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Ribbing patterns in inertial rotary drag-out
No full textInternational audienceWe report pattern formation in an otherwise non-uniform and unsteady flow arising in high-speed liquid entrainment conditions on the outer wall of a wide rotating drum. We show that the coating flow in this rotary dragout undergoes axial modulations to form an array of roughly vertical thin liquid sheets which slowly drift from the middle of the drum towards its sidewalls. Thus, the number of sheets fluctuates in time such that the most probable rib spacing varies ever so slightly with the speed, and a little less weakly with the viscosity. We propose that these axial patterns are generated due to a primary instability driven by an adverse pressure gradient in the meniscus region of the rotary drag-out flow, similar to the directional Saffman–Taylor instability, as is wellknown for ribbing in film-splitting flows. Rib spacing based on this mechanistic model turns out to be proportional to the capillary length, wherein the scaling factor can be determined based on existing models for film entrainment at both low and large capillary numbers. In addition, we performed direct numerical simulations, which reproduce the experimental phenomenology and the associated wavelength. We further include two numerical cases wherein either the liquid density or the liquid surface tension is quadrupled while keeping all other parameters identical with experiments. The rib spacings of these cases are in agreement with the predictions of our model
23 - Nanomechanics of tribologically transformed surfaces
No full textInternational audienceTribologically transformed structures (TTSs), or surfaces, refer to materials that undergo significant changes at their extreme surfaces due to intense tribological loadings. The size of these transformed structures can vary from a few microns to tenths of microns, depending on factors such as contact geometry, the bodies in contact, and the applied loads. The purpose of this chapter is to provide a concise overview of recent advancements in the use of nanomechanical testing to quantify the mechanical properties of tribologically transformed surfaces (TTSs). The chapter explores the methodology for assessing the yield strength and thermal stability of TTS through nanoindentation and microcompression experiments. Additionally, it focuses on the application of nanomechanical testing to glaze layers, which are tribofilms induced by high-temperature fretting loading. Finally, the chapter discusses anticipated developments in this field in the near future
Group schemes over LG-rings and applications to cancellation theorems and Azumaya algebras
No full textInternational audienceWe prove several results on reductive group schemes over LG-rings, e.g., existence of maximal tori and conjugacy of parabolic subgroups. These were proven in SGA3 for the special case of semilocal rings. We apply these results to establish cancellation theorems for hermitian and quadratic forms over LG-rings and show that the Brauer classes of Azumaya algebras over connected LG-rings have a unique representative and allow Brauer decomposition
From Kähler Ricci solitons to Calabi-Yau Kähler cones
No full text28 pages. Comments are welcome!We show that if is a smooth Fano manifold which caries a K\"ahler Ricci soliton, then the canonical cone of the product of with a complex projective space of sufficiently large dimension is a Calabi--Yau cone. This can be seen as an asymptotic version of a conjecture by Mabuchi and Nikagawa. This result is obtained by the openness of the set of weight functions over the momentum polytope of a given smooth Fano manifold, for which a -soliton exists. We discuss other ramifications of this approach, including a Licherowicz type obstruction to the existence of a K\"ahler Ricci soliton and a Fujita type volume bound for the existence of a -soliton
New Lower Bounds for the (Near) Critical Ising and φ^4 Models’ Two-Point Functions
No full textInternational audienceWe study the nearest-neighbour Ising and φ 4 models on Z d with d ≥ 3 and obtain new lower bounds on their two-point functions at (and near) criticality. Together with the classical infrared bound, these bounds turn into up to constant estimates when d ≥ 5. When d = 4, we obtain an "almost" sharp lower bound corrected by a logarithmic factor. As a consequence of these results, we show that η = 0 and ν = 1/2 when d ≥ 4, where η is the critical exponent associated with the decay of the model's two-point function at criticality and ν is the critical exponent of the correlation length ξ(β). When d = 3, we improve previous results and obtain that η ≤ 1/2. As a byproduct of our proofs, we also derive the blow-up at criticality of the so-called bubble diagram when d = 3, 4
Impact of communication modalities on social presence and regulation processes in a collaborative game
No full textInternational audienceIn the digital era, leveraging communication technologies to foster collaborative learning is of utmost importance. This study explores the impact of different communication modalities, such as text, audio and video, on social presence and regulation processes within a computer-supported collaborative learning (CSCL) environment. Using learning analytics, we examine the influences of these modalities on collaboration and derive recommendations for their optimized use in the design of future CSCL environments. Our findings reveal a significant impact of communication modalities on the sense of social presence and regulation of collaborative activities. Audio communication results in enhanced co-presence, psychobehavioral accessibility, and better regulation processes compared to video and text modalities, indicating that audio is the most suitable modality in collaborative virtual environments for decision-making tasks. Conversely, video communication still facilitated strategic planning and enhanced self-regulation. Chat communication showed the lowest sense of social presence, yet improvements over time suggest that participants adapt to this modality, enhancing their collaborative efficiency.</div
A new wave-based structural identification framework for estimating material properties of honeycomb sandwich structural components
No full textInternational audienceWave-based structural identification for real honeycomb sandwich structures has become an important research focus. However, most existing wave-based identification methods suffers from experimental uncertainties and a limited frequency range of applicability. To this end, we present a new wave-based structural identification framework, which includes two promising material identification methods – linear and nonlinear – suitable for honeycomb sandwich structures. The advantages of the identification process are reflected on two aspects: Firstly, the Algebraic Wavenumber Identification (AWI) technique reliably extracts complex wavenumbers over a wide frequency range under stochastic conditions, serving as input for the identification process. Secondly, a novel frequency-dependent, stepwise estimation strategy is proposed for honeycomb sandwich structures, greatly enhancing the precision of material parameter determination. Noteworthy, the proposed structural identifications enable the recovery of both equivalent dynamic and static mechanical properties. The experimental applications on a real beam, plate, and shell are presented. Key results show that (1) The proposed stepwise strategy reduces the relative error of wavenumbers of the tested beam to below 3.5%, improving parameter accuracy and ensuring estimation success; (2) For the tested plate, the estimated Young’s modulus of skins, shear modulus of the core, and dynamic Hooke’s matrix demonstrate satisfied precision; (3) It is the first to extract mechanical parameters of real curved structures using wave-based propagation parameters
TruShare: base de données clé-valeur confidentielle pour environnements non-fiable
No full textInternational audienceKey-Value Stores (KVSs), commonly used for storing sensitive data, face significant security challenges when deployed in untrusted cloud environments. These environments are susceptible to various types of attacks exploiting a compromised OS or running a side-channel attacks. To protect sensitive data from these types of attacks distributed TEE-based KVSs have been proposed. However, these solutions are still vulnerable to side channel attacks that may compromise any node and leak all its data at once. Active defense mechanisms against side channel attacks, such as Oblivious RAM, are impractical to deploy due to their significant performance overhead or the requirement for additional hardware (e.g., FPGA). Consequently, these defense mechanisms are often skipped in favor of performance in most existing TEE-based KVSs, which weakens their security. To address this issue, we present TruShare, a practical distributed in-memory KVS that integrates TEEs (Intel SGX) and Shamir Secret Sharing (SS) to provide security against highprivileged spywares while tolerating side-channel attacks on a fraction of storage nodes, without requiring costly active defense mechanisms. We implemented TruShare and evaluated its performance using 25 Microsoft Azure VMs. Compared to its closest competitors, TruShare considers a stronger threat model while providing more practical performance than solutions relying on active defense mechanisms against side-channel attacks
Coexistence of Two Equilibrium Configurations in 2D turbulence
No full textInternational audienceIn the past, two families of statistical mechanics approaches have been applied to the two-dimensional Euler equations. The first one is formulated in Fourier space and considers the Galerkin truncated dynamics. The other one is formulated in physical space and considers either point-vortices or coarse-grained vorticity. We show that in a Galerkin truncated system both methods describe a part of the flow. A condensate can be identified assuming that it can be characterized by an unspecified functional relation between the vorticity and the stream-function. It is shown, a posteriori, that this function is a hyperbolic sine relation, as predicted by point-vortex statistical mechanics. The energy spectrum associated with the condensate is well described by an exponential function and the tails of the probability density function of the vorticity are following a power law. Analytical arguments to explain these observations are proposed. After removing the condensate, the remaining field can be described by Fourier-statistical mechanics
