Search

This work is devoted to the study of the conditions under which a drop directed normally towards a superheated or isothermal smooth substrate prevents the initial contact with the solid by skating over a micrometre-sized vapour or air layer. The results have been obtained analysing the gas flow at the spatio-temporal region where the maximum liquid pressure is attained, which is also where and when the minimum values of the film thickness are reached. For the common case in which WeSt−1/6≳1, where We=ρlU2R/γ and St=ρlUR/ηa denote, respectively, the Weber and Stokes numbers, we find that capillary effects are negligible and the ratio between the minimum film thickness and the local drop radius of curvature is hm/R∝St−7/6, with ρl, γ, ηa, U and R indicating the liquid density, interfacial tension coefficient, gas viscosity, impact velocity and drop radius, respectively. In contrast, when WeSt−1/6≲1, capillary effects can no longer be neglected and hm/R∝We−1/3St−10/9. The predicted values of the minimum film thickness are compared with published experimental data, finding good agreement between predictions and measurements for the cases of both isothermal and superheated substrates. In addition, using mass conservation, we have also deduced an equation providing the minimum value of the substrate temperature for which a cylindrical central vapour bubble of constant height hd/R∝St−2/3, with hd≫hm, grows radially at the wetting velocity deduced in Riboux & Gordillo (Phys. Rev. Lett., vol. 113, 2014, 024507). The predicted values are in good agreement with the dynamic Leidenfrost temperatures reported by Shirota et al. (Phys. Rev. Lett., vol. 116, 2016, 064501). Ministerio de Ciencia e Innovación PID2020-115655G

Article number A50 Here, we present experimental results of water and ethanol drops of radii R, density ρ and interfacial tension coefficient σ, impacting with a velocity V over different types of sandpapers containing particles of characteristic diameter ε embedded in their surfaces. It is shown that the transition from spreading to splashing at normal atmospheric conditions can be classified depending on the value of the parameter ε/Ht Weε = We(ε/R), with We = ρV2R/σ the Weber number and Ht indicating the initial thickness of the thin film – the lamella – which is ejected along the substrate once the drop touches the solid. When Weε 1 and the liquid wets the substrate, the critical value of the Weber number above which the drop splashes, Wec, can be predicted using the results in Gordillo & Riboux (J. Fluid Mech., vol. 871, 2019, R3) once the angle the advancing rim forms with the substrate, α, is expressed as a decreasing function of the static advancing contact angle. The calculated values of Wec for the case of water drops impacting over rough substrates are smaller than the corresponding ones for smooth substrates, in agreement with experimental observations. Moreover, if the liquid does not wet the substrate, it is also shown that the splash velocity can be predicted using the theory for superhydrophobic substrates in Quintero, Riboux & Gordillo (J. Fluid Mech., vol. 870, 2019, 175–188). For those cases in which Weε 1 and the liquid wets the substrate, we demonstrate that the critical Weber number for splashing decreases with ε as Wec ∝ (R cos θ0/ε)3/5, with θ0 the value of the Young contact angle. Ministerio de Economía y Competitividad (España) DPI2017-88201-C3-1-R

Periodicity in evolution of premixed methane–air V-shaped flames in the space domain is investigated experimentally. The experiments were performed using the Mie scattering and Particle Image Velocimetry techniques. Three Reynolds numbers of 510, 790, and 1057 along with two fuel–air equivalence ratios of 0.6 and 0.7 were tested in the experiments. The analyses were performed using the Proper Orthogonal Decomposition (POD) technique for the flame front position as well as the velocity data pertaining to non-reacting flow condition. The POD analysis shows that the spectral characteristics of the mode shapes associated with the velocity and the flame front position data feature similarities; however, the corresponding temporal coefficients are significantly different. Specifically, the POD mode shapes pertaining to both velocity and flame front position data feature dominant instabilities. It was shown that the normalized wave number pertaining to these instabilities are similar and equal to the Strouhal number corresponding to non-reacting flow over a circular cylinder. Comparison of the normalized temporal coefficients show that, for the flame front position data, the normalized first and second coefficients are mainly centered close to the origin; however, those associated with the velocity data are positioned around a unity radius circle. This was argued to be linked to the ratio of the corresponding first and second eigenvalues. Specifically, it was shown that, as this ratio approached to unity, the signal energy becomes distributed between the first and the second POD modes. As a result, the normalized temporal coefficients follow a circular pattern.

International audience; In this study, a combined experimental and numerical investigation of a toroidal vortex interacting with a stagnation premixed flame is carried out with the aim of quantifying the ability of such a vortex to stretch the flame. By scrutinizing the literature, it was found that, although inferred from exactly similar numerical simulations, existing parametric expressions for the efficiency function (the ratio of the flame stretch to vortex strain) do not agree in the way the latter should behave when the ratio of the vortex rotational velocity U θ to the laminar flame speed S L is increased. These expressions also appear to be unequally accurate when compared to experimental data and do not feature the non monotonic evolution of the efficiency function with U θ / S L which is observed in both experimental data and numerical simulations of a ‘isothermal’ propagating interface. In addition, whilst previous studies have focused only on the impact of U θ / S L and R v / δL ( R v being the vortex typical size and δL the laminar flame thickness) our study reveals the importance of other parameters, the most important of which being the residence time of the vortex associated with its convection velocity. These results yield a new formulation for the efficiency function which compares favorably well with experimental data.

Although traditional validation process has historically produced sufficiently safe and reliable systems, they may be not sufficient for emerging Guidance Navigation & Control (GNC) systems, for new requirements for exploration missions or for next generation launchers. Spacecraft autonomy and safety requirements are always increasing for such missions, therefore, the number of tests, their complexity and their duration also increase. The main consequence is the increase of the level of effort (and the cost) required to manage and perform the VV thus, current V&V technologies may either be inadequate or will simply become unaffordable. In this paper, we give an overview of the SAFE-V enhanced framework that upgrades the whole traditional Design Verification and Validation (DVV) strategies for advanced GNC systems: from specifications, validation plan, validation methods and tools, design to analysis. To illustrate the potential interests of this framework, an application of multi-objective optimization control design is presented showing iterative design cost reduction and performance improvement with fixed or varying level of requirement.

International audience; The main objective of the present study is to explore the degree of interactions between turbulence and the flame front in the context of lean premixed combustion. Premixed methane Bunsen flames are investigated using Mie scattering at three different pressure magnitudes (0.1, 0.2, and 0.3 MPa) and equivalence ratios (0.6, 0.7, 0.8) through six operating conditions at the same mean bulk velocity (3.6 m/s). Nearly homogeneous and isotropic turbulence is generated by a multi-scale grid consisting of three distinct perforated plates. The multi-scale nature of the flame front is characterized using an original method based on Proper Orthogonal Decomposition (POD). Particular emphasis is shed to the scales responsible for the flame brush and the wrinkling of the flame front. The flame front curvilinear length is analyzed as a function of the mode (or equivalently the scale) and yields the well known Richardson plot that is usually observed using fractal analysis. The inner and outer cut-off length-scales, together with the fractal dimension can thus be evaluated using the present method. An analytical expression for unambiguously assessing the latter three parameters is introduced and compares favorably well with experimental data. Second, the reconstruction of the flame using only the first few modes suggests that, the flame brush is characteristic of a rather large-scale phenomena whilst the smallest scales act in decreasing the total flame brush.

International audience; We propose a model for assessing the unresolved wrinkling factor in the large eddy simulation of turbulent premixed combustion. It relies essentially on a power-law dependence of the wrinkling factor on the filter size and an original expression for the ‘active’ corrugating strain rate. The latter is written as the turbulent strain multiplied by an efficiency function that accounts for viscous effects and the kinematic constraint of Peters. This yields functional expressions for the fractal dimension and the inner cut-off length scale, the latter being (i) filter-size independent and (ii) consistent with the Damköhler asymptotic behaviours at both large and small Karlovitz numbers. A new expression for the wrinkling factor that incorporates finite Reynolds number effects is further proposed. Finally, the model is successfully assessed on an experimental filtered database.

Polynomial surrogate models of an industrial standard, closed-loop launcher model during an atmospheric flight phase are developed. A probabilistic collocation method is applied to derive surrogate models of varying complexity for wind perturbations occurring at different time instances during the atmospheric flight phase. These models are employed in optimisation-based worst-case analysis to significantly reduce the computational overhead while maintaining the accuracy of the computed worst-case uncertain parameter combinations. Comparisons of worst-case analysis results obtained using standard simulation models and their polynomial surrogate equivalents demonstrate significant potential of the proposed approach for radically reducing the computational burden associated with the industrial verification and validation process.

The robustness of an industrial standard, closed-loop launcher model during an atmospheric flight phase is assessed in terms of robust safety margins in a multi-dimensional parameter space made up of various rigid and bending mode uncertainties. A single-axis launcher model with time-varying aerodynamic and thruster efficiency parameters, along with an H ∞ controller and bending mode filter is considered. A constrained hybrid optimisation approach is adopted to determine the safety margins for different wind perturbations acting at different time instances of the flight. In addition, optimisation-based analysis is performed to determine the worst-case uncertain parameter combination for the launcher during the atmospheric flight phase. Probability constraints on the uncertain parameter combination satisfying the dispersion requirements are considered during the analysis.

International audience; This study focuses on the geometrical properties of turbulent flame fronts and other interfaces. Toward that end, we use an original tool based on proper orthogonal decomposition (POD), which is applied to the interface spatial coordinates. The focus is mainly on the degree of roughness of the flame front, which is quantified through the scale dependence of its coverage arclength. POD is first validated by comparing with the caliper technique. Fractal characteristics are extracted in an unambiguous fashion using a parametric expression which appears to be impressively well suited for representing Richardson plots. Then it is shown that, for the range of Reynolds numbers investigated here, the scale-by-scale contribution to the arclength does not comply with scale similarity, irrespectively of the type of similarity which is invoked. The finite ratios between large and small scales, referred to as finite Reynolds number effects, are likely to explain this observation. In this context, the Reynolds number that ought to be achieved for a proper inertial range to be discernible, and for scale similarity to be likely to apply, is calculated. Fractal characteristics of flame folding are compared to available predictions. It is confirmed that the inner cutoff satisfactorily correlates with the Kolmogorov scale while the outer cutoff appears to be proportional to the integral length scale. However, the scaling for the fractal dimension is much less obvious. It is argued that much higher Reynolds numbers have to be reached for drawing firm statements about the evolution (or constancy) of the fractal dimension with respect to flame and flow parameters. Finally, a heuristic phenomenology of corrugated interfaces is highlighted. The degree of generality of the latter phenomenology is confirmed by comparing the folding of different interfaces including a turbulent-nonturbulent interface, a liquid jet destabilized by a surrounding air jet, a cavitating flow, and an isoscalar evolving in a turbulent medium. The latter outcome is likely to have strong implications for modeling the corrugation of turbulent interfaces occurring in many physical situations.

Dissertação (mestrado)—Universidade de Brasília, Instituto de Ciências Sociais, 2011. A ilha de Amantaní constitui um dos principais atrativos turísticos da região de Puno, nos Andes peruanos. A ilha se diferencia dos outros destinos turísticos da região ao propor uma imersão cultural na qual os turistas se hospedam nas casas de famílias hospedeiras. Eles recebem, assim, hospedagem e alimentação, além de conviverem com a família, em complemento da visita rápida à ilha. Este turismo é conhecido localmente como ―turismo vivencial‖ e permite desvendar e viver o quotidiano de uma cultura autêntica, localizada no meio do lago Titicaca, a mais de 3.000 metros de altitude. Há poucos anos, Amantaní faz parte de um vasto programa turístico implementado pelo Estado, em vista de diversificar a oferta turística no país e desenvolver as comunidades camponesas rurais: o programa de Turismo Rural Comunitário. Esta dissertação tem como principal objetivo analisar o conceito de autenticidade relacionado com o encontro direto entre uma população nativa e os turistas estrangeiros. Nesse sentido, ela se propõe a analisar o significado da experiência turística para as famílias camponesas que hospedam os turistas no seu lar, permitindo dessa forma certa introspecção sobre sua intimidade a qual, antes de ser uma experiência vivida, é representada por meio de uma ordem discursiva articulada ao redor da autenticidade. Para tanto, Goffman (1973) e MacCannell (1976) foram as referências privilegiadas para a compreensão da situação social criada através da interação entre quem é de dentro e de fora. Além disso, foi realizada pesquisa de campo como forma de levantamento dos dados etnográficos. Concluiu-se que a experiência turística reflete a lógica da busca por uma alteridade construída discursivamente. Essa lógica se renova através de novos conceitos, tais como a ―colonização da intimidade‖ e ―desejo colonial‖, os quais, apesar de surgirem localmente, são fomentados, no âmbito nacional e global, pelas políticas do Estado peruano. _______________________________________________________________________________ ABSTRACT Amantani Island constitutes one of the main touristic attractions of the region of Puno, in the Peruvian Highlands. The island distinguishes from other local touristic destinations since it offers a cultural immersion which provides tourists with a roof in family houses. These tourists visiting the island receive accommodation and food, and have the opportunity to live together with the native family in addition to the quick visit of the island. This tourism is locally known as ―vivencial tourism‖ and enables to discover and live the day-to-day life in an authentic culture, situated in the middle of Titicaca Lake, at more than 3.000 meters above sea level. Amantaní has been integrated in a vast touristic program implemented by the State for a few years, in order to diversify the national touristic offer and to develop the peasant rural communities. Such program is known as the Rural Community Tourism. The main purpose of this master thesis is to analyse the concept of authenticity linked with the direct encounter between native population and foreign tourists. In that way, it proposes to analyse the meaning of the touristic experience for the peasant families who accommodate the tourists at home. It enables a certain insight on their intimacy. Before being lived, this experience is represented by a discursive order articulated around the authenticity notion. Goffman (1973) and MacCannell (1976) have been the privilegiate references to understand the social situation created through the interaction between insiders and outsiders. Moreover, a fieldwork was conducted so as to bring up the ethnographic data. It has been concluded that the touristic experience reflects the logic of the quest for the other discursively constructed. This logic is renovated through new concepts like the ―colonization of intimacy‖ and ―colonial desire‖, which are produced locally but are promoted in a national and global scale, by the policies of Peruvian State.

Modern game playing programs use opening books in order to perform better. Generating opening books automatically in combination with an αβ program has been well studied. A challenge is to generate automatically an opening book for the new Monte-Carlo Tree Search (MCTS) algorithms. In this article, we tackle this issue by combining two level of MCTS. The resulting algorithm is called Meta-MCTS. Instead of applying a simulation strategy, it uses an MCTS program to play a simulated game. We describe two Meta-MCTS algorithms: the first one, Quasi Best-First, favors exploitation. The second one, Beta Distribution Sampling, favors exploration. Our approach is generic and can be used for general game playing. Itwill be particularly usefulwhen there is off-line time avail- able. In order to evaluate the performances of these algorithms, we generated and tested 99 Go opening books.

International audience

International audience

International audience; Many projects centered on new floating wind turbines as Vertical Axis Wind Turbines (VAWTs) whose advantages are magnified for offshore applications, are being developed to adapt the extraction of wind energy to the most difficult conditions associated with deep waters. The OWLWIND project which is starting is centered on such a machine with a twin rotors architecture; its first challenge is to raise peak VAWT rotor efficiencies, which is still lower than peak Horizontal Axis Wind Turbines (HAWTs) efficiencies. Each rotor has an innovative design, reducing parasitic drags. Performances of an isolated rotor have given quite encouraging results. A second test phase on the properties of the whole machine is currently conducted via a model in a wind tunnel.