Thèses sur le sujet « Rectangular cylinders »
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1
Steggel, Nathan. « A numerical investigation of the flow around rectangular cylinders ». Thesis, University of Surrey, 1998. http://epubs.surrey.ac.uk/771936/.
Texte intégralRésumé :
The viscous flow around rectangles defined by afterbody length, B, and cross-stream dimension, A, is investigated through a hybrid discrete vortex method. For uniform flow conditions the effects of varying the side ratio, BIA, the angle of incidence, a, and the Reynolds number, Re, are all considered. Pulsating flow results are reported for rectangular cylinders with B/A values of 0.62, 1.0, 2.0 and 3.0, a B/A=1.0 cylinder inclined at 45° and a circular cylinder. At a fixed Reynolds number, Re=200, the variation of drag coefficient with side ratio shows CD increasing with decreasing B/A. This contrasts with the known result at higher Reynolds number, 104
2
Venugopal, Vengatesan. « Hydrodynamic force coefficients for rectangular cylinders in waves and currents ». Thesis, University of Glasgow, 2002. http://theses.gla.ac.uk/6351/.
Texte intégralRésumé :
The research into hydrodynamic loading on ocean structures is concentrated mostly on circular cross section members and relatively limited work has been carried out on wave loading on rectangular sections, particularly in waves and currents. This research work is therefore carried out focussing on the evaluation of hydrodynamic force coefficients for sharp edged rectangular cylinders of various cross-sections (aspect ratios), subjected to waves and currents. Three cylinders with three different cross-sections are constructed and tested vertically, as surface piercing and horizontally, as fully submerged with the cylinder axis parallel to the wave crests. The aspect ratios considered for this investigation are 1.0, 112, 2/1, 3/4 and 4/3. The length of each cylinder is 2000mm. The sectional loadings are measured on a 100mm section, which is located at the mid-length of the cylinder. The forces are measured using a force measuring system, which consists of load cells, capable of measuring wave and current forces. The in-line & transverse forces (for vertical cylinders) and horizontal & vertical forces (for horizontal cylinders) have been measured. For horizontal cylinder, to study the effect of depth of variation on submergence of the cylinder, the tests are carried out for two depths of submergence. The experiments are carried out at the Hydrodynamic Laboratory, Department of Naval Architecture and Ocean Engineering, University of Glasgow. The tests are carried out in a water depth of 2.2m with regular and random waves for low Keulegan-Carpenter (KC) number up to 4.5 and the Reynolds number varied from 6.397xl03 to 1.18xl05 • The combined wave and current effect has been produced by towing the cylinders in regular waves, along and opposite to the wave direction at speeds of ± 0.1 mis, ± 0.2 mls and ± 0.3 mls. Based on Morison's equation, the relationship between inertia and drag coefficients are evaluated and are presented as a function of KC number for various values of frequency parameter, {3. For the vertical cylinders, the drag coefficients decrease and inertia coefficients increase with increase in KC number up to the range of KC tested for all the cylinders. For the horizontally submerged cylinders, the drag coefficients showed a similar trend to vertical cylinders, whereas the inertia coefficients decrease with increase in KC number for all the cylinders. This reduction in inertia force is attributed to the presence of a circulating flow [Chaplin (1984)] around the cylinders. The random wave results are consistent with regular wave results and the measured and computed force spectrum compares quite well. While computing the force coefficients in the case of combined waves and currents, only the wave particle velocity is used, as the inclusion of current velocity tends to produce unreliable drag force coefficients. For vertical cylinders, the drag and the inertia coefficients in combined waves and currents are lower than the drag and the inertia coefficients obtained in waves alone. For horizontal cylinders the drag coefficients are larger than those obtained for waves alone and the inertia coefficients are smaller than those measured in waves alone. The Morison's equation with computed drag and inertia coefficients has been found to predict the measured forces well for smaller KC numbers. However, the comparison between measured and computed positive peak forces indicate that the computed forces are underestimated. It is suggested that if the wave particle kinematics are directly measured, this discrepancy between measured and computed forces might well be reduced. Wave excitation forces are also reported in non-dimensional forms in the diffraction regime, using 3D-Green function method. Wave induced pressure distribution around the cylinder in regular waves have been measured and are reported as normalised pressures. Wave run-up on the cylinder surfaces has been measured and simple empirical formulae are presented for run-up calculations on the cylinder surfaces. The results of this investigation show that the cylinder aspect ratio plays major role on hydrodynamic force coefficients, dynamic pressure distribution and on wave run-up on cylinder surfaces.
3
Namiranian, Farshid. « Fluctuating lift on cylinders of rectangular cross section in smooth and turbulent flows ». Thesis, University of British Columbia, 1985. http://hdl.handle.net/2429/25126.
Texte intégralRésumé :
This thesis presents an experimental investigation of the fluctuating lift (or side force) coefficient on fixed two dimensional rectangular cylinders for various free stream turbulence intensities and scales. The measurements are made using turbulence producing devices such as grids and circular rods placed upstream of the stagnation line of the model. Measurements are reported for three fixed rectangular prisms with B/H of .5, .67 and 1 where H is the frontal dimension and B is the streamwise width of the body.
The method of measurement made it possible to vary the body span so that the correlation of the fluctuating side force over the body span could be investigated. It was shown that for low turbulence intensity, the spanwise correlation of the fluctuating side force over the square cylinders decreases by a large amount with increasing span. For higher turbulence intensity this decrease was reduced, and for U'/U≃10% there was essentially no decrease of fluctuating lift coefficient with increase of span.Applied Science, Faculty of
Mechanical Engineering, Department of
Graduate
4
Tran, Thanh Tinh. « Développement d'une méthode hybride RANS-LES temporelle pour la simulation de sillages d'obstacles cylindriques ». Phd thesis, ISAE-ENSMA Ecole Nationale Supérieure de Mécanique et d'Aérotechique - Poitiers, 2013. http://tel.archives-ouvertes.fr/tel-00823281.
Texte intégralRésumé :
Dans le domaine de la modélisation des écoulements turbulents, les approche hybrides RANS/LES ont reçu récemment beaucoup d'attention car ils combinent le coût de calcul raisonnable du RANS et la précision de la LES.Parmi elles, le TPITM (Temporal Partially Integrated Transport Model) est une approche hybride RANS/LES temporelle qui surmonte les inconsistances du raccordement continu du RANS et de la LES grâce à un formalisme de filtrage temporel. Cependant, le modèle TPITM est relativement difficile à mettre en œuvre et, en particulier, nécessite l'utilisation d'une correction dynamique, contrairement à d'autres approches, notamment la DES (Detached Eddy Simulation).Cette thèse propose alors une approche hybride RANS/LES similaire à la DES, mais basée sur un filtrage temporel, déduite du modèle TPITM par équivalence, c'est-à-dire en imposant la même partition entre énergies résolue et modélisée. Ce modèle HTLES (Hybrid Temporal LES) combine les caractéristiques de la DES (facilité de mise en œuvre) et du TPITM (formalisme consistant, justification théorique des coefficients).Après calibration en turbulence homogène, l'approche est appliquée à des cas d'écoulements autour de cylindres carrés puis rectangulaires. La modélisation des tensions de sous-filtre est une adaptation au contexte hybride du modèle RANS k-wSST.
5
Tamrakar, Swaraj. « Analytical Model Study of Flow Through Trapezoidal, Half-Trapezoidal and Rectangular Channels with Submerged and Un-submerged Rigid Cylinders ». OpenSIUC, 2014. https://opensiuc.lib.siu.edu/theses/1380.
Texte intégralRésumé :
For this study, two analytical models were developed for predicting the depth-averaged velocity distribution (U) in trapezoidal, half-trapezoidal and rectangular channels with submerged and unsubmerged rigid cylinders. The first model uses linear superposition of momentum defects (MDS) and mass conservation, and is referred to as the MDS model. The second model uses linear superposition of velocity defects (VDS) and mass conservation, and is referred to as the VDS model. For implementing either the VDS or MDS model, a criterion is required for considering the wake created by an individual cylinder to be fully dissipated (i.e., a cutoff criterion). Also, implementing the MDS model requires numerical integration. Analyses were conducted to identify suitable cutoff criterion and an appropriate subinterval size for the numerical integration. Data from a physical model study conducted in a flume with a half-trapezoidal channel section was used to calibrate and validate the models. Data from a physical model study conducted in a rectangular channel section was also used to validate the model. Predicted values of U from the VDS and MDS models were within the range of ± 20 % of the trapezoidal channel section validation data. The models failed to accurately predict U for the rectangular channel section data. It is concluded that the models developed herein should be used only for half-trapezoidal channel sections. With respect to the trapezoidal channel section validation data, the MDS model yields a sum of squared errors that is 36% less than that yielded by the VDS model. Therefore, the MDS model is regarded as the best model overall for computing U in half-trapezoidal channel sections.
6
Massai, Tommaso [Verfasser], et Prof Klaus [Akademischer Betreuer] Thiele. « On the Interaction between vortex-induced vibration and galloping in rectangular cylinders of low side ratio / Tommaso Massai ; Betreuer : Prof. Klaus Thiele ». Braunschweig : Technische Universität Braunschweig, 2015. http://d-nb.info/1175818984/34.
Texte intégral
7
Shrestha, Sanjeet. « WAKE CHARACTERISTICS DOWNSTREAM OF A RIGID UNSUBMERGED CYLINDER IN A RECTANGULAR CHANNEL ». OpenSIUC, 2014. https://opensiuc.lib.siu.edu/theses/1379.
Texte intégralRésumé :
Wakes form when a fluid flows past stationary objects. Within the wake, the velocity of the fluid is reduced, that is, there is a velocity defect. Limited information is available for predicting the magnitude of the velocity defects and the width of wakes that form behind rigid cylinders in open-channel flows. Therefore, the objective of this study is to develop relationships for predicting wake geometry downstream of a rigid unsubmerged cylinder. This study was conducted in a 0.46-m-wide rectangular channel with either medium sand (median size, d50 = 0.33 mm) or fine gravel (d50 = 6.35 mm) bed material. A wooden dowel (6.35 mm to 38.1 mm in diameter) was used as a rigid cylinder. Point velocity data was collected for varied hydraulic conditions; the data was used to compute the maximum velocity defect (umax) and half-width of the wake (2b1/2) at sections located different distances downstream from the cylinder (&Deltax). The data were also used to compute parameters for models that predict umax and 2b1/2 as a function of x. It was expected that the study results would yield two equations: one for predicting umax and 2b1/2; instead, the parameters varied with cylinder size and bed material. Since the results were not as expected, evaluation tests were performed to determine why the parameters varied. The evaluation tests (pump configuration tests, point-velocity repetition-tests, point-velocity sampling-time tests, and boundary-layer-development tests) revealed that the turbulent boundary layer (the layer of fluid where the effects of viscosity are significant) was not fully developed in most of the sections where point velocity data was collected. In addition, it was determined that the point-velocity sampling-time of 100 s was not sufficient for keeping the velocity measurement within 2% of the long-term mean velocity (the mean velocity based on a 500-s sample); to achieve this goal it is recommended that a 200-s sampling time should be used.
8
Luong, Sanh B. « Numerical investigations of turbulent flow past a rectangular cylinder with active flow control ». Thesis, California State University, Long Beach, 2016. http://pqdtopen.proquest.com/#viewpdf?dispub=10004172.
Texte intégralRésumé :
The objective of the present research was to investigate the effects of rotating circular cylinders to control high intensity wind load. This research used computational fluid dynamics (CFD) to simulate high Reynolds number gust-like wind load condition for a transient duration of 12 seconds across a three-dimensional rectangular cylinder with dimension of 240x15x7 meters and aspect ratio (Breadth/Height) of 2.3. An array of 20 circular cylinders was positioned along the leading edges of the rectangular bridge cylinder. The research analyzed turbulent flow characteristics across the top and bottom deck surfaces and the development of wake region during two cases: 1) stationary cylinders and 2) rotated cylinders at 400 RPM or velocity ratio of λ = 1.33. The Strouhal number flow characteristics of 0.08 and 0.17 for aspect ratio of 2 to 3 analyzed in this study were found to be in agreements with published literature.
9
Nguyen, Dinh Tung. « Vortex-induced vibration of a 5:1 rectangular cylinder : new computational and mathematical modelling approaches ». Thesis, University of Nottingham, 2017. http://eprints.nottingham.ac.uk/43008/.
Texte intégralRésumé :
As a the limit-cycle oscillation, vortex-induced vibration (VIV) does not cause catastrophic failure but it can lead to fatigue in long and slender structures and structural elements, especially for long span bridges. Assessing this behaviour during the design stage is therefore very important to ensure the safety and serviceability of a structure. Currently, this task requires very time-consuming wind tunnel or computational simulation since a reliable mathematical model is not available. Moreover, knowledge of the underlying physical mechanism of the VIV and, particularly, of the turbulence-induced effect on the VIV is insufficient. Turbulence is normally considered to produce suppressing effects on the VIV; however, this influence appears to depend on cross sections and a comprehensive explanation is yet to be found. This issue can be resulted from some limitation that most wind tunnel or computational studies have used sectional models. The flow field is therefore dominated by 2D flow features. In this research study, the 5:1 rectangular cylinder is selected as the case study since it is considered as the generic bride deck geometry. Using the wind tunnel at the University of Nottingham, a series of wind tunnel tests using a static and elastically supported sectional model is conducted in smooth flow. This wind tunnel study is complemented by a computational study of a static and dynamic sectional model; the computational simulations are carried out using the Computational Fluid Dynamics software OpenFOAM and the High Performance Computer system at the University of Nottingham. A Fluid-structure-interaction (FSI) solver is built to model the heaving VIV. By comparing the surface pressure measurement between these two studies, it uncovers the two separate flow mechanisms and associated flow features, which are both responsible for the VIV. The series of wind tunnel static and dynamic tests is also repeated in different turbulent flow regimes. By analysing the forces, moment, surface pressure and structural response, it reveals the mechanism of the turbulence-induced effect on the aerodynamic characteristics as well as on VIV. By improving the proposed FSI solver, a novel computational approach is introduced to simulate the VIV of a flexible 5:1 rectangular cylinder excited at the first bending mode shape. Employing the Proper Orthogonal Decomposition (POD) technique and comparing against results of the sectional model, some emerging span-wise flow features are revealed together with their influences on the mechanism of the bending VIV. The Hartlen and Currie mathematical model for the VIV is generalised so that it is able to simulate the VIV response of a 3D flexible structure. Such modifications and improvements are originated from and assessed by results of the computational simulation of the flexible model. A case study of the Great Belt East bridge is then carried out to verify this modified model.
10
Hellou, Mustapha. « Etude numérique et expérimentale de l'écoulement à structure cellulaire engendré par la rotation d'un cylindre dans un canal ». Poitiers, 1988. http://www.theses.fr/1988POIT2267.
Texte intégralRésumé :
Etude du decollement de l'ecoulement et de la formation de cellules. Structure et caracteristiques geometriques de ces cellules. Analyse du champ hydrodynamique. Calcul numerique base sur l'ecriture des conditions des conditions aux limites par la methode des moindres carres. Mise au point d'une technique de visualisation par intermittence pendant de longues durees, utilisant les traceurs solides
11
Wang, Jiaqi. « Aerodynamic performance of bluff bodies with openings on side surface ». Kyoto University, 2020. http://hdl.handle.net/2433/253252.
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12
Lee, Jaehyung. « Study on aerodynamic interference and unsteady pressure field around B/D=4 rectangular cylinder based on proper orthogonal decomposition ». 京都大学 (Kyoto University), 2006. http://hdl.handle.net/2433/136143.
Texte intégralRésumé :
Kyoto University (京都大学)0048
新制・課程博士
博士(工学)
甲第12587号
工博第2700号
新制||工||1388(附属図書館)
UT51-2006-S595
京都大学大学院工学研究科社会基盤工学専攻
(主査)教授 松本 勝, 教授 河井 宏允, 助教授 白土 博通, 教授 田村 武
学位規則第4条第1項該当
13
Deschamps, Jessica. « Contrôle actif d'un jet d'air rectangulaire via l'introduction de cylindres animés d'un mouvement de rotation uniforme ». Toulouse, INPT, 2002. http://www.theses.fr/2002INPT044H.
Texte intégralRésumé :
Cette étude expérimentale constitue une étape complémentaire / intermédiaire à une étude numérique qui a montré que le forçage d'un jet convectivement instable, au moyen de sources de vorticité contrôlées, conduisait à une augmentation du mélange et au changement de la nature de l'écoulement vers un mode globalement instable. La première étape a visé à créer une nouveau générateur de jet et un système électromécanique de contrôle d'écoulement. Ce dernier est basé sur la mise en rotation uniforme de deux cylindres. Pour caractériser le jet d'air rectangulaire, de rapport de forme 4:1, sans contrôle et avec contrôle passif / actif, l'anémométrie à simple fil chaud a été utilisée. Quelques visualisations de l'écoulement ont également été réalisées. Au final, la base de donnée a permis de confronter les résultats du jet naturel à ceux du jet avec contrôle passif / actif. Les différentes configurations du jet avec contrôle ont mis en lumière les influences de la position, du sens de rotation et de la vitesse de rotation des cylindres.
14
Batolo, Leod Paul. « Structure de l'écoulement en aval d'une cavité rectangulaire sur un cylindre placé longitudinalement dans un écoulement turbulent développé ». Valenciennes, 1996. https://ged.uphf.fr/nuxeo/site/esupversions/b93bde98-b810-43bf-acb6-3f88b352a9bd.
Texte intégralRésumé :
Ce travail porte sur l'analyse de la structure de l'écoulement en aval d'une cavité. On considère un écoulement incident axisymétrique de couche limite turbulente et développée. Le contrôle de la couche limite incidente à la cavité a été mené par une approche numérique. L'étude expérimentale de la structure de l'écoulement en aval de la cavité est effectuée par visualisation, par polarographie pour les mesures du frottement pariétal et par anémométrie laser à effet doppler pour la détermination du champ de vitesses. Les mesures du frottement pariétal associées aux visualisations permettent de distinguer nettement trois zones en aval de la cavité: une zone de décélération qui marque le début du rétablissement de l'écoulement, une deuxième superposée à la première et caractérisée par une accélération due au replaquage sur le cylindre des lignes de courant, et une troisième zone avec une décélération toujours relative au rétablissement de l'écoulement. La variation du frottement pariétal dépend du nombre de Reynolds de l'écoulement, de l'élancement de la cavité et de la position considérée en aval de la cavité. L'analyse des variations du frottement pariétal exclut l'existence d'un quelconque écoulement de retour en aval de la cavité, confirmant ainsi l'observation issue des visualisations. En aval de la cavité, la distance de rétablissement de l'écoulement dépend de la vitesse extérieure et de l'ouverture de la cavité. Les mesures du champ de vitesse en aval de la cavité montrent que les perturbations engendrées par la cavité restent confinées à la zone interne de la couche limite. Et l'analyse spectrale des signaux du frottement pariétal ne révèle aucune fréquence caractéristique suffisamment énergétique pour que l'on puisse affirmer l'existence de phénomènes d'oscillations lies à la présence de la cavité
15
Scolan, Yves-Marie. « Contribution à l'étude des non-linéarités de surface libre : deux cas d'application : clapotis dans un bassin rectangulaire, diffraction du second ordre sur un groupe de cylindres verticaux ». Paris 6, 1989. http://www.theses.fr/1989PA066452.
Texte intégralRésumé :
On considere ici des ecoulements potentiels a surface libre. Deux cas d'application sont envisages. On tente de resoudre le probleme complet des non-linearites de surface libre dans un bassin rectangulaire par une methode d'elements finis. Ce probleme, formule en terme de potentiel des vitesses et elevation de surface libre, est instationnaire; on discretise le systeme differentiel couple par un schema implicite. On suit les mouvements de la surface libre en remaillant, a chaque pas de temps, le domaine de calcul. Il en resulte des instabilites numeriques. Les traitements envisages peuvent entrainer une baisse du niveau moyen ou une diminution importante de l'energie du systeme; la methode n'est validee que pour des regimes faiblement non lineaires. La deuxieme partie est consacree a la diffraction au second ordre d'une onde plane progressive sur un groupe de cylindres verticaux. Le probleme aux limites est resolu dans le cadre d'une methode integrale. La recherche de conditions de radiation permet d'introduire les notions d'ondes liees et libres. On raccorde la solution interieure aux deux composantes exterieures par une methode des moindres carres. Des criteres permettent d'optimiser ce raccord. Des resultats numeriques sont presentes pour les cas d'un ou plusieurs cylindres.
16
Scolan, Yves-Marie. « Contribution à l'étude des non-linéarités de surface libre deux cas d'application, clapotis dans un bassin rectangulaire, diffraction au second ordre sur un groupe de cylindres verticaux ». Grenoble : ANRT, 1989. http://catalogue.bnf.fr/ark:/12148/cb375938705.
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17
Wu, Yi-Chiao, et 吳易樵. « Effect of Aspect Ratio in Flow Past a Rectangular Cylinder and Tandem Rectangular Cylinders in a Plane Channel ». Thesis, 2010. http://ndltd.ncl.edu.tw/handle/71480936221953825697.
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18
Lee, Wen-hsin, et 李文心. « Numerical Simulation of Two-dimensional Rectangular Cylinders ». Thesis, 2010. http://ndltd.ncl.edu.tw/handle/41958572296960463276.
Texte intégralRésumé :
碩士國立中央大學
土木工程研究所
99
The unsteady forces and wake flows of two-dimensional rectangular cylinders are numerically simulated by using the Computational Fluid Dynamics (CFD) model. The turbulent flow was computed by the Large Eddy Simulation (LES) model, and the Partial Cell Treatment (PCT) was used to handle the moving solid. The aerodynamic forces, and wake flow of stationary cylinders with different aspect ratios L/D = 1, 3, 5, 10 (L is the length of the rectangular cylinder and D is the height of the cylinder) were investigated in details. The results indicated that the distribution of time-average pressure coefficients on the cylinder wall is related to the reattachment phenomenon. This study also investigated the wall effect on the flow field around a stationary rectangular cylinder L/D = 5. The rectangular cylinder was placed close to a solid wall with different gap ratio. It is found that as the gap ratio S/D (S is the distance from the wall to the lower face of cylinder) decreased, the mean pressure coefficient on upper and lower walls became asymmetric, and the time-averaged lift coefficient increased as the gap ratio decreased. Finally, the wake flows of oscillating cylinders were simulated by varying oscillation frequency and amplitude, the resulting drag and lift coefficient of the cylinders are analyzed systematically. The numerical results reveal that for square cylinder (L/D = 1), the time-averaged drag and lift coefficient reach maximum value for in-line oscillation when the forced oscillation frequency equal to the vortex shedding frequency (lock-in region). Also, the transversely oscillated cylinder has the maximum drag coefficient in the lock-in region. The rms lift coefficient of rectangular cylinder of aspect ratio L/D = 10 increased as the oscillation amplitude increased.
19
Xiao, Jian Xiong, et 蕭健雄. « Numerical simulation of transient flow past rectangular cylinder and two cylinders in tandem arrangement ». Thesis, 1995. http://ndltd.ncl.edu.tw/handle/96711606617360407946.
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20
Tien, Cheng Ho, et 田正禾. « Numerical Simulation of Transient Flow Past Rectangular Cylinder and Two Cylinders in Tandem Arrangement ». Thesis, 2005. http://ndltd.ncl.edu.tw/handle/02303391776047731025.
Texte intégralRésumé :
碩士國立中興大學
機械工程學系
93
ABSTRACT The purpose of this study is to analyze the dynamical flow characteristics over a single isolated square cylinder and two identical square cylinders in tandem arrangement with variable spacing between two cylinders. For the case of the flow over two cylinders, the spacing between two cylinders ranges from 2 to 5 widths of the cylinder are examined and the flow for all cases are investigated at the Reynolds number 100. Also, the loading induced on the cylinders including the drag and the lift coefficients is determined. In the process of the computation, the vortex sheet is first used to simulate the surface of the cylinders by enforcing zero normal component of velocity at the surface. Then, the no-slip condition at the solid surface is satisfied by producing free vorticity near the surface. Next, the Biot-Savart Law of Induced Velocities is applied to determine the velocity field within the computational domain. And finally, the transport of the free vorticity in the flow field is fulfilled by satisfying the vorticity transport equation. In this study, our computations show that the spacing between two cylinders will have the effect on the formation of the Karman vortex trail and on the magnitude of the force exerted on the cylinders. It also shows that two different modes of flow characteristics are observed based on the different spacing between two cylinders. The first type of flow characteristics is occurred when the spacing is set to be double or triple of the width of the square cylinder. In this case, there will be no Karman vortex trail found between two cylinders, but the vortex trail is found shedding from the downstream cylinder periodically. The second type of flow characteristics is occurred when the space of two cylinders is set to be 4 times or 5 times of the width of the cylinder. In this case, both the upstream cylinder and the downstream cylinder will result in the occurrence of the Karman Vortex trail periodically. The vortex trail induced by the upstream cylinder will cause the downstream cylinder to have a very strong vibration of fluid property, such as the drag and the lift coefficients.
21
Lin, Chien-Chung, et 林建中. « Vortex induced transverse and torsional oscillations of rectangular cylinders ». Thesis, 1999. http://ndltd.ncl.edu.tw/handle/83622309539609570352.
Texte intégralRésumé :
碩士淡江大學
水資源及環境工程學系
87
The high-rise building is the tendency of construction development in modern cities. Due to the consideration of beauty and multi-dimensional space variation in the building, the eccentricity problem of the structure system cannot be neglected in the design. The behavior of high-rise buildings caused by the wind and the relationship between the high-rise building and the wind field are the main research topics we have to understand. In order to control the influence of the wind on the building, this research simplifies the high-rise building into a 2-D rectangular section. In order to observe the aeroelastic responses of the rectangular cylinders, we applied the method of the free vibration in the wind tunnel, used rectangular cylinders with different depth-to-height ratios changed the frequency ratio of vibration and torsion, and applied the control factor to obtain different eccentricity of the cylinders in the along-wind direction. The experimental results show that the biggest fluctuating displacement of the rectangular cylinders happens when the depth-to-height ratio is 0.6 (B/D=0.6) in the across-wind direction. Without eccentricity, the change of the frequency ratio does not influence the fluctuating displacement response. When the frequency ratio is 1.0 and the mass center is moved either above the geometric center or below the geometric center, the fluctuating displacement is decreased in the across-wind direction. The behavior becomes more clear when the eccentricity increases. The magnitude of the fluctuating displacement response in the transverse wind direction is directly affected by the magnitude of the torsional direction response. The torsional direction fluctuating response increases dramatically when the eccentricity is increased and the frequency ratio approaches 1.0. The fluctuating lift coefficient of the rectangular cylinders increases as the depth-to-height ratio is increased. It reaches the maximum value at the critical depth-to-height ratio of 0.6 (B/D=0.6). The across-wind fluctuating displacement when B/D=1.0 will be magnified to the same amount of the fluctuating displacement response when B/D=0.6. This is most possibly caused by a strong synchronization between the fluctuating displacement response and fluctuating lift.
22
Tsay, whitwen, et 蔡惠文. « Simulation on Turbulent Flows Around Two-Dimensional Rectangular Cylinders ». Thesis, 1996. http://ndltd.ncl.edu.tw/handle/78471696546015286353.
Texte intégralRésumé :
碩士國立中興大學
土木工程學系
84
Investigations on flow around a rectangular bluff body has been an important subject in building research . To evaluate the dynamic response of a structure, the flow effects on the building (such as the variations and mean values of lift and drag) and the related flow characteristics are necessary during the design stage. So far, these are mostly obtained by wind-tunnel model testings. However, the accuracy of the experimental results usually suffers from scale effect. Besides, the achievement of the flow data is generally costly. Therefore, the application of high-speed computation in flow simulation then becomes an important way of the analyses. In the study , a numerical method is established to simulate the unsteady turbulent flow around a two-dimensional rectangular body. In the calculation, a weakly-compressible-flow method incorporated with a space-average large-eddy- simulation technique is adopted. By varying the aspect ratio of the building (the ratio of the along-wind dimension to the across-wind dimension ),the predictions of the flow show good agreement with the availableexperimental data.
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Chuang, Tsung-Wen, et 莊崇文. « Thermosolutal Convection in a Rectangular Enclosure with Triple Cylinders ». Thesis, 2003. http://ndltd.ncl.edu.tw/handle/43502800346812077050.
Texte intégralRésumé :
碩士元智大學
機械工程學系
91
ABSTRACT The purpose of present study is to investigate flows pattern, temperature and concentration distributions resulting from buoyancy force due to a combination of temperature and species convection effects in a rectangular enclosure with triple cylinders inside. In the process of crystal growth, the non-homogeneous phenomenon would become more and more serious. So to know the sufficient understanding of heat and mass transports is indispensable. In this experiment, the temperature and concentration differences are made between three cylinders and the insulation is kept all over the margin. Cupric Sulphate solution is selected as the working fluid. Two constant temperature baths keep the temperature gradient. Concentration gradient is controlled via electrochemical system. In order to visualize the flow, the shadowgraph technique is used. Electrical measurements will be used to obtain mass transfer rates and to infer a Sherwood number. We can find that in the cooperating case, the interface is shown due to the fluid accumulation after imposing the mass transport. However, in the opposing case, the salt finger is found owing to the difference between the thermal diffusive velocity and mass diffusive velocity. This study can be divided into two parts: (1)The enclosure is placed horizontally, and convection is established where one cylinder is inserted right up to the other two. (2)The enclosure is placed vertically, and convection is established where one cylinder is inserted next to the other two. The range of the parameters in this study are Ar=0.75 Gr,t=5.9 ×104 ~ 1.18 ×105 Gr,m=3.16 ×105 N=-5.35~5.35 Sc=1700~2300 Pr=6.0~7.0
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Li, Jing-Yi, et 李兢翌. « Numerical Simulation of Two-Dimensional Flow Past Rectangular Cylinders ». Thesis, 1994. http://ndltd.ncl.edu.tw/handle/27038074670827501087.
Texte intégral
25
TSAI, CHAI-CHANG, et 蔡佳璋. « DRAG REDUCTION OF RECTANGULAR CYLINDERS AT LOW REYNOLDS NUMBERS ». Thesis, 1994. http://ndltd.ncl.edu.tw/handle/13672522144121318431.
Texte intégralRésumé :
碩士國立成功大學
航空太空工程學系
82
The drag of a rectangular cylinder immersed in air stream is the sum ofpressure drag and viscous drag. The pressure drag is caused by the flow separation that introduces a pressure differences between the front and the back faces of the cylin- der. The viscous drag is caused by the viscosity. The purpose of this work is to investigate the effect of a pair of flat plate guided vanes on the drag reduction of rectangular cylin- ders at low Reynolds numbers. The vanes are located symmetric- ally above the top and bottom faces of the cylinder. It is ex- pected that the guided vanes will reduce or even eliminate the separation bubble on the top and bottom faces. This in turn will increase the back pressure and reduce the pressure drag. The method of artificial compressibility is used to modify the incompressible Navier-Stokes equations from elliptic-para- bolictype to hyperbolic-parabolic. An upwind finite-volume met- hod is used to discretize the governing equations and implicit time marching with subiterations are used to obtain time accur- acy. Roe''s approximate Riemann solver is used to obtain invisc- id fluxes. Gauss theorem is used to compute the velocity gradi- ent on the cell faces. The viscous fluxes on the cell faces are then calculated.
26
Liau, Fuu-Bin, et 廖甫斌. « Aerodynamics of Three dimensional Rectangular Cylinders under Acrosswind Vibrations ». Thesis, 1996. http://ndltd.ncl.edu.tw/handle/08972966137143870328.
Texte intégralRésumé :
碩士淡江大學
土木工程學系
84
Due to slenderness and less rigidity , wind loads become the dominate lateral force for highrice building design . Excessive building motion could introduce extra motion related force and cause aerodynamic instability , especially in the acrosswind direction .The aerodynamic characteristics of oscillation retangular cylinders in turbulentboundary layer flows were studied through wind tunnel model test . Experimental parameters such as: depth-to-width ratio , flow features , vibration amplitudeand vibration frequency were used to study the motion effects on aerodynamic damping , mean and r.m.s. force coefficients , power spectral densities , spatial correlation and local pressure of the retangular cylinders . The results of oscillating model tests indicate that for model with depth-to- width ratio smaller than critical value acrosswind oscillation will amplify the so call "synchronization" phenomenon , enlarge the lock-in range , increase the spanwise correlation length and mean and r.m.s. force coefficients of the rectangular cylinders .The non-dimensionalized aerodynamic stiffness andaerodynamic damping coefficients were obtained based upon the acrosswind oscillation models tests . Aerodynamic stiffness and aerodynamic damping coefficients coupled with forced spectra obtained through stationary pressure models tests were used to estimated the acrosswind response .The results show that , the effects of aerodynamic damping of the rectangular cylinders which depth-to- width ratio is smaller than critical depth-to-width ratio is very strong.
27
PERROTTA, ANDREA. « A study of wall-injected flows into closed–open rectangular cylinders ». Doctoral thesis, 2018. http://hdl.handle.net/11573/1083685.
Texte intégralRésumé :
The present work concerns with the study of the fluid motion within a channel when a gas is injected from its wall. This kind of flow appears in a wide variety of engineering applications and it is realized either with low or high injection velocity. The most fruitful scientific production about this argument comes from the propulsive community. In fact, a Solid Rocket Motor (SRM) is a hollowed cylinder, whose inner walls burn and hence inject gas into a channel. Thus, propulsive applications involves wall-injected flows and combustion. Actually, the most of combustion takes place in a very thin limited region near the propellant (the flame) and it is often neglected in modeling. However, pressure and temperature are extremely high in a SRM. Therefore, due to this extreme condition, real firing tests provide a number of dataset limited to the number of probes. Usually, sophisticated pressure transducers, withstanding high temperature, placed at the head-end and aft-end of the motor and ultra-sound pads are employed in order to gauge pressure oscillations and monitor the radial regression of the propellant surface. As stated in [1], it is extremely hard to equip model rocket motor to allow for visual access to its interior and also punctual temperature are of limited interest. A first simplification had been to employ a cold flow injected into a hollow cylinder. Although it has been very useful to confirm theoretical predicted instabilities, also this experimental equipment was limited to punctual measurements. A further simplification leads to a closed-open rectangular cylinder, which can be seen as a bi-dimensional approximation of what happens in three-dimensions. Despite this kind of configuration have been developed, moreover with visual access, it has been mostly used for visualizations and no global flowfield measurements were conducted.
In this work, a closed-open rectangular cylinder has been adopted in order to obtain a global velocity flowfield by means of Particle Image Velocimetry (PIV). The test section of the experiment is a 24 cm-long, 4 cm-wide and 2 cm-high rectangular channel into which a mixture of air and oil droplets, necessary to the PIV measure- ment, is injected through a porous material in order to simulate the propellant gas injection. As the propellant heterogeneity, the injection due to the flame presents a non-trivial spatial pattern and unsteady temporal behavior. Therefore, the porosity of the adopted material has been investigated in order to understand whether it yields a simulative phenomenon of the propellant flame evolution and, if so, which is the mechanism behind it. Since porosity implies morphological structures of the material of the dimensions of tens of a micron, a high resolution measuring technique is required for the analysis of the flow generated from the injection through this porous material. Hence, Single Pixel Ensemble Correlation PIV, which has a resolution as small as one pixel, has been applied in addition to the more classical Window Correlation technique, which has instead a resolution related to the window size, usually around 16 pixel. The velocity field of the whole channel, reconstructed by means of Window Correlation PIV, is then compared with analytical models opportunely presented. The influence of the flow structure due to porosity has been analyzed. The injection Reynolds number, based on the injection velocity and the height of the channel, for the present set-up is around 100 and the flowfield does not seems to show any transition to turbulence within the full length of the channel.
The presence of corners and cavities is a fundamental point of interest for a better comprehension of the internal fluid dynamics of a SRM. Therefore, a second configuration has been taken into account. It presents a ninety degrees corner backward facing step, on the injecting wall, doubling the port area, and a subsequent non-porous, movable block forming a cavity. Investigations have been performed for different cavity lengths and in presence or not of injection from the bottom of the cavity itself. This aspect is strictly related to the flow structure at the aft region of a segmented grain with a tubular segment at the head followed by a star-shaped segment. In fact, the star-shaped segment and the nozzle form a cavity that injects until the propellant of the star-shaped segment burns, thereafter the cavity becomes inert and the flow is due to the tubular segment. The capability of a numerical scheme to accurately solve acoustic waves is fundamental in this circumstance, because the acoustical interaction plays a crucial role in the fluid dynamics of cavities.
Definitely, numerical simulations provide a more complete outline and will be also employed in order to compare and go beyond the limitations of the experimental apparatus. A numerical code has been developed implementing high order centered finite difference schemes for the compressible Navier-Stokes equations. Since the physical phenomenon is confined into a bounded region with several boundary con- ditions, the Navier-Stokes Characteristics Boundary Conditions (NSCBC) technique has been adopted. It consists in a local one-dimensional approximation near the boundary, where the method of the characteristics is adopted in order to adequately compute the quantities deriving from the boundary conditions. Since a fourth-order finite difference scheme has been adopted, a selective low-pass filtering process was mandatory in order to overcome the spurious naturally arising oscillations. More- over, the coefficients of the schemes and filters have been chosen so that the scheme minimizes the dispersion error in order to solve the acoustical waves as accurately as possible.
28
Chung, Cheng-Yang, et 鍾政洋. « Numerical Investigations of Aerodynamic Force on Rectangular Cylinders in Oscillating Approaching Flow ». Thesis, 2007. http://ndltd.ncl.edu.tw/handle/91746993901630869728.
Texte intégralRésumé :
碩士中興大學
土木工程學系所
95
The unsteady forces on rectangular cylinders selected aspect ratios in oscillating flows at a zero attack angle with non-zero mean velocities were investigated numerically in a two-dimensional sense. At a high Reynolds number, the major parameters of the flow around a cylinder with a sinusoidal motion were the amplitude ratio of the approaching-flow velocity (AR) and the Keulegan-Carpenter number (KC), which varied respectively from 0.1 to 0.7 and 0.5 to 16. The resulting time-series wind loads on the cylinder for various AR and KC values were analyzed systematically to examine the flow effect of the rectangular cylinders. To predict the unsteady turbulent flow around the rectangular cylinders, a weakly-compressible-flow method together with a subgrid-scale turbulence model was adopted. The resulting wind load histories were compared with the Morison equation to further find out the the corresponding values of Cd and Cd~ in the theoretical expressions based on a least-square estimation. Results show that the numerical predictions are in good agreement with the available experimental results. Although Morison equation fails to correctly describe the time variation of lift in the case of a zero attack angle, it well describes the of the resulting drag histories in the oscillating approaching flow. Finally, the maximum drag is found when resonance occurs or when the period of the oscillating approaching flow equals the shedding period of the cylinders.
29
YANG, WEI, et 楊威. « The development of separation shear layer of rectangular cylinders in two-dimensional turbulence flows ». Thesis, 1993. http://ndltd.ncl.edu.tw/handle/62016357110189912723.
Texte intégralRésumé :
碩士淡江大學
水資源及環境工程研究所
81
1-1 前言 1-2 研究動機 第二章 文獻回顧 2-1 端板效應 2-2 雷諾數效應 2-3 紊流強度的影響 2-4 紊流尺度的影響 2-5 高寬比的影響 第三章 實驗設備與資料分析 3-1 風洞 3-2 柵板 3-3 模型的設計 3-4 定位系統 3-5 紊流強度尺度 3-6 紊流長度尺度 3-7 散漫數據處理 第四章 量測儀器 4-1 壓力轉換計的率定 4-2 熱膜探針的率定 4-3 X 型熱膜探針的率定 4-4 列膜探針的率定 4-5 管線系統的轉換函數 第五章 實驗結果與討論 5-1 模型與流場參數的配置 5-2 模型側面壓力係數的分布 5-2-A 側面平均壓力隨紊流強度的變化 5-2-B 側面擾動壓力隨紊流強度的變化 5-2-C 紊流長度尺度與模型側面的壓力分布關係 5-2-D 模型高寬比與模型側面的壓力分布關係 5-3 模型側面時間與空間相關函數 5-2-A 紊流強度的影響 5-2-B 紊流長度尺度的影響 5-2-C 模型高寬比的影響 5-2-D 模型高寬比對史特嚇數的影響 5-4 模型側面分離剪力層的速度剖面 5-5 分離剪力層的在接觸現象 5-6 分離剪力層的厚度 第六章 結論與建議 6-1 結論 6-2 建議 參考文獻
30
Du, Xiang-Ning, et 杜祥寧. « Numerical Simulation of Flow around Two-dimensional Rectangular Cylinders with Uniform Turbulent Approaching Flows ». Thesis, 2015. http://ndltd.ncl.edu.tw/handle/34464249794894350075.
Texte intégralRésumé :
碩士國立中興大學
土木工程學系所
103
In the study, a numerical method is established to simulate the unsteady turbulent flow around a two-dimensional rectangle body. At a high Reynolds number, the major parameters of the flow around a cylinder with turbulence intensity and turbulence length scale from laminar to 10% and 0.25D to 1.0D, where D is the width of the rectangle cylinder. To predict the unsteady turbulent flow around the rectangle cylinders (B/D=1、2、3、4), a weakly-compressible-flow method together with a space-average large-eddy-simulation subgrid-scale turbulence model. the turbulent approaching-flow is produced based on the MDSRFG method to properly adjust the prescribed turbulence characteristics. We are verified the validity of the numerical predictions with wind tunnel experiments for build a numerical model.
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ZHENG, CHU-TIAN, et 鄭楚田. « Reflection and Transmission of EM Fields through Single Array of Rectangular Dielectric Cylinders of Various Materials ». Thesis, 2018. http://ndltd.ncl.edu.tw/handle/49j4pr.
Texte intégralRésumé :
碩士吳鳳科技大學
光機電暨材料研究所
106
This thesis is focusedupon the numerical simulation of the reflection and transmission of the electromagnetic (EM) fields propagating through a single array of rectangular dielectric cylinders by applying the method of characteristics. By observing the reflected and transmitted EM from and through array of rectangular dielectric cylinders, information about the shielding effects of such structure can become available.Thisstudy uses rectangular cylinders that are made of nonmagneticdielectric materials whose dielectric constants are 4, 9, and 16; the short side (facing the incidence) of cylinder is 1 mm in length, long side can be either 1or2mm; space between cylinders is measured 2 mm. The incident Gaussian EM pulse, both ends are truncated at a level of 10–5, has a width of5 psand a spatial span of 14.4mm. The EM pulse therefore has a maximum frequency content of about 152.7 GHz corresponding to a minimum wavelength of about 1.96 mm. The numerical results demonstrated include the electric and magnetic fields at the center ofcylinder, the reflected electric fields, the transmitted electric fields, and their spectra.
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MASSAI, TOMMASO. « On the Interaction between vortex-induced vibrations and galloping in rectangular cylinders of low side ratio ». Doctoral thesis, 2015. http://hdl.handle.net/2158/1026282.
Texte intégralRésumé :
The thesis deals with the interaction between vortex-induced vibrations (VIV) and galloping for rectangular cylinders of low side ratio, which is defined as the body width on the body depth facing the fluid flow (SR=B/D). In particular, the interaction mechanism has been characterized for a wide range of Reynolds numbers (Re) and mass ratios (m*), aiming to provide a complete description of the response in several flow situations (smooth and turbulent, air and water) for bodies which exhibited, or were known to have, a pronounced proneness to this type of instability. This type of flow-induced vibrations phenomenon occurs for particular combinations of both aerodynamic and dynamic characteristics of a system. The study consists in two main parts, both of them aimed at proposing a complete framework for scientific and designing purposes. The first one investigates the phenomenon occurring in sectional models purposely designed and experimentally tested, whereas the second one is devoted to the implementation of a predictive model for the interaction, implying also further experimental measures to assess the model key-parameter.
Several sectional models of a SR=1.5 have been tested given that this section demonstrated to be particularly prone to the interaction between VIV and galloping. Nevertheless, the majority of former literature investigations were performed on the square section. The response features of such a phenomenon are still not fully understood. In order to have a deeper insight and to give a complete description of the interaction, the present investigation was conducted focusing particularly on the SR=1.5 rectangular section: this is a soft oscillator respect to the incipient instability, while the same rotated section with an angle of attack of 90°, that is SR=0.67, is generally referred to as a hard-type one. Results in air flow showed peculiar amplitude response curves differently shaped depending on Re, m* and corners sharpness accuracy. Results in water flow showed the response in amplitude and frequency to be strongly influenced by the abrupt change of m*, recalling the different responses in air and water flow regime reported in literature for a circular cylinder, though related to VIV only. SR=0.67 shows a completely different response, although remaining, differently from air flow measurements, a soft oscillator. Further tests on m* variation constituted an integration for the data so far available in literature about these sections.
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MARRA, ANTONINO MARIA. « Risk assessment of bridge decks prone to vortex induced vibrations ». Doctoral thesis, 2011. http://hdl.handle.net/2158/819534.
Texte intégral
34
Chen, Ti-Hung, et 陳荻閎. « A study on the characteristics of surface pressures of rectangular cylinders in uniform turbulent flow and numerical verification ». Thesis, 2004. http://ndltd.ncl.edu.tw/handle/45140530125534532431.
Texte intégralRésumé :
碩士國立中興大學
土木工程學系
92
Abstract A series of aerodynamic experiments on 2D rectangular cylinders with variant depth-to-width ratio (D/B) were conducted to investigate the aerodynamic characteristics, included surface pressures coefficients, probability distribution type and independent peak events. The numerical computation procedures, with LES schemes, were also adopted to verify the results of aerodynamic experiments. The results shown, the drag coefficients will be decreased as the depth-to-width ratio is increased, and the trends will be changed with small variant angle of approaching flow. The pressure coefficients of side face and rear face of cylinders with small depth-to-width ratio were affected by the turbulence intensity of approaching flow, but the affection will be inversed in the flow field with small angle of approaching flow. The K-S test method was adopted to identify the distribution type of probability of wind pressures. The results have shown the PDF will close to normal-type and log-normal type for most cases but the pressure signals at side face and rear face with small depth-to-width ratio. The surface pressures of cylinders, depth-to-width equal to 1.0 and 4.0, were also measured with small angle of approaching flow. The result shown, the PDF of side face pressures will close to normal and log-normal type. The increasing of turbulent intensity will promote the type of PDF of surface pressures closer to normal and log-normal type. The probability distribution of independent peak events at side face and rear face of cylinders will be more uniform as the turbulent intensity increased, but the values will be slightly decreased. With small angle of wind and the side face change slightly into the front face, the probability of independent peak events will be decreased but in the same time will be increased at the rear face. For the ensemble average of peak events, only the front face will be affected by the change of turbulent intensity. The PDF of independent peak events were found to be identical to the type I distribution type. The numerical simulation results were quite similar to the result of aerodynamic experiments in the condition of small angle of approaching flow. For the cases of angle larger than 10 degree and high turbulence intensity, there were lots of differences between the simulated results and experiments results. Keyword: 2D rectangular cylinders, wind tunnel test, surface pressures, computational fluid dynamics
35
YOU, YUAN-QUAN, et 游原泉. « A new feasible approach to 3-D location and orientation determination for cylinders using a single image with a rectangular standard mark ». Thesis, 1991. http://ndltd.ncl.edu.tw/handle/02575667515940851654.
Texte intégral
36
Sadeghipour, Sakineh (Parvaneh). « The Effects of Porous Surfaces on the Control of Flows over Bluff Bodies of Circular, Square and Rectangular Cross-Sections ». Thesis, 2018. https://vuir.vu.edu.au/38655/.
Texte intégralRésumé :
Bluff bodies are an inextricable part of life. They are found abundantly in engineered structures. When exposed to high velocity fluid flows, bluff bodies cause the flow to separate from their surfaces, which result in vortices shed from the bodies. Vortex shedding gives rise to large unsteady forces in the wake of the bluff bodies. The control of such forces is of great importance in engineering design because they reduce drag forces, vibrations and the generation of noise. There is evidence that the application of porous media to the surfaces of bluff bodies immersed in turbulent fluid flows has a profound effect on the associated aerodynamic phenomena. This idea is explored in this thesis by performing a series of experiments and numerical simulations on cylinders treated with porous media. The findings contribute to existing knowledge by providing researchers in the field with a trove of new, and carefully obtained experimental data on the flow fields generated by bluff bodies that have been modified by incorporating regions of porous media. These data will provide theoreticians with benchmark cases against which they can evaluate their mathematical models. The study is significant for two principal reasons. Firstly, it provides new insights into the fundamental mechanisms of the fluid dynamics of flows around bluff bodies, and secondly it has useful practical implications for the design of a multitude of systems immersed in turbulent flow fields. The novelty of the research is that it develops the idea of porous treatment by investigating the effects of the replacement of a part of a bluff body with a porous material and determining the efficacy of such treatment in the control of flow. The benefits include reduction of drag, vibration and noise, which in turn leads to the reduction in fuel consumption of vehicles and therefore a decline in the emissions of greenhouse gases.
In the experiments, cylinders that have circular, square and rectangular cross sections have been studied. Well-established benchmarks were set by measuring the flow fields around bare solid aluminium cylinders. The flow fields were also established around circular cylinders that had been encased in porous media, and which retained the same dimensions of the bare cylinder. The square and rectangular bluff bodies were modified so that their upstream and downstream halves consisted of solid and porous materials respectively. As a result, their leading and trailing edges were respectively solid and porous. The porous media consisted of open cell polyurethane. Three formulations were used that had permeabilities of 4.64×10−7 m2, 5.70×10−8 m2 and 6.87×10−8 m2. The corresponding porosities were 91.8%, 86.2% and 82.1%. Experiments were performed in a wind tunnel operating under conditions that resulted in a Reynolds number of about 53000 based on the diameter of the circular cylinder and the dimensions of the leading edges of the square and rectangular bodies. The flow streamlines, wake velocity profiles and Reynolds stresses were determined by means of Particle Image Velocimetry (PIV).
The experimental investigations revealed that the porous media applied to the circular cylinder had the most effect on the wake, and the largest effect was observed in the wake generated by the most permeable material. The principal effect of the porous media was to cause the eyes of the recirculation region to be located further downstream. This effect is apparent in all three geometries considered, but the effect of the permeability is increasingly diminished in the square and rectangular geometries. The presence of the porous media also had a significant effect on the Reynolds stresses. For example, immediately downstream of the cylinders higher normal stresses in the direction of flow are generated by bare cylinders compared with those generated by the cylinders modified by porous media. The effect of the porous media applied to all three geometries is to damp the normal stresses further downstream of the bluff bodies.
Experiments provided results, which were used as benchmarks for numerical simulations. A CFD code, CFX using an SST-RANS turbulence model was validated against the experimental results and further investigations were performed numerically. Square cylinders with and without porous materials were further examined and compared with each other to expand our understanding of the flow structures around the cylinders. A porous material with a measured permeability equal to 4.64×10−7 m2 and a resistance loss coefficient equal to 1658 m−1 was used. The same configurations in the experiments, namely the bare square cylinder and the square cylinder with its trailing half composed of porous material have been examined. Furthermore, the effect of the disposition of porous material on the flow behaviour and drag force was studied by examining a square cylinder consisted of porous material at its leading half. The flow behaviour in the wake and in the boundary region, pressure distributions and drag forces were obtained in each case and compared with each other.
Investigations of the flow around the bare and porous-treated square cylinders disclosed that the trailing-half porous cylinder is the most effective in the control of flow. This was demonstrated through the location of the recirculation bubbles, and the drag force. Maximum shift in the recirculation bubbles occurred in the trailing-half porous cylinder. The biggest development in the boundary layer was also observed in the trailing half-porous cylinder. The presence of porous material both at the leading and trailing half of the cylinders resulted in a decrease in the drag force. The maximum reduction in the drag coefficient was achieved with the trailing-half porous cylinder by 13% compared to the bare cylinder. In addition, the use of porous materials significantly altered the pressure distributions around the cylinders, when the highest pressure gradient was found for the leading-half porous cylinder.
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Lin, Chih Ping, et 林志坪. « Numerical Simulation of Transient Flow Past Rectangular Cylinder ». Thesis, 1998. http://ndltd.ncl.edu.tw/handle/81383723311670083299.
Texte intégralRésumé :
碩士國立中興大學
機械工程學系
86
The main purpose of this study is to analyze dynamical processesof flow field over a single cylinder and also to investigatethe force induced on cylinder.In the research,the vortex sheet is used to simulate the surface of cylinder by enforcing zeronormal component of velocity at the surface.The no-slip conditionis satisfied by producing free vorticity near the surface.Then,the Biot-savart Law of induced vorticities is applied to determinethe velocity.Finally,the transport of free vorticity in the flowfield is determined by solving the vorticity transport equation.In the equation,two Reynolds numbers,which are 100 and 150 are used for the flow over single cylinder.The computational resultsshow that lift and the pressure-drag coefficient increase withReynolds number.But total drag coefficient decrease with Reynoldsnumber.
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Chueh, Ho-Cheng, et 闕河誠. « Experimental Investigation of Vortex and Rectangular Cylinder Interaction ». Thesis, 1999. http://ndltd.ncl.edu.tw/handle/66138523324937045885.
Texte intégralRésumé :
碩士國立中興大學
機械工程學系
86
This study investigates the interaction of an incident vortex withrectangular cylinders of different side (chord-to-thickness) ratios by theuse of flow visualization and surface pressure measurements. Theexperiment was carried out in an open-type wind tunnel. The side ratiosof the rectangular cylinders are 3,6 and 10. During the Karman vortexstreet and rectangular cylinder interaction, it is observed that thegeneration of the low frequency is accompanied by enlargement andshrinkage of the bubble and also by a flapping motion of the shear layernear the separation line. Associated with the secondary vortexgeneration and growth of the flow is a "jitter", or irregularity, in thegrowth and amalgamation of the vortical structures. The experimental study also looks at the interaction between a freevortex and the cylinder. A two-dimensional counter-clockwise rotatingvortex was formed by employed a vortex generator that pitched in atailored nonsinusiodal schedule. The experimental results show that asthe vortex approaching the leading edge of the rectangular cylinder thelift of the cylinder increases due to an upwash effect. The lift decreasesas the vortex passes over the rectangular cylinder due to a downwasheffect. The amplitude of the transient lift was found to be dependent onthe transverse offset of the vortex generator with respect to therectangular cylinder. Because the counter-clockwise rotating vortexconveys slightly upward, the lift amplitude induced by the vortex thatpasses below the cylinder appears to have a large value than that for thevortex passing above the cylinder.
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Cheng, J. J., et 陳智育. « Investigations of Flow Structures around a Rectangular Cylinder ». Thesis, 1993. http://ndltd.ncl.edu.tw/handle/12760366722271185827.
Texte intégral
40
CHEN, SHAO-LAI, et 陳紹來. « Acrosswind forces on 2D rectangular cylinder in homogeneous turbulent flows ». Thesis, 1991. http://ndltd.ncl.edu.tw/handle/22104320943935431291.
Texte intégral
41
YING-MIN, CHENG, et 鄭英民. « Experimental Study on Separated Flow Structures around a Rectangular Cylinder ». Thesis, 2005. http://ndltd.ncl.edu.tw/handle/68664098632715329040.
Texte intégralRésumé :
碩士國立中興大學
土木工程學系
93
ABSTRACT The purpose of the present study is to make clear flow characteristics in the separated flow around a rectangular cylinder with a width-to-height ratio of 2. According to studies, there is a special phenomenon that the vortex shedding frequency has a sudden drop in terms of Strouhal number when the Reynolds numbers range from 400 to 600. In this study, The simultaneous measurement technique which combines the fiber laser Doppler velocimetry (FLDV) and particle image velocimetry (PIV) was used. Based upon this technique the information of both time and spatial domains can be shown simultaneously. Moreover, the energy history of each shedding frequency can be described in time-frequency domain by using the Wavelet transform. The characteristics can also be shown by the flow visualization method around the model. First, flow visualization in the water channel demonstrated the flow characteristics of separated flow structures, such as the location of reattachment points. Second, there were two main frequencies with apparent peaks in the power spectrum for 400≦Re≦600 by using Fast Fourier transform (FFT) to analyze the time history of velocity signal. The energy history of component shedding frequency in time-frequency domain was also shown using Wavelet transform. The relationship between the full-field velocity map and shedding frequency was clearly identified and presented. According to the experimental results, the relationship between the characteristics of flow field and shedding frequency were found. The flow field can be classified into two categories: (1) the shedding frequency is high when reattachment point occurs; (2) the shedding frequency is low when reattachment does not occurs.
42
Huang, Yu-Bin, et 黃昱賓. « Numerical Simulation of Turbulent Flow Around a 5:1 Rectangular Cylinder ». Thesis, 2011. http://ndltd.ncl.edu.tw/handle/41097527219032172424.
Texte intégralRésumé :
碩士淡江大學
航空太空工程學系碩士班
99
In this study, we use the numerical method to simulate the turbulent flow around a rectangular cylinder with chord-to-depth ratio equal to 5. The aim of this benchmark is to provide a contribution to the analysis of the turbulent. This BARC (a Benchmark on the Aerodynamics of a Rectangular 5:1 Cylinder) problem originated with the Italian National Association for Wind Engineering (ANIV). In spite of the simple geometry, it is believed that the problem is interested not only for the purpose of fundamental research, but also to provide useful information on the aerodynamics of a wide range of bluff bodies (e.g. high-rise buildings). The governing equations are Navier-Stokes equations coupled with K-ε-E turbulence mode equation together with near-wall treatment (wall function). Computations are performed on a nonuniform and staggered MAC (marker-and-cell) grid system. The transport equations are discretized by three steps. In the first step, forward time and central space is applied, and then tensor term is added to stabilize the solution and to reach second order accuracy in time. Finally, FRAM procedure is introduced to remove nonphysical oscillations. First, we calculate the turbulent flow over cube problems. The computed results show that the scheme is suitable for simulation of turbulent flow. Second, we simulated the flow around a 5:1 rectangular cylinder and vortex shedding past rectangular cylinder problem. The computed results indicate that present study gives good agreement with other studies from BARC database and FLUENT (commercial CFD software). Finally, we also calculate the 1:1, 2:1, 3:1 and 4:1 rectangular cylinder problem and we compare result with the flow situation and vortex shedding in 5:1 rectangular cylinder problem.
43
Chung, Chun-Hsuan, et 鍾駿軒. « Large Eddy Simulation of Hydrodynamic Loading on Fully Submerged Rectangular Cylinder ». Thesis, 2014. http://ndltd.ncl.edu.tw/handle/92150207035175420162.
Texte intégralRésumé :
碩士國立中央大學
土木工程學系
102
In recent years, the numbers of floods have increased due to the global warming. The bridge engineers used the historical flood levels to design, so it may underestimate the water level during the future flood. This study focuses on the interaction between the free surface flow and a submerged cylinder with rectangular cross-section. The turbulent flow and pressure distribution around the cylinder were computed by the Large Eddy Simulation (LES) model with the Volume of fluid (VOF) method to track the water surface. The experimental results of Malavasi &; Guadagnini (2007) and water-surface measured in this study were compared with the simulation results to verify the numerical results. Then the verified LES model was utilized to examine the influences of Reynolds number, Froude number, blockage ratio and submergence ratio on the drag and lift coefficients. The simulation results demonstrated that the drag coefficient is dependent on the Froude number, rather than the Reynolds number. For sub-critical flow (deck Froude number FrD < 0.52), due to the blockage effect, the drag coefficient increases as the blockage ratio increases. For trans-critical flow (FrD 0.78), because of the wave-induced drag, the drag coefficient is larger than that in sub-critical flow with the same blockage ratio. On the other hand, the lift coefficient is a function of the deck Froude number and the submergence ratio h*. The separation shear flow on the upper side of the cylinder was constrained by the water surface when h* = 2.0, and resulted in an asymmetric pressure distribution on the upper and lower sides of the cylinder, subsequently generate a downward force on the cylinder.
44
Danny et 何勇緯. « Transient analysis of two-dimensional rectangular plate and cylinder fin with constant temperature ». Thesis, 2000. http://ndltd.ncl.edu.tw/handle/16259839470389832516.
Texte intégralRésumé :
碩士逢甲大學
應用數學系
88
Abstract Discussing the constant temperature of the transient state suddenly transfer to the root of two-dimensional rectangular plate and cylinder fin. The top、bottom and tip of the rectangular plate fin all have different heat convection effect. According to different Biot Number, there have different temperature distribution and heat transfer state. The analysis of the heat transfer of the two dimensional rectangular and two dimensional cylinder fin via using eigenfunction expansion method and Zero Bessel function to solving the analysis solution. In other hand, we transfer the govern equation into difference equation. At the same time, we solve the numerical solution and analysis solution by the program of Matlab. Compare the relative error of analysis and numerical solution. And solve the analysis and numerical solution and non-dimensional heat transfer rate. The temperature distributions of the rectangular plate have apparent differentia with different the Biot Number and thickness. The temperature will gradually descend with raising the Biot Number. When the Biot Number is very small, the slope of he center temperature distribution of the different thick fin is still identical. The variable of temperature would be difficultly lost with more large thickness. The temporal heat transfer rate will be difficult to reach steady state with arising thickness. The temperature distributions of the cylinder fin have apparent differentia with different the Biot Number and thickness. And the condition of the cylinder fin is the same as the rectangular plate.
45
Ni, Chien-Wu, et 倪建武. « Transient analysis of two-dimensional rectangular plate and cylinder fin with periodic heat ». Thesis, 2000. http://ndltd.ncl.edu.tw/handle/76452350120381979177.
Texte intégralRésumé :
碩士逢甲大學
應用數學系
88
Discussing the periodic heat of the transient state suddenly transfer to the root of two-dimensional rectangular plate fin and cylinder fin. The upside、underside and tip of the rectangular plate fin all have different heat convection effect. According to different Biot Number, there have different temperature distribution and heat transfer state. (1)The analysis of the heat transfer of the two dimensional rectangular via using eigenfunction expansion method to fit、top and bottom boundary condition and to compact variable. And to utilize coordinate transform making the homogeneous of right and left boundary condition of the two dimensional rectangular plate. Similarly, to utilize eigenfunction expansion method to satisfy the right and left boundary condition of the two dimensional rectangular plate and to compact variable. The resolute answer of the two dimensional rectangular plate would be achieved via fundamental the differential equation method of the single variable. (2)When we research the analysis of the heat transfer of the two-dimensional cylinder fin, as result of the symmetry of the cylinder fin, we only consider one chamfer. The research method primarily uses zero Bessel function to transfer the equation of the cylinder fin to dimensionless heat transfer equation. satisfy that the center temperature is finite , so we could resolve by using function to be eigenfunction expansion. It would be satisfied the right and left boundary condition of the two-dimensional cylinder fin via the eigenfunction function expansion. The analytical answer of the two dimensional cylinder would be solved by using a general method of single variable differential equation. (3)To solve the analysis and numerical solution of rectangular plate fin and cylinder fin using the method of numerical analysis. Compare the equivalent of analysis and numerical solution and non-dimensional heat transfer rate. The center temperature distributions of the rectangular plate have apparent differentia with different the Biot Number and thickness. The center temperature will gradually descend with raising the Biot Number. When the Biot Number of the end arise, then the temperature distribution of the end will show constringency status. The center temperature distributions of the cylinder fin have apparent differentia with different the Biot Number and thickness. When the Biot Number of the end arise, then the temperature distribution of the end will show constringency status. When the Biot Number is very small, the slope of he center temperature distribution of the different thick fin is still identical. The temporal heat transfer rate will be difficult to reach steady state with arising thickness.
46
Miao, Lin Chun, et 林純妙. « Transient analysis of two-dimensional rectangular plate and cylinder fin with heat flux ». Thesis, 2000. http://ndltd.ncl.edu.tw/handle/72762366778049522880.
Texte intégralRésumé :
碩士逢甲大學
應用數學系
88
Discussing the heat flux of the transient state suddenly transfer to the root of two-dimensional rectangular plate fin and cylinder fin. The upside、underside and tip of the rectangular plate fin all have different heat convection effect. According to different Biot Number, there have different temperature distribution and heat transfer state. (1)The analysis of the heat transfer of the two dimensional rectangular via using eigenfunction expansion method to fit、top and bottom boundary condition and to compact variable. And to utilize coordinate transform making the homogeneous of right and left boundary condition of the two dimensional rectangular plate. Similarly, to utilize eigenfunction expansion method to satisfy the right and left boundary condition of two dimensional rectangular plate and to compact variable. The resolute answer of the two dimensional rectangular plate would be achieved via fundamental the differential equation method of the single variable. (2)When we research the analysis of the heat transfer of the two dimensional cylinder fin, as result of the symmetry of the cylinder fin, we only consider one chamfer. The research method primarily uses zero Bessel function to transfer the equation of the cylinder fin to dimensionless heat transfer equation. satisfy that the center temperature is finite , so we could resolve by using function to be eigenfunction expansion. It would be satisfied the right and left boundary condition of the two dimensional cylinder fin via the eigenfunction function expansion. The analytical answer of the two dimensional cylinder would be solved by using a general method of single variable differential equation. To solve the analysis and numerical solution using the method of numerical analysis and the program of Matlab. Compare the equivalent of analysis and numerical solution and non-dimensional heat transfer rate. The center temperature distributions of the rectangular plate have apparent differentia with different the Biot Number and thickness. The center temperature will gradually descend with raising the Biot Number. When the Biot Number of the end arise , then the temperature distribution of the end will show constringency status. When the Biot Number is very small , the slope of he center temperature distribution of the different thick fin is still identical. The diversity of temperature would be more with more small thickness. Heat quantity would be faster accumulated and lost. The temporal heat transfer rate will be difficult to reach steady state with arising thickness. The center temperature distributions of the cylinder fin have apparent differentia with different the Biot Number and thickness. The center temperature will gradually descend with raising the Biot Number. The heat quantity would be more accumulated with more large thickness. When the Biot Number of the end arise , then the temperature distribution of the end will show constringency status. When the Biot Number is very small , the slope of he center temperature distribution of the different thick fin is still identical. The temporal heat transfer rate will be difficult to reach steady state with arising thickness.
47
Jiuh-Cheng, Chen, et 陳鉅昌. « Simulation on The Plume Dispersion Around A Rectangular Cylinder in Boundary-layer Flows ». Thesis, 1999. http://ndltd.ncl.edu.tw/handle/34461926722201697885.
Texte intégralRésumé :
碩士國立中興大學
土木工程學系
87
ABSTRACT Keywords: (1) Computational Wind Engineering (2) Large Eddy Simulation (3) Dispersion In the evaluation of the environmental influence of a high-rise building, the effect of exhaust dispersion is another important issue other than the surrounding wind characteristics. The exhaust dispersion in the atmospheric boundary layer is concerned with the atmospheric stability and the ground roughness. If there are obstacles in the flow, the exhaust dispersion will be affected with aerodynamic wake effect. Hence the exhaust dispersion is closed together with the flow condition. While the former researchers considering the problems, they usually utilize the survey of experimental and field data. Compared to experimental and field tests, the application of numerical simulations can yield complete design data in a more economical way. The objective of the study is to investigate numerically the exhaust dispersion around a rectangular cylinder in boundary-layer flows. By varying the dynamic terms of the approaching flow; cylinder aspect ratio; and the pollutants released positions, the resulting flow and exhaust dispersion characteristics are analyzed. Results show that the cylinder aspect ratio is the most the most important factor in affecting the exhausted dispersion. When the Reynolds number of the flow is greater than 104, the normalized solutions of the flow becomes Reynolds-number independent. When a pollutant is released at a lower location of the front surface or the back surface of the cylinder, it tends to be trapped, resulting in high concentration areas near the ground respectively before and after the cylinder. As it is released near the leading edge of the top surface, however, the pollutant would be carried downstream by the vortex flow.
48
Lin, Chin-Piao, et 林金標. « Numerical Study of Heat Transfer around the Rotating Circular Cylinder in Rectangular Ponds ». Thesis, 2013. http://ndltd.ncl.edu.tw/handle/91344180696878294811.
Texte intégral
49
Liao, Feng-Liang, et 廖峰諒. « DSMC Analysis of Rarefied Gas Flow Over a Rectangular Cylinder at All Knudsen Numbers ». Thesis, 1999. http://ndltd.ncl.edu.tw/handle/73652006990688870107.
Texte intégralRésumé :
碩士大同工學院
機械工程研究所
87
The present study is concerned with the flow behavior of the rarefied gas over a rectangular cylinder at all Knudsen numbers. Attention will be focused on the transition regime between the continuous flow (low Knudsen number) and the molecular flow (high Knudsen number). The Direct Simulation Monte Carlo Method (DSMC) is employed to examine the distributions of density, velocity, and temperature for the external flow fields as well as the pressure, skin friction, and net heat transfer coefficients on the body surfaces at all Knudsen numbers. In order to check the validity of the simulated program, a comparison between the obtained results and the existing information has been made. The present analysis includes the subsonic flow, supersonic flow, and micro-scale flow. For subsonic and supersonic flows, the length l of the square cylinder is 0.06 m, the Mach number (Ma) ranges from 0.85 to 8, and the Knudsen number (Kn) is in the range 0.01<=Kn<=1.0, while l is fixed at 0.1 or 10 micro-meter, Ma is fixed at 0.15, and Kn is in the range 0.01<=Kn<=1.5 for micro-scale flow.
50
LEI, TAI-XIONG, et 雷泰雄. « The effects of turbulence on the velocity field of a 2-D rectangular cylinder ». Thesis, 1991. http://ndltd.ncl.edu.tw/handle/57415707541218226585.
Texte intégral