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Futrzynski, Romain. "Drag reduction using plasma actuators." Licentiate thesis, KTH, Farkost och flyg, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-161409.
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This thesis is motivated by the application of active flow control on the cabin of trucks, thereby providing a new means of drag reduction. Particularly, the work presented strives to identify how plasma actuators can be used to reduce the drag caused by the detachment of the flow around the A-pillars. This is achieved by conducting numerical simulations, and is part of a larger project that also includes experimental. The effect of plasma actuators is modeled through a body force, which adds very little computational cost and is suitable for implementation in most CFD solvers. The spatial distribution of this force is described by coefficients which have been optimized against experimental data, and the model was shown to be able to accurately reproduce the wall jet created by a single plasma actuator in a no-flow condition. A half cylinder geometry - a simplified geometry for the A-pillar of a truck - was used in a preliminary Large Eddy Simulation (LES) study that showed that the actuator alone, operated continuously, was not sufficient to achieve a significant reduction of the drag. Nevertheless, a significant drag reduction was obtained by simply increasing the strength of the body force to a higher value, showing that this type of actuation remains relevant for the reduction of drag. In the course of finding ways to improve the efficiency of the actuator, dynamic mode decomposition was investigated as a post-processing tool to extract structures in the flow. Such structures are identified by their spatial location and frequency, and might help to understand how the actuator should be used to maximize drag reduction. Thus a parallel code for dynamic mode decomposition was developed in order to facilitate the treatment of the large amounts of data obtained by LES. This code and LES itself were thereafter evaluated in the case of a pulsating channel flow. By using the dynamic mode decomposition it was possible to accurately extract oscillating profiles at the forcing frequency, although harmonics with lower amplitude compared to the turbulence intensity could not be obtained.Denna avhandling behandlar tillämpningen av aktiv strömningskontroll för lastbilshytter, vilket är en ny metod för minskning av luftmotståndet. Mer i detalj är det övergripande målet att visa på hur plasmaaktuatorer kan användas för att minska luftmotståndet orsakat av avlösningen runt A-stolparna. In denna avhandling studeras detta genom numeriska simuleringar. Arbetet är en del av ett projekt där även experimentella försök görs. Effekten av plasmaaktuatorer modelleras genom en masskraft, vilket inte ger nämnvärd ökning av beräkningstiden och är lämplig för implementering i de flesta CFD-lösare. Den rumsliga fördelningen av kraften bestäms av koefficienter vilka i detta arbete beräknades utifrån experimentella data. Modellen har visat sig kunna återskapa en stråle nära väggen med god noggrannhet av en enskild plasmaaktuator för en halvcylinder utan strömning. Samma geometri - en halvcylinder som här används som förenklad geometri av A-stolpen på en lastbil - användes i en preliminär LES studie som visade att enbart aktuatorn vid kontinuerlig drift inte var tillräckligt för att uppnå en signifikant minskning av luftmotståndet. En signifikant minskning av luftmotståndet erhölls genom att helt enkelt öka styrkan på kraften, vilket visats att denna typ av strömningskontroll är relevant för minskning av luftmotståndet. I syfte att förbättra effektiviteten hos aktuatorn, studerades dynamic mode decomposition, som ett verktyg för efterbehandling för att få fram flödesstrukturer. Dessa strukturer identifieras genom deras rumsupplösning och frekvens och kan hjälpa till att förstå hur aktuatorerna bör användas för att minska luftmotståndet. En parallelliserad kod för dynamic mode decomposition utvecklades för att underlätta efterbehandlingen av de stora datamängder som fås från LES-beräkningarna. Slutligen, utvärderades denna kod och LES-beräkningar på ett strömningsfall med pulserande kanalflöde. Metoden, dynamic mode decomposition, visade sig kunna extrahera de oscillerande flödesprofilerna med hög noggrannhet för den påtvingade frekvensen. Övertoner med lägre amplitud jämfört med turbulensintensiteten kunde dock inte erhållas.
QC 20150312
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Kulmatova, Dilafruz. "Turbulent drag reduction by additives." Paris 6, 2013. http://www.theses.fr/2013PA066480.
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The addition of a minute amount of polymer or surfactant additive to a turbulent fluid flow can result in a large reduction in the frictional drag in pipes and channels. Over the past decades, numerous studies have been carried out on drag reducing additives (DRA). DRA have been successfully applied for potential benefits in various industrial processes, including oil well operations, heating and cooling water circuits, marine and biomedical systems. The use of additives to enhance flow in petroleum pipelines has received the greatest attention due to its great commercial success in reducting cost and energy consumption. Although this effect has been known for almost half a century, the detailed mechanism of drag reduction have still not been clearly identified and is still a subject of ongoing controversy. The aim of this thesis is to develop an understanding of the role of drag reducing agents and to explain the nature of drag reduction mechanism. This could have an impact on the design of efficient pumping systems, the design of drag-reducing agent that are more stable over time, and the modeling of mixing processes that could be an important consideration in designing practical systemsL'ajout d'une quantité infime d'un polymère ou d'un additif tensioactif à un flux turbulent de fluide peut causer une forte diminution de la friction dans les tuyaux et les canalisations. Ces dix dernières années, de nombreuses études ont été réalisées sur les agents réducteurs de friction (ARF). Les ARF sont utilisés pour leurs effets bénéfiques dans de nombreux procédés industriels, tels que l'extraction de pétrole, le chauffage et le refroidissement de circuits de circulation d'eau ainsi que dans des systèmes marins et biomédicaux. L'utilisation d'additifs pour améliorer l'écoulement dans les canalisations de pétrole a été particulièrement étudiée, en raison de son succès commercial en terme de réductions de couts et de consommation d'énergie. Bien que l'action de ces additifs est connue depuis presque cinquante ans, le mécanisme détaillé de la réduction des frictions n'a pas été clairement identifié et est encore sujet à controverses. Le but de cette étude est d'apporter une explication au rôle de ces agents en matière de réduction des frictions, et d'expliquer la nature ce mécanisme. Les résultats présentés ici peuvent influencer significativement la conception des systèmes de pompes, le développement d'agent réducteurs de friction plus stables ainsi que la modélisation de procédés mixtes qui pourraient devenir une considération majeure dans le design de systèmes réels
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Snelling, Diana. "Surfactant drag reduction using mixed counterions." Connect to resource, 2006. http://hdl.handle.net/1811/6447.
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Thesis (Honors)--Ohio State University, 2006.Title from first page of PDF file. Document formatted into pages: contains 36 p.; also includes graphics. Includes bibliographical references (p. 31-32). Available online via Ohio State University's Knowledge Bank.
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Jukes, Timothy N. "Turbulent drag reduction using surface plasma." Thesis, University of Nottingham, 2007. http://eprints.nottingham.ac.uk/12160/.
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An experimental investigation has been undertaken in a wind tunnel to study the induced airflow and drag reduction capability of AC glow discharge plasma actuators. Plasma is the fourth state of matter whereby a medium, such as air, is ionized creating a system of electrons, ions and neutral particles. Surface glow discharge plasma actuators have recently become a topic for flow control due to their ability to exert a body force near the wall of an aerodynamic object which can create or alter a flow. The exact nature of this force is not well understood, although the current state of knowledge is that the phenomenon results from the presence of charged plasma particles in a highly non-uniform electric field. Such actuators are lightweight, fully electronic (needing no moving parts or complicated ducting), have high bandwidth and high energy density. The manufacture of plasma actuators is relatively cheap and they can be easily retrofitted to existing surfaces. The first part of this study aims at characterising the airflow induced by surface plasma actuators in initially static air. Ambient air temperature and velocity profiles are presented around a variety of actuators in order to understand the nature of the induced flow for various parameters such as applied voltage, frequency, actuator geometry and material. It is found that the plasma actuator creates a laminar wall jet along the surface of the material on which it is placed. The second part of the study aims at using plasma actuators to reduce skin-friction drag in a fully developed turbulent boundary layer. Actuators are designed to induce spanwise forcing near the wall, oscillating in time. Thermal anemometry measurements within the boundary layer are presented. These show that the surface plasma can cause a skin-friction drag reduction of up to 45% due to the creation of streamwise vortices which interact with, and disrupt the near-wall turbulence production cycle.
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Wang, Cheng. "Aerodynamics drag reduction of commercial trucks." Master's thesis, University of Cape Town, 2000. http://hdl.handle.net/11427/5456.
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Bibliography: leaves 71-74This thesis deals with the airflow over a double trailer Gull Wing truck, with a view to reducing the drag of the truck. To investigate the flow over the truck, a 1:20 scale double trailer truck model was designed and constructed from chipboard for wind tunnel experiments. The overall size of the model is 1100 mm long, 130 mm wide and 215 mm high. A same scale numerical model was also built for computational simulations.
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Wise, D. J. "Disc actuators for turbulent drag reduction." Thesis, University of Sheffield, 2015. http://etheses.whiterose.ac.uk/9216/.
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Rowan, Scott A. "Viscous drag reduction in a scramjet combustor /." St. Lucia, Qld, 2003. http://www.library.uq.edu.au/pdfserve.php?image=thesisabs/absthe17438.pdf.
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Shi, Haifeng. "Surfactant Drag Reduction and Heat Transfer Enhancement." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1343664380.
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Khosh, Aghdam Sohrab. "Turbulent drag reduction through wall-forcing methods." Thesis, University of Sheffield, 2016. http://etheses.whiterose.ac.uk/12589/.
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The constraints brought about by environmental and economical issues have been key elements for devising new techniques for skin-friction drag reduction in turbulent flows. Several methodologies have been applied during the last thirty years. These methods can be categorised as active, passive, closed or open-loop. In general, these techniques are mathematically modelled, then tested in experimental settings and numerical simulations. The numerical model for this study was based on the resolution of the full spatio-temporal scales through Direct Numerical Simulation (DNS). With the advent of powerful high-end computing systems endowed with several thousands of processors and relying on distributed memory programming, the performance deadlock due to highly resolved DNS is progressively being overcome. To study in a first principal basis a flow, DNS based on an efficient flow solver called Incompact3d has been relied on more particularly focusing on the development of a large array of flow control techniques. Motivated by extensive discussion in the literature, by experimental evidence and byrecent direct numerical simulations, we study flows over hydrophobic surfaces with shear-dependent slip lengths and we report their drag-reduction properties. The laminar channel-flow and pipe-flow solutions are derived and the effects of hydrophobicity are quantified by the decrease of the streamwise pressure gradient for constant mass flow rate and by the increase of the mass flow rate for constant streamwise pressure gradient. The nonlinear Lyapunov stability analysis is employed on the three-dimensional channel flow with walls featuring shear-dependent slip lengths. The feedback law extracted through the stability analysis is recognized for the first time to coincide with the slip-length model used to represent the hydrophobic surfaces, thereby providing a precise physical interpretation for the feedback law advanced by Balogh et al. (2001). The theoretical framework by K. Fukagata, N. Kasagi, and P. Koumoutsakos is employed to model the drag-reduction effect engendered by the shear-dependent slip-length surfaces and the theoretical drag-reduction values are in very good agreement with our direct numerical simulation data. The turbulent drag reduction is measured as a function of the hydrophobic-surface parameters and is found to be a function of the time- and space-averaged slip length, irrespectively of the local and instantaneous slip behaviour at the wall. For slip parameters and flow conditions that could be realized in the laboratory, the maximum computed turbulent drag reduction is 50% and the drag reduction effect degrades when slip along the spanwise direction is considered. The power spent by the turbulent flow on the hydrophobic walls is computed for the first time and is found to be a non-negligible portion of the power saved through drag reduction, thereby recognizing the hydrophobic surfaces as a passive-absorbing drag-reduction method. The turbulent flow is further investigated through flow visualizations and statistics of the relevant quantities, such as vorticity and strain rates. When rescaled in drag-reduction viscous units, the streamwise vortices over the hydrophobic surface are strongly altered, while the low-speed streaks maintain their characteristic spanwise spacing. We finally show that the reduction of vortex stretching and enstrophy production is primarily caused by the eigenvectors of the strain rate tensor orienting perpendicularly to the vorticity vector. In a second phase, several drag-reduction techniques were implemented and benchmarked. This step was motivated by the drag-reducing potential benefits of combined active-active and active-passive techniques compared to those taken separately. With this objective in mind, three control techniques were selected and categorized as primary and secondary. The primary control method consisted in an array of steady rotating discs or rings embedded at the walls of the channel flow. The secondary methods consisting of opposition control or constant-slip hydrophobic surfaces were used to complement the primary one. It was found that the combination of the the combination of these techniques did not result in the sum of the contributions of each technique taken separately. In addition to these studies and developments within the code, various techniques for analysing the results have been implemented and used which are presenting a novel aspect for the within the flow control area: probabilistic and eigenvalue methods. All these methods are now part of a full-fledge revised version of the code and can be used in parallel. An extensive guide has also been written for future users of the code for flow control problems.
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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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Oggiano, Luca. "Drag reduction and aerodynamic performances in Olympic sports." Doctoral thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for energi- og prosessteknikk, 2010. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-12110.
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In sports where high speed is involved, races are often won by milliseconds. Any advantage can then be important in order to reach the success. The drag acting on the athletes is often the highest force that the athletes have to fight against and, even a small reduction of drag, can create an advantage in terms of performances. However, in sports like ski jumping, the aerodynamic involved gets to be more complex, involving drag and lift force. Wind tunnel measurements have been carried out in the last century in order to understand the physics behind phenomena linked to sport activities (for example ball aerodynamics) or in order to optimize postures and materials. With the performances enhancement as final goal the aerodynamics behind a number of sports have been previously studied. Posture optimization, low drag bycicles, skin suits or even the recent and famous Speedo swimming suits are only some of the achievements of the research carried out. In the present thesis, a wide approach to the topic with particular focus on textile aerodynamics has been used. The thesis has then be divided into two main areas: A research Area 1 named Textiles and their effect on the aerodynamics of athletes and referred RA1 where the influence of textiles and clothing equipement on the drag acting against the athletes have been studied and a Research Area 2 named Performances and Prototyping where more practical examples of how aerodynamics can directy affect athletes performances are given and exposed. In RA1 the topography of textiles have been studied and the surface structure properties has been linked to the aerodynamic properties with particular regards to drag reduction and turbulence tripping. In order to simplify the case the athlete’s body has been simplified as a serie of cylindrical shapes and tests have been carried out mostly on cylinders. Effect of yaw angle, different speed, different diameter, different roughness, different material and distance between body parts have been analyzed. At the same time, test on existing suits have been carried out and a mathematical model in order to estimate performances in speed skating has been made. In RA2 different side projects have been carried out and the results can be summarized as follow: Effects of body weight in ski jumping has been analyzed in order to figure out if the new rules imposed by the FIS (International Ski Federation) were effective in order to reduce the increasing problem of anorexia amongst ski jumpers. Wind tunnel measurements were carried out in order to find the aerodynamic forces acting on a ski jumper in his flight path. The experimental data were then implemented into a mathematical model which is able to simulate the in-run and the flight path. In cycling, the attention was focused on the posture assumed by the cyclists with the goal of reducing the drag while keeping a good biomechanical efficiency. The rules imposed by UCI (International Cycling Union) set the boundaries. However, a impressively good result has been obtained focusing the attention on each athlete and finding a subjective optimum posture for each of the athletes tested. A low drag ski boot have been designed with a airfoiled shape which permitted to obtain an impressive drag reduction on the total drag acting on a downhill skier. Speed skating suits have been tested in order to quantify the influence of different model suits on skating performances. The suit used by torwegian Olympic team of ski-cross has been designed using the knowledge acquired and presented in RA1. An impressive drag reduction has been obtained and it helped two norwegian athletes to win a silver and a bronze medal at theWinter Olympic Games in Vancouver 2010. As previously mentioned, the research areas are: Research Area 1 - Textiles and their effect on the aerodynamics of athletes Research Area 2 - Performances and prototyping The main contributions are: P1: Reducing the Athlete’s aerodynamics P2: Experimental analysis on parameters affecting drag force on athletes P3: Aerodynamic and comfort properties of single jersey textiles for high speed sports P4: Aerodynamic behavior of single sport jersey fabrics with different roughness and cover factors P5: Effect of different skin suits on speed skating performances P6: Aerodynamic optimization and energy saving of cycling postures for international elite level cyclists P7: Effects of body weight on Ski Jumping performances under the new FIS rules P8: Airfolied design for alpine skiers boots P9: Aerodynamic and Comfort Characteristics of A Double Layer Knitted Fabric Assembly for High Speed Winter Sports P10: A Low Drag Suit For Ski-Cross Competitions
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Grabowski, Daniel W. "Drag reduction in pipe flows with polymer additives /." Online version of thesis, 1990. http://hdl.handle.net/1850/10599.
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Ghebali, Sacha. "Turbulent drag reduction by oblique wavy wall undulations." Thesis, Imperial College London, 2018. http://hdl.handle.net/10044/1/63827.
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The turbulent flow past a horizontal wavy wall, positioned at an angle to the main flow direction, is investigated by means of Direct Numerical Simulations with the purpose of reducing the drag. The concept aims to emulate the active control by spanwise wall oscillations---known for its high drag-reducing effectiveness---by use of a passive device. The latter takes advantage of the large characteristic spatial wavelength of the active method, which is a crucial aspect for the potential practical implementation on commercial aircraft. Imparting wall oscillations in the form of a standing wave $w_w = A_\textrm{SSL} \sin\left(\frac{2\pi}{\lambda_x}x\right)$ gives rise to a so-called Spatial Stokes Layer (SSL), resulting in a shear-strain layer, which induces a strong suppression of the near-wall turbulence, thereby leading to drag reduction. A skewed wavy wall described by $h_w = A_w \sin\left(\frac{2\pi}{\lambda_x}x + \frac{2\pi}{\lambda_z}z\right)$ is considered, so as to produce a shear-strain layer that is similar to that of the SSL in featuring the same streamwise wavelength $\lambda_x$. The main points of resemblance between the wavy wall and SSL are investigated, and then contrasted through the identification of significant differences. A reduced-order model is formulated to aid the DNS exploration of the parameter space (wave height, flow angle and wavelength) in the search for the optimal flow configuration. The validity of the assumptions made in the model, as well as its ability to predict the main flow properties, are examined by comparison to the DNS results. Arising from a DNS exploration of the 3D parameter space, a configuration yielding approximately 1\% drag reduction is identified. The response of the flow properties and the drag to variations in the wave height, flow angle and wavelength on the flow properties is reported and analysed. Major emphasis is placed on quantifying the influence of numerical accuracy on the predicted drag-reduction margin and on the computational efforts required to achieve this margin.
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Koury, Eddie. "Drag reduction by polymer solutions in riblet pipes." Thesis, Massachusetts Institute of Technology, 1995. http://hdl.handle.net/1721.1/11372.
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Choi, Wonjae. "Micro-textured surfaces for omniphobicity and drag-reduction." Thesis, Massachusetts Institute of Technology, 2009. http://hdl.handle.net/1721.1/49756.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2009.Includes bibliographical references.
When a liquid droplet contacts a surface possessing the appropriate combination of surface texture and solid surface energy, the liquid may not penetrate into the surface texture. Instead, the droplet sits partially on air, forming a solid-liquid-air composite interface. Entrapped air pockets of the composite interface are known to cause various interesting phenomena such as super-repellency, low hysteresis, and liquid slip. The current thesis research aims to extend the understandings about the role of surface texture on the static and dynamic behavior of the liquids forming composite interfaces on these textured surfaces. The first part of this thesis investigates the ability of surfaces with re-entrant texture, i.e., topography which bends back on itself, to promote the formation of composite interfaces against low surface tension liquids and display omniphobicity, i.e., repellency against a wide range of liquids regardless of their surface tension values. Surfaces that display contact angles of 0* > 1500 with liquids having appreciably lower surface tensions are attained by the incorporation of re-entrant surface curvature. The second part is an expansion of the classic Cassie-Baxter model that is widely used to predict the apparent contact angles on composite interfaces. A liquid droplet sitting on composite interfaces shows a range of apparent contact angles and corresponding contact angle hysteresis when the drop advances or recedes. The Cassie-Baxter model only predicts a single value of the apparent contact angle, and consequently, the model is inherently unable to provide an explanation for the hysteretic behavior of the liquid droplet.
(cont.) In this thesis, the classic Cassie-Baxter relation is extended to predict the contact angle hysteresis observed on textured surfaces as well. In the final part of this thesis, the drag-reducing ability of composite interfaces will be investigated. It will be shown that the robustness, i.e., the resistance against a pressure perturbation, of composite interfaces and the amount of drag-reduction on these interfaces are negatively coupled. An approach to decouple these two important characteristics will be proposed based on the concept of dual-scale textures.
by Wonjae Choi.
Ph.D.
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Gouder, Kevin. "Turbulent friction drag reduction using electroactive polymer surfaces." Thesis, Imperial College London, 2011. http://hdl.handle.net/10044/1/6911.
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Both experiments and numerical simulations have provided evidence that an initially fully developed two-dimensional boundary layer, subjected to a sudden spanwise forcing, exhibits a decrease of turbulent quantities such as the Reynolds shear stress, turbulent kinetic energy and turbulent friction drag. In past experiments and investigations, such forcing has traditionally been in the form of spanwise wall oscillations, spanwise travelling Lorentz forcing, superimposed spanwise pressure gradients and spanwise travelling waves of an inplane flexible wall. The aim of this work is to take the idea a step further and develop an active surface which locally executes the motions described above making such a system more readily deployable. Two surfaces were developed: both executing in-plane local oscillations with amplitude close to or larger than the mean streak spacing in a turbulent flow, but based on two different technologies, electroactive polymers in the dielectric form of actuation and electromagnetic motor forcing. The effect of these two surfaces was confined to wall-normal heights on the order of the linear sublayer of the turbulent boundary layer, and frequency and wavelength similar to those reported in literature. Extensive hot-wire measurements, some PIV measurements and direct measurement of friction drag using a bespoke drag balance are presented for the systematic variation of the relevant parameters for turbulent friction drag reduction. Electroactive polymers (EAP) are able to undergo relatively large deflections at high frequencies. Developments in the field of EAP such as static and dynamic characterisation of the EAP membranes in use in this work, development of robust electrodes and their characterisation, in-house manufacturing of thin silicone membranes and post-processing of pre-built silicone membranes are presented. Numerical studies of the optimum pre-strain values and of the optimum electrode to passive portions width ratios are presented. Actuator development techniques including EAP membrane pre-stretch in a bespoke jig, EAP membrane pre-conditioning to go past the Mullins' effect, electrode preparation procedure and deposition, and frame preparation are presented. Actuator characterisation results including analysis of multi-flash photographs and laser profilometer scans for in-plane and out-plane deflections at different frequencies are also presented.
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Wu, Gangwei. "Drag reduction in large diameter hydraulic capsule pipeline /." free to MU campus, to others for purchase, 1998. http://wwwlib.umi.com/cr/mo/fullcit?p9904874.
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Ge, Wu. "Studies on the Nanostructure, Rheology and Drag Reduction Characteristics of Drag Reducing Cationic Surfactant Solutions." The Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=osu1230589917.
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Jimenez, Bernal Jose Alfredo. "Microbubble drag reduction phenomenon study in a channel flow." Texas A&M University, 2004. http://hdl.handle.net/1969.1/2784.
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An experimental study on drag reduction by injection of microbubbles was performed in the upper wall of a rectangular channel at Re = 5128. Particle Image Velocimetry measurement technique (PIV) was used to obtain instantaneous velocity fields in the x-y plane. Microbubbles, with an average diameter of 30??m, were produced by electrolysis
using platinum wires with a diameter of 76 ??m. They were injected in the buffer layer producing several different values of local void fraction. A maximum drag reduction of 38.45% was attained with a local void fraction of 4.8 %. The pressure drop in the test station was measured by a reluctance pressure transducer. Several parameters such as velocity profile, turbulent intensities, skewness, flatness, joint probability density function (JPDF), enstrophy, one and two-dimensional energy spectra were evaluated. The results indicate that microbubbles reduced the intermittency of the streamwise fluctuating component in the region near the wall. At the same time they destroy or reduce the vortical structures regions (high shear zones) close to the wall. They also redistribute the energy among different eddy sizes. An energy shift from larger wavenumbers to lower wavenumbers is observed in the near wall region (buffer layer). However, outside this region, the opposite trend takes place. The JPDF results indicate that there is a decrease in the correlation between the streamwise and the normal fluctuating velocities, resulting in a reduction of the Reynolds stresses. The results of this study indicate that pursuing drag reduction by injection of microbubbles in the buffer layer could result in great saving of energy and money.
The high wavenumber region of the one dimensional wavenumber spectra was evaluated from PIV spatial information, where the maximum wavenumber depends on the streamwise length (for streamwise wavenumber) of the recorded image and the minimum wavenumber depends on the distance between vectors. On the other hand, the low wavenumber region was calculated from the PIV temporal information by assuming Taylor??s frozen hypothesis. This new approach allows obtaining the energy distribution of a wider wavenumber region.
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Koberg, Henrik. "Turbulence control for drag reduction with active wall deformation." Thesis, Imperial College London, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.486911.
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Ecological and economical considerations motivate the search for ways to reduce the skin friction drag in turbulent flows. Several numerical studies have shown that wall shear stress can be lowered at low Reynolds numbers by applying a small amount of wall transpiration. In this study it is investigated how another type of actuation, active wall deformation, could be used to yield a similar effect. . Ã??Ã?? First. discrete time-dependent wall deformation is studied in laminar flow. Lacking background turbulence, the baseflow allows clear identification of the flow perturbation. The analysis reveals that a downward moving actuator is surrounded by a region of negative wall-normal velocity and vice versa. Comparably less intuitive are the vorticity fields which often display complicated structures. A similar, subsequent study in turbulent flow shows that, indeed, active wall deformation can restruckJre wall turbulence. Seised on thesA findings, a series of experiments were conducted on opposition control. This scheme . aims at opposing the velocity sensed away from the wall by imposing velocity of opposite direction at the wall. By locally deforming the wall accordingly, skin friction reductions of up to 15% are observed. Parameters critical to the performance of the control scheme, such as actuation scales and deformation limiters, are identified and analysed. As Reynolds number and actuation scales are much smaller than in practical applications, the results are of limited applicability but encouraging for prospective drag reduction at higher Reynolds numbers. In a separate study a novel control method based on non-linear global stabilisation of the perturbed Navier-Stokes equations was tested. Using body forcing over the entire domain as actuation. the flow
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Littlewood, Rob. "Novel methods of drag reduction for squareback road vehicles." Thesis, Loughborough University, 2013. https://dspace.lboro.ac.uk/2134/12534.
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Road vehicles are still largely a consumer product and as such the styling of a vehicle becomes a significant factor in how commercially successful a vehicle will become. The influence of styling combined with the numerous other factors to consider in a vehicle development programme means that the optimum aerodynamic package is not possible in real world applications. Aerodynamicists are continually looking for more discrete and innovative ways to reduce the drag of a vehicle. The current thesis adds to this work by investigating the influence of active flow control devices on the aerodynamic drag of square back style road vehicles. A number of different types of flow control are reviewed and the performance of synthetic jets and pulsed jets are investigated on a simple 2D cylinder flow case experimentally. A simplified ¼ scale vehicle model is equipped with active flow control actuators and their effects on the body drag investigated. The influence of the global wake size and the smaller scale in-wake structures on vehicle drag is investigated and discussed. Modification of a large vortex structure in the lower half of the wake is found to be a dominant mechanism by which model base pressure can be influenced. The total gains in power available are calculated and the potential for incorporating active flow control devices in current road vehicles is reviewed. Due to practicality limitations the active flow control devices are currently ruled out for implementation on a road vehicle. The knowledge gained about the vehicle model wake flow topology is later used to create drag reductions using a simple and discrete passive device. The passive modifications act to support claims made about the influence of in wake structures on the global base pressures and vehicle drag. The devices are also tested at full scale where modifications to the vehicle body forces were also observed.
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Whiteman, Jacob T. "Active Flow Control Schemes for Bluff Body Drag Reduction." The Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1452184221.
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Soeiro, Pereira Anselmo. "Transient aspects of the polymer induced drag reduction phenomenon." Thesis, Lille 1, 2016. http://www.theses.fr/2016LIL10138/document.
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La dilution en faible concentration de chaînes polymériques longues dans un fluide newtonien peut réduire la traînée turbulente, phénomène nommé ici DR (drag reduction). Les polymères s’étirent et s’enroulent successivement, en interaction avec les structures turbulentes, imposant à DR un comportement transitoire. Il en résulte que la DR traverse trois stades. Lors du premier, la DR démarre à zéro et descend à des valeurs négatives en raison d’un étirement considérable du polymère au début du processus, ce qui exige de l’énergie de l’écoulement. Une fois atteint le niveau minimal de réduction de la traînée, les polymères commencent leur cycle d’étirement-enroulement et la DR augmente en réponse au développement de structures turbulentes, pour en arriver à une valeur maximale, menant au début du deuxième stade. Cependant, les polymères peuvent subir une dégradation mécanique à la suite d’un étirement polymérique intense. Lorsque la dégradation polymérique devient assez prononcée, la DR redescend pour atteindre une valeur finale qui indique que la dégradation s’est arrêtée. Le processus de dégradation polymérique caractérise le troisième stade. Dans le présent travail, ces trois stades sont examinés à l’aide de simulations numériques directes d’écoulements turbulents viscoélastiques FENE-P en géométries du type Poiseuille plan et Couette plan, sur un large éventail de nombres de Reynolds, de nombres de Weissenberg et d’extension maximale de la chaîne polymérique. Les deux premiers stades sont étudiés à partir des analyses tensorielle, énergétique et spectrale. Un nouveau modèle de dégradation polymérique est proposé afin de reproduire numériquement le stade finalThe addition of a small amount of polymers of high molecular weight can lead to a pressure drop decrease in turbulent flows. The polymers successively stretch and coil by interacting with the turbulent structures, which imposes a transient behaviour on the drag reduction (DR). As a result, DR undergoes three stages over time: A, B, and C. In stage A, DR departs from zero and assumes negative values due to a significant polymer stretching at the beginning of the process, which requires energy from the flow. After the minimum DR is reached, the polymers start their coil-stretch cycle and DR increases in response to the development of turbulent structures, achieving a maximum value, which makes for the beginning of stage B. However, during their coil-stretch cycle, polymers can be mechanically degraded as a result of an intense polymer stretching, which reduces their ability to act as energy exchange agents. Hence, when polymer degradation becomes pronounced, DR decreases until achieving a final value. The polymer degradation process characterizes the stage C. In the present work, numerical analyses are conducted aiming to investigate the stages A, B and C. The transient aspects of the polymer induced drag reduction phenomenon are explored with the aid of direct numerical simulations of turbulent plane Poiseulle and Couette flows of viscoelastic FENE-P fluids taking into account a large range of Reynolds number, Weissenberg number and maximum polymer molecule extensibility. Stages A and B are carefully studied from tensor, energy budget and spectral perspectives. A polymer scission model is developed in order to numerically reproduce the stage C
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Yang, Qiang. "Turbulent skin-friction drag reduction control by spanwise motion." Thesis, University of Warwick, 2016. http://wrap.warwick.ac.uk/88438/.
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Three types of actuators, i.e., oscillating walls, Lorentz force actuators and DBD plasma actuators, were used to actively control turbulent boundary layer for the turbulent skin-friction drag reduction with Direct Numerical Simulations. The main object is to understand drag reduction mechanism in simple spanwise wall oscillation case (Jung et al., 1992), then implement the control using more practical Lorentz force actuators and plasma actuators. A large amount of 40 ± 2%, 30 ± 2% and 20±2% drag reduction was observed at Rer = 200 turbulent channel for oscillating walls, Lorentz force actuators and plasma actuators, respectively. Different configurations for Lorentz force and plasma actuators were intensively studied, with a new configuration proposed for DBD plasma actuators. The present study suggests a good prospective of skin-friction drag reduction by using Lorentz force actuators for ocean transportation, and DBD plasma actuators for land and air transportation. However, no net energy saving was obtained for both actuators considering the fluid power required for flow control, and this situation was even worse if the electric efficiency of the actuators was accounted for. For all three types of actuators, the interaction between the actuators and the near wall turbulent structure is presented using ensemble averaged method. DNS control cases were also performed at moderate Reynolds numbers, i.e, Rer = 800 and 1600, to understand the role of recently discovered very large scale motions (VLSMs). The result suggests that the control of the VLSMs in the outer region is necessary for maximising drag reduction at high Reynolds number turbulent flows.
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Hackenberg, Petra. "Numerical optimization of the suction distribution for laminar flow control aerofoils." Thesis, University of Southampton, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.241170.
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Rioual, Jean-Luc. "The automatic control of boundary layer transition." Thesis, University of Southampton, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.259625.
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Qi, Yunying. "Investigation of Relationships among Microstructure, Rheology, Drag Reduction and Heat transfer of Drag Reducing Surfactant Solutions." The Ohio State University, 2002. http://rave.ohiolink.edu/etdc/view?acc_num=osu1036712806.
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Sood, Arun. "A study of drag reduction and convective heat transfer reduction in turbulent flow through pipes." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp03/NQ31534.pdf.
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Xu, Jin. "High Reynolds number simulation and drag reduction techniques : a thesis /." View online version; access limited to Brown University users, 2005. http://wwwlib.umi.com/dissertations/fullcit/3174698.
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Yeshala, Nandita. "A coupled lattice Boltzmann-Navier-Stokes methodology for drag reduction." Diss., Georgia Institute of Technology, 2010. http://hdl.handle.net/1853/37097.
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Helicopter performance is greatly influenced by its drag. Pylons, fuselage, landing gear, and especially the rotor hub of a helicopter experience large separated flow regions, even under steady level flight conditions the vehicle has been designed for, contributing to the helicopter drag. Several passive and active flow control concepts have been studied for reducing helicopter drag. While passive flow control methods reduce drag, they do so at one optimized design condition. Therefore, passive drag reduction methods may not work for helicopters that operate under widely varying flight conditions. Active flow control (AFC) methods overcome this disadvantage and consequently are widely being pursued.
The present investigator has studied some of these AFC methods using computational fluid dynamics (CFD) techniques and has found synthetic (or pulsed) jets as one of the more effective drag reduction devices. Two bluff bodies, representative of helicopter components, have been studied and the mechanism behind drag reduction has been analyzed. It was found that the increase in momentum due to the jet, and a resultant reduction in the separated flow region, is the main reason for drag reduction in these configurations. In comparison with steady jets, synthetic jets were found to use less power for a greater drag reduction.
The flow inside these synthetic jet devices is incompressible. It is computationally inefficient to use compressible flow solvers in incompressible regions. In such regions, using Lattice Boltzmann equations (LBE) is more suitable compared to solving the incompressible Navier-Stokes equations. The length scales close to the synthetic jet devices are very small. LBE may be used to better resolve these small length scale regions. However, using LBE throughout the whole domain would be computationally expensive since the grid spacing in the LBE solver has to be of the order of the mean free path. To address this need, a coupled Lattice Boltzmann-Navier-Stokes (LB-NS) methodology has been developed.
The LBE solver has been successfully validated in a standalone manner for several benchmark cases. The solver has also been shown to be of second order accuracy. This LBE solver has been subsequently coupled with an existing Navier-Stokes (NS) solver. Validation of the coupled methodology has been done for analytical problems with known closed form solution.
This LB-NS methodology is further used to simulate the flow past a cylinder where synthetic jet devices have been used to reduce drag. The LBE solver is used in the cavity of the synthetic jet nozzle while the NS solver is employed in the rest of the domain. The cylinder configuration was chosen to demonstrate drag reduction on helicopter hub shape geometries. Significant drag reduction is observed when synthetic jets are used, compared to the baseline no flow control case.
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Dominguez, Ontiveros Elvis Efren. "Wall-pressure and PIV analysis for microbubble drag reduction investigation." Texas A&M University, 2004. http://hdl.handle.net/1969.1/2582.
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The effects of microbubbles injection in the boundary layer of a turbulent channel flow are investigated. Electrolysis demonstrated to be an effective method to produce microbubbles with an average diameter of 30 ??m and allowed the placement of microbubbles at desired locations within the boundary layer.
Measurement of velocity fluctuations and the instantaneous wall shear stress were carried out in a channel flow facility.
The wall shear stress is an important parameter that can help with the characterization of the boundary layer. This parameter can be obtained indirectly by the measurement of the flow pressure at the wall.
The wall shear stress in the channel was measured by means of three different independent methods: measurement of the pressure gradient by a differential pressure transducer, Particle Image Velocimetry (PIV), and an optical wall shear stress sensor. The three methods showed reasonable agreement of the wall shear stress values for single-phase flow. However, differences as skin friction reductions were observed when the microbubbles were injected. Several measurements of wall-pressure were taken at various Reynolds numbers that ranged from 300 up to 6154. No significant drag reduction was observed for flows in the laminar range; however, a drag reduction of about 16% was detected for turbulent Reynolds numbers.
The wall-pressure measurements were shown to be a powerful tool for the measurement of drag reduction, which could help with the design of systems capable of controlling the skin friction based on feedback given by the wall-pressure signal.
The proposed measurement system designed in this work has capabilities for application in such diverse fields as multiphase flows, drag reduction, stratified flows, heat transfer among others. The synchronization between independent systems and apparatus has the potential to bring insight about the complicated phenomena involved in the nature of fluid flows.
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Waring, John. "Reduction of drag of a submerged swimmer using vortex generators." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2000. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape3/PQDD_0017/MQ48464.pdf.
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Zadrazil, Ivan. "various aspects of polymer induced drag reduction in turbulent flow." Thesis, Imperial College London, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.537570.
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Raul, Vishal Vinod. "Analysis of F-duct drag reduction system in Formula 1." Thesis, Wichita State University, 2013. http://hdl.handle.net/10057/10643.
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During the Formula 1 auto racing season of 2010, team McLaren developed an aerodynamic device called an F-Duct. This device was an ingenious way of out-thinking the regulations of the Federation Internationale de l'Automobile (FIA) by providing a means of notably controlling car performance. More specifically, the F-Duct is a device that allows a driver to alter air flow over the car's rear wing in order to gain higher speed on long straight sections of the race track (up to 10 kmph). In general, Formula 1 teams do not share technological details about their cars in order to keep an advantage over other teams. Available public information is vague in nature and does not provide any technical details. This thesis studied the existing F-Duct system and its requirements, including the location of key components and their functions. First, a working concept was proposed, and a simplified model was prepared to test the concept. Wind tunnel testing was performed on a physical model and the data analyzed. Continuous improvements were made to achieve drag reduction through passive flow control. Finally, an improved model was prepared, drag reduction was observed, and wind tunnel data presented.Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Aerospace Engineering
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Pang, Junguo. "Turbulence control for drag reduction by spanwise Lorentz-force oscillation." Thesis, University of Nottingham, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.430318.
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VASQUEZ, JULIO RAUL SIERRA. "DRAG REDUCTION IN LAMINAR FLOW BY LUBRICATION OF GROOVED WALLS." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2009. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=15347@1.
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COORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIORObjetivo: Uma parte significativa das reservas mundiais de petróleo é encontrada na forma de óleos pesados. Estes óleos pesados possuem alta viscosidade de 100 - 10000 cP, que torna seu transporte altamente complexo e custoso. Vários métodos foram desenvolvidos para reduzir a perda de carga de escoamentos laminares de óleos de alta viscosidade. Entre os mais utilizados, pode-se citar o bombeio de um fluido de baixa viscosidade perto da parede do tubo com o óleo viscoso sendo transportado no centro, conhecido como core-annular flow. Neste trabalho, uma alternativa ao core-annular flow é estudada. O método é baseado na utilização de micro ranhuras da parede do duto preenchidas com um liquido de baixa viscosidade. Este método tem o potencial de evitar alguns dos problemas que ocorrem no uso de core-annular flow. A análise do efeito das diferentes propriedades dos fluidos, condições de operação, geometria das ranhuras na perda de carga do escoamento foi feita através de um estudo numérico e experimental. Resultados indicam as limitações e potencialidade do uso de micro ranhuras na reduçãode perda de carga de escoamento laminar.
Objective: A significant portion of the world oil reserves is found in the form of heavy oil. These oils have a high values of viscosity around 100-10000 cP, that makes their transportation complex and expensive. Several methods have been developed to reduce the pressure drop in laminar flows of high viscosity oils. Among them is the solution of pumping a liquid of lower viscosity near the pipe wall with the high viscosity oil flowing in the center. This method is known as Core-annular flow. In this work, an alternative to core-annular flow is studied. The method is based on the use of micro grooves in the pipe wall filled with a liquid of smaller viscosity. This method has the potential to elude some problems that occur with the core-annular flow method. The analysis of the drag reduction effect as a function of different fluids properties, operational conditions and geometry of the grooved walls was made using a numerical and experimental approach. Results indicate the limitations and potential of using micro grooves for drag reduction in laminar flows of high viscosity fluids.
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Rajappan, Anoop. "Skin friction drag reduction in turbulent flows using superhydrophobic surfaces." Thesis, Massachusetts Institute of Technology, 2017. http://hdl.handle.net/1721.1/113959.
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Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2017.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 127-136).
The use of randomly textured superhydrophobic surfaces have recently gained interest as a potential approach for the passive reduction of skinfriction on the hull of ships, submarines and underwater projectiles. When submerged in water, these surfaces trap a layer of air (or 'plastron') within their texture, which allows the external fluid to partially slip over the boundary, decreasing the net frictional shear stress on the wall. Five prototype drag-reducing surfaces were evaluated experimentally as possible candidates for turbulent drag reduction applications, using a combination of flow tests, surface profile measurements, and contact angle goniometry. Three of these were randomly rough superhydrophobic textures, produced by easily scalable mechanical and chemical surface treatment processes; the other two surfaces consisted of periodic streamwise grooves, filled with air or a low viscosity liquid lubricant. The skin friction characteristics of all five prototype surfaces were measured in fully turbulent flow inside a custom-built Taylor-Couette apparatus for Reynolds numbers in the range 1.64 x 104 by Anoop Rajappan.
S.M.
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Raayai, Ardakani Shabnam. "Geometry mediated drag reduction using riblets and wrinkled surface textures." Thesis, Massachusetts Institute of Technology, 2018. http://hdl.handle.net/1721.1/115612.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2018.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 281-291).
The surfaces of many plants and animals are covered with a variety of micro-textures such as ribs or 3D tubules which can control surface-mediated properties such as skin friction. Inspired by the drag reducing ability of these natural structures, especially the ribbed features on shark denticles, passive drag reduction strategies such as micro-fabricated riblet surfaces have been developed and studied. Microgroove textures on the surface of objects such as hulls, wings or inner surface of pipes which are aligned in the stream-wise direction have been shown to reduce the wall friction by 4 - 8%. The mechanisms suggested for this form of drag reduction are viscous retardation of the flow in the grooves (both laminar and turbulent) and the displacement of vortical structures away from the wall in turbulent flows. Due to their effectiveness in altering the boundary layer structure and reducing the viscous drag force, use of riblets have been banned in various competitions such as the America's Cup. The current thesis work is partly focused on theoretical and numerical modelling (using the open source CFD package OpenFOAM) of the evolution of viscous boundary layers in the presence of various-shaped riblets (V-grooves as well as sinusoidal wrinkled surfaces) in high Reynolds laminar flow. We explore the effect of the dimensionless height to spacing of the grooves (aspect ratio) as well as the length of the wetted surface in the streamwise direction and how these change the total drag compared with a corresponding flat wall. We show that riblets retard the viscous flow inside the grooves and reduce the shear stress inside the grooves. But for this reduction to result in overall drag reduction, the riblet wall needs to be longer than a critical length. The total drag reduction achieved is a non-monotonic function of the aspect ratio of the riblets, with aspect ratios of order unity offering the largest reduction in the total drag. To eliminate the role of entrance effects, we additionally investigate the effect of stream-wise aligned riblet structures on fully-developed Taylor-Couette flow. We perform both experimental studies as well as time-dependent numerical simulations in both the laminar Couette and the Taylor vortex regime. We again explore the effect of the size of the riblets with respect to the geometry of the Taylor-Couette cell, as well as the aspect ratio of the riblet grooves and the shape of the grooves (V-groove, Rectangular, semi-circular, etc.). For the experiments, the cylindrical textured rotors are fabricated using 3D printing techniques and the rest of the Taylor-Couette cell is custom built using CNC machining. The test cell is then aligned and mounted on a stress-controlled rheometer to measure the velocity and the torque on the rotating inner cylinder. The numerical studies are performed using the open source CFD software package OpenFOAM to compare results and understand the physical mechanisms contributing to this drag reduction phenomenon. Again we observe a non-monotonic behavior for the reduction in torque as a function of the aspect ratio of the riblets tested, similar to the trend observed in the boundary layer analysis and we discuss the effect of changing the geometry of the flow as well as the riblet spacing on the changes in the total torque. When viewed holistically the results of these two studies show that, through careful design, a net reduction in viscous drag force can be robustly realized on micro-textured surfaces in high Reynolds number laminar flows through complex changes in near-wall stream-wise velocity profiles even in the absence of turbulent effects. The understanding of these changes can be effective in guiding the design of internal flows (pipes or ducts) and external flows (such as ship hulls, micro air vehicles or unmanned underwater vehicles) that are tailored and optimized to result in low frictional drag over the entire wetted surface in both laminar and turbulent regions.
by Shabnam Raayai Ardakani.
Ph. D.
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Jaskolski, Corey (Corey John) 1974. "Experimental implementation of Lorentz force actuators for hydrodynamic drag reduction." Thesis, Massachusetts Institute of Technology, 2002. http://hdl.handle.net/1721.1/85727.
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Gruncell, Brian. "Superhydrophobic surfaces and their potential application to hydrodynamic drag reduction." Thesis, University of Southampton, 2014. https://eprints.soton.ac.uk/363781/.
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Superhydrophobic surfaces appear frequently in the natural world, for example allowing insects to respire underwater and plants, such as the lotus leaf, to have self-cleaning properties. Attempts to mimic these superhydrophobic surfaces have been successful on nano- and micro-scales, with increased effciency of water flowing through micro-channels when the walls are superhydrophobic. This thesis is focused on the proposed use of superhydrophobic surfaces to reduce drag on a much larger scale, applicable to small water craft such as canoes and yachts. The potential for drag reduction using superhydrophobic surfaces arises from the ability of such surfaces to retain an air-layer or plastron on the surface. The presence of a plastron results in slip and reduced shear at the surface, producing a drag reduction. This potential drag reduction is explored through numerical simulations and experimental testing. Computational Fluid Dynamics is used to explore the effect of slip on flow separation and viscous drag, allowing the potential drag reduction mechanisms to be explored. A range of superhydrophobic surfaces have been developed and characterised based on their roughness, contact angle and ability to retain a plastron. Confocal microscopy is used to generate the first high resolution 3D images of the air-water interface on a superhydrophobic surface over a large area. These images confirm the presence of a plastron on the surfaces and help contribute to the understanding of optimal design of superhydrophobic surfaces. These surfaces are explored experimentally in a towing tank with a repeatability of better than 1%. Refinement of the surface design leads to the presence of a plastron producing a relative drag reduction of up to 3% for hydrophobic sand, up to 10% for hydrophobic ridges and up to 15% for a hydrophobic mesh. Overall, superhydrophobic surfaces are shown to be capable of producing a relative drag reduction when a plastron is retained on the surface, although with the penalty of increased roughness-induced drag component. The drag reduction is shown to be linked to both the structure of the surface, and the quality and thickness of the plastron. It is demonstrated that it is difficult to retain a plastron over long immersion periods and manufacturing constraints currently limit applicability.
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Miller, Simon James. "Adaptive wing structures for aeroelastic drag reduction and loads alleviation." Thesis, University of Manchester, 2011. https://www.research.manchester.ac.uk/portal/en/theses/adaptive-wing-structures-for-aeroelastic-drag-reduction-and-loadsalleviation(562181ed-7153-44cb-b0c7-9bfe1f79ae0f).html.
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An investigation into two distinct novel adaptive structures concepts is performed with a view to improving the aerodynamic efficiency of aircraft wings.The main focus of the work is on the development of a rotating spars concept that enables the adaptive aeroelastic shape control of aircraft wings in order to reduce drag. By altering the orientation of the internal wing structure, it becomes possible to control the flexural and torsional stiffnesses of the wing, as well as the position of the elastic axis. It follows then that control of the aeroelastic deformation is also possible. Consequently, the aerodynamic performance can be tailored, and more specifically the lift-to-drag ratio can be maximised through continuous adjustment of the structure.To gain a thorough understanding of the effect of the concept on a wing, an assumed modes static aeroelastic model is developed, and studies are performed using this. These studies establish guidelines with regards to the effective design of a wing incorporating the rotating spars concept. The findings of these studies are then used to establish a baseline design for a wind tunnel model. A finite element model of this is constructed and aeroelastic analyses are used to improve the model and arrive at the final experimental wing design. The wind tunnel tests confirm analytical trends and the robustness of an approach to automaticallyadapt the structure to maintain an aerodynamic performance objective.The remainder of the work investigates the application of an all-moving wing tip device with an adaptive torsional stiffness attachment as a passive loads alleviation system. Through consideration of the attachment stiffness and position, it is possible to tune the device throughout flight in order to minimise the loads that are introduced into the aircraft structure in response to a gust or manoeuvre. A dynamic aeroelastic wing model incorporating the device is developed and used to perform parameter studies; this gives an insight into the sizing and placement of the device. Next, a finite element representation of a conceptual High Altitude Long Endurance (HALE) aircraft is used as a baseline platform for the device. Aeroelastic analyses are performed for the baseline and modified models to investigate the effect of the attachment stiffness and position on the gust response and aeroelastic stability of the system. The reduced loading within thestructure of the modified aircraft then enables the model to be optimised in order to reduce the mass of the aircraft.
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Waring, John (John Chester Stephen) Carleton University Dissertation Engineering Mechanical and Aerospace. "Reduction of drag of a submerged swimmer using vortex generators." Ottawa, 1999.
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Angle, Gerald M. "Aerodynamic drag reduction of a racing motorcycle through vortex generation." Morgantown, W. Va. : [West Virginia University Libraries], 2002. http://etd.wvu.edu/templates/showETD.cfm?recnum=2643.
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Thesis (M.S.)--West Virginia University, 2002.Title from document title page. Document formatted into pages; contains xvi, 137 p. : ill. (some col.). Vita. Includes abstract. Includes bibliographical references (p. 87-89).
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V'Dovec, Taylor. "Implementation of travelling waves on lifting surface for drag reduction." Thesis, Massachusetts Institute of Technology, 2019. https://hdl.handle.net/1721.1/123253.
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Thesis: S.B., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2019Cataloged from PDF version of thesis.
Includes bibliographical references (page 27).
NACA series airfoils are the standard cross-section shapes for lift-generating surfaces widely used in both aeronautic and aquatic transportation. As with any wing, the key design goal of such airfoils is to attain a large lift-to-drag ratio, namely increasing the lift force while reducing the drag force. At moderate angles of attack, boundary layer separation begins near the trailing edge, and moves up to the leading edge as the angle increases. Such a phenomenon raises the form drag significantly, and a method of pushing the separation point back towards the trailing edge could improve the overall efficiency of the foil tremendously. One such new approach is to introduce a travelling wave on the lifting surface, a technique inspired by the undulatory motion of fish. In order to manufacture and test such a surface, first the necessary design specifications must be determined. Using NACA0012 and NACA0018 wings at a Reynolds number of 1 x 10 5, key parameters including the location of the point of separation on the wing, the angles of attack where the surface would be most effective, and the characteristics of the travelling wave itself were determined.
by Taylor V'Dovec.
S.B.
S.B. Massachusetts Institute of Technology, Department of Mechanical Engineering
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Suryanarayana, G. K. "On The Reduction Of Drag Of a Sphere By Natural Ventilation." Thesis, Indian Institute of Science, 1995. https://etd.iisc.ac.in/handle/2005/125.
Full textAbstract:
The problem of bluff body flows and the drag associated with them has been the subject of numerous investigations in the literature. In the two-dimensional case, the flow past a circular cylinder has been most widely studied both experimentally and computationally. As a result, a well documented understanding of the gross features of the near-wake around a circular cylinder exists in the literature. In contrast, very little is understood on the general features of three-dimensional bluff body near-wakes, except that the vortex shedding is known to be less intense.
Control or management of bluff body flows, both from the point of view of drag reduction as well as suppressing unsteady forces caused by vortex shedding, has been an area of considerable interest in engineering applications. The basic aim in the different control methods involves direct or indirect manipulation (or modification) of the near-wake structure leading to weakening or inhibition of vortex shedding. Many passive and energetic techniques (such as splitter plates, base and trailing edge modifications and base bleed) have been effective in the two-dimensional case in increasing the base pressure, leading to varying amounts of drag reduction; a large body of this work is centered around circular cylinders because of direct relevance in applications.
The present work is an attempt to understand some of the major aspects of the near-wake structure of a sphere and to control the same for drag reduction employing a passive technique. Many of the passive control techniques found useful in two-dimensional flows are not appropriate in the context of a sphere. In this thesis, the effects of natural ventilation on the wake and drag of a sphere at low speeds have been studied experimentally in some detail. Natural bleed into the base is created when the stagnation and base regions of a sphere are connected through an internal duct. Although natural ventilation has features broadly similar to the well known base-bleed technique (both involve addition of mass, momentum and energy into the near-wake), there are many significant differences between the two methods; for example, in base bleed, the mass flow injected can be controlled independent of the outer flow, whereas in natural ventilation, it is determined by an interaction between the internal and the external flow around the body.
Experiments have been conducted in both wind and water tunnels, which covered a wide range of Reynolds number (ReDj based on the diameter of the sphere) from of 1.7 x 103 to 8.5 x 105 with natural boundary layer transition. The ratio of the frontal vent area to the maximum cross sectional area of the sphere was varied from 1% to 2.25% and the effect of the internal duct geometry, including a convergent and a divergent duct was examined as well. After preliminary force measurements involving different duct geometries and vent areas, it was decided to make detailed measurements with a straight (parallel) duct with a vent area ratio of 2.25%. Extensive flow visualization studies involving dye-flow, hydrogen bubble, surface oil-flow and laser-light-sheet techniques were employed to gain insight into many aspects of the near-wake structure and the flow on the surface of the sphere. Measurements made included model static pressures, drag force using a strain gauge balance and velocity profiles in the near-wake and internal flow through the vent. In addition, wake vortex shedding frequency was measured using a hotwire.
In the subcritical range of Reynolds numbers (ReD< 2 x 105), the near-wake of the sphere (without ventilation) was found to be vortex shedding, with laminar separation occurring around a value of0s = 80° (where 0s is the angle between the stagnation point and separation location). In contrast, there was little evidence of vortex shedding in the supercritical range (ReD> 4 x 105), consistent with many earlier observations in the literature; however, flow visualization studies in the near-wake clearly showed the existence of a three-dimensional vortex-like structure exhibiting random rotations about the streamwise axis. In this range of Reynolds numbers, surface flow visualization studies indicated the existence of a laminar separation bubble which was followed by a transitional/turbulent reattachment and an ultimate separation around 0S = 145°. All the above observations are broadly consistent with the results available in the literature.
With ventilation at subcritical Reynolds numbers, the pressure distributions on the sphere including in the base region was only weakly altered, resulting in a marginal reduction in the total drag; because of the higher pressure difference between the stagnation and base regions, the mean velocity in the vent-flow was about 0.9 times the free-stream velocity. As may be expected, there was little change in the location of laminar separation on the sphere and the vortex shedding frequency was virtually unaltered due to ventilation. The relatively small effects on pressure distribution and drag suggest weak interaction between the vent-flow and the separated shear layer in the subcritical regime. The time-averaged near-wake flow revealed a stagnation point occurring between the vent-flow and the reverse flow in the near-wake, along with the formation of a torroidal vortex between the stagnation point and the near-wake closure; these features bear some resemblance to those observed with base bleed from a blunt base.
With ventilation in the supercritical range of Reynolds numbers (ReD > 4 x 105), significant reduction in the total drag, of as much as 65%, was observed from force measurements. Pressure distributions showed higher pressures in the separated flow zone (consistent with reduced drag) as a result of which the internal mass and the mean velocity of the vent-flow were lower (0.69 times the free-stream velocity) compared to the value in the subcritical flow regime. Flow visualization studies clearly showed that the three-dimensional rotating structure (associated with the wake of the unvented sphere) was significantly modified by ventilation, leading to more symmetric and steady near-wake features. The larger effects on pressure distribution and drag suggest strong interaction between the vent-flow and the separated shear layer, promoted by their close proximity. The comparison of power spectral density of u1 signals in the near-wake showed significant reduction in the amplitude at all frequencies, consistent with observations from flow visualization studies. The time-averaged near-wake flow features a pair of counterrotating ring vortices which are trapped between the outer separated shear layer and the vent-flow shear layer; such a mean flow pattern is qualitatively similar to that behind an axisymmetric base with a central jet with unequal freestream velocities in the jet and outer flow.
This study strongly suggests that natural ventilation can provide significant total drag reduction provided the vent-flow is in close proximity of the separated shear layer promoting a strong interaction between them. Drag reduction is associated with more symmetric and relatively steady near-wake features in contrast with the unvented sphere.
46
Suryanarayana, G. K. "On The Reduction Of Drag Of a Sphere By Natural Ventilation." Thesis, Indian Institute of Science, 1995. http://hdl.handle.net/2005/125.
Full textAbstract:
The problem of bluff body flows and the drag associated with them has been the subject of numerous investigations in the literature. In the two-dimensional case, the flow past a circular cylinder has been most widely studied both experimentally and computationally. As a result, a well documented understanding of the gross features of the near-wake around a circular cylinder exists in the literature. In contrast, very little is understood on the general features of three-dimensional bluff body near-wakes, except that the vortex shedding is known to be less intense.
Control or management of bluff body flows, both from the point of view of drag reduction as well as suppressing unsteady forces caused by vortex shedding, has been an area of considerable interest in engineering applications. The basic aim in the different control methods involves direct or indirect manipulation (or modification) of the near-wake structure leading to weakening or inhibition of vortex shedding. Many passive and energetic techniques (such as splitter plates, base and trailing edge modifications and base bleed) have been effective in the two-dimensional case in increasing the base pressure, leading to varying amounts of drag reduction; a large body of this work is centered around circular cylinders because of direct relevance in applications.
The present work is an attempt to understand some of the major aspects of the near-wake structure of a sphere and to control the same for drag reduction employing a passive technique. Many of the passive control techniques found useful in two-dimensional flows are not appropriate in the context of a sphere. In this thesis, the effects of natural ventilation on the wake and drag of a sphere at low speeds have been studied experimentally in some detail. Natural bleed into the base is created when the stagnation and base regions of a sphere are connected through an internal duct. Although natural ventilation has features broadly similar to the well known base-bleed technique (both involve addition of mass, momentum and energy into the near-wake), there are many significant differences between the two methods; for example, in base bleed, the mass flow injected can be controlled independent of the outer flow, whereas in natural ventilation, it is determined by an interaction between the internal and the external flow around the body.
Experiments have been conducted in both wind and water tunnels, which covered a wide range of Reynolds number (ReDj based on the diameter of the sphere) from of 1.7 x 103 to 8.5 x 105 with natural boundary layer transition. The ratio of the frontal vent area to the maximum cross sectional area of the sphere was varied from 1% to 2.25% and the effect of the internal duct geometry, including a convergent and a divergent duct was examined as well. After preliminary force measurements involving different duct geometries and vent areas, it was decided to make detailed measurements with a straight (parallel) duct with a vent area ratio of 2.25%. Extensive flow visualization studies involving dye-flow, hydrogen bubble, surface oil-flow and laser-light-sheet techniques were employed to gain insight into many aspects of the near-wake structure and the flow on the surface of the sphere. Measurements made included model static pressures, drag force using a strain gauge balance and velocity profiles in the near-wake and internal flow through the vent. In addition, wake vortex shedding frequency was measured using a hotwire.
In the subcritical range of Reynolds numbers (ReD< 2 x 105), the near-wake of the sphere (without ventilation) was found to be vortex shedding, with laminar separation occurring around a value of0s = 80° (where 0s is the angle between the stagnation point and separation location). In contrast, there was little evidence of vortex shedding in the supercritical range (ReD> 4 x 105), consistent with many earlier observations in the literature; however, flow visualization studies in the near-wake clearly showed the existence of a three-dimensional vortex-like structure exhibiting random rotations about the streamwise axis. In this range of Reynolds numbers, surface flow visualization studies indicated the existence of a laminar separation bubble which was followed by a transitional/turbulent reattachment and an ultimate separation around 0S = 145°. All the above observations are broadly consistent with the results available in the literature.
With ventilation at subcritical Reynolds numbers, the pressure distributions on the sphere including in the base region was only weakly altered, resulting in a marginal reduction in the total drag; because of the higher pressure difference between the stagnation and base regions, the mean velocity in the vent-flow was about 0.9 times the free-stream velocity. As may be expected, there was little change in the location of laminar separation on the sphere and the vortex shedding frequency was virtually unaltered due to ventilation. The relatively small effects on pressure distribution and drag suggest weak interaction between the vent-flow and the separated shear layer in the subcritical regime. The time-averaged near-wake flow revealed a stagnation point occurring between the vent-flow and the reverse flow in the near-wake, along with the formation of a torroidal vortex between the stagnation point and the near-wake closure; these features bear some resemblance to those observed with base bleed from a blunt base.
With ventilation in the supercritical range of Reynolds numbers (ReD > 4 x 105), significant reduction in the total drag, of as much as 65%, was observed from force measurements. Pressure distributions showed higher pressures in the separated flow zone (consistent with reduced drag) as a result of which the internal mass and the mean velocity of the vent-flow were lower (0.69 times the free-stream velocity) compared to the value in the subcritical flow regime. Flow visualization studies clearly showed that the three-dimensional rotating structure (associated with the wake of the unvented sphere) was significantly modified by ventilation, leading to more symmetric and steady near-wake features. The larger effects on pressure distribution and drag suggest strong interaction between the vent-flow and the separated shear layer, promoted by their close proximity. The comparison of power spectral density of u1 signals in the near-wake showed significant reduction in the amplitude at all frequencies, consistent with observations from flow visualization studies. The time-averaged near-wake flow features a pair of counterrotating ring vortices which are trapped between the outer separated shear layer and the vent-flow shear layer; such a mean flow pattern is qualitatively similar to that behind an axisymmetric base with a central jet with unequal freestream velocities in the jet and outer flow.
This study strongly suggests that natural ventilation can provide significant total drag reduction provided the vent-flow is in close proximity of the separated shear layer promoting a strong interaction between them. Drag reduction is associated with more symmetric and relatively steady near-wake features in contrast with the unvented sphere.
47
Bruce, R. J. "Trailing edge strips to reduce the drag of slender wings." Thesis, Cranfield University, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.380467.
Full text
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Gutierrez, Torres Claudia del Carmen. "Modification of turbulent structure in channel flows by microbubble injection close to the wall." Texas A&M University, 2004. http://hdl.handle.net/1969.1/2783.
Full textAbstract:
An investigation of turbulent structure modification of a boundary layer for a fully developed channel flow by microbubble injection close to the upper wall was carried out using Particle Image Velocimetry (PIV). Two-dimensional velocity components in an x-y plane at Reynolds number of 5128 based on the half height of the channel and bulk velocity were measured. Microbubbles, with an average diameter of 30 ??m were produced by electrolysis and injected in the buffer layer. Different values of the void fraction were attained and used to evaluate the effects of the presence of microbubbles and their concentration within the boundary layer.
A reduction in drag was observed due to the injection of microbubbles. Drag reduction augments as the value of the void fraction increases. Furthermore, increases in both the non-dimensional values of streamwise and normal turbulent intensities, normalized by the friction velocity were observed with the void fraction growth.
A gradual decrease in the Reynolds shear stresses was achieved as the void fraction increases. This effect is due to a ??decorrelation?? or ??decoupling?? between the streamwise and normal fluctuating velocities.
Modifications in the length and time scales due to the presence of microbubbles were detected by calculating two-point correlation coefficients in one and two dimensions and the autocorrelation coefficient at various locations within the measurement zone. Streamline length and time scales were increased. On the contrary, the normal length and time scales were decreased.
The vorticity and strain rate values decreased with the injection of microbubbles. Turbulent energy production was also decreased within the boundary layer.
Quadrant analysis was used to find out the contribution of the u?? and v?? fluctuating velocity components to the Reynolds stress. The presence of microbubbles reduces the contribution to the Reynolds stresses by Q4 events (sweeps), which are responsible for the production of skin friction. Vortical structure detection in the measurement area was pursued. The structure with and without the microbubble injection is compared.
In this study the presence of microbubbles within the boundary layer has produced several modifications in the flow structure as well as reduction in the drag.
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PEREIRA, PATRICK PAQUELET. "EXPERIMENTAL STUDY OF DRAG REDUCTION IN PIPE FLOW WITH POLYMER ADDITIVE." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2009. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=15416@1.
Full textAbstract:
O presente trabalho descreve os experimentos realizados para estudar os
efeitos da adição de polímeros sobre os perfis de velocidade de um escoamento
turbulento desenvolvido. O objetivo é estudar os mecanismos de redução de
arraste observando os efeitos causados nos perfis de velocidade e intensidade de
turbulência utilizando a velocimetria por imagem de partículas (PIV) na sua forma
estereoscópica como técnica experimental para medição do campo completo de
velocidade tri-dimensional. Para a realização dos experimentos foi especialmente
projetada e construída uma seção de testes que permitisse a realização de medidas
de queda de pressão, além da medição de velocidades utilizando a técnica óptica
SPIV. Conseguiu-se uma redução no atrito da ordem de 50% antes da degradação
do polímero. Foi possível mostrar que o efeito da redução de arraste pela adição
de polímeros depende diretamente do número de Reynolds do escoamento. Para
Reynolds baixo (< 10(3)) não foi observada redução na perda de carga. No campo
médio de velocidades foi observado um alargamento da camada de transição para
o escoamento com a adição de polímeros, gerando um deslocamento do perfil de
velocidades na região logarítmica.The present work is an experimental study of the characteristics of a drag reduced turbulent pipe flow. The main goal of the study is to measure the differences in the mean velocity and turbulence profiles between the Newtonian flow and the drag reduced flow using the stereoscopic particle image velocimetry (SPIV). It has been designed and built a loop test to perform the experiments. The loop test should allow the measurement of the pressure drop and the velocity field with the SPIV. It has been achieved a drag reduction of 50% before the polymer degradation. It was shown that drag reduction strongly depends on the Reynolds number. For low Reynolds number (< 10(3)) there was no drag reduction. In the mean velocity field it has been seen that the buffer layer is thickened, with causes an upward shift of the logarithmic profile for the drag reduced flow.
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Wagger, David Leonard 1963. "Turbulent flow enhancement by polyelectrolyte additives : mechanistic implications for drag reduction." Thesis, Massachusetts Institute of Technology, 1992. http://hdl.handle.net/1721.1/13125.
Full text