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Acreman, D. M. "Galaxy wakes." Thesis, University of Birmingham, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.403583.
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Cicatelli, Giancarlo. "Time warying wake flow characteristics behind turbine blade wakes." Doctoral thesis, Universite Libre de Bruxelles, 1998. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/212070.
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Nilsson, Karl. "Numerical computations of wind turbine wakes and wake interaction." Doctoral thesis, KTH, Stabilitet, Transition, Kontroll, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-166658.
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When wind turbines are placed in farms, wake effects reduce the overall power production. Also, turbine loads are significantly increased since turbulence levels are high within the wake flow. Therefore, when planning for a wind farm, it is imperative to have an understanding of the flow conditions in the farm in order to estimate the power losses and to optimize the durability of the turbines to be selected for the farm. The possibilities granted by numerical modeling and the development of computational capabilities give an opportunity to study these flow conditions in detail, which has been the key focus of this doctoral work. The actuator disc method is used for predicting the power production of the Lillgrund wind farm. The results of the simulations are compared to measurements from the actual wind farm, which are found to be in very good agreement. However, some uncertainties are identified in the modeling of the turbine. One of the uncertainties is that a generic rotor is used in the Lillgrund case. In order to quantify the errors resulting from this generalization three different rotor configurations are simulated in various flow conditions. Generally, it can be stated that the choice of rotor configuration is not crucial if the intention of the simulations is to compute the mean wake characteristics subject to turbulent inflow. Another uncertainty is that the turbines in the Lillgrund case were simulated without a power controller. Therefore, a power controller is implemented and used in simulations. Generally, the controller reduces the thrust of the turbines, reduces turbulence intensity and increases velocity levels in the wake flow. However, the use of a controller was observed to have a small impact on the power production. The effects of using the technique of imposing pregenerated turbulence and a prescribed boundary layer in the simulation are analyzed in order to verify its applicability in very long domains. It is observed that close to the plane of imposed turbulence, the conditions are mainly dependent on the imposed turbulence while far downstream the turbulence, regardless of its initial characteristics, is in near equilibrium with the prescribed wind shear. The actuator line method is validated using measurements of the near wake behind the MEXICO rotor. The analysis is performed by comparing position, size and circulation of the tip vortices, as well as velocity distributions in the wake flow. The simulations and measurements are generally found to be in good agreement apart from the tip vortex size, which is greatly overestimated in the simulations.QC 20150519
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Baille, Kevin. "Fine-scale structures in Saturn's rings waves, wakes and ghosts." Doctoral diss., University of Central Florida, 2011. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/4840.
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The Cassini mission provided wonderful tools to explore Saturn, its satellites and its rings system. The UVIS instrument allowed stellar occultation observations of structures in the rings with the best resolution available (around 10 meters depending on geometry and navigation), bringing our understanding of the physics of the rings to the next level. In particular, we have been able to observe, dissect, model and test the interactions between the satellites and the rings. We first looked at kilometer-wide structures generated by resonances with satellites orbiting outside the main rings. The observation of structures in the C ring and their association with a few new resonances allowed us to estimate some constraints on the physical characteristics of the rings. However, most of our observed structures could not be explained with simple resonances with external satellites and some other mechanism has to be involved. We located four density waves associated with the Mimas 4:1, the Atlas 2:1, the Mimas 6:2 and the Pandora 4:2 Inner Lindblad Resonances and one bending wave excited by the Titan -1:0 Inner Vertical Resonance. We could estimate a range of surface mass density from 0.22 ([plus or minus]0.03) to 1.42 ([plus or minus]0.21) g cm[super-2] and mass extinction coefficient from 0.13 ([plus or minus]0.03) to 0.28 ([plus or minus]0.06) cm[super2] g[super-1]. These mass extinction coefficient values are higher than those found in the A ring (0.01 - 0.02 cm[super2] g[super-1]) and in the Cassini Division (0.07 - 0.12 cm[super2] g[super-1] from Colwell et al. (2009), implying smaller particle sizes in the C ring. We can therefore imagine that the particles composing these different rings have either different origins or that their size distributions are not primordial and have evolved differently.; Using numerical simulations for the propeller formation, we estimate that our observed moonlets belong to a population of bigger particles than the one we thought was composing the rings: Zebker et al. (1985) described the ring particles population as following a power-law size distribution with cumulative index around 1.75 in the Cassini Division and 2.1 in the C ring. We believe propeller boulders follow a power-law with a cumulative index of 0.6 in the C ring and 0.8 in the Cassini Division. The question of whether these boulders are young, ephemeral and accreted inside the Roche limit or long-lived and maybe formed outisde by fragmentation of a larger body before migrating inward in the disk, remains a mystery. Accretion and fragmentation process are not yet well constrained and we can hope that Cassini extended mission will still provide a lot of information about it.; We also estimate the mass of the C ring to be between 3.7 ([plus or minus]0.9) x 10[super16] kg and 7.9 ([plus or minus]2.0) x 10[super16] kg, equivalent to a moon of 28.0 ([plus or minus]2.3) km to 36.2 ([plus or minus]3.0) km radius (a little larger than Pan or Atlas) with a density comparable to the two moons (400 kg m[super-3]). From the wave damping length and the ring viscosity, we also estimate the vertical thickness of the C ring to be between 1.9 ([plus or minus]0.4) m and 5.6 ([plus or minus]1.4) m, which is consistent with the vertical thickness of the Cassini Division (2 - 20 m) from Tiscareno et al. (2007) and Colwell et al. (2009). Conducting similar analysis in the A, B rings and in the Cassini Division, we were able to estimate consistent masses with previous works for the these rings. We then investigated possible interactions between the rings and potential embedded satellites. Looking for satellite footprints, we estimated the possibility that some observed features in the Huygens Ringlet could be wakes of an embedded moon in the Huygens gap. We discredited the idea that these structures could actually be satellite wakes by estimating the possible position of such a satellite. Finally, we observed a whole population of narrow and clear holes in the C ring and the Cassini Division. Modeling these holes as depletion zones opened by the interaction of a moonlet inside the disk material (this signature is called a "propeller"), we could estimate a distribution of the meter-sized to house-sized objects in these rings. Similar objects, though an order of magnitude larger, have been visually identified in the A ring. In the C ring, we have signatures of boulders which sizes are estimated between 1.5 and 14.5 m, whereas similar measures in the Cassini Division provide moonlet sizes between 0.36 and 58.1 m.ID: 030422748; System requirements: World Wide Web browser and PDF reader.; Mode of access: World Wide Web.; Thesis (Ph.D.)--University of Central Florida, 2011.; Includes bibliographical references (p. 269-295).
Ph.D.
Doctorate
Physics
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Planetary Science
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Nedic, Jovan. "Fractal-generated wakes." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/12632.
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This thesis aims at getting a better understanding of the properties, scalings and similarities of turbulent axisymmetric wakes, as well as possible applications that arise from the information learnt. Over the last 60 years, axisymmetric wakes have been generated using axisymmetric bodies, such as disks, spheres and bodies of revolution, and key parameters such as the drag coefficient, shedding frequency and similarity and scaling of the wake width and velocity deficit have been documented and verified by numerous experimental and numerical studies. However, in this thesis the aim is to use asymmetric wake generators to generate the axisymmetric wakes and see if this has any effect on the results. These asymmetric wake generators are made up of a square plate and a number of fractal plates, where the perimeter of the plates can be increased by as much as 16 times that of the square. As well as increasing the perimeter, the irregularity, or fractal dimension, is also increased. It is found that the drag coefficient of the fractal plates is increased to beyond the values observed for regular polygons and a theory is presented that could explain this possible change in the drag coefficient, whereby the drag coefficient is the product of the volume of the wake and the dissipation of the turbulent kinetic energy within the wake. Wake profiles were taken over a moderate downstream distance of up to 50l, where l is the characteristic length of the plates, defined as the square root of the frontal area. Using the measured integral width of the wake directly, it was found that the volume of the wake decreased with increasing fractal dimension and iteration. Using these values, the similarity and scaling of the wake was carried out and a new high local Reynolds number scaling for turbulent axisymmetric wakes was discovered and for which the data from the fractal plates fit very well. The intensity of the vortex shedding is also shown to decrease with increasing perimeter and fractal dimension and it is found that the rate at which these vortices are shed is the same for all plates if the characteristic length is used to normalise the frequency. It is also discussed how the decrease in the energy of the vortex shedding is linked to the volume of the wake. Finally, the use of fractal geometries to manipulate the wake to reduce noise is also investigated, with emphasis placed on various aspects of an aircrafts wing.
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Rigas, Georgios. "Modelling of turbulent wakes." Thesis, Imperial College London, 2014. http://hdl.handle.net/10044/1/26590.
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The dynamics of the turbulent three-dimensional wake generated by an axisymmetric bluff body with blunt trailing edge are experimentally and theoretically investigated at a diameter based Reynolds number of 188,000. A detailed analysis of the base pressure measurements shows that the large scale structures of the turbulent three-dimensional wake retain the structure of the laminar instabilities observed in the transitional regimes, in a statistical sense. These persisting instabilities at the turbulent regime, are associated with spatial and temporal symmetry breaking, giving rise to spatial reflectional symmetry and quasi-periodic vortex shedding. The influence of turbulence recovers the lost temporal and spatial symmetries in the long-time average. It is shown that the turbulent spatial dynamics are reproduced by a simple stochastic model the deterministic part of which accounts for the spatial symmetry breaking and gives rise to steady large scale structures through a supercritical pitchfork bifurcation, and the stochastic part modelling in a phenomenological sense the turbulent fluctuations acting on the large scale structures. The axisymmetric body wake is further investigated when axisymmetric slot-jet zero-net-mass-flux forcing is applied on the rear base. Landau-like models that capture the weakly nonlinear interaction between the global vortex shedding mode and axisymmetric forcing are derived from the phase-averaged Navier-Stokes equations. The Landau-like models describe accurately the forced response by means of measured base pressure of the global vortex shedding mode. With the present analysis it is demonstrated that the concept of weakly nonlinear global modes can be extended to a fully turbulent flow, far from the critical bifurcation Reynolds number, and a general framework for the description of systems with broken symmetries---giving rise to global dynamics---and turbulent dynamics is provided. The novel results presented here advance the understanding of the dynamics of three-dimensional turbulent wakes and pave the way for turbulence prediction and control.
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Smith, David Andrew Robert. "Pertubation of vortex wakes for amelioration of the vortex wake hazard." Thesis, Imperial College London, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.406670.
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Onur, Cagla. "Acoustic Tracking Of Ship Wakes." Phd thesis, METU, 2013. http://etd.lib.metu.edu.tr/upload/12615656/index.pdf.
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Theories about ship wake structure, bubble dynamics, acoustic propagation through bubble clouds, backscattering and target strength of bubble clouds have been investigated and related Matlab simulations have been carried on. Research has been carried on algorithms for ship wake acoustic detection and tracking. Particle filter method has been simulated with Matlab for target tracking using wake echo measurements. Simulation results are promising.
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Rind, Elad. "Turbulent wakes in turbulent streams." Thesis, University of Southampton, 2011. https://eprints.soton.ac.uk/193955/.
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Direct numerical simulation and wind tunnel experiments have been used to study the effects of free-stream turbulence on axisymmetric wakes. In both cases the wake was introduced to various turbulent streams having various levels of turbulence intensity and length scales. It was found that the presence of the free-stream turbulence changes the wake’s decay rate and does not allow self-similarity to occur (unless maybe very far downstream and way beyond the current measurements reached). Also, the free-stream turbulence was found to be causing a significant transformation in the turbulence structure inside the wake, where the latter was found to be gradually evolving towards the former. Last, the fact that the two approaches were modelling two different problems led to some differences in their results emphasising the importance of the flow structure around the wake generating body in shaping the far wake region.
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Parslew, Ben. "Simulating avian wingbeats and wakes." Thesis, University of Manchester, 2012. https://www.research.manchester.ac.uk/portal/en/theses/simulating-avian-wingbeats-and-wakes(038035c2-fe9e-4104-92dc-544579c7011b).html.
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Analytical models of avian flight have previously been used to predict mechanical and metabolic power consumption during cruise. These models are limited, in that they neglect details of wing kinematics, and model power by assuming a fixed or rotary wing (actuator disk) weight support mechanism. Theoretical methods that incorporate wing kinematics potentially offer more accurate predictions of power consumption by calculating instantaneous aerodynamic loads on the wing. However, the success of these models inherently depends on the availability and accuracy of experimental kinematic data. The predictive simulation approach offers an alternative strategy, whereby kinematics are neither neglected nor measured experimentally, but calculated as part of the solution procedure. This thesis describes the development of a predictive tool for simulating avian wingbeat kinematics and wakes. The tool is designed in a modular format, in order to be extensible for future research in the biomechanics community. The primary simulation module is an inverse dynamic avian wing model that predicts aerodynamic forces and mechanical power consumption for given wing kinematics. The model is constructed from previous experimental studies of avian wing biomechanics. Wing motion is defined through joint kinematic time histories, and aerodynamic forces are predicted using blade element momentum theory. Mechanical power consumption at the shoulder joint is derived from both aerodynamic and inertial torque components associated with the shoulder joint rotation rate. An optimisation module is developed to determine wing kinematics that generate aerodynamic loads for propulsion and weight support in given flight conditions, while minimising mechanical power consumption. For minimum power cruise, optimisation reveals numerous local minima solutions that exhibit large variations in wing kinematics. Validation of the model against wind tunnel data shows that optimised solutions capture qualitative trends in wing kinematics with varying cruise speed. Sensitivity analyses show that the model outputs are most affected by the defined maximum lift coefficient and wing length, whereby perturbations in these parameters lead to significant changes in the predicted amount of upstroke wing retraction. Optimised solutions for allometrically scaled bird models show only small differences in predicted advance ratio, which is consistent with field study observations. Accelerating and climbing flight solutions also show similar qualitative trends in wing kinematics to experimental measurements, including a reduction in stroke plane inclination for increasing acceleration or climb angle. The model predicts that both climb angle and climb speed should be greater for birds with more available instantaneous mechanical power. Simulations of the wake using a discrete vortex model capture fundamental features of the wake geometry that have been observed experimentally. Reconstruction of the velocity field shows that this method overpredicts induced velocity in retracting-wing wakes, and should therefore only be applied to extended-wing phases of an avian wingbeat.
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Chan, Tak-yee Andy. "The interaction of laminar far wake with a free surface /." Hong Kong : University of Hong Kong, 1997. http://sunzi.lib.hku.hk/hkuto/record.jsp?B1856544X.
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Medici, Davide. "Wind turbine wakes : controland vortex shedding." Licentiate thesis, KTH, Mechanics, 2004. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-361.
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Wind tunnel studies of the wake behind a model wind turbine have been made in order to get a better understanding of wake development as well as the possibility to predict the power output from downstream turbines working in the wake of an upstream one. Both two-component hot-wire anemometry as well as particle image velocimetry (PIV) have been used to map the flow field. All three velocity components were measured both for the turbine rotor normal to the oncoming flow as well as with the turbine inclined to the free stream direction (the yaw angle was varied from 0 to 30 degrees). The measurements showed, as expected, a wake rotation in the opposite direction to that of the turbine. A yawed turbine is found to clearly deflect the wake flow to the side showing the potential of controlling the wake position by yawing the turbine. The power output of a yawed turbine was found to vary nearly as the square of the cosine of the yaw angle. The possibility to use active wake control by yawing an upstream turbine was evaluated and was shown to have a potential to increase the power output significantly for certain configurations. An unexpected feature of the flow was that spectra from the time signals showed the appearance of a low frequency fluctuation both in the wake and in the flow outside. This fluctuation was found both with and without free stream turbulence and also with a yawed turbine. The non-dimensional frequency (Strouhal number) was independent of the free-stream velocity and turbulence level but increases with the yaw angle. However the low frequency fluctuations were only observed when the tip speed ratio (or equivalently the drag coefficient) was high. This is in agreement with the idea that the turbine shed structures as a bluff body. It is hypothesized that the observed meandering of wakes in field measurements is due to this shedding.
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Williams, Patrick Richard Giles. "Transition to turbulence in cylinder wakes." Thesis, University of Warwick, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.487916.
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The laminar to turbulent transition of a circular cylinder wake was investigated experimentally, examining a range of features leading to an aperiodic three-dimensional structure and a possible mechanism causing the route to turbulence was postulated. Following the analysis of the Stuart-Landau equations defining a Hopf bifurcation at the primary instability, a new relationship between the Strouhal number (St) and the Reynolds number (Re) was formed, providing agreement with a range of equations defined experimentally. The St-Re relationship was examined for a range of mechanisms modifying the cylinder's end termination and a variation in the free-stream turbulence level. It was ascertained that the end termination had a direct influence on the wake dynamics, producing a change in the wake structure leading to the possibility of parallel vortex shedding and also a modification of the critical Reynolds number. Following the introduction of specific boundary conditions, it was ascertained that the critical Reynolds number could be delayed to a value of 192. Although an increased level of free-stream turbulence demonstrated a reduction in the critical Reynolds number for all end conditions imposed. Within a small range of Reynolds numbers prior to transition, random wake irregularities \vere identified within the laminar velocity signal. These were ascertained to be generated from finite three-dimensional perturbations within the wake, introduced by the method of end termination and the increased level of free-stream turbulence. Following the analysis of these intermittent aperiodic bursts within the transition regime, they were demonstrated to relate to the occurrence of turbulent flow prior to transition. This produced an alternate switching between the two states relating to the hysteretic bifurcation to the mode A wake structure. A possible transition mechanism causing a route to turbulence was considered, relating to the statistics of these intermittent bursts. Leading to a proposal that the cylinder wake transition undergoes a noise induced dynamic forcing, which is consistent with on-off intermittency.
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Potylitsin, Pavel G. "Columnar vortex stability in island wakes." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2000. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp02/NQ53839.pdf.
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Ivanell, Stefan S. A. "Numerical computations of wind turbine wakes." Licentiate thesis, Stockholm : Royal Institute of Technology, 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-316.
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Drake, Nick. "Wakes and dynamics of galaxy clusters." Thesis, University of Southampton, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.326792.
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Agoropoulos, D. "Interactions between wakes and boundary layers." Thesis, University of Cambridge, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.372874.
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Bereketab, Semere. "Complex Equilibrium of Laterally Curved Wakes." Thesis, Virginia Tech, 1999. http://hdl.handle.net/10919/35806.
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Turbulent wakes generated from an aircraft or submarine vehicles has been of main interest to researchers due to the broad band noise associated with such wakes. One such case is the noise generated by spiral vortices shed of from one blade interacting with another oncoming blade of helicopter rotor. Consequently, researchers have been trying to understand the basic physics and evolution of such wakes. Although there has been numerous studies done on plane wakes, there has been little research being done on laterally curved wakes.
Single and two-point velocity measurements were taken on a plane and laterally curved turbulent wakes to understand the evolution and effect of lateral curvature into the far wake region. The analyses provide useful information in modeling curved or spiral wakes such as turbulence field surrounding tip vortices shed from a wing. In order to achieve our objectives, the Virginia Tech 3’ x 2’ subsonic wind tunnel was used to take velocity measurements of toroidal ring model and a straight cylinder as a control case. Velocity measurements were done using four sensor hot-wire anemometers, to obtain all mean velocity, Reynolds stress, triple product components of the turbulence field. Single point, spectra and two-point measurements of the wakes were performed throughout the development into the far wake region. The single point results reveal the universality of the mean axial velocity, however the Reynolds stresses and triple products were not universal illustrating that the turbulence field has its own length and velocity scales different from that of the mean flow. The effect of lateral curvature is mainly evidenced in the early development of the curved ring wake. The turbulent energy budget reveals similar trend for both wakes and plane wake achieves approximate equilibrium. The spectra result reveals for the plane wake that self-preservation is achieved for all scales of motion, while the ring wake does not achieve such a state. While the longitudinal correlations of both wakes are similar in form, in general difference in form and orientation prevailed over all indicating the difference in the turbulent structure of both wakes. Linear stochastic estimation reveals the presence of spanwise and double-roller eddy structures in the plane wake and only spanwise eddies were detected for the ring wake.Master of Science
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Ivanell, Stefan S. A. "Numerical computations of wind turbine wakes." Doctoral thesis, Stockholm : Royal Institute of Technology, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-9916.
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Shi, Shaoping. "Large-eddy simulation of ship wakes." Morgantown, W. Va. : [West Virginia University Libraries], 2001. http://etd.wvu.edu/templates/showETD.cfm?recnum=2217.
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Thesis (Ph. D.)--West Virginia University, 2001.Title from document title page. Document formatted into pages; contains xv, 211 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 200-211).
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Farr, Thomas D. "The effects of atmospheric and wake turbulence on wind turbines and wind turbine wakes." Thesis, University of Surrey, 2015. http://epubs.surrey.ac.uk/807177/.
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Wind tunnel studies using model wind turbines have been used to investigate the effects and characteristics of neutral and unstable atmospheric boundary layers on their operation and wake behaviour. Wind turbine arrays have also been arranged to observe the effect of wake interaction. Single-point two-component and two-point single-component velocity measurements have been made using laser Doppler anemometry in conjunction with cold-wire anemometry to interrogate the modelled boundary layer. The manufacture and installation of a second traverse mechanism in the wind tunnel was necessary to perform the two-point measurements, along with the development of laboratory software for control and data analysis. In order to allow for measurements of turbine performance, a current sensor was developed so that correlations could be made between velocity and torque fluctuations. Investigation of larger arrays, up to 12 turbines, required the production of additional turbines and installation and subsequent integration of the associated control systems. Measurements made in the neutral flow conditions show that there is an increasing correlation between the upstream turbulence and torque fluctuations with proximity to the turbine, especially in the wake of another turbine where the flow is rapidly evolving. Two-point velocity measurements, with a lateral separation, have shown that there is little effect of the turbine on the correlation of the flow over the rotor disc. Analysis of data from this type of measurement also shows that in an array of four aligned turbines, the spatial structures reach an equilibrium state and are of larger size after the second turbine. Furthermore, the velocity-torque correlation magnitude decreases after the first turbine, but then increases with distance through the array owing to the increased correlation over the rotor disc, although not to the level observed for the first turbine. The turbulence approaching the first turbine behaves in a frozen-flow manner, but this is not true for the second and subsequent turbines, although the idea of convection time still applies. Measurements made in the modelled unstable atmospheric boundary layer show that the length and time scales are changed in the flow, in addition to the alteration of the profiles of mean velocity and Reynolds stresses. The increased turbulence caused by the convective boundary layer increases the rate of wake deficit recovery and does not result in the same spatial structures as the neutral conditions. Temperature effects are of secondary importance with regard to wake and turbine behaviour, with the main driving force behind the performance being the increased turbulence levels.
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Marquardt, Matthew William Stern Frederick Longo Joseph. "Effects of waves and the free surface on a surface-piercing flat-plate turbulent boundary layer and wake." [Iowa City, Iowa] : University of Iowa, 2009. http://ir.uiowa.edu/etd/404.
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Chan, Fung Chi. "Investigation of water wakes in shallow environment /." View abstract or full-text, 2005. http://library.ust.hk/cgi/db/thesis.pl?CIVL%202005%20CHANFC.
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Fransson, Jens H. M. "Flow control of boundary lagers and wakes." Doctoral thesis, KTH, Mekanik, 2003. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3664.
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Both experimental and theoretical studies have beenconsidered on flat plate boundary layers as well as on wakesbehind porous cylinders. The main thread in this work iscontrol, which is applied passively and actively on boundarylayers in order to inhibit or postpone transition toturbulence; and actively through the cylinder surface in orderto effect the wakecharacteristics. An experimental set-up for the generation of the asymptoticsuction boundary layer (ASBL) has been constructed. This studyis the first, ever, that report a boundary layer flow ofconstant boundary layer thickness over a distance of 2 metres.Experimental measurements in the evolution region, from theBlasius boundary layer (BBL) to the ASBL, as well as in theASBL are in excellent agreement with boundary layer analysis.The stability of the ASBL has experimentally been tested, bothto Tollmien-Schlichting waves as well as to free streamturbulence (FST), for relatively low Reynolds numbers (Re). For the former disturbances good agreement is foundfor the streamwise amplitude profiles and the phase velocitywhen compared with linear spatial stability theory. However,the energy decay factor predicted by theory is slightlyoverestimated compared to the experimental findings. The latterdisturbances are known to engender streamwise elongated regionsof high and low speeds of fluid, denoted streaks, in a BBL.This type of spanwise structures have been shown to appear inthe ASBL as well, with the same spanwise wavelength as in theBBL, despite the fact that the boundary layer thickness issubstantially reduced in the ASBL case. The spanwise wavenumberof the optimal perturbation in the ASBL has been calculated andis β = 0.53, when normalized with the displacementthickness. The spanwise scale of the streaks decreases withincreasing turbulence intensity (Tu) and approaches the scale given by optimalperturbation theory. This has been shown for the BBL case aswell. The initial energy growth of FST induced disturbances hasexperimentally been found to grow linearly as Tu2Rexin the BBL, the transitional Reynolds numberto vary as Tu-2, and the intermittency function to have a relativelywell-defined distribution, valid for all Tu. The wake behind a porous cylinder subject to continuoussuction or blowing has been studied, where amongst other thingsthe Strouhal number (St) has been shown to increase strongly with suction,namely, up to 50% for a suction rate of 2.5% of the free streamvelocity. In contrast, blowing shows a decrease ofStof around 25% for a blowing rate of 5% of the freestream velocity in the considered Reynolds number range. Keywords:Laminar-turbulent transition, asymptoticsuction boundary layer, free stream turbulence,Tollmien-Schlichting wave, stability, flow control, cylinderwake.QC 20100607
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Rødningsby, Anders. "Multitarget Multisensor Trackingin the Presence of Wakes." Doctoral thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for teknisk kybernetikk, 2010. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-11913.
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TARGET tracking is an essential requirement for surveillance and control systems to interpret the environment. This environment may contain multiple targets, and the environmental information may be obtained by multiple sensors in a multitarget multisensor tracking system. In this thesis we focus on targets which, in addition to reflecting signals themselves, also have a trailing path behind them, called a wake. This wake causes additional measurements to those originating from the target. When the measurements are processed, the estimated track can be misled and sometimes lose the real target because of the wake. This problem becomes even more severe in multitarget environments where targets are operating close to each other in the presence of wakes. In this thesis a probabilistic model is developed which reflects the probability that a false measurement originates from the wake behind a target. This wake model is integrated in the probabilistic data association filter (PDAF) to improve the track continuity for tracking single targets. The modified PDAF is further extended to handle multiple targets in the presence of wakes by using a probabilistic wake model for each of the targets in the multitarget environment that has a wake behind it. These single wake models are combined to form a joint wake model which augments the joint probabilistic data association filter (JPDAF) for both coupled and decoupled filtering. The wake-originated measurements may also cause confusion in the track initiation. To prevent this problem, a clustering method is proposed based on morphological operators which allows tracks to be initialized based on two-point differencing of the cluster centroids from succeeding scans. The modified PDAF is tested on data of a real scuba diver with an open breathing system. In this case the air bubbles produced by the diver form a wake which extends far behind the diver. The experiment showed that the above modifications of the PDAF improved the track continuity significantly. Finally, a relatively extensive simulation, based on real scuba diver data, is presented. Four different multitarget multisensor tracking scenarios are simulated, considering two targets with wakes that are: 1. Crossing each other. 2. Moving in parallel to each other. 3. One following after another. 4. Meeting and then passing each other. The results of these simulation scenarios show that the presented modifications improve the tracking performance, and the probability of lost tracks is significantly reduced. The targets are observed by two sensors, and it is shown that tracks estimated in a centralized fusion configuration are better than the local tracks estimated using data from individual sensors only. It is also shown that applying the wake model to targets that do not generate a wake, yields almost no deterioration of the tracking performance.
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Blomhoff, Hedda Paulsen. "An experimental investigation of wind turbine wakes." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for energi- og prosessteknikk, 2012. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-18412.
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In the present study the wake behind a scaled; Horizontal Axis Wind Turbine (HAWT) has been investigated. The experiments were performed at the Department of Energy and Process Engineering, at the Norwegian University of Science and Technology, NTNU. The turbine was installed in the wind tunnel at the department and measurements were performed at several distances behind the turbine to examine the development of the flow. A five-hole pitot probe was applied as measurement instrument. The instrument made it possible to calculate both size and direction of the velocity components.Through the experiments, characteristic curves of the turbine and grid measurements over the cross-section of the wind tunnel, were obtained. The power and thrust coefficients were measured against the local velocity ratio at the tip of the blade, the 'Tip Speed Ratio' (TSR). The power coefficient had a peak at TSR=5,5. The maximum value at this point was 0,45. The highest measured thrust coefficient was 1,15, achieved at TSR=10,3. During the experiments the turbine operated at optimal conditions, at the highest obtained power coefficient.Measurements behind the turbine found that the axial velocity distribution developed as expected. A significant velocity deficit was measured in the wake behind the turbine, which gradually decreased with increased distance to the turbine. Due to the presence of the hub and tower, the middle of the wake was characterized by disturbances. Moving down the wake the profile got more symmetric. The tangential profile was almost symmetric about the origin, right behind the turbine, but drifted to the left at increased distance downstream. Contribution from the tangential components were gradually reduced further down the wake.The tower shadow moved with the rotation of the wake, in clockwise direction, as a region of lower velocities than the prevailing wake. Further downstream the tower shadow merged with the surrounding wake.The rotational axis relocated in the wake behind the turbine. Downstream, measurements showed that the center of rotation moved to the left of the origin. To investigate if the tower was responsible for the experienced downshift, an additional tower was mounted to the tunnel roof, above the turbine. The additional tower created symmetry about the hub and gave a symmetric development of the flow field. Thus, it was concluded that the tower was responsible for the relocation of the rotational axis.
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Emerson, Benjamin L. "Dynamical characteristics of reacting bluff body wakes." Diss., Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/49073.
Full textAbstract:
Combustion instability plagues the combustion community in a wide range of applications. This un-solved problem is especially prevalent and expensive in aerospace propulsion and ground power generation. The challenges associated with understanding and predicting combustion instability lie in the flame response to the acoustic field. One of the more complicated flame response mechanisms is the velocity coupled flame response, where the flame responds dynamically to the acoustic velocity as well as the vortically induced velocity field excited by the acoustics. This vortically induced, or hydrodynamic, velocity field holds critical importance to the flame response but is computationally expensive to predict, often requiring high fidelity CFD computations. Furthermore, its behavior can be a strong function of the numerous flow parameters that change over the operability map of a combustor.
This research focuses on a nominally two dimensional bluff body combustor, which has rich hydrodynamic stability behavior with a manageable number of stability parameters. The work focuses first on experimentally characterizing the dynamical flow and flame behavior. Next, the research shifts focus toward hydrodynamic stability theory, using it to explain the physical phenomena observed in the experimental work. Additionally, the hydrodynamic stability work shows how the use of simple, model analysis can identify the important stability parameters and elucidate their governing physical roles. Finally, the research explores the forced response of the flow and flame while systematically varying the underlying hydrodynamic stability characteristics. In the case of longitudinal combustion instability of highly preheated bluff body combustors, it shows that conditions where an acoustic mode frequency equals the hydrodynamic global mode frequency are not especially dangerous from a combustion instability standpoint, and may actually have a reduced heat release response. This demonstrates the very non-intuitive role that the natural hydrodynamic flow stability plays in the forced heat release response of the flame.
For the fluid mechanics community, this work contributes to the detailed understanding of both unforced and forced bluff body combustor dynamics, and shows how each is influenced by the underlying hydrodynamics. In particular, it emphasizes the role of the density-shear layer offset, and shows how its extreme sensitivity leads to complicated flow dynamics. For the flow-combustor community as a whole, the work reviews a pre-existing method to obtain the important flow stability parameters, and demonstrates a novel way to link those parameters to the governing flow physics. For the combustion instability community, this thesis emphasizes the importance of the hydrodynamic stability characteristics of the flow, and concludes by offering a paradigm for consideration of the hydrodynamics in a combustion instability problem.
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Moroney, Gerard. "Internal wave wakes in stratified shear flows." Thesis, University of Liverpool, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.399177.
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Eriksson, Ola. "Numerical Computations of Wakes Behind Wind Farms." Licentiate thesis, Uppsala universitet, Luft-, vatten och landskapslära, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-255859.
Full textAbstract:
More and larger wind farms are planned offshore. As the most suitable build sites are limited wind farms will be constructed near to each other in so called wind farm clusters. Behind the wind turbines in these farms there is a disrupted flow of air called a wake that is characterized by reduced wind speed and increased turbulence. These individual turbine wakes combine to form a farm wake that can travel a long distance. In wind farm clusters farm to farm interaction will occur, i.e. the long distance wake from one wind farm will impact the wind conditions for other farms in the surrounding area. The thesis contains numerical studies of these long distance wakes. In this study Large Eddy Simulations (LES) using an Actuator Disc method (ACD) are used. A prescribed boundary layer is used where the wind shear is introduced using body forces. The turbulence, based on the Mann model, is introduced as fluctuating body forces upstream of the farm. A neutral atmosphere is assumed. The applied method has earlier been used for studies of wake effects inside farms but not for the longer distances needed for farm to farm interaction. Numerical studies are performed to get better knowledge about the use of this model for long distance wakes. The first study compares the simulation results with measurements behind an existing farm. Three parameter studies are thereafter setup to analyze how to best use the model. The first parameter study examines numerical and physical parameters in the model. The second one looks at the extension of the domain and turbulence as well as the characteristics of the flow far downstream. The third one gathers information on the downstream development of turbulence with different combinations of wind shear and turbulence level. The impact of placing the turbines at different distances from the turbulence plane is also studied. Finally a second study of an existing wind farm is performed and compared with a mesoscale model. The model is shown to be relevant also for studies of long distance wakes. Combining LES with a mesoscale model can be of interest.
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Karlsson, Christian. "Regression Models of 3D Wakes for Propellers." Thesis, Karlstads universitet, Institutionen för ingenjörsvetenskap och fysik (from 2013), 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:kau:diva-67785.
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In this work, regression models for the wake field entering a propeller at certain axial andnominal position have been proposed. Wakes are non-uniform flows following a body immersedin a viscous fluid. We have proposed models for the axial and tangential velocity distribution asfunctions of ship hull and propeller measures. The regression models were modelled using Fourierseries and parameter estimations based on skewed-Gaussian and sine functions. The wake fieldis an important parameter in propeller design. The regression models are based on experimentaldata provided by the Rolls-Royce Hydrodynamic Research Center in Kristinehamn. Also we havestudied the flow in the axial velocity distribution in the propeller plane using the coherent structurecoloring method. The coherent structure coloring is used to study coherent patterns by looking atfluid particle kinematics. Using this type of analysis, we observed that the velocity distributionbehaves kinematically similar in the different regions of the wake distribution, which according tothe coherent structure coloring indicate coherence.I det här arbetet, har regressionsmodeller för medströmsfältet in i en propeller vid viss axielloch nominell position utvecklats. Medströmsfältet är ojämn strömning efter en kropp nedsänkt i enviskös vätska. Vi har föreslagit modeller för axiell och tangentiell hastighetsfördelning som funktionerför fartygsskrov-och propeller-parametrar. Regressionsmodellerna modellerades med hjälpav Fourier-serier och parameterskattning baserade på skeva Gaussfördelningar och sinusfunktioner.Medströmsfältet är en viktig parameter i propeller design. Regressionsmodellerna är baserade påexperimentella data från Rolls-Royces Hydrodynamiska Forskningscenter i Kristinehamn. Vi harockså studerat flödet i axialhastighetsfördelningen i propellplanet med hjälp av den koherenta struktureringsfärgmetoden.Den koherenta struktureringsfärgmetoden används för att studera koherentamönster genom att titta på vätskepartikelkinematik. Med hjälp av denna typ av analys observeradevi att hastighetsfördelningen uppför sig kinematiskt lika i de olika regionerna i medströmsfältet,vilket enligt koherenta strukturfärgmetoden indikerar koherens.
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Costis, Christopher E. "Separation and wakes over three-dimensional bodies." Diss., Virginia Polytechnic Institute and State University, 1985. http://hdl.handle.net/10919/54745.
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The laminar flow over a prolate spheroid was investigated via flow visualization and Laser-Doppler Velocimetry. Experiments were conducted in a water tunnel and the flow was visualized with dyes. The measurement of three-dimensional boundary layers required a special design of the laser optics. Attention was focused in the neighborhood of three-dimensional separation.
The Vortex-Lattice method was employed to calculate the inviscid flow and the development of separated vortex sheets over a prolate spheroid. An approximate-method based on the assumption of local similarity was used to solve the boundary layer equations and calculate the line of open separation. A condition of vortex shedding along separation is proposed. The two schemes, viscous and inviscid, interact through the line of separation which is allowed to displace as the wake grows. Results are compared with flow visualization data for laminar separation and pressure data for turbulent separation.Ph. D.
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Somero, John Ryan. "Structure and Persistence of Surface Ship Wakes." Thesis, Virginia Tech, 2021. http://hdl.handle.net/10919/101989.
Full textAbstract:
It has long been known that ship wakes are observable by synthetic aperture radar. However,
incomplete physical understanding has prevented the development of simulation tools
that can predict both the structure and persistence of wakes in the ocean environment.
It is the focus of this work to develop an end-to-end multi-scale modeling-and-simulation
methodology that captures the known physics between the source of disturbance and the
sensor. This includes turbulent hydrodynamics, free-surface effects, environmental forcing
through Langmuir-type circulations, generation of surface currents and redistribution of
surface-active substances, surface-roughness modification, and simulation of the signature
generated by reflection and scattering of electromagnetic waves from the ocean surface. The
end-to-end methodology is based upon several customized computational fluid dynamics
solvers and empirical models which are linked together. The unsteady Reynolds-averaged
Navier-Stokes equations, including models for the Craik-Leibovich vortex force and near surface
Reynolds-stress anisotropy, are solved at full-scale Reynolds and Froude numbers on
domains that extend tens of kilometers behind the ship. A parametric study is undertaken to
explore the effects of ship heading, ship propulsion, ocean-wave amplitude and wavelength,
and the relative importance of Langmuir-type circulations vs. near-surface Reynolds-stress
anisotropy on the generation of surface currents that are transverse to the wake centerline.
Due to the vortex force, the structure of the persistent wake is shown to be a function of
the relative angle between the ambient long-wavelength swell and the ship heading. Ships
operating in head seas observe 1-3 streaks, while ships operating in following seas observe
2 symmetric streaks. Ships operating in calm seas generate similar wakes to those in following
seas, but with reduced wake width and persistence. In addition to the structure of
the persistent wake, the far wake is shown to be dominated by ship-induced turbulence and
surface-current gradients generating a wide center wake. The redistribution of surface-active
substances by surface currents is simulated using a scalar-transport model on the ocean surface.
Simulation of surface-roughness modification is accomplished by solving a wave-action balance
model which accounts for the relative change in the ambient wave-spectrum by the
surface currents and the damping-effects of surface-active substances and turbulence. Simulated
returns from synthetic aperture radar are generated with two methods implemented.
The first method generates a perfect SAR image where the instrument and platform based
errors are neglected, but the impact of a randomized ocean field on the radar cross section
is considered. The second method simulates the full SAR process including signal detection
and processing. Comparisons are made to full-scale field experiments with good agreement
between the structure of the persistent wake and observed SAR imagery.1
It has long been known that ship wakes are observable by synthetic aperture radar. However, incomplete physical understanding has prevented the development of simulation tools that can predict both the structure and persistence of wakes in the ocean environment, which is critical to understanding both the design and operation of maritime remote sensors as well as providing tactically relevant operational guidance and awareness of the maritime domain. It is the focus of this work to develop an end-to-end multi-scale modeling-and simulation methodology that captures the known physics between the source of disturbance and the sensor. This includes turbulent hydrodynamics, free-surface effects, environmental forcing, generation of surface currents and redistribution of surface-active substances, surface-roughness modification, and simulation of the signature from the ocean surface. The end-to-end methodology is based upon several customized computational fluid dynamics solvers and empirical models. The unsteady Reynolds-averaged Navier-Stokes equations, including models to account for environmental effects and near-surface turbulence, are solved at full-scale on domains that extend tens of kilometers behind the ship. A parametric study is undertaken to explore the effects of ship heading, ship propulsion, ocean-wave amplitude and wavelength, and the relative importance of environmental forcing vs. near-surface turbulence on the generation of surface currents that are transverse to the wake centerline. Due to the environmental forcing, the structure of the persistent wake is shown to be a function of the relative angle between the ambient long-wavelength swell and the ship heading. Ships operating in head seas observe 1-3 streaks, while ships operating in following seas observe 2 symmetric streaks. Ships operating in calm seas generate similar wakes to those in following seas, but with reduced wake width and persistence. In addition to the structure of the persistent wake, the far wake is shown to be dominated by ship-induced turbulence and surface-current gradients generating a wide center wake. The redistribution of surface films by surface currents is simulated using a scalar-transport model on the ocean surface. Simulation of surface-roughness modification is accomplished by solving a wave-action-balance model which accounts for the relative change in the ambient surface profile by the surface currents and the damping-effects of surface-active substances and turbulence. Simulated returns from synthetic aperture radar are generated with two methods implemented. The first method generates a perfect SAR image where the instrument and platform based errors are neglected, but the impact of a randomized ocean field on the radar cross section is considered. The second method simulates the full SAR process including signal detection and processing. Comparisons are made to full-scale field experiments with good agreement between the structure of the persistent wake and observed SAR imagery.
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Follin, Gordon J. "The turbulence structure of trailing vortex wakes." Thesis, This resource online, 1996. http://scholar.lib.vt.edu/theses/available/etd-09042008-063119/.
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Harris, Paul Jeffrey 1970. "Numerical investigation of transitional compressible plane wakes." Diss., The University of Arizona, 1997. http://hdl.handle.net/10150/282531.
Full textAbstract:
Air flow in the wake region of a two-dimensional (plane) body with a blunt base has been studied using numerical simulations. The objective of this study is (1) to observe the behavior of large dynamic structures in the plane wake at several Mach numbers from low (almost incompressible) up to M = 2.46 and examine their effect on the base pressure, and (2) to address the nature of the instability in the shear layers bounding the wake flow at M = 2.46 and observe the structures that arise from this instability. A code was developed for this study which solves the compressible Navier-Stokes equations in two or three dimensions. This code may be used for either Direct Numerical Simulations (DNS) or Large Eddy Simulations (LES). A spatial model is used, with the computational domain arranged around the trailing edge of a two-dimensional flat plate with a blunt base. Two-dimensional simulations were carried out at Mach numbers of M = 0.25, M = 1.20, and M = 2.46. At all Mach numbers, the flow was found to be unstable with respect to sinuous (antisymmetric) disturbances, with the critical Reynolds number increasing with increasing Mach number. These disturbances grow to a periodic state, and a Karman vortex street is formed. Examination of the supersonic cases revealed that expansion fans in the flow at the corners are the primary cause of the low base pressure, and that disruptions in the expansions raise the base pressure. At M = 2.46 and Reynolds numbers starting at Re = 100, 000, an intermittent shear layer instability was also found, excited by sinuous disturbances. The two instability 2 modes interact to produce a chaotic behavior. Above Re = 200, 000, the shear layer instability appears close to the base without sinuous disturbances, forming rows of vortices in the shear layers. Preliminary three-dimensional simulations were carried out at M = 2.46, examining the variation in the growth rate of three-dimensional disturbances with spanwise wavelength.
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Sofia, Alessandro. "The effect of wakes on leading edge loss /." Zürich : ETH, Swiss Federal Institute of Technology, IET, Institute of Energy Technology, LSM, Turbomachinery Laboratory, 2006. http://e-collection.ethbib.ethz.ch/show?type=dipl&nr=284.
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Seim, Bjarte Grytli. "Study of Hydrofoil Wakes Using PIV and CFD." Thesis, Norwegian University of Science and Technology, Department of Energy and Process Engineering, 2009. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-9929.
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In this master thesis the wake of a hydrofoil have been investigated using PIV. The main goal of this work have been to investigate how vortex generators can create mixing and smoothing of the velocity deficit in hydrofoil wakes. This study is motivated by the rotor stator interactions in Francis turbines with the idea that smoother wakes from the stator can reduce the forces on the rotor and hence increase the life span of Francis turbines. A literature survey of foil theory and wake flows have been carried out. This survey motivated the use of a normalization of the velocity in the wake. Experimental work was carried out at the water tunnel facility at Saint Anthony Falls Laboratory at the University of Minnesota. Tests were performed on a NACA0015 hydrofoil with four different vortex generator configurations, for a range of different angles of attack and velocities. Lift and drag forces on the hydrofoil was measured using a force balance. Because the drag measurement had poor accuracy, it could not be used to compare the different vortex generator configurations in terms of drag. As a result the drag was investigated using the velocity deficit in the wakes. The quality of this analysis have been discussed with the use of CFD. CFD is also used to gain insight into how pressure and velocity is distributed in the water tunnel. The PIV images from the tests have been processed into vector fields with the commercial PIV software DaVis7. For analyzing the PIV data further, different post-processing schemes in DaVis7 was investigated together with programs developed in Matlab. In order to compare the wakes resulting from the use of different vortex generators with measurable quantities, the use of a standard wake profile has been investigated. The standard wake profile is symmetrical and could hence only describe wake measurements done at an angle of attack close to $0^{circ}$. Furthermore it turned out that most vortex generators resulted in a wake that could not be described with the standard wake profile. The vortex generator configurations that gave the best smoothing of the hydrofoil wake for the investigated operation points turned out to be a $1unit{mm}$ V-shaped vortex generator. This vortex generator also caused less drag than than the other vortex generators tested. However, the use of vortex generators resulted in increased drag compared to the plain hydrofoil for the analyzed operating points. The velocity deficit in the wake is shown to get so well smoothed out for some tested cases that it is considered worth while to continue the investigation on vortex generators capability to increase the lifespan of Francis turbines.
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Hyman, Mark C. "The simulation of surface ship micro-bubble wakes." Diss., This resource online, 1990. http://scholar.lib.vt.edu/theses/available/etd-08252008-161944/.
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Moghadam, A. H. K. "Three-dimensional interaction of wakes and boundary layers." Thesis, University of Cambridge, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.384545.
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Brücker, Christoph [Verfasser]. "Dynamic Interaction in Bluff Body Wakes / Christoph Brücker." Aachen : Shaker, 2006. http://d-nb.info/1186584696/34.
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Gough, Tim. "Low Reynolds number turbulent boundary layers and wakes." Thesis, University of Surrey, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.360949.
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Haakonsen, Christian Bernt 1985. "Kinetic electron phenomena in dense magnetized plasma wakes." Thesis, Massachusetts Institute of Technology, 2015. http://hdl.handle.net/1721.1/103664.
Full textAbstract:
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2015.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 135-143).
Flow past an obstacle by dense magnetized plasma, having both Debye-length and gyroradii smaller than the obstacle, is explored using particle-in-cell (PIC) simulations. These simulations are relevant to a wide range of physical settings, ranging from the moon in the (supersonic) solar wind to Mach probes in (subsonic) tokamak plasmas. For supersonic flow, the evolution of the resulting elongated wake is captured with high-resolution 1D simulations, using kinetic electrons with realistic mass. This leads to the discovery of a novel wake phenomenon, where electron holes spawned from a narrow dimple in the velocity-distribution grow to large velocity extents, leading to disruption of the ion beams present in the wake. Those beams are the result of shadowing by the obstacle, which also occurs for electrons in what is a less elongated forewake, lying outside the traditional wake. This forewake is explored with 2D simulations, also using kinetic electrons with realistic mass, and it is found that drift-energization near the obstacle can significantly modify the electron distribution in some regions. Most significantly, drift-energization appears to quite robustly generate a slope-reversal of the electron velocity-distribution, which is expected to become unstable; this phenomenon thus provides a novel drive for forewake instability. 2D simulations at subsonic flow are used in an initial investigation of whether kinetic electron effects also impact the stability of wakes at slower flow. It is found that kinetic electrons do trigger disruption of the ion beams in the wake, as in the (supersonic) 1D simulations, but the hole-growth phenomenon cannot be conclusively implicated because a highly artificial electron mass needed to be used. In summary, the understanding of kinetic electron effects as dense magnetized plasma flows past an obstacle is greatly enhanced, uncovering a number of novel phenomena with implications for the stability of the resulting wake and forewake
by Christian Bernt Haakonsen.
Ph. D.
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Brucker, Kyle Ashley. "Numerical investigation of momentumless wakes in stratified fluids." Diss., [La Jolla] : University of California, San Diego, 2009. http://wwwlib.umi.com/cr/ucsd/fullcit?p3359191.
Full textAbstract:
Thesis (Ph. D.)--University of California, San Diego, 2009.Title from first page of PDF file (viewed July 14, 2009). Available via ProQuest Digital Dissertations. Vita. Includes bibliographical references (p. 171-180).
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Marasli, Barsam. "Spatially traveling waves in a two-dimensional turbulent wake." Diss., The University of Arizona, 1989. http://hdl.handle.net/10150/184811.
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Hot-wire measurements taken in the turbulent wake of a flat plate are presented. Symmetrical and antisymmetrical perturbations at various amplitudes and frequencies were introduced into the wake by small flap oscillations. As predicted by linear stability theory, the sinuous (antisymmetric) mode was observed to be more significant than the varicose (symmetric) mode. When the amplitude of the perturbation was low, the spatial development of the introduced coherent perturbation was predicted well by linear stability theory. At high forcing levels, the wake spreading showed dramatic deviations from the well known square-root behavior of the unforced case. Measured coherent Reynolds stresses changed sign in the neighborhood of the neutral point of the perturbation, as predicted by the linear theory. However, the linear theory failed to predict the disturbance amplitude and transverse shapes close to the neutral point. Some nonlinear aspects of the evolution of instabilities in the wake are discussed. Theoretical predictions of the mean flow distortion and the generation of the first harmonic are compared to experimental measurements. Given the unforced flow and the amplitude of the fundamental wave, the mean flow distortion and the amplitude of the first harmonic are predicted remarkably well.
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Chan, Tak-yee Andy, and 陳德儀. "The interaction of laminar far wake with a free surface." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1997. http://hub.hku.hk/bib/B30101098.
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Del, Guercio Gerardo. "Optimal streaks amplification in wakes and vortex shedding control." Phd thesis, Toulouse, INPT, 2014. http://oatao.univ-toulouse.fr/12244/1/Del_Guercio.pdf.
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We compute optimal energy growths leading to streamwise streaks in parallel, weakly non-parallel and the circular cylinder wakes. We find that very large energy amplifications can be sustained by these wakes. The energy amplifications increase with the spanwise wavelength of the perturbations except in the circular cylinder wake where maximum energy growths are reached for λz ≈ 5 − 7 D. The optimally amplified structures are streamwise streaks. When forced with finite amplitudes these streaks are shown, in parallel wakes, to be able to completely suppress the absolute instability. The global instability of the weakly non-parallel and the circular cylinder wakes can be completely suppressed with moderate streaks amplitudes. The energy required to stabilize the wake is much reduced when optimal perturbations are used, and it is shown to be always smaller than the one that would be required if a 2D control was used. It is also shown that the sensitivity of the global mode growth rate is quadratic and that therefore usual first order sensitivity analyses are unable to predict the high efficiency of the control-by-streaks strategy.
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Brock, Jerry S. "A modified Baldwin-Lomax turbulence model for turbomachinery wakes." Thesis, This resource online, 1991. http://scholar.lib.vt.edu/theses/available/etd-09052009-040231/.
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Bennett, William Paul. "A time accurate computational analysis of two-dimensional wakes." Thesis, University of Leicester, 2005. http://hdl.handle.net/2381/7362.
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Ramsey, William Durand. "Boundary integral methods for lifting bodies with vortex wakes." Thesis, Massachusetts Institute of Technology, 1996. http://hdl.handle.net/1721.1/10583.
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Sandberg, Richard D. "Numerical investigation of transitional and turbulent supersonic axisymmetric wakes." Diss., The University of Arizona, 2004. http://hdl.handle.net/10150/280743.
Full textAbstract:
Transitional and turbulent supersonic axisymmetric wakes are investigated by conducting various numerical experiments. The main objective is to identify hydrodynamic instability mechanisms in the flow at M = 2.46 for several Reynolds numbers, and relating these to coherent structures that are found from various visualization techniques. The premise for this approach is the assumption that flow instabilities lead to the formation of coherent structures. The effect of these structures on the mean flow is of particular interest, as they strongly affect the base drag. Three high-order accurate compressible codes were developed in cylindrical coordinates for this research: A spatial Navier-Stokes (N-S) code to conduct Direct Numerical Simulations (DNS), a linearized N-S code for linear stability investigations using two-dimensional basic states, and a temporal N-S code for performing local stability analyses. The ability of numerical simulations to deliberately exclude physical effects is exploited. This includes intentionally eliminating certain azimuthal/helical modes by employing DNS for various circumferential domain-sizes. With this approach, the impact of structures associated with certain modes on the global wake-behavior can be scrutinized. It is concluded that azimuthal modes with low wavenumbers are responsible for a flat mean base-pressure distribution and that k = 2 and k = 4 are the dominant modes in the trailing wake, producing a four-lobe wake pattern. Complementary spatial and temporal calculations are carried out to investigate whether instabilities are of local or global nature. Circumstantial evidence is presented that absolutely unstable global modes within the recirculation region coexist with convectively unstable shear-layer modes. The flow is found to be absolutely unstable with respect to modes k > 0 for ReD > 5,000 and with respect to the axisymmetric mode for ReD > 100,000. Furthermore, it is investigated whether flow control measures designed to weaken the naturally most significant modes can decrease the base drag. Finally, the novel Flow Simulation Methodology (FSM), using state-of-the-art turbulence closures, is shown to reproduce DNS results at a fraction of the computational cost.
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Tourbier, Dietmar 1964. "Numerical investigation of transitional and turbulent compressible axisymmetric wakes." Diss., The University of Arizona, 1996. http://hdl.handle.net/10150/282242.
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A numerical method has been developed for solving the complete compressible Navier-Stokes equations. The method is applicable for Direct Numerical Simulations (DNS) and Large-Eddy Simulations (LES) and was used here to study the evolution of three-dimensional disturbances in the laminar and turbulent near wake of axisymmetric bluff bodies with a blunt base in supersonic flows. The main objective of this research is to investigate the time dependent behavior of these disturbances and their influence on and interaction with the global flow field. The equations are solved in a cylindrical coordinate system using finite difference approximations of fourth-order accuracy in axial and radial directions and and a fourth-order accurate explicit Runge-Kutta scheme for the time integration. A pseudo-spectral method is employed in the azimuthal direction. Direct Numerical Simulations (DNS) were performed for a subsonic free stream Mach number of M ͚ = 0.2 and for supersonic free stream Mach numbers of M ͚ = 1.2 and M ͚ = 2.46. Large-Eddy Simulations (LES) were carried out for a subsonic free stream Mach number of M ͚ = 0.2 and a global Reynolds number of ReD = 2,000 and for a supersonic free stream Mach number of M ͚ = 2.46 and global Reynolds numbers of ReD = 30,000 and ReD = 100,000. Comparison of the instantaneous flow field for subsonic calculations with water channel experiments and incompressible simulations show good qualitative agreement. An absolute instability with regard to helical disturbances was found for the subsonic flow at ReD = 1,000 and for the supersonic flows for M ͚ = 1.2 and ReD ≥ 4,000 and for M ͚ = 2.46 and ReD ≥ 30,000. Small disturbances appear in the flow field near the corner of the base. As the disturbances are propagating downstream they grow and form intense vortical structures. These structures have a strong influence on the flow field, which results in a drastic change of the base pressure distribution and thus of the base drag.