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1
Benson, M. "Flow past bluff bodies." Thesis, University of Manchester, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.382725.
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Armstrong, Brian Jeffrey. "Unsteady flow over bluff bodies." Thesis, University of Edinburgh, 1985. http://hdl.handle.net/1842/11409.
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Polpitiye, Sisira J. "Unsteady fluid flow around certain bluff bodies." Thesis, University of Leicester, 1986. http://hdl.handle.net/2381/34813.
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It is shown in this thesis that fluid dynamic forces on unsteadily moving bluff bodies depend on the history of motion as much as on the velocity and acceleration of motion. An empirical relationship between the motion of the body and the resulting force is obtained by analysing the effect of the history of motion on the fluid dynamic force at any instant. The fluid dynamic force, velocity and acceleration are obtained as functions of time, by oscillating test models in water while they are being towed at constant speed. The test models used are: 1. a two-dimensional circular cylinder, 2. a rectangular block with square frontal area and fineness ratio of 3:1, 3. a cruciform parachute canopy with arm ratio of 4:1, and 4. a ring-slot parachute canopy. The functions by which the history of flow affects the future forces, are evaluated by using the Convolution Integral. The results show that the effects due to history of both velocity and acceleration are by no means negligible, that is the velocity and the acceleration at a specific time prior to any instant is so domineering that the fluid dynamic force can approximately be expressed as being delayed by this period of time. This 'time-delay', or time lag (as opposed to phase-lag) in the part of the measured force is found to be independent of the frequency of excitation. In the light of this evidence, a prediction model is suggested for estimating unsteady fluid forces. The data required for the application of this prediction model are obtained experimentally. Chapter One of this thesis gives a brief explanation of the historical background of unsteady fluid dynamics. The effects of acceleration on the fluid dynamic force, in both ideal and real fluids, are discussed in Chapter Two. Explained in Chapter Three are the techniques used for building the force prediction model, and data acquisition. The experimental procedure is explained in Chapter Four. Chapter Five gives the empirical form of the prediction model, and some data that are used in association with this model.
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Peers, Edward. "Physics-based aeroacoustic modelling of bluff-bodies." Thesis, University of Southampton, 2009. https://eprints.soton.ac.uk/71651/.
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In this work physics-based modelling of bluff-body noise was performed with application to landing gear noise production. The landing gear is a primary contributor to airframe noise during approach. Noise is primarily generated from the unsteady pressures resulting from the turbulent flow around various components. The research was initiated in response to the need for an improved understanding of landing gear noise prediction tools. A computational approach was adopted so that the noise generating physics of the problem could be captured. Governing laws were solved numerically to predict the noise source characteristics and the resulting acoustic far-field. Three-dimensional compressible Navier-Stokes simulations were performed to solve the unsteady turbulent near-field flow and the acoustic analogy was used to predict the resulting far-field acoustic pressure. The flow solver included a high-order computational aeroacoustics code adopting large-eddy simulation, whilst a Ffowcs Williams and Hawkings solver was used for the acoustic prediction. Circular cylinders in various configurations were selected to represent basic landing gear struts and results were used to form a modelling database. Initially, cylinders at various Reynolds numbers were investigated in cross-flow to determine the noise characteristics of a simple model strut. The work was extended to investigate the effect of strut alignment to the flow by simulating cylinders in yaw. The effect of yaw was shown to modify the peak level and frequency of far-field noise spectra. Component interaction effects were then investigated by simulating cylinders in tandem arrangements. The resulting aerodynamic and far-field noise characteristics were shown to be complex and extremely sensitive to the separation distance between the cylinders. Finally, a prediction model was developed and validated by comparing predictions against theory and measurements of the noise radiated by a simple two-wheel landing gear model. The results demonstrated the capability of the model to accurately predict correct spectral and directivity characteristics.
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Vallès, Brice. "Computational Study of Vortex Shedding Behind Bluff Bodies." Doctoral thesis, Norwegian University of Science and Technology, Faculty of Engineering Science and Technology, 2001. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-567.
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A computational study of vortex shedding behind bluff bodies is presented. The main focus of this work is the study of vortex shedding and wake flow behaviour behind tapered circular cylinders in the laminar flow regime. However,a stepped circular cylinder in laminar flow as well as uniform circular cylinders in the turbulent flow regime have been investigated as well.
In the laminar flow regime, the flow behaviour behind tapered and stepped circular cylinders is rather different than behind a uniform circular cylinder. Because the tapered and the stepped circular cylinders are three-dimensional objects, as opposed to uniform circular cylinder swhich are nominally two dimensional, secondary flows are generated along the cylinder span causing complex three-flow patterns which are stil not completely understood. The main motivation of the thesis is thus to contribute to improve knowledge of vortex shedding phenomena.
This thesis is consists of two main parts. Firstly, a brief description of the work is given, as well as the numerical tools used, the vortex shedding phenomenon in laminar and turbulent flow regimes. In addition some of the results are presented. Secondly, the appendices in which published and submitted papers on the subject are enclosed. The aim of the first part is to provide details and/or questions which are not included in the later papers in order to form a more comprehensive document.
Paper III reprinted with kind permission of Elsevier, sciencedirect.com
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Narasimhamurthy, Vagesh D. "Wake dynamics of flow past tapered bluff bodies." Doctoral thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for energi- og prosessteknikk, 2008. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-2324.
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Vernet, Julie A. "Plasma actuators for separation control on bluff bodies." Doctoral thesis, KTH, Mekanik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-203208.
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ORAZI, MATTEO. "Bluff Bodies Flow Control using Innovative Piezoelectric Actuators." Doctoral thesis, Politecnico di Torino, 2014. http://hdl.handle.net/11583/2562349.
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An active flow control technique based on “smart-tabs” is proposed to delay flow separation on a circular cylinder and on a simplified bi-dimensional automotive geometry body. The actuators are retractable and orientable multilayer piezoelectric tabs which protrude perpendicularly from the model surface. They are mounted along the spanwise direction with constant spacing. The effectiveness of the control was tested in pre-critical and in post-critical regime by evaluating the effects of several control parameters of the tabs like frequency, amplitude, height, angular position and plate incidence with respect to the local flow. Measurements of the mean static pressure distribution around the cylinder were used to estimate the pressure drag and lift coefficient. To take into account friction and the parasite drag of the actuators wake analysis was performed. The maximum drag reduction achieved in the pre-critical regime was of the order of 30%, whereas in the post-critical regime was about 15%, both for the highest forcing available. In pre-critical condition the active forcing plays an important role on drag and pressure fluctuations reduction. Spectral analysis of the signals from instantaneous pressure transducers (electret microphones) indicated an almost complete suppression of the vortex shedding in active forcing conditions. In post-critical regime, instead, most of the effects are due to passive forcing limiting the active contribution to a maximum of 3%. The results related to the automotive geometry confirm the ones obtained on the circular cylinder, with a maximum attained drag reduction of 13.4% with only small contributions due to active forcing. To evaluate the potentialities of the smart-tabs in real applications power absorption measurements are reported together with energy budget considerations. Finally, the identification of the key non-dimensional control parameters, was performed.
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SARDU, COSTANTINO. "Active flow control around simplified 3D bluff bodies." Doctoral thesis, Politecnico di Torino, 2017. http://hdl.handle.net/11583/2676916.
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In the present work is illustrated a flow control technique designed with the aim of reducing
the aerodynamic drag of a three-dimensional car model with a square-back rear part. The
technique is based on continuous jets disposed in the periphery of the rear surface of the
model. The design process of the entire investigation is illustrated. As a first step, a
preliminary CFD study was performed to verify the effectiveness of the technique and its
sensitivity to the flow control parameters. This CFD analysis assisted the design and the set
up of the physical experiment. Several different measurements were performed and the entire
chain of measurement is widely described evidencing the methodology. The assessment of
this experimental methodology is also presented, which allowed to properly calibrate the
different acquisition parameters and to reach adequate level of accuracy and repeatability of
the measurements. Subsequently, the experimental investigation with the several diagnostic
measurements allowed to understand and explain the flow characteristics. The wide range of
flow control configurations evidenced, in fact, the effect that the forcing have on the pressure
distribution, the flow structures and, consequently, on the drag value. The wake different
sensitivity on the flow control parameters highlighted the most effective configuration which
led to a drag reduction up to 12.6%. The analysis of pressure fluctuations, supported by the
smoke visualization and the POD analysis evidenced the main flow structures present in
the wake, their typical frequencies and their relation on the drag values. Finally the energy
budget of the entire flow control system is presented in order to highlight the most efficient
configurations and evidence the multi-criteria aspect of the active flow control technique
presented.
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Mushyam, Aditya. "Effects of flow control on flow past bluff bodies." Doctoral thesis, Universitat Politècnica de Catalunya, 2016. http://hdl.handle.net/10803/398031.
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The aim of this Ph.D. thesis, "Effects of flow control on flow past bluff bodies" is to study and analyzes the effects of Flow Control on various aspects and proverbs of flow past bluff bodies. Flow control techniques are presently researched and studied across the world to gauge their effectiveness in altering flow properties and patterns in different flow scenarios. The inspiration for the present work is derived from automobile industry, as analyzing the flow over backward step and inclined step shall help in understanding the characteristics of the rear vehicle wake. Since a considerable percentage of the energy needed to propel the vehicle is dissipated by the vorticity generated in the rear of the vehicle, hence it is of utmost importance to understand the properties of the wake. In the present study various possibilities were explored for application and examine effectiveness of active flow control techniques in bluff bodies to increase the aerodynamic efficiency. The primary aim of the study is to reduce the drag acting on bluff bodies by determining an effective active flow control technique and configuration.
Two different kinds of Active Flow Control techniques have been used in this thesis namely, "Zero Net Mass Flow Actuator" and "Fluidic Actuator". The characteristics of laminar and turbulent boundary layer were analyzed and the effectiveness of flow control in delaying the detachment of the boundary layer was analyzed. Numerical simulation programs were developed in C language for simulating various cases in the thesis and a 3D LES turbulence modeling finite volume code was developed using Smagorinsky-Lilly model in C language for the simulating the flow over backward step in turbulence regime. All the numerical simulation codas were validated with previous research works. The thesis is divided into five chapters analyzing flow over square cylinder, backward step and inclined step geometries in 2D and 3D in laminar and turbulent regimesL'objectiu de aquesta tesi doctoral, "Effects offlow control on flow past bluff bodies", és estudiar i analitzar els efectes del control de flux sobre diferents aspectes del flux al voltant de cossos genèrics, Bluff bodies. Les tècniques de control de flux estan en la actualitat sota recerca a nivell mundial, doncs és necessari estudiar l'efectivitat del flux aportat sobre les propietats i els patrons vermiculars del corrent de flux principal. El present treball està inspirat en la indústria del automòbil, així l’anàlisi del flux sobre un esgraó o be una superfície inclinada, ha de permetre entendre les característiques de l'estela de flux a la part del darrera de un vehicle automòbil. Aquest estudi es especialment rellevant quan es considera que una quantitat important de l'energia necessària per desplaçar un vehicle es dissipa gràcies a les estructures vorticulars formades a la part de darrera del vehicle. En aquesta tesi s'han examinat diverses possibilitats de aplicació de les tècniques de control actiu de flux per millorar la efectivitat a l'hora de incrementar la eficiència aerodinàmica sobre cossos genèrics, Bluff bodies. La finalitat bàsica de aquest estudi, és reduir les forces de arrossegament sobre cossos genèrics, gràcies a la determinació de la tècnica de control de fluid més efectiva i la seva implementació pràctica. Dues tècniques diferents de control actiu de fluid han sigut avaluades en aquesta tesi, la definida per un actuador amb flux net nul, "Zero Not Mass Flow Actuator", i la definida per la utilització de un actuador amb un flux net positiu, o be actuador fluídic, "Fluidic Actuator". Les característiques de la capa límit laminar i turbulenta han sigut analitzades així com la efectivitat del control de flux respecte el desplaçament aigües avall del despreniment de la capa límit. Tots els programes utilitzats per toles les simulacions numèriques han sigut creats per el autor de la tesi en llenguatge C, per el que fa referència a les simulacions tridimensionals en règim turbulent, el model de turbulència LES implementat
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Heatlie, Fiona. "Turbulent flow around bluff bodies at the floodplain edge." Thesis, Loughborough University, 2010. https://dspace.lboro.ac.uk/2134/8517.
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This thesis examines the flow around bluff bodies placed at the floodplain edge in a compound, open channel. The floodplain edge location is associated with a strong shear layer between lower velocity floodplain flow and high velocity flow in the main channel. The drag force exerted by a bluff body is dependant on the way in which the flow separates around the body and subsequently recovers but the drag coefficients typically used to represent the effects of bluff bodies are based on experiments on bodies in geometrically simple channels. The differences induced in the wake structures and therefore in the drag coefficients of bluff bodies when they are placed in the shear layer at the floodplain edge are little understood. In this study, experimental data is gathered that allows direct comparison of the wakes of identical bluff bodies, both emergent (surface-piercing) and submerged, in simple and compound open channels. For the compound channel scenarios, for both single and multiple block arrangements, turbulence data is also reported. These results are augmented using a computational model based on the solution of the 3D Reynolds Averaged Navier Stokes equations, using a non-linear turbulence model. The results show that the changes induced in the wake structures due to their location at the floodplain edge of the compound channel can have a significant effect on the drag coefficient. For the emergent bodies, the proximity of the deep main channel flow is shown to impact in a complex manner upon the processes of reattachment and re-separation, changing the formation of vorticity in the wake. For the submerged bodies, this is complicated by asymmetry in the same processes on the block top. For both body types, separation on the main channel side results in the creation of a strong axial circulation at the floodplain edge and the decay of the wake is asymmetrically affected by the differing behaviour of the turbulence on the two sides.
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Nichols, A. R. "Vortex shedding from two rectangular bluff bodies in tandem." Thesis, Teesside University, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.383278.
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Harland, David George. "Unsteady flow around bluff bodies spanning thin rectangular ducts." Thesis, Imperial College London, 2017. http://hdl.handle.net/10044/1/57508.
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This project consists of a two-pronged computational and experimental approach to the study of flow in closed, thin rectangular ducts with a partial cubic blockage. Results are presented at three different bulk Reynolds numbers, ReD = 5600, 10400 and 15600, based on the channel height, which is also the blockage dimension. The new experimental data produced consists of fluctuating pressure measurements at the cube surface, with 2D-2C PIV snapshots captured simultaneously in the wake region. In addition to this, DNS data is produced at the lowest Reynolds number of ReD = 5600, allowing more detailed comparisons where PIV laser access was not possible. Comparisons are drawn between the data and URANS CFD simulations. A literature review and preliminary testing process narrowed down the considered URANS models to the two-layer k−ε model and the Elliptic Blending Reynolds Stress Model, or EBRSM. In the light of the new data, these two URANS models are compared in order to better understand their strengths and weaknesses. Particular regard is given to the prediction of large-scale unsteady behaviour, with a focus on vortex shedding. This unsteady phenomenon was found to be present and to have a significant effect on the flow in the near-cube and wake regions. Results show that certain aspects of this behaviour are captured with only limited accuracy by the URANS models tested. As a result, inaccuracies are also found in the mean simulated velocity fields. The shortcomings appear more pronounced at higher flow rates. At a given flow rate, they are more severe in regions of the flow where organised unsteadiness is large relative to the mean values. It is suggested that inaccuracies in mean URANS predictions are a result of limitations in model capability for unsteady flows, and that validation cases may be pertinent to address this.
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Assi, Gustavo R. S. "Mechanisms for flow-induced vibration of interfering bluff bodies." Thesis, Imperial College London, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.508992.
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Saxena, Vivek. "Turbulence distortion around leading edges and its effect on boundary layer." Thesis, University of Cambridge, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.360934.
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Wolf, Claus Christian [Verfasser]. "The subsonic near-wake of bluff bodies / Claus Christian Wolf." Aachen : Hochschulbibliothek der Rheinisch-Westfälischen Technischen Hochschule Aachen, 2014. http://d-nb.info/105142805X/34.
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Castriotta, Matteo. "Numerical assessment of wind tunnel blockage effects for bluff bodies." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2021.
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The blockage effect due to the presence of wind tunnel walls represents a big obstacle to the accuracy of the acquired data during testing. The flow constriction around the model
depends primarly on wind tunnel dimensions and shape of the tested model.
In this thesis, a numerical study of the correlation between a change in vehicle length and wind tunnel blockage effect is performed on a simple car model known as Ahmed body. To
do that, two case scenarios are simulated: an open case, used as reference for the comparison, and a wind tunnel case, to simulate the effects of blockage.
A comparison between RANS, DDES and SAS is perfomed to choose the simulation technique. Furthermore, the
accuracy of different blockage correction methods available in literature is analyzed.
RANS exhibits a good agreement with DDES and SAS results with the latter that is proven to better reproduce the main flow feature of the Ahmed body with 25° of slant angle. In particular, it is able to predict the separation bubble over the slant but not the exact separation point while, in RANS and DDES, no separation bubble is exhibited.
However, RANS is chosen to investigate wind tunnel blockage because it represents a trade-off between computational cost and quality of the simulation.
It has been observed an enhanced blockage effect when the length of the test vehicle is increased keeping the same width-to-heigth ratio. In particular the increment in drag coefficient, net of the skin friction contribution, is larger than the lift one which is almost independent on the model length.
Lastly, the comparison of wind tunnel correction methods has shown that the accuracy is highly dependent on the model that is studied with most of the methods that fail to predict the correct change in drag coefficient when the vehicle length is increased.
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Alomar, Antoni. "Investigation into noise emitted by bluff bodies with large roughness." Thesis, University of Southampton, 2013. https://eprints.soton.ac.uk/355960/.
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A set of wind tunnel experiments were performed to study the effect of large surface roughness on circular cylinder noise, with the goal of improving landing gear noise predictions. Roughness increases vortex shedding noise levels, and shifts the peak to a lower Strouhal number. The noise levels in the fall-off range also increase, but no significant change in the fall-off rate is observed. The decrease of the vortex shedding peak frequency has been associated with early detachment caused by the effect of roughness on the TBLs, which is in agreement with previous experimental studies with smaller roughness. The high frequency range of the spectrum revealed a broadband, Strouhal-based peak, which is caused by roughness noise generated on the upstream face of the cylinder. The peak Strouhal number is well predicted by Howe's model using the maximum outer velocity around the cylinder. Cylindrical roughness presents a weaker roughness noise peak, but higher noise levels for higher frequencies, and is thought to be caused by sharp edge separation. A bluff body roughness noise model has been developed based on the model of Howe and a Green's function tailored to the bluff body geometry, calculated using the Boundary Element Method. The application to rough circular cylinders using a at wall (ZPG) TBL model shows good agreement with experiments for downstream observers, but the model overpredicts the levels in over-head observers. The disagreement is thought to be due to inaccuracy of the at wall TBL model. The transition from smooth regime to rough regime was studied experimentally by partially covering the cylinder with distributed roughness in spanwise uniform configurations. Transition regarding vortex shedding happens mainly when roughness is added or removed around the separation region. The results agree with the fact that roughness changes the separation location by perturbing the TBL close to separation. Sparse and dense two-dimensional roughness on a circular cylinder, studied using CFD, have similar effects than distributed roughness regarding the vortex shedding peak level and frequency.
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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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Cross, Caleb Nathaniel. "Combustion heat release effects on asymmetric vortex shedding from bluff bodies." Diss., Georgia Institute of Technology, 2011. http://hdl.handle.net/1853/42772.
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Combustion systems utilizing bluff bodies to stabilize the combustion processes can experience oscillatory heat release due to the alternate shedding of coherent, von Kármán vortices under certain operating conditions. This phenomenon needs to be understood in greater detail, since unsteady burning due to vortex shedding can lead to combustion instabilities and flame extinction in practical combustion systems. The primary objective of this study was to elucidate the influence of combustion process heat release upon the Bénard-von Kármán (BVK) instability in reacting bluff body wakes. For this purpose, spatial and temporal heat release distributions in bluff body-stabilized combustion of liquid Jet-A fuel with high-temperature, vitiated air were characterized over a wide range of operating conditions. Upon comparing the spatial and temporal heat release distributions, the fuel entrainment and subsequent heat release in the near-wake were found to strongly influence the onset and amplitude of the BVK instability. As the amount of heat release in the near-wake decreased, the BVK instability increased in amplitude. This was attributed to the corresponding decrease in the local density gradient across the reacting shear layers, which resulted in less damping of vorticity due to gas expansion.
The experimental results were compared to the results of a parallel, linear stability analysis in order to further understand the influence of the combustion processes in the near-wake upon the wake instability characteristics. The results of this analysis support the postulate that oscillatory heat release due to BVK vortex shedding is the result of local absolute instability in the near-wake, which is eliminated only if the temperature rise across the reacting shear layers is sufficiently high. Furthermore, the results of this thesis demonstrate that non-uniform fuelling of the near-wake reaction zone increases the likelihood of absolutely unstable, BVK flame dynamics due to the possibility of near-unity products-to-reactants density ratios locally, especially when the reactants temperature is high.
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Owen, John Clifford. "Passive control of vortex shedding in the wakes of bluff bodies." Thesis, Imperial College London, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.364563.
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Haack, Sarah Jo. "Flow control using plasma and synthetic jet actuators on bluff bodies." College Park, Md.: University of Maryland, 2007. http://hdl.handle.net/1903/7421.
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Thesis (M.S.) -- University of Maryland, College Park, 2007.Thesis research directed by: Dept. of Aerospace Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.
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Prosser, Daniel T. "Advanced computational techniques for unsteady aerodynamic-dynamic interactions of bluff bodies." Diss., Georgia Institute of Technology, 2015. http://hdl.handle.net/1853/53899.
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Interactions between the aerodynamics and dynamics of bluff bodies are important in many engineering applications, including suspension bridges, tall buildings, oil platforms, wind turbine towers, air drops, and construction with cranes. In the rotorcraft field, bluff bodies are commonly suspended underneath the vehicle by tethers. This approach is often the only practical way to deliver a payload in a reasonable amount of time in disaster relief efforts, search-and-rescue operations, and military operations. However, currently a fundamental understanding of the aerodynamics of these bluff bodies is lacking, and accurate dynamic simulation models for predicting the safe flying speed are not available. In order to address these shortcomings, two main advancements are presented in this thesis.
The aerodynamics of several three-dimensional canonical bluff bodies are examined over a range of Reynolds numbers representative of wind-tunnel-scale to full-scale models. Numerical experiments are utilized, with a focus on uncertainty analysis and validation of the computations. Mean and unsteady forces and moments for these bluff bodies have been evaluated, and empirical models of the shear layer characteristics have been extracted to quantify the behaviors and provide predictive capability. In addition, a physics-based reduced-order simulation model has been developed for bluff bodies. The physics-based approach is necessary to ensure that the predicted behavior of new configurations is accurate, and it is made possible by the breakthroughs in three-dimensional bluff body aerodynamics presented in this thesis. The integrated aerodynamic forces and moments and dynamic behavior predicted by model are extensively validated with data from wind tunnels, flight tests, and high-fidelity computations. Furthermore, successful stability predictions for tethered loads are demonstrated. The model is applicable to the simulation of any generic bluff body configuration, is readily extensible, and has low computational cost.
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Liaw, Kai. "Simulation of flow around bluff bodies and bridge deck sections using CFD." Thesis, University of Nottingham, 2005. http://eprints.nottingham.ac.uk/10125/.
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This thesis focuses on the simulation of flow around bluff bodies and bridge deck sections, in which unsteady nature and vortex shedding of flow are commonly found, using computational fluid dynamics (CFD). Various turbulence models have been tested to develop understanding and proper modelling techniques for the flow around such bodies. Throughout the thesis, the turbulence models employed, mainly large eddy simulation (LES) and detached eddy simulation (DES), have been validated through comparative study with experimental work. The major part of the work discusses flow around bluff bodies ranging from a simple circular cylinder, a square cylinder to rectangular sections with various aspect ratios (1:2 to 1:8). The research section concentrates on modelling flow characteristics around bluff bodies to investigate the impact of fluid flow on them. This aids in the understanding of a more complex flow around bridge deck sections. The thesis combines investigation and discussion of the vortex shedding nature on the flow around bluff bodies, in which the simulations are done using advanced modelling techniques on high performance computing system. Work also includes a sectional wind tunnel test of the bridge deck section for the comparative study with the numerical solution. Finally, the conclusions outline the achievements and findings of the work done in this thesis and give recommendations for further research on the topic.
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McColgan, Andrew Hamilton. "An experimental investigation into the wake structure of streamlined and bluff bodies." Thesis, Heriot-Watt University, 2004. http://hdl.handle.net/10399/358.
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Hamidy, Eghbal. "The structure of wakes of 3D bluff bodies in proximity to the ground." Thesis, Imperial College London, 1991. http://hdl.handle.net/10044/1/7603.
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Darekar, Rupad. "Drag reduction on bluff bodies by the introduction of a wavy stagnation face." Thesis, Imperial College London, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.367727.
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Frucht, Yaacov I. "High Reynolds number incompressible flow simulation about parachute canopies and similar bluff bodies." Thesis, University of Leicester, 1987. http://hdl.handle.net/2381/9152.
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A model for the flow around bluff bodies has been developed. It is applied to an investigation of parachute canopy aerodynamic characteristics. Since the model assumes an axisymmetric incompressible high Reynolds number flow, it is only applicable to the calculation of aerodynamic characteristics at zero angle of attack. The flow is assumed to separate from the canopy at its surface discontinuity, i.e. the canopy hemline. The vorticity created in the boundary layer over the canopy upper surface is carried downstream, forming a free shear layer. In the flow field vorticity is confined to the this shear layer, outside it the flow is irrotational. Consequently, in this part of the fluid field a velocity potential can be defined. The wake flow created by bluff canopies is found to consist of a cluster of vortex rings which are shed periodically to the wake. Consequently, the axial aerodynamic force developed on the canopy will exhibit periodic behaviour. The resulting Strouhal number, has been determined to be about 0.13, based on the canopy projected area diameter. For all axisymmetric bluff canopies considered the calculated mean axial force coefficient, based on the canopy projected diameter, was found to be between 1.20 and 1.45. These values, together with the calculated pressure distribution and the wake flow periodicity, are in good agreement with known experiments. For parachute canopies performing an oscillatory axial motion the calculated results compare well with experimental data. However, it is shown that Morison's formula for this axial force is, generally, inadequate. Limited calculations of axial forces developed on the inflating parachute canopies agree with the sparse experimental data available. In the model the real flow field is simulated, basically, by a potential model. The canopy surface is replaced by a vortex ring panel lattice. Each panel contains a circular bound vortex ring which is located at one quarter panel length. For each panel the flow boundary conditions on the canopy surface are fulfilled along a control circle at three quarters of the panel length. A standing eddy which is generated by the high back-flow developed near the canopy hemline, on the canopy under surface is simulated by a standing vortex ring. The simulation of a two-dimensional discrete vortex separated wake is extended to the axisymmetric case by representing the separated wake with axisymmetric discrete vortex rings. The free shear layer emanating from the canopy hemline is represented by discrete free vortex rings which leave the canopy surface tangentially. At each time step in the calculation process a newly-created vortex ring is shed to the wake. In the vortex modelling of the separated wake a number of new elements have been introduced: -improvement of the near wake simulation by accounting for the standing eddy on the canopy under surface; -a simple method of calculating the newly created vortex ring strength & location; -reduction of the free parameters from two, the time step and the number of panels representing the canopy surface to one, i.e. the number of panels. Further model validation & implementation have been suggested. Methods of model development for asymmetric canopy representation have been discussed.
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Greiner, Christopher Mark. "Unsteady hot-wire and hot-film wake measurements of automobile-like bluff bodies." Thesis, Massachusetts Institute of Technology, 1989. http://hdl.handle.net/1721.1/42210.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1990.Title as it appears in the M.I.T. Graduate List, Feb. 1990: Unsteady hot-film and hot-wire wake measurements of automobile-like bluff bodies.
Includes bibliographical references (leaves 213-214).
by Christopher Mark Greiner.
Ph.D.
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Hind, Andrew Kenneth. "An investigation into the flow fields around bluff bodies and artificial heart valves." Thesis, University of Edinburgh, 1997. http://hdl.handle.net/1842/15029.
Full textAbstract:
A milk analogue for clotting blood was developed by Lewis. This was then used by Christy, and subsequently Marosek, to investigate the clot deposited in the region of a series of bluff bodies and heart valves in both steady and pulsatile flows. Bodies of revolution, such as a teardrop, were used to simplify the flow structures present in the flow, making the relation of flow features to the deposition of clot more amenable to analysis. Christy concluded that stasis was a necessary but not sufficient condition for thrombus deposition. A quantitative flow visualisation technique known as particle image velocimetry (PIV) has been developed and applied to the flows investigated by Christy but with water as the test fluid. Steady flow PIV investigations around a Björk-Shiley valve showed significant three dimensional structure which varied markedly over time. In pulsatile flows, both at and below physiologically relevant flowrates, significant variation was observed between recordings made at the same point in successive pulse cycles. This may indicate that the assumptions underlying the sampling methods used in LDA investigations to estimate the Reynolds stresses downstream of heart valve prostheses in pulsatile flows are flawed. In pulsatile flows of a period of 1 Hz and a mean volumetric flowrate of 6.7x10-5 m3/s, the maximum bulk viscous shear stresses determined using PIV were of the order of 0.05 N/m2 with maximum values of 0.5 N/m2 near the wall, well below the value expected to cause lysis of red blood cells even for extended exposure. In the same physiologically relevant flow conditions, the maximum viscous shear stress at the trailing tip of the valve designed by Dr Norman Macleod was again of the order of 0.5 N/m2. This level of wall shear is at the lower end of the range over which behavioural and shape changes are induced in endothelial cells and the adhesion of platelets to endothelial cells may be promoted. Particle image velocimetry is a powerful augmentation to the range of techniques available for assessing the performance of heart valve prostheses in vitro.
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Cannon, Steven Cary. "Large-scale structures and the spatial evolution of wakes behind axisymmetric bluff bodies." Diss., The University of Arizona, 1991. http://hdl.handle.net/10150/185647.
Full textAbstract:
The wakes behind a variety ofaxisymmenic bodies were investigated with flow visualization and hot-wire measurements. The main objective of this study was to correlate changes in the time-averaged features of the wake with changes in the characteristics of the coherent large-scale structure resulting from changing the solidity of the wake generator or by introducing periodic disturbances to force the wake. The use of an axisymmenic probe holder permitted instantaneous decomposition of the axialvelocity field into temporal and azimuthal Fourier modes. Increases in the body solidity resulted in nearly proportional increases in the size of the mean wake boundary. For the non-shedding (low body solidity) wakes, flow visualization shows that the amplitude of discernible large-scale structure is small in comparison to the wake diameter, and there is no evidence of a recirculation region. For the shedding wakes, flow visualization reveals large-scale structure with amplitude that is comparable in size to the wake diameter, and a recirculation region is observed that oscillates in axial extent Fourier analysis of velocity measurements discloses that the temporal scale of the coherent large-scale structure for a non-shedding wake decreases in value with downstream distance while the corresponding scale for a shedding wake is constant Significant changes in the both the time-averaged features of the wake and in the large-scale structure usually occurred only when the forcing frequency was near the natural shedding frequency (within ± 25%). Those time-averaged features changed by forcing include the drag, the mean- and variance-profile shapes, and the size of the wake. The mean profile was observed to change from a regular shape to that which resembles a variance profile. Forcing results in a number of peaks being present in 2-D spectra plots, most of which are the result of non-linear interactions of the forcing wave with the natural shedding frequency. The flow visualization reveals that those peaks which are harmonics or subharmonics of the forcing frequency may be more prominent than the forcing frequency if they are closer to the natural shedding frequency of the unforced wake.
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Campioli, Theresa Lynn. "Assessment of Formulations for Numerical Solutions of Low Speed, Unsteady, Turbulent Flows over Bluff Bodies." Thesis, Virginia Tech, 2005. http://hdl.handle.net/10919/32141.
Full textAbstract:
Two algorithms commonly used for solving low-speed flow fields are evaluated using an unsteady turbulent flow formulation. The first algorithm is the method of artificial compressibility which solves the incompressible Navier-Stokes equations. The second is a preconditioned system for solving the compressible Navier-Stokes equations. Both algorithms have been implemented into GASP Version 4, which is the flow solver used in this investigation. Unsteady numerical simulations of unsteady, 2-D flow over square cylinders are performed with comparisons made to experimental data. Cases studied include both a single-cylinder and a three-cylinder configuration. Two turbulence models are also used in the computations, namely the Spalart-Allmaras model and the Wilcox k-ω (1998) model. The following output data was used for comparison: aerodynamic forces, mean pressure coefficient, Strouhal number, mean velocity magnitude and turbulence intensity. The main results can be summarized as follows. First, the predictions are more sensitive to the turbulence model choice than to the choice of algorithm. The Spalart-Allmaras model overall produced better results with both algorithms than the Wilcox k-ω model. Second, the artificial compressibility algorithm produced slightly more consistent results compared with experiment.Master of Science
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Bhattacharya, Samik Ahmed Anwar. "Effect of three dimensional forcing on the wake of a circular cylinder." Auburn, Ala, 2009. http://hdl.handle.net/10415/1852.
Full text
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Ghadiri, Dehkordi Behzad. "Numerical simulation of the viscous flow around bluff bodies via the random Vortex method." Thesis, McGill University, 1993. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=41598.
Full textAbstract:
The viscous, incompressible and laminar flow around bluff bodies is analysed via a random vortex method using a hybrid (Eulerian-Lagrangian) scheme. The Navier-Stokes equations, written in the form of vorticity equations, are split into nonlinear convection and linear diffusion parts according to a fractional step method. Vorticity is modelled by discrete point vortices. Vortices are created on the surface to satisfy the zero tangential velocity boundary condition. Random walks are applied to the vortices to simulate the diffusion equation. Vortices are then convected in an inviscid calculation in the Lagrangian scheme to simulate the convection equation. The velocity of each vortex is calculated using the Clould-in-Cell method.The random vortex method is developed to simulate the flow around a circular cylinder with a splitter plate. A conformal transformation is used to map the cylinder with a splitter plate to a unit circular cylinder. The force coefficients and Strouhal number are presented for various lengths of the splitter plate and are in good agreement with experiments.
The random vortex method has also been developed for the flow around a circular cylinder vibrating transverse to the flow in a channel. The results are presented for various values of Reynolds number, forcing frequency and vibration amplitudes. The effect of blockage on the drag coefficient is presented. The drag coefficient and vortex shedding frequency are shown to compare favourably with experimental results. The flow around a vibrating cylinder with a splitter plate is also simulated. The results are presented for various values of forcing frequency and lengths of the splitter plate.
The flow around multiple stationary cylinders is also simulated. Also, results are presented for the flow around two cylinders, one of which is forced to oscillate in a transverse direction. The numerical results of Strouhal number are in reasonable agreement with experiments.
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Khan, Tahir Saeed. "Stabilizing lattice Boltzmann simulation of flows past bluff bodies by introduction of Ehrenfests' limiters." Thesis, University of Leicester, 2011. http://hdl.handle.net/2381/9173.
Full textAbstract:
The lattice Boltzmann method (LBM) have emerged as an alternative computational approach to the conventional computational fluid dynamics (CFD). Despite being computationally efficient and popular numerical method for simulation of complex fluid flow, the LBM exhibits severe instabilities in near-grid scale hydrodynamics where sharp gradients are present. Further, since the LBM often uses uniform cartesian lattices in space, the curved boundaries are usually approximated by a series of stairs that also causes computational inaccuracy in the method. An interpolation-based treatment is introduced for the curved boundaries by Mei et al. One of the recipe to stabilize the LBM is the introduction of Ehrenfests' step. The objective of this work is to investigate the efficiency of the LBM with Ehrenfests' steps for the flows around curved bluff bodies. For this purpose, we have combined the curved boundary treatment of Mei et al. and the LBM with Ehrenfests' steps and developed an efficient numerical scheme. To test the validity of our numerical scheme we have simulated the two-dimensional flow around a circular cylinder and an airfoil for a wide range of low to high Reynolds numbers (Re ≤ 30, 000). We will show that the LBM with Ehrenfests' steps can quantitatively capture the Strouhal-Reynolds number relationship and the drag coefficient without any need for explicit sub-grid scale modeling. Comparisons with the experimental and numerical results show that this model is a good candidate for the turbulence modeling of fluids around bluff bodies.
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Corlett, Anne Emily. "A near wake study of vortex shedding from bluff bodies at low Reynolds numbers." Thesis, University of Edinburgh, 1996. http://hdl.handle.net/1842/13465.
Full textAbstract:
Vortex shedding in the near wakes behind a flat plate and a circular cylinder is studied for Reynolds numbers, R, between 80 and 235. Particle Image Velocimetry (PIV) is used to generate velocity maps of the surface flow from which the vorticity fields are found. The majority of results are taken within the two-dimensional shedding regime for which R < 180. Vortex strength calculations of the Karman vortices show that there is little variation of this quantity with Reynolds number for both bodies, and that, for a circular cylinder, there is no discontinuity at a Reynolds number of 100 as found by Green & Gerrard (1991). When the Reynolds number is increased to above 180, no major changes are found. However, dye visualisations of the flow within the water indicate the presence of three-dimensional transitional modes. The effect of rotation, at Reynolds numbers 100 and 200, on the vortex shedding from a flat plate is also investigated. All the flow characteristics are modified by the rotation. As the ratio of rotational to translational velocity (spin parameter, S) is increased the vortices become weaker and the wakes become increasingly asymmetrical. At Reynolds number 00, vortex shedding ceases by S = 2.4. Examination of the mean streamwise velocity profiles shows that the velocity deficit decreases with increasing spin parameter, and indeed for S = 4.5 Re = 100, the mean velocity has a jet profile. Thus rotation is shown to be an effective means of wake control.
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Kao, Shiung-po. "Modified source-type flame model and vorticity generated by the flame and bluff bodies." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/37751.
Full textAbstract:
A numerical model is developed to simulate the wrinkled laminar flame sheet flapping in weakly turbulent premixed combustion. The wrinkled laminar flame sheet is represented by a discrete distribution of volume sources called source disks. These source disks are utilized to produce the acceleration of combustion products behind the flame sheet. The laminar flame speed is allowed to vary according to flame stretch. A modified source model is proposed against the background of the existing source model's physically unrealistic symmetric expansion in both the upstream and the downstream directions. This flame model also includes flame-generated vorticity which is associated with the increasing entropy intrinsic to any system going through an irreversible process. The flame-generated vorticity is treated as discrete vortex disks. Vorticity created on the surface of the flame holder is computed with the vortex sheet method and diffuses into the surrounding flow in the form of vortex disks. The freestream turbulence is simulated by injecting vortex disks into an initially uniform freestream.
Flame-flow interactions are studied when a thin circular cylinder, a large circular cylinder, and a flat plate normal to freestream are used as flame holders. Results sho\v that the modified source model gives more accurate prediction of flame angle than the existing source model does, the relative errors can be reduced by as much as four times. The modified source model also produces velocity profiles closer to those found in experiment, the deviations are cut by half at most sampling points in the flow. The vorticity shed from a thin circular cylinder flame stabilizer is found to only influence downstream regions very close to the cylinder. The eddy shedding behind a bluff body flame holder is suppressed in reacting flow simulations and the computed recirculating zone in a reacting flow is nearly half as long as that in a cold flow. When the relative size of the flame holder is one order of magnitude larger than the thickness of flame sheet, the vorticity shed by the flame holder can no longer be neglected. Flame wrinkling and flame extinction caused by vortical fluid motion behind the flame holder are found through numerical simulation.Ph. D.
38
Clapperton, Surfleet Ben Lewis. "Drag reduction of bluff bodies by passive control of boundary layer transition and separation." Thesis, Imperial College London, 2016. http://hdl.handle.net/10044/1/48456.
Full textAbstract:
In many sporting activities the athletes and their associated equipment often operate in a range of Reynolds number, Re, that is close to the critical regime where the boundary layer flow undergoes transition from a laminar to a turbulent state. It is well known from numerous studies on circular cylinder and sphere flows that boundary layer transition can be forced to occur at lower Re, and hence drag reduced by delaying flow separation, through the application of, for example, roughness to the surface. This thesis is aimed at increasing understanding of how passive flow control methods might be employed to influence boundary layer flow in order to reduce the drag of bluff bodies. A wind tunnel based research programme was undertaken to study these aspects, including a review of a selection of commonly studied boundary layer tripping methods. The main body of the thesis is devoted to the investigation of two novel passive flow control concepts, developed for this research, which were found to significantly reduce the drag coefficient in the sub-critical Re flow regime of a plain cylinder. Of these two concepts, the main research focus was on identifying the drag reducing mechanisms of a system of passive, continuously blowing jets. It was found that the interaction of the jets and cross-flow induced a very high frequency instability which leads to the downstream formation of tornado-like vortices which are shed into, and identified in the near-wake. It is postulated that the introduction of stream-wise vorticity into the separating shear layer develops favourable drag reducing mechanisms. Discrete cylindrical surface protrusions were additionally found to develop similar effects on flow topology and drag reduction, and were more effective than the passive jets at low Reynolds numbers.
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Ikram, Zaheer. "Numerical investigation of the effects of free-surface flow past submerged bluff and streamlined bodies." Thesis, Queen Mary, University of London, 2012. http://qmro.qmul.ac.uk/xmlui/handle/123456789/8477.
Full textAbstract:
The last decade has been marked by a continuous growth in computational power, which has allowed for elaborate modelling techniques like LES and DES applied at engineering scales which generally imply a Reynolds number in excess of 1×106. One field that has seen a rapid growth in use of numerical methods in design and performance analysis is the naval sector, especially in the design of submarines where the free-surface boundary plays an important role. This thesis is devoted to the study of free-surface flow past submerged bodies, with the objective of numerically studying free-surface flow past a submarine fairwater at periscope depth near actual operating conditions. This work is motivated by DSTL, who have reported that near periscope depth submarines exhibit an increased drag (private communication). In this work both LES and DES modelling approaches are also utilized, while the submerged body is accounted through the use of an immersed boundary method and the free-surface is through the use of a part moving mesh. The thesis is split in two main parts. The first part of the thesis focuses on low and moderate Reynolds number flow about a submerged cube (bluff body) for various submergence depths. Two configurations are examined one being that of a single cube in a uniform flow, while the other is that of a matrix of equally spaced cubes. Results show that for both cases a reduction in submergence depth causes the forces, the fluctuation in the forces and shedding frequencies to alter, while the level of interaction between vortices and free-surface to increase. The second part of this thesis focuses on the main study of free-surface flow past a submerged fairwater at a high and near operating Reynolds number. It is found that for both Reynolds numbers the flow behaviour shows little change, while the effects of reducing submergence depth results in the forces, the force variation and shedding frequency to increase. Surface waves are found to disperse by an angle of up to 40° for all submergence depth and are of the Kelvin wave kind. No direct interaction between vortical structures shed from the fairwater and free-surface are found, whilst the increases in forces acting on the fairwater are directly attributed due to wave motion.
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Vlachos, Pavlos P. "An Experimental Spatio-Temporal Analysis of Separated Flows Over Bluff Bodies Using Quantitative Flow Visualization." Diss., Virginia Tech, 2000. http://hdl.handle.net/10919/28675.
Full textAbstract:
In order to study three-dimensional unsteady turbulent flow fields such as the wakes of bluff bodies, a Digital Particle Image Velocimetry (DPIV) system was developed. This system allows non-intrusive two-dimensional and time varying velocity measurements. Software and hardware modifications necessary to enhance the capabilities of the system were preformed, resulting in increased frequency resolution. However, due to hardware limitations and limitations inherited from the implementation of the method, space resolution is reduced. Subsequently, digital image processing tools to improve the space resolutions were developed. The advantages and limitations of the method for the study of turbulent flows are presented in detail.
The developed system is employed in the documentation of time-varying turbulent flow fields. Initially we study the spanwise variation of the near wake of a low-aspect ratio, surface-mounted, circular cylinder piercing a free surface. The asymmetry of the end conditions combined with the natural unsteadiness of the vortex shedding generates a very complex flow filed which is difficult to study with conventional methods. By employing the aforementioned system we are able to reveal a departure of the two-dimensional character of the flow in the form of oblique vortex shedding. The effect of free surface on the vortex formation length and on the vortex reconnection process is documented. Near the free surface the alternate mode of vortex shedding is suppressed, leading to simultaneous shedding of vortices in the wake. Indications of vortex dislocations and change of the vortex axis in order to reconnect to the free surface are observed. Finally, a novel approach of reconstructing the three-dimensional, time -varying volume of the flow field by obtaining simultaneous measurements of Laser Doppler Velocimetry and Particle Image Velocimetry planes is presented.
The same field is investigated with focus on the streamwise structures. Three-dimensional streamwise vortical structures are known to exist due to instabilities of plane shear layers. Similar streamwise vortices, also known as braid vortices have been observed in the past in the wake of circular cylinders with symmetric boundary conditions. The present spatio-temporal analysis demonstrated coexistence of two types of streamwise vortices in the wake, bilge and braid type of vortices. These may be due to the three dimensionality introduced by the free surface. In addition, the sufficient time resolution allowed the detection of the primary Von-Karman vortex through a plane of interrogation normal to the free stream, thus revealing the spanwise variation of the vortex shedding and its evolution at different downstream stations.
The combination of the effect of the asymmetric boundary conditions with a free surface is investigated by adding one more source of three-dimensionality in terms of inclination of the cylinder axis. Hydrogen-bubble and particle-flow visualizations are preformed in combination with Laser-Doppler Velocimetry measurements. From both qualitative and quantitative results the effects of inclination and Froude number are documented. It is proved that the vortex shedding is suppressed for high values of the Froude number, however the inclination counteracts the vortex suppression and favors the vortex shedding mechanism. In addition, in the region of the no-slip boundary condition the flow is dominated by the effect of the horseshoe vortex.
The case of a three-dimensional separated flow over a surface-mounted prism is investigated using a modified version of the system. The character of the separated from the leading edge corner shear layer and the formed separation bubble are documented in space and time along the mid-plane of symmetry of the body. Three different flows corresponding to different Reynolds numbers are studied. The unsteadiness of the flow is presented indicating a pseudo-periodic character. Large-scale, low-frequency oscillations of the shear layer that have been observed in the past using point measurement methods are now confirmed by means of a whole field velocity measurement, technique allowing a holistic view of the flow. In addition, the unsteadiness of the point of reattachment is associated with the flapping of the shear layer and the shedding of vorticity in the wake. Finally, it is demonstrated that the apparent vortex shedding mechanism of such flows is dependent on the interaction of the primary vortex of the separation bubble with a secondary vortex formed by the separation of the reverse flow boundary layer. By performing measurements with such time and space resolution the inadequacy of time averaged or point measurement methods for the treatment of such complex and unsteady flow fields becomes evident.
In final case we employ Particle-Image Velocimetry to show the effect of unsteady excitation on two-dimensional separated flow over a sharp edged airfoil. It is proved that such an approach can be used to effectively control and organize the character of the flow, potentially leading to lift increase and drug reduction of bluff bodiesPh. D.
41
Dahan, Jeremy. "Linear feedback control for form-drag reduction on bluff bodies with a blunt trailing edge." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/11594.
Full textAbstract:
The work described in this thesis is a computational investigation applying linear feedback control to reduce form-drag on bluff bodies with a blunt trailing edge. For such bodies, a large portion of the aerodynamic drag is associated with an unsteady separated region or wake downstream of the body. The development of tractable feedback strategies to control unsteady wakes promises strong benefits, both in terms of industrial applications and for furthering our understanding of the flow mechanisms at play. For this purpose, large-eddy simulations are carried out where a linear feedback controller targets an increase in the mean pressure force on the rear (base) of the body. The flows over two distinct geometries are examined: a backward-facing step and a bluff body with a rounded leading edge, often referred to as a D-shaped body. The control is effected by zero-net-mass-flux slot jets, responding to sensors located on the body base. Open-loop characterization provides information on the effects of actuation and some physical insight into the relation between the base pressure and wake dynamics. System identification is used to obtain a low-order model of the flow's response to actuation that can be used for control. The control strategy is based on the premise that reducing the fluctuations in the near-wake will cause an increase in the mean base pressure, hence a reduction in form-drag. The controllers are designed with classical frequency-domain methods, using a sensitivity transfer function to attenuate the size of the pressure force fluctuations. The influence of parameters such as the Reynolds number and the location and type of actuators is studied. For all cases, low-order linear feedback controllers successfully reduce the pressure force fluctuations and achieve sensible drag reductions. They do so with higher efficiency than the open-loop forcing considered. Uncertainties in the model and flow conditions can be to some extent mitigated by the robustness of the controller. The results support the conjecture linking the fluctuating and mean base pressure, although it is observed that further work is needed before such an approach can be used for optimization.
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Trip, Renzo. "An experimental study on the wake behind a rectangular forebody with variable inlet conditions." Licentiate thesis, KTH, Strömningsfysik, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-142859.
Full textAbstract:
The wake behind a rectangular forebody with variable inlet conditions is investigated. The perforated surface of the two-dimensional rectangular forebody, with a smooth leading edge and a blunt trailing edge, allows for boundary layer modification by means of wall suction. The test section, of which the rectangular forebody is the main part, is experimentally evaluated with a series of hot-wire and Prandtl tube measurements in the boundary layer and the wake. For a suction coefficient of Γ>9, corresponding to 0.9% suction of the free stream velocity, the asymptotic suction boundary layer (ASBL) is obtained at the trailing edge of the forebody for laminar boundary layers (Rex=1.6×105−3.8×105). The key feature of the ASBL, a spatially invariant boundary thickness which can be modified independent of the Reynolds number, is used to perform a unique parametrical study. Turbulent boundary layers (Rex=4.5×105−3.0×106) subject to wall suction are also investigated. For a critical suction coefficient Γcrit, which depends on Rex, the boundary layer relaminarizes. Strong evidence is found to support the hypothesis that turbulent boundary layers will ultimately attain the ASBL as well, provided that the wall suction is strong enough. The effect of the modulated laminar and turbulent boundary layers on the wake characteristics is studied. The shape of the mean wake velocity profile, scaled with the velocity deficit U0and the wake half width ∆y1/2, is found tobe independent of x/h, for x/h> 6 and Reh >6.7×103. The wake width is shown to scale with the effective thickness of the body h+2δ1, where the ratio is expected to vary with the downstream location. A decrease of the displacement thickness leads to a decrease of the base pressure, with Cp,b = −0.36 in the ASBL limit. The Strouhal number based on the effective thickness becomes Sth+2δ1 ≈ 0.29 in the ASBL limit and independent of the plate thickness (h) Reynolds number, in the range Reh = 2.9×103 − 6.7×103. For the turbulent boundary Sth+2δ1 is found to be 25% lower, which shows that the wake characteristics depend on the state of the boundary layer at the trailing edge. The total drag is found to be reduced by as much as 30% for Reh = 2.7×104 when a wall normal velocity of only 3.5% of the free stream velocity is applied. Wall suction successively reduces the total drag with increasing wall suction, at least in the Reynolds number rangeReh = 8.0×103−5.5×104.QC 20140312
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Trein, Cristiano Augusto. "Study on the Unsteady Pressure Characteristics of Bluff Bodies focusing on Flutter Stabilization of Long-Span Bridges." 京都大学 (Kyoto University), 2009. http://hdl.handle.net/2433/85380.
Full textAbstract:
With the increase of the spans in the last decades, new concepts of projects and perceptions about bridges around the world have imposed the need of the development of new design techniques. Aerodynamic instabilities, which were not being of concern not long time ago, started to demand special care during all phases of the projects. Among them, coupledflutter instability has gained remarkable importance, due to its catastrophic consequences and impacts on the safety of the structure. Concerning coupled-flutter, aerodynamic derivatives have been regarded as fundamental components for the assessment of the stability of long-span bridges, resulting in that the focus during the proposition of the bridge deck geometry has been put essentially on the control of those empirical indices. The relationships between bridge deck geometric characteristics and the values assumed by those indices are recognized to be non-linear and straightforward approaches for their prompt evaluation are still not available. Because of that, trial and error experimental techniques based on wind tunnel tests have been regarded as main tools in the search of geometric configurations of bridge decks that satisfy the flutter stability requirements from the aerodynamic derivatives point of view. Since aerodynamic derivatives are resultant from the unsteady pressure characteristics developed along the bridge deck, it must be more rational to search for relationships between deck geometry and flutter stabilization from the unsteady pressure characteristics point of view. However, the complex geometric characteristics associated to modern longspan bridges may impose difficulties on the pressure measurements in wind tunnel tests, becoming counter-productive. So, the direct evaluation of the aerodynamic derivatives through alternative methods has been preferred in wind tunnel tests. In this research, the impacts of the deck geometric characteristics on the aerodynamic derivatives and, consequently, on flutter stabilization are assessed from the unsteady pressure characteristics point of view. Rectangular cross-sections arranged in single box and twobox configurations are used as base geometries and the influences of different geometric singularities (geometric modifications as leading edges, vertical plates and slots) on the unsteady pressure characteristics of those cylinders are systematized through the proposition of semi-empirical formulations. The knowledge generated herein may serve as a base for the development of a design framework based on the direct manipulation of the unsteady pressure characteristics of the deck through the use of geometric singularities, aiming the flutter stabilization by controlling indirectly the aerodynamic derivatives. Such a framework is to be used in substitution of some stages of wind tunnel test campaigns, and its feasibility is evaluated along the study. It was concluded that the relationships of geometric singularities with the unsteady pressure characteristics are much more linear and predictable than their relationships with the aerodynamic derivatives, opening a new methodology for the proposition of geometric improvements. Data regarding the geometries studied herein are also provided along the study, as reference for future development.Kyoto University (京都大学)
0048
新制・課程博士
博士(工学)
甲第14914号
工博第3141号
新制||工||1471(附属図書館)
27352
UT51-2009-M828
京都大学大学院工学研究科社会基盤工学専攻
(主査)教授 白土 博通, 教授 杉浦 邦征, 教授 河井 宏允
学位規則第4条第1項該当
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Cazzonato, Guilherme. "Modelagem matemática do escoamento turbulento em canal axissimétrico: uma abordagem com software livre." reponame:Repositório Institucional da UNIJUI, 2016. http://bibliodigital.unijui.edu.br:8080/xmlui/handle/123456789/3629.
Full textAbstract:
A presente pesquisa apresenta um estudo do escoamento turbulento ocasionado
pela interação entre “bluff-bodies” - corpos geométricos que podem ser instalados
em turbinas aeronaves como estabilizadores de chamas que geram zonas
de recirculação e, consequentemente, aumentam a eficácia da queima do combustível.
Este trabalho utilizou o software livre de Computational Fluid Dynamics
(CFD) OpenFoam para realizar simulações computacionais que permitiram
a análise do comportamento do ar em relação às zonas de recirculação geradas
pela inserção dos corpos em forma de cone e disco. Foram realizadas simulações
envolvendo diferentes valores de espessura (em mm) destes corpos e velocidades
do escoamento (em m/s). Para analisar o comportamento do escoamento turbulento
foram utilizados alguns dos modelos matemáticos mais comuns em estudos
envolvendo o fenômeno da turbulência, k-E, k-W, k-ERN G, k-WS ST , fato que
permitiu que fosse feita a comparação entre os dados obtidos computacionalmente
e os experimentais previamente obtidos na literatura. Pode-se dizer que,
entre os modelos estudados, o que mostrou maior conformidade com os dados
experimentais foi o k-W SST.85 f.
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Fallenius, Bengt E. G. "Experimental design and vortex analyses in turbulent wake flows." Doctoral thesis, KTH, Strömningsfysik, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-46203.
Full textAbstract:
A new experimental setup for studies on wake flow instability and its control that successfully has been designed and manufactured, is introduced and de- scribed. The main body is a dual-sided flat plate with an elliptic leading edge and a blunt trailing edge. Permeable surfaces enable boundary layer suction and/or blowing that introduce the unique feature of adjusting the inlet condition of the wake created behind the plate. This, in combination with a trailing edge that is easily modified, makes it an ideal experiment for studies of different control methods for the wake flow instability as well as extensive parameter studies. Experimental validation of the setup has been performed by means of measurements of the wake symmetry and boundary layer velocity profiles at the trailing edge. Some preliminary results on the Strouhal number versus different inlet conditions are reported. Additionally, an in-house vortex detection (VD) program has been developed in order to detect, analyse and compare small-scale vortical structures in instantaneous velocity fields from flow measurements. This will be a powerful tool for comparison of wake characteristics for varying inlet conditions and control methods in the new experimental setup. Measurements from three completely separate experimental setups with different geometries and flow cases, have been analysed by the VD-program. i. In order to obtain improved ventilation we have studied the effect of pulsating inflow into a closed volume compared to having the inflow at a constant flow rate. We show that the number of small-scale eddies is significantly increased and that the stagnation zones are reduced in size, which enhances the mixing. ii. Instantaneous velocity fields in the wake behind a porous cylinder subjected to suction or blowing through the entire cylinder surface have also been analysed using the VD-program. The results show that the major change for different levels of blowing or suction is the location of vortices while the most common vortex size and strength are essentially unchanged. iii. Another study on how the geometry of a V-shaped mixer in a pipe flow affects the mixing have also been examined, where no general differences were found between different thicknesses, why a thickness that is favourable from an acoustic point of view can be chosen. We also propose a new method, using global mode analysis on experimental data, showing that randomly ordered snapshots of the velocity field behind the porous cylinder can be re-ordered and phase-averaged.QC 20111108
Active control of vortex shedding behind bluff bodies
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Pantalone, Matteo. "Simulation techniques for the aerodynamic study of automotive components." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2018.
Find full textAbstract:
When it comes to vehicle aerodynamics, wheels have always a primary importance due to their high drag contribution, expecially if they cannot be shrouded. For a vehichle such
as a F1 car, wheel total drag contribution reaches high percentage, such as 30-40%.
This study has the purpose to investigate the aerodynamic behaviour of a non-deformable rotating flat sided wheel without hubs. The testcase is supposed to have dimensions of a 2018 F1 front tyre. A Computational Fluid Dynamics (CFD) approach, by using Open-FOAM (OF), is used to catch the main flow features, vortex structures and forces involved.
A lot of time is spent to get a good mesh around the wheel with OF internal mesher due to the ground presence. Widely used Spalart-Allmaras, κ-ω SST, Realizable κ-ε codes
are implemented first. Then the analysis is concentrated to κ-ω SST LM and several built-in DES models: κ-ω SST DES, Spalart-Allmaras DES, Spalart-Allmaras DDES. In
the transtional model the goal is to look for laminar-turbulent boundary layer transition.
In the DES/DDES approaches it is desiderable to see more detailed flow fields and a reduction of turbulent viscosity in regions where the mesh is fine enough to perform a local LES.
As a consequence, comparisons between models output data and catching capabilities are made. Furthermore, the feasibility of using a DES approach with respect to RANS in automotive problems is analyzed in the case mesh grids don’t allow a fine wall resolution.
Experimental data matching the testcase considered don’t exist but literature, however, provides a good agreement with the results for similar researches.
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Piccoli, Guilherme Luiz. "Análise numérica na Engenharia do Vento Computacional empregando computação de alto desempenho e simulação de grandes escalas." reponame:Biblioteca Digital de Teses e Dissertações da UFRGS, 2009. http://hdl.handle.net/10183/17561.
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O presente trabalho tem como objetivo o desenvolvimento de um sistema voltado à solução de problemas relacionados à Engenharia do Vento Computacional. Para o tratamento das estruturas turbulentas, a Simulação das Grandes Escalas é empregada. Esta metodologia resolve diretamente as estruturas que governam a dinâmica local do escoamento (grandes escalas) e utiliza modelos para resolver as escalas com características mais universais (pequenas escalas). Neste estudo, os efeitos sub-malha são obtidos a partir do modelo clássico de Smagorinsky. Na análise numérica, o método dos elementos finitos é avaliado a partir da utilização de elementos hexaédricos e uma formulação baseada nas equações governantes de escoamentos quase-incompressíveis. Para reduzir o requerimento de memória computacional, esquemas explícitos para solução de sistemas de equações são empregados. O primeiro aspecto a ser abordado para o desenvolvimento do sistema proposto é a redução do tempo de processamento. Partindo do algoritmo desenvolvido por [Petry, 2002], desenvolvese um estudo a cerca de técnicas computacionais de alto desempenho visando acelerar o processamento dos problemas. Assim, apresenta-se um comparativo entre alocações estática e dinâmica de vetores e matrizes, juntamente a implementação do paralelismo de memória compartilhada utilizando diretivas OpenMP. A verificação do aumento da velocidade de processamento é desenvolvida simulando o escoamento em um domínio contendo um corpo imerso aerodinamicamente rombudo. As técnicas utilizadas permitiram a obtenção de um aumento de aproximadamente cinco vezes em relação ao código originalmente avaliado. Uma importante dificuldade na avaliação de escoamentos externos está na solução numérica de problemas advectivo-dominantes. O esquema de Taylor-Galerkin explícito-iterativo, originalmente presente no código e validado para escoamentos internos, mostrou-se inadequado para avaliação do escoamento externo proposto, apresentando perturbações no campo de pressões e não convergindo para a solução correta do problema. Estas instabilidades persistiram em uma versão alternativa desenvolvida, a qual utilizava funções de interpolação de igual ordem para solução da pressão e velocidade. Para uma análise de escoamentos não confinados, é implementado o esquema temporal de dois passos utilizando funções de interpolação para velocidade e pressão de mesma ordem. Esta configuração apresentou resultados físicos de boa qualidade e importante redução no tempo de processamento. Após a identificação da alternativa que permitiu a avaliação dos resultados sem a presença de perturbações, apresenta-se a análise do escoamento sobre um prisma quadrado bidimensional, privilegiando o monitoramento da velocidade, pressão e energia cinética total da turbulência na linha central do domínio e nas proximidades do obstáculo. Esta avaliação é efetuada em malhas com configurações uniformes e irregulares para um número de Reynolds igual a 22000.Development of a system to solve problems related to Computational Wind Engineering is the main aim of this work. In order to treat turbulent structures, Large Eddy Simulation is employed. This methodology compute directly scales governing local flow dynamics (large eddies) and it use models to solve those with universal character (small eddies). In this study, the sub-grid effects are considered using the standard Smagorinsky model. In the numerical analysis, hexahedral finite elements are used and a formulation based on the governing equations of quasi-compressible flows. To reduce the computational memory request, explicit schemes to solve the equations system are used. In order to reduce CPU time, an algorithm developed by [Petry, 2002] is evaluated and high-performance techniques aiming to accelerate the problem solution are studied. Thus, it is showed a comparison between dynamic and static allocations of vectors and matrices associated to the implementation of shared-memory parallelization using OpenMP directives. The speed up verification is developed simulating the flow around an immersed bluff body. As a consequence of the techniques employed here, an acceleration of approximately five times with respect of the original computational code is obtained. An important difficulty in the external flow evaluation is the numerical solution of convection dominated flows. The Taylor-Galerkin explicit-iterative scheme, (originally used by the program), which was validated for confined flows, did not present good results for external flows simulations and pressure field perturbations were observed. These instabilities were persevered even in an alternative version, where interpolations functions with the same order were used to compute velocity and pressure (in the original version, constant pressure field at element level were employed). To analyze unbounded flows accurately, a two-step explicit scheme using velocity and pressure interpolation functions with the same order was implemented. This configuration presented physical results with good quality and achieve an important reduction in the processing time. After identification of the best alternative without perturbations of the pressure field, the numerical simulation of the flow around a two-dimensional square cylinder was investigated favoring velocity, pressure and total kinetic energy evaluations along the mid line of the domain and in the obstacle vicinity. These evaluations were effectuated with uniform and stretched meshes for a Reynolds number equal to 22000.
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Haffner, Yann. "Manipulation of Three-Dimensional Turbulent Wakes for Aerodynamic Drag Reduction Mechanics of bluff body drag reduction during trnasient near wake reversals Unsteady Coanda Effect and Drag Reduction of a Turbulent Wake Manipulation of Three-Dimensional Asymmetries of a Turbulent Wake for Drag Reduction Large-Scale Asymmetries of a Turbulent Wake: Insights and Closed-Loop Control for Drag Reduction." Thesis, Chasseneuil-du-Poitou, Ecole nationale supérieure de mécanique et d'aérotechnique, 2020. http://www.theses.fr/2020ESMA0006.
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Une combinaison de moyens passifs et actifs de contrôle d'écoulement est utilisée pour réduire la traînée aérodynamique produite par le sillage turbulent d'une géométrie simplifiée de véhicule à culot droit. Ces sillages sont caractérisés par deux aspects principaux : une traînée de pression importante liée à la séparation massive de l'écoulement, et des asymétries à grande échelle. Ces dernières, se manifestant sous forme de dynamique bimodale ou de brisure de symétrie permanente, contribuent pour environ 10% de la traînée de pression. L'étude des basculements de sillage transitoires en dynamique bimodale s'opérant au travers d'états symétriques du sillage permet d'isoler le mécanisme responsable de l'augmentation de traînée des états à brisure de symétrie. Une interaction et un couplage entre l'écoulement de recirculation issu d'un côté et la couche cisaillée opposée propre aux états à brisure de symétrie déclenche et amplifie les instabilités de couche cisaillée, ce qui conduit à une augmentation de l'écoulement d'entraînement et de la traînée. Il est montré que ce mécanisme est caractéristique des sillages de corps à culot droit.Une stratégie de contrôle actif de l'écoulement combinant des jets pulsés émis tangentiellement aux bords de fuite et de surfaces courbées miniatures affleurantes est utilisée pour réduire la traînée de pression de la géométrie. Le recollement de l'écoulement sur les surfaces courbées résulte en un rétreint fluidique du sillage se traduisant par une réduction de trainée jusqu'à 12%, indépendamment de l'asymétrie initiale du sillage, et est notablement influencé par l'échelle de temps caractéristique de l'instationnarité du forçage. Une combinaison minutieuse entre l'échelle de temps du forçage et la taille caractéristique des surfaces courbées permet d'exploiter tout le potentiel de réduction de traînée de cet effet Coanda instationnaire comme le montre un modèle simple d'écoulement permettant la mise en évidence de lois d'échelles caractérisant le phénomène. De plus, un forçage localisé selon certaines arêtes seulement permet d'interagir avec les asymétries à grande échelle du sillage et impacte de manière très différente la traînée selon l'équilibre su sillage non-forcé. La symétrisation du sillage résultant d'un forçage asymétrique permet une réduction de traînée d'environ 7% à coup énergétique réduit. Des éléments clefs sont donnés concernant l'adaptation de la localisation du contrôle pour une réduction de traînée en présence de différentes asymétries du sillage. Comme le changement d'équilibre global du sillage résulte de changements géométriques et d'écoulement mineurs, des stratégies de contrôle adaptives et robustes sont essentielles pour les applications dans l'industrie automobileCombination of passive and active flow control are used to experimentally reduce the aerodynamic drag produced by the turbulent wake past a simplified vehicle geometry with a blunt base. Such wakes are characterized by two main features: important pressure drag linked to the massive flow separation, and large-scale asymmetries. The latter,manifesting as bi-modal dynamics or permanent symmetry-breaking, are shown to contribute for around 10% of the pressure drag. The study of the transient wake reversais occurring in bi-modal dynamics though symmetric states enables to isolate the flow mechanism responsible for increased drag in symmetry-breaking states. An interaction and coupling between the recirculating flow from one side and the shear-layer from opposite side peculiar to symmetry-breaking states triggers shear-layer instabilities and their amplification leading to increased flow entrainment and drag.This mechanism is shown to be characteristic of the wakes of blunt bodies.An active flow control strategy combining tangential pulsed jets along the trailing-edges and small flush-mounted curved surfaces is used to reduce the pressure drag of the geometry. The flow reattachment and separation on thecurved surfaces results in a fluidic boat-tailing of the wake leading to drag reductions up to 12%, independently of the unforced large-scale asymmetry of the wake, and is noticeably influenced by the time-scale of unsteadiness of the forcing. Careful combination between forcing time-scale and size of the curved surfaces is needed to achieve ail thepotential of this unsteady Coanda effect in drag reduction as shown from a simple flow model providing scaling laws of the phenomenon. The model provided allows for an extension of the flow control mechanism to separated flows moregenerally. Furthermore, forcing along only selected edges enables to interact with the large-scale wake asymmetries and has very different impact on the drag depending on the unforced wake equilibrium. Symmetrisation of the wake through asymmetric forcing leads to 7% drag reduction at a reduced energetic cost. Key ingredients are provided to adapt forcing strategies for drag reduction in presence of various wake asymmetries. As global wake equilibrium changes result from minor geometric and flow conditions changes, adaptive and robust flow control strategies are essential for industrial automotive applications
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Coelho, Jairo Fernando de Lima. "Desenvolvimento de uma instalação experimental para estudo de fenômenos de interação fluido-estrutura." reponame:Repositório Institucional da FURG, 2008. http://repositorio.furg.br/handle/1/3437.
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Dissertação(mestrado) - Universidade Federal do Rio Grande, Programa de Pós-Graduação em Engenharia Oceânica, Escola de Engenharia, 2008.Submitted by Lilian M. Silva (lilianmadeirasilva@hotmail.com) on 2013-04-19T22:08:47Z No. of bitstreams: 1 Desenvolvimento De Uma Instalação Experimental Para Estudo De Fenômenos De Interação Fluido-estrutura.pdf: 1830430 bytes, checksum: fbf20ca5696e6496fdf803ca403c6632 (MD5)
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O estudo experimental de problemas de interação fluido-estrutura é de fundamental importância para o entendimento dos processos dinâmicos envolvidos. Esses processos podem ser bastante complexos em função da alta não-linearidade do problema, especialmente para os casos em que o acoplamento entre a dinâmica do fluido e a dinâmica da estrutura é forte, ou para casos em que ocorram instabilidades dinâmica tanto no fluido quanto na estrutura. O estudo experimental de fenômenosde interação fluido-estrutura necessita de instalações de laboratório capazes de impor movimento relativo entre o fluido e a estrutura. Neste contexto, nesta dissertação apresentam-se o projeto, a construção e a avaliação de um canal de reboque otimizado para o estudo da interação fluido-estrutura em corpos rombudos cilíndricos de alta razão de aspecto. A instalação consiste de um canal de 16 m de comprimento com seção transversal 0,71m de largura por 0,79 m de altura e de uma plataforma de reboque para movimento da estrutura sobre o fluido. Além do movimento retilíneo, a plataforma de reboque é capaz de impor, simultaneamente, movimentos oscilatórios em até dois grupos de estruturas de forma independente. Esta capacidade permite o estudo da influência da amplitude, da freqüência e da fase do movimento oscilatório na interação entre o fluido e as estruturas. A dissertação contém uma discussão sobre a formulação geral dos problemas de interação fluido-estrutura e os principais parâmetros adimensionais relevantes. São tratados os problemas específicos da instalação experimental tais como especificação dos parâmetros hidrodinâmicos, projeto das estruturas de suporte do canal e da plataforma de reboque, o projeto mecânico da plataforma de reboque e uma breve descrição do sistema de controle dos movimentos da plataforma, bem como apresentam-se a descrição dos ensaios de avaliação de desempenho do sistema e a discussão dos resultados. Finalmente, a dissertação apresenta as conclusõesdos aspectos construtivos e capacidade da instalação em uma faixa do número de Reynolds, bem como as sugestões para trabalhos futuros.
The experimental study of fluid-structure interaction problems is of fundamental importance for the understanding of the dynamic phenomena involved. The dynamics of these phenomena can be very complex as a result of the high non-linearity of the problem, in particular for the cases with strong coupling between the dynamic of the fluid and the dynamic of the structure, or in cases in which instabilities in the fluid or in the structure are present. The experimental study of fluid-structure interaction requires laboratory facilities capable of impose relative motion between the fluid and the structure. In this context, that work presents the design, the construction and the evaluation of a towing canal optimized for the study of the interaction fluid-structure in cylindrical bluff bodies of high aspect ratio. The facility consists of 16 m long canal with cross section 0.71 m of width by 0.79 m of height and a towing platform for the motion of the structure in the fluid. In addition to the linear motion, the towing platform is capable of impose simultaneously independent transverse oscillatory motion in up to two structure groups. This capacityallows the study of the influence of the amplitude, of the frequency and of the phase of the oscillatory motion in the interaction between the fluid and the structure. This work presents a discussion on the general formulation of the fluid-structure interaction problems and on the main dimensionless parameters that are relevant to the problem. Next, it deals with the specification of the experimental facility such as hydrodynamic parameters, design of structures for supporting the canal itself and the towing platform, the mechanical design of the towing platform and a brief description of the system of control of the towing platformmotions. Next, it presents the description of the evaluation tests for the platform performance and the discussion of the results. Finally, the work presents the conclusions of the aspects of capacity building and installation on a range of Reynolds number, as well as suggestions for future work.
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de, Rooy S. C. "Improved efficiencies in flame weeding." Lincoln University, 1992. http://hdl.handle.net/10182/18.
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Possible areas of improving the efficiencies of the Lincoln University flame weeder are identified and investigated. The Hoffmann burner initially used in the Lincoln University flame weeder was found not to entrain sufficient air to allow complete combustion of the LPG used. A new burner, the Modified Lincoln University burner, was designed to improve the entrainment of air. Results show that the new design entrained sufficient air to theoretically allow complete combustion of the LPG, and this resulted in a 22.7% increase in heat output per Kg of LPG used over the Hoffmann burner. Temperature x time exposure constants required to kill weeds 0 - 15, 15 - 30, and 30 - 45 mm in size, were found to be respectively 750, 882, and 989 degrees Celsius.Seconds. These constants can be used to calculate the maximum speed of travel an operator can use a flame weeder at, once the temperature profile underneath its shields are established at various travel speeds, and therefore ensure that the flame weeder is used at its maximum efficiency. The constants can also be used to establish the cost efficiency of any flame weeder (in $/Ha), depending on the size of the weeds to be treated. The materials and methods used in establishing the temperature x time exposure constants can be used to establish the temperature x time exposure constant of any weed species at any size.