducing a no-slip surface. The effect on the end cell is found to be minimal, while the central cell shedding frequency is reduced. Comparisons with published data on a cylinder with aspect ratio L/D = 25 and two free ends shows that both the reduction of aspect ratio and the introduction of the no-slip boundary condition contributes to the reduced shedding frequency.
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AlRefaie, Abdulaziz Mohammed. "Flow control around circular cylinder : ventilation holes method /." Connect to full text in OhioLINK ETD Center, 2009. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=toledo1260201547.
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Thesis (M.S.)--University of Toledo, 2009.Typescript. "Submitted as partial fulfillment of the requirements for the Master of Science Degree in Mechanical Engineering." "A thesis entitled"--at head of title. Bibliography: leaves 51-54.
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AlRefaie, Abdulaziz M. "Flow Control Around Circular Cylinder: Ventilation holes Method." University of Toledo / OhioLINK, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1260201547.
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Bjørkli, Rune. "Numerical Simulations of Viscous Flow Around Stepped Circular Cylinder." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for marin teknikk, 2012. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-18612.
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A stepped cylinder could be a desired design for an offshore buoy or SPAR platform. The geometry of a stepped cylinder consists of a small diameter cylinder (d) placed on top of a large diameter cylinder (D). This master thesis has investigated numerically the flow around a stepped cylinder with different diameter ratios (d/D) for a Reynolds number, ReD = 150. The commercial software Fluent v13.0 by Ansys was used for the numerical investigation.The aim of the study has been exploring the nearby wake flow as well as the region where the two cylinders are joined. The hydrodynamic forces and vortex shedding frequency have been analysed and compared for the four different diameter ratios: d/D = 0.3, 0.5, 0.8 and 0.9The major part of the published papers on the topic is based on experimental studies. Only two papers are based on numerical studies, having considered d/D = 0.5 exclusively. Earlier studies have focused mainly on vortex shedding in the wake flow, omitting the forces acting on the stepped cylinder.The stepped cylinder has been modelled using the software GAMBIT. A convergence study investigating the domain size and element density was conducted to ensure a grid independent solution. Special attention was directed at the step region to fully resolve the complex flow in this region. The numerical model was verified to be in good agreement with previous experimental- and numerical studies.For the stepped cylinders significant spanwise velocity was detected in the step region. For d/D = 0.3 and 0.5, upflow was detected over the leading edge of the step whereas downwash characterised the trailing edge of the step. Similarities to the flow around a finite length cylinder could be drawn for d/D = 0.3 and 0.5, whereas d/D = 0.8 and 0.9 resembled that of a straight cylinder.The step was found to affect the wake flow ≈ 10D into D independent of diameter ratio. The wake flow behind the small diameter cylinder was less affected by the step than the large for d/D < 0.5. In the step region two distinct streamwise vortices were detected. A pair of edge vortices as well as a junction vortex were readily detected for d/D = 0.3 and 0.5. The junction vortex was not detected for d/D = 0.8 and 0.9 due to the small step change in diameter.The drag force on D was found to increase as d/D increased. Similarly, the amplitude of the lift force was also found to increase as d/D increased. The mean drag-coefficient varied along the span with peaks in the local drag-coefficient observed in close vicinity of the step.Regular spanwise vortex shedding was detected away from the step at a frequency similar to that of a straight cylinder. In the step region, located mainly on D, a cell of lower vortex shedding frequency was detected for d/D = 0.3 and 0.5. As d/D increased this cell seemed to disappear. Suppression of regular vortex shedding close to the step for d/D = 0.3 was observed for ReD = 150, 300 and 600. For ReD > 150 the large spanwise vortex structures were still discernible, but the presence of small-scale streamwise vortices complicated the flow.
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Engelbreth, Knut Inge. "Viscous flow around a circular cylinder near a plane wall." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for marin teknikk, 2011. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-15482.
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Marine pipelines placed at or near the seabed are exposed to currents and waves. In the presence of a sedimentary seabed, the interaction between the surrounding water motions and the pipeline may cause erosion of sediments beneath the pipeline. Due to this erosion or due to the installation procedure, free spans may occur in sections along the span of the pipeline. A simplified description of the flow is obtained by approximating the seabed as a plane and impermeable wall. Most of the previous experimental and numerical studies on this flow are performed at Reynolds numbers, Re, in the range 103-105 as this range is of most relevance for offshore engineering purposes. Numerical modeling at such Reynolds numbers generally involves different kinds of artificial flow modeling. In this study, the software OpenFOAM is applied for numerical simulations of the viscous flow around a circular cylinder at Re= 100. At this Reynolds number the flow around an unconfined circular cylinder is characterized by vortex shedding and yet turbulence is avoided, which provides the possibility of accurate calculations of the flow. The flow is investigated with reference to visualizations of pressure, velocity and vorticity; and the flow characteristics are quantified in terms of drag and lift acting on the cylinder and vortex shedding frequency. Numerical simulations of an unconfined cylinder in uniform cross-flow constitutes the basis of the present study, and these simulations are included in an attempt to validate the results and the applied computational method. The results for this flow are within the scatter of the reported values in the literature. In this study, the flow around a cylinder near a plane wall is of main interest. Three different gap ratios G/D=0:2;0:5;1:0 are applied. The results are generally in accordance with published data from numerical simulations; best agreement is found at G/D=0:2 and G/D= 0:5. The results are supportive to the suggestion of vortex shedding suppression to be caused by the interaction between the lee-side recirculating flow and the gap flow, which inhibits large-scale vortex roll-up. Further, the results at G/D= 1:0 indicates cancellation of opposite signed vorticity in the near-wall region, in accordance with suggestions in the literature. Additionally, the geometry of the wall is altered, introducing a hollow below the cylinder. This shape imitates a fully developed scour profile. These simulations are expected to bring new results to this topic of research. The flow is characterized by evident vortex shedding. Further, at this gap a distinct mean lift in the direction towards the wall is observed and both the drag coefficient and the frequency of vortex shedding is reduced as compared to the flow around a cylinder in uniform cross-flow. The obtained results exhibits similarity to published experimental data for the flow at Re= 1:104. Two-dimensional simulations are performed for all of the flow configurations, and a few three-dimensional simulations are performed for a cylinder in uniform cross-flow and a cylinder located a distance G/D= 0:5 from a plane wall. Due to the two-dimensional flow patterns, insignificant differences are found between the two-dimensional and three- dimensional simulations. In this study, emphasize is given to the influences on the solution of the following numerical parameters: time step, domain size, grid geometry, element size and element spacing. These parameters are thoroughly investigated in terms of convergence studies. Also included in this thesis is a review of some features of these flows and an overview of the governing equations, OpenFOAM and the applied solver icoFoam.
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Thingbø, Sunniva Selstad. "Simulation of viscous Flow around a circular Cylinder with STAR-CCM+." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for marin teknikk, 2013. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-22378.
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In this thesis, three-dimensional modelling of the flow around a circular cylinder isaccomplished. Two cases are considered. The first (Case 1) is a cylinder in steady,uniform current subjected to Reynolds number 3900. The second case (Case 2) isa cylinder in the vicinity of a rigid wall, also at Reynolds number 3900. For thesecond case, both a boundary layer velocity profile (Case 2a) and a uniform inletvelocity profile (Case 2b) is simulated. The gap-to-diameter ratio, e/D, is set to 0.2 for Case 2. Large eddy simulations (LES) with Smagorinsky subgrid scale (SGS)model are applied to simulate the flow. LES have ability to resolve fine structuresin the turbulent wake of the cylinder. A mesh convergence study is accomplishedfor Case 2a.In the first part of the thesis, the background theory for the case is given in addition to information about the applied pre- and postprocessing tools. In the lastpart of the thesis, the case set-up is described and the results are presented anddiscussed. Velocity profiles in the cylinder wake, hydrodynamic values and pressuredistribution on the cylinder wall are investigated to give a better understandingof the physics in the cases. The results are compared to published experimentalmeasurements and numerical studies.For the first case, the results tend to agree well with published research. The softwaresystem with LES and the Smagorinsky subgrid scale model does successfullysimulate the flow in the boundary layers, the shear layers and the near wake. Theresults are interesting in the context of flow bifurcation for at cylinder subjectedto Reynolds number 3900.The vortex shedding is suppressed for both Case 2a and Case 2b, as expected. Forthe cylinder in Case 2a, a decreased drag coefficient is observed when comparedto Case 1. This is not observed for the cylinder in Case 2b. The two cases areobserved to have an increased mean lift coefficient caused by the vicinity of the wall. However, for Case 2b, the increase is of larger magnitude. This is addressed to the pressure distribution on the cylinder surface. There are also observed differences in the wake statistics for Case 2a and Case 2b. It is concluded that both gap-todiameter ratio and boundary layer thickness have a significant influence on the flow around a circular cylinder.
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Fredrickson, Kent Allen. "Numerical study of non-impulsively started flow around a circular cylinder." Thesis, Monterey, California : Naval Postgraduate School, 1990. http://handle.dtic.mil/100.2/ADA238051.
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Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, September 1990.Thesis Advisor(s): Sarpkaya, Turgut. "September 1990." Description based on title screen as viewed on December 16, 2009. DTIC Identifier(s): Channel flow, cylinders, wake, impulsive flow, theses. Author(s) subject terms: Hydrodynamics, numerical calculations, wake, cylinder, impulsive flow. Includes bibliographical references (p. 65-66). Also available in print.
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Zhao, Xingyuan. "Cross-flow around and stability of multiple circular cylinders." Thesis, University of Strathclyde, 2010. http://oleg.lib.strath.ac.uk:80/R/?func=dbin-jump-full&object_id=13231.
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Panchal, Jay K. "Flow around a rotating circular cylinder with an end plate near a plane wall boundary." Thesis, California State University, Long Beach, 2013. http://pqdtopen.proquest.com/#viewpdf?dispub=1522645.
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The objective of the present study is to investigate the characteristics of a flow around a rotating circular cylinder with and without an end plate near a wall boundary. The different cases which are taken into consideration in the current investigations were with gap ratios of 0.1d, 0.5d, 1.0d, 1.5d and 2.0d. A symmetric end plate is attached behind the rotating circular cylinder at a distance of 0.1d from the cylinder and a gap ratio of 1.5d. We performed Computational Fluid Dynamics (CFD) simulation of the flow around a rotating circular cylinder near a plane wall boundary using a CFD solver, STAR-CCM+. Free-stream velocity is kept constant at 5 m/s and the Reynolds number calculated is 3.24X104. We then studied the flow characteristics such as lift and drag generated on the circular cylinder with and without an end plate and the wake structure. We observed that the vortex suppression is increased when the gap ratio is reduced, i.e., when the circular cylinder is nearer to the plane wall boundary. As the gap ratio increases the drag force generated decreases and the lift force increases considerably. In the case of rotating circular cylinder with an end plate, the wake area has moved upwards and the lift generated has increased manifold.
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Javaid, Muhammad Salik. "Measurement of pressure distribution around a circular cylinder on a plane wall in oscillatory flow." Diss., Georgia Institute of Technology, 1994. http://hdl.handle.net/1853/20733.
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Tremblay, Frederic. "Direct and large eddy simulation of flow around a circular cylinder at subcritical Reynolds numbers." [S.l.] : [s.n.], 2001. http://deposit.ddb.de/cgi-bin/dokserv?idn=963771507.
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Ortega, Mariana Silva. "Suppression of vortex-induced vibration of a circular cylinder with fixed and rotating control cylinders." Universidade de São Paulo, 2015. http://www.teses.usp.br/teses/disponiveis/3/3135/tde-15072016-152949/.
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The offshore oil industry is engaged in the development of new floating platforms, such as Spar, semi-submersible, tension-leg, FPSO and monocolumn for the exploration of deep and ultra-deep waters. Some of these floating systems have circular cross sections (or cross sections of other bluff geometries) being susceptible to vortex-induced vibrations (VIV). Vortex shedding behind a bluff body can be altered, suppressed or controlled over a limited range of Reynolds numbers. Various flow-control techniques, which result in the reduction of drag and unsteady forces, have been suggested and tested in simple geometries. One such method is the moving-surface boundary layer control (MSBC), in which smaller control rotating cylinders are placed close to the bluff body. This method is considered as an inspiration for the present experimental investigation of VIV suppression for omni-directional flows. In this context, three different configurations have been assembled to compare the effect of suppression on a plain cylinder surrounded by two, four and eight control cylinders distributed symmetrically around it. Experiments were carried out with static models and models free to oscillate in one-degree-of-freedom with fixed and rotating control cylinders. Experiments with a plain cylinder were performed to serve as reference. Displacements, drag and lift forces were measured. The position of the control cylinders proved to be an important parameter to VIV suppression. Configurations with two control cylinders increased lift and drag forces. In contrast, configurations of four and eight control cylinders showed to be more effective to suppress VIV. Furthermore the results for all the cases of the configuration of eight fixed control cylinders presented a reduction of displacement amplitude, lift and drag forces when compared to a plain cylinder. However, when the control cylinders were actuated, the two cases with rotating control cylinders increased drag force when compared to fixed control cylinders.A indústria offshore está envolvida no desenvolvimento de novas plataformas flutuantes como Spar, semi-submersível, TLP, FPSO e monocoluna para a exploração de águas profundas e ultra-profundas. Alguns destes sistemas flutuantes têm seções transversais circulares (ou de outras seções rombudas) sendo susceptíveis à vibrações induzidas por vórtices (VIV). A esteira de vórtices desprendida de um corpo rombudo pode ser alterada ou suprimida ao longo de uma faixa de número de Reynolds. Várias técnicas de controle do escoamento foram sugeridas e testadas em geometrias simples, resultando na redução de forças de sustentação e arrasto. Um desses métodos é o controle de camada limite por superfícies móveis (CCLSM), no qual cilindrinhos rotativos de controle são colocados próximos ao corpo rombudo. Neste trabalho, este método foi abordado através de uma investigação experimental como um supressor de VIV para o escoamento omnidirecional. Neste escopo três diferentes configurações foram montadas para comparar o efeito de supressão sobre um cilindro liso rodeado por dois, quatro e oito cilindros de controle, distribuídos simetricamente em torno dele. Foram realizados ensaios com o modelo estático, ensaios de VIV em um grau de liberdade com cilindros de controle fixos e rotativos. Foram medidos deslocamento e forças de sustentação e arrasto. Os resultados mostraram que a posição dos cilindros de controle é um parâmetro importante para a supressão de VIV. A configuração com dois cilindros de controle aumentou as forças de sustentação e arrasto. Diferentemente, as configurações de quatro e oito cilindros de controle mostraram-se mais eficazes para suprimir VIV. Além disso, todos os casos da configuração de oito cilindros de controle fixos apresentaram redução nas amplitudes de vibração e nas forças de sustentação e arrasto, quando comparados com um cilindro liso. No entanto, quando os cilindros de controle foram acionados para rotacionar, mostrou-se um aumento na força de arrasto em relação aos cilindros de controle fixos.
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Kuehtz, Silvana. "Experimental investigation of oscillatory flow around circular cylinders at low β numbers." Thesis, Imperial College London, 1996. http://hdl.handle.net/10044/1/8866.
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Al-Rubaiy, Ahmed Abed Ali Ghaidan. "The effect of surface roughness and free stream turbulence on the flow and heat transfer around a circular cylinder." Thesis, University of Sheffield, 2018. http://etheses.whiterose.ac.uk/20219/.
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Carmo, Bruno Souza. "On wake interference in the flow around two circular cylinders : direct stability analysis and flow-induced vibrations." Thesis, Imperial College London, 2009. http://hdl.handle.net/10044/1/4697.
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The flow around two identical circular cylinders, arranged in configurations where one of the cylinders is immersed in the wake of the other, is studied using numerical simulations. Two aspects of such flows were considered. The first is the stability of nominally two-dimensional time-periodic wakes to three-dimensional perturbations. We investigated flows around tandem and staggered arrangements with diverse centre-to-centre distances. Direct stability analysis and numerical simulations were employed, and the results are compared to those obtained for the flow around an isolated cylinder. The onsets of the three-dimensional instabilities were calculated and the unstable modes are fully described. In addition, we assess the nonlinear character of the bifurcations and physical mechanisms are proposed to explain the instabilities. The second aspect considered in this thesis is the flowinduced vibration experienced by a rigid cylinder when it is mounted on an elastic base and immersed in the wake of a fixed identical cylinder. Tandem arrangements with centre-to-centre distances varying from 1.5 to 8 cylinder diameters were tested. The flow was simulated using an Arbitrary Lagrangian-Eulerian approach that coupled the solution of the structure equations with that of the flow. Two- and three- dimensional simulations were performed to assess the mutual influence between the three-dimensional flow structures in the wake and the motion of the cylinder. The response of the downstream cylinder is compared to that of an elastically-mounted rigid isolated cylinder. Based on the simulation results we propose physical mechanisms to explain the origin of the excitation.
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Korkischko, Ivan. "Investigação experimental e analítica do escoamento ao redor de cilindro circular com supressores de VIV." Universidade de São Paulo, 2011. http://www.teses.usp.br/teses/disponiveis/3/3150/tde-08122011-150201/.
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O escoamento de um fluido ao redor de corpos rombudos configura-se em um dos principais problemas da mecânica dos fluidos. As investigações deste fenômeno são baseadas em técnicas analíticas, numéricas e experimentais. Cada abordagem apresenta suas vantagens e desvantagens, sendo impossível a plena caracterização do escoamento através de uma única técnica para a maioria das geometrias de corpo e condições de escoamento. É neste contexto que se insere esta tese, que compreende um estudo fundamental sobre a dinâmica de formação e desprendimento de vórtices baseado em técnicas experimentais. Primeiramente, realiza-se um estudo de estabilidade hidrodinâmica do escoamento em torno de cilindro circular via modelo de Ginzburg-Landau. Os resultados experimentais deste estudo serviram para validar simulações numéricas e a principal técnica experimental utilizada foi a Velocimetria por Imagem de Partículas (PIV). A decomposição de campos de velocidade em série de Fourier permite verificar a hierarquia sugerida pela solução assintótica da bifurcação de Hopf. Além disso, os resultados também indicam quando a tridimensionalidade passa a ser importante no escoamento. A tridimensionalidade do escoamento está intimamente ligada ao fenômeno de vibração induzida por vórtices (VIV), que exerce efeitos danosos em uma grande quantidade de estruturas sujeitas a escoamentos fluidos. Com o objetivo de suprimir as VIV, realizou-se um estudo paramétrico do supressor do tipo strakes helicoidal, que é bastante empregado na indústria offshore. Verificou-se que os strakes modificam as características tridimensionais da esteira de um cilindro. Apesar da eficiência dos strakes na supressão de VIV, eles possuem algumas limitações inerentes às características hidrodinâmicas do escoamento. Sendo assim, apresenta-se uma técnica que reduz as tridimensionalidades de modo a tornar o escoamento bidimensional, com a possibilidade de controle ativo de malha fechada. O método de controle de camada limite por superfícies móveis (CCLSM) suprime VIV e diminui o arrasto atrasando a separação da camada limite do cilindro através da injeção de quantidade de movimento angular pelos cilindros de controle rotativos.The fluid flow around bluff bodies is one of the main problems in fluid mechanics. The investigations of this phenomenon are based on analytical, numerical and experimental techniques. Each technique presents advantages and disadvantages, being impossible the comprehensive characterization of the flow though only one technique for the majority of body geometries and flow conditions. Within this context, the present thesis is proposed, which comprehends a fundamental study about the vortex shedding dynamics based on experimental techniques. Firstly, a hydrodynamic stability study of the flow around a circular cylinder is performed using the Ginzburg-Landau model. The experimental results of this study allowed the validation of numerical simulations and the main experimental technique employed was the Particle Image Velocimetry (PIV). The Fourier series decomposition of velocity fields permits to verify the hierarch suggested by the asymptotic solution of the Hopf bifurcation. Additionally, the results also indicate when the three-dimensionalities become important in the flow. The flow three-dimensionality is closely associated to the vortex-induced vibration (VIV) phenomenon, which exerts damaging effects on a great quantity of structures subjected to fluid flows. In order to suppress VIV, it was realized a parametric investigation of the helical strakes, that are commonly employed in the offshore industry. One verifies that the strakes modify the three-dimensional characteristics of the cylinder wake. Despite of the strake efficiency regarding the VIV suppression, the strakes have some limitations that are inherent to the flow hydrodynamic features. Consequently, it is presented a technique that reduces the three-dimensionalities in order to create a two-dimensional flow, with the possibility of closed-loop active control. The moving surface boundary-layer control (MSBC) suppresses VIV and reduces the drag delaying the cylinder boundary-layer separation through the injection of angular momentum by the rotational control cylinders.
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Lavinas, Pedro Nery. "Escoamento ao redor de um cilindro circular: derivação da equação de Landau a partir das equações de Navier-Stokes." Universidade de São Paulo, 2010. http://www.teses.usp.br/teses/disponiveis/3/3150/tde-10012011-134141/.
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Este trabalho aborda o escoamento incompressível ao redor de um cilindro circular. A tese que se quer defender, com base em experimentos numéricos, é: A equação de Landau pode ser obtida a partir das equações de Navier-Stokes por uma análise de estabilidade não-linear global. A teoria produz um procedimento bem-definido para determinação dos coeficientes da equação de Landau, permitindo assim a sua interpretação como um modelo simplificado (equações reduzidas de Navier-Stokes) para a predição das forças aplicadas pelo fluido ao cilindro, que podem ser comparados com resultados experimentais. O modelo não-linear se baseia em uma teoria assintótica que, como se sabe, tem sua faixa de validade no espaço de parâmetros determinada a posteriori, por meio da própria comparação com dados de laboratório. Resultados na faixa 46 <= Re <= 80 são apresentados. Descobriu-se, que a faixa de aplicabilidade da teoria como aqui exposta é restrita, não excedendo em muito o valor crítico do número de Reynolds. Argumentos são expostos para justificar esta afirmação e possíveis maneiras de modificar a teoria para estender esta faixa são apresentadas. São reportados, ainda, teoria e resultados sobre um novo tipo de condição de contorno,denominado impedância fluida, que permite reduzir o tamanho do domínio de cálculo necessário para simulação de escoamentos externos, comparativamente à comumente utilizada condição de outflow. Neste caso, abordou-se a faixa 20 <= Re <= 600.This work adresses the incompressible flow around a circular cylinder. What we want to prove, based on numerical experiments, reads: The Landau equation can be derived from the Navier-Stokes equations by means of a global nonlinear stability analysis. The theory leads to a procedure for calculating numerically the coefficients of these equation, thus permitting their interpretation as a simplified model - reduced Navier-Stokes equation - for the prediction of the forces applied by the fluid on the cylindrical structure, which can be compared against experimental data. The nonlinear model is based on an asymptotic theory which, as is known, has its validity range in the parameter space determined a posteriori. The focus lies in the range 46 <= Re <= 80. It was found that the theorys applicability range as presented here is restricted to a small neighborhood of Rec. This affirmation in justified and possible means of modifying the theory in order to enlarge this range are proposed. Theory and results concerning a new type of boundary condition called fluid impedance are also reported, permitting the reduction of the domain size necessary for simulating external flows, comparatively to the commonly used outflow condition. In this case, the range 20 <= Re <= 600 was considered.
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Leong, Chi Seng. "Gravity current around circular cylinder." Thesis, University of Macau, 2010. http://umaclib3.umac.mo/record=b2182938.
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Cicolin, Murilo Marangon. "Supressão da vibração induzida por vórtices de cilindros com malha permeável." Universidade de São Paulo, 2015. http://www.teses.usp.br/teses/disponiveis/3/3135/tde-07032016-102934/.
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O fenômeno de vibração induzida por vórtices (VIV) é particularmente danoso para estruturas submarinas como risers de exploração de petróleo. A maneira mais usual de se atenuarem os efeitos de VIV é instalar um supressor, como por exemplo strakes ou fairings. Dentre esses, foi desenvolvido por All Brow Universal Components um supressor chamado Ventilated Trousers (VT), que consiste em uma malha permeável feita de uma rede flexível e dezenas de bobbins. Através de um estudo experimental, procurou-se investigar os mecanismos hidrodinâmicos pelos quais o supressor V T funciona. Foram construídos três modelos diferentes de supressores: um modelo idêntico ao V T e duas malhas dele derivadas, alterando-se a geometria dos bobbins e a distribuição destes ao redor da malha. Foram realizados ensaios com o modelo xo e ensaios de VIV em um grau de liberdade alterando-se o amortecimento estrutural. Foram medidos deslocamento e forças de sustentação e arrasto. Os resultados mostraram que o supressor do tipo V T reduz as amplitudes de vibração, força de sustentação e arrasto quando comparados com um cilindro oscilando. No entanto, aumenta a força de arrasto quando comparado com o cilindro xo. A geometria da malha mostrou-se de grande importância para a supressão de VIV. Modelos que possuem o disco externo no bobbin impedem o surgimento de folga entre o modelo e o cilindro, além de aumentar o amortecimento hidrodinâmico. Três hipóteses foram levantadas para explicar o funcionamento do supressor V T. A primeira diz que a supressão é provocada pelo aumento do amortecimento hidrodinâmico. Os ensaios mostraram que, de fato, o supressor V T aumenta o amortecimento e, consequentemente, diminui as VIV. No entanto, somente esse efeito não explica toda a supressão obtida. As outras hipóteses, relacionadas à alterações bi e tridimensionais da esteira, foram avaliadas, porém não se pode afirmar que alguma delas seja isoladamente responsável por produzir o mecanismo hidrodinâmico de supressão.The phenomenon of vortex-induced vibration (VIV) is particularly harmful to submarine structures such as risers used for oil extraction. The most usual way to attenuate the effects of VIV is the installation of suppressors, like strakes or fairings. Among them, All Brow Universal Components developed a VIV suppressor called Ventilated Trousers (VT), which consist of a permeable mesh made of a flexible net and tens of bobbins. Three different models of suppressors based on permeable meshes have been assembled with the objective to understand the hydrodynamic mechanism behind the suppression: one model identical to the VT and two meshes with different bobbin geometries and distribution. Tests were carried out with xed models and models free to oscillate in one degree of freedom varying the structural damping. Displacements, drag and lift forces were measured. Results showed that the VT suppressor reduced vibration amplitudes, lift and drag forces when compared to an oscillating circular cylinder. However, it increased drag force when compared to a fixed circular cylinder. The mesh geometry proved to be important to VIV suppression. Models that had an external disc on the bobbins avoided the appearance of a gap between the model and the cylinder. Three hypotheses were formulated to explain how the VT suppressor works. The first one says that the increase on hydrodynamic damping is responsible for suppression. In fact, tests showed that the VT increased hydrodynamic damping and, consequently, reduced the VIV response. However, this effect alone does not explain the suppression as a whole. The other two hypotheses related to two-dimensional and three-dimensional wake changes were evaluated, but it cannot be stated that any of them, on its own, is responsible for the whole of the suppression mechanism.
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Gioria, Rafael dos Santos. "Estudo da estabilidade secundária da esteira de um cilindro em oscilação forçada." Universidade de São Paulo, 2010. http://www.teses.usp.br/teses/disponiveis/3/3150/tde-11082010-123926/.
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Esta tese apresenta o estudo da transição para um escoamento tridimensional da esteira de um cilindro oscilando. Esta é a transição secundária do escoamento ao redor de um cilindro, sendo a primária a própria esteira de von Kármán. A investigação é realizada na mesma faixa de número de Reynolds (Re) que ocorre a transição da esteira de um cilindro fixo: 200 Re 400. O estudo envolve simulações numéricas diretas bi- e tridimensionais do escoamento incompressível ao redor de um cilindro oscilando usando o método dos elementos espectrais. A transição também é analisada através do estudo de estabilidade linear do escoamento. O método de Gaston Floquet é adequado para a análise de estabilidade a perturbações tridimensionais devido à periodicidade característica da esteira de von Kármán. Além disto, o método é mais geral e não é aplicado somente a equações autônomas com soluções periódicas: ele também pode ser usado em análises cujo campo base é estacionário como a análise da transição primária da esteira de um cilindro. Mostra-se que a transição da esteira para tridimensionalidade é influenciada pela oscilação do cilindro. As oscilações podem atrasar a transição quando em amplitudes baixas e com escoamento a um número de Reynolds até 260. Em outros casos, a transição é similar à observada no escoamento ao redor um cilindro fixo. Além disso, quando há mudança de padrões de desprendimento de vórtices devido às oscilações, desencadeiam-se modos instáveis diferentes dos observados na esteira de um cilindro fixo. A comparação dos resultados da análise de estabilidade de Floquet com simulações numéricas diretas e experimentos publicados mostra na maioria dos casos que, apesar de ser uma análise de estabilidade linear, muitas características da análise persistem além do limiar de estabilidade. As situações com discrepâncias são identificadas nesta tese, como padrão de desprendimento diferentes em simulações bi- e tridimensionais na mesma situação. As causa das discrepâncias são estudadas através das simulações numéricas diretas e considerações sobre o sistema linearizado e a abrangência da análise de estabilidade. Em adição, mostra-se que há um limiar de amplitude de oscilação acima do qual a dinâmica da esteira, e portanto a transição secundária, é afetada. Abaixo deste limiar, retoma-se o comportamento do escoamento ao redor de um cilindro fixo.This thesis presents the investigation of the transition to a three-dimensional flow in the wake of an oscillating circular cylinder. This is the secondary transition of the flow around a circular cylinder, while the primary transition leads to the von Kármán wake. The investigative work is performed in the Reynolds number (Re) range which occurs the secondary transition of the wake o a circular cylinder: 200 Re 400. The study consists of two- and three-dimensional direct numerical simulations of the incompressible flow around an oscillating circular cylinder by means of the spectral/hp method. The transition is also analyzed through linear stability study of the flow. The Gaston Floquet method is an appropriate method for this linear stability analysis to three-dimensional perturbations due to the typical periodicity of the von Kármán wake. Furthermore, the Floquet method is of a general kind and it is not specifically applied to autonomous equation with periodic solutions: it can be employed in analysis with a stationary base flow like the primary transition of the wake of a circular cylinder. The thesis shows that the transition to a three-dimensional wake is affected by the cylinder oscillation. The oscillations can delay the transition when they have low amplitudes and the flow has Reynolds number below 260. In other situations, the transition is similar to that observed in the flow of a fixed circular cylinder. In addition, when there is a change in the vortex shedding pattern due to the oscillatory motion, the observed unstable modes are different than those in a wake of a fixed circular cylinder. The comparison of the results from the Floquet stabilty analysis with the ones from direct numerical simulations and published experiments shows that in most cases, despite the linearization for the stability analysis, many features of the analysis persist beyond the instability threshold. The discrepant features observed are specified in this thesis, e.g. vortex shedding patterns different in two- and three-dimensional simulations with the same parameters. The cause of the discrepancies are studied through direct numerical simulations and careful considerations on the linearized system and the range of the stability analysis. In addition, this thesis shows that there is a threshold amplitude of oscillation for which the wake dynamics, therefore the secondary transition, is affected. Below this threshold, the wake behaves the same way as in the flow around a fixed circular cylinder.
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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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Tabrizi, Seyed Pariviz Alavi. "Jet impingement onto a circular cylinder." Thesis, University of Liverpool, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.263841.
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Fung, Ka Shuen. "Separated and reattaching flow on a circular cylinder." Thesis, Imperial College London, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.339118.
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盧建弘 and Kin-wang Lo. "The flow dynamics of an asymmetric circular cylinder." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1996. http://hub.hku.hk/bib/B31235116.
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Lo, Kin-wang. "The flow dynamics of an asymmetric circular cylinder /." Hong Kong : University of Hong Kong, 1996. http://sunzi.lib.hku.hk/hkuto/record.jsp?B17490741.
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Hanson, Craig D. "Numerical Analysis of Oscillating Flow about a Circular Cylinder." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1995. http://handle.dtic.mil/100.2/ADA305867.
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Lotshaw, John E. "Numerical analysis of oscillating flow about a circular cylinder." Thesis, Monterey, California. Naval Postgraduate School, 1992. http://hdl.handle.net/10945/23993.
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Pattenden, Richard John. "An investigation of the flow around a truncated cylinder." Thesis, University of Southampton, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.409906.
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Ortega, Lopez Miguel Dario. "Exploring and Envisioning Periodic Laminar Flow Around a Cylinder." Thesis, Virginia Tech, 2009. http://hdl.handle.net/10919/32236.
Full textAbstract:
It is well known that for small Reynolds numbers, flow around a cylinder is laminar and stable.
For larger Reynolds numbers, although the flow regime remains laminar, the formation of
complex periodic structures appear downstream. The cyclic nature of this periodic flow is well
characterized by the vortex shedding frequency and Strouhal number. However, complexities of
these periodic structures downstream continue to be a topic of research. Periodic laminar 2D
incompressible viscous flow around a cylinder is simulated using OpenFoam, an open source
computational fluid dynamics program. To better understand these complex structures
downstream, a customized computer graphical tool, VerFlow-V.01, was created to analyze and
study OpenFoam simulation results. This study includes an investigation of calculating the
details of drag and lift coefficients for the cylinder using mathematical models that integrate
properties in subdomains, an approach not previously explored to the knowledge of the author.
Numerical integration is accomplished using a finite difference approach for solving surface and
contour integrals in subdomains of interest. Special attention is given to pressure and to the
second invariant of the velocity gradient, as they have a clear mathematical relationship, which is
consistent with results previously published. A customized visual data analysis tool, called
VerFlow-V.01, allowed investigators to compare simulation data variables in a variety of useful
ways, revealing details not previously understood. Main subroutines and a userâ s manual are
included as appendices to encourage reproducibility and future development of the numerical,
analytical and graphical models developed here. Together these models resulted in a new
understanding of periodic laminar flow around a cylinder. A unique approach was developed to
qualitatively understand the origins of drag and lift coefficients associated with properties
mapped as images in subdomains of interest downstream. These results explain the development
of convergent, eddy, and stream zones embedded in flow fields downstream.Master of Science
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Smith, Heather D. "Modeling the flow and scour around an immovable cylinder." The Ohio State University, 2004. http://rave.ohiolink.edu/etdc/view?acc_num=osu1303136105.
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Begum, Assma. "Numerical Investigations of Flow Around a Wire-wrapped Rotating Cylinder." Thesis, California State University, Long Beach, 2018. http://pqdtopen.proquest.com/#viewpdf?dispub=10786676.
Full textAbstract:
Numerical investigations of flow past rotating circular cylinders with and without wires wrapped on the surface of the cylinder were studied using Computational Fluid Dynamics (CFD). The flow characteristics such as flow separation, shedding of the primary and secondary vortices, and drag coefficients were investigated. The software STAR CCM+ from Siemens PLM was used in all investigations. Three-dimensional Unsteady Reynolds Average Navier Stokes (URANS) equations were utilized. The free stream mean velocity was constant at 10 m/sec, which corresponded to an approximate Reynolds number based on cylinder’s diameter of 32,000. The results are presented for cylinders with and without wires at varying rotation rates α of 0, 0.5, and 1. This is represented by α, the ratio of the tangential velocity at the cylinder to that of the free stream velocity of the flow. As the rotation rate increased from 0 to 1, the drag coefficient for the smooth rotating cylinder reduced, while the drag coefficient for the wire-wrapped cylinder increased. The wire-wrapped cylinder produced significantly higher lift when compared with the corresponding value for the smooth cylinder. Increasing the rotation rate increases the lift and lift to drag ratio.
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Gsell, Simon. "Vortex-induced vibrations of a rigid circular cylinder." Phd thesis, Toulouse, INPT, 2016. http://oatao.univ-toulouse.fr/17430/1/gsell.pdf.
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When a flexible body with bluff cross-section is immersed in a flow, the unsteady fluid forces accompanying the vortex-shedding phenomenon may lead to structural vibrations. These vortexinduced vibrations (VIV) occur through a mechanism of synchronization between flow unsteadiness and body displacement, referred to as lock-in. VIV are detrimental to many industrial systems, but may also be used as mechanical energy converter in the context of flow energy harvesting. In the present work, the VIV of a rigid circular cylinder mounted on a elastic support are investigated in various configurations on the basis of direct numerical simulations. Four aspects are studied. (i) The interaction mechanisms are first examined in the early turbulent regime, where VIV have been rarely studied numerically. The Reynolds number, based on the cylinder diameter and oncoming flow velocity, is set to 3900. A combined analysis of the structural responses and fluid forcing in the case where the cylinder is free to oscillate in the in-line and cross-flow directions, i.e. the directions parallel and perpendicular to the oncoming flow, is performed over a range of the reduced velocity (inverse of the oscillator natural frequency). (ii) The system behavior when the body is free to oscillate in a single direction only is also explored, shedding some light on the possible interactions between in-line and cross-flow motions in the two-degree-of-freedom (2-dof) case; the analysis shows for instance how large-amplitude in-line oscillations may be induced by cross-flow motion. (iii) The three-dimensional wake developing downstream of the oscillating body is analyzed in the 2-dof case as well as in the fixed body case. The spanwise flow patterns, which are analyzed quantitatively in terms of wavelength and amplitude of vorticity fluctuations, are differently altered in the shear-layer and wake regions, when the body oscillates. (iv) The last physical configuration involves a cylinder immersed in a flow linearly sheared in the cross-flow direction. The impact of the symmetry breaking induced by the shear, on the flow-structure system behavior, is explored. Different interaction regimes are uncovered in the shear rate - reduced velocity domain. Some of them are associated with a profound reconfiguration of the wake and a major alteration of the fluid forces.
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Canabes, Jose Patricio Gallardo. "Direct Numerical Simulation of the Flow Past a Curved Circular Cylinder." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for marin teknikk, 2010. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-11466.
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Offshore marine applications often include configurations of cylindrical structures that produce complex three-dimensional flow features. Catenary risers, for instance, can create complex flow patterns when subjected to hydrodynamic loads. In recent published studies, the shape of a catenary riser has been approximated by a quarter segment of a ring followed by a horizontal extension, obtaining a curved circular cylinder. In the present Master thesis, Direct Numerical Simulations at Re = 100 and 500 have been conducted in order to study the flow past such geometry. The main flow direction was parallel to the plane of curvature of the cylinder and directed towards the convex face of the quarter-of-ring. Additionally, a sheared incoming flow has been considered in the analysis by imposing a linearly varying velocity profile at the inlet. The shedding mechanism observed in uniform flow was similar to that reported in previous published studies. One single shedding frequency prevailed along the entire span of the cylinder at Re = 100 and 500. Moreover, the vortex cores at Re = 100 were normal to the flow direction and exhibited slight distortions as they were convected downstream, whereas at Re = 500 the wake topology was characterized by three-dimensional structures of smaller scale. A sheared inflow, on the other hand, gave rise to an oblique and cellular vortex shedding pattern with two cells of different shedding frequencies. The strong slanting of the vortices, as well as the cellular pattern, was clearly induced by the variation of the local Reynolds number along the front stagnation point. The basic knowledge gained from this thesis appear as very promising in the context of marine structures, it is therefore expected that this work will constitute a basis for further investigations considering this type of geometry.
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湯健東 and Kin-tung Dick Thung. "Digital PIV techniques for studies of circular cylinder under oscillating flow." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2000. http://hub.hku.hk/bib/B31223473.
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Christopoulos, George P. "Oscillating-flow wind tunnel studies for a circulation control circular cylinder." Thesis, Monterey, California. Naval Postgraduate School, 1991. http://hdl.handle.net/10945/28435.
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Thung, Kin-tung Dick. "Digital PIV techniques for studies of circular cylinder under oscillating flow /." Hong Kong : University of Hong Kong, 2000. http://sunzi.lib.hku.hk/hkuto/record.jsp?B22029783.
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Cardell, Gregory Scott Roshko A. "Flow past a circular cylinder with a permeable wake splitter plate /." Diss., Pasadena, Calif. : California Institute of Technology, 1993. http://resolver.caltech.edu/CaltechETD:etd-04012005-092116.
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Ackerman, Jonathan R. "Unsteady energy separation and base pressure distributions in subsonic crossflow around a circular cylinder." Thesis, University of Leicester, 2004. http://hdl.handle.net/2381/30214.
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Eckert and Weise first measured time-averaged energy separation behind a circular cylinder in 1943- Although a significant amount of work has been carried out on the subject since Eckert and Weise's discovery no time-resolved measurements have been made of the phenomenon at high subsonic Mach numbers. The aim of this project was to make these measurements, along with those of surface pressure. Energy separation and base pressure are investigated at high subsonic Mach numbers, behind a circular cylinder in crossflow, for the first time.;The measurement of energy separation has involved developing a novel operating procedure for a high frequency response thin film total temperature probe, allowing it to be heated in stagnant conditions while keeping the wind tunnel running, hi addition the analysis of the results has involved the development of a fully automated phase lock averaging routine.;The principal original contribution of this work is to demonstrate clearly that unsteady energy separation occurs as a result of vortex shedding at high subsonic Mach numbers. The time-resolved measurements show how the areas of increased and decreased total temperature and total pressure are related. The results also give, a good qualitative description of the shape of the vortex street, showing the presence of interconnecting ribs and the areas of maximum entropy increase and thus drag creation.;The surface pressure results have permitted the study of how Strouhal number, drag, base drag and vortex shedding mechanisms change with Mach number. Of particular note they show that the vortex mechanism present in the permanent shock wave regime does not replace that present in the intermediate shock wave regime but rather develops from it.
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"Flow around a circular cylinder with a free end." Thesis, 2003. http://hdl.handle.net/10388/etd-07252011-090143.
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A circular cylinder with a free end is one of the simplest possible three dimensional structures, yet flow around such a body is complex and not well understood. Numerous previous studies have examined this flow situation; however there is disagreement within the literature about the exact nature of the flow behaviour. The primary dispute is whether a pair of counter-rotating trailing vortices exists near the free end or whether the Kármán vortices are inclined near the cylinder free end and attach onto the body. There is a distinct lack of quantitative wake velocity results from previous work. Therefore, the focus of this study is to obtain such results in order to determine the structure of the wake.
Velocity measurements were made in the wake of various circular cylinders mounted vertically perpendicular to a horizontal ground plane. Experiments were conducted in the subcritical Reynolds number regime at ReD = 6x104 using a seven-hole pressure probe capable of determining the time-averaged velocity vector. From the literature, there is evidence that the flow pattern is sensitive to the cylinder aspect ratio and boundary layer thickness relative to the cylinder height. Therefore, the aspect ratio was varied from 3 to 9 by changing the cylinder height while keeping the boundary layer height to cylinder diameter ratio constant at 2.5. For the higher aspect ratio cylinders, results indicate that there are streamwise vortical structures near both ends of the circular cylinder with the vortex pair near the tip of the cylinder much stronger than near the base. As well, thermal anemometry measurements show that the periodicity of the wake weakens near the cylinder ends, with the peak of the power spectrum of the velocity fluctuations being more broadbanded. This suggests that there is indeed a
complicated streamwise vortex structure near the free end that is likely composed of
inclined Kármán vortices interacting with a strong downwash flow. The strong downwash flow toward the ground plane in this region would act to increase the vorticity of the inclined Kármán vortices while moving the vortices toward the ground plane as they travel downstream. The circulation of the vortex structures becomes weaker downstream and the structures grow in size as additional fluid is entrained. There is also a slight spreading of the tip structures as they travel downstream. When the cylinder aspect ratio is reduced to 3, the lower vortical structures disappear while the strong tip vortex structures remain and a vortex shedding peak is still present in the power spectrum of velocity fluctuations very near the ground plane.
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Lin, Wei-Cheng, and 林瑋城. "Horseshoe Flow Structure Around Junction of a Circular Cylinder and Wall." Thesis, 2013. http://ndltd.ncl.edu.tw/handle/01952527382621699353.
Full textAbstract:
碩士國立臺灣科技大學
機械工程系
101
The effects of Reynolds number and boundary layer displacement thickness on the flow field characteristics near the junction between the circular cylinder and the wall were experimentally investigated in a water towing tank. The qualitative and quantitative characteristic flow behaviors near the junction of the circular cylinder were measured by particle tracking flow visualization method and particle image velocimetry (PIV), respectively. By observing the flow patterns of horseshoe vortex appearing in the vertical symmetry plane upstream the circular cylinder, four characteristic flow modes were identified in the domain of Reynolds number and boundary layer displacement thickness—single-vortex mode, dual-vortex mode, triple-vortex mode, and unsteady-vortex mode. Near the wall surface upstream the circular cylinder, the horseshoe vortex and freestream in the horizontal plane induced two flows approaching each other and then departed away from a specific point so that a four-way saddle is formed. In the downstream area, a bifurcation line appeared in the vertical symmetry plane and two counter-rotating recirculation bubbles exhibited in the horizontal plane. The frequency characteristics of unsteady flow structure at various characteristic flow modes in the wake region were examined by extracting the time-series velocity data from PIV measurement. No peak frequency was detected near the wall surface.
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Chen, Dave C., and 陳慶. "Controlling Horseshoe Flow Structure around Juncture of a Circular Cylinder and Wall." Thesis, 2014. http://ndltd.ncl.edu.tw/handle/5e5sqc.
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碩士國立臺灣科技大學
機械工程系
102
The flow around the upstream area of the juncture of a wall and an obstacle extruding from the wall may present interesting vortical flow structure which was commonly termed the horseshoe or necklace vortices. The vortical flow structure usually induced negative effects in the applications such as the heat transfer efficiency of heat exchangers, scour of river bed for the bridge columns, aerodynamic performance of wings, etc. There was a necessity to eliminate the vortical flow structure around the obstacle/wall juncture. This study focused on characterizing the effects of two flow control technologies on the flow characteristics upstream the circular cylinder/flat plate juncture. The laser-light-sheet flow visualization technique and the time-evolving photographing method were employed to obtain the dynamic images of flow patterns. The first method was the rod-control method—a small circular rod placing horizontally in front of the juncture. The second method was the upstream filling method—a pyramid-shaped small block was installed at the juncture. The rod-control method was proved ineffectively in eliminating the horseshoe vortices. Three characteristic flow patterns (vortical flow, reverse flow, and forward flow) appeared when the pyramid block was installed, depending on the geometric configurations and dimensions of the pyramid block as well as the Reynolds number. Operating the pyramid block in the vortical flow regime, the horseshoe vortical flow structure still existed. Operating the pyramid block in the forward flow regime, the horseshoe flow structure was completely eliminated. The reverse flow regime was a status for transition from the vortical flow regime to forward flow regime.
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Jia-HuaTsai and 蔡佳樺. "Investigations of Flow Around a Finite Circular Cylinder at Critical Reynolds Numbers." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/m69j69.
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碩士國立成功大學
航空太空工程學系
106
This study aims on the characteristics of flow field of a finite circular cylinder surface at critical Reynolds number. Experiments were made in a low-speed wind tunnel to measure the pressure signals of ±90° and 180° on a finite circular cylinder surface. The critical regime can be divided into four regions, which are the transition of subcritical regime to one-bubble regime, stable one-bubble regime, the transition of one-bubble regime to two-bubble regime and stable two-bubble regime. The instantaneous flow characteristics in the critical transition range can be discussed with the distributions of the real-time “Cpb ＆ the difference of Cp±90°” data. In addition, the drag coefficient and the lift coefficient in the critical Reynolds number of a finite cylinder and a two-dimensional cylinder are measured by a two-dimensional force balance, and drag crisis phenomenon is also discussed. From the experimental results, the critical transition phenomenon is delayed to higher Reynolds number for the finite smooth circular cylinder model, compared to that of the 2D smooth circular cylinder model. However, the situation was opposite for rough surface of 2D circular cylinder model and finite circular cylinder model. The instantaneous flow characteristics in the critical transition range can be discussed with the distributions of the real-time “CD ＆ CL” data. Finally, we analyzed the pressure signals and force signals by using the method of fast Fourier transform and ensemble empirical mode decomposition. We also used correlation analysis to get more evidences for the three-dimensional wake flow field.
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lin, sing-shing, and 林欣洵. "Study of the Flow Characteristics around a Circular Cylinder near a Plane Boundary." Thesis, 2004. http://ndltd.ncl.edu.tw/handle/77181471638660467491.
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碩士國立中興大學
土木工程學系
92
The flow characteristics around a circular cylinder near a plane boundary were investigated experimentally using particle image velocimetry (PIV), fiber laser Doppler velocimetry (FLDV) and two flow visualization techniques. The Reynolds number, based on the free stream velocity and the diameter of the cylinder, ranges from 100 to 1800. By changing the distance between the cylinder and the plate (G/D) and the thickness of boundary layer (δ), the flow field near the circular cylinder and the relationship between the characteristics of the vortex in front of the cylinder, the strength of the gap flow and the shedding frequency of the vortices behind the cylinder were all discussed. The results show that: (1) The change in the shedding frequency of vortices behind the cylinder, the increase in the momentum of the gap flow and the size of the vortex in front of the cylinder are closely inter-related. (2) As G/D < 0.2, the behavior of the wake is quite different from those of G/D≧0.2 because the gap flow is not strong enough to interact with the flow passing over the top of the cylinder.
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Meng-ChiaoChen and 陳孟巧. "Investigations of Transition Phenomenon of the Flow around a Circular Cylinder in the Critical Regime." Thesis, 2011. http://ndltd.ncl.edu.tw/handle/86375561413599384706.
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Lin, Mu-chen, and 林慕塵. "Investigation of Flow Around a Circular Cylinder Near a Plane Wall by Immeresed Boundary Method." Thesis, 2008. http://ndltd.ncl.edu.tw/handle/96178085247833846530.
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碩士逢甲大學
水利工程與資源保育研究所
96
We present a numerical method using an immersed boundary method for solving flow field around a circular cylinder near a plane wall. Immersed boundary method could deal with irregular boundary effectively, and it did not need grid generation technique. So that we can solve the flow problem using finite difference method with a Cartesian-Grid system. We slove two-dimensional incompressible Navier–Stokes equations with a virtual force term. Pressure term solved by Pressure correction method, and that similitude Projection method which to derive the pressure Poisson equation, then the continuity equation satisfy automatically. We can solve the velocity of the Navier–Stokes equations by explicit Adams-Bashforth scheme, when the pressure be solved. The backward-facing step flow problem was used to test the accuracy of immersed boundary method. We find that the simulation result of Pressure correction method approximated experiment result better than Projection method, and it can reduce the value of the continuous equation effectively. Then investigation of flow around a circular cylinder near a plane wall. To observe flow filed at different Reynolds numbers and different gaps between cylinder and plane wall. The result compares with reference that Roughly agrees in the change of the whole trend.
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"The Effect of a Splitter Plate on the Flow around a Surface-Mounted Finite Circular Cylinder." Thesis, 2011. http://hdl.handle.net/10388/ETD-2011-09-171.
Full textAbstract:
Splitter plates are passive flow control devices for reducing drag and suppressing vortex shedding from bluff bodies. Most studies of splitter plates involve the flow around an “infinite” circular cylinder, however, in the present study the flow around a surface-mounted finite-height circular cylinder, with a wake-mounted splitter plate, was studied experimentally in a low-speed wind tunnel using a force balance and single-component hot-wire anemometry. Four circular cylinders of aspect ratios AR = 9, 7, 5 and 3 were tested for a Reynolds number range of Re = 1.9×10^4 to 8.2×10^4. The splitter plates had lengths, relative to the cylinder diameter, of L/D = 1, 1.5, 2, 3, 5 and 7, thicknesses ranging from T/D = 0.10 and 0.15, and were the same height as the cylinder being tested. The cylinders were partially immersed in a flat-plate turbulent boundary layer, where the range of boundary layer thickness relative to the cylinder diameter was δ/D = 1.4 to 1.5.
Measurements were made of the mean drag force coefficient, the Strouhal number at the mid-height position, and the Strouhal number and power spectra along the cylinder height. For all four finite circular cylinders, the splitter plates were effective at reducing the magnitude of the Strouhal number, and weakening or even suppressing vortex shedding, depending on the specific combination of AR and L/D. Compared to the case of an infinite circular cylinder, the splitter plate is less effective at reducing the mean drag force coefficient of a finite circular cylinder. The largest drag reduction was obtained for the cylinder of AR = 9 and splitter plates of L/D = 1 to 3, while negligible drag reduction occurred for the shorter cylinders.
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