Extraction of 3D vortex structures from a turbulent puff in a pipe using two-color illumination and flakes

A novel visualization technique was proposed to extract the three-dimensional vortex structure of a turbulent puff, which is a local turbulence event that is observed in pipe flows at relatively low Reynolds numbers. The technique is based on multi-color illumination of microscopic flakes that are suspended in the flow, which makes structural visualization more informative than conventional monochrome approaches. A special optical arrangement of two laser sheets, colored green and blue, was established for the circular pipe. Based on an image analysis sequence, the internal structure of the puff is reconstructed as a cross-sectional temporal 3D image consisting of voxels with unicolor degrees between green and blue, where an individual single vortex is extracted as a pair of two-color stripes. This allows quantification of the azimuthal wavenumber of the vortical structure that characterizes the puff. The wavenumber results agreed well with the results of previous studies, thus supporting the applicability of the proposed visualization technique.

Random Dynamics from Time Series of Rotating Fluid

A random dynamics is extracted from time series of laminar-turbulent transition in rotating fluid\ud in an open cylinder. We focus on the dynamics of the surface height in the central region and\ud measure switching dynamics between different quasi-stationary states and intensity of underlying\ud turbulence. Density of return map is constructed from an one dimensional map with an stochastic\ud term from the experimental data. It is shown that the random dynamics whose noise amplitude\ud depends on the slow variable describes the observed macroscopic features of rotating fluid in terms\ud of noise-induced phenomena

A statistical study of spots in torsional Couette flow

International audienceThis article presents some results on the statistical behavior of localized structures - called "spots" - that propagate in the flow between a rotating and a stationary disc when those are very close one to the other. Under these conditions the rotating disc flow belongs to the Couette flow family and is called the torsional Couette flow. Some visualizations of its transition to turbulence have already revealed the propagation of these spots [Schouveiler et al., J. Fluid Mech. 443, pp. 329-350, 2001] from the rim of the disc towards its center. Using flow visualizations and an original image analysis, the present study aims to better describe the characteristics of the spots whose number continuously increases with the Reynolds number until they invade the whole flow. Moreover, we propose a statistical model that predicts an error function shape for the probability to observe a spot at a given radial position. This prediction is confirmed by an image analysis of the flow and the stability curve of torsional Couette flow is deduced from these observations

Continuous monitoring of groundwater radon for evaluating chemical and structural properties and fluid flow variations of shallow aquifer systems

We have carried out continuous monitoring of radon concentration in groundwater at eight wells in Nishinomiya City, Hyogo Prefecture, Japan. The shallowest major aquifer, which we call the first aquifer, extends widely at a depth about 3-5 meters in the region. We pay our major attention to the first aquifer, because it provides us with abundant high quality water, which has long been utilized for brewing Sake. We made the radon monitoring at 5 wells tapping the first aquifer. The first aquifer can be characterized by very high radon concentration and its large temporal variation. Time-averaged values of radon concentration at 4 shallow wells were about 70-80 Bq/l, which is almost the highest value ever reported for natural water throughout the Japanese Islands. These high radon concentrations can be attributed to uranium-rich sediments in the aquifer, which had been brought from the Rokko Mountains region on the north. Large temporal variations of radon concentration observed at the shallow wells probably reflect the heterogeneous distribution of the grain size of sediments, which makes the groundwater flow unstable. We examined radon concentrations in deeper aquifers at three wells with depths of 8-17 m for comparison. Absolute values of radon concentrations and their temporal variations at the deeper wells are smaller than those in the first aquifer, which suggests the small contributions of uranium-rich sands to the sediments compared with the first aquifer. The radon concentration at a well with a depth of 16 m showed periodical variations responding to the ocean tide, which demonstrates that very small signals of crustal deformations can be detected by the continuous monitoring of groundwater radon

Bubble fragmentation dynamics in a subsonic Venturi tube for the design of a compact microbubble generator

Microbubble generators are in wide demand in industry following the discovery of a number of new functions of microbubble mixtures. This paper deals with a Venturi tube microbubble generator in which air bubbles at the inlet are fragmented in the diverging part of the tube. In contrast with past studies, we here regulated the flow subsonic so that fragmentation occurred without the help of pressure shock waves. Counting the microbubbles in image processing, we found that a single bubble fragmented into 20–400 microbubbles depending on the Weber number. The power efficiency is found to range from 30 to 50 percent and insensitive to the liquid viscosity. The mechanism of subsonic fragmentation is elucidated adopting particle tracking velocimetry, in association with a theoretical description of the translational motion and the shape oscillation of the bubble. The key event was found to be the bubble’s rapid slipback in the diverging part of the Venturi tube due to a positive pressure gradient. This provides a function that prevents large bubbles from being released from the subsonic Venturi tube

MEDINA, MARÍA E HIJJOS [Material gráfico]

Copia digital. Madrid : Ministerio de Educación, Cultura y Deporte, 201

Transition vers la turbulence de l'écoulement de Couette de torsion

Cette étude présente le comportement statistique de structures localisées – appelées « spots »- qui se propagent dans l'écoulement entre un disque tournant et un disque stationnaire quand ceux-ci sont très proches l'un de l'autre. Dans ces conditions l'écoulement s 'appelle l'écoulement de Couette de torsion. En utilisant une analyse d'images originale, la présente étude permet de décrire les caractéristiques des spots dont le nombre augmente avec le nombre de Reynolds jusqu'à ce que ceux-ci envahissent totalement l'écoulement. Nous proposons un modèle statistique qui prévoit une forme de type « fonction erreur » pour la probabilité d'observation d'un spot à une position radiale donnée. Cette prévision est confirmée par l'analyse d'images et la courbe de stabilité de l'écoulement de Couette de torsion est déduite de ces observations. Cette courbe présente un comportement en loi de puissance du type (Re-Rec)-3/2 , compatible avec les courbes de stabilité des autres écoulements de Couette

Surface switching statistics of rotating fluid: Disk-rim gap effects

We examined the influence of internal noise on the irregular switching of the shape of the free surface of fluids in an open cylindrical vessel driven by a bottom disk rotating at constant speed [Suzuki, Iima, and Hayase, Phys. Fluids 18, 101701 (2006)]. A slight increase in the disk-rim gap (less than 3% of the disk radius) was established experimentally to cause significant changes in this system, specifically, frequent appearance of the surface descending event connecting a nonaxisymmetric shape in strong mixing flow (turbulent flow) and an axisymmetric shape in laminar flow, as well as a shift in critical Reynolds number that define the characteristic states. The physical mechanism underlying the change is analyzed in terms of flow characteristics in the disk-rim gap, which acts as a noise source, and a mathematical model established from measurements of the surface height fluctuations with noise term