2,366 research outputs found
Control of flow separation over an aerofoil by external acoustic excitation at a high Reynolds number
The effectiveness of acoustic excitation as a means of flow control at high Reynolds
number turbulent flows is investigated numerically by using Improved Delayed Detached
Eddy Simulations. Previous studies on low Reynolds number laminar flows have shown
that acoustic excitation can substantially suppress flow separation for specific effective
frequency and amplitude ranges. However, the effect of acoustic excitation on higher
Reynolds number turbulent flow separation has not yet been explored due to limitations
on appropriate fidelity computational methods or experimental facility constraints. Therefore,
this paper addresses this research gap. A NACA (National Advisory Committee for
Aeronautics) 0015 aerofoil profile at 1 million Reynolds number based on the aerofoil
chord length is used for the investigations. Acoustic excitation is applied to the baseline
flow field in the form of transient boundary conditions at the computational domain inlet.
A parametric study revealed that the effective sound frequency range shows a Gaussian
distribution around the frequency of the dominant disturbances in the baseline flow. A
maximum of ⌠43% increase in lift-to-drag ratio is observed for the most effective excitation
frequency F+ = 1.0 at a constant excitation amplitude of Am = 1.8%. The effect
of excitation amplitude follows an asymptotic trend with a maximum effective excitation
amplitude above which the gains are not significant. A fully reattached flow is observed
for the highest excitation level considered (Am = 10%), that results in ⌠120% rise in
aerofoil lift-to-drag coefficient. Overall, the findings of the current work demonstrate the
higher Reynolds number effectiveness of acoustic excitation on separated turbulent flows,
thereby paving the way for application in realistic flow scenarios observed in aircraft and
gas turbine engine flow fields
A kinetic Fokker-Planck algorithm for simulating multiscale gas flows
Numerical, aerodynamic analysis of spacecraft requires the modeling of rarefied hypersonic flows. Such flow regimes are usually dominated by broad shock waves and strong expansion flows. In such areas of the flow the gas is far from its equilibrium state and therefore conventional modeling approaches such as the Euler or Navier-Stokes equations cannot be used. Instead, non-equilibrium modeling approaches must be applied. While most non-equilibrium flow solvers are computationally expensive, a recently introduced kinetic Fokker-Planck (FP) method shows the potential of describing non-equilibrium flows with satisfactory accuracy and, at the same time, significantly reducing computational costs. However, the application of kinetic FP solvers was so far still limited to simple, single species gases.
The aim of this study is to extend the capabilities of the kinetic FP approach for describing complex gas flows. Particular attention is paid to the modeling of non-equilibrium aerodynamics, as it is relevant for describing spacecraft related gas flows.
Methods for describing polyatomic species as well as gas mixtures within the kinetic FP framework are constructed. All models are intensively validated by comparison to already established numerical methods, as well as in comparison to experimental studies.
Excited energy states are modeled by a stochastic jump process described by a master equation. This approach allows the description of both continuous and discrete energy levels. Gas mixtures are modeled based on the hard-sphere and variable hard-sphere collision potentials. For both cases, FP models are constructed for an arbitrary number of species. The efficiency of the described models is investigated and different strategies are proposed to use kinetic FP methods efficiently.
The expansion of synthetic air from an axially symmetric orifice is numerically reproduced using the developed models and results are compared with experimental measurements. Although the numerical simulations capture several magnitudes of Knudsen numbers, from the continuum flow in the reservoir up to the free-molecular far field, good agreement between simulation and experiment is seen
Neural Residual Radiance Fields for Streamably Free-Viewpoint Videos
The success of the Neural Radiance Fields (NeRFs) for modeling and free-view
rendering static objects has inspired numerous attempts on dynamic scenes.
Current techniques that utilize neural rendering for facilitating free-view
videos (FVVs) are restricted to either offline rendering or are capable of
processing only brief sequences with minimal motion. In this paper, we present
a novel technique, Residual Radiance Field or ReRF, as a highly compact neural
representation to achieve real-time FVV rendering on long-duration dynamic
scenes. ReRF explicitly models the residual information between adjacent
timestamps in the spatial-temporal feature space, with a global
coordinate-based tiny MLP as the feature decoder. Specifically, ReRF employs a
compact motion grid along with a residual feature grid to exploit inter-frame
feature similarities. We show such a strategy can handle large motions without
sacrificing quality. We further present a sequential training scheme to
maintain the smoothness and the sparsity of the motion/residual grids. Based on
ReRF, we design a special FVV codec that achieves three orders of magnitudes
compression rate and provides a companion ReRF player to support online
streaming of long-duration FVVs of dynamic scenes. Extensive experiments
demonstrate the effectiveness of ReRF for compactly representing dynamic
radiance fields, enabling an unprecedented free-viewpoint viewing experience in
speed and quality.Comment: Accepted by CVPR 2023. Project page, see
https://aoliao12138.github.io/ReRF
Oral hygiene practice: the experiences and perspectives of older adults
Increasing longevity has brought with it evolving trends in dental health. Older adults are, now more than ever, maintaining their natural teeth longer in life. The benefits of preserving a natural dentition are considerable and among other advantages have shown to increase oneâs quality of life. Alongside the maintenance of natural teeth many older people have partial or complete dentures to care for and maintain, therefore oral care has the potential to become more complicated. Rates of periodontal disease are recognised to increase substantially with age while rates of dental decay in the older population increase. These concerns have been raised by experts in the field and the World Health Organisation have urged countries to implement suitable strategies to improve the oral health of older people.
This study seeks to investigate the experience of oral hygiene maintenance and explore the challenges that older people are faced with when trying to maintain appropriate oral hygiene practices at home. The opinions of independently living older adults were sought as little is known about their personal oral health practices and associated experiences.
In an attempt to understand the potential impact ageing may have on oral hygiene practices, the aims of this research comprise;
1. Explore the narratives of older adults regarding the maintenance of natural teeth in older age.
2. Identify the most common practices adopted by older adults to care for their natural dentition and explore the associated behaviours and influences.
3. Determine if there are challenges which impact oral hygiene behaviour/practice later in life.
4. Theorize about this phenomenon in order to provide better insight and understanding of how teeth are cared for/maintained in old age.
5. A constructivist grounded theory methodology was adopted which allowed the generation of substantive theory - A theory of ageing and perspectives on oral hygiene (APOH). It is hoped this theory will assist in providing an insight and better understanding of the common oral hygiene behaviours and practices of the independently living, older adult population. As well as this, the theory developed offers an interpretive explanation of why older adults may fail to identify problems in their personal practice The perspectives of this older cohort were gathered from 2 separate semi structured interviews. A 2 week oral hygiene diary was also maintained by participants in-between in an attempt to further pursue the everyday thoughts and experiences/practices of older people maintaining an oral health routine at home.
Three core categories were developed from the data and were intimately linked with one contributing to or impacting upon the next. The 1st core category identified that the meaning and significance of dental health, as well as the experience of personal oral hygiene, changes and evolves over the course of an individualâs life. The 2nd core category highlights how natural teeth become a prized possession and oral hygiene self-care is perceived to improve significantly by independent older adults, later in life. As a result, and detailed in the 3rd and final core category, challenges impeding practice were much less of a concern and rarely considered among this ageing cohort.
âA theory of ageing and perspectives on oral hygieneâ contributes to knowledge by explaining the viewpoints of the independent older adult regarding personal oral hygiene, providing a better understanding of common practices. It also explains how and why older adults may become less aware of issues within their personal oral hygiene practice, later in life
Improved Panning on Non-Equidistant Loudspeakers with Direct Sound Level Compensation
Loudspeaker rendering techniques that create phantom sound sources often
assume an equidistant loudspeaker layout. Typical home setups might not fulfill
this condition as loudspeakers deviate from canonical positions, thus requiring
a corresponding calibration. The standard approach is to compensate for delays
and to match the loudness of each loudspeaker at the listener's location. It
was found that a shift of the phantom image occurs when this calibration
procedure is applied and one of a pair of loudspeakers is significantly closer
to the listener than the other. In this paper, a novel approach to panning on
non-equidistant loudspeaker layouts is presented whereby the panning position
is governed by the direct sound and the perceived loudness is governed by the
full impulse response. Subjective listening tests are presented that validate
the approach and quantify the perceived effect of the compensation. In a setup
where the standard calibration leads to an average error of 10 degrees, the
proposed direct sound compensation largely returns the phantom source to its
intended position.Comment: 10 pages. Accepted for presentation in AES Convention 155 (2023
SantĂ© cardiomĂ©tabolique, paramĂštres inflammatoires et faisabilitĂ© dâune intervention nutritionnelle en oncologie pĂ©diatrique
GrĂące aux progrĂšs mĂ©dicaux, le taux de survie Ă 5 ans des enfants et des adolescents diagnostiquĂ©s dâun cancer est maintenant dâenviron 85%. MalgrĂ© ces chiffres encourageants, Ă lâĂąge adulte, les survivants dâun cancer pĂ©diatrique sont Ă risque de dĂ©velopper plusieurs problĂšmes de santĂ© dont des maladies cardiovasculaires et des complications cardiomĂ©taboliques (CM) comme de l'hypertension, une rĂ©sistance Ă l'insuline, de la dyslipidĂ©mie et de l'obĂ©sitĂ© abdominale. Devant cette rĂ©alitĂ©, il importe dâenrichir les connaissances quant Ă lâĂ©volution et lâĂ©tiologie de ces sĂ©quelles tĂŽt dans la trajectoire du cancer et de la survivance afin dâamĂ©liorer la prise en charge prĂ©coce des patients. Pourtant, Ă ce jour, seulement quelques Ă©tudes ont dĂ©crit la santĂ© CM de patients Ă court terme aprĂšs la fin des traitements et les facteurs associĂ©s au dĂ©veloppement hĂątif de ces complications demeurent mĂ©connus.
Le cancer et ses traitements causent un Ă©tat pro-inflammatoire & pro-oxydant susceptible dâentraĂźner le dĂ©veloppement de complications CM. En parallĂšle, la pĂ©riode de traitement est caractĂ©risĂ©e par des changements dans les habitudes alimentaires, un comportement sĂ©dentaire et une augmentation de lâindice de masse corporelle (IMC), qui peuvent persister aprĂšs la fin des traitements. Ă long terme chez les survivants de cancers pĂ©diatriques, le statut dâobĂ©sitĂ© Ă la fin des traitements ainsi que certains biomarqueurs inflammatoires ont Ă©tĂ© associĂ©s Ă la prĂ©sence de complications CM. De plus, la composition des HDL est diffĂ©rente Ă long terme chez les survivants de la leucĂ©mie lymphoblastique aiguĂ« comparativement Ă celle de contrĂŽles sains. Ătant donnĂ© lâimportance de tous ces facteurs, il est primordial de les dĂ©crire Ă court terme, aprĂšs la fin des traitements.
Par ailleurs, lâobĂ©sitĂ© au moment du diagnostic et lâaugmentation de lâIMC pendant le traitement influencent nĂ©gativement le pronostic et lâoccurrence dâeffets secondaires graves durant les traitements. Toutefois, la plupart des interventions nutritionnelles en oncologie pĂ©diatrique ne tiennent pas en compte la problĂ©matique du gain de poids significatif durant les traitements. Ainsi, il semble important dâĂ©valuer la possibilitĂ© dâimplanter des stratĂ©gies misant sur la promotion d'habitudes de vie saines tĂŽt aprĂšs le diagnostic dâun cancer pĂ©diatrique afin dâamĂ©liorer l'Ă©tat nutritionnel, la qualitĂ© de vie et possiblement la santĂ© CM des patients Ă court et Ă long terme.
Les travaux de cette thĂšse ont pour but de : i) dĂ©crire la santĂ© CM de patients en oncologie pĂ©diatrique peu de temps aprĂšs les traitements, ii) Ă©laborer, par une revue de littĂ©rature, sur le rĂŽle de lâĂ©tat inflammatoire et oxydant reliĂ© au cancer de lâenfant et ses traitements dans le dĂ©veloppement des complications CM, iii) Ă©valuer les associations entre lâĂ©volution de lâIMC durant la trajectoire de soins dâun cancer pĂ©diatrique et les complications CM ainsi que le statut inflammatoire; vi) dĂ©tailler la composition lipidique et protĂ©ique des HDL des patients peu de temps aprĂšs la fin des traitements du cancer pĂ©diatrique et; v) dĂ©terminer la faisabilitĂ© dâune intervention nutritionnelle prĂ©coce chez des enfants et des adolescents nouvellement diagnostiquĂ©s dâun cancer.
Pour ce faire, deux cohortes de patients ont Ă©tĂ© recrutĂ©es dans le cadre de lâĂ©tude VIE (Valorisation, Implication, Ăducation) au CHU Sainte-Justine. Dâabord, 80 patients suivis en hĂ©mato-oncologie ont Ă©tĂ© rencontrĂ©s en moyenne 1,4 ± 0,8 an aprĂšs la fin de leur traitement contre un cancer pĂ©diatrique. De ce groupe, 56,3 % Ă©taient des filles, 43,8% avaient Ă©tĂ© traitĂ©s pour une leucĂ©mie. L'Ăąge moyen lors de la rencontre Ă©tait de 11,8 ans (min - max: 4,5 - 21,0). La proportion de complications CM observĂ©e Ă©tait de 26,3 % pour la pression artĂ©rielle (PA) Ă©levĂ©e, 8,1 % pour le prĂ©diabĂšte, 35,0 % pour la dyslipidĂ©mie et 11,5 % pour lâobĂ©sitĂ©. Les adolescents (â„ 10 ans au diagnostic) Ă©taient plus susceptibles d'avoir une PA Ă©levĂ©e, une dyslipidĂ©mie et de cumuler â„ 2 complications CM que les enfants. Ătre en surpoids ou obĂšse aprĂšs le traitement Ă©tait associĂ© Ă des niveaux plus Ă©levĂ©s d'insuline, dâHOMA-IR, de leptine et du ratio leptine/adiponectine du plasma. Chez les patients en surpoids ou obĂšses Ă la fin du traitement, le changement de lâIMC a Ă©tĂ© reliĂ© au niveau dâadipokines (leptine et ratio leptine/adiponectine) aprĂšs les traitements. De plus, les fractions de HDL3 Ă©taient enrichies en triglycĂ©rides chez les patients prĂ©sentant une dyslipidĂ©mie Ă lâĂ©valuation par rapport aux normolipidiques et chez ceux ayant Ă©tĂ© traitĂ©s avec des doses de doxorubicine â„ 90 mg/m2 par rapport Ă des doses infĂ©rieures.
ParallĂšlement, une intervention nutritionnelle d'un an, comprenant une Ă©valuation initiale et six visites de suivi tous les deux mois, a Ă©tĂ© effectuĂ©e auprĂšs de 61 participants. De ceux-ci, 51,6% Ă©taient des garçons, lâĂąge moyen Ă©tait de 8,5 ans et le temps moyen entre le dĂ©but de lâintervention et le diagnostic Ă©tait de 13,2 semaines. AprĂšs 1 an dâintervention, le taux de rĂ©tention Ă©taient de 72,6 %, 258 rencontres ont Ă©tĂ© menĂ©es sur 362 planifiĂ©es (taux de prĂ©sence 71,6%) et la moitiĂ© des participants (50,8 %) avaient participĂ© Ă au moins 4 rencontres de suivi.
En conclusion, peu aprĂšs le traitement dâun cancer pĂ©diatrique, la santĂ© CM est influencĂ©e par lâĂąge au diagnostic et lâĂ©volution de lâIMC pendant les traitements, et le mĂ©tabolisme des HDL tant par lâĂąge que les doses de doxorubicine reçues. Les biomarqueurs du statut inflammatoire peuvent servir dâindicateur de la santĂ© CM chez ces patients. Par ailleurs, lâimplantation dâune intervention nutritionnelle impliquant les patients et leurs parents tĂŽt aprĂšs le diagnostic dâun cancer pĂ©diatrique est faisable et constitue une stratĂ©gie Ă prioriser afin dâoptimiser la santĂ© CM de cette population Ă court et Ă long terme. Dans leur ensemble, nos travaux contribuent Ă amĂ©liorer la prise en charge et les mĂ©thodes dâinterventions auprĂšs des enfants et des adolescents diagnostiquĂ©s dâun cancer.Due to medical advances, the 5-year survival rate for children and adolescents diagnosed with cancer is now approximately 85%. Despite these encouraging statistics, in adulthood, survivors of pediatric cancer are at risk of developing several health problems including cardiovascular disease and cardiometabolic complications (CM) such as hypertension, insulin resistance, dyslipidemia and abdominal obesity. In this context, it is important to increase the knowledge of the evolution and etiology of these sequelae to improve early management. However, to date, only a few studies have described the CM health of patients in the short term after the end of treatment and the factors associated with the early development of these complications remain unknown.
Cancer and its treatment cause a pro-inflammatory & pro-oxidative state that can lead to the development of CM complications. In parallel, the treatment period is characterized by changes in dietary habits, sedentary behavior, and increased body mass index (BMI), which may persist after treatment ends. In the long term in pediatric cancer survivors, obesity status at the end of treatment and some inflammatory biomarkers have been associated with the presence of CM complications. In addition, HDL composition is different in long-term survivors of acute lymphoblastic leukemia compared with healthy controls. Given the importance of these factors, it is critical to describe them in the short term, after the end of treatment.
In addition, obesity at diagnosis and increasing BMI during treatment negatively influence prognosis and the occurrence of serious side effects during treatment. However, most nutritional interventions in pediatric oncology do not address the issue of significant weight gain during treatment. Thus, it seems important to evaluate the possibility of implementing strategies focusing on the promotion of healthy lifestyle habits early after the diagnosis of pediatric cancer in order to improve the nutritional status, quality of life and possibly the CM health of patients in the short and long term.
The work in this thesis aims to: i) describe the CM health of pediatric oncology patients shortly after treatment, ii) elaborate, through a literature review, on the role of inflammatory and oxidative status related to pediatric cancer and its treatments in the development of CM complications, iii) evaluate the associations between changes in BMI during the pediatric cancer care trajectory and CM complications as well as inflammatory status; vi) detail the lipid and protein composition of patients' HDL shortly after completion of pediatric cancer treatments and; v) determine the feasibility of early nutritional intervention in children and adolescents newly diagnosed with cancer.
Two cohorts of patients were recruited as part of the VIE (Valorisation, Implication, Education) study at CHU Sainte-Justine. First, 80 patients followed in hemato-oncology were met on average 1.4 ± 0.8 year after the end of their treatment for pediatric cancer. Of this group, 56.3% were girls, 43.8% had been treated for leukemia. The mean age at encounter was 11.8 years (min - max: 4.5 - 21.0). The proportion of CM complications observed was 26.3% for high blood pressure (BP), 8.1% for prediabetes, 35.0% for dyslipidemia, and 11.5% for obesity. Adolescents (â„ 10 years of age at diagnosis) were more likely to have elevated BP, dyslipidemia, and â„ 2 CM complications than children. Being overweight or obese after treatment was associated with higher levels of insulin, HOMA-IR, leptin, and plasma leptin/adiponectin ratio. In patients who were overweight or obese at the end of treatment, the change in BMI was related to the level of adipokines (leptin and leptin/adiponectin ratio) after treatments. In addition, HDL3 fractions were enriched in triglycerides in patients who were dyslipidemic at assessment compared with the normolipidics, and in those who had been treated with doxorubicin doses â„90 mg/m2 compared to lower doses.
In parallel, a one-year nutrition intervention, including an initial assessment and six follow-up visits every two months, was conducted with 61 participants. Of these, 51.6% were boys, the mean age was 8.5 years, and the mean time from intervention initiation to diagnosis was 13.2 weeks. After 1 year of intervention, the retention rate was 72.6%, 258 visits were held out of the 362 planned (71.6% attendance rate) and half of the participants (50.8%) had attended at least 4 follow-up visits.
In conclusion, shortly after treatment of pediatric cancer, CM health is influenced by age at diagnosis and BMI changes during treatment, and HDL metabolism by both age and doses of doxorubicin received. Biomarkers of inflammatory status may serve as an indicator of CM health in these patients. Furthermore, implementation of a nutritional intervention involving patients and parents early after diagnosis of pediatric cancer is feasible and is a strategy to prioritize to optimize CM health in this population in the short and long term. Taken together, our work contributes to improve the management and intervention methods for children and adolescents diagnosed with cancer
Design and optimisation of solar sail orbits in proximity of asteroids
A solar sail is a large reflective membrane which is capable of producing thrust for a spacecraft by the reflection of sunlight. Such a propellant-less propulsion system can offer solutions to high-energy missions which would be impossible for conventional propulsion systems. As a result, this technology has been proposed by many authors as the ideal candidate for a multiple asteroid rendezvous mission.
At the time of writing, there are more than 30,000 known near-Earth asteroids (NEAs) alone.
Adding to this those contained in the main belt and elsewhere in the solar system, the abundance of these small rocky worlds becomes apparent. Focusing only on the NEAs, there are many reasons for interest in missions to these bodies. In the first instance, they represent the earliest building blocks of the rocky worlds of the solar system, and are often still in pristine condition, similar to how they would have been since these earliest moments. As such, there is massive scientific interest in visiting and extracting samples of their constituent materials. There is another community which is also interested in the extraction of these materials: the future asteroid miners. This mining could provide propellant for deep space missions, materials for in-space infrastructure and potentially also in the return of minerals which are rare on Earth, and so of great value. However, although these bodies provide many opportunities, they are not without threat. Although the frequency of impacts of large bodies capable of causing considerable damage to Earth-based infrastructure is relatively low, there are still recent examples of just such events. With the potential for large scale loss of life due to an asteroid impacting populated areas, the science of planetary defence requires greater knowledge of the make-up of these bodies. Yet another reason for mission designers to examine further options in achieving efficient missions to these bodies.
It would be beneficial, in terms of cost, for a single spacecraft to be able to carry out a mission to multiple asteroids. Such a high-energy mission is ideally suited to the solar sail. Although the literature has provided many works on orbital transfers to multiple bodies, the operation of the sail when in proximity of the asteroid has not received quite as much attention. It is in this phase of the mission, where the science objectives would be carried out, that this thesis focuses. There are numerous challenges which the sail faces in the near-asteroid environment. These include the irregular gravity field, the strength of the acceleration provided by the sail in a relatively weak gravitational field, the often fast rotational velocities of the asteroid and higher demands on slew rates for the sail due to the shorter period of low-altitude orbits.
The work will consider three main proximity phases. The first operation is in the control of an orbit using the solar sail in an irregular gravity field. In this operation, the sail must counter the perturbative effects of a non-spherical body. This manifests in the rotation of the orbit node line, referred to as nodal regression. A new tool, referred to as the Control Transition Matrix (CTM), which aids in forcing a periodic orbit solution over multiple orbits is then presented. The second operation deals with the control of a sail at the point of and subsequent to the deployment of a lander and during the deployment of a series of small ChipSat probes. The landing conditions for deployments from various locations around the asteroid are analysed before the deployment is presented from a low-asteroid orbit. The control of the sail along a nominal orbit while the lander is still on-board is presented before the sail control subsequent to the lander deployment is considered. Given the high velocity impacts for a ballistic lander deployed at large distances from the surface, an alternative mission scenario of the deployment of small ChipSat probes is presented. These probes are envisaged to carry out their science goals during the descent and so the landing conditions are less important. The final operation is in the gravitational capture of the sail around the asteroid. This work provides a preliminary analysis of the capability of the sail in achieving this by using a simple on/off control law. Following this, a more detailed two-phase approach is presented. In the first âinitial captureâ phase, the sail uses the value of Jacobi constant in the 3 body system as a guide to reduce the orbit radius to within a defined region. After this, the âorbit shapingâ phase aims to circularise the orbit at this radius. Subsequently, preliminary investigations into an optimal approach are presented.
In controlling the effects due to the non-spherical asteroid shape, an optimally controlled solution, where a minimum effort control law is sought, is presented. Following this, a novel method of updating a control law was successfully applied to force a periodic orbit. In the work carried out on lander deployment, it was found that the sail was capable of maintaining a periodic orbit after the point of lander separation by application of time-delay feedback control. For the deployment of a series of small probes, it was found that maintaining a fixed attitude for the sail during the deployment was not considerably different in station-keeping performance compared with LQR control, and performed this with no effort required of the sail. Finally, in the work on capture, the two-phase approach provided successful capture trajectories down to the desired orbit radius. The work showed that, for reducing size of asteroid, there was a reduction in the time to capture. This is due to the fact that the same size of sail is used in the weakening gravity field of each asteroid. This makes the sail relatively more powerful and so able to affect quicker capture. It was also seen that long period capture trajectories are compounded by the need for the sail to spend periods of time waiting for the position of the Sun relative to the orbit to be in such a way as to permit the capture operations to proceed. There was also the successful demonstration of an optimally controlled capture which minimised the orbit semi-parameter over one orbit revolution.
The work contained in this thesis provides preliminary analysis for the consideration of future solar sail mission designers in the proximity operations of a sail near an asteroid. The findings presented here have shown that the sail can be of considerable utility in these proximity operations. They also present challenges to the mission designer given the continuous thrust that they may provide. Where a powerful sail may benefit the interplanetary phase of a mission in reaching many more asteroids further from the Earth, this can also present a challenge in the relatively weak asteroid gravitational field. However, these challenges are not insurmountable and so the sail remains a promising option for these high-energy missions
Numerical simulation of combustion instability: flame thickening and boundary conditions
Combustion-driven instabilities are a significant barrier for progress for many avenues of immense practical relevance in engineering devices, such as next generation gas turbines geared towards minimising pollutant emissions being susceptible to thermoacoustic instabilities. Numerical simulations of such reactive systems must try to balance a dynamic interplay between cost, complexity, and retention of system physics. As such, new computational tools of relevance to Large Eddy Simulation (LES) of compressible, reactive flows are proposed and evaluated.
High order flow solvers are susceptible to spurious noise generation at boundaries which can be very detrimental for combustion simulations. Therefore Navier-Stokes Characteristic Boundary conditions are also reviewed and an extension to axisymmetric configurations proposed. Limitations and lingering open questions in the field are highlighted.
A modified Artificially Thickened Flame (ATF) model coupled with a novel dynamic formulation is shown to preserve flame-turbulence interaction across a wide range of canonical configurations. The approach does not require efficiency functions which can be difficult to determine, impact accuracy and have limited regimes of validity. The method is supplemented with novel reverse transforms and scaling laws for relevant post-processing from the thickened to unthickened state. This is implemented into a wider Adaptive Mesh Refinement (AMR) context to deliver a unified LES-AMR-ATF framework. The model is validated in a range of test case showing noticeable improvements over conventional LES alternatives.
The proposed modifications allow meaningful inferences about flame structure that conventionally may have been restricted to the domain of Direct Numerical Simulation. This allows studying the changes in small-scale flow and scalar topologies during flame-flame interaction. The approach is applied to a dual flame burner setup, where simulations show inclusion of a neighbouring burner increases compressive flow topologies as compared to a lone flame. This may lead to favouring convex scalar structures that are potentially responsible for the increase in counter-normal flame-flame interactions observed in experiments.Open Acces
Cryogenic silicon Fabry-Perot resonator with Al0.92Ga0.08As/GaAs mirror coatings
The sensitivity and stability of today's most precise optical interferometers, like gravitational wave detectors and ultra-stable lasers, are fundamentally limited by thermodynamically induced length fluctuations of high-reflectivity mirror coatings. Among them, Brownian thermal noise related to internal friction is the dominant contribution and can be reduced by using coating materials with lower mechanical loss. Owing to their low mechanical losses, AlGaAs/GaAs crystalline mirror coatings are expected to reduce this limit set by conventional dielectric coatings as demonstrated from a room temperature measurement. However, due to the high noise contributions from other resonator constituents in previous study, accurate characterization of the noise of crystalline coatings has yet been possible.
In this work, the first detailed study on the spatial and temporal noise properties of crystalline coatings at an unprecedented level of precision is presented. This was achieved by using these novel coatings in a cryogenic silicon Fabry-Perot resonator operating at a temperature of 124 K and at a wavelength of 1.5 ”m. To observe the expected low fractional frequency instability of mod Ï_y=1x10^-17 imposed significant challenges in suppressing technical noise contributions. With methods and experimental setups described in this work, technical noise contributions were suppressed to a level well below the predicted coating noise. Nevertheless, the measured frequency was significantly higher than the predicted thermal and the total technical noise, which indicates the existence of excess noise in crystalline coatings.
To disentangle the different excess noise sources, a sophisticated interrogation scheme, which investigates spatiotemporal correlations between different cavity eigenmodes by stabilizing two independent lasers simultaneously on the resonator, was developed. With this interrogation scheme, noise mechanisms related to the large birefringence mode splitting in these coatings were discovered. Upon a step change of optical power, anticorrelated frequency transient responses between the two birefringence-induced polarization eigenmodes of the silicon resonator were measured. The frequency noise induced by power fluctuations from this photo-birefringent effect was reduced to a neglectable level by active stabilization of optical power. However, anticorrelated spontaneous frequency fluctuations between the two polarization eigenmodes were still observed, indicating intrinsic birefringence fluctuations. To cancel this dominant excess noise - birefringent noise - in the crystalline coating, a dual-frequency locking technique was developed to stabilize the laser to the average of both polarization eigenmodes. With this technique, the expected low Brownian thermal noise was verified, but at the same time, this revealed another novel global excess noise with a correlation length larger than the mode diameter of 1 mm. This excess noise currently limits the frequency stability of the new cryogenic silicon resonator at a level comparable to dielectric coatings. Due to its large correlation length, increasing the beam size will only marginally reduce the noise level.
In future ultra-sensitive interferometers using similar coatings based on semiconductor materials, these novel noise contributions discovered in this thesis must be carefully considered
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