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1
Giannetti, Flavio. "Boundary layer receptivity." Thesis, University of Cambridge, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.620646.
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2
Brotherton-Ratcliffe, Rupert Victor. "Boundary-layer effects in liquid-layer flows." Thesis, University College London (University of London), 1987. http://discovery.ucl.ac.uk/1317966/.
Testo completoAbstract (sommario):
In this thesis we describe various regimes of practical and theoretical significance that arise in the laminar two-dimensional flow of a layer of an incompressible viscous fluid over a solid surface at high Reynolds number. In Part I we consider steady flows over a distorted rigid surface. Almost uniform flows are considered first, when the distortion is sufficient to provoke a viscous-inviscid interaction, and therefore boundary-layer separation. The two cases of supercritical and subcritical flow have quite distinct features, and are discussed separately. The governing equations in each case require a numerical treatment in general, but analytical progress has been made in certain important regimes e. g. when the distortion is relatively small and linearisation of the problem is possible. Next, the grossly separated motion of fully-developed flows over large obstacles, with dimensions of the order of the depth of the liquid layer, is studied on the basis of inviscid Kirchhoff free-streamline theory. Some comparisons of the theory with recent experiments are also given. In Part II we discuss unsteady and instability aspects of two-dimensional flow over a flat surface. It is shown that viscous and mean flow effects can combine to give instability in some cases, whereas previous studies have only found viscous effects to be stabilising. Unsteadiness of a two-layer fluid flow, with fluids of different viscosity and density, and incorporating surface tension effects, is also discussed. In Part III, deviating from the above theme slightly, we discuss briefly the steady, high-Reynolds-number flow in an asymmetric branching channel, again in the context of viscous-inviscid interactions. The asymmetry is found to force a large-scale response both up- and downstream of the start of the bifurcation. The aim is to find the pressure distributions on the channel walls and on the dividing body. This requires the use of a Wiener-Hopf technique in view of the mixed boundary conditions.
3
Yuile, Adam. "Swept boundary layer transition." Thesis, University of Liverpool, 2013. http://livrepository.liverpool.ac.uk/14613/.
Testo completoAbstract (sommario):
Boundary layer transition has been investigated for incompressible three-dimensional mean flows on a flat plate with a 60° swept leading edge for a nominally zero, a positive, and a negative pressure gradient for three freestream turbulence intensities using a low speed blower tunnel with a 1.22 x 0.61 m working section at the University of Liverpool. The freestream turbulence intensities were generated using grids upstream of the leading edge, producing turbulence levels of approximately 0.2 %, 1.25 % and 3.25 %. For each of these nine (3 x 3) test cases detailed boundary layer traverses were obtained at ten streamwise measurement stations, at a fixed spanwise location, using single-wire constant temperature hot-wire anemometry techniques and digital signal processing. The location for the onset and end of transition was obtained for each case, in terms of distance from the leading edge and local momentum thickness Reynolds number. These results are compared with the 2-D unswept empirical transition correlations of Abu-Ghannam and Shaw (1980) and the differences in the results between the two flows are highlighted. It was found that transition starts and ends earlier than for similar unswept flows, complementing the transition observations of Gray (1952) for swept wings. Further to this the receptivity of the swept boundary layers to freestream turbulence (in the bypass transition regime) was determined by comparing near wall and local freestream spectra, for the pre-transitional boundary layers. These experimental results were compared with numerical predictions from a fourth order accurate computational fluid dynamics method which considered a multitude of perturbation waveforms. This numerical approach was also able to identify the waveform frequency and orientation combinations which drive receptivity in swept boundary layer transition and indicate the manner in which receptivity scales with momentum thickness Reynolds number. It was found that the most receptive waveforms correspond to the streamwise streaks which are frequently observed in flow visualisations and direct numerical simulation studies of pre-transitional boundary layers. Additionally it was also found that the numerical receptivities to freestream turbulence were highest for the positive pressure gradient and, in contrast, lowest for the negative pressure gradient – a similar finding to that in 2-D boundary layers. Transition was seen to commence prior to the advent of the intended non-zero pressure gradients in the experiments and thus direct comparisons are not strictly available. The results obtained, and synthesis undertaken for this thesis, contribute towards an improved understanding of the transition process, particularly with respect to receptivity, in regard to flat plates with swept leading edges in various pressure gradients and highlight the differences between swept and unswept flows. Furthermore, additional avenues have been identified for future work on more complicated topologies where potential problems have also been highlighted.
4
Kral, Linda Dee. "Numerical investigation of transition control of a flat plate boundary layer." Diss., The University of Arizona, 1988. http://hdl.handle.net/10150/184621.
Testo completoAbstract (sommario):
A numerical model has been developed for investigating boundary layer transition control for a three-dimensional flat plate boundary layer. Control of a periodically forced boundary layer in an incompressible fluid is studied using surface heating techniques. The spatially evolving boundary layer is simulated. The Navier-Stokes and energy equations are integrated using a fully implicit finite difference/spectral method. The Navier-Stokes equations are in vorticity-velocity form and are coupled with the energy equation through the viscosity dependence on temperature. Both passive and active methods of control by surface heating are investigated. In passive methods of control, wall heating is employed to alter the stability characteristics of the mean flow. Both uniform and nonuniform surface temperature distributions are studied. In the active control investigations, temperature perturbations are introduced locally along finite heater strips to directly attenuate the instability waves in the flow. A feedback control loop is employed in which a downstream sensor is used to monitor wall shear stress fluctuations. Passive control of small amplitude two-dimensional Tollmien-Schlichting waves and three-dimensional oblique waves are numerically simulated with both uniform and nonuniform passive heating applied. Strong reductions in both amplitude levels and amplification rates are achieved. Active control of small amplitude two-dimensional and three-dimensional disturbances is also numerically simulated. With proper phase control, in phase reinforcement and out of phase attenuation is demonstrated. A receptivity study is performed to study how localized temperature perturbations are generated into Tollmien-Schlichting waves. It is shown that narrow heater strips are more receptive in that they maximize the amplitude level of the disturbances in the flow. It is also found that the local temperature fluctuations cause mainly a strong normal gradient in spanwise vorticity. Control of the early stages of the nonlinear breakdown process is also investigated. Uniform passive control is applied to both the fundamental and sub-harmonic routes to turbulence. A strong reduction in amplitude levels and growth rates results. In particular, the three-dimensional growth rates are significantly reduced below the uncontrolled levels. Active control of the fundamental breakdown process is also numerically simulated. Control is achieved using either a two-dimensional or three-dimensional control input.
5
Andersson, Paul. "Modelling of boundary layer stability." Doctoral thesis, KTH, Mechanics, 1999. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-2888.
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6
Vogl, Stefanie. "Tropical Cyclone Boundary-Layer Models." Diss., lmu, 2009. http://nbn-resolving.de/urn:nbn:de:bvb:19-102740.
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7
Graham, William Richard. "Boundary-layer noise and vibration." Thesis, University of Cambridge, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.308331.
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8
Lea, Adam Stuart Robert. "Boundary layer flow over hills." Thesis, University of Leeds, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.400175.
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9
Riley, S. "Three-dimensional boundary layer transition." Thesis, University of Liverpool, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.356291.
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10
Gardiner, I. D. "Transition in boundary layer flows." Thesis, University of Abertay Dundee, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.376973.
Testo completoAbstract (sommario):
An experimental investigation of transition in boundary layer flows under the influence of various freestream conditions is described. Velocity profiles are obtained automatically by means of a stepper-motor driven traverse mechanism which carries a hot wire probe connected to a constant temperature anemometer and associated instrumentation. This was achieved by use of a data acquisition and control facility centred around a microcomputer with a Eurocard rack mounted extension. The automatic boundary layer traverse is software controlled and the data obtained is stored in a disc file for subsequent analysis and graphical display. As an integral part of this facility a successful method of obtaining reliable intermittency values from a hot wire signal was developed. The influence of freestream turbulence and pressure gradient upon transition within a boundary layer developing on a flat plate is elucidated by a series of controlled experiments. From the data accumulated, the concept of statistical similarity in transition regions is extended to include moderate non-zero pressure gradients, with the streamwise mean intermittency distribution described by the normal distribution function. An original correlation which accounts for the influence of freestream turbulence in zero pressure gradient flows, and the combined influence of freestream turbulence and pressure gradient in adverse pressure gradient flows, on the transition length Reynolds number R, is presented. (The limited amount of favourable pressure gradient data precluded the extension of the correlation to include favourable pressure gradient flows). A further original contribution was the derivation of an intermittency weighted function which describes the development of the boundary layer energy thickness through the transition region. A general boundary layer integral prediction scheme based on existing established integral techniques for the laminar and turbulent boundary layers with an intermittency modelled transition region, has been developed and applied successfully to a range of test data.
11
Grimaldi, Margaret Elizabeth. "Roughness-induced boundary layer transition." Thesis, Massachusetts Institute of Technology, 1994. http://hdl.handle.net/1721.1/47353.
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12
Dullaway, Scott N. M. "A VHF boundary-layer radar /." Title page, contents and abstract only, 1999. http://web4.library.adelaide.edu.au/theses/09SM/09smd883.pdf.
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13
Ailinger, Kevin Gerard. "Measurements of surface shear stresses under a three-dimensional turbulent boundary layer using oil-film laser interferometry." Thesis, This resource online, 1990. http://scholar.lib.vt.edu/theses/available/etd-11012008-063040/.
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14
Holloway, Simon. "Remote sensing measurements of the atmospheric boundary layer : development of a nocturnal boundary layer temperature LiDAR." Thesis, University of Manchester, 2018. https://www.research.manchester.ac.uk/portal/en/theses/remote-sensing-measurements-of-the-atmospheric-boundary-layer-development-of-a-nocturnal-boundary-layer-temperature-lidar(3de0f22b-8165-4c22-b38e-296062b37940).html.
Testo completoAbstract (sommario):
A LiDAR instrument to monitor the evolution of the urban boundary layer capping inversion over Manchester has been developed from a previous instrument. This LiDAR uses a frequency-tripled Nd:YAG laser, operating at low pulse energy but high repetition frequency. Rotational Raman scattering of this laser light is parsed into two channels by narrowband interference filters, before detection by photomultiplier tubes operating in photon-counting mode. The receiving telescope was refocused to operate in the boundary layer, and an interference filter was replaced following modelling work. The calibrations of this instrument use locally-launched sondes to determine corrections due to operating in the near-field region of the receiving telescope. The LiDAR receiver was thoroughly calibrated under laboratory conditions to construct a lookup table. Locally-launched sondes were used to correct for mirror shading by instrument components, as well as constrain the overlap function of the BLT. A temperature resolution of better than 0.4K arising from Poisson noise was achieved for data collected for the mean temperature profile measured over the course of a night, with temperature inversions being identifiable down to a height of 500m. A total temperature error of less than 3K was achieved by taking the whole-night mean, which is significantly less than the size of the smallest identified temperature inversion (7.6±2K). The LiDAR instrument data was compared with locally-launched sondes to validate the collected data, agreeing with the sonde measurements to within the uncertainty of the instrument. A WRF model temperature output was compared to both the BLT and sonde data and found to poorly capture the boundary layer temperature profile. The inversion strength was always underestimated by several K, and when the inversion height is below 300m the model underestimates the inversion height by 100-500m.
15
Paget, Aaron Christopher. "On determining the hurricane boundary layer." Tallahassee, Florida : Florida State University, 2009. http://etd.lib.fsu.edu/theses/available/etd-11092009-170223/.
Testo completoAbstract (sommario):
Thesis (M.S.)--Florida State University, 2009.<br>Advisor: Paul Ruscher, Florida State University, College of Arts and Sciences, Dept. of Meteorology. Title and description from dissertation home page (viewed on May 10, 2010). Document formatted into pages; contains xi, 46 pages. Includes bibliographical references.
16
Högberg, Markus. "Optimal Control of Boundary Layer Transition." Doctoral thesis, KTH, Mechanics, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3245.
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17
Lögdberg, Ola. "Turbulent Boundary Layer Separation and Control." Doctoral thesis, KTH, Linné Flow Center, FLOW, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-9821.
Testo completoAbstract (sommario):
Boundary layer separation is an unwanted phenomenon in most technical applications, as for instance on airplane wings, ground vehicles and in internal flow systems. If separation occurs, it causes loss of lift, higher drag and energy losses. It is thus essential to develop methods to eliminate or delay separation.In the present experimental work streamwise vortices are introduced in turbulent boundary layers to transport higher momentum fluid towards the wall. This enables the boundary layer to stay attached at larger pressure gradients. First the adverse pressure gradient (APG) separation bubbles that are to be eliminated are studied. It is shown that, independent of pressure gradient, the mean velocity defect profiles are self-similar when the scaling proposed by Zagarola and Smits is applied to the data. Then vortex pairs and arrays of vortices of different initial strength are studied in zero pressure gradient (ZPG). Vane-type vortex generators (VGs) are used to generate counter-rotating vortex pairs, and it is shown that the vortex core trajectories scale with the VG height h and the spanwise spacing of the blades. Also the streamwise evolution of the turbulent quantities scale with h. As the vortices are convected downstream they seem to move towards a equidistant state, where the distance from the vortex centres to the wall is half the spanwise distance between two vortices. Yawing the VGs up to 20° do not change the generated circulation of a VG pair. After the ZPG measurements, the VGs where applied in the APG mentioned above. It is shown that that the circulation needed to eliminate separation is nearly independent of the pressure gradient and that the streamwise position of the VG array relative to the separated region is not critical to the control effect. In a similar APG jet vortex generators (VGJs) are shown to as effective as the passive VGs. The ratio VR of jet velocity and test section inlet velocity is varied and a control effectiveness optimum is found for VR=5. At 40° yaw the VGJs have only lost approximately 20% of the control effect. For pulsed VGJs the pulsing frequency, the duty cycle and VR were varied. It was shown that to achieve maximum control effect the injected mass flow rate should be as large as possible, within an optimal range of jet VRs. For a given injected mass flow rate, the important parameter was shown to be the injection time t1. A non-dimensional injection time is defined as t1+ = t1Ujet/d, where d is the jet orifice diameter. Here, the optimal t1+ was 100-200.<br>QC 20100825
18
Högberg, Markus. "Optimal control of boundary layer transition /." Stockholm, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3245.
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19
Berlin, Stellan. "Oblique waves in boundary layer transition." Doctoral thesis, Stockholm, 1998. http://www.lib.kth.se/abs98/berl0529.pdf.
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20
Li, Yong. "Active control of boundary layer instabilities." Thesis, Queen Mary, University of London, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.413644.
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21
Anderson, Philip Stuart. "Aspects of the Antarctic boundary layer." Thesis, Lancaster University, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239832.
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22
Fowler, Alison M. "Assimilation of misplaced boundary layer features." Thesis, University of Reading, 2010. http://centaur.reading.ac.uk/24800/.
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23
Hystad, Ida. "Numerical Modelling of Turbulent Boundary Layer." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for marin teknikk, 2014. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-26365.
Testo completoAbstract (sommario):
Most physical problems involving viscous fluid flows are characterized by turbulence where instabilities and large velocity gradients generate fluctuations in the flow field. Towed sonar arrays are exposed to turbulence in the boundary layer formed around the cable. Problems are related to the cable rotating around its own axis due to variations in tension force caused by the towing vehicle. Numerical calculations of a pressure driven flow along a cylinder are performed for the purpose of investigating the turbulent boundary layer around the cable. In this study, the numerical software OpenFOAM has been used in order to solve the flow field. The Reynolds Average Navier-Stokes (RANS) approach was applied, providing a time-average solution of the flow quantities. The results were used in a comparative study with data obtained from Large Eddy Simulation (LES). Simulations were carried out for two Reynolds numbers based on the shear velocity; Re_tau=[240,550]. The cylinder was assigned two different rotational velocities in addition to a case with zero rotation. Results show that the normalized mean velocity profile is in good agreement with the universal law-of-the-wall and previous published data. Comparison with LES data indicated good agreement with Reynolds shear stresses and the normalized mean velocities in the case of a non-rotating cylinder. However, deviations were observed when rotation was applied. In order to ensure the quality of the numerical results a convergence study was performed. Special attention was paid to the near-wall region in order to capture all levels of the boundary layer.
24
Chen, J. M. "Boundary layer resistances of artificial leaves." Thesis, University of Reading, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.371436.
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25
Hachem, Farouk H. "Boundary layer transition on concave surfaces." Thesis, University of Liverpool, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.279702.
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26
Nash, Emma Clare. "Boundary layer instability noise on aerofoils." Thesis, University of Bristol, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.337698.
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27
Leoutsakos, George. "Boundary layer transition on concave surfaces." Thesis, Imperial College London, 1987. http://hdl.handle.net/10044/1/47060.
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28
Doyle, Brian Patrick. "Propulsion via buoyancy driven boundary layer." Thesis, Massachusetts Institute of Technology, 2011. http://hdl.handle.net/1721.1/87938.
Testo completoAbstract (sommario):
Thesis: S.B., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2011.<br>Cataloged from PDF version of thesis.<br>Includes bibliographical references (page 31).<br>Heating a sloped surface generates a well-studied boundary layer flow, but the resulting surface forces have never been studied in propulsion applications. We built a triangular wedge to test this effect by mounting a resistive heating pad to one of its conducting sloped surfaces. We submerge the wedge within a two-layer water stratification, turn the heater on and track the wedge's motion. We have observed a propulsion speed of 0.613 ± 0.042 mm/s with a temperature difference between the heated surface and ambient fluid of 4°C. We also use theory and numerics to predict the propulsion speed and predicted a speed of 1.43 mm/s, within an order of magnitude of the observed results, and thus our model was validated by the experiments.<br>by Brian Patrick Doyle.<br>S.B.
29
Sargeant, Matthew Alan. "Boundary layer ingestion for advanced airframes." Thesis, University of Cambridge, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.612183.
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30
Liu, Yu. "Surface roughness generated boundary layer noise." Thesis, University of Cambridge, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.612403.
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31
Ozkan, Musa. "Boundary layer transition over rotating disks." Thesis, University of Warwick, 2016. http://wrap.warwick.ac.uk/87170/.
Testo completoAbstract (sommario):
This thesis summarizes results of a combined numerical and experimental study investigating the effects of surface roughness, and of the geometry of the ow domain (confinement) on the boundary{layer transition over rotating disks. Numerically, a three{dimensional enclosed cavity ow in a rotor{stator flow configuration is simulated. The effects induced by surface roughness of the rotor disk and the effects induced by the stator geometry enclosing the ow domain are investigated. The steady{state velocity pro les of the boundary{layer ow on the rotating disk are obtained, subjected to a linear stability analysis and compared to relevant data from the literature. Experimentally, the ow over rotating disks is studied for smooth disk surfaces and for disks with concentric grooves representing distributed roughness. The disks are mounted submerged inside a water {filled tank. Due to the surrounding perimeter wall of the tank and the liquid surface this arrangement resembles the classic rotor{stator flow configuration. Comprehensive measurements of the boundary{layer ow and its laminar{turbulent transition were performed by means of an hot{ lm anemometer. The computational results suggest that, for the rotor{stator ow investigated, the roughness{induced effects are very similar to the geometry{induced effects, both in nature and magnitude. This suggests that it may be di cult to distinguish between both effects in experiments where the ow domain is restricted. Nevertheless, in comparison to previous hot{ lm measurements employing the same experimental facility, the data of the current study have been significantly improved by means of introducing a new calibration technique. The new experimental data discussed here confirm recent theoretical results of our research group in that they corroborate that an increase in the roughness level can reduce the number of stationary vortices and also stabilize the Type{I (cross{ ow) instability mode. However, the detailed analysis of the experimental data, in comparison to the theoretically predicted magnitude of the roughness{induced and the geometry{induced effects, reveal that future studies would greatly bene t from the availability of a new air{based rotating{disk apparatus.
32
Frank, Donya P. "Wave-Current Bottom Boundary Layer Interactions." The Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=osu1229087949.
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33
Yahiaoui, Mondher. "Numerical studies in boundary-layer stability /." The Ohio State University, 1993. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487848891511753.
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34
Su, Yi-Chung. "Longitudinal vortices in boundary layer transition /." The Ohio State University, 1999. http://rave.ohiolink.edu/etdc/view?acc_num=osu1488190109870444.
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35
Lögdberg, Ola. "Turbulent boundary layer separation and control /." Stockholm : Mekanik, Kungliga Tekniska högskolan, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-9821.
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36
MacKinnon, Andrew David. "VHF Boundary Layer Radar and RASS." Title page, abstract and table of contents, 2001. http://hdl.handle.net/2440/37807.
Testo completoAbstract (sommario):
This thesis describes the refinements, modifications and additions to a prototype Very High Frequency (VHF) Boundary Layer (BL) Spaced Antenna (SA) radar initially installed at the University of Adelaide's Buckland Park field site in 1997. Previous radar observations of the lowest few kilometres of the atmosphere, in particular the Atmospheric Boundary Layer, have used Ultra-High Frequency (UHF) radars. Unlike VHF radars, UHF radars are extremely sensitive to hydro-meteors and have difficulty in distinguishing clear-air echoes from precipitation returns. The advantages and requirements of using a VHF radar to observe the lowest heights is discussed in conjunction with some of the limitations. The successful operation of the system over long periods has enabled in-depth investigation of the performance of the system in a variety of conditions and locations. Observations were made from as low as 300m and as high as 8 km, dependent upon conditions. Comparisons between the radar and alternative wind measuring devices were carried out and examined. The antenna system of the radar is a critical component which was analysed in depth and subsequently re-designed. Through the use of numerical models and mea- surements, evaluation of different designs was accomplished. Further calibration of the remaining components of the full system has enabled estimations of the absolute received power. Additional parameters which can be derived with a calibrated radar were compared with values obtained by other authors, giving favourable results. Full Correlation Analysis (FCA) is the predominant technique used in this work. A brief discussion of the background theory and parameters which can be measured is described. A simple one-dimensional model was developed and combined with a 'radar backscatter model' to investigate potential sources of errors in the parameters determined using FCA with the VHF Boundary Layer Radar. In particular, underes- timations in the wind velocity were examined. The integration of a Radio Acoustic Sounding System (RASS) to obtain tempera- ture profiles is discussed. The theory of RASS measurements including the limitations and considerations which are required for the VHF BL radar are given. The difficulties encountered trying to implement such a system and the subsequent success using a Stratospheric Tropospheric (ST) Profiler in place of the BL radar is presented. Taken as a whole this thesis shows the success of the VHF BL to obtain mea- surements from as low as 300m. The validation of this prototype radar provides an alternative and, in certain situations, a superior device with which to study the lower troposphere.<br>Thesis (Ph.D.)--Department of Physics and Mathematical Physics, 2001.
37
Ha, Siew-Mun. "An experimental study of coherent structures in a three-dimensional turbulent boundary layer." Diss., This resource online, 1993. http://scholar.lib.vt.edu/theses/available/etd-07122007-103942/.
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38
Breitfeld, Oliver. "Improving the performance of aerofoil sections using momentum transfer via a secondary flow." Thesis, University of South Wales, 2002. https://pure.southwales.ac.uk/en/studentthesis/improving-the-performance-of-aerofoil-sections-using-momentum-transfer-via-a-secondary-flow(a3627a17-2b56-4e3a-9c73-64e0e105f3d6).html.
Testo completoAbstract (sommario):
Aerodynamic flow control can improve aerofoil performance by influencing the natural growth of boundary layers, which develop on the surface of vehicles moving in viscous fluids. Many active and passive techniques have been developed to reduce drag and/or increase the lift of aerofoil sections. The work presented in this thesis is concerned with the active excitation of the boundary layer on the suction side of aerofoil sections through momentum transfer via a secondary flow. The secondary flow was achieved by air passing through an air breathing device (ABD) which was implemented in the aerofoil surface. This resulted in an almost tangential and uni-directional fluid interaction. Numerical and experimental work showed a beneficial influence of the secondary flow on the aerodynamic characteristics of the studied aerofoil sections. A Taguchi analysis was initially used to confirm findings from previous work on the use of an ABD on a NACA0012 aerofoil section. The resulting parameter ranking showed general agreement with previous data in that the most important parameters are the gap-size i.e. the length over which the two fluids are in contact and the velocity gradient between the two fluids. However, it also raised questions that required an additional in-depth analysis of the parameters governing the flow control process. Due to the greater importance to the modern aviation industry of the NACA65-415 aerofoil section this particular cambered aerofoil section was used for further investigations. This study highlighted the importance of the velocity gradient between the main and secondary flows as well as the location of interaction of the ABD. In addition the gap-size is also important. Consideration of the power requirements for the ABD indicated that this may limit exploitation of the device. An evolutionary search strategy based on genetic algorithms, was employed to optimize the air breathing geometry. This optimisation produced non-intuitive geometries which revealed the importance of promoting an inner fluid recirculation in the device. Finally experimental data in a closed loop wind-tunnel showed trends which were in general agreement with the numerical predictions. However, the measurements indicated significantly greater enhancements of lift forces than those predicted by thenumerical investigation.
39
Halatchev, Iordan Atanassov. "Hydrodynamic stability of boundary-layer flows in the presence of mass transfer." Title page, contents and abstract only, 2000. http://web4.library.adelaide.edu.au/theses/09PH/09phh157.pdf.
Testo completoAbstract (sommario):
Bibliography: leaves 201-207. This thesis presents studies of the non-linear mass-transfer kinetics and a linear analysis of the hydrodynamic stability of systems under conditions of intense interfacial mass transfer.
40
Blay, Carreras Estel. "Transitional periods of the atmospheric boundary layer." Doctoral thesis, Universitat Politècnica de Catalunya, 2014. http://hdl.handle.net/10803/277380.
Testo completoAbstract (sommario):
The atmospheric boundary layer is the part of the troposphere influenced by the presence of the surface, and where most weather phenomena occur. During the day, with fair weather conditions, a convective boundary layer exists. In contrast, during the night, a stable boundary layer appears. It is important to note that the evolution from a convective boundary layer to a stable boundary layer and vice versa happens through two transitional processes. Due to its complexity and the rapid variability, there is a lack of studies about the morning or afternoon/evening transitions.
This thesis wants to solve some of the uncertainties related with the morning and afternoon/evening transition of the atmospheric boundary layer. It is based on observations from the project Boundary-Layer Late Afternoon and Sunset Turbulence (BLLAST) and numerical simulation experiments developed with mixed-layer and large-eddy simulation models.
In this thesis, we develop an analysis focused on the role played by the residual layer during the morning transition and by the large-scale subsidence on the evolution of the boundary layer. DALES numerical experiments that include the residual layer are capable of modeling the observed sudden increase of the boundary-layer depth during the morning transition and the subsequent evolution of the boundary layer. These simulations show a large increase in the entrainment buoyancy flux when the residual layer is incorporated into the mixed layer. We also examine how the inclusion of the residual layer above a shallow convective boundary layer modifies the turbulent kinetic energy budget. Large-scale subsidence mainly acts when the boundary layer is fully developed and, for the studied day, it is necessary to consider this in order to reproduce the afternoon observations. Finally, we also investigate how CO2 stored the previous night in the residual layer plays a fundamental role in the evolution of the CO2 mixing ratio during the following day.
Moreover, we hypothesize that during the evening transition a delay exists between the instant when the buoyancy flux goes to zero and the time when the local gradient of the virtual potential temperature changes sign contradict the assumption in which are base the gradient-based turbulence models. The results from this work confirm and quantify a flux-gradient delay. Specifically, the observed values of the delay are between approximately 30 and 80 min. The existence of the delay and its duration can be explained by considering the convective time and the competition of forces associated with the classical Rayleigh-Bénard problem. This combined theory predicts that the last eddy formed should produce a delay when the sensible heat flux changes sign during the evening transition. It appears that this last eddy is decelerated through the action of turbulent momentum and thermal diffusivity, and that the delay is related to the convective turnover time. Observations indicate that, as horizontal shear becomes more important, the delay time apparently increases to values greater than the convective turnover time.
Finally, we study the existence and characteristics of Lifted Temperature Minimum (LTM) during the evening transition. The study shows that LTM can be detected in calm conditions already during day¿night transition, several hours earlier than the usual time of occurrence reported by previous works. These conditions are fulfilled when weak synoptic forcing exists and the local flow shifts from valley to mountain breeze in a relatively complex orography. Under these special conditions, turbulence becomes a crucial parameter to determine the ideal conditions for observing LTM. Additionally, the correlation of longwave radiation measured at 0.8 m and estimated at the ground varies when the LTM is observed. Therefore, LTM is also related to a change in the atmospheric radiative characteristics under calm conditions.<br>La capa límit atmosfèrica és la part de la troposfera influenciada per la presència de la superfície terrestre, i on es produeixen la majoria dels fenòmens meteorològics. Durant el dia, en condicions de bon temps, es forma una capa límit convectiva. En canvi, durant la nit, apareix una capa límit estable. L'evolució d'una capa límit convectiva a una capa límit estable i viceversa passa a través de dos processos de transició. A causa de la seva complexitat i la ràpida variació, hi ha una manca d'estudis sobre les transicions del dia a la nit o viceversa.
Aquesta tesi vol resoldre algunes de les incerteses relacionades amb les transicions de la capa límit atmosfèrica. La tesi es basa en les observacions obtingudes durant la campanya Boundary-Layer Late Afternoon and Sunset Turbulence i simulacions numèriques desenvolupades amb dos models: un model de capa de mescla i un model de tipus large-eddy simulation.
Primerament, es desenvolupa una anàlisi centrada en el paper de la capa residual durant la transició entre la nit i el dia i en el paper de la subsidència en l'evolució de la capa límit. Les simulacions que inclouen la capa residual són capaces de modelar l’augment sobtat de l’alçada de la capa límit durant aquesta transició i també la posterior evolució de la capa límit. Aquestes simulacions mostren un gran augment del flux que entra des de la atmosfera lliure quan la capa residual s'incorpora a la capa convectiva. També s’analitza els efectes de considerar la capa residual en el balanç d'energia cinètica turbulenta. La subsidència actua principalment quan la capa límit està totalment desenvolupada i , pel dia seleccionat, cal tenir-la en compte per tal de reproduir les observacions durant la tarda. Finalment, també investiguem com el diòxid de carboni (CO2) emmagatzemat a la capa residual juga un paper fonamental en l'evolució del CO2 durant el dia.
En segon lloc, s'analitza el fet de que durant la transició del dia a la nit hi ha una demora entre el moment en què el flux de flotació esdevé zero i el moment en què el gradient de la temperatura potencial virtual canvia de signa. Aquest fet contradiu el supòsit en què estan basats els models de simulació. Els resultats d'aquest treball confirmen i quantifiquen aquest demora.
Específicament, els valors observats de la demora són aproximadament d'entre 30 i 80 min. L'existència de la demora i la seva durada es pot explicar a través del temps convectiu i de les forces associades amb el problema de Rayleigh-Bénard. La teoria proposada considera que l'últim remolí format abans de cessament del flux de superfície produeix el retard en el canvi de signe del gradient del flux de calor. Alhora aquest últim remoli es pot frenar a través de l'acció de la viscositat i la difusivitat tèrmica, i el retard està relacionat amb el temps que dura el recorregut del remolí. Les observacions indiquen que, com més important és el cisallament de vent horitzontal, el temps de retard aparentment augmenta a valors més grans que el temps convectiu.
Finalment , s'estudia l'existència i característiques d'un Mínim Elevat de Temperatura (LTM) durant la transició entre el dia i la nit. L'estudi mostra que el LTM es pot detectar en condicions de calma durant aquesta transició hores abans de l'hora d’observació descrita en els treballs anteriors. Aquestes condicions de calma es compleixen quan les forces sinòptiques són febles i el vent local canvia de direcció en una orografia relativament complexa (durant el dia prové de la vall i durant la nit de la muntanya). En aquestes condicions especials, la turbulència es converteix en un paràmetre fonamental per determinar les condicions ideals per a l'observació de LTM. A més, la correlació de la radiació d'ona llarga mesurada a 0,8 m i estimada a terra varia quan s'observa el LTM. Per tant, el LTM també està relacionada amb un canvi de les característiques radiatives de l’atmosfera en condicions de calma.
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Hubrich, Klaus. "Highly loaded compressor with boundary layer suction /." Lausanne : EPFL, 2005. http://library.epfl.ch/theses/?nr=3237.
Testo completo
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Schafer, Robert. "Tropical island boundary layer structure and development /." [Sydney : University of Technology, Sydney], 1998. http://grison.colorado.edu/Robert/paper/phd.pdf.
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Teakle, Ian Alan Lester. "Coastal boundary layer and sediment transport modelling /." [St. Lucia, Qld.], 2006. http://www.library.uq.edu.au/pdfserve.php?image=thesisabs/absthe19317.pdf.
Testo completo
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Orton, Nigel Paul. "Boundary Layer Energy Transport in Plasma Devices." NCSU, 2000. http://www.lib.ncsu.edu/theses/available/etd-20000427-184805.
Testo completoAbstract (sommario):
<p>The purpose of this research was to develop a model of boundary-layer energy transport in electric launchers, and perform a numerical simulation to investigate the influence of turbulence, thermal radiation and ablation on energy flux to plasma-facing surfaces. The model combines boundary-layer conservation equations with a k-omega turbulence model and multi-group radiation transport, and uses plasma models for fluid properties such as viscosity, thermal conductivity and specific heat capacity. The resulting TURBFIRE computer code is the most comprehensive simulation to date of boundary-layer turbulence and radiation transport in electric launcher plasmas.<p>TURBFIRE was run for cases with and without ablation. Temperature and velocity profiles are presented for all code runs, as are values of heat flux to the wall. The results indicate that both radiation transport and turbulence are important mechanisms of energy transport in the boundary layer, and therefore that both should be modeled in future simulations. Additionally, heat flux to the wall via both conduction and radiation was found to be significant for all cases run. Other authors have theorized that conduction could be neglected, but the current results show that this is not the case near the wall.<p>This research is also novel for its advances in computational fluid dynamics (CFD). The energy equation was written in terms of internal energy and discretized in a manner more implicit than in typical CFD codes. These changes were necessary to enable the code to accurately calculate heat capacity, which changes greatly with temperature for even weakly-ionized plasmas. Additionally, zero-gradient boundary conditions were used at the free stream for the turbulent kinetic energy and its dissipation rate (k and omega). Experimentally determined freestream values of k and omega are typically used in CFD codes, but these data are not available for most plasma devices.<P>
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Miller, Teresa S. "Turbulent boundary layer models for acoustic analysis." Diss., Wichita State University, 2011. http://hdl.handle.net/10057/3933.
Testo completoAbstract (sommario):
An analysis of the three types of turbulent boundary layer (TBL) models for acoustic analysis is presented because current preferred models over-predict TBL contributions to aircraft interior noise predictions. The mean square pressure is a measure of the total energy due to the pressure fluctuations beneath a turbulent boundary layer. The single point wall pressure spectrum sorts the energy into frequencies. The normalized wavenumber-frequency spectrum sorts the energy into wavenumbers. The pressure fluctuations beneath a turbulent boundary layer are found by solving the Poisson equation. In this work, the Poisson equation is solved both numerically and analytically using data from an LES/DES simulation. The numerical solution uses the point Gauss-Seidel method and has reasonable results. The analytical solution uses an eigenvalue expansion method that is less successful. The empirical mean square pressure models predict a relatively large spread in the pressure fluctuation values. It is difficult to draw any meaningful conclusions on which mean square pressure model is preferred when compared to data from the Spirit AeroSystems 6x6 duct. The single point wall pressure spectrum models are evaluated and the two more modern models of Smol’yakov and Goody seem to perform the best. These models are also compared to data from the Spirit AeroSystems 6x6 duct. The spectrum at low frequencies rolled off similar to the Goody model. This analysis indicates that the Goody model is the appropriate single point wall pressure spectrum model for aircraft applications. Important features of the normalized wavenumber-frequency spectrum models are presented and can be classified as either separable or non-separable. Separable models in the Corcos normalized wavenumber-frequency spectrum model class tend to over-predict the response for a range of cases. Both the non-separable Chase 1 and Smol’yakov-Tkachenko models appear to match the M.I.T. low noise, low turbulence wind tunnel data throughout the range of comparison. The Smol’yakov-Tkachenko model does not lend itself to straight forward Fourier transforms needed by the acoustic models. But the Chase 1 model can be converted from wavenumber-frequency spectrum to the cross spectrum, so it is the preferred model for aircraft applications. Therefore, the preferred turbulent boundary layer models for aircraft interior noise predictions are the single point wall pressure spectrum model of Goody and the normalized wavenumber-frequency spectrum model of Chase 1.<br>Dissertation (Ph.D.)--Wichita State University, College of Engineering, Dept. of Aerospace Engineering
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Dyachenko, Evgueniya, and Nikolai Tarkhanov. "Degeneration of boundary layer at singular points." Universität Potsdam, 2012. http://opus.kobv.de/ubp/volltexte/2012/6013/.
Testo completoAbstract (sommario):
We study the Dirichlet problem in a bounded plane domain for the heat
equation with small parameter multiplying the derivative in t. The behaviour of solution at characteristic points of the boundary is of special interest.
The behaviour is well understood if a characteristic line is tangent to the boundary with contact degree at least 2. We allow the boundary to not only have contact of degree less than 2 with a characteristic line but also a cuspidal singularity at a characteristic point. We construct an asymptotic solution of the problem near the characteristic point to describe how the boundary layer degenerates.
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Shaikh, F. N. "Turbulent spots in a transitional boundary layer." Thesis, University of Cambridge, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.319596.
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Moller, Sarah Julia. "Nitrogen oxides in the remote boundary layer." Thesis, University of York, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.535054.
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Boutle, Ian. "Boundary-Layer Processes in Mid-latitude Cyclones." Thesis, University of Reading, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.520124.
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Harvey, Natalie Jane. "Boundary-layer type classification and pollutant mixing." Thesis, University of Reading, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.654495.
Testo completo