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Yassin, Menna. "Steady State Hydroplaning Risk Analysis and Evaluation of Unsteady State Effects." Scholar Commons, 2019. https://scholarcommons.usf.edu/etd/7990.
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Hydroplaning is a major concern on high speed roadways during heavy rainfall events. Hydroplaning tools are widely used by designers to reduce their roadway’s hydroplaning potential, therefore reducing the possibilities of severe crashes. This dissertation presents two methodologies for improving the prediction of hydroplaning potential.
The first phase focused on improving an existing widely used software called PAVDRN. Using multiple datasets from the Florida Department of Transportation, the author filtered the data using specific criteria to leave only truly dynamic hydroplaning crashes. The author then evaluated PAVDRN’s prediction capabilities and assessed its reliability in predicting a hydroplaning crash. Using past accident statistics, the author accounted for extraneous factors that are difficult to capture, such as driver behavior, and obtained probability factors for a more realistic estimate of hydroplaning risk on roadways. The second phase focused on improving the modeling technique used in hydroplaning prediction tools. Currently when assessing a roadway’s hydroplaning potential, the roadside drainage is not considered in the analysis. The author modeled a combined pavement-drainage system using a 1D/2D method to better capture the effects of roadside drainage, especially in the events of flooding. The methodology used in modeling successfully captures the backwater effects that are caused under critical flooding conditions. Lastly the author created a new tool (MY-PAVDTCH) to provide design engineers with updated waterfilm thickness values under roadside drainage flooded conditions.
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Zeiger, Matthew D. "The Dynamic Character of the Flow Over a 3.5 Caliber Tangent-Ogive Cylinder in Steady and Maneuvering States at High Incidence." Diss., Virginia Tech, 2014. http://hdl.handle.net/10919/25150.
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Although complex, inconsistent and fickle, the time-averaged flow over a stationary slender forebody is generally well-understood. However, the nature of unsteady, time-varying flows over slender forebodies - whether due to the natural unsteadiness or forced maneuvering - is not well-understood. This body of work documents three experimental investigations into the unsteadiness of the flow over a 3.5 caliber tangent-ogive cylinder at high angles of incidence. The goal of the investigations is to characterize the natural and forced flow unsteadiness, using a variety of experimental tools.
In the first investigation, flow data are collected over a stationary model in a water tunnel. Particle-Image Velocimetry (PIV) is employed to acquire time-dependent planes of velocity data with the model at several angles of attack. It is discovered that the asymmetric flow associated with the tangent-ogive forebody exhibits a large degree of unsteadiness, especially for data planes located far from the forebody tip. Vortex shedding of the type exhibited by a circular cylinder in crossflow is observed, but this shedding is skewed by the presence of the tip, the shedding process does not require equal periods of time from each side of the body, and this results in a time-averaged flowfield that is asymmetric, as expected. The rms values of the time-averaged velocity, as well as the turbulent kinetic energy and axial vorticity are calculated.
In the second investigation, surface pressure data are acquired from several circumferential rings of pressure ports located on two models undergoing ramp coning motions in two different wind tunnel facilities. The surface pressure data are integrated to determine the sectional yaw forces. Coning motions were performed at several different reduced frequencies, and pneumatic control actuation from the nose was employed. The chosen control actuation method used a small mass flow rate ejected very close to the forebody tip, so as to leverage the inherent convective instability. The data resulting from these tests were analyzed in order to determine how the coning motions affect the distribution of surface pressure and yaw forces, how quickly the flow reacts to the motion, and the extent of control authority of the pneumatic actuation. It was discovered that the yaw forces increase in the direction of the motion for small reduced frequencies, but in the direction opposite to the motion for large reduced frequencies. The effects of the motion tend to dominate the control method, at least for the reduced frequencies and setup tested in the low-speed wind tunnel. The results from the high-speed testing with transitional separation give a preliminary indication that the control method could have sufficient control authority when the reduced frequencies are low.
The third investigation involves tangent-ogive cylinder undergoing a pitching maneuver in a water tunnel. Laser-Doppler Velocimetry (LDV) is used in order to map out several planes of velocity data as the model is pitched. The LDV data is used to calculate vorticity and turbulent kinetic energy. Variables that are proportional to the flow asymmetry and proximity to the steady-state flow are defined. All of these variables are displayed as a function of time and space (where appropriate). The delay in the development of the asymmetry and the flow progression to the steady state are determined to be a function of pitch-axis location. The propagation velocity of the convective asymmetry is faster than expected, most likely because of the increased axial velocity in the vortex cores. Vortex breakdown of one of the vortices is observed, with loss of axial velocity and dilution of the vorticity over a large area. The cause of this phenomenon is not yet understood, but it is reminiscent of vortex breakdown over delta wings.Ph. D.
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Grooms, Daniel Douglas. "Optimization of hybrid dynamic/steady-state processes using process integration." [College Station, Tex. : Texas A&M University, 2006. http://hdl.handle.net/1969.1/ETD-TAMU-1752.
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Mishra, Bikash. "Model development of a polymer electrolyte membrane fuel cell to predict steady and unsteady behavior." Diss., Mississippi State : Mississippi State University, 2008. http://library.msstate.edu/etd/show.asp?etd=etd-08132008-103611.
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Hutton, Bridget Margaret. "Evaluation of a vibrating plate extraction column by application of steady state and unsteady state backflow models." Thesis, Stellenbosch : Stellenbosch University, 2001. http://hdl.handle.net/10019.1/52262.
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Thesis (MScEng)--University of Stellenbosch, 2001.ENGLISH ABSTRACT: Liquid-liquid extraction is a branch of solvent extraction that employs addition of an immiscible solvent, as a separating agent, to a liquid feed. Various types of equipment can be used, however if the process requires more than three stages, typically an extraction column, operated in a countercurrent manner would be employed. In order to scale-up and design a commercial extraction column, it is necessary to quantify the extraction system hydrodynamics and mass transfer characteristics. The principal objectives of countercurrently operated extraction columns concern the mass transfer rate and permissible throughput. The performance of a countercurrent extraction column can be adversely affected by axial mixing, which disturbs countercurrent plug flow. Various methods have been devised whereby it is possible to evaluate the performance of a column extraction and scale it up to ensure that the commercial operation achieves the same separation achieved on a pilot scale. Classical axial dispersion models allow quantification of axial mixing and mass transfer rates. Two Backflow models were derived to describe the performance of a Vibrating Plate Extraction (VPE) Column, one for steady state and one for unsteady state operation. The steady state model consisted of a series of simultaneous equations, which were solved using the Excel solver function. The unsteady state model consisted of 54 ordinary differential equations, which were solved stagewise using a fourth order Runge Kutta procedure. The steady state model was based on a dissociation extraction process, whereby meta-cresol (mcresol) was separated from para-hydroxy-benzaldehyde (PHB). The process used the differing de-protonation constants of the two components and the fact that the solubility of the ionic species of each was low in the organic solvent. The extraction system was quantified using a combination of acid-base and extraction theory. Experimentally determined concentration profiles, measured along the length of the column, were force fitted to the model, thereby allowing determination of the model parameters. The mass transfer coefficients ranged between 0.0098 and 0.189 Imin, and it was found that backmixing of the dispersed phase was negligible, while that of the continuous phase was low (varying between 0 and 0.3). The unsteady state model, used to describe the dynamic response of a VPE, was based on a system whereby tert-butyl hydroquinone (TBHQ) was recovered from a purge stream. Conductivity measurements of the raffinate were used to determine the residence time distribution in the column, and hence allowed determination of the extent of axial mixing. It was preferable that the column be operated with minimum settler volumes, otherwise buffering in the settlers occurred, thereby masking axial mixing effects. This method did not facilitate accurate determination of backmixing, at least two other conductivity measurements in the column needed to be measured.
AFRIKAANSE OPSOMMING: Vloeistof-vloeistofekstraksie is 'n vertakking van oplosmiddelekstraksie wat gebruik maak van die toevoeging van 'n onmengbare oplosmiddel as 'n skeidingagent tot die vloeistofvoer. Verskeie tipes apparaat kan gebruik word, maar as die proses meer as drie stadia vereis, sal 'n ekstraksiekolom, in teenstroom bedryf, tipies gebruik word. Om dit moontlik te maak om 'n kommersiele ekstraksiekolom te skalleer en te bedryf, moet die ekstraksiesisteem se hidrodinamika en massa-oordragkarakteristieke gekwantifiseer word. Die hoof doelwitte van ekstraksiekolomme wat teenstroom bedryf word, gaan om die massa-oordrag en toelaatbare deurset. Die skeidingsdoeltreffendheid van 'n teenstroom ekstraksiekolom kan nadelig beinvloed word deur aksiale vermenging, wat teenstroom propvloei versteur. Verskeie metodes is voorgestel wat dit moontlik maak om die doeltreffendheid van 'n ekstraksiekolom te evalueer en te verseker dat dieselfde skeiding verkry word vir 'n kornmersiele aanleg as vir 'n loodsaanleg. Klassieke aksiale dispersiemodelle laat kwantitatiewe berekening van aksiale vermenging en massa-oordragtempos toe. Twee terugvloeimodelle is afgelei om die werksverrigting van 'n Vibrerende Plaat Ekstraksiekolom (VPE) te beskryf. Die gestadidge toestand model bestaan uit 'n stelsel gelyktydige vergelykings wat opgelos is d.m.v. Excel. Die ongestadige toestand model bestaan uit 54 gewone differensiaalvergelykings, wat stapsgewys opgelos is d.m.v. die vierde orde Runge-Kutta metode. Die gestadigde teostand model is gebaseer op 'n dissosiasie ekstraksieproses, waardeur m-kresol geskei is van p-hidroksiebensaldehied (PHB). Die proses maak gebruik van die verskillende protoneringskonstantes van die twee verbindings en die feit dat die oplosbaarheid van beide die ioniese spesies laag is in die organiese oplosmiddel. Die ekstraksiestelsel is gekwantifiseer deur gebruik te maak van 'n kombinasie van suur-basis- en ekstraksieteorie. Die model is gepas op eksperimenteel bepaalde konsentrasieprofiele, gemeet langs die lengte van die kolom. Die massa-oordragkoeffisiente het waardes aangeneem tussen 0.0098 en 0.189 Imin en daar is gevind dat die terugvermenging van die verspreide fase weglaatbaar was, terwyl die van die kontinue fase laag was (tussen 0 en 0.3). Die ongestadige toestand model wat gebruik is om die dinamiese respons van die VPE te beskryf, is gebaseer op 'n stelsel waar tert-butielhidrokinoon (TBHQ) herwin is vanuit 'n bloeistroom. Geleidingsmetings van die raffinaat is gebruik om die residensietydverspreiding in die kolom te bepaaI en het derhalwe toegelaat dat die mate van aksiale vermenging bepaaI kon word. Die kolom moet by voorkeur met minimale skeiervolumes bedryf word, anders is daar 'n buffereffek in die skeiers, wat die aksiale vermenging verskuiI. Hierdie metode Iaat nie die akkurate bepaling van terugvermenging toe nie en minstens twee ander geleidingsmetings in die kolom was benodig.
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McDougall, Steven Robert. "The application of network modelling techniques to steady-and-unsteady-state multiphase flow in porous media." Thesis, Heriot-Watt University, 1994. http://hdl.handle.net/10399/1406.
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Elsheikh, Mutasim Mohamed. "A Numerical and Experimental Investigation of Flow Induced Noise In Hydraulic Counterbalance Valves." Scholar Commons, 2015. http://scholarcommons.usf.edu/etd/5683.
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The main objective of this study is to explore the complex fluid flow phenomena that result in the generation of a high frequency noise in counterbalance valves through an experimental and numerical investigation of the flow. Once the influence of the different components involved in noise generation is established, a secondary objective is the introduction of design modifications that eliminate the undesired effect without altering the operation envelope or the performance of the valve.
A hydraulic test bench was used to carry out an experimental investigation of the noise generation process. A computer based data acquisition system was used to record pressure fluctuations, flowrates and hydraulic oil temperatures in a production valve under a variety of operational conditions. Extensive experimental measurements and numerical modeling lead to the hypothesis that noise generation is the result of an acoustic resonance triggered by shear layer instability at the valve inlet. The pressure gradients developed when the shear layer entrains the stagnant fluid in the valve main cavity cause the layer to become unstable and oscillate. The oscillation frequency will depend on a great number of factors such as valve geometry, pressure and velocity gradients and the density and viscosity of the fluid. It is postulated that the observed noise is generated when this frequency matches one of the resonant frequencies of the valve cavity.
The proposed mechanism is theoretically poorly understood and well beyond simplified analysis, its accurate numerical simulation is computational very intensive requiring sophisticated CFD codes. The numerical investigation was carried out using STAR–CCM+, a commercially available CFD code featuring 3-D capabilities and sophisticated turbulence modeling. Streamline, pressure, velocity-vector and velocity-scalar plots were obtained for several valve configurations using steady and unsteady state flow simulations.
An experimental and numerical analysis of an alternative valve geometry was carried out. Experimental results demonstrated a greatly reduced instability range. The numerical analysis of the unsteady behavior of the shear-layer streamlines for both valves yielded results that were compatible with the experimental work.
The results of this investigation promise a great positive impact on the design of this type of hydraulic valves.
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Green, Henry Gregory. "Characteristics of steady and unsteady jets." Thesis, Imperial College London, 1987. http://hdl.handle.net/10044/1/38338.
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Abu-Khiran, E. "Steady and unsteady flow losses in automotive exhaust." Thesis, Queen's University Belfast, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.268980.
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Khezzar, Lyes. "Experiments with steady and unsteady confined turbulent flows." Thesis, Imperial College London, 1987. http://hdl.handle.net/10044/1/46926.
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Hart, James R. "Longitudinal dispersion in steady and unsteady pipe flow." Thesis, University of Warwick, 2013. http://wrap.warwick.ac.uk/57725/.
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The longitudinal dispersion coefficient describes the change in characteristics of a solute cloud, as it travels along the longitudinal axis of a flow. Within potable water networks, it is important to be able quantify this parameter, to predict the fate of solutes introduced into the network. Current water quality models assume steady, highly turbulent flow [Tzatchkov et al., 2009]. However, this assumption is not valid for the network's periphery, where water leaves the main network and comes to the point of consumption. Here, the flow can be both unsteady and turbulent, transitional or laminar [Buchberger et al., 2003; Blokker et al., 2008, 2010]. Taylor [1954] proposed a now classical expression to predict the longitudinal dispersion coefficient within steady, turbulent pipe flow. However, experimental data has shown significant deviation from his prediction for Re < 20000. Within the present work, new experimental data is presented for steady and unsteady flows for a range of discharges corresponding to 2000 < Re < 50000. From this experimental work, results describing the mixing processes through steady and unsteady turbulent and transitional pipe flow are presented, as well as an explana- tion as to why Taylor's theory fails to predict experimental data for Re < 20000. In addition, a simple numerical model is proposed for steady flow for 2000 < Re < 50000. The model extends Taylor's analysis to predict the longitudinal disper- sion coefficient in a manner more consistent with experimental data for Re < 20000. Furthermore, the model is extended for use within unsteady flow.
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Wintenberger, Eric Hornung H. G. "Application of steady and unsteady detonation waves to propulsion /." Diss., Pasadena, Calif. : California Institute of Technology, 2004. http://resolver.caltech.edu/CaltechETD:etd-04222004-121013.
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Hajilouy-Benisi, A. "Radial inflow turbine : performance characteristics under steady and unsteady flow." Thesis, Imperial College London, 1993. http://hdl.handle.net/10044/1/7426.
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Tewnion, Angus Jamieson. "Steady and unsteady flow simulations in a circular closed conduit." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp04/mq20810.pdf.
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Chen, Hua. "Steady and unsteady performance of vaneless casing radial-inflow turbines." Thesis, University of Manchester, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.291004.
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Cabrita, Pedro Miguel. "Steady and unsteady features of twin-jet STOVL ground effects." Thesis, Cranfield University, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.426157.
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Granlund, Kenneth Ove. "Steady and Unsteady Maneuvering Forces and Moments on Slender Bodies." Diss., Virginia Tech, 2009. http://hdl.handle.net/10919/26912.
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Forces and moments have been measured on slender bodies in both static angle conditions as well as rapid time-dependent large amplitude maneuvers with the Dynamic Pitch Plunge Roll (DyPPiR) apparatus.
Lateral and transversal forces as well as all three moments have been measured at static angles of attack and sideslip and unsteady pitch ramp maneuvers at a fixed point of rotation at the quarter length of the body. The two bodies are the DARPA Suboff generic submarine shape and a non-Body-of-Revolution scalene ellipsoid with a constant cross-section midbody. An analytical two-mode equation has been shown to accurately describe the normal force and pitch moment as well as side force and yaw moment for the ellipsoid body. It is based on the observation that the center of pressure for the cross-flow contribution is at a fixed location. For the Suboff body, this assumption is invalid. Unsteady forces and moments can be measured to a very small magnitude of uncertainty and were found to differ from steady forces and moments at the time-instantaneous flow angle during the motion.
Ph. D.
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Bekiaris, Nikolaos Morari Manfred Morari Manfred. "Multiple steady states in distillation /." Diss., Pasadena, Calif. : California Institute of Technology, 1995. http://resolver.caltech.edu/CaltechETD:etd-09122007-075846.
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Hakeem, Imtiaz. "Steady and unsteady performance of mixed-flow turbines for automotive turbochargers." Thesis, Imperial College London, 1995. http://hdl.handle.net/10044/1/7402.
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Grigore, Daniel. "Analysis of steady and unsteady flows past fixed or oscillating wings." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape8/PQDD_0024/MQ50613.pdf.
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Abdo, Mohammed. "Analysis of steady and unsteady flows past fixed and oscillating airfoils." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2000. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape4/PQDD_0032/MQ64209.pdf.
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Abdo, Mohammed. "Theoretical and computational analysis of airfoils in steady and unsteady flows." Thesis, McGill University, 2004. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=84871.
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This dissertation studies three aspects of airfoil flows: (i) second-order theoretical solutions of airfoils in steady flows; (ii) unsteady solutions for oscillating flexible airfoils; and (iii) numerical analysis of airfoil flows at low Reynolds numbers.The first part presents simple and efficient analytical solutions in closed form for the velocity and pressure distributions on airfoils of arbitrary shapes in steady flows, which are obtained using special singularities in the expression of the fluid velocity. A second-order accurate method is first developed for airfoils in inviscid incompressible flows to simultaneously solve the symmetric and anti-symmetric flow components defined by coupled boundary conditions. Then, the method is extended to take into account the viscous and compressibility effects on the pressure distribution. The resulting solutions were found to be in very good agreement with the available exact solutions (for specific airfoils), and with numerical and experimental results at various Mach and Reynolds numbers and moderate angles of attack.
The second part presents a new method of solution for the analysis of unsteady incompressible flows past oscillating rigid and flexible airfoils. The method has been successfully validated by comparison with the results obtained by Theodorsen and by Postel and Leppert for rigid airfoil and aileron oscillations in translation and rotation. The aerodynamic stiffness, damping and virtual mass contributions are specifically determined, as required in the aeroelastic studies. In all cases studied, this method led to very efficient and simple analytical solutions in closed form.
The third part presents an efficient numerical method for the incompressible flows past airfoils at low Reynolds numbers, which are of interest for micro-aircraft applications. The present analysis is based on a pseudo-time integration method using artificial compressibility to accurately solve the Navier-Stokes equations. Solutions are obtained with this method for airfoils at various incidences and very low Reynolds numbers between 400 and 6000. A detailed analysis is presented for the influence of the Reynolds number, incidence and airfoil shape on the pressure distribution, lift and drag coefficients. The flow separation is especially studied; the separation and reattachment positions are compared for various airfoil shapes, incidences and Reynolds numbers.
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Grigore, Daniel. "Analysis of steady and unsteady flows past fixed or oscillating wings." Thesis, McGill University, 1998. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=21296.
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In the last decades, several numerical methods have been developed for the study of steady and unsteady flows past fixed or oscillating wings. Several studies were also conducted to analyze the structural oscillations of wings subjected to aerodynamic loading and to explore the possibilities of suppressing the aeroelastic oscillations of the wings, for example by using piezoelectric strips. However, it was difficult to use efficiently these numerical methods for the analysis of wing oscillations including the effect of the vibration-suppressing elements.This work presents an analytical method for the analysis of steady and unsteady flows past fixed or oscillating wings, which may be efficiently used for complex problems involving the structural dynamics analysis of the wing. The method is first validated for steady flows past fixed wings. Then results are presented for unsteady flows past oscillating wings in comparison with previous results.
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Bennetts, Tony Andrew. "Local scour around bridge piers under steady and unsteady flow conditions." Thesis, University of Hertfordshire, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.394076.
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Redolfi, Marco. "Sediment transport and morphology of braided rivers : steady and unsteady regime." Thesis, Queen Mary, University of London, 2015. http://qmro.qmul.ac.uk/xmlui/handle/123456789/8857.
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Braided rivers are complex, fascinating fluvial pattern, which represent the natural state of many gravel and sand bed rivers. Both natural and human causes may force a change in the boundary conditions, and consequently impact the river functionality. Detailed knowledge on the consequent morphological response is important in order to define management strategies which combine different needs, from protection of human activities and infrastructures to preservation of the ecological and biological richness. During the last decades, research has made significant advance to the description of this complex system, thanks to flume investigations, development of new survey techniques and, to a lesser extent, numerical and analytical solutions of mathematical models (e.g. Ashmore, 2013). Despite that, many relevant questions, concerning the braided morphodynamics at different spatial and temporal scales (from the unit process scale, to the reach scale, and eventually to the catchment scale) remain unanswered. For example, quantitative analysis of the morphological response to varying external controls still requires investigation and needs the definition of suitable, stage-independent braiding indicators. In addition, the morphodynamics of the fundamental processes, such as bifurcations, also needs further analysis of the driving mechanisms. General aim of the present study is to develop new methods to exploit, in an integrated way, the potential of the new possibilities offered by advanced monitoring techniques, laboratory models, numerical schemes and analytical solutions. The final goal is to fill some gaps in the present knowledge, which could ultimately provide scientific support to river management policies. We adopted analytical perturbation approaches to solve the two-dimensional shallow water model; we performed laboratory simulations on a large, mobilebed flume; we analysed existing topographic measurements from LiDAR and Terrestrial Laser scanning Devices; and we simulated numerically the river hydrodynamics. Within each of the six, independent, research chapters, we interconnected results from the different approaches and methodologies, in order to take advantage of their potential. Summarising, the more relevant and novel outcomes of the present work can be listed as follows:(i) We explored the morphological changes during a sequence of flood events in a natural braided river (Rees River, NZ) and we proposed a morphological method to assess the sediment transport rate. In particular we propose a semi-automatic method for estimating the particles path-length (Ashmore and Church, 1998) on the basis of the size of the deposition patches, which can be identified on the basis of DEM of differences. Comparison with results of numerical simulation confirmed that such an approach can reproduce the response of the bedload rate to floods of different duration and magnitude. (ii) We developed a new indicator of the reach-scale morphology and, on the basis of existing laboratory experiments, we explored its dependence, under regime conditions, to the controlling factors: slope, discharge, confinement width, grain size. In spite of its synthetic nature, this simple indicator embeds the information needed to estimate the variability of the Shield stress throughout the braided network, and consequently enables to assess the transport-rate and its variation with the driving discharge. (iii) We investigated, through flume experiments, the effect of the flow unsteadiness on the sediment transport in a braided river. This is possible only by following a statistical approach based on multiple repetitions of the same flow hydrograph. Results revealed that for confined network an hysteresis of the bedload response occurs, which leads to higher sediment transport during increasing flow, whereas relatively unconfined networks always show quasi-equilibrium transport rates. (iv) A second set of laboratory experiments provided information on the morphodynamics of a braided network subject to variations of the sediment supply. We proposed a simple diffusive model to quantify the evolution of the one-dimensional bed elevation profile. Such simple approach, albeit having a limited range of practical applications, represents the first attempt to quantify this process and enables to study the relevant temporal and spatial scales of the phenomenon. (v) We solved analytically the two-dimensional morphodynamic model for a gravel-bed river bifurcation. This furnishes a rigorous proof to the idea proposed by Bertoldi and Tubino (2007) to interpret the morphological response of bifurcation in light of the theory of the morphodynamic influence. The analytical approach enables to investigate the fundamental mechanics which leads to balance, and unbalance, configurations and, from a more practical point of view, allows for a better prediction of the instability point than the existing 1D models (e.g. Bolla Pittaluga et al., 2003).
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Rice, James Quackenbush. "Investigation of a two-dimensional hydrofoil in steady and unsteady flows." Thesis, Massachusetts Institute of Technology, 1991. http://hdl.handle.net/1721.1/27996.
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Wang, Le. "Bedload sediment transport and bed evolution in steady and unsteady flows." Thesis, Heriot-Watt University, 2016. http://hdl.handle.net/10399/3099.
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Until recently, bedload sediment transport has generally been investigated under steady flow conditions, which does not represent the typical sediment transport scenarios observed within natural fluvial channels. These channels often experience significant variations in bedload transport and bed morphological evolution under unsteady flow conditions, especially during flood hydrograph events. At present, however, there is a distinct lack of understanding of the interaction between hydrograph flow properties and the variability in bedload transport and bed evolution. Within the current study, different series of parametric experiments were conducted in two glass-walled, tilting flume facilities with the main aim of investigating the response of uniform and graded sediment beds to a range of different unsteady flow hydrograph conditions, and compare this to the corresponding response under equivalent steady flow conditions. The novelty of this research against previous studies is highlighted in terms of (i) the identification of the individual influence of hydrograph-related parameters (shape , unsteadiness HG and total water work Wk) on unsteady sediment transport characteristics (e.g. hysteresis and temporal lag) and the resulting bed evolution; (ii) the use of natural-shaped hydrographs (single and double peak) rather than stepped, triangular or trapezoidal hydrographs; (iii) the direct comparison between bedload sediment transport yields and corresponding bed evolution under unsteady hydrograph and equivalent (volume) steady flows; and (iv) enhanced understanding of influence of intergranular effects (e.g. particle exposure and/or hiding) on observed bedload sediment transport behaviour in unsteady and equivalent steady flows. Experiments conducted with both uniform (course sand, d = 1.95mm) and graded (fine gravel, d50 = 2.64mm, 5.00mm) beds not only confirm the clockwise and counter-clockwise hysteresis in measured bed load transport rates, but also reveal no/mixed hysteresis patterns as a transitional condition between these clockwise and counter-clockwise hystereses for bedload transport measured under unsteady flow hydrographs. For graded sediment transport, the intergranular effect on temporal lag and hysteresis patterns for transport of classified fine, medium and coarse size classes within graded (unimodal, bimodal and fine-grained) sediment beds in unsteady flow hydrographs is also revealed. Specifically, the coarse size class tends to respond preferentially to the changing flow and exhibits clockwise hysteresis in general; whereas the fine size class tends to become more active later and demonstrates reduced clockwise, no/mixed or counter-clockwise hysteresis. Corresponding temporal variations and hysteresis patterns in the median db50 size of the bedload transport load are also observed, with the db50 value generally reaching its peaking value during the rising hydrograph limb and indicating a clockwise hysteresis over the hydrograph duration. Bedforms (i.e. dunes and alternate bars) are found to develop only within uniform sediment bed (d = 1.95mm) under both steady and unsteady flows tested. A relevant analysis of the formative conditions under which these bedforms develop is presented. By contrast, no bedforms are observed to form within graded sediment beds; only slight bed evolution (e.g. bed aggradation up to +5mm at upstream) is identified for fine-grained sediment bed (d50 = 2.64mm) runs. Additionally, the median size of the bed surface ds50 is shown to coarsen during the antecedent flow conditions within all graded sediment beds (compared to d50 in specified particle size distributions). This surface coarsening persists over the duration of both steady and unsteady flow events tested in current study [e.g. run-averaged ds50 values: (i) unimodal grading, ds50 = 6.51mm (> d50 = 5mm); (ii) bimodal grading, ds50 = 6.74mm (> d50 = 5mm); and (iii) fine-grained grading, ds50 = 3.62mm (> d50 = 2.64mm)]. The temporal variations in the median bedload size db50 (< ds50 throughout) for both the steady and unsteady flow runs, are also responsible for explaining the development and persistence of the surface armour layer. Bedload transport and bed evolution for uniform and graded sediment beds under steady flows are measured to provide a benchmark for direct comparison with equivalent bedload rates and yields, bedform development, and variations in bedload and bed surface grain size composition under unsteady flows. The difference in effects of unsteady and steady flow is found to be significant for bedload rate (with the unsteady-steady bedload ratio Kq = 0.23 – 14.95), bedload yields (i.e. yield ratio us = 1.11 – 13.56), bedforms (i.e. dunes, alternate bars and etc.) and bedload composition (i.e. db50 ratio Kb = 0.38 – 1.56), but less significant for bed surface composition (i.e. ds50 ratio Ks = 0.92 – 1.33). Further, the individual influence of hydrograph related parameters (shape , unsteadiness HG, total water work Wk) on bedload yields, yield distribution over the rising and falling two limbs of single-peaked hydrographs, and on of bedform characteristics (i.e. type, dimensions) is also evaluated. In more complex flow hydrographs, such as double-peaked hydrographs, the equivalent bedload transport and bed evolution characteristics are also derived. Finally, the presence of specific grain size classes within the graded sediment beds and their influence on the transport of other six classes is considered from comparison of sediment transport characteristics for (i) the unimodal and bimodal sediment mixtures (with the same d50 = 5.0mm), and (ii) the uniform (d = 1.95mm) and fine-grained sediment mixture (d50 = 2.64mm), which arise from relative grain exposure and sheltering effects under the same steady and unsteady flow events. An idealised, theoretical analysis of the hydrodynamic forces and moments associated with these potential impacts is provided. Most importantly, the transport of fine particles is found to vary depending upon the presence state of coarser particle sizes in the surface layer (i.e. stationary or mobile to varying rates), which reveals the mechanism for selective fractional transport within the graded sediment bed under equivalent steady and unsteady hydrograph flow conditions.
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Whitfield, Cindy Carol. "Steady and Unsteady Force and Moment Data on a DARPA2 Submarine." Thesis, Virginia Tech, 1999. http://hdl.handle.net/10919/34333.
Full textAbstract:
Steady and unsteady force and moment experiments were conducted in the Virginia Tech Stability wind tunnel using the Dynamic Plunge-Pitch-Roll (DyPPiR) model mount to perform rapid time-dependent,high-excursion maneuvers. The experiments were performed for a DARPA2 submarine model using three widely spaced 2-force-component loadcells and three tri-axial accelerometers to extract the aerodynamic loads.The DARPA2 model was tested with different body configurations in two different test sections. The body configurations for both the steady and unsteady experiments were the bare body hull, body with sail, body with stern appendages, and body with sail and stern appendages. Tests were done using trips on the bow and sail and with no trips. The bare hull configuration with no trips was the only body configuration tested in the six-foot-square test section with solid walls. All body configurations were tested in the six-foot-square test section with slotted walls that were used to reduce the blockage effects produced by the DyPPiR and model.
The steady experiments were performed over a range of angles of attack and roll positions. Data were acquired through the series of angles the body encountered during the unsteady testing (-26° < ± <+26° ). The data for the tripped bare hull gave symmetric results while the data acquired for the bare hull with no trips did not. In the unsteady experiments the model was pitched in ramp maneuvers about the 1/4 chord location of the sail from 0° to -25° and from +25° to 0° in 0.3 seconds. Sine wave maneuvers at 3 Hz were also performed, plunging the model up and down with an amplitude of ±0.375 inches. The steady data agreed within uncertainties with previous data that were limited to the David Taylor Research Center (DTRC). There was a higher level of confidence in the steady data taken with trips due to the symmetry of the data. Effects of the sail and/or stern appendages were studied using the steady and unsteady data, but no quantitative value could be calculated due to the uncertainties. The unsteady data
were modeled with a quasi-steady time-lag model, and all the unsteady data were found to lead the quasi-steady data. The unsteady data did have oscillations, but the overall aerodynamic trend was still present. The uncertainties were too large to discuss effects of any appendages, however.
Master of Science
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Redolfi, Marco. "Sediment transport and morphology of braided rivers: steady and unsteady regime." Doctoral thesis, Università degli studi di Trento, 2014. http://hdl.handle.net/11572/314279.
Full textAbstract:
Braided rivers are complex, fascinating fluvial pattern, which represent the natural state of many gravel and sand bed rivers. Both natural and human causes may force a change in the boundary conditions, and consequently impact the river functionality. Detailed knowledge on the consequent morphological response is important in order to define management strategies which combine different needs, from protection of human activities and infrastructures to preservation of the ecological and biological richness. During the last decades, research has made significant advance to the description of this complex system, thanks to flume investigations, development of new survey techniques and, to a lesser extent, numerical and analytical solutions of mathematical models (e.g. Ashmore_2013). Despite that, many relevant questions, concerning the braided morphodynamics at different spatial and temporal scales (from the unit process scale, to the reach scale, and eventually to the catchment scale) remain unanswered. For example, quantitative analysis of the morphological response to varying external controls still requires investigation and needs the definition of suitable, stage-independent braiding indicators. In addition, the morphodynamics of the fundamental processes, such as bifurcations, also needs further analysis of the driving mechanisms. General aim of the present study is to develop new methods to exploit, in an integrated way, the potential of the new possibilities offered by advanced monitoring techniques, laboratory models, numerical schemes and analytical solutions. The final goal is to fill some gaps in the present knowledge, which could ultimately provide scientific support to river management policies. We adopted analytical perturbation approaches to solve the two-dimensional shallow water model; we performed laboratory simulations on a large, mobile-bed flume; we analysed existing topographic measurements from LiDAR and Terrestrial Laser scanning Devices; and we simulated numerically the river hydrodynamics. Within each of the six, independent, research chapters, we interconnected results from the different approaches and methodologies, in order to take advantage of their potential. Summarising, the more relevant and novel outcomes of the present work can be listed as follows: 1) We explored the morphological changes during a sequence of flood events in a natural braided river (Rees River, NZ)and we proposed a morphological method to assess the sediment transport rate. In particular we propose a semi-automatic method for estimating the particles path-length (Ashmore and Church, 1998) on the basis of the size of the deposition patches, which can be identified on the basis of DEM of differences. Comparison with results of numerical simulation confirmed that such an approach can reproduce the response of the bedload rate to floods of different duration and magnitude. 2) We developed a new indicator of the reach-scale morphology and, on the basis of existing laboratory experiments, we explored its dependence, under regime conditions, to the controlling factors: slope, discharge, confinement width, grain size. In spite of its synthetic nature, this simple indicator embeds the information needed to estimate the variability of the Shield stress throughout the braided network, and consequently enables to assess the transport-rate and its variation with the driving discharge. 3) We investigated, through flume experiments, the effect of the flow unsteadiness on the sediment transport in a braided river. This is possible only by following a statistical approach based on multiple repetitions of the same flow hydrograph. Results revealed that for confined network an hysteresis of the bedload response occurs, which leads to higher sediment transport during increasing flow, whereas relatively unconfined networks always show quasi-equilibrium transport rates. 4) A second set of laboratory experiments provided information on the morphodynamics of a braided network subject to variations of the sediment supply. We proposed a simple diffusive model to quantify the evolution of the one-dimensional bed elevation profile. Such simple approach, albeit having a limited range of practical applications, represents the first attempt to quantify this process and enables to study the relevant temporal and spatial scales of the phenomenon. 5) We solved analytically the two-dimensional morphodynamic model for a gravel-bed river bifurcation. This furnishes a rigorous proof to the idea proposed by Bertoldi and Tubino (2007) to interpret the morphological response of bifurcation in light of the theory of the morphodynamic influence. The analytical approach enables to investigate the fundamental mechanics which leads to balance, and unbalance, configurations and, from a more practical point of view, allows for a better prediction of the instability point than the existing 1D models (e.g. Bolla Pittaluga et al., 2003).
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McLain, Brian K. "Steady and unsteady aerodynamic flow studies over a 1303 UCAV configuration." Thesis, Monterey, California : Naval Postgraduate School, 2009. http://edocs.nps.edu/npspubs/scholarly/theses/2009/Sep/09Sep%5FMcLain.pdf.
Full textAbstract:
Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, September 2009.Thesis Advisor(s): Chandrasekhara, M. S. "September 2009." Description based on title screen as viewed on November 5, 2009. Author(s) subject terms: Unmanned Combat Air Vehicle, UCAV, UCAV 1303 Includes bibliographical references (p. 57-58). Also available in print.
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Melissa, Matthew (Matthew J. ). "Steady states in rapidly evolving populations." Thesis, Massachusetts Institute of Technology, 2017. http://hdl.handle.net/1721.1/115020.
Full textAbstract:
Thesis: S.M., Massachusetts Institute of Technology, Department of Physics, 2017.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 63-68).
Populations are subject to mutations conferring beneficial effects, as well as mutations conferring deleterious effects. Even if deleterious mutations occur much more frequently than beneficial mutations, the contribution of deleterious mutations to the overall rate of change of the population-wide mean fitness may be limited, due to the efficient action of selection. However, in particularly rapidly evolving populations, the stochastic accumulation of deleterious mutations may negate a significant fraction of the fitness increments provided by beneficial mutations, or even result in an overall decrease in fitness over time. Here we obtain a constraint on beneficial and deleterious mutation rates and selection pressures in order for positive adaptation to counterbalance fitness decline via Muller's ratchet. The steady state that separates parameter regimes of positive adaptation and negative adaptation is characterized by appealing to the exponential dependence of fixation probabilities on fitness effect sizes. We consider a range of fitness-mediated epistatic interactions and the corresponding implications regarding the existence, location and stability of long-term evolutionary fixed points.
by Matthew Melissa.
S.M.
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Miermont, Frédéric. "Influence des compensations série des lignes et des compensateurs statiques de puissance sur les réseaux de l'avenir." Grenoble INPG, 1989. http://www.theses.fr/1989INPG0118.
Full textAbstract:
La compensation série des lignes et les compensateurs statiques de puissance permettent d'améliorer sensiblement la fiabilité et la rentabilité des réseaux d'énergie, sans avoir recours pour cela, à la mise en service de nouvelles lignes de transport. Cependant, leurs interactions avec un réseau et les éléments qui le constituent pose un problème. Après avoir examiné l'intérêt et les inconvénients de ces deux procédés en régime permanent, nous avons déduit que la compensation statique de puissance était la solution la plus adaptée au réseau Français. Elle permet, par une meilleure gestion de l'énergie réactive, de réduire les pertes ohmiques dans les lignes et d'assurer une régulation fine de la tension. Pour compléter ces travaux, nous avons entrepris une étude en régime transitoire de ces deux moyens de compensation d'énergie réactive. Pour ce faire, un système à compensation série a été modélisé dans un cas simple, ce qui nous a permis de mettre en relief le rôle important des protections de ces systèmes. Enfin, nous avons élaboré un modèle de réactance régulée par thyristors munie de son régulateur de tension et de son générateur d'impulsions, dont nous avions fait l'étude séparément. L'ensemble a été regroupé et inséré dans un logiciel de simulation de transitoires d'Electricité de France MORGAT. Une première série d'essais a permis de valider le modèle
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Paniagua, Guillermo. "Investigation of the steady and unsteady performance of a transonic HP turbine." Doctoral thesis, Universite Libre de Bruxelles, 2002. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/211407.
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Starr, Maxwell. "High-order methods for steady, unsteady and transitional flow over a cylinder." Thesis, Cranfield University, 2012. http://dspace.lib.cranfield.ac.uk/handle/1826/7859.
Full textAbstract:
In this thesis, the flow around a cylinder is chosen as a test case for higher-order
numerical reconstruction techniques. No direct comparison of these higher-order
methods has been carried out for this particular test case. Especially for low Mach
number, incompressible flows with Implicit Large Eddy Simulation method. The
cylinder test case is both a proven test case in literature, as well as a test case
that can be scaled up in terms of flow speed with other parameters remaining
unchanged. The scaling of flow speed around the cylinder allows ease of flow
regime change. Thus the flow was modelled in this thesis from laminar flow to
turbulent flow, going through a transitional regime in between.
The simulations were set up such that numerical reconstruction methods could
be directly compared to one another at the range of flow speeds, and subsequently
in both two-dimensional and three-dimensional flows. The numerical
reconstruction methods for the ILES cases ranged from first order reconstruction
through to higher-order methods as high as ninth-order (in the weighted
essentially non-oscillatory scheme). With the speed of computation for the twodimensional
simulations, it was possible to test all of these schemes directly with
one another. However, three-dimensional simulations require a significantly greater
CPU run-time. Therefore, based on the results of the two-dimensional simulations,
a group of the higher-order methods were chosen for continuing analysis
in the three-dimensional simulations.
In the laminar flow regime, all the numerical schemes agreed very well with
literature data. As the flow speed increased, discrepancies started to appear in
the results, to varying degrees based on the flow speed, the numerical scheme
used, and the dimensionality of the flow. An analysis of the results showed that
two-dimensional simulations were suitable up to Reynolds 300. From this flow speed onwards, three-dimensional simulations are deemed necessary. At lower
Reynolds number flows the two-dimensional simulations provided good predictions
of the flow. At the higher Reynolds numbers, the 3D simulations outperformed
the 2D simulations.
Specific numerical reconstruction schemes were found to perform better at
certain aspects of the flow. For example, the coefficients around the cylinder or
the velocities in the wake varied based solely on the numerical scheme used.
Finally, during the course of the post-processing of the simulations, a spectral
analysis was carried out. The flow field was analysed at specific points in the
wake (ranging from near, medium and far wake). The spectral analysis proved
suitable for examining the fluctuations in the wake of the cylinder, showing the
redistribution of energy in the wake towards higher frequencies. In addition, the
wake showed increased power densities for the fluctuations as the flow moved
away from the cylinder, before then decreasing again as dissipation into the surrounding
flow occurred.
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Popp, Oliver. "Steady and Unsteady Heat Transfer in a Film Cooled Transonic Turbine Cascade." Diss., Virginia Tech, 1999. http://hdl.handle.net/10919/28513.
Full textAbstract:
The unsteady interaction of shock waves emerging from the trailing edge of modern turbine nozzle guide vanes and impinging on downstream rotor blades is modeled in a linear cascade. The Reynolds number based on blade chord and exit conditions (5*10^6) and the exit Mach number (1.2) are representative of modern engine operating conditions. The relative motion of shocks and blades is simulated by sending a shock wave along the leading edges of the linear cascade instead of moving the blades through an array of stationary shock waves. The blade geometry is a generic version of a modern high turning rotor blade with transonic exit conditions. The blade is equipped with a showerhead film cooling scheme. Heat flux, surface pressure and surface temperature are measured at six locations on the suction side of the central blade. Pressure measurements are taken with Kulite XCQ-062-50a high frequency pressure transducers. Heat flux data is obtained with Vatell HFM-7/L high speed heat flux sensors. High speed heat flux and pressure data are recorded during the time of the shock impact with and without film cooling. The data is analyzed in detail to find the relative magnitudes of the shock effect on the heat transfer coefficient and the recovery temperature or adiabatic wall temperature (in the presence of film cooling).
It is shown that the variations of the heat transfer coefficient and the film effectiveness are less significant than the variations of recovery temperature. The effect of the shock is found to be similar in the cases with and without film cooling. In both cases the variation of recovery temperature induced by the shock is shown to be the main contribution to the overall unsteady heat flux.
The unsteady heat flux is compared to results from different prediction models published in the literature. The best agreement of data and prediction is found for a model that assumes a constant heat transfer coefficient and a temperature difference calculated from the unsteady surface pressure assuming an isentropic compression.Ph. D.
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Santuz, Alessandro. "Extracting muscle synergies from human steady and unsteady locomotion: methods and experiments." Doctoral thesis, Humboldt-Universität zu Berlin, 2018. http://dx.doi.org/10.18452/19351.
Full textAbstract:
Die Notwendigkeit, sich über unebene, sich ständig verändernde Gelände zu bewegen, gehört zu unserem täglichen Leben. Das zentrale Nervensystem muss daher eine erhöhte Menge an Information integrieren, um mit der Unvorhersehbarkeit äußerer Störungen zurechtkommen zu können. Die Folge dieser erhöhten Beanspruchung könnte eine flexible Kombination der modularen Organisation von Bewegungssteuerung sein. Auf Kosten der Genauigkeit der Bewegung wäre es so möglich, dass das System reagiert, indem es die Robustheit (Fähigkeit mit Fehlern umzugehen) seiner Steuerung erhöht. Jedoch sind die Strategien, die das zentrale Nervensystem zur Organisation der Bewegung verwendet, immer noch schlecht verstanden. Eine Möglichkeit besteht darin, dass Bewegungen zustande kommen durch eine kleine Anzahl linear kombinierter Aktivierungsmuster (Muskelsynergien).
Unter den verschiedenen Möglichkeiten der Bewegungsstörung sind das Weglassen von Schuhen und die Verwendung von unebenen Oberflächen zwei gebräuchliche Optionen. In einem ersten Schritt habe ich eine gründliche Analyse der Methoden durchgeführt, die nützlich sind für a) die Auswertung von raumzeitlichen Gangparametern mithilfe von Daten der plantaren Druckverteilung und b) die Extraktion von Muskelsynergien mittels nicht-negativer Matrixfaktorisierung. Anschließend habe ich die modulare Organisation von c) beschut und barfuß Laufen und d) Laufband Gehen und Laufen über ebener und unebener Oberfläche analysiert. Im Vergleich zum gestörten Zustand zeigte das Barfußlaufen eine zeitlichen Verschiebung der zeitabhängigen Muskelaktivierungspatterns (Motor Primitives) und eine Reorganisation der zeitunabhängigen Koeffizienten (Motor Modules). Zusammenfassend, konserviert Fortbewegung über unebener Oberfläche, im Vergleich zu ebener, Motor Modules, während Motor Primitives im Allgemeinen breiter werden. Diese Ergebnisse unterstützen die Idee einer erhöhten Robustheit in der motorischen Kontrolle während der instabilen Fortbewegung.The need to move over uneven, continuously changing terrains is part of our daily life. Thus, the central nervous system must integrate an augmented amount of information in order to be able to cope with the unpredictability of external disturbances. A consequence of this increased demand might be a flexible recombination of the modular organisation of movement creation and control. At the expense of motion’s accuracy, it is possible that the system responds by increasing its control’s robustness (i.e. ability to cope with errors). However, the strategies employed by the central nervous system to organise movement are still poorly understood. One possibility is that movements are constructed through a small amount of linearly combined patterns of activations, called muscle synergies. Amongst the several possibilities of perturbing locomotion, the removal of footwear and the use of uneven surfaces are two valid options. In a first step, I conducted a thorough analysis of the methodologies useful for a) the evaluation of spatiotemporal gait parameters using plantar pressure distribution data and b) the extraction of muscle synergies using non-negative matrix factorisation. Afterwards, I analysed the modular organisation of c) shod and barefoot running and d) walking and running over an even- and an uneven-surface treadmill. The modular organisation of locomotion, assessed through the extraction of muscle synergies, changed when perturbations were introduced. Compared to the shod condition, barefoot running underwent, mostly due to the different foot strike pattern, a reorganisation of the time-independent coefficients (motor modules) and a time-shift of the time-dependent muscle activation patterns (motor primitives). Uneven-surface locomotion, compared to even-surface, conserved motor modules, while motor primitives were generally wider, confirming the idea of an increased robustness in motor control during unsteady locomotion.
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Hytopoulos, Evangelos. "Laminar, steady and unsteady flow over inclined plates in two and three dimensions." Thesis, This resource online, 1990. http://scholar.lib.vt.edu/theses/available/etd-06082009-170928/.
Full text
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She, Kaiming. "Numerical studies of vortex shedding from bluff cylinders in steady and unsteady flows." Thesis, Heriot-Watt University, 1987. http://hdl.handle.net/10399/992.
Full text
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Shapiro, Andrew P. "Steady and unsteady heat and mass transfer through porous media with phase change." Thesis, Massachusetts Institute of Technology, 1987. http://hdl.handle.net/1721.1/31004.
Full textAbstract:
Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1987.MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING
Bibliography: leaves 138-139.
by Andrew P. Shapiro.
M.S.
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Stettler, Jeffrey W. (Jeffrey Wayne). "Steady and unsteady dynamics of an azimuthing podded propulsor related to vehicle maneuvering." Thesis, Massachusetts Institute of Technology, 2004. http://hdl.handle.net/1721.1/33164.
Full textAbstract:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 2004.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Includes bibliographical references (p. 173-177).
While the implementation of azimuthing propulsors powered by internal electric motors (often called "podded propulsors") into the commercial ship market has been swift, the understanding of their hydrodynamics through research, particularly in the area of maneuvering performance, has been very limited. This thesis research investigates the steady and unsteady dynamic maneuvering forces associated with an azimuthing podded propulsor, and provides supporting theoretical insight toward understanding their mechanisms and prediction. Because of the wide range of potential applications of azimuthing podded propulsion in the marine field, dynamic force phenomena applicable to maneuverability of both large and small scale vehicles are investigated. These include quasi-steady vectored maneuvering forces, of importance to all maneuvering vehicles or ships, as well as unsteady or transient maneuvering forces, which have more significance to the maneuverability of smaller vehicles, particularly for precision control applications. The ultimate goal of the research is to provide a comprehensive understanding of the maneuvering forces associated with an azimuthing podded propulsor, such that future maneuvering and control applications, and computational fluid dynamics studies in the field, can be appropriately focused. The research efforts are focused in four main areas. First, a number of relevant dynamic models for the maneuvering of a surface vehicle with an azimuthing propulsor are developed. Second, an extensive test program measures and characterizes the nature of quasi-steady vectored maneuvering forces associated with a podded propulsor in azimuth to ±180⁰ for the entire range of forward propeller speeds, as well as unsteady or transient maneuvering forces due to rapid changes in azimuth angle or propeller rate.
(cont.) This test program is aimed at quantifying the steady and unsteady parameters associated with the developed dynamic models. Third, two flow visualization techniques are utilized to visualize, document, and correlate the helical wake characteristics, velocities and forces for both quasi-steady and unsteady propulsor states. A new fluorescent paint flow visualization technique is developed and applied for small, moderate and4 large propulsor azimuth angles, and a laser particle image velocimetry (PIV) technique is adapted for small and moderate propulsor azimuth angles. Finally, a set of comprehensive physics-based models are developed to foster the understanding of the mechanisms associated with the steady and unsteady force dynamics. The quasi-steady models are based upon a combination of momentum-based, blade-element, and vortex wake propeller theories, as applied to an azimuthing podded propulsor. The unsteady force models are based upon unsteady wake or "dynamic inflow" methods. Additionally, an interesting phenomenon associated with the formation of a vortex ring during rapid propeller rate increase is presented and discussed. The steady and unsteady test results, flow visualizations, and theoretical models, are shown to be consistent in terms of the magnitudes and character of the azimuthing maneuvering forces. Limited comparisons of quasi-steady propulsor forces at small, moderate and large azimuth angles are also made with forces predicted by a modified combined blade-element-momentum (BEM) method, as well as the unsteady vortex-lattice propeller code MPUF-3A, with and without modified inflows to account for propulsor pod wake. The results illustrate inherent complexities related to use of existing computational fluid dynamics tools with azimuthing podded propulsors.
by Jeffrey W. Stettler.
Ph.D.
41
Hytopoulos, Evangelos. "A turbulence model for steady and unsteady boundary layers in strong pressure gradients." Diss., Virginia Tech, 1994. http://hdl.handle.net/10919/40119.
Full textAbstract:
A new turbulence model designed for two-dimensional, steady and unsteady boundary layers in strong adverse pressure gradients is described. The model is developed in a rational way based on an understanding of the flow physics obtained from recent experimental observations. The turbulent shear stress is given by a mixing length model, but the variation of the mixing length in the outer region is not constant; it varies according to an integral form of the turbulence kinetic-energy equation. This approach allows for the history effects of the turbulence to be taken into account in an approximate but rational way. The form of the near-wall mixing length model is derived based on the rigorous distribution of the shear stress near the wall, and it takes into account the pressure and convection terms which become important in strong adverse pressure gradients. Since the significance of the normal stresses in turbulent kinetic-energy production is increasing as separation is approached, a model accounting for this contribution is incorporated. The model is calibrated using available experimental data. These data also indicate a change in turbulence structure near and through separation. Such a change can be significant and is accounted for here using an empirical function. The complete model was tested against steady and unsteady, two-dimensional experimental cases with adverse pressure gradient up to separation. Improved predictions compared to those obtained with other turbulence models were demonstrated. The general and rational approach that led to the derivation of the model allows the straightforward extension of the model in the region of separation. The further extension to steady and unsteady, three-dimensional cases is indicated.Ph. D.
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Taroyan, Yeghiazar. "MHD waves and resonant interactions in steady states." Thesis, University of Sheffield, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.397503.
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Meile, Tobias. "Influence of macro-roughness of walls on steady and unsteady flow in a channel /." Lausanne : EPFL, 2007. http://library.epfl.ch/theses/?nr=3952.
Full textAbstract:
Thèse Ecole polytechnique fédérale de Lausanne EPFL, no 3952 (2007), Faculté de l'environnement naturel, architectural et construit ENAC, Section de génie civil, Institut des infrastructures, des ressources et de l'environnement ICARE (Laboratoire de constructions hydrauliques LCH). Dir.: Anton Schleiss.
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Nuhalt, Abdullah O. "Numerical simulation of feedback control of aerodynamic configurations in steady and unsteady ground effects." Diss., Virginia Polytechnic Institute and State University, 1988. http://hdl.handle.net/10919/82627.
Full textAbstract:
A general numerical simulation of closely coupled lifting surfaces in steady and unsteady ground effects was developed. This model was coupled with the equations of motion to simulate aerodynamic-dynamic interaction. The resulting model was then coupled with a feedback-control law to form a general nonlinear unsteady numerical simulation of control of an aircraft in and out of ground effect.
The aerodynamic model is based on the general unsteady vortex-lattice method and the method of images. It is not restricted by planform, angle of attack, sink rate, dihedral angle, twist, camber, etc. as long as stall or vortex bursting does not occur. In addition, it has the versatility to model steady and unsteady aerodynamic interference. The present model can be used to simulate any prescribed flare and to model the effects of cross and/or head winds near the ground.
The present results show the influences of various parameters on the aerodynamic coefficients for both steady and unsteady flows. Generally, the ground increases the aerodynamic coefficients; the greater the sink rates, the stronger the effects. Increasing the aspect ratio increases both the steady and unsteady ground effects. An exception is a large aspect-ratio wing with large camber. The present results are generally in close agreement with limited exact solutions and experimental data.
In the aerodynamic-dynamic simulation, the equations of motion were solved by Hammlng's predictor-corrector method. The aircraft, air stream, and control surfaces were treated as a single dynamic system. The entire set of governing equations was solved simultaneously and interactively. The aerodynamic-dynamic model was used to study a configuration that resembles a Cessna 182 airplane. The ground lowers the effectiveness of the tail in controlling pitch, increases the lift and drag, and makes the hinge-moment less negative. Proportional and rate control laws were used in a feedback system to control pitch. One set of gains was used in and out of ground effect. For the same control input, the pitch angle responds faster and overshoots more near the ground than it does far from the ground. The present results demonstrate the feasibility of using the current simulation to model more complicated motions and the Importance of including the unsteady ground effects when analyzing the performance of an airplane during a landing maneuver.Ph. D.
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Rateitschak, Katia. "Construction of nonequilibrium steady states: Thermostating by determministic scattering." Doctoral thesis, Universite Libre de Bruxelles, 2001. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/211654.
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Buckley, Matthew. "Magnetoconvection in sunspot umbrae : steady and oscillatory localised states." Thesis, University of Newcastle upon Tyne, 2013. http://hdl.handle.net/10443/2130.
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Astrophysical observations of the solar photosphere uncover a wealth of detailed structures that arise from the interaction of vigorously convecting plasma and the internally generated magnetic elds. The most prominent features are sunspots, which exhibit sub-structures on a range of scales. Speci cally within the umbra is an intensity pattern consisting of individual small bright points, referred to as umbral dots. These states are thought to indicate the presence of localised magnetoconvective motions. This thesis discusses the applications of magnetoconvection to the umbra, with the aim of investigating the occurrence of steady and oscillatory localised states known as convectons. Convectons are isolated convective plumes from which magnetic ux is at least partially expelled. In two-dimensional Boussinesq magnetoconvection we examine both a simpli ed model, in which the vertical structure has been reduced, and a fully-resolved model. In performing parametric surveys of the steady modes we attempt to understand how localised states di er between the two models. Examining the oscillatory localised cells we locate, for the rst time, these states in the fully-resolved system. Both of these models are horizontally periodic. We nd that by altering these horizontal boundaries so that they are impermeable to uid motions does not impede the existence of these states but leads to the additional existence of a new set of solutions that are localised at the boundaries. To examine the bifurcation structure of these states we develop a numerical continuation model. However, due to the limitations of the continuation program, AUTO-07p, this model has restricted symmetries and impermeable horizontal boundaries. Despite these simpli cations the symmetries of the model ensure that convectons can still be found and in addition allows the examination of the wall states. The remainder of this thesis focuses on compressible magnetoconvection. In studying oscillatory convectons in two-dimensions we nd a new type of oscillation not found in the Boussinesq models. This state no longer retains Boussinesq point symmetry but has more gentle extended up ows characteristic of a three-dimensional cylindrical plume. In three dimensions a new type of steady convecton is found with a broken symmetry such that the cross-section corresponds to a single overturning roll.
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Jia, Yabo. "Numerical simulation of steady states associated with thermomechanical processes." Thesis, Lyon, 2020. http://www.theses.fr/2020LYSEE007.
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De nombreux procédés de fabrication thermomécanique comme le laminage, le soudage ou encore l’usinage mettent en jeu soit des sollicitations mobiles par rapport à la matière fixe, soit de la matière mobile par rapport à des sollicitations fixes. Dans tous les cas, après un régime transitoire en général assez court, les champs thermiques, métallurgiques et mécaniques associés à ces procédés atteignent un état stationnaire. La recherche de ces états stationnaires à l’aide de la méthode des éléments finis classique nécessite de mettre en œuvre des modèles complexes et couteux où les sollicitations se déplacent par rapport à la matière (ou l’inverse). La recherche directe des états stationnaires a fait l’objet de nombreux travaux de recherche ces trente dernières années. Des méthodes sont aujourd’hui disponibles et pour certaines sont proposées dans des codes de calcul du commerce. Ainsi, une option de calcul dite repère mobile proposée par différents auteurs est disponible dans le logiciel SYSWELD. Cette méthode permet de calculer les états thermique, métallurgique et mécanique stationnaires associés à un procédé de soudage, en résolvant un problème de diffusion-convection en thermique et en intégrant, en mécanique, les équations constitutives du comportement du matériau le long des lignes de courant. Si cette méthode a été utilisée avec succès dans de nombreuses applications, elle présente néanmoins quelques limitations. Ainsi le maillage doit être structuré et la convergence des calculs est en général assez lente. Nous proposons dans cette thèse de résoudre le problème mécanique dans un repère lié aux sollicitations, en nous appuyant sur une méthode de calcul par éléments finis reposant sur l’intégration nodale et la technique SCNI (Stabilized Conforming Numerical Integration). Cette méthode permet l’utilisation de maillages en tétraèdres (ou triangles en 2D) sans rencontrer de problème de verrouillage volumique résultant de l’incompressibilité plastique associée au critère de plasticité de von Mises. Plutôt que de rechercher directement l’état stationnaire, l’idée générale est ici de construire l’état stationnaire à partir d’une analyse transitoire en faisant entrer pas à pas la matière par l’amont et en la faisant sortir par l’aval d’un maillage fixe par rapport aux sollicitations et de taille limitée. L’état stationnaire n’est donc atteint qu’au bout d’un certain temps d’analyse. Après une introduction générale (Chapitre 1) et un état de l’art sur les méthodes existantes (Chapitre 2), nous présentons une approche de simulation du mouvement de matière dans le cadre de la méthode des éléments finis classique sur un problème de soudage (Chapitre 3). Nous y proposons également des conditions aux limites thermiques pertinentes pour calculer directement la distribution de températures en régime stationnaire. La méthode des éléments finis reposant sur l’intégration nodale est ensuite décrite au Chapitre 4. Les avantages et inconvénients de la méthode sont discutés. La méthode est validée sur une application en grandes déformations élastoplastiques, un problème de flexion et une simulation thermomécanique de soudage. La méthode des éléments finis reposant sur l’intégration nodale est alors développée pour prendre en compte un mouvement de matière (Chapitre 5). Trois types de mouvement sont considérés : en translation, circulaire et en hélice. Différentes méthodes de transport de champ sont abordées et discutées ainsi que le couplage thermomécanique. Des perspectives à ce travail sont proposées au Chapitre 6. Les perspectives envisagées visent d’une part à améliorer la méthode proposée et d’autre part, à développer la méthode pour simuler d’autres procédés. Une première application de la méthode à la simulation de la coupe orthogonale y est présentéeIn the numerous thermomechanical manufacturing processes such as rolling, welding, or even machining involve either moving loads with respect to the fixed material or moving material with respect to fixed loads. In all cases, after a transient regime which is generally quite short, the thermal, metallurgical, and mechanical fields associated with these processes reach a steady state. The search for these stationary states using the classical finite element method requires the implementation of complex and expensive models where the loads move with respect to the material (or vice versa). The steady-state simulation in one increment has been the subject of much researches over the past thirty years. Methods are now available and some are integrated into calculation codes commercial. Thus, a so-called Moving Reference Frame method proposed by various authors is available in the SYSWELD software. This method makes it possible to calculate the steady-state of thermal, metallurgical, and mechanical states associated with a welding process, by solving a thermal diffusion-convection problem in thermal-metallurgy and by integrating, in mechanics, the constitutive equations of the material along the streamline. Moreover, this method has been used successfully in many applications, it nevertheless has some limitations. Thus the mesh must be structured and the convergence of computations is generally quite slow. In this thesis, we propose to solve the mechanical problem in a frame linked to the solicitations, by relying on a finite element calculation method based on nodal integration and the SCNI (Stabilized Conforming Numerical Integration) technique. This method allows the use of tetrahedron meshes (or 2D triangles) without encountering a locking problem resulting from the plastic incompressibility associated with the von Mises plasticity criterion. Rather than directly calculating the steady-state, the general idea here is to construct the steady-state from a transient analysis by bringing material step by step upstream and by making it exit downstream of a fixed mesh related to the solicitations and of the limited mesh size. The steady-state is therefore only achieved after certain steps of analysis. Apart from a general introduction (Chapter 1) and a state of the art on the existing methods (Chapter 2), we present an approach of simulation of the movement of material within the framework of the classical finite element method on a welding problem (Chapter 3). We also provide relevant thermal boundary conditions for directly calculating the steady-state of temperature distribution. The finite element method based on the nodal integration technique is then described in Chapter 4. The advantages and disadvantages of the method are discussed. The nodal-integration-based finite element is validated by comparing its simulation results with classical finite element methods in large elastoplastic strains, a bending problem, and a thermomechanical simulation of welding. The nodal-integration-based finite element is then developed and applied to simulate material motion (Chapter 5). Three types of movement are considered: translational, circular, and helical. Different methods of field transport are approached and discussed as well as thermomechanical coupling. Perspectives for this work are presented in Chapter 6. The envisaged perspectives aim, on the one hand, to improve the proposed method and on the other hand, to develop the method to simulate other processes. A first application of the material motion method to the simulation of the orthogonal cut is presented there
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Zagnoli, Daniel Anthony. "A Numerical Study of Deposition in a Full Turbine Stage Using Steady and Unsteady Methods." The Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1429796426.
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Araki, Yoshikazu. "Stability of Steady States and Steady-State Limit of Elastoplastic Trusses under Quasi-Static Cyclic Loading." Kyoto University, 1997. http://hdl.handle.net/2433/77862.
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Joussellin, Florence. "Générateur à induction à métal liquide : aspects théoriques et expérimentaux." Grenoble 1, 1986. http://www.theses.fr/1986GRE10124.
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Etude theorique portant sur l'analyse des conditions d'auto-excitation et sur l'efficacite du transfert d'energie qui caracterise le rendement electrique du generateur. Le probleme, traite analytiquement pour l'etude du regime transitoire et l'influence du desequilibre des phases. Resultats experimentaux obtenus avec une boucle a mercure essentiellement a verifier les conditions d'auto-excitation. Les mesures ont permis de valider, avec une bonne precision, l'ensemble des resultats theoriques et ont conduit a une premiere approche de dimensionnement d'un generateur fonctionnant au lithium dont l'objectif est la production d'electricite dans l'espace