Дисертації з теми "Wake structures"
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1
Lam, Fung. "Induced drag and wake structures behind wings." Thesis, University of Cambridge, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.316745.
2
Muld, Tomas W. "Analysis of Flow Structures in Wake Flows for Train Aerodynamics." Licentiate thesis, KTH, Mechanics, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-12746.
Анотація:
Train transportation is a vital part of the transportation system of today anddue to its safe and environmental friendly concept it will be even more impor-tant in the future. The speeds of trains have increased continuously and withhigher speeds the aerodynamic effects become even more important. One aero-dynamic effect that is of vital importance for passengers’ and track workers’safety is slipstream, i.e. the flow that is dragged by the train. Earlier ex-perimental studies have found that for high-speed passenger trains the largestslipstream velocities occur in the wake. Therefore the work in this thesis isdevoted to wake flows. First a test case, a surface-mounted cube, is simulatedto test the analysis methodology that is later applied to a train geometry, theAerodynamic Train Model (ATM). Results on both geometries are comparedwith other studies, which are either numerical or experimental. The comparisonfor the cube between simulated results and other studies is satisfactory, whiledue to a trip wire in the experiment the results for the ATM do not match.The computed flow fields are used to compute the POD and Koopman modes.For the cube this is done in two regions of the flow, one to compare with a priorpublished study Manhart & Wengle (1993) and another covering more of theflow and especially the wake of the cube. For the ATM, a region containing theimportant flow structures is identified in the wake, by looking at instantaneousand fluctuating velocities. To ensure converged POD modes two methods toinvestigate the convergence are proposed, tested and applied. Analysis of themodes enables the identification of the important flow structures. The flowtopologies of the two geometries are very different and the flow structures arealso different, but the same methodology can be applied in both cases. For thesurface-mounted cube, three groups of flow structures are found. First groupis the mean flow and then two kinds of perturbations around the mean flow.The first perturbation is at the edge of the wake, relating to the shear layerbetween the free stream and the disturbed flow. The second perturbation isinside the wake and is the convection of vortices. These groups would then betypical of the separation bubble that exists in the wake of the cube. For theATM the main flow topology consists of two counter rotating vortices. Thiscan be seen in the decomposed modes, which, except for the mean flow, almostonly contain flow structures relating to these vortices.
QC 20100518
Gröna Tåget
3
Ryan, Kris. "The analysis of wake structures behind stationary, freely oscillating and tethered cylinders." Monash University, Dept. of Mechanical Engineering, 2004. http://arrow.monash.edu.au/hdl/1959.1/9605.
4
Muld, Tomas W. "Slipstream and Flow Structures in the Near Wake of High-Speed Trains." Doctoral thesis, KTH, Farkost och flyg, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-94182.
Анотація:
Train transportation is a vital part of the transportation system of today. Asthe speed of the trains increase, the aerodynamic effects become more impor-tant. One aerodynamic effect that is of vital importance for passengers’ andtrack workers’ safety is slipstream, i.e. the induced velocities by the train.Safety requirements for slipstream are regulated in the Technical Specificationsfor Interoperability (TSI). Earlier experimental studies have found that forhigh-speed passenger trains the largest slipstream velocities occur in the wake.Therefore, in order to study slipstream of high-speed trains, the work in thisthesis is devoted to wake flows. First a test case, a surface-mounted cube, issimulated to test the analysis methodology that is later applied to two differ-ent train geometries, the Aerodynamic Train Model (ATM) and the CRH1.The flow is simulated with Delayed-Detached Eddy Simulation (DDES) andthe computed flow field is decomposed into modes with Proper Orthogonal De-composition (POD) and Dynamic Mode Decomposition (DMD). The computedmodes on the surface-mounted cube compare well with prior studies, whichvalidates the use of DDES together with POD/DMD. To ensure that enoughsnapshots are used to compute the POD and DMD modes, a method to inves-tigate the convergence is proposed for each decomposition method. It is foundthat there is a separation bubble behind the CRH1 and two counter-rotatingvortices behind the ATM. Even though the two geometries have different flowtopologies, the dominant flow structure in the wake in terms of energy is thesame, namely vortex shedding. Vortex shedding is also found to be the mostimportant flow structure for slipstream, at the TSI position. In addition, threeconfigurations of the ATM with different number of cars are simulated, in orderto investigate the effect of the size of the boundary layer on the flow structures.The most dominant structure is the same for all configurations, however, theStrouhal number decreases as the momentum thickness increases. The velocityin ground fixed probes are extracted from the flow, in order to investigate theslipstream velocity defined by the TSI. A large scatter in peak position andvalue for the different probes are found. Investigating the mean velocity atdifferent distances from the train side wall, indicates that wider versions of thesame train will create larger slipstream velocities.QC 20120530
5
Krueger, Matthew J. "Three-dimensional vortical structures in the wake of a flexible flapping foil." Thesis, Massachusetts Institute of Technology, 2005. http://hdl.handle.net/1721.1/32950.
Анотація:
Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2005.Includes bibliographical references (leaf 22).
This project aims to gain a qualitative view of the three-dimensional vortical structures of a flexible flapping foil at Reynolds number 164. Flexible foils were fabricated, coated with fluorescent dye, and towed with heave and pitch in a large glass tank. The foil cross section is a NACA 0030 foil shape, and the foil has an aspect ratio of 3. Pictures where taken of the vortical structures from planform, wingtip, and isometric views over a range of Strouhal number and kinematic parameters. Results are compared to previous experimental and numerical studies.
by Matthew J. Krueger.
S.B.
6
Zhong, Shan. "An interferometric study of organized structures in compressible turbulent flows." Thesis, University of Cambridge, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.319551.
7
Karunakaran, Arvind. "Truce structures : examining cross-professional coordination in the wake of technological and institutional change." Thesis, Massachusetts Institute of Technology, 2018. http://hdl.handle.net/1721.1/118004.
Анотація:
Thesis: Ph. D., Massachusetts Institute of Technology, Sloan School of Management, 2018.Cataloged from PDF version of thesis.
Includes bibliographical references.
This research examines the structures, processes, and mechanisms that facilitate cross-professional coordination during periods of technological and institutional change. My study draws on a 24-month ethnographic study, combined with historical data and quantitative analysis, of 911 emergency management organizations in the United States. In Chapter 2, I focus on the mechanisms to facilitate cross-professional coordination in conditions that are marked by protracted jurisdictional conflicts. My findings articulate the importance of truce structures - an ensemble of truce roles and organizational forms - that are intended to address protracted jurisdictional conflicts among symmetrical professions such as police officers and firefighters. I further find that the coevolution of truce roles and organizational forms resulted in the emergence of a specific truce profession - in this case, that of 911 Public Safety Telecommunicators. The truce profession serves to triage, direct, and channel contested tasks among the conflicting professions without bringing those professions into direct contact with each during the initial stages of coordination when the "definition of the situation" is getting worked out. In Chapter 3, I turn to examining how the truce professionals navigate what I call status-authority asymmetry in order to effectively coordinate with the focal professionals. Conducting within-shift comparisons of coordination encounters between 911 dispatchers and police officers, I identify that the bounded publicization tactic performed via the open radio channel allows dispatchers to generate peer knowledge about individual non-compliance. Through this process, dispatchers navigate the status-authority asymmetry and orchestrate effective cross-professional coordination. My focus in Chapter 4 shifts to examining how truce professionals respond to the public's increased digital scrutiny, and consider the consequences for organizational accountability. My findings suggest that the public's increased use of mobile phones and social media to monitor and report on organizations and their workers can, under some conditions, end up worsening accountability. I unpack the processes that generate this paradox of public accountability, showing how these processes reshape the work of truce professionals and produce a vicious cycle of coordination that worsens organizational accountability. I end with a concluding chapter that discusses the implications of my dissertation for research on cross-professional coordination, accountability, and technological change.
by Arvind Karunakaran.
Ph. D.
8
Carmer, Carl Friedrich v. "Shallow turbulent wake flows momentum and mass transfer due to large-scale coherent vortical structures /." Karlsruhe : Univ.-Verl, 2005. http://deposit.d-nb.de/cgi-bin/dokserv?idn=976439034.
9
Sutkowy, Mark Louis Jr. "Relationship between Rotor Wake Structures and Performance Characteristics over a Range of Low-Reynolds Number Conditions." The Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1534768619864476.
10
Sönmez, Nurcan. "Investigating Wind Data and Configuration of Wind Turbines for a Turning Floating Platform." Thesis, KTH, Mekanik, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-148957.
Анотація:
Wake interactions on a floating platform for offshore wind energy applications were investigated.The study is performed in collaboration with Hexicon AB which has a patent family for innovative floating platforms, which are able to turn automatically. The Jensen model is used for wake effect calculations and the simulations were performed in MATLAB. The present study starts with wind speed and wind direction data analysis for the specific site that Hexicon AB plans to construct its first platform. Data analysis is followed by wake interaction studies for H4-24MW type Hexicon AB platform. Wake interaction simulations were performed for three different cases. Fixed turbine and platform, Nacelle yawing and fixed platform and Nacelle yawing and turned platform. Different cases were investigated in order to see wake interactions for different wind directions. Wind direction effect on wake interactions were performed between _90_ and 90_ with an increment of 10_. After having the simulation results for Nacelle yawing and turned platform case the results were compared with ANSYS - CFX simulations results. The results didn’t match exactly but they were very close, which is an indicator to the validity of the Jensen Model. After finding out the possible behavior of wake interactions for different wind directions, power calculations were performed for the same three cases. In order to perform the power calculations the wake interactions for different wind directions were taken into account. In case of platform turning it was assumed that power losses were caused both by wake interactions and in case of thrusters activation. The losses that would be caused by different thrust forces on the turbine blades were not included. The last study was performed to suggest different layouts. In the second case, Nacelle yawing and fixed platform, it was found out that nacelle yawing for most of the angles is not possible because it creates wake regions in front of the rotor area. It was decided to propose new turbine configurations on the platform which are tolerant to different nacelle yawing angles. The simulations were run without considering any constructions limitations, meaning that the availability of platform structure was not included. The study is ended by performing some probabilistic results for platform turning behavior.
11
羅志永 and Chi-wing Law. "The flow structures and vortex interaction in the subcritical regime in the near wake of a circular cylinder." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1999. http://hub.hku.hk/bib/B31238816.
12
Gieseke, Thomas J. "Identification of coherent structures in the turbulent wake of a circular cylinder with comparison to linear instability modes /." The Ohio State University, 1993. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487844948077581.
13
Law, Chi-wing. "The flow structures and Vortex Internation in the subcritical regime in the near wake of a circular cylinder /." Hong Kong : University of Hong Kong, 1999. http://sunzi.lib.hku.hk/hkuto/record.jsp?B20971667.
14
Carmer, Carl Friedrich v. [Verfasser]. "Shallow turbulent wake flows : momentum and mass transfer due to large-scale coherent vortical structures / von Carl Friedrich v. Carmer." Karlsruhe : Univ.-Verl. Karlsruhe, 2005. http://d-nb.info/976439034/34.
15
Clemons, Lucas A. "An experimental study of the vortex structures in the wake of a piezoelectric flapping plate for Nano Air Vehicle applications." [Ames, Iowa : Iowa State University], 2009. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:1468074.
16
Newberry, James. "A Damage Assessment and Wind Loading Analysis of Residential Structures Built Post-1996 in Punta Gorda in the Wake of Hurricane Charley." Scholar Commons, 2006. http://scholarcommons.usf.edu/etd/3749.
Анотація:
One of the communities in the path of Hurricane Charley as it came ashore
August 13, 2004, was Punta Gorda, recording gusts up to 145 mph. This project
utilizes aerial photos taken approximately 10 days after the storm battered the
area, using a digital photography program. Focusing on the one-story residential
structures (houses) of the Punta Gorda area, a damage assessment could be
made of the area’s homes, and how they stood up to the storm. This study
focused further on homes built after major changes to the local/state building
codes went into effect (starting in 1996) after the devastation left in south Florida
by Hurricane Andrew in 1992. After selecting approximately 20 damaged
houses, damaged from wind loading only, an analysis of these houses (or types
of houses) could then be undertaken complying with the most current
building/wind codes used at the time of Charley’s landfall. Furthermore, by
looking at the pictures, and using reports outlining the types of damage seen
from the storm, the building/wind codes could then be checked for their
effectiveness.
After performing a wind loading analysis on houses similar to those seen in
the selected pictures, and using the wind code provisions of ASCE 7-98,
calculations show a substantial increase in local wind pressure to various zones
of the roof. High pressure zones of the roof included the ridges of the gable and
hipped style roofs, as well as the corners and the edges.
More emphasis needs to be placed on the installation of the clay tiles
(mandated by certain deed-restricted subdivisions of Punta Gorda). If the tiles
are ripped off from the wind, then the roof sheathing becomes exposed to the
environment, and if this becomes damaged, rain leaking down into the interior of
the house would cause additional damage.
17
Armaly, Majd. "Dynamics of coherent structures over wind turbine blade." Electronic Thesis or Diss., Normandie, 2024. http://www.theses.fr/2024NORMR005.
Анотація:
L’objectif principal de cette thèse est d’évaluer quantitativement le comportement dynamique de l’écoulement d’air turbulent autour d’un profil aérodynamique NACA en fonction de l’écoulement en amont. L’écoulement en amont est constitué d’une famille de structures cohérentes, chacune ayant ses propres échelles temporelles et spatiales. En outre, cet écoulement en amont peut inclure un cisaillement moyen dû à une couche limite à grande échelle. Ces situations se rencontrent couramment dans les parcs d’éoliennes et de turbines marines, où le sillage généré par une turbine devient l’écoulement en amont de la suivante, et où la couche limite atmosphérique peut influencer la dynamique de l’écoulement de l’air du profil aérodynamique. La recherche aérodynamique sur les éoliennes s’est principalement concentrée sur la réponse générale des profils aérodynamiques et sur l’écoulement en aval. Cependant, les profils aérodynamiques ne sont pas encore pris en compte dans un écoulement en amont avec la présence d’une famille de structures cohérentes. Cette étude prend en compte les aspects spatiaux et temporels, ce qui est crucial pour comprendre comment l’énergie cinétique transportée par ces mouvements cohérents est capturée dans le sillage. La nouveauté du projet est de fournir une analyse complémentaire et plus détaillée des interactions entre le profil aérodynamique et l’écoulement en amont, basée sur l’analyse de l’intermittence des conditions en amont. Cela permet de mieux comprendre la réponse dynamique de l’aile en aval et la distribution de l’énergie cinétique sur l’aile. Pour atteindre cet objectif, les distributions de pression (structures cohérentes) doivent être moyennées en phase avec un conditionnement basé sur la dynamique du flux en amont. Ce type d’analyse représente une nouvelle approche et nécessite le développement de méthodes spécialisées à appliquer à des cas complexes. L’un des principaux domaines d’investigation du projet est l’étude de l’énergie cinétique turbulente totale (TKE) lorsque les paquets de structures cohérentes en amont interagissent avec le profil aérodynamique en aval. En outre, d’autres recherches peuvent être menées sur l’énergie cinétique turbulente, en considérant les interactions entre les différentes composantes des fluctuations, telles que les interactions entre le mouvement cohérent et le mouvement aléatoire. Ce travail de thèse est intégré dans le cadre du projet DYNEOL (ANR-17-CE06-0020) qui est financé par l’Agence Nationale de la Recherche (ANR). Le projet est une recherche collaborative associant quatre laboratoires françaisThe primary objective of the thesis is to quantitatively assess the dynamic behavior of turbulent airflow around a NACA airfoil as a function of the upstream flow. The upstream flow consists of a family of coherent structures, each with its own distinct temporal and spatial scales. Additionally, this upstream flow may include a mean shear due to a large scale boundary layer. These situations are commonly encountered in wind and marine turbine farms, where the wake generated by one turbine becomes the upstream flow for the next one, and where the atmospheric boundary layer can influence the dynamics of the airfoil’s airflow. Aerodynamic research on wind turbines has primarily focused on the general response of airfoils, and the flow downstream. However, airfoils are not considered yet in an upstream flow with the presence of a family of coherent structures. This study takes into account the spatial and temporal aspects, which is crucial for understanding how efficiently the kinetic energy carried by these coherent motions is captured within the wake. The novelty of the project is to provide a complementary and more detailed analysis of the airfoil-upstream flow interactions based on the analysis of the intermittency of the upstream conditions. This helps to gain insights into the dynamic response ofthe downstream airfoil and the distribution of kinetic energy over the airfoil. To achieve this goal, pressure distributions (coherent structures) must be phase-averaged with a conditioning based on the dynamics of the upstream flow. This type of analysis representsa novel approach and requires the development of specialized methods to be applied to such complex cases. One of the key areas of investigation within the project is the study of the total turbulent kinetic energy (TKE) when the upstream coherent structure packets interact with the downstream airfoil. Additionally, further research can be involved into turbulent kinetic energy, considering the interactions between different components of fluctuations, such as the interactions between coherent and random motion. This thesis work is integrated in the framework of DYNEOL (ANR-17-CE06-0020) project that is funded by the French National Agency of Research (ANR). The project is a collaborative research involving four French laboratories, including the CORIA laboratory
18
Wu, Xiong-Jian. "Motion and wave load analyses of large offshore structures and special vessels in waves." Thesis, Brunel University, 1990. http://bura.brunel.ac.uk/handle/2438/7865.
Анотація:
Predictions of the environmental loading and induced motional and structural responses are among the most important aspects in the overall design process of offshore structures and ships. In this thesis, attention is focused on the wave loads and excited bodily motion responses of large offshore structures and special vessels. With the aim of improving the existing theoretical methods to provide techniques of theoretical effectiveness, computational efficiency, and engineering practicality in marine and offshore applications, the thesis concentrates upon describing fundamental and essential aspects in the physical phenomenon associated with wave-structure interactions and deriving new methods and techniques to analyse offshore structures and unconventional ships of practical interest. The total wave force arising from such a wave-structural interaction is assumed to be a simple superposition of the potential and the viscous flow force components. The linear potential forces are solved by the Green function integral equation whilst the viscous forces are estimated based on the Morison's damping formula. Forms of the Green function integral equation and the associated Green function are given systematically for various practical cases. The relevant two-dimensional versions are then derived by a transformation procedure. Techniques are developed to solve the integral equation numerically including the interior integral formulation and, in particular, to tackle the mathematical difficulties at irregular frequencies. In applying the integral equations to solve problems with various offshore structures and special vessels, some modified, improved or simplified methods are proposed. At first, simplified method is derived for predictions of the surge, sway and yaw motions of elongated bodies of full sectional geometry or structures with shallow draft. Then, a new shallow draft theory is described for both three- and two-dimensional cases with inclusion of the finite draft effect. Furthermore, a three-dimensional strip method is formulated where the end effects of the body are fully taken into account. Finally, an approximation to the horizontal mean drift forces of multi-column offshore structures are presented. Some new findings are also discussed including the multiple resonances occurring in the motions of multi-hulled marine structures due to the wave-body interaction, the mutual cancellation effect of the diffraction and the radiation forces arising from a full shaped slender body, and so on. Further to those verification studies for individual methods developed, more comprehensive example investigations are given related to two industrial applications. One is a derrick barge semi-submersible with zero forward speed; and the other, a SWATH ship with considerable speed. By correlation of all the proposed approaches with available analytical, numerical and experimental data, the thesis tries to demonstrate a principle that as long as principal physical aspects in the wave-structure interaction problem are properly treated, an appropriately modified or simplified method works, performs well and, sometimes, even better.
19
Giacobello, Matteo. "Wake structure of a transversely rotating sphere at moderate Reynolds numbers." Online version, 2005. http://repository.unimelb.edu.au/10187/2840.
Анотація:
Over the last century, the problem of a viscous flow past a sphere has received on-going attention due to its many engineering applications. These include combustion processes, sediment transport processes and atmospheric flow problems, where the sphere serves as a good model for more general bluff body particles. In these environments, particles may be subjected to both translational and rotational velocities. The objective of this study was to investigate the effect that sphere rotation, about an axis transverse to the freestream flow, has on the characteristics or the vertical wake structure and the forces exerted on the sphere. That was achieved by solving the time-dependant, incompressible Navier Stokes equations, using a highly accurate Fourier Chebyshev spectral collation method.
20
Grice, James Robert. "Prediction of extreme wave-structure interactions for multi-columned structures in deep water." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:dd7320c1-7121-4ea7-827f-527af9405e9a.
Анотація:
With a continuing and rising demand for hydrocarbons, the energy companies are installing infrastructure ever further offshore, where such infrastructure is often exposed to extreme waves. This thesis explores some aspects of wave-structure interaction, particularly the maximum water surface elevation increase in severe storms due to these local interactions. The effects on wave-structure interactions of column cross-sectional shape are investigated using linear and second-order wave diffraction theory. For multi-column structures, the excitation of locally resonant wave modes (near-trapping) is studied for several column cross-sectional shapes, and a simple method for estimating the surface elevation mode shape is given. The structure of the quadratic transfer functions for second-order sum wave elevation is investigated and an approximation assuming these QTFs are flat perpendicular to the leading diagonal is shown to be adequate for the first few lowest frequency modes. NewWave-type focused wave groups can be used as a more realistic model of extreme ocean waves. A Net Amplification Factor based on the NewWave model is given as an efficient tool for finding the incident frequencies most likely to cause a violent wave-structure interaction and where these violent responses are likely to occur. Statistics are collected from Monte Carlo type simulations of random waves to verify the use of the Net Amplification Factor. Going beyond linear calculations, surface elevation statistics are collected to second-order and a `designer' wave is found to model the most extreme surface elevation responses. A `designer' wave can be identified at required levels of return period to help to understand the relative size of harmonic components in extreme waves. The methods developed with a fixed body are then applied to an identical hull which is freely floating, and the responses between the fixed and moving cases are compared. The vertical heave motion of a semi-submersible in-phase with the incident wave crests is shown to lead to a much lower probability of water-deck impact for the same hull shape restrained vertically. The signal processing methods developed are also applied to a single column to allow comparison with experimental results. Individual harmonic components of the hydrodynamic force are identified up to at least the fifth harmonic. Stokes scaling is shown to hold even for the most violent interactions. It is also shown that the higher harmonic components of the hydrodynamic force can be reconstructed from just the fundamental force time history, and a transfer function in the form of a single phase and an amplitude for each harmonic. The force is also reconstructed well to second-order from the surface elevation time history using diffraction transfer functions. Finally, possible causes of damage to a platform high above mean water level in the North Sea are investigated.
21
Somero, John Ryan. "Structure and Persistence of Surface Ship Wakes." Thesis, Virginia Tech, 2021. http://hdl.handle.net/10919/101989.
Анотація:
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.
22
Abdolmaleki, Kourosh. "Modelling of wave impact on offshore structures." University of Western Australia. School of Mechanical Engineering, 2007. http://theses.library.uwa.edu.au/adt-WU2008.0055.
Анотація:
[Truncated abstract] The hydrodynamics of wave impact on offshore structures is not well understood. Wave impacts often involve large deformations of water free-surface. Therefore, a wave impact problem is usually combined with a free-surface problem. The complexity is expanded when the body exposed to a wave impact is allowed to move. The nonlinear interactions between a moving body and fluid is a complicated process that has been a dilemma in the engineering design of offshore and coastal structures for a long time. This thesis used experimental and numerical means to develop further understanding of the wave impact problems as well as to create a numerical tool suitable for simulation of such problems. The study included the consideration of moving boundaries in order to include the coupled interactions of the body and fluid. The thesis is organized into two experimental and numerical parts. There is a lack of benchmarking experimental data for studying fluid-structure interactions with moving boundaries. In the experimental part of this research, novel experiments were, therefore, designed and performed that were useful for validation of the numerical developments. By considering a dynamical system with only one degree of freedom, the complexity of the experiments performed was minimal. The setup included a plate that was attached to the bottom of a flume via a hinge and tethered by two springs from the top one at each side. The experiments modelled fluid-structure interactions in three subsets. The first subset studied a highly nonlinear decay test, which resembled a harsh wave impact (or slam) incident. The second subset included waves overtopping on the vertically restrained plate. In the third subset, the plate was free to oscillate and was excited by the same waves. The wave overtopping the plate resembled the physics of the green water on fixed and moving structures. An analytical solution based on linear potential theory was provided for comparison with experimental results. ... In simulation of the nonlinear decay test, the SPH results captured the frequency variation in plate oscillations, which indicated that the radiation forces (added mass and damping forces) were calculated satisfactorily. In simulation of the nonlinear waves, the waves progressed in the flume similar to the physical experiments and the total energy of the system was conserved with an error of 0.025% of the total initial energy. The wave-plate interactions were successfully modelled by SPH. The simulations included wave run-up and shipping of water for fixed and oscillating plate cases. The effects of the plate oscillations on the flow regime are also discussed in detail. The combination of experimental and numerical investigation provided further understanding of wave impact problems. The novel design of the experiments extended the study to moving boundaries in small scale. The use of SPH eliminated the difficulties of dealing with free-surface problems so that the focus of study could be placed on the impact forces on fixed and moving bodies.
23
Louis, Stephen. "Continuer à irriguer quand les lacs-réservoirs de barrage souffrent de taux de sédimentation sévères - Recommandations d'amélioration de la gestion du principal canal d'irrigation alimenté par l'ouvrage répartiteur de Canneau (Haïti)." Doctoral thesis, Universite Libre de Bruxelles, 2019. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/288808.
Анотація:
L’État haïtien, pour faire face à l’insécurité alimentaire que connaît sa population (à croissance rapide et à faible revenu), s’appuie particulièrement sur la Vallée du département de l’Artibonite qui constitue depuis toujours le véritable grenier agricole du pays, en fournissant, à elle seule, plus de 80% de la production rizicole nationale. Cette production agricole assure non seulement les besoins alimentaires de la population locale, mais également ceux particuliers des départements voisins (Ouest, Nord et Centre).L’irrigation de cette vaste plaine agricole (32400 ha de terres agricoles irrigables) est garantie, à plus de 75%, par le réseau d’irrigation établi en rive Gauche de l'ouvrage-partiteur de Canneau, alimenté par un Canal principal (CMRG) ayant un débit nominal de 40 m3/s.Néanmoins, ce Partiteur, source d’approvisionnement exclusive du CMRG, est lui-même régulé par le Barrage-réservoir de Péligre qui se trouve à 70 km plus en amont sur le fleuve de l’Artibonite. En termes hydrauliques, nos travaux ont d'abord examiné la situation du réservoir de Péligre. Alors qu'au moment de la construction il était initialement prévu d’y stocker 607 Mm3, il ne reste plus aujourd'hui qu'à peine 40% de cette capacité utile, en raison des dépôts sédimentaires qui se sont constitués année après année derrière le barrage.Cette sédimentation spectaculaire (due à l’érosion des bassins versants amont fort dégradés), combinée aux déficits pluviométriques, provoque en période d’étiage une rareté d’eau, des lâchers insuffisants et donc des déficits en eau utile envoyée en amont de ce Partiteur de Canneau. Cela impacte significativement le réseau d’irrigation aval, dont le CMRG. Nos travaux ont montré que la situation est d'autant plus critique que les débits eux-mêmes, en amont comme en aval du Partiteur, sont en fait très mal connus et devraient faire l'objet d'approches méthodologiques plus rigoureuses que celles déployées sur site actuellement.Aussi, face à ces constats, de nouvelles règles de distribution de l’eau s’imposent, pour continuer à irriguer et espérer obtenir un rendement agricole acceptable (souhaitable).Notre travail s'est ainsi donné pour objectif de contribuer à la mise en place des nouvelles règles de gestion de l’eau (répartition) au sein du réseau d’irrigation alimenté par le CMRG, pour continuer à fournir l’eau à l’irrigant en quantité acceptable (et connue). Cette amélioration de gestion a été envisagée à la fois en amont et en aval de ce réseau d’irrigation, particulièrement en ses différents nœuds-clés (Canal principal et Canal secondaire).La démarche méthodologique adoptée pour relever ce défi majeur s’appuie notamment sur un système d’information hydro-morpho-sédimentaire actualisé et de qualité. Il s’agit d’une base de données, riche en observations de hauteurs d’eau (lues aux stations limnimétriques), vitesses de surface au flotteur, champs de vitesse explorés au moyen d’un courantomètre et en données bathymétriques et granulométriques des tronçons des canaux étudiés, appréciées respectivement au moyen d’un GPS différentiel et du tamisage à sec.Les résultats fort encourageants obtenus permettent d'acquérir une meilleure compréhension du système et une amélioration particulière du réseau d’irrigation en rive gauche du Partiteur de Canneau. En s’appuyant sur les historiques de sédimentation du Lac-réservoir de Péligre (de 1960 à 2016), nous présentons un document de synthèse sur la sédimentation du Lac-réservoir de Péligre. Ce document met notamment en exergue le taux de sédimentation sévère de ce dernier (5.47 Mm3/an), qui continue d’augmenter encore aujourd’hui, ainsi que les conséquences de celui-ci sur les débits turbinés et la disponibilité de l’eau en amont du Partiteur de Canneau.Nous mettons également en évidence les formes irrégulières (Lit-non prismatique) des tronçons des canaux étudiés, via une vue axonométrique des profils en travers (issus de l’étude bathymétrique) des canaux d’irrigation en terre battue étudiés. Puis, nous présentons de manière détaillée le caractère très hétérogène des dépôts sédimentaires de ces derniers, à partir d’une analyse des représentations en Log-Probit des résultats du tamisage, construites au moyen du logiciel GrandPlots.En nous appuyant sur les mesures expérimentales des contraintes de Reynolds et des profils instantanés de vitesse (pris à intervalle de 64 ms), tirés de la base de données EPFL, nous avons montré qu’il faut absolument travailler dans les 18% inférieurs de la colonne d’eau (z/h<0.18) et en mode déficitaire, dans un écoulement turbulent comme celui-là, pour extraire de façon représentative et pertinente une pente expérimentale u*/, comme indicateur de u*.À l’issue d’un examen détaillé de la distribution verticale de la vitesse au canal secondaire FNE, nous validons un DMLWL (Dip-Modified-log-wake-law) à la fois en amont et en aval du réseau. Nous montrons que ceci permet de modéliser le Dip-phenomenon observé systématiquement au sein des profils explorés in situ. Nous proposons une relation entre le coefficient d’inégale répartition de la vitesse à la verticale αv (de Prony) et l’aspect ratio (W/h) pour tout le réseau d’irrigation étudié ;ceci afin d’obtenir une vitesse débitante (Ū), simplement à partir d’une prise de vitesse au flotteur, dans l’axe central d’écoulement.À partir des débits quantifiés à la section de référence du CMRG, via l’équation de continuité (Q=AŪ), nous fournissons un Abaque, diagramme à 3 entrées (débit (Qp), charge amont (H0) et ouverture de vanne (hv)), permettant aux vanniers de connaitre les débits au pont de fer correspondant aux différentes ouvertures de vanne et celui pour lequel le trop-plein (retour des eaux excédentaires vers le fleuve de l’Artibonite) commence à fonctionner.À l’égard des opérateurs locaux et gestionnaires du système, nous mettons enfin à disposition, des méthodes/outils simples et efficaces leur permettant de quantifier finement le débit au Canal principal en amont ainsi qu’au canal secondaire en aval, simplement à partir d’une mesure de hauteur d’eau (h) et de vitesse de surface au flotteur (Us).Doctorat en Sciences agronomiques et ingénierie biologique
info:eu-repo/semantics/nonPublished
24
Miranda, Joseph A. "The structure of a trailing vortex wake." Thesis, This resource online, 1996. http://scholar.lib.vt.edu/theses/available/etd-01312009-063252/.
25
Attarian, Vatche. "Long-term structural health monitoring of plate-like structures using distributed guided wave sensors." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/24840.
Анотація:
Aircraft, containers, and storage tanks contain plate-like structures that are safety critical. The structures often undergo non-destructive inspections. The inspection frequency tends to be over-conservatively high, and it may be possible to reduce the intervals between inspections to realize cost savings. This goal can possibly be realized by automated structural health monitoring (SHM) of structures using sparse active guided wave sensor arrays. Guided waves are sensitive to small defects and can propagate long distances across feature dense plates. Thus, a guided wave SHM system that enables reliable detection of critical defects or monitoring of their growth can potentially be used to reduce the frequency of inspections for real structures. Industrial guided wave SHM systems must be reliable throughout prolonged exposure to temperature, humidity, and loading changes encountered in operation. Research at Imperial College shows that temperature compensation and subtraction between monitored guided wave signals and baselines acquired from healthy plates enables detection of 1.5% reflection change over areas ~1 m^2 in the presence of thermal swings and uniform liquid layers. These results and findings from scattering studies indicate it may be possible to detect reflections from hole type defects and notches affecting structures during their operation. An issue is that demonstrations of SHM system capabilities have only been shown in controlled laboratory tests within short periods following baseline acquisition. There is concern whether sustained exposure to service conditions will subject transducer elements to irreversible changes and introduce variability in baseline subtraction results that would mask signals due to slowly growing damage. This thesis studies the reliability of guided wave SHM for monitoring plate-like structures over longer time periods. The theoretical characteristics of the fundamental Lamb waves and their use to monitor and detect damage are reviewed. Strategies for sensing and signal processing are described alongside experimental validation of their performance. The effectiveness of the SHM system is tested in experiments where damage-free plates are exposed to British weather as well as thermal variations in an environmental chamber. The monitoring capabilities of bonded piezoelectric sensors are quantified and compared to the performance achieved using electromagnetic acoustic transducers. Experimental results and findings from simulations of bonded piezoelectric transduction establish that performances achieved with bonded sensors degrade due to variations in the properties of adhesives used to attach sensors to plates. EMATs are relatively stable and capable of enabling detection of 1.5% reflection change at points away from the edges of plates after sustained exposure to thermal cycling loads.
26
Hazell, Jonathan. "New slow wave structures for travelling wave tubes." Thesis, Imperial College London, 2017. http://hdl.handle.net/10044/1/59703.
Анотація:
This thesis covers the design of slow wave structures for travelling wave tubes, with a specific focus on those that could be used for operation at millimetre or shorter wavelengths. Serpentine and a coupled cavity photonic crystal structure are covered in detail, together with the interaction between the electromagnetic waves they support and the electron gun and magnetic beam focusing systems needed for a travelling wave interaction. In Chapter 2, the existing small-signal theory of the travelling wave interaction is introduced and applied to a serpentine travelling wave tube. A set of synthesis equations for the serpentine structure are then derived from the analysis and verified with simulation. In Chapter 3, possible improvements to the serpentine structure for high frequency operation, and operation on harmonics other than the fundamental (for both the phase and the interaction impedance) are considered. From the investigation it can be concluded that higher harmonics allow a larger beam current than the fundamental. In Chapter 4, slow wave structures based on photonic crystals are proposed for use in travelling wave tubes. A specific photonic crystal arrangement – the coupled resonator optical waveguide (CROW) - that does not appear to have been studied previously in this application is then investigated. The conclusion is that a CROW is suitable for use in a travelling wave tube and is significantly more manufacturable than existing approaches. In Chapter 5, the design of a full electron beam system for use with both the original and the improved slow wave structures is presented. The design of an electron gun, cathode and collimating magnet using an immersed flow insertion are all covered in detail. In Chapter 6, conclusions are drawn and avenues for possible future work are presented.
27
Lehmann, Lutz. "Wave propagation in infinite domains : with applications to structure interaction /." Berlin [u.a.] : Springer, 2007. http://www.gbv.de/dms/bs/toc/523903820.pdf.
28
Md, Noar Nor. "Wave impacts on rectangular structures." Thesis, Brunel University, 2012. http://bura.brunel.ac.uk/handle/2438/6609.
Анотація:
There is a good deal of uncertainty and sensitivity in the results for wave impact. In a practical situation, many parameters such as the wave climate will not be known with any accuracy especially the frequency and severity of wave breaking. Even if the wave spectrum is known, this is usually recorded offshore, requiring same sort of (linear) transfer function to estimate the wave climate at the seawall. What is more, the higher spectral moments will generally be unknown. Wave breaking, according to linear wave theory, is known to depend on the wave spectrum, see Srokosz (1986) and Greenhow (1989). Not only is the wave climate unknown, but the aeration of the water will also be subject to uncertainty. This affects rather dramatically the speed of sound in the water/bubble mixture and hence the value of the acoustic pressure that acts as a maximum cutoff for pressure calculated by any incompressible model. The results are also highly sensitive to the angle of alignment of the wave front and seawall. Here we consider the worst case scenario of perfect alignment. Given the above, it seems sensible to exploit the simple pressure impulse model used in this thesis. Thus Cooker (1990) proposed using the pressure impulse P(x, y) that is the time integral of the pressure over the duration of the impact. This results in a simplified, but much more stable, model of wave impact on the coastal structures, and forms the basis of this thesis, as follows: Chapter 1 is an overview about this topic, a brief summary of the work which will follow and a summary of the contribution of this thesis. Chapter 2 gives a literature review of wave impact, theoretically and experimentally. The topics covered include total impulse, moment impulse and overtopping. A summary of the present state of the theory and Cooker’s model is also presented in Chapter 2. In Chapter 3 and Chapter 4, we extend the work of Greenhow (2006). He studied the berm and ditch problems, see Chapter 3, and the missing block problem in Chapter 4, and solved the problems by using a basis function method. I solve these problems in nondimensionlised variables by using a hybrid collocation method in Chapter 3 and by using the same method as Greenhow (2006) in Chapter 4. The works are extended by calculating the total impulse and moment impulse, and the maximum pressure arising from the wave impact for each problem. These quantities will be very helpful from a practical point of view for engineers and designers of seawalls. The mathematical equations governing the fluid motion and its boundary conditions are presented. The deck problem together with the mathematical formulation and boundary conditions for the problem is presented in Chapters 5 and 6 by using a hybrid collocation method. For this case, the basis function method fails due to hyperbolic terms in these formulations growing exponentially. The formulations also include a secular term, not present in Cooker’s formulation. For Chapter 5, the wave hits the wall in a horizontal direction and for Chapter 6, the wave hits beneath the deck in a vertical direction. These problems are important for offshore structures where providing adequate freeboard for decks contributes very significantly to the cost of the structure. Chapter 7 looks at what happens when we have a vertical baffle. The mathematical formulation and the boundary conditions for four cases of baffles which have different positions are presented in this chapter. We use a basis function method to solve the mathematical formulation, and total impulse and moment impulse are investigated for each problem. These problems are not, perhaps, very relevant to coastal structures. However, they are pertinent to wave impacts in sloshing tanks where baffles are used to detune the natural tank frequencies away from environmental driving frequencies (e.g ship roll due to wave action) and to damp the oscillations by shedding vortices. They also provide useful information for the design of oscillating water column wave energy devices. Finally, conclusions from the research and recommendations for future work are presented in Chapter 8.
29
Sander, Tavallaey Shiva. "Wave propagation in sandwich structures /." Stockholm, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3088.
30
Shareef, Mohamed. "Wave overtopping of coastal structures." Thesis, University of Liverpool, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.415752.
31
Swift, R. H. "Wave forces on coastal structures." Thesis, Open University, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.371247.
32
Topliss, Margaret E. "Water wave impact on structures." Thesis, University of Bristol, 1994. http://hdl.handle.net/1983/2fa7ba69-7867-4cd0-8b3a-de4de97f98db.
33
Roe, Eric Allen. "Wave Propagation in Complex Structures." OpenSIUC, 2010. https://opensiuc.lib.siu.edu/theses/380.
Анотація:
The main focus of this research was to gain an understanding as to how waves propagate through structures. Lamb's Problem was studied on an isometric half plane, where numerical results were obtained. The calculated wavefronts for this problem were in agreement to the numerical results. When a distributed pressure is applied on an isometric half plane, after a long period of time, the wavefronts look as if a point force was applied on the half plane. Waves propagating through an orthotropic material were obtained numerically; it was found that Huygens' Principle cannot be used to calculate the wavefronts. The impact of spherical and cylindrical projectiles on glass plates was studied next. The waves introduced into the material were calculated using Finite Element Analysis, and compared to calculated wavefronts using Snell's Law, where they were found to be in agreement with one another. The effects of circular and square discontinuities were also studied, where a creeping wave that is produced after a wave propagates past a circular hole is explained. A sandwich beam was also modeled using FEA, where the wavefronts were obtained, and were found to be in agreement with calculated wavefronts. The displacement of the bottom layer of the sandwich beam was obtained numerically; it was found that the bending of the beam occurs at the same time as whether the middle layer is present or not.
34
Narisetti, Raj K. "Wave propagation in nonlinear periodic structures." Diss., Georgia Institute of Technology, 2010. http://hdl.handle.net/1853/39643.
Анотація:
A periodic structure consists of spatially repeating unit cells. From man-made multi-span bridges to naturally occurring atomic lattices, periodic structures are ubiquitous. The periodicity can be exploited to generate frequency bands within which elastic wave propagation is impeded. A limitation to the linear periodic structure is that the filtering properties depend only on the structural design and periodicity which implies that the dispersion characteristics are fixed unless the overall structure or the periodicity is altered.
The current research focuses on wave propagation in nonlinear periodic structures to explore tunability in filtering properties such as bandgaps, cut-off frequencies and response directionality. The first part of the research documents amplitude-dependent dispersion properties of weakly nonlinear periodic media through a general perturbation approach. The perturbation approach allows closed-form estimation of the effects of weak nonlinearities on wave propagation. Variation in bandstructure and bandgaps lead to tunable filtering and directional behavior. The latter is due to anisotropy in nonlinear interaction that generates low response regions, or "dead zones," within the structure.The general perturbation approach developed has also been applied to evaluate dispersion in a complex nonlinear periodic structure which is discretized using Finite Elements. The second part of the research focuses on wave dispersion in strongly nonlinear periodic structures which includes pre-compressed granular media as an example. Plane wave dispersion is studied through the harmonic balance method and it is shown that the cut-off frequencies and bandgaps vary significantly with wave amplitude. Acoustic wave beaming phenomenon is also observed in pre-compressed two-dimensional hexagonally packed granular media. Numerical simulations of wave propagation in finite lattices also demonstrated amplitude-dependent bandstructures and directional behavior so far observed.
35
Xu, Ruo Feng. "Miniaturized guided wave structures and applications." Thesis, University of Kent, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.529405.
36
Klepsvik, Jonny. "Nonlinear wave loads on offshore structures." Thesis, Massachusetts Institute of Technology, 1995. http://hdl.handle.net/1721.1/36062.
37
Bucca, Steven E. "Wave propagation in lossy waveguide structures." Thesis, Virginia Tech, 1989. http://hdl.handle.net/10919/44110.
Анотація:
In this thesis a numerical technique is developed determining the propagation constant
in waveguides and transmission lines. The technique accounts for both dielectric and conductor
losses in a guide having an arbitrary cross section and uses a full-wave solution process. A set of
coupled, vector integral equations which characterize the system are derived. The equations
enforce the necessary boundary conditions on the tangential electric and magnetic fields at the
boundaries separating the conductors and dielectrics.
The method of moments (MOM) technique is used to cast the equations into a
numerically solvable form. Computed results for various waveguide structures are compared to
known or perturbed results for three well-known structures. However, the program is more
general and may be applied to other cross-sections. Finally, possible future extensions of the
work is presented.Master of Science
38
Voisey, Ruth. "Multiple wave scattering by quasiperiodic structures." Thesis, University of Manchester, 2014. https://www.research.manchester.ac.uk/portal/en/theses/multiple-wave-scattering-by-quasiperiodic-structures(1c366ad1-443a-4667-9d03-db77487ab1d1).html.
Анотація:
Understanding the phenomenon of wave scattering by random media is a ubiquitous problem that has instigated extensive research in the field. This thesis focuses on wave scattering by quasiperiodic media as an alternative approach to provide insight into the effects of structural aperiodicity on the propagation of the waves. Quasiperiodic structures are aperiodic yet ordered so have attributes that make them beneficial to explore. Quasiperiodic lattices are also used to model the atomic structures of quasicrystals; materials that have been found to have a multitude of applications due to their unusual characteristics. The research in this thesis is motivated by both the mathematical and physical benefits of quasiperiodic structures and aims to bring together the two important and distinct fields of research: waves in heterogeneous media and quasiperiodic lattices. A review of the past literature in the area has highlighted research that would be beneficial to the applied mathematics community. Thus, particular attention is paid towards developing rigorous mathematical algorithms for the construction of several quasiperiodic lattices of interest and further investigation is made into the development of periodic structures that can be used to model quasiperiodic media. By employing established methods in multiple scattering new techniques are developed to predict and approximate wave propagation through finite and infinite arrays of isotropic scatterers with quasiperiodic distributions. Recursive formulae are derived that can be used to calculate rapidly the propagation through one- and two-dimensional arrays with a one-dimensional Fibonacci chain distribution. These formulae are applied, in addition to existing tools for two-dimensional multiple scattering, to form comparisons between the propagation in one- and two-dimensional quasiperiodic structures and their periodic approximations. The quasiperiodic distributions under consideration are governed by the Fibonacci, the square Fibonacci and the Penrose lattices. Finally, novel formulae are derived that allow the calculation of Bloch-type waves, and their properties, in infinite periodic structures that can approximate the properties of waves in large, or infinite, quasiperiodic media.
39
Molinaro, Nicholas Joseph. "The Two Point Correlation Structure of a Cylinder Wake." Thesis, Virginia Tech, 2017. http://hdl.handle.net/10919/78296.
Анотація:
In this study the complete four dimensional space time correlation function was measured in the wake of an untripped circular cylinder at a Reynolds number of 60 000. This correlation serves as the complete inflow boundary condition for an open rotor ingesting inhomogeneous turbulence. An important aspect of the turbulence ingestion problem is understanding how different inflow boundary conditions effect the sound produced by a rotor. In the present study the turbulence structure of two plane wakes were compared. Measurements completed by a previous study in the wake of a NACA 0012 airfoil were compared with the measurements completed by the present study in the wake of a cylinder. The mean flows of both plane wakes were found to be very similar, however the Reynolds stress profiles show that the cylinder wake is substantially more turbulent. The structures of the two-point correlation function in each wake are also similar, although the cylinder wake had greater maximum correlation values and was correlated at greater separations. The two-point correlation was used along with proper orthogonal decomposition to compute the average instantaneous velocity fields of both wake flows. These velocity fields represent the average eddy structures present in each wake flow. The eddy structure comparisons show that the structures in the cylinder wake are larger and better correlated at longer time delays.Master of Science
40
Hart, Douglas P. Acosta Allan J. Brennen Christopher E. "Cavitation and wake structure of unsteady tip vortex flows /." Diss., Pasadena, Calif. : California Institute of Technology, 1993. http://resolver.caltech.edu/CaltechETD:etd-03272007-131947.
41
Erdozain, Jack Jr. "Long wave infrared structured light." Thesis, Massachusetts Institute of Technology, 2019. https://hdl.handle.net/1721.1/121664.
Анотація:
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Thesis: M. Eng., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2019
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 69-72).
3D sensing technologies have been exploited for many applications in autonomous vehicles, manufacturing, and consumer products, however, existing techniques may suer in challenging conditions, where scattering due to fog, dust, or smoke is present. While light in the visible and near IR spectrum are heavily affected by by common scattering mediums, long-wave infrared (also known as thermal IR) experiences less scattering, especially when scattering particles are small. This thesis proposes and demonstrates a structured light scanning approach which operates in the long-wave IR Spectrum. We believe that structured light technique with long-wave IR leads to more robust 3D sensing in some challenging environments. In this thesis the conceptual framework behind the technology is presented, the technology is demonstrated, and testing of long-wave infrared projection is compared to the visible spectrum.
by Jack Erdozain Jr.
M. Eng.
M.Eng. Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science
42
Sander, Tavallaey Shiva. "Wave propagation in sandwich structure." Doctoral thesis, KTH, Vehicle Engineering, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3088.
43
Akkala, James Marcus. "Flow structure and performance of a flexible plunging airfoil." Thesis, University of Iowa, 2013. https://ir.uiowa.edu/etd/2433.
Анотація:
An investigation was performed with the intent of characterizing the effect of flexibility on a plunging airfoil, over a parameter space applicable to birds and flapping MAVs. The kinematics of the motion was determined using of a high speed camera, and the deformations and strains involved in the motion were examined. The vortex dynamics associated with the plunging motion were mapped out using particle image velocimetry (PIV), and categorized according to the behavior of the leading edge vortex (LEV). The development and shedding process of the LEVs was also studied, along with their flow trajectories. Results of the flexible airfoils were compared to similar cases performed with a rigid airfoil, so as to determine the effects caused by flexibility. Aerodynamic loads of the airfoils were also measured using a force sensor, and the recorded thrust, lift and power coefficients were analyzed for dependencies, as was the overall propulsive efficiency. Thrust and power coefficients were found to scale with the Strouhal number defined by the trialing edge amplitude, causing the data of the flexible airfoils to collapse down to a single curve. The lift coefficient was likewise found to scale with trailing edge Strouhal number; however, its data tended to collapse down to a linear relationship. On the other hand, the wake classification and the propulsive efficiency were more successfully scaled by the reduced frequency of the motion. The circulation of the LEV was determined in each case and the resulting data was scaled using a parameter developed for this specific study, which provided significant collapse of the data throughout the entire parameter space tested.
44
Chau, Fun-pang. "Numerical methods in wave loading of large offshore structures /." [Hong Kong] : University of Hong Kong, 1985. http://sunzi.lib.hku.hk/hkuto/record.jsp?B12315916.
45
Levine, Ross M. "Ultrasonic guided wave imaging via sparse reconstruction." Diss., Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/51829.
Анотація:
Structural health monitoring (SHM) is concerned with the continuous, long-term assessment of structural integrity. One commonly investigated SHM technique uses guided ultrasonic waves, which travel through the structure and interact with damage. Measured signals are then analyzed in software for detection, estimation, and characterization of damage. One common configuration for such a system uses a spatially-distributed array of fixed piezoelectric transducers, which is inexpensive and can cover large areas. Typically, one or more sets of prerecorded baseline signals are measured when the structure is in a known state, with imaging methods operating on differences between follow-up measurements and these baselines. Presented here is a new class of SHM spatially-distributed array algorithms that rely on sparse reconstruction. For this problem, damage over a region of interest (ROI) is considered to be sparse. Two different techniques are demonstrated here. The first, which relies on sparse reconstruction, uses an a priori assumption of scattering behavior to generate a redundant dictionary where each column corresponds to a pixel in the ROI. The second method extends this concept by using multidimensional models for each pixel, with each pixel corresponding to a "block" in the dictionary matrix; this method does not require advance knowledge of scattering behavior. Analysis and experimental results presented demonstrate the validity of the sparsity assumption. Experiments show that images generated with sparse methods are superior to those created with delay-and-sum methods; the techniques here are shown to be tolerant of propagation model mismatch. The block-sparse method described here also allows the extraction of scattering patterns, which can be used for damage characterization.
46
Beard, Malcolm David. "Guided wave inspection of embedded cylindrical structures." Thesis, Imperial College London, 2002. http://hdl.handle.net/10044/1/8849.
47
Khajooeizadeh, Arash. "Metal-insulator-semiconductor (MIS) slow-wave structures." Thesis, McGill University, 2006. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=99773.
Анотація:
Metal-insulator-semiconductors (MIS) are one of the most basic elements in the digital and microwave circuits. Theoretical and experimental investigation has shown that the dominant mode of propagation in MIS is not a quasi-TEM mode at all frequencies. It has been shown that an MIS transmission line is in fact a slow-wave structure in a certain frequency range. Slow-wave structures offer a large effective permittivity, therefore, can be employed to create large propagation delay and to reduce the guided wavelength. These characteristics can be utilized to design compact passive elements such as delay lines, phase shifters and filters. MIS structures can be easily fabricated using the current semiconductor technology. As well, they can be designed to operate in the slow-wave region for implementation of miniaturized passive components. In this thesis, first, an MIS interconnect is designed, fabricated and measured. Then, the transmission line characteristics are extracted from simulated and measured results. Subsequently, the MIS line parameters are used in designing a compact slow-wave meander line structure, which is compatible with silicon-based-packaging (SBP) solutions used in modern system-in-package/system-on-package (SIP/SOP) technologies.
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Oakes, Melanie Kathleen. "Violent sea wave impacts on coastal structures." Thesis, University of East Anglia, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.410085.
Анотація:
The damage caused to many coastal structures during storms is suspected to be due to very high pressures from steep and breaking water waves. In this thesis we restrict attention to block work seawalls with vertical faces, with the aim of interpreting the type of violent flows measured and photographed by E.S. Chan and W.K. Melville, in a study reported in Proc. R. Soc. Lond. A, 147: 95-131, 1988. We use two different approaches to model the impact of a sea wave against a coastal structure. The first approach involves applying pressure impulse theory, the integration of pressure over time, to the two dimensional model of a wave with a vertical face impacting a vertical wall. Fourier solutions are found for the pressure impulse, and convergent series are used to express the distribution of the pressure impulse on the wall and the bed. The location and value of the maximum pressure is demonstrated to be strongly dependent on the height of the impacting wave front. For the second approach we use potential flow theory applied to two-dimensional and axisymmetric flows in infinite depth. The two dimensional flow is also considered in finite depth. The velocity potential is prescribed on the free surface for an initial time and for finite depth the seabed forms a rigid boundary. We use a Maclaurin series in time for the velocity potential, where the series coefficients satisfy a Dirichlet problem with a flat free surface, and solve the boundary value problem analytically using a Greens function approach. In the two-dimensional infinite depth case, we also apply complex function theory when the boundary data is a rational function. Analytical results are achieved for all cases, and it is established that both the presence of the bed and the axisymmetric effects are significant in the predicted pressures. Comparisons with experimental results show some agreement
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Sharma, Rahul. "Dynamic wave-modes in solar chromospheric structures." Thesis, University of Sheffield, 2017. http://etheses.whiterose.ac.uk/21212/.
Анотація:
Sun’s outer atmosphere is a million degree hotter than it’s visible surface, which is not understood with any of the known laws of thermodynamics and remains an intriguing problem for the astrophysics in general. It is now believed that most of the energy dissipation phenomenon occurs at the interface-region, which is a highly dynamic, gravitationally stratified, nonlinear, inhomogeneous environment, where the plasma-β varies from large, across unity, to very small. Previous studies, suggests that energy from lower solar atmosphere is transported up higher in corona by waves and oscillations through small-scale thin magnetic flux-tube structures, that populate the interface-region. This thesis primarily focuses on the identification and the understanding of the coupled linear/nonlinear wave-modes that are confined in the observed flux-tube structures. High-resolution imaging-spectroscopy data from the ground-based telescope is used to get an unprecedented view of the spicule structures within the complex chromospheric environment. Innovative analysis techniques were developed, for the first time, to investigate the three-dimensional (3D) ensemble of the observed kinematic components. The subsequent analysis at both, pixel- and the tube-scale, provided important insight into the nonlinear evolution of the coherent wave-modes, along with their consequent affects on the ambient solar atmosphere. Key findings include, the accurate interpretation of the observed spectroscopic (Doppler) velocity profiles, which were akin for both torsional Alfvén and kink wave modes. It was shown that the kinematic behavior of the kink wave-mode is not entirely transverse, but also has associated rotational component, due to displaced surrounding plasma. Also, the various observed kinematic components (transverse, cross-sectional width, azimuthal torsion) which, till-date, were observed independent to each other were found strongly coupled, with definitive phase-relationships. Furthermore, the non- helical evolution of the coupled dynamical components across the interface region, was found, due to the presence of a plethora of nonlinear wave phenomenon. The analysis, presented in this thesis, on the dynamics in the solar chromosphere, can provide the vital clues and insight into the mechanisms responsible for the transfer and dissipation of energy.
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Li, Tian. "Spin wave propagation in ferromagnetic nano-structures." Doctoral thesis, Kyoto University, 2021. http://hdl.handle.net/2433/263500.