Flow and air-entrainment around partially submerged vertical cylinders

In this study, a partially submerged vertical cylinder is moved at constant velocity through water, which is initially at rest. During the motion, the wake behind the cylinder induces free-surface deformation. Eleven cylinders, with diameters from $D=1.4$ to 16 cm, were tested at two different conditions: (i) constant immersed height $h$ and (ii) constant $h/D$. The range of translation velocities and diameters are in the regime of turbulent wake with experiments carried out for $4500<Re<240 \,000$ and $0.2<Fr<2.4$, where $Re$ and $Fr$ are the Reynolds and Froude numbers based on $D$. The focus here is on drag force measurements and relatively strong free-surface deformation up to air-entrainment. Specifically, two modes of air-entraiment have been uncovered: (i) in the cavity along the cylinder wall and (ii) in the wake of the cylinder. A scaling for the critical velocity for air-entrainment in the cavity has been observed in agreement with a simple model. Furthermore, for $Fr>1.2$, the drag force varies linearly with $Fr$

Transition to turbulence in slowly divergent pipe flow

The results of a combined experimental and numerical study of the flow in slowly diverging pipes are presented. Interestingly, an axisymmetric conical recirculation cell has been observed. The conditions for its existence and the length of the cell are simulated for a range of diverging angles and expansion ratios. There is a critical velocity for the appearance of this state. When the flow rate increases further, a subcritical transition for localized turbulence arises. The transition and relaminarization experiments described here quantify the extent of turbulence. The findings suggest that the transition scenario in slowly diverging pipes is a combination of stages similar to those observed in sudden expansions and in straight circular pipe flow.Comment: 8 pages, 5 figure

Packings of deformable spheres

International audienceWe present an experimental study of disordered packings of deformable spheres. Fluorescent hydrogel spheres immersed in water together with a tomography technique enabled the imaging of the three-dimensional arrangement. The mechanical behavior of single spheres subjected to compression is first examined. Then the properties of packings of a randomized collection of deformable spheres in a box with a moving lid are tested. The transition to a state where the packing withstands finite stresses before yielding is observed. Starting from random packed states, the power law dependence of the normal force versus packing fraction or strain at different velocities is quantified. Furthermore, a compression-decompression sequence at low velocities resulted in rearrangements of the spheres. At larger packing fractions, a saturation of the mean coordination number took place, indicating the deformation and faceting of the spheres

Introdução a um Estudo sobre o "Estudo V - Die Reihe Courante "- para Piano

O Estudo V ("die Reihe - Courante") foi escrito em 1992. Como qualquer Estudo que se preze, e tomando como referência histórica os exemplos magistrais de Chopin, Liszt ou Debussy, uma peça com este título deve conter dois vetores fundamentais, a saber: ser um "estudo" simultaneamente de execução para o instrumento respectivo (neste e naqueles casos, o piano) e para o compositor igualmente, como laboratório de novas experiências e dilatação dos seus limites técnico-expressivos. Neste artigo, procuramos analisar sumariamente estes dois aspectos (obviamente, complementares entre si) no “Estudo V”. No plano da técnica instrumental, ele pretende constituir um verdadeiro "estudo" de novos processos de execução, bem como um aprofundamento de princípios técnicos composicionais. Quanto ao que se refere à problemática específica do compositor, como "estudo" de conceitos e processos, há que realçar, aspectos como a) a função tímbrica (colorística) do intervalo; b) o encadeamento poli-temporal das secções constituintes da peça; c) a valorização do registro como fator determinante da estrutura composicional; d) a criação de espaços ambíguos ("entrelaçados"); e) a utilização de zonas de âmbito fixo e oscilante; f) as novas relações instauradas entre planos distintos simultâneos, hierarquicamente distribuídos; g) por fim, é de assinalar a presença constante e quase avassaladora da série ("die Reihe"), constituindo esta o núcleo, a raiz e a fonte primacial da obra, ao mesmo tempo que assume uma função emblemática e simbólica

Extensional Viscosity of Immiscible Polymers Multinanolayer Films: Signature of the Interphase

The measurement of interfacial mechanical or rheological properties in polymer blends is a challenging task, as well as providing a quantitative link between these properties and the interfacial nanostructure. Here, we perform a systematic study of the extensional rheology of multilayer films of an immsicible polymer pair, polystyrene and poly(methyl methacrylate). We take advantage of multinanolayer coextrusion to increase the number of interfaces up to thousands, consequently magnifying the interfacial response of the films. The transient elongational response is compared to an addivity rule model based on the summation of the contribution of each polymer as well as the interfacial one. At low strain rates, the model reproduces the transient extensional viscosity up to strain-thinning, while at larger ones, the extra stress exceeds the prediction based on constant interfacial tension. This extra-contribution is attributed to an interphase modulus on the order of 1-10 MPa, which increases with strain rate following a power-law with an exponent 1/3. Extensional rheology of multinanolayer films is then an efficient combination to go beyond interfacial tension and measure quantitatively the interfacial rheology of immiscible polymer blends.Comment: 6 figure

Étude des propriétés d'adhésion à l'interface entre deux films de polytétrafluoroéthylène (PTFE)

National audienceCette étude vise à déterminer la fenêtre de paramètres optimum (pression, temps, température) pour réaliser le soudage entre deux surfaces de PTFE et comprendre les mécanismes d'adhésion entrant en jeu. Les principaux mécanismes responsables de l'adhésion à l'interface polymère/polymère sont présentés puis leur caractérisation expérimentale est exposée. Abstract This study aims to determine the optimum parameters (pressure, time, temperature) to achieve welding between two PTFE surfaces and to understand the adhesion mechanisms involved. The main mechanisms responsible for adhesion of polymer-to-polymer interface are presented and then their experimental characterization is exposed

Numerical and experimental studies of the flow around a partially submerged vertical cylinder

International audienceL'écoulement et l'injection d'air autour d'un cylindre vertical partiellement immergé, en translation, est étudié expérimentalement et numériquement. Le régime d'écoulement est turbulent avec 15 000 < Re < 60 000 et 0.4 < F r < 1.7, où Re et F r sont les nombres de Reynolds et de Froude adimensionnés à l'aide du diamètre D du cylindre. Une cavité à l'aval du cylindre est observée. L'objectif de ce travail est d'étudier les efforts de traînée, l'élévation de la surface libre et la vitesse critique à partir de laquelle il y a entraînement d'air. Un bon accord entre les expériences et les simulations a été trouvé, évaluant ainsi la profondeur de la cavité, le coefficient de traînée et la vitesse critique d'injection d'air. Abstract : The flow around a vertical cylinder piercing the free-surface is studied experimentally and numerically. The cylinder has a free-end and the range of velocities are in the regime of turbulent wake with experiments and simulations carried out for 15 000 < Re < 60 000 and 0.4 < F r < 1.7, where Re and F r are the Reynolds and Froude numbers based on the cylinder diameter D. A cavity downstream the cylinder is observed. The focus here is on drag force measurement, free-surface elevation, and critical velocity for air-entrainment. Specifically, a good agreement between experiments and simulations is obtained for the cavity depth, the drag coefficient and the critical velocity

Drop dynamics of viscoelastic filaments

The stretching of viscoelastic polymer solutions close to breakup can create attached drops on a filament, whose properties and dynamics are little understood. The stretching of capillary bridges and the consecutive filament, until its breakup, can be quantified using diameter-space-time diagrams, which demonstrate hierarchy, as well as asymmetry of satellite drops around a big central drop. All drops experience migration, oscillation, and merging. In addition, the position of the minimum diameter on the filament is determined, along with the number of drops, their positions, the diameters of drops and the filament breakup time. The maximum number of drops on the filament can be predicted using the Deborah number. The diagrams also quantify the large Hencky strains in the filaments before pinch-off. The obtained minimum diameter is used to measure the extensional viscosity, which indicates the effect of polymer concentration and direction of filament thinning

Features of Transition to Turbulence in Sudden Expansion Pipe Flows

The complex flow features resulting from the laminar-turbulent transition (LTT) in a sudden expansion pipe flow, with expansion ratio of 1:2 subjected to an inlet vortex perturbation is investigated by means of direct numerical simulations (DNS). It is shown that the threshold for LTT described by a power law scaling with -3 exponent that links the perturbation intensity to the subcritical transitional Reynolds number. Additionally, a new type of instability is found within a narrow range of flow parameters. This instability originates from the region of intense shear rate which is a result of the flow symmetry breakdown. Unlike the fast transition, usually reported in the literature, the new instability emerges gradually from a laminar state and appears to be chaotic and strongly unsteady. Additionally, the simulations show a hysteresis mode transition due to the reestablishment of the recirculation zone in a certain range of Reynolds numbers. The latter depends on (i) the initial and final quasi-steady states, (ii) the observation time and (iii) the number of intermediate steps taken when increasing and decreasing the Reynolds number

Instability and rupture of sheared viscous liquid nanofilms

Liquid nanofilms are ubiquitous in nature and technology, and their equilibrium and out-of-equilibrium dynamics are key to a multitude of phenomena and processes. We numerically study the evolution and rupture of viscous nanometric films, incorporating the effects of surface tension, van der waals forces, thermal fluctuations and viscous shear. We show that thermal fluctuations create perturbations that can trigger film rupture, but they do not significantly affect the growth rate of the perturbations. The film rupture time can be predicted from a linear stability analysis of the governing thin film equation, by considering the most unstable wavelength and the thermal roughness. Furthermore, applying a sufficiently large unidirectional shear can stabilise large perturbations, creating a finite-amplitude travelling wave instead of film rupture. In contrast, in three dimensions, unidirectional shear does not inhibit rupture, as perturbations are not suppressed in the direction perpendicular to the applied shear. However, if the direction of shear varies in time, the growth of large perturbations is prevented in all directions, and rupture can hence be impeded