Reynolds-dependence of turbulent skin-friction drag reduction induced by spanwise forcing

This paper examines how increasing the value of the Reynolds number $Re$ affects the ability of spanwise-forcing techniques to yield turbulent skin-friction drag reduction. The considered forcing is based on the streamwise-travelling waves of spanwise wall velocity (Quadrio {\em et al. J. Fluid Mech.}, vol. 627, 2009, pp. 161--178). The study builds upon an extensive drag-reduction database created with Direct Numerical Simulation of a turbulent channel flow for two, 5-fold separated values of $Re$, namely $Re_\tau=200$ and $Re_\tau=1000$. The sheer size of the database, which for the first time systematically addresses the amplitude of the forcing, allows a comprehensive view of the drag-reducing characteristics of the travelling waves, and enables a detailed description of the changes occurring when $Re$ increases. The effect of using a viscous scaling based on the friction velocity of either the non-controlled flow or the drag-reduced flow is described. In analogy with other wall-based drag reduction techniques, like for example riblets, the performance of the travelling waves is well described by a vertical shift of the logarithmic portion of the mean streamwise velocity profile. Except when $Re$ is very low, this shift remains constant with $Re$, at odds with the percentage reduction of the friction coefficient, which is known to present a mild, logarithmic decline. Our new data agree with the available literature, which is however mostly based on low-$Re$ information and hence predicts a quick drop of maximum drag reduction with $Re$. The present study supports a more optimistic scenario, where for an airplane at flight Reynolds numbers a drag reduction of nearly 30\% would still be possible thanks to the travelling waves

Performance losses of drag-reducing spanwise forcing at moderate values of the Reynolds number

A fundamental problem in the field of turbulent skin-friction drag reduction is to determine the performance of the available control techniques at high values of the Reynolds number $Re$. We consider active, predetermined strategies based on spanwise forcing (oscillating wall and streamwise-traveling waves applied to a plane channel flow), and explore via Direct Numerical Simulations (DNS) up to $Re_\tau=2100$ the rate at which their performance deteriorates as $Re$ is increased. To be able to carry out a comprehensive parameter study, we limit the computational cost of the simulations by adjusting the size of the computational domain in the homogeneous directions, compromising between faster computations and the increased need of time-averaging the fluctuating space-mean wall shear-stress. Our results, corroborated by a few full-scale DNS, suggest a scenario where drag reduction degrades with $Re$ at a rate that varies according to the parameters of the wall forcing. In agreement with already available information, keeping them at their low-$Re$ optimal value produces a relatively quick decrease of drag reduction. However, at higher $Re$ the optimal parameters shift towards other regions of the parameter space, and these regions turn out to be much less sensitive to $Re$. Once this shift is accounted for, drag reduction decreases with $Re$ at a markedly slower rate. If the slightly favorable trend of the energy required to create the forcing is considered, a chance emerges for positive net energy savings also at large values of the Reynolds number.Comment: Revised version: change of title, revised intro, small improvements to figures and tex

La Saga de Bernard Assiniwi, ou comment faire revivre les Béothuks

Bernard Assiniwi, en 1971, a été le premier auteur amérindien à publier un ouvrage en français largement distribué au Québec. Son dernier roman La Saga des Béothuks (Leméac/Actes Sud, 1996) raconte l’histoire des Béothuks de Terre-Neuve depuis l’an 1000 environ jusqu’à 1829, date du décès de la dernière femme de cette nation. La rencontre et le métissage sont au coeur de cette saga. Le passage de l’oralité à l’écriture, la modification des traditions ancestrales, les nouvelles habitudes sexuelles, les thématiques, les personnages montrent, dans le récit, différentes facettes des changements apportés par l’arrivée des européens. Le présent article analysera La Saga des Béothuks, car cet ouvrage est représentatif de la manière dont plusieurs auteurs amérindiens ont marié la tradition orale amérindienne et la tradition écrite romanesque européenne. Après une brève présentation de l’auteur, je me pencherai sur la structure du roman, sa thématique originale, sa riche typologie de personnages, et son utilisation de la langue béothuk. Je terminerai en examinant le style ironique et tranchant de Bernard Assiniwi.In 1971, Bernard Assiniwi became the first Amerindian author to publish a French language work that was broadly distributed in Québec. His last novel, The Beothuk Saga (McClelland & Stewart, 2000), recounts the story of Newfoundland’s Beothuk peoples from about the year 1000 until 1829, when the last woman of the Beothuk nation died. Encounters and miscegenation are at the heart of this saga. The passage from orality to writing, the transformation of ancestral traditions, new sexual habits, themes and characters—the story draws a picture of the changes effected by the arrival of Europeans. In this article, The Beothuk Saga is analyzed as a work that represents the way many Amerindian authors joined Amerindian oral tradition with the European tradition of novelistic writing. After a brief presentation of the author, I will look at the novel’s structure, its original thematic, the rich typology of its characters, and its use of Beothuk language. Finally, I will examine Bernard Assiniwi’s ironic and cutting style

Sugar and chromosome stability: Clastogenic effects of sugars in vitamin B6-deficient cells

Pyridoxal 5'-phosphate (PLP), the active form of vitamin B6, has been implicated in preventing human pathologies, such as diabetes and cancer. However, the mechanisms underlying the beneficial effects of PLP are still unclear. Using Drosophila as a model system, we show that PLP deficiency caused either by mutations in the pyridoxal kinase-coding gene (dPdxk) or by vitamin B6 antagonists results in chromosome aberrations (CABs). The CAB frequency in PLP-depleted cells was strongly enhanced by sucrose, glucose or fructose treatments, and dPdxk mutant cells consistently displayed higher glucose contents than their wild type counterparts, an effect that is at least in part a consequence of an acquired insulin resistance. Together, our results indicate that a high intracellular level of glucose has a dramatic clastogenic effect if combined with PLP deficiency. This is likely due to an elevated level of Advanced Glycation End-products (AGE) formation. Treatment of dPdxk mutant cells with alpha lipoic acid (ALA) lowered both AGE formation and CAB frequency, suggesting a possible AGE-CAB cause-effect relationship. The clastogenic effect of glucose in PLP-depleted cells is evolutionarily conserved. RNAi-mediated silencing of PDXK in human cells or treatments with PLP inhibitors resulted in chromosome breakage, which was potentiated by glucose and reduced by ALA. These results suggest that patients with concomitant hyperglycemia and vitamin B6 deficiency may suffer chromosome damage. This might impact on cancer risk, as CABs are a well-known tumorigenic factor

Global energy budgets in turbulent Couette and Poiseuille flows

Turbulent plane Poiseuille and Couette flows share the same geometry, but produce their flow rate owing to different external drivers: pressure gradient and shear, respectively. By looking at integral energy fluxes, we pose and answer the question as to which flow performs better at creating flow rate. We define a flow efficiency, which quantifies the fraction of power used to produce flow rate instead of being wasted as a turbulent overhead; effectiveness, instead, describes the amount of flow rate produced by a given power. The work by Gatti et al. (J. Fluid Mech., vol. 857, 2018, pp. 345–373), where the constant power input concept was developed to compare turbulent Poiseuille flows with drag reduction, is here extended to compare different flows. By decomposing the mean velocity field into a laminar contribution and a deviation, analytical expressions are derived which are the energy-flux equivalents of the FIK identity. These concepts are applied to literature data supplemented by a new set of direct numerical simulations, to find that Couette flows are less efficient but more effective than Poiseuille flows. The reason is traced to the more effective laminar component of Couette flows, which compensates for their higher turbulent activity. It is also observed that, when the fluctuating fields of the two flows are fed with the same total power fraction, Couette flows dissipate a smaller percentage of it via turbulent dissipation. A decomposition of the fluctuating field into large and small scales explains this feature: Couette flows develop stronger large-scale structures, which alter the mean flow while contributing less significantly to dissipation

Literatura nativa do Quebec de 1970 a 2022

This article provides an overview of Quebec's French-speaking native literature, which is currently becoming more widespread. Its birth and the obstacles encountered over the years are analyzed, with a late affirmation, after the 1970s, due to the historically conflicting relationship between the indigenous peoples and the Quebec school system, and facilitated by the relative sedentarization provided by the creation of the first reservations. We demonstrate how its original manifestations, of a more political and essayistic nature, evolved to encompass several other literary genres, asserting itself today as a space for research and aesthetic creation. We identifie the main themes addressed by the writers, their characteristics and style. We addresse the difficulties of publishing and disseminating the works, and various limitations that lead authors to write mainly in French and not in their native languages. Finally, the achievements of native writers in this historical path are highlighted, as well as the legitimation strategies they adopted and the literary infrastructure they built.O presente artigo traça um panorama geral da literatura nativa francófona do Quebec, que atualmente tem se tornado mais difundida. Analisa-se seu nascimento e os obstáculos por ela encontrados ao longo dos anos, com uma afirmação tardia, após os anos 1970, em razão da relação historicamente conflituosa dos povos autóctones com o sistema escolar quebequense, e facilitada pela relativa sedentarização propiciada pela criação das primeiras reservas. Demonstra-se como suas manifestações originais, de cunho mais político e ensaístico, evoluíram para abarcar diversos outros gêneros literários, afirmando-se hoje como espaço de pesquisa e criação estética. Identifica-se os principais temas abordados pelos escritores, suas características e estilo. Aborda-se as dificuldades de publicação e difusão das obras, e diversas limitações que levam os autores a escrever sobretudo em língua francesa e não em suas línguas nativas. Por fim, sublinha-se as conquistas dos escritores nativos nesse percurso histórico, as estratégias de legitimação por eles adotadas e a infraestrutura literária que construíram

The Drosophila Protein Asp Is Involved in Microtubule Organization during Spindle Formation and Cytokinesis

Abnormal spindle (Asp) is a 220-kD microtubule-associated protein from Drosophila that has been suggested to be involved in microtubule nucleation from the centrosome. Here, we show that Asp is enriched at the poles of meiotic and mitotic spindles and localizes to the minus ends of central spindle microtubules. Localization to these structures is independent of a functional centrosome. Moreover, colchicine treatment disrupts Asp localization to the centrosome, indicating that Asp is not an integral centrosomal protein. In both meiotic and mitotic divisions of asp mutants, microtubule nucleation occurs from the centrosome, and γ-tubulin localizes correctly. However, spindle pole focusing and organization are severely affected. By examining cells that carry mutations both in asp and in asterless, a gene required for centrosome function, we have determined the role of Asp in the absence of centrosomes. Phenotypic analysis of these double mutants shows that Asp is required for the aggregation of microtubules into focused spindle poles, reinforcing the conclusion that its function at the spindle poles is independent of any putative role in microtubule nucleation. Our data also suggest that Asp has a role in the formation of the central spindle. The inability of asp mutants to correctly organize the central spindle leads to disruption of the contractile ring machinery and failure in cytokinesis

Structure function tensor equations in inhomogeneous turbulence

Exact budget equations for the second-order structure function tensor ⟨u$_{i}$u$_{j}$⟩, where u is the difference of the i th fluctuating velocity component between two points, are used to study the two-point statistics of velocity fluctuations in inhomogeneous turbulence. The anisotropic generalised Kolmogorov equations (AGKE) describe the production, transport, redistribution and dissipation of every Reynolds stress component occurring simultaneously among different scales and in space, i.e. along directions of statistical inhomogeneity. The AGKE are effective to study the inter-component and multi-scale processes of turbulence. In contrast to more classic approaches, such as those based on the spectral decomposition of the velocity field, the AGKE provide a natural definition of scales in the inhomogeneous directions, and describe fluxes across such scales too. Compared to the generalised Kolmogorov equation, which is recovered as their half-trace, the AGKE can describe inter-component energy transfers occurring via the pressure–strain term and contain also budget equations for the off-diagonal components of ⟨u$_{i}$u$_{j}$⟩. The non-trivial physical interpretation of the AGKE terms is demonstrated with three examples. First, the near-wall cycle of a turbulent channel flow at a friction Reynolds number of Re$_{}$ = 200 is considered. The off-diagonal component ⟨-uυ⟩, which cannot be interpreted in terms of scale energy, is discussed in detail. Wall-normal scales in the outer turbulence cycle are then discussed by applying the AGKE to channel flows at Re$_{}$ = 500 and 1000. In a third example, the AGKE are computed for a separating and reattaching flow. The process of spanwise-vortex formation in the reverse boundary layer within the separation bubble is discussed for the first time

A construction of a robust family of exponential attractors

Given a dissipative strongly continuous semigroup depending on some parameters, we construct a family of exponential attractors which is robust, in the sense of the symmetric Hausdorff distance, with respect to (even singular) perturbations