Dissertations / Theses on the topic 'Atmospheric turbulence'
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Nicholls, Thomas William. "Atmospheric turbulence measurements relevant to adaptive optics." Thesis, Imperial College London, 1996. http://hdl.handle.net/10044/1/11487.
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Song, Xuegui. "Subcarrier optical wireless communications in atmospheric turbulence." Thesis, University of British Columbia, 2014. http://hdl.handle.net/2429/50010.
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In this thesis, we focus on performance study of subcarrier optical wireless communications (OWC) systems in atmospheric turbulence. Laser beam propagating through turbulence channel induces scintillation and phase aberration, which can result in significant performance degradation. The error rate performance of subcarrier OWC systems with lower order phase-shift keying (PSK) over the Gamma-Gamma turbulence channels is investigated using a direct integration method. Such analysis is generalized to M-ary PSK modulated OWC systems over strong atmospheric turbulence channels through a moment generating function approach. Since noncoherent or differentially coherent modulation schemes do not require carrier phase information at the receiver, such modulation schemes can be employed in scenarios where the carrier phase information cannot be tracked accurately. Error rate expressions of such OWC systems with noncoherent or differentially coherent modulation schemes over the Gamma-Gamma turbulence channels are also developed. Asymptotic relative performance of subcarrier OWC systems with noncoherent or differentially coherent modulation schemes with respect to those using coherent modulation schemes is quantified analytically.
Besides atmospheric turbulence, other sources can also introduce performance degradation to an outdoor OWC system. Through our analysis, it is shown that pointing errors can also degrade the performance of an OWC system significantly. The effect of pointing errors together with the Gamma-Gamma turbulence channel can be evaluated using the analytical tools we developed. As another performance impairment source, the effect of carrier phase synchronization error on performance of subcarrier binary PSK and quadrature PSK systems over atmospheric turbulence channels is also studied. In order to quantify the asymptotic noise reference loss for OWC systems over the lognormal channels, a novel auxiliary random variable technique is introduced.
Since the performance of outdoor subcarrier OWC systems is found to be impaired severely under strong turbulence conditions, spatial diversity techniques are introduced to mitigate the effects of turbulence induced fading. Multiple-input multiple-output OWC systems with repetition code and the Alamouti type orthogonal space-time block code are considered for the Gamma-Gamma turbulence channels. The performance analysis confirms that repetition code outperforms orthogonal space-time block code although both schemes achieve full diversity.Applied Science, Faculty of
Engineering, School of (Okanagan)
Graduate
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Robinson, C. G. "The effect of atmospheric turbulence of trains." Thesis, University of Nottingham, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.235563.
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Ayers, Geoffrey Robert. "Correlation techniques for imaging through atmospheric turbulence." Thesis, Imperial College London, 1989. http://hdl.handle.net/10044/1/47343.
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Charrier, Benoit. "Drag considerations for flight in atmospheric turbulence." Thesis, Virginia Polytechnic Institute and State University, 1989. http://hdl.handle.net/10919/52080.
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The distribution of lift between the wing and tail surfaces of a conventional aircraft is examined in order to determine the combination that would produce the minimum drag for a given lift. Further, the center of gravity (CG) position which gives the desired lift distribution and at the same time, maintains aircraft trim is determined.
Furthermore, a classic set of non-linear equations of motion for longitudinal flight is reduced to a set of linear equations by linearization. The location of the CG of the aircraft is then changed and a linear feedback control law is used to retain the dynamic characteristic (flying qualities) of the airplane. The response of the aircraft to an external disturbance such as a gust (modeled with a stochastic process) is studied in terms of drag versus CG position.
Finally, it is shown that the position of the CG for minimum drag should be determined with consideration of the expected atmospheric turbulence.Master of Science
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Canavero, Flavio Giovanni. "Variability of atmospheric pressure spectra in the Po Valley." Diss., Georgia Institute of Technology, 1986. http://hdl.handle.net/1853/26029.
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Mohr, Judy Lynette. "Atmospheric Turbulence Characterisation Using Scintillation Detection and Ranging." Thesis, University of Canterbury. Department of Physics and Astronomy, 2009. http://hdl.handle.net/10092/3195.
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Astronomical images taken by ground-based telescopes are subject to aberrations induced by the Earth's atmosphere. Adaptive optics (AO) provides a real-time solution to compensate for aberrated wavefronts. The University of Canterbury would like to install an AO system on the 1-m McLellan telescope at Mount John University Observatory (MJUO). The research presented in this thesis is the first step towards this goal. To design an effective AO system it is important to understand the characteristics of the optical turbulence present at a site. Scintillation detection and ranging (SCIDAR) is a remote sensing method capable of measuring the refractive index structure constant, Cn2(h), and the wind velocity profile, V(h). The dominant near ground turbulence (NGT) at MJUO required the use of both pupil-plane and generalised SCIDAR. A purpose-built SCIDAR system was designed and constructed at low cost, using primarily off-the-shelf components. UC-SCIDAR saw first light at MJUO in 2003, and has since undergone several revisions. The current version employs two channels for simultaneous pupil-plane and generalised SCIDAR measurements, and is very portable. Through the use of a different mounting plate the system could be easily placed onto any telescope. Cn2(h) profiling utilised standard analysis techniques. V(h) profiling using data from a 1-m telescope is not common, and existing analysis techniques were extended to provide meaningful V(h) profiles, via the use of partial triplet analysis. Cn2(h) profiling between 2005 and 2007 indicate strong NGT and a weak turbulent layer located at 12 - 14 km above sea level, associated with the tropopause region. During calm weather conditions, an additional layer was detected at 6 - 7 km above sea level. V(h) profiles suggest that the tropopause layer velocity is nominally 12 - 30 m/s, and that NGT velocities range from 2 m/s to over 20 m/s, dependent on weather. Little seasonal variation was detected in either Cn2(h) or V(h) profiles. The average coherence length, $r_0$, was found to be 12+-5 cm and 7+-1 cm for pupil-plane and generalised measurements respectively, for a wavelength of 589 nm. The average isoplanatic angle, $\theta_0$, was 1.5+-0.5 arcseconds and 1.1+-0.4 arcseconds for pupil-plane and generalised profiles respectively. No seasonal trends could be established in the measurements for the Greenwood frequency, $f_G$, due to gaps present in the V(h) profiles obtained. A modified Hufnagel-Valley (HV) model was developed to describe the Cn2(h) profiles at MJUO. The estimated $r_0$ from the model is 6 cm for a wavelength of 589 nm, corresponding to an uncompensated angular resolution, $\theta_{res}$, of 2.5 arcseconds. $\theta_0$ is 0.9 arcseconds. A series of V(h) models were developed, based on the Greenwood wind model with an additional Gaussian peak located at low altitudes, to encompass the various V(h) profiles seen at MJUO. Using the modified HV model for Cn2(h) profiles and the suggested model for V(h) profiles in the presence of moderate ground wind speeds, $f_G$ is estimated at 79 Hz. The Tyler frequency, $f_T$, is estimated at 11 Hz. Due to financial considerations, it is suggested that the initial AO design for MJUO focuses on the correction of tip/tilt only, utilising self-guiding, as it is unlikely that any suitable guide stars would be sufficiently close to the science object. The low $f_T$ suggests that an AO system with a bandwidth in the order of 60 Hz would be adequate for tip/tilt correction.
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Turner, Barry John. "Studies of atmospheric turbulence using the wavelet transform." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape11/PQDD_0018/NQ50273.pdf.
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Niu, Mingbo. "Coherent optical wireless communications over atmospheric turbulence channels." Thesis, University of British Columbia, 2012. http://hdl.handle.net/2429/43813.
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Recent advances in free-space optics have made outdoor optical wireless communication (OWC)
an attractive solution to the “last-mile” problem of broadband access networks. Significant chal-
lenges can, however, arise for OWC links with increased levels of atmospheric turbulence from
time-varying temperatures and pressures. As a promising alternative to the current generation of
on-off keying (OOK) direct detection based OWC system, the coherent OWC system is studied
in this thesis for a variety of turbulence conditions. Since coherent OWC system performance is
found to be impaired severely under strong turbulence conditions, spatial diversity techniques, e.g.,
maximum ratio combining (MRC), equal gain combining (EGC), and selection combining (SC),
are adopted to overcome turbulence impacts. The results are then generalized to Gamma-Gamma
turbulence for MRC and EGC with perfect channel or phase estimation. The impacts of phase
noise compensation error on coherent OWC system performance are investigated, and it is found
that such impacts can be small when the standard deviation of the phase noise compensation error
is kept below twenty degrees. A postdetection EGC scheme using differential phase-shift keying
(PSK) is proposed and is shown to be a viable alternative to overcome phase noise impacts.
The subcarrier intensity modulation (SIM) based OWC system has been proposed as another
alternative to the OOK system. With a unified average signal-to-noise ratio definition, system per-
formance is compared for coherent and SIM links over the Gamma-Gamma turbulence channels.
Closed-form error rate expressions are derived for coherent and SIM systems using MRC, EGC and
SC schemes. It is found that the coherent systems outperform the SIM systems significantly. The
benefits of coherent systems come chiefly from the large local oscillator power which eliminates
the effects of the thermal and ambient noises that dominate in SIM systems.
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Lazeroms, Werner. "Turbulence modelling applied to the atmospheric boundary layer." Doctoral thesis, KTH, Turbulens, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-166806.
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Turbulent flows affected by buoyancy lie at the basis of many applications, both within engineering and the atmospheric sciences. A prominent example of such an application is the atmospheric boundary layer, the lowest layer of the atmosphere, in which many physical processes are heavily influenced by both stably stratified and convective turbulent transport. Modelling these turbulent flows correctly, especially in the presence of stable stratification, has proven to be a great challenge and forms an important problem in the context of climate models. In this thesis, we address this issue considering an advanced class of turbulence models, the so-called explicit algebraic models.In the presence of buoyancy forces, a mutual coupling between the Reynolds stresses and the turbulent heat flux exists, which makes it difficult to derive a fully explicit turbulence model. A method to overcome this problem is presented based on earlier studies for cases without buoyancy. Fully explicit and robust models are derived for turbulence in two-dimensional mean flows with buoyancy and shown to give good predictions compared with various data from direct numerical simulations (DNS), most notably in the case of stably stratified turbulent channel flow. Special attention is given to the problem of determining the production-to-dissipation ratio of turbulent kinetic energy, for which the exact equation cannot be solved analytically. A robust approximative method is presented to calculate this quantity, which is important for obtaining a consistent formulation of the model.The turbulence model derived in this way is applied to the atmospheric boundary layer in the form of two idealized test cases. First, we consider a purely stably stratified boundary layer in the context of the well-known GABLS1 study. The model is shown to give good predictions in this case compared to data from large-eddy simulation (LES). The second test case represents a full diurnal cycle containing both stable stratification and convective motions. In this case, the current model yields interesting dynamical features that cannot be captured by simpler models. These results are meant as a first step towards a more thorough investigation of the pros and cons of explicit algebraic models in the context of the atmospheric boundary layer, for which additional LES data are required.QC 20150522
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Vaucher, Gail M. Tirrell. "Correlation of atmospheric optical turbulence and meteorological measurements." Thesis, Monterey, California. Naval Postgraduate School, 1989. http://hdl.handle.net/10945/25793.
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Bizinos, Nicholas. "Passenger comfort during formation flight within atmospheric turbulence." Master's thesis, University of Cape Town, 2012. http://hdl.handle.net/11427/12050.
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Includes abstract.Includes bibliographical references.
Formation flight is currently being investigated as a means to reduce drag and improve fuel efficiency in commercial aviation. In light of this, the potential for passenger discomfort due to the formation flying through free air turbulence was considered in this study. In an attempt to approximately ascertain the increase in discomfort, a simple formation flight aerodynamic model for two aircraft in formation was developed. The wing trailing vortices were assumed to shift in an ideal fashion within atmospheric turbulence resulting in aerodynamic disturbance loads acting on the trailing aircraft. As the sensitivity of the human body to vibrations is frequency dependent, spectral representation of atmospheric turbulence was incorporated. Monte Carlo simulations were done for various levels of turbulence intensity.
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Lee, Christopher Francis. "Use of wind profilers to quantify atmospheric turbulence." Thesis, University of Manchester, 2011. https://www.research.manchester.ac.uk/portal/en/theses/use-of-wind-profilers-to-quantify-atmospheric-turbulence(d6a12ed2-533a-4dae-9f0d-747bc0b4c725).html.
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Doppler radar wind profilers are already widely used to measure atmospheric winds throughout the free troposphere and stratosphere. Several methods have been developed to quantify atmospheric turbulence with such radars, but to date they have remained largely un-tested; this thesis presents the first comprehensive validation of one such method. Conventional in-situ measurements of turbulence have been concentrated in the surface layer, with some aircraft and balloon platforms measuring at higher altitudes on a case study basis. Radars offer the opportunity to measure turbulence near continuously, and at a range of altitudes, to provide the first long term observations of atmospheric turbulence above the surface layer. Two radars were used in this study, a Mesosphere-Stratosphere-Troposphere (MST) radar, at Capel Dewi, West Wales, and the Facility for Ground Based Atmospheric Measurements (FGAM) mobile boundary layer profiler. In-situ measurements were made using aircraft and tethered-balloon borne turbulence probes. The spectral width method was chosen for detailed testing, which uses the width of a radar's Doppler spectrum as a measure of atmospheric velocity variance. Broader Doppler spectra indicate stronger turbulence. To obtain Gaussian Doppler spectra (a requirement of the spectral width method), combination of between five and seven consecutive spectra was required. Individual MST spectra were particularly non-Gaussian, because of the sparse nature of turbulence at its observation altitudes. The width of Gaussian fits to the Doppler spectrum were compared to those from the `raw' spectrum, to ensure that non-atmospheric signals were not measured. Corrections for non-turbulent broadening, such as beam broadening, and signal processing, were investigated. Shear broadening was found to be small, and the errors in its calculation large, so no corrections for wind shear were applied. Beam broadening was found to be the dominant broadening contribution, and also contributed the largest uncertainty to spectral widths. Corrected spectral widths were found to correlate with aircraft measurements for both radars. Observing spectral widths over time periods of 40 and 60 minutes for the boundary layer profiler and MST radar respectively, gave the best measure of turbulence intensity and variability. Median spectral widths gave the best average over that period, with two-sigma limits (where sigma is the standard deviation of spectral widths) giving the best representation of the variability in turbulence. Turbulent kinetic energies were derived from spectral widths; typical boundary layer values were 0.13 m 2.s (-2) with a two-sigma range of 0.04-0.25 m 2.s (-2), and peaked at 0.21 m 2.s (-2) with a two-sigma range of 0.08-0.61 m 2.s (-2). Turbulent kinetic energy dissipation rates were also calculated from spectral widths, requiring radiosonde measurements of atmospheric stability. Dissipation rates compared well width aircraft measurements, reaching peaks of 1x10 (-3) m 2.s (-3) within 200 m of the ground, and decreasing to 1-2x10 (-5) m 2.s (-3) near the boundary layer capping inversion. Typical boundary layer values were between 1-3x10 (-4) m 2.s (-3). Those values are in close agreement with dissipation rates from previous studies.
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Li, Xiang-Yu. "Numerical approaches to droplet growth in atmospheric turbulence." Licentiate thesis, Stockholms universitet, Meteorologiska institutionen (MISU), 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-129868.
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The bottleneck problem of cloud droplet growth is one of the most challenging problems in cloud physics. Cloud droplet growth is neither dominated by con-densation nor gravitational collision in the size range of 15–40 μm in radius. Turbulence-generated collision has been thought to be the mechanism to bridge the size gap, i.e., the bottleneck problem. This study develops the numerical approaches to study droplet growth in atmospheric turbulence and investigates the turbulence effect on cloud droplet growth. The collision process of in-ertial particles in turbulence is strongly nonlinear, which motivates the study of two distinct numerical schemes. An Eulerian-based numerical formulation for the Smoluchowski equation in multi-dimensions and a Monte Carlo-type Lagrangian scheme have been developed to study the combined collision and condensation processes. We first investigate the accuracy and reliability of the two schemes in a purely gravitational field and then in a straining flow. Discrepancies between different schemes are most strongly exposed when con-densation and coagulation are studied separately, while their combined effects tend to result in smaller discrepancies. We find that for pure collision simulated by the Eulerian scheme, the mean particle radius slows down using finer massbins, especially for collisions caused by different terminal velocities. For the case of Lagrangian scheme, it is independent of grid resolution at early times and weakly dependent at later times. Comparing the size spectra simulated by the two schemes, we find that the agreement is excellent at early times. For pure condensation, we find that the numerical solution of condensation by the Lagrangian model is consistent with the analytical solution in early times. The Lagrangian schemes are generally found to be superior over the Eulerian one interms of computational performance. Moreover, the growth of cloud droplets in a turbulent environment is investigated as well. The agreement between the two schemes is excellent for both mean radius and size spectra, which gives us further insights into the accuracy of solving this strongly coupled nonlinear system. Turbulence broadens the size spectra of cloud droplets with increasing Reynolds number.
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Nichols-Pagel, Gerald A. "Relating measured turbulence and meteorological predictions /." Thesis, Connect to this title online; UW restricted, 2005. http://hdl.handle.net/1773/7137.
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Zink, Florian. "Gravity waves and turbulence in the lower atmosphere /." Title page, contents and abstract only, 2000. http://web4.library.adelaide.edu.au/theses/09PH/09phz778.pdf.
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Waite, Michael L. "On vortical and wave motion in stratified turbulence." Thesis, McGill University, 2005. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=86061.
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The dynamics of the atmospheric mesoscale and oceanic submesoscale are characterized by strong stratification and weak rotation, and the resulting energy spectra, though surprisingly universal, are still poorly understood. The aim of this work is to study the nonlinear dynamics and interactions of vortical motion (with potential vorticity) and internal inertia-gravity waves in the idealized context of homogeneous stratified turbulence, and to examine the extent to which they can account for the observations.We consider separately turbulence generated by vortical motion and internal waves using a combination of theory and numerical simulations. When vortical motion dominates the flow, the statistical mechanical equilibrium of the Boussinesq equations points to the absence of an inverse cascade of vortical energy. Instead, energy leaks into waves and cascades downscale. Our simulations show that the kz spectrum of vortical energy is flat out to kz ~ N/U (where N is the Brunt-Vaisala frequency and U is the root mean square velocity), which is consistent with the asymptotic limiting equations but not with the observations. Steeper spectra are obtained when waves are forced, but they are nevertheless shallower than the observations. At sufficiently strong stratifications, the wave energy spectra are found to be sensitive to the resolution of wave breaking and the presence of vortical motion. Bumpy spectra are obtained when no breaking occurs, but interactions with vortical modes cause the bumps to disappear. Overall, these results indicate that the observed atmosphere and ocean energy spectra are not universal properties of stratified turbulence, and theories for the spectra must take other factors into account.
In both the vortical and wave-dominated cases, U/N emerges as a key vertical length scale. Overturning is generated only when U/N is larger than the dissipation scale. Furthermore, in the vortical case, it is the scale at which different layers are coupled together. When rotation is introduced, the coupling scale evolves from U/N to the quasi-geostrophic scale (f/N)L (where f is the Coriolis parameter and L is the horizontal scale). This transition occurs at a relatively large Rossby number of O(1).
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Lim, Steven S. M. "Investigation of outer length scale in optical turbulence." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 2003. http://library.nps.navy.mil/uhtbin/hyperion-image/03Dec%5FLim.pdf.
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Thesis (M.S. in Combat Systems Technology)--Naval Postgraduate School, December 2003.Thesis advisor(s): D.L. Walters, D.K. Miller. Includes bibliographical references (p. 35-36). Also available online.
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Leblanc, Sylvain G. "Studies of turbulence with a wind profiler." Thesis, McGill University, 1994. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=22755.
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In Doppler radar analysis of the atmosphere the spectrum width is rarely used but it contains information about turbulence. Turbulence is not the only effect that contributes to the broadening of the spectrum. Another effect is the cross-beam wind, which can be dominant in broad-beam radars such as wind profilers. Once this effect is removed, the so-called residual width then serves as an indication of turbulence. A large snowstorm is used in this study for the computation of the residual width. Strong wind and wind shear were observed during the storm. The time-height pattern of residual width bears a close resemblance to that of wind shear. This supports the interpretation of the residual width as being an indication of turbulence induced by wind shear. Energy dissipation rates are also estimated for the snowstorm. In some regions values as large as 800 cm$ rm sp2 s sp{-3}$ are observed. These are large, but within the range of what has been reported by others. The same techniques were applied to the study of clear-air turbulence to relate radar reflectivity with turbulence.
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Riaz, Jamshed. "A simulation model of atmospheric turbulence for rotorcraft applications." Diss., Georgia Institute of Technology, 1992. http://hdl.handle.net/1853/12940.
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Palmer, R. H. "An atmospheric turbulence measurement system for an unmanned aircraft." Thesis, Cranfield University, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.392259.
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Cohn, Stephen Arthur. "Investigations of atmospheric turbulence with radar and thermodynamic soundings." Thesis, Massachusetts Institute of Technology, 1991. http://hdl.handle.net/1721.1/17296.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1992.Includes bibliographical references (p. 237-244).
by Stephen Arthur Cohn.
Ph.D.
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Boppe, Ravi Shankar. "Structure of turbulence in the marine atmospheric surface layer." Diss., Virginia Tech, 1995. http://hdl.handle.net/10919/37461.
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Turbulence research in the laboratory has confirmed the existence of quasi-coherent structures amidst the chaos of a turbulent boundary layer. It has been observed that a quasi-periodic phenomena called “bursting” accounts for a major contribution to the turbulent Reynolds stress and the production of turbulent kinetic energy. Bursting is the term used for a sequence of events, where a low-speed streak of fluid from the near wall region lifts away from the wall, slowly at first, and then rapidly moves away from the wall as it convects downstream where it becomes unstable and breaks up violently upon interaction with the outer flow. This ejection of low speed fluid into the mean flow is responsible for locally high values of turbulent kinetic energy. Although a great deal is known about these structures in laboratory flows, little has been done to investigate if such structures are universal in turbulent flows, i.e., their existence in large Reynolds number flows such as the turbulent air flow over the ocean. It would seem, intuitively, that such structures, if present in the marine atmospheric boundary layer, would play a major role in the transfer of momentum, mass and heat across the air-sea interface. It is speculated that these motions may also be associated with large scale organized motions in wall bounded turbulent shear flows. The effort aimed at elucidating the physics underlying such structures would be invaluable in contributing to our understanding of the air-sea flux mechanism.
In this dissertation, standard ejection detection schemes like the quadrant, the VITA and the modified u-level techniques have been applied to turbulent wind data measured over the ocean to confirm the existence of burst like structures. The proportions of contributions to the Reynolds stress from the four quadrants of the u’w’ plane are in close agreement with the corresponding contributions for a laboratory flow. Ejection detection followed by the grouping of ejections into bursts yielded a mean burst period of 47 s, at a height of 8.2 m above the water surface, where the mean wind velocity was 6.74 m/s. This burst period corresponds well with the peaks obtained from the autocorrelation of the streamwise velocity signal and the first moment of the stress spectrum, confirming the quasi-periodic nature of this phenomena. Furthermore, phase averages of these events show a structure which is similar to the structure of events detected in laboratory flows.
The ejection periods are seen to decrease with increasing wind speed. The burst periods decrease at first with increasing wind speed and then appear to attain a constant value after a wind speed of 6-7 m/s. This has been attributed to the breakdown of the grouping algorithm at higher wind speeds. Ejection and burst frequencies exhibit no discernible dependence on the surface wave field.
Ejection and sweep motions have been studied at various length scales. The original velocity signal is bandpass filtered for various frequency bands. For each band, the percentage contributions to the Reynolds stress from the quadrants of the u'w’ plane are close to the corresponding quadrant contributions of the other bands. This indicates similar turbulence structure at different scales. The velocity signals for each band have been normalized by their root mean square (RMS) value. Visualizing the signals on nondimensional time shows the signals from each band to be very similar. These results can also be interpreted as evidence for the ejection and sweep motions existing simultaneously at different scales, indicating the fractal nature of these events.
Large scale motions, which appear to be associated with ejection and sweep motions, have been identified in the marine atmospheric surface layer using velocity probe measurements at multiple heights. Visualizing these velocity signals suggests that the organized features extend across the depth of the surface layer. Converting the temporal signals to spatial fluctuations suggests that these structures are inclined at an angle while convecting downstream. The inclination angle near the surface (z < 18 m) is approximately 15° and it increases with increasing height to about 45° when z = 45 m. The spatial velocity fluctuations also indicate that these organized features may be large transverse vortical arches.Ph. D.
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Kellerer, Aglaé. "Assessing time scales of atmospheric turbulence at observatory sites." Paris 7, 2007. http://www.theses.fr/2007PA077085.
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La vie sur Terre est rendue possible grâce au rideau protecteur que constitue l'atmosphère avec sa masse équivalente à dix mètres d'eau, l'atmosphère est une condition préalable à l'apparition de la vie. Mais ce rideau rend la vie difficile aux astronomes qui préféreraient avoir une vue directe sur l'Univers. Il est donc indispensable de caractériser et de spécifier les conditions atmosphériques qui permettent la meilleure utilisation des systèmes d'optiques adaptatives et des interféromètres - les interféromètres, dont le potentiel est ici illustré à travers les observations d'un système de trois étoiles, δ Velorum. Le temps de cohérence de la turbulence est un paramètre essentiel qui détermine la sensibilité des interféromètres et la performance des systèmes d'optiques adaptatives. Il existe plusieurs instruments qui mesurent le temps de cohérence ou des paramètres reliés, mais tous ces instruments ont des limitations intrinsèques: ou bien ils nécessitent de grands télescopes, ou bien l'analyse des données est complexe, ou encore la méthode n'est sensible qu'à une partie de la turbulence. C'est pourquoi les campagnes de tests de sites et de monitoring reposent principalement sur la mesure du seeing, avec des instruments comme le Differential Image Motion Monitor, DIMM. Pour palier à ce manque, nous avons proposé un instrument pour mesurer le temps de cohérence: le Fast Defocus Monitor, FADE. La méthode consiste à transformer l'image d'une étoile, à travers un petit télescope de 0. 35 m de diamètre, en un anneau fin. La turbulence cause alors des variations temporelles du rayon de l'anneau, dont la vitesse et l'amplitude sont reliées au temps de cohérence et au seeing. Cette méthode est présentée ici avec les résultats de premières observationsThe research brought together in this thesis is concerned with the distortion of astronomical observations due to atmospheric fluctuations. These fluctuations are especially critical for the technique of interferometry whose potential is here exemplified by the study of δ Velorum. The focus of the research is, accordingly, on improved methods to assess the changing viewing conditions at existing observatories, as well as to determine the suitability of potential observatory sites. Site-testing and site-monitoring missions are usually directed at the assessment of the Fried parameter with instruments such as the Differential Image Motion Monitor, DIMM. An estimation of the coherence time requires then, in addition, wind-speed measurements by weather stations. A more refined evaluation is obtained with instruments such as the Multi Aperture Scintillation Sensor, MASS, that measure the altitude profiles of the index structure constant, C 2 n, and the wind speed with a resolution of about 500 m, and infer the coherence time from the integrated turbulence profiles. The main error in the estimated coherence time results from the turbulence below 500 m altitude not being accounted for. To avoid these complexities, we suggest the direct measurement of a quantity proportional to the coherence time. The variance of the defocus velocity is a suitable option, because it can be evaluated through fast and continuous sampling of the atmospheric defocus coefficient. The concept of a Fast Defocus Monitor, FADE, an instrument using a small telescope, some simple optics and a fast camera is described, and first measurements are presented. The final aim is to use FADE for site monitoring and site testing campaigns. A particularly challenging and interesting project will be to monitor the coherence time at Dome C
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Douds, Jeffrey T. "Investigation of outer length scale in optical turbulence using an acoustic sounder." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 2004. http://library.nps.navy.mil/uhtbin/hyperion/04Sept%5FDouds.pdf.
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Rune, Anna. "Turbulence Structure of Marine Stratocumulus." Thesis, Uppsala universitet, Institutionen för geovetenskaper, 2000. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-392951.
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Aircraft measurements are analysed from the “First Lagrangian” of the Atlantic Stratocumulus Transition Experiment (ASTEX) from south east of the Azores Islands. In this experiment, Lagrangian strategy was used and the marine air mass, that advected southward, was followed during 12 to 14 June 1992. During the experiment, the stratocumulus clouds transitioned into thin and broken stratocumulus with cumulus cloud penetrating from below. To characterise the vertical structure in the marine boundary layer the buoyancy fluxes, the variances, the turbulent kinetic energy, the momentum fluxes and humidity fluxes were examined. The buoyancy flux profiles were used to discover the decoupling of the stratocumulus and the sub-cloud layer. Turbulence analysis for all five flights shows that the cloud layer were decoupled from the underlying layer. In the cloud layer the buoyancy production due to longwave radiative cooling at cloud top, was the main source for driving the turbulence. In the sub-cloud layer, the variances indicate wind shear to be the main generator of turbulence for the first two days. Then, as sea surface temperature increases, buoyancy produced turbulence was more pronounced. The u-, v- and w-spectra and cospectra of wθ and uw give insight into the typical eddy sizes, and how the peak wavelengths vary with height. The peak wavelengths in sub-cloud and cloud layer were larger than layer depth and u- and v-spectral peak wavelengths often larger than the peak wavelength from w-spectra. While peak wavelengths in the sub-cloud layer vary with the height above the surface, they are approximately invariant with height in the cloud layer.
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Jansen, Malte F. (Malte Friedrich). "Equilibration of an atmosphere by geostrophic turbulence." Thesis, Massachusetts Institute of Technology, 2013. http://hdl.handle.net/1721.1/79329.
Full textAbstract:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2013.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 183-187).
A major question for climate studies is to quantify the role of turbulent eddy fluxes in maintaining the observed atmospheric mean state. It has been argued that eddy fluxes keep the mid-latitude atmosphere in a state that is marginally critical to the deepest mode of baroclinic instability, which provides a powerful constraint on the response of the atmosphere to changes in external forcing. A similar criterion does, however not hold in the Southern Ocean, a region whose dynamics are otherwise very similar to the mid-latitude atmosphere. This thesis resolves this apparent contradiction, using a combination of theoretical considerations and eddy-resolving numerical simulations. It is shown that the adjustment of the extra-tropical troposphere to states of marginal criticality does not follow from a fundamental constraint, but is rather the result of the particular parameters characterizing Earth's troposphere. Both marginally critical and strongly supercritical zonal mean flows can be obtained in planetary atmospheres if external parameters are varied. We argue that changes in the equilibrated mean state over a wide range of simulations can better be understood in terms of a balance between the diabatic forcing and the eddy driven overturning circulation. Using a diffusive closure for the eddy flux of potential vorticity, we can relate the eddy-driven overturning transport to properties of the mean flow, and derive scaling relations for both the baroclinicity and vertical stratification of the equilibrated state.
by Malte F. Jansen.
Ph.D.
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Mandock, Randal Lee Nicholas. "A multiple beam sodar for the measurement of atmospheric turbulence." Diss., Georgia Institute of Technology, 1997. http://hdl.handle.net/1853/25869.
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Bell, Martin J. "Wind pumping in a snow pack related to atmospheric turbulence." Thesis, University of Canterbury. Chemical and Process Engineering, 1993. http://hdl.handle.net/10092/6013.
Full textAbstract:
An extensive investigation has been carried out to evaluate the possible influence of wind pumping on snow metamorphism, and hence snow stability. Wind pumping refers to a pulsile forced flow of air in a snow pack due to surface pressure fluctuations.
An extensive review of atmospheric turbulence from related disciplines was carried out in order to identify the characteristics of turbulence in mountain terrain. This was followed by an experimental programme to characterise and measure the pressure fluctuations on the snow surface in the mountain terrain of Arthur's Pass National Park. Frequencies between 0.0005 and 1 Hz were studied and power spectral/correlation function analysis was made. Gust exceedance statistics were also calculated.
The turbulence intensities and rms pressure fluctuations were found to be significantly larger than those reported in the literature. The spectral analysis showed similar form to other documented spectra, but was shifted substantially toward lower frequencies. The same low frequency components, and a semi-periodic component was evident in the autocorrelation analysis. It was suggested that these features arise from turbulent structures shedding off large upstream structures and/or gravity wave activity.
Following this, laboratory tests on beds packed with granular materials were carried out to study wind pumping airflow dynamics and the influence of wind pumping airflow on diffusive mass transfer.
A mathematical basis for the dissipation of the surface pressure fluctuations in a permeable bed has been experimentally verified in the laboratory. Three one-dimensional models proposed were then applied to a wide variety of snow pack conditions. One model (finite-sealed) was found to be widely applicable to wind pumping modelling in the seasonal snow pack. A linear approximation to this model is applicable to most seasonal snow packs.
Sublimating naphthalene was used as a tracer in a set of laboratory experiments to measure the enhanced diffusive mass transfer rates in a packed column due to wind pumping. Compared to stagnant air diffusion rate, an enhancement of one to two orders of magnitude was measured in mass loss rates. The mass loss rate enhancement depended strongly on both the period and magnitude of the applied wind pumping. The mass loss per cycle showed a large, linear dependence on cyclic displacement. This showed that the naphthalene mass loss process was 1-D planar convective/diffusion controlled. A slight dependence on peak velocity was also found. This indicated that the naphthalene mass loss process had a small dependence on boundary layer diffusion.
Finally, cold laboratory experiments were carried out to directly study the effects of wind pumping in snow in a controlled environment. A number of strength tests were applied. The cone penetrometer is recommended as the most useful strength testing tool in this type of analysis.
Pressure fluctuations associated with gale force winds (60 to 90 km/hr) were used to provide strong wind pumping conditions. Two types of experiments were run, both in high density snow (=350 kg/m3):
Initially bonded grains had moderate to high temperature gradients imposed on them to evaluate any enhancements in the faceting process.
Grains without any initial bonding had low temperature gradients imposed upon them to evaluate any enhancements in the rounding process.
It was found that wind pumping enhanced the rate of the process in both cases.
There was a significant wind pumping enhancement of strength loss at high temperature gradients. The influence of wind pumping on the growth rates of depth hoar appears to be almost as significant as the influence of the temperature gradient magnitude. This wind pumping enhancement is most likely to occur when there is a large temperature difference near the surface of the snow pack between the air and snow or between two layers of snow.
There was a significant wind pumping enhancement of strength increase at low temperature gradients. The effect of wind pumping on the development of snow strength appears to be more important than the effect of elapsed time. The differences in enhancement with and without wind pumping appear to be larger at the beginning of the bonding process. It is conceivable that the wind pumping could play a major role in the initial bonding. It is therefore concluded that wind pumping is possibly a major contributing mechanism in the development of wind slabs.
Hence it can be concluded that wind pumping significantly enhances dry snow metamorphism.
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Chandrasekaran, Nivedita. "Bidirectional adaptive optics architectures for optical communication through atmospheric turbulence." Thesis, Massachusetts Institute of Technology, 2016. http://hdl.handle.net/1721.1/107323.
Full textAbstract:
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2016.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 223-231).
Free-space optical (FSO) classical communication links can provide high data rates vital for successfully serving the world's exponentially growing demand for bandwidth, while FSO quantum key distribution (QKD) links allow information-theoretic rather than computational secure communication between two parties. Unlike fiber-optic classical communication and QKD links, FSO links can do so with minimal up-front investments in infrastructure. Setting aside absorption and scattering losses along the propagation path, optical links with a terrestrial terminal will still experience the deleterious effects of clear-weather turbulence, namely beam spread, beam wander, angle-of-arrival spread, and scintillation, which leads to low end-to-end power transfer from the transmitter to receiver. Decreases in the power transfer result in lower communication rates and may result in no secure-key rate for the loss-sensitive QKD communication protocols. Adaptive optics holds the best promise for mitigating, if not completely compensating for, these turbulence-induced degradations. Nevertheless, despite adaptive optics being a richly developed field, theoretical studies of adaptive optics systems have not fully exploited the reciprocal nature of propagation through atmospheric turbulence. It is known that applying ideal, full-wave adaptive optics at both the transmitter and receiver of a free-space optical link can guarantee scintillation-free power transmission when operation is deep in the near-field power transfer regime. Buoyed by the advent of enabling technologies like scalable Mach-Zehnder interferometer arrays and coherent receiver arrays, this thesis: (1) introduces both a full-wave and phase-only bidirectional adaptive optics (BDAO) protocol; (2) assesses the ergodic performances of FSO classical and QKD communication links utilizing these BDAO systems using both theoretical performance bounds as well as turbulence simulation results; and (3) provides an initial study of the noise and time-dynamics the BDAO protocol can tolerate while still achieving near-optimal classical communication or QKD rates.
by Nivedita Chandrasekaran.
Ph. D.
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Halsig, Sebastian [Verfasser]. "Atmospheric refraction and turbulence in VLBI data analysis / Sebastian Halsig." Bonn : Universitäts- und Landesbibliothek Bonn, 2018. http://d-nb.info/116067339X/34.
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Yang, Zhijun. "Incoherent Imaging in the Presence of Atmospheric Turbulence and Refractivity." University of Dayton / OhioLINK, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=dayton1500284170323378.
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Schumm, Bryce Eric. "Effect of Atmospheric Turbulence on Synthetic Aperture Ladar Imaging Performance." University of Dayton / OhioLINK, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=dayton1509259463652724.
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Patel, Chinmay Karsandas. "Energy extraction from atmospheric turbulence to improve flight vehicle performance /." May be available electronically:, 2007. http://proquest.umi.com/login?COPT=REJTPTU1MTUmSU5UPTAmVkVSPTI=&clientId=12498.
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Bruno, Olimpia. "Turbulence models in the atmospheric boundary layer under convective conditions." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2014. http://amslaurea.unibo.it/6579/.
Full textAbstract:
In this work a modelization of the turbulence in the atmospheric boundary
layer, under convective condition, is made. For this aim, the equations that
describe the atmospheric motion are expressed through Reynolds averages
and, then, they need closures.
This work consists in modifying the TKE-l closure used in the BOLAM
(Bologna Limited Area Model) forecast model. In particular, the single
column model extracted from BOLAM is used, which is modified to obtain
other three different closure schemes: a non-local term is added to the flux-
gradient relations used to close the second order moments present in the
evolution equation of the turbulent kinetic energy, so that the flux-gradient
relations become more suitable for simulating an unstable boundary layer.
Furthermore, a comparison among the results obtained from the single
column model, the ones obtained from the three new schemes and the
observations provided by the known case in literature ”GABLS2” is made.
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Andrae, Ulf. "Turbulence structures in a non-stationary marine atmospheric boundary layer." Thesis, Uppsala universitet, Meteorologiska institutionen, 1996. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-392332.
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The vertical structure in the coastal marine atmosphere has been investigated using data from aircraft measurements performed along the Blekinge coast. The present data are from the third of October 1990. The main feature is fairly homogeneous horizontal conditions and a subceeing boundary layer which lowers from 600 meters down to about 50 meters during the day. The turbulence were found to be in a decreasing state. The turbulence parameters were normalized using normal stationary scaling, in order to compare with other results.
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Ghate, Virendra Prakash. "Turbulence and Mass-Transports in Stratocumulus Clouds." Scholarly Repository, 2009. http://scholarlyrepository.miami.edu/oa_dissertations/265.
Full textAbstract:
Boundary layer (BL) stratocumulus clouds are an important factor in the earth's radiation budget due to their high albedo and low cloud top heights. Continental BL stratocumulus clouds are closely coupled to the diurnal cycle and the turbulence in the BL affecting the surface energy and moisture budgets. In this study the turbulence and mass-transport structures in continental BL stratocumulus clouds are studied using data from the Atmospheric Radiation Measurements (ARM)'s Southern Great Plains (SGP) observing facility located at Lamont, Oklahoma. High temporal (4 sec) and spatial (45 m) resolution observations from a vertically pointing 35 GHz cloud Doppler radar were used to obtain the in-cloud vertical velocity probability density function (pdf) in the absence of precipitation size hydrometeors. A total of 70 hours of radar data were analyzed to report halfhourly statistics of vertical velocity variance, skewness, updraft fraction, downdraft and velocity binned mass-flux at five cloud depth normalized levels. The variance showed a general decrease with increase in height in the cloud layer while the skewness is weakly positive in the cloud layer and negative near cloud top. The updraft fraction decreases with height with the decrease mainly occurring in the upper half of the cloud layer. The downdraft fraction increases with decrease in height with the increase being almost linear. The velocity of eddies responsible for maximum mass-transport decreases from of 0.4 ms-1 near cloud base to 0.2 ms-1 near cloud top. The half-hour periods were then classified based on the surface buoyancy flux as stable or unstable and it was found that the variance near cloud top is higher during the stable periods as compared to the unstable periods. Classification was also made based on the cloud depth to BL depth ratio (CBR) being greater or less than 0.3. The variance profile was similar for the classification while the skewness was almost zero during periods with CBR less 0.3 and positive during periods with CBR greater than 0.3. A 14 hour period of stratocumulus cloud on March 25, 2005 was analyzed to study the diurnal changes in the turbulence structure and mass transports. The variance near cloud base during the day time when the BL turbulence is primarily due to surface buoyancy production was higher than during the nighttime when the BL turbulence is driven by radiative cooling near the cloud top. Output from a one dimensional radiative transfer model was analyzed to study the vertical structure of the radiative fluxes. A radiative velocity scale analogous to the surface convective velocity scale is proposed to assess the relative importance of radiative cooling near cloud top in generating turbulence compared with the surface buoyancy production. An attempt was also made to calculate the hourly liquid water flux by combining the high temporal resolution (20 sec) liquid water content estimates from the radar reflectivity and a microwave radiometer with the radar observed vertical velocity. The liquid water flux was found to peak at a level below the cloud top and show a divergence with height that was similar to that from model simulations.
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Spyksma, Kyle. "On the dynamics and predictability of moist turbulence." Thesis, McGill University, 2007. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=103296.
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In this thesis I present a simple, computationally-inexpensive moist turbulence model in order to study the differences between moist and dry turbulence. The model is validated by comparing a moist-bubble simulation with ones presented in Grabowski and Clark (1993) using a more-sophisticated model. We show that the outputs compare well and that our model can easily be extended to higher resolutions due to its simplified equations and uncomplicated implementation. Measurements of liquid water content spectra from the 3843 validation run are shown having shallow slopes, implying that moist processes require high resolution. Consideration is also given to the issue of Gibb's oscillations near sharp gradients, such as at a cloud boundary. It is shown that, due to our high resolutions, the dynamics of our model are not seriously affected if corrections are not made to address them.The model is used to study the small-scale predictability and dynamics of moist and dry shallow convective turbulence. Although moist flows are less predictable than their associated dry flows, we can account for the differences via a simple scaling. Using large-scale (the root-mean-squared vorticity) and small-scale (the dissipation wavenumber, kd) measures, we can reconcile classical predictability statistics from both wet and dry runs, with different lapse rates and relative humidities.
Finally, I present a more thorough investigation of the dynamical differences between wet and dry convective turbulence, and then consider the very small-scale (ℓ ≲ 10 m) variability of liquid water content and compare our high-resolution simulation results to existing in situ cumulus-cloud observations. We find that there is a small decrease in the spatial intermittency of vorticity in wet runs relative to dry ones. This is consistent with the idea that evaporation of the liquid water in the clouds reduces the instabilities that would lead to the most intense vortices. At the same time, the liquid water content spectra show that in these areas of intense mixing and cloud decay, the characteristic scale of variability is shifted to smaller scales compared to a passive scalar. Further integrations in which the convective forcing is removed show that as the amount of liquid water decreases through evaporation, there is delayed decay of the smallest scales of the cloud. These findings may explain the small-scale shallow liquid water content spectra from cumulus-cloud fly-through measurements reported in Davis et al. (1999).
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黎敦楠 and Tun-nam Lai. "Turbulent transport of airborne pollutant near a low hill." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2002. http://hub.hku.hk/bib/B31227491.
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Bartello, Peter. "Numerical models of turbulence with a wide range of scales." Thesis, McGill University, 1988. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=75855.
Full textAbstract:
Severe D-1 directional Fourier truncation of the equations for D-dimensional (D = 2, 3) incompressible flow leads to a small number of coupled PDE's in one space dimension with the same inviscid quadratic invariants as the original set. Numerically generated equilibria for inviscid truncated versions of the reduced systems agree with predicted energy-enstrophy (D = 2) and energy (D = 3) equipartition spectra based on statistical mechanical equilibrium. Viscous calculations for forced and decaying turbulence at high Reynolds number are consistent with k$ sp{-5/3}$ inverse energy and k$ sp{-3}$ direct enstrophy cascading inertial ranges in two dimensions and with a k$ sp{-5/3}$ direct energy cascading inertial range in three dimensions. The power-law behaviour is clear in the two-dimensional system, but less convincing in three dimensions due to the persistence of a shallow spectral range at low wavenumbers.Dissipation-range intermittency consisting of spatially intermittent vorticity gradients (D = 2) and vorticity (D = 3) is observed. In contrast to full numerical simulations, there is no tendency for isolated coherent structures to emerge in two dimensions. The model consequently mimics some, but not all, of the properties of the full set.
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Campbell, Laurence. "Stellar scintillation and its use in atmospheric measurements /." Title page, contents and summary only, 1991. http://web4.library.adelaide.edu.au/theses/09PH/09phc1885.pdf.
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Horne, Jimmy D. "Modeling optical turbulence with coamps during two observation periods at Vandenberg AFB /." Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 2004. http://library.nps.navy.mil/uhtbin/hyperion/04Mar%5FHorne.pdf.
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Thesis (M.S. in Meteorology and Physical Oceanography)--Naval Postgraduate School, March 2004.Thesis advisor(s): Douglas Miller. Includes bibliographical references (p. 67-70). Also available online.
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Valero, Marcos E. "Comparative Analysis of Atmospheric Turbulence-induced Laser Power Fluctuations in a Monolithic and Tiled Optical Receiver System." University of Dayton / OhioLINK, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=dayton1616587433479341.
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Yenice, Yusuf E. "Effects of atmospheric turbulence on ground-satellite optical communications and countermeasures." Thesis, University of Surrey, 1999. http://epubs.surrey.ac.uk/843086/.
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The theoretical advantages of ground-satellite optical communication systems can only be exploited, if means can be found to circumvent the limitations due to atmospheric effects. Atmospheric turbulence dominates the analysis and design of these links. The effects of atmospheric turbulence on the performance of ground-satellite optical communication systems and possible countermeasures are investigated in this thesis. The design and analysis of any optical system operating in the atmosphere requires empirical investigations of atmospheric turbulence conditions at the system's location. Stellar observations provide a useful and convenient means for this purpose in the case of ground-satellite optical communications. The available techniques are reviewed. The experimental setup for a particular technique is described and initial results are presented. We were involved in the first ever ground-satellite optical communication experiments conducted between Japanese Engineering Test Satellite VI and a ground station in Tokyo. One issue which has not yet been satisfactorily resolved is the probability density function (PDF) of intensity fluctuations. It was theoretically shown that the PDF approaches a negative exponential in the very strong turbulence region. Experimental evidence is presented in support of this prediction. The ETS-VI experiment results also confirm that too large a beam size can have significantly deleterious effects on fading performance. Early analyses predicted drastic reductions in uplink on-axis scintillation variance with increasing beam size. As the beam size is increased, the scintillation variance gradient off the beam centre becomes large, and eventually the limitation of the first-order theory is exceeded. An explicit limit on the beam size is identified in this thesis: the beam radius must not exceed a third of the coherence scale. Analyses also predict that appropriately converging the beam results in less scintillation compared to a collimated beam. During the ETS-VI experiment we were not able confirm this prediction. Supplements to the first-order theory also suggest that converging beams behave very similarly to collimated beams. This makes the uplink beam size the single most important adjustable parameter. Possible countermeasures to the atmospheric turbulence effects are identified and reviewed separately for the downlink and the uplink. It is emphasized that uplink transmitter beam size is a crucial design parameter and its optimum value changes continuously according to changing turbulence conditions along the propagation path. A previous study concluded that the optimum beam size is of the order of the coherence scale. It is shown that the optimum size is critically dependent on beam wander and pointing accuracy, and can in fact be much smaller. A novel countermeasure is proposed in which the uplink transmitter beam size is controlled in real time in response to measured turbulence parameters to maximize mean intensity and minimize fluctuations at the satellite receiver.
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Ito, Keiichi. "Range and endurance optimization of a sailplane gliding through atmospheric turbulence." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape11/PQDD_0001/MQ40898.pdf.
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Costello, Mark Francis. "A theory for the analysis of rotorcraft operating in atmospheric turbulence." Diss., Georgia Institute of Technology, 1992. http://hdl.handle.net/1853/12893.
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Rall, William Jeffrey. "Investigation of a CCD camera for measurements of optical atmospheric turbulence." Thesis, Monterey, California. Naval Postgraduate School, 1992. http://hdl.handle.net/10945/23935.
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Ijaz, Muhammad. "Experimental characterisation and modelling of atmospheric fog and turbulence in FSO." Thesis, Northumbria University, 2013. http://nrl.northumbria.ac.uk/15255/.
Full textAbstract:
Free space optical (FSO) communication uses visible or infrared (IR) wavelengths to broadcast high-speed data wirelessly through the atmospheric channel. The performance of FSO communications is mainly dependent on the unpredictable atmospheric channel such as fog, smoke and temperature dependent turbulence. However, as the real outdoor atmosphere (ROA) is time varying and heterogeneous in nature as well as depending on the magnitude and intensity of different weather conditions, carrying out a proper link assessment under specific weather conditions becomes a challenging task. Investigation and modelling the ROA under diverse atmospheric conditions is still a great challenge in FSO communications. Hence a dedicated indoor atmospheric chamber is designed and built to produce controlled atmosphere as necessary to mimic the ROA as closely as possible. The experimental results indicate that the fog attenuation is wavelength dependent for all visibility V ranges, which contradicts the Kim model for V < 0.5 km. The obtained result validates that Kim model needs to be revised for V < 0.5 km in order to correctly predict the wavelength dependent fog attenuation. Also, there are no experimental data and empirical model available for FSO links in diverse smoke conditions, which are common in urban areas. Therefore, a new empirical model is proposed to evaluate the wavelength dependent fog and smoke attenuation by reconsidering the q value as a function of wavelength rather than visibility. The BER performance of an FSO system is theoretically and experimentally evaluated for OOK- NRZ, OOK-RZ and 4-PPM formats for Ethernet line data-rates from light to dense fog conditions. A BER of 10-6 (Q-factor ≈ 4.7) is achieved at dense fog (transmittance, T = 0.33) condition using 4-PPM than OOK-NRZ and OOK-RZ modulation schemes due to its high peak-to-average power ratio albeit at the expense of doubling the bandwidth. The effects of fog on OOK-NRZ, 4-PAM and BPSK are also experimentally investigated. In comparison to 4-PAM and OOK-NRZ signals, the BPSK modulation signalling format is more robust against the effects of fog. Moreover, the effects of using different average transmitted optical communication powers Popton the T and the received Q-factor using the OOK-NRZ modulation scheme are also investigated for light and dense fog conditions. The results show that for an FSO system operating at a Q-factor of 4.7 (for BER = 10-6), the required Q-factor is achieved at T of 48% under the thick fog condition by increasing Popt to 1.07 dBm, whereas the values of T are 55% and ~70% for the transmit power of 0.56 dBm and -0.7 dBm, respectively. The experimental characterisation and investigation of the atmospheric turbulence effect on the Ethernet and Fast-Ethernet FSO link is reported using different modulation schemes. The experiment is carried out in a controlled laboratory environment where turbulence is generated in a dedicated indoor atmospheric chamber. The atmospheric chamber is calibrated to mimic an outdoor turbulence conditions and the measured data are verified against the theoretical predictions. The experiment also demonstrates methods to control the turbulence levels and determine the equivalence between the indoor and outdoor FSO links. The results show that the connectivity of Ethernet and Fast-Ethernet links are highly sensitive to atmospheric turbulence. The results also show that the BPSK and OOK-NRZ modulation signalling formats are more robust against the weak atmospheric turbulence conditions than PAM signal.
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Liu, Jeng-Shiung. "Improvement of an acoustic sounder device used to measure atmospheric turbulence." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 2004. http://library.nps.navy.mil/uhtbin/hyperion/04Dec%5FLiu.pdf.
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Thesis (M.S. in Applied Physics)--Naval Postgraduate School, Dec. 2004.Thesis Advisor(s): Thomas J. Hofler, Donald L. Walters. Includes bibliographical references (p. 51-52) Also available online.
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Marulanda, Acosta Valentina. "Quantum Key Distribution through atmospheric turbulence : secure satellite-to-ground links." Electronic Thesis or Diss., Sorbonne université, 2023. http://www.theses.fr/2023SORUS378.
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Les exigences sans cesse croissantes des systèmes de télécommunication modernes en termes de débit, ainsi que la menace imminente que pose l’augmentation de la puissance de calcul des ordinateurs modernes sur les méthodes cryptographiques actuelles, font de la transmission sécurisée des données à la fois une exigence essentielle et un grand défi, et donc un domaine d'étude très actif. La distribution quantique des clés (QKD) permet l'échange de clés cryptographiques dont le niveau de sécurité ne dépend pas de la complexité d'un algorithme mathématique mais repose intrinsèquement sur l'exploitation des propriétés de la mécanique quantique. Cependant, le déploiement des systèmes QKD via des réseaux fibrés terrestres, est fortement limité en distance, et n'atteint que quelques centaines de kilomètres, en raison de l'atténuation exponentielle subie par les signaux transmis par fibre optique. Les méthodes d'amplification des répéteurs de communications optiques classiques ne sont pas compatibles avec un signal quantique, et en raison du manque de maturité technologique concernant les répéteurs quantiques, les relais satellite se présentent comme une alternative intéressante pour l'établissement de liaisons quantiques intercontinentales sécurisées. Nous présentons ici, dans le contexte d’un lien QKD descendant entre un satellite en orbite basse et le sol, un modèle complet du canal atmosphérique satellite-sol prenant conjointement en compte la turbulence, sa correction partielle par optique adaptative (OA) les pertes géométriques et les fluctuations de pointage à bord du satellite. Nous utilisons ce modèle pour évaluer les performances de trois protocoles QKD - à variables continues et à variables discrètes, avec des photons uniques ou intriqués - pour différentes conditions de turbulence, différents degrés de correction par OA, différents scénarios de configuration du lien (diamètre télescope, altitude du satellite…) et en prenant en compte les effets de taille finie. Les résultats obtenus montrent l’intérêt de l’utilisation d’un système d’OA : en effet , la performance en termes de taux de génération de clé de tous les protocoles analysés s’améliore en considérant une correction par OA. Cette augmentation du taux de clé est particulièrement significative pour les scénarios de forte turbulence, d’opération diurne et pour le protocole QKD à variables continues (CV). L’apport de l’OA est de plus démontré et quantifié dans une configuration très prometteuse exploitant l’émission de deux photons intriqués vers deux stations sol depuis un relais satellite qui n’est pas forcément de confiance. Afin de valider nos résultats de simulation, nous avons aussi commencé à implémenter un banc de test expérimental à partir d’une émulation simplifiée du canal atmosphérique et d’un système CV-QKD. Nous expliquons les difficultés rencontrées pendant cette mise en œuvre ainsi que les solutions proposées et des idées sur les perspectives de l’étudeThe ever-growing demands of modern telecommunication systems in terms of data rates as well as the impending threat of the increasing computing power of modern computers, make the secure transmission of data an essential requirement and thus a very active field of study. Quantum key distribution (QKD) allows for the exchange of cryptographic keys whose security level does not depend on the complexity of a mathematical algorithm but rather relies on exploiting the properties of quantum mechanics cite{scarani2009}. Depending on the protocol, the key bits will be encoded either on the superposition of modes of individual photons, such as polarization modes, as is the case for the discrete variable protocols (DV) or they will be encoded into the quadratures of a very low flux electromagnetic field as it happens in the continuous variable protocols (CV). While offering security levels unattainable by classical means, QKD protocols in their terrestrial implementation are severely limited in distance reaching only several hundred kilometers because of the exponential attenuation suffered by fiber-transmitted signals. Since the amplification methods of classical optical communications repeaters are not compatible with a signal that is quantum in nature, and because of the current lack of technological maturity regarding quantum repeaters, satellite relays present an interesting alternative for the establishment of secure intercontinental quantum links. A study by Dequal et al. upon which a part of the present study is based on, examines the possibility of performing a continuous variable key exchange between a satellite and a ground station by proposing a modeling of the propagation channel accounting for the effects of beam wandering, a fluctuating atmospheric transmission and a fixed loss due to single mode fiber coupling. It is as an in-depth continuation of this analysis that this simulation study was initially developed. Taking into account in particular the effects of propagation through the turbulent atmosphere on the spatial coherence of the optical signal, as well as expanding on the protocols taken into account. Adaptive optics (AO) are able to partially correct some of the aforementioned propagation effects. A typical AO system consists of a feedback loop containing elements capable of measuring and correcting wavefront aberrations in real time and we will focus our efforts in analyzing the effect of such a system in the performance of several protocols of quantum key distribution under different scenarios