17 research outputs found

    Riblets in the viscous sublayer : Optimal Shape Design of Microstructures

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    Previous research has established that a smooth surface has not necessarily minimal drag: Many experiments by different laboratories, e.g. NASA and DLR Berlin, indicate that an extra surface layer with tiny grooves aligned in the stream-wise direction can be used to reduce the drag. The aim of this project is to find the optimal shape of such microstructures on surfaces of submerged bodies. We assume that these microstructures remain in the viscous sublayer where the flow equations are the 3D incompressible, steady state Navier-Stokes equations with a Couette in- and outflow determinated through two boundary conditions, the no-slip condition on the lower boundary and the friction condition on the upper one. The objective function of our optimization problem is the tangential drag force, which we want to minimize. Solving this problem is difficult because of the rough boundary, which causes a big amount of data. We apply homogenization theory and replace the rough boundary by a smooth one, where the right boundary conditions have been determined. Furthermore, our optimization problem can be simplified using this approximation and we end up minimizing a scalar size, the Navier constant, which is calculated using the velocity of an auxiliary boundary layer equation. To solve the optimization problem we use sensitivity-based optimization methods. The sensitivity is calculated analytically and we use it to determine the gradient of the cost function with respect to the design variable. A minimum is sought by using the steepest descent algorithm with step size according to Armijo rule. The necessary optimality conditions are derived and a sequence of admissible domains is built which tends to the optimal solution. The state equations are solved numerically using finite elements on unstructured grids and multigrid algorithms. The results obtained with this approach give us a drag reduction of approximately 2-6% relative to the drag of the smooth configuration

    Optimal energy growth and optimal control in swept Hiemenz flow

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    International audienceThe objective of the study is first to examine the optimal transient growth of Görtler-Hämmerlin perturbations in swept Hiemenz flow. This configuration constitutes a model of the flow in the attachment-line boundary layer at the leading-edge of swept wings. The optimal blowing and suction at the wall which minimizes the energy of the optimal perturbations is then determined. An adjoint-based optimization procedure applicable to both problems is devised, which relies on the maximization or minimization of a suitable objective functional. The variational analysis is carried out in the framework of the set of linear partial differential equations governing the chordwise and wall-normal velocity fluctuations. Energy amplifications of up to three orders of magnitude are achieved at low spanwise wavenumbers(k ~ 2000) and large sweep Reynolds number (Re ~ 2000) Optimal perturbations consist of spanwise travelling chordwise vortices, with a vorticity distribution which is inclined against the sweep. Transient growth arises from the tilting of the vorticity distribution by the spanwise shear via a two-dimensional Orr mechanism acting in the basic flow dividing plane. Two distinct regimes have been identified: for k ? 0.25, vortex dipoles are formed which induce large spanwise perturbation velocities; for k ? 0.25, dipoles are not observed and only the Orr mechanism remains active. The optimal wall blowing control yields for instance an 80% decrease of the maximum perturbation kinetic energy reached by optimal disturbances at Re = 550 and k = 0.25 The optimal wall blowing pattern consists of spanwise travelling waves which follow the naturally occurring vortices and qualitatively act in the same manner as a more simple constant gain feedback control strategy. © 2006 Cambridge University Press

    Proceedings of the Twenty Second Nordic Seminar on Computational Mechanics

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    Vortex Instability and Transient Growth

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    The dynamics of vortex flow is studied theoretically and numerically. Starting from a local analysis, where the perturbation in the vortex flow is Fourier decomposed in both radial and azimuthal directions, a modified Chebyshev polynomial method is used to discretize the linearized governing operator. The spectrum of the operator is divided into three groups: discrete spectrum, free-stream spectrum and potential spectrum. The first can be unstable while the latter two are always stable but highly non-normal. The non-normality of the spectra is quantitatively investigated by calculating the transient growth via singular value decomposition of the operator. It is observed that there is significant transient energy growth induced by the non-normality of continuous spectra. The non-normality study is then extended to a global analysis, in which the perturbation is decomposed in the radial or azimuthal direction. The governing equations are discretized through a spectral/hp element method and the maximum energy growth is calculated via an Arnoldi method. In the azimuthally-decomposed case, the development of the optimal perturbation drives the vortex to vibrate while in the stream-wise-decomposed case, the transient effects induce a string of bubbles along the axis of the vortex. A further transient growth study is conducted in the context of a co-rotating vortex pair. It is noted that the development of optimal perturbations accelerates the vortex merging process. Finally, the transient growth study is extended to a sensitivity analysis of the vortex flow to inflow perturbations. An augmented Lagrangian function is built to optimize the inflow perturbations which maximize the energy inside the domain over a fixed time interval

    Synthesis and study of fluorescent molecular dyes

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    PhD ThesisUses for fluorescent dyes are diverse and increasingly important with compounds having many uses in medicinal, chemical and physical fields - amongst others. The creation of new fluorescent dyes helps to push the boundaries of molecular photonics alongside the further study of the underlying principles involved in the systems. This thesis concentrates largely on the synthesis and characterisation aspects of novel fluorescent dyes, though the analysis of the resultant photophysical data also features prominently. Chapter 1 is an introduction to fluorescence from some of the more basic principles involved in the field. A discussion and comparison of intrinsic and extrinsic fluorescent dyes is followed by a brief discussion of a series of examples of fluorescent molecular sensors. As bodipy dyes feature heavily throughout the thesis the second half of the introduction is focused solely on this topic. This half of the chapter centres around the synthetic approaches towards bodipy, modifications to the bodipy core and the resulting photophysics. Photo-induced electron transfer and fluorescence energy transfer is introduced from basic principles along with selected literature examples that demonstrate these processes in systems that incorporate bodipy. Chapter 2 discusses the synthesis and photophysics of a new class of fluorescent dyes based on a highly substituted terephthalate core. The initial aim of the chapter was to create fluorescent systems based on a xanthene core, this was found to be non-fluorescent. As such attention was turned towards a terephthalate intermediate which demonstrated strong and highly red shifted fluorescence in solution, as well as solid state fluorescence. A meso-perfluorinated phenyl ring causes a marked increase in fluorescence quantum yield, along with a pronounced red-shift, relative to the equivalent meso-phenyl variant. This observation lead to a series of Fn-aryl (n = 1,2,3,5) bodipy dyes being synthesised. Chapter 3 subsequently investigates the relationship between the number and position of fluorine atoms on the aryl moiety, and the resultant photophysical measurements. High fluorescence quantum yields were observed with ortho-substitution of fluorine atoms, a trend that was mirrored with the fluorescence lifetime. Mono-ortho fluorine substitution iii of the aryl group was also found to make the bodipy prochiral pathing the way towards axially chiral bodipy compounds. Chapter 4 follows on from chapter 4 by taking prochiral bodipy compounds to there chiral conclusion. In this chapter several synthetic approaches towards axially chiral (AxC) bodipy compounds are discussed. Included in these synthetic approaches is a completely novel route towards asymmetric bodipy cores thus AxC-bodipy compounds. This chapter represents the first examples of AxC bodipy compounds to exist with future developments in the field aimed at enhanced fluorescence sensing in chiral media and facile enantiomeric determination via circularly-polarised fluorescence measurements. Chapter 5 is an in-depth experimental section where the synthesis and characterisation of each compound is detailed. Also provided are the details for each chemical used, purification and drying methods for each solvent used and techniques used for proper characterisation of all of the compounds

    Nanopartículas multifuncionais para imagens de RM fluorescência

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    Doutoramento em QuimicaIn the past few years a new generation of multifunctional nanoparticles (NPs) has been proposed for biomedical applications, whose structure is more complex than the structure of their predecessor monofunctional counterparts. The development of these novel NPs aims at enabling or improving the performance in imaging, diagnosis and therapeutic applications. The structure of such NPs comprises several components exhibiting various functionalities that enable the nanoparticles to perform multiple tasks simultaneously, such as active targeting of certain cells or compartmentalization, imaging and delivery of active drugs. This thesis presents two types of bimodal bio-imaging probes and describes their physical and chemical properties, namely their texture, structure, and 1H dynamics and relaxometry, in order to evaluate their potential as MRI contrast agents. The photoluminescence properties of these probes are studied, aiming at assessing their interest as optical contrast agents. These materials combine the properties of the trivalent lanthanide (Ln3+) complexes and nanoparticles, offering an excellent solution for bimodal imaging. The designed T1- type contrast agent are SiO2@APS/DTPA:Gd:Ln or SiO2@APS/PMN:Gd:Ln (Ln= Eu or Tb) systems, bearing the active magnetic center (Gd3+) and the optically-active ions (Eu3+ and Tb3+) on the surface of silica NPs. Concerning the relaxometry properties, moderate r1 increases and significant r2 increases are observed in the NPs presence, especially at high magnetic fields, due to susceptibility effects on r2. The Eu3+ ions reside in a single low-symmetry site, and the photoluminescence emission is not influenced by the simultaneous presence of Gd3+ and Eu3+. The presence of Tb3+, rather than Eu3+ ion, further increases r1 but decreases r2. The uptake of these NPs by living cells is fast and results in an intensity increase in the T1-weighted MRI images. The optical features of the NPs in cellular pellets are also studied and confirm the potential of these new nanoprobes as bimodal imaging agents. This thesis further reports on a T2 contrast agent consisting of core-shell NPs with a silica shell surrounding an iron oxide core. The thickness of this silica shell has a significant impact on the r2 and r2* relaxivities, and a tentative model is proposed to explain this finding. The cell viability and the mitochondrial dehydrogenase expression given by the microglial cells are also evaluated.Nos últimos anos, surgiu uma nova geração de nanopartículas (NPs) multifuncionais destinada a aplicações biomédicas, mais concretamente para uso em técnicas de diagnóstico, reconstrução celular e em diversas aplicações terapêuticas. Em relação às suas antecessoras, estas novas nanopartículas apresentam uma estrutura mais elaborada, integrando vários componentes ativos com diferentes funcionalidades que, em princípio, permitem realizar diversas tarefas em simultâneo (como o direcionamento ativo para determinadas células ou compartimentos celulares, imagem e libertação de fármacos). Estas nanopartículas são designadas, por isso, de nanopartículas multifuncionais. A presente dissertação relata o desenvolvimento de dois tipos de sondas bimodais e as propriedades físico-químicas destas, nomeadamente a sua textura, estrutura e relaxometria e dinâmica de 1H , com o objectivo de avaliar o seu potencial como agentes de contraste para Imagem por Ressonância Magnética Nuclear (IRM). São, também, apresentados estudos de fotoluminescência que permitem avaliar o potencial daquelas sondas para serem usadas como agentes de contraste óptico. Estes materiais combinam as propriedades dos complexos de lantanídeos trivalentes (Ln3+) e das NPs funcionando, assim, como agentes bimodais. Foram desenvolvidos os seguintes sistemas fotoluminescentes e com contraste T1 em IRM em que os iões Ln3+ magnetica (Gd3+) e opticamente (Eu3+, Tb3+) activos se encontram à superfície das NPs de sílica: SiO2@APS/DTPA:Gd:Ln e SiO2@APS/Pyd-DTPA:Gd:Ln (Ln = Eu ou Tb). No que respeita às propriedades de relaxometria, na presença destas NPs observa-se um aumento moderado de r1 e considerável de r2, especialmente a campos magnéticos altos (devido aos efeitos de susceptibilidade para r2). Os iões Eu3+ apresentam um único ambiente local de baixa simetria, sendo que a emissão de fotoluminescência não é influenciada pela presença simultânea de Gd3+ e Eu3+. Verificou-se que a presença de Tb3+ (em lugar do ião Eu3+) aumenta ainda mais o valor r1, diminuindo r2. A internalização das NPs em células vivas é rápida e resulta num aumento de intensidade nas imagens ponderadas em T1. Foram estudadas as características ópticas de pastilhas de células (“cellular pellets”) contendo NPs, tendo-se confirmado o interesse das novas sondas propostas enquanto agentes para imagem bimodal. Esta dissertação relata, ainda, agentes com contraste em T2 para IRM, que consistem em um sistema núcleo-coroa (“core-shell”) em que é ajustável a espessura da coroa de sílica envolvendo o núcleo de óxido de ferro. A espessura do revestimento de sílica tem um efeito significativo sobre a relaxividade r2 e r2* , sendo aqui proposto um modelo para explicar este comportamento. A viabilidade celular e a expressão da desidrogenase mitocondrial das células da microglia foram também avaliadas

    HASTECS: Hybrid Aircraft: reSearch on Thermal and Electric Components and Systems

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    In 2019, transportation was the fastest growing sector, contributing to environmental degradation. Finding sustainable solutions that pollute less is a key element in solving this problem, particularly for the aviation sector, which accounts for around 2-3% of global CO2 emissions. With the advent of Covid-19, air traffic seems to have come to a fairly permanent halt, but this pandemic reinforces the need to move towards a "cleaner sky" and respect for the environment, which is the objective of the Clean Sky2 program (H2020 EU), the context in which the HASTECS project has been launched in September 2016

    Women in Bioorganic Chemistry

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    Issues relating to the gender schism and its effect on the career advancement of women in the Academy, especially in the field of STEM disciplines, deserve our attention and the efforts of all the scientific community to mitigate the gender gap. In order to embrace gender equality, recognize the career progression of women, and to celebrate the achievements of women in the field of bioorganic chemistry, we present contributions both from highly renowned female scientists and young female researchers who are in the early stages of their careers. This Special Issue includes fifteen manuscripts, including eleven high-quality research articles and four comprehensive review articles in the area of bioorganic chemistry, published from mid-2020 to early 2022. The scope of this Special Issue covers a wide range of topics at the organic chemistry–biology interface, including the synthesis and derivatization of natural compounds and their analogues, and the investigation of their biological activities in the human health field (for instance as antitumorals, antioxidants and antimicrobial agents), as well as their possible application in the crop protection field as agrochemicals. An example of nanoparticle-based biomaterial is also included. The techniques employed, besides organic synthesis, are in silico studies (docking procedures and molecular modeling), FT-IR spectroscopy, laser diffraction, PET, fluorescence, STD-NMR studies, enzymatic evaluation, experiments on cell lines and in vivo studies on mice

    Antioxidants in Age-Related Diseases and Anti-Aging Strategies

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    Aging is a complex, only partially understood process, and is a key risk factor in the development of noncommunicable ailments that greatly impact human quality of life. One of the causes of most age-related diseases is an imbalance in the oxidation-reduction reactions, with the accumulation of reactive species, which are major contributors to cellular senescence. The book contributes to the knowledge connected to the prevention or treatment of these pathological conditions. It focuses on the mechanisms by which oxidative stress and inflammatory factors could cause the genesis and progression of age-associated diseases, and on new strategies for delaying and altering aging
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