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Path instability on a sphere towed at constant speed

JOURNAL OF FLUIDS AND STRUCTURES 2015年 58卷 99-108页

作者： Obligado, Martin Machicoane, Nathanael Chouippe, Agathe Volk, Romain Uhlmann, Markus Bourgoin, Mickael Univ London Imperial Coll Sci Technol & Med Dept Aeronaut Turbulence Mixing & Flow Control Grp London SW7 2AZ England Univ Paris 11 CNRS Lab FAST F-91405 Orsay France Karlsruhe Inst Technol Inst Hydromech D-76131 Karlsruhe Germany Univ Lyon UMR CNRS 5672 ENS Lyon Lab Phys Lyon France Univ Grenoble CNRS UJF G INP Lab Ecoulements Geophys & Ind UMR5519 F-38041 Grenoble 09 France CNRS Ecole Normale Super Lyon Lab Phys F-69364 Lyon 07 France Univ Lyon F-69364 Lyon 07 France

The dynamics of towed objects in a fluid environment is of interest for many practical situations. In this paper we report results for wake instabilities of spheres towed in a water tank. Six particles of different diameter and/or density ratio have been investigated, towed at 5 different constant velocities. The explored density ratios lay within Gamma is an element of [1.06;2.56], with particle Reynolds numbers Re-p is an element of [100;1200], corresponding to Galileo numbers in the range Ga is an element of [1300;8000]. We introduce a surrogate Galileo number Ga* that, by taking into account the towing force applied to the particle, allows a comparison with the case of free falling/ascending spheres. Using innovative 3D tracking techniques, the three-dimensional trajectory of each particle is reconstructed. The wake instability for the studied particles is found to be associated to a 3D helicoidal motion with an elliptical cross section in the plane perpendicular to the towing direction. The 3D oscillatory motion was found independent of the particle density ratio, with a threshold of the order of Re-p(c) similar to 355 (or Ga*(c) similar to 245). This threshold is slightly larger than the one found for the free falling particles' transition to 3D chaotic motions (Re-p(c) similar to 310 or Ga-c similar to 225). (c) 2015 Elsevier Ltd. All rights reserved.

关键词： Towed spheres Wake instabilities 3D tracking

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Experimental detection of superclusters of water droplets in homogeneous isotropic turbulence

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EPL 2015年第5期112卷 54004-54004页

作者： Obligado, Martin Cartellier, Alain Bourgoin, Mickael Univ London Imperial Coll Sci Technol & Med Dept Aeronaut Turbulence Mixing & Flow Control Grp London England Univ Grenoble UMR5519 CNRS UJF G INP Lab Ecoulements Geophys & Ind F-38041 Grenoble 09 France Ecole Normale Super Lyon CNRS Lab Phys F-69364 Lyon 07 France Univ Lyon F-69364 Lyon 07 France

We present a new technique to study preferential concentration of inertial particles in (active) grid generated turbulence. This method, based on the use of a Taylor hypothesis combined to high-speed imaging, allows to reconstruct unprecedentedly long fields of particles and therefore to analyse new large-scale features, not easily resolvable with standard high-speed imaging methods. We first show that the new approach robustly reproduces results on particles clustering previously reported from standard methods. We then extend the analysis to show the first evidence of superclustering (existence of clusters of clusters) of inertial particles in turbulence and present the first characterization of superclusters. Copyright (C) EPLA, 2015

关键词： Turbulent flows Multiphase and stratified flows Instrumentation and measurement methods in fluid dynamics

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Preferential concentration of heavy particles in turbulence

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JOURNAL OF turbulence 2014年第5期15卷 293-310页

作者： Obligado, Martin Teitelbaum, Tomas Cartellier, Alain Mininni, Pablo Bourgoin, Mickael Univ London Imperial Coll Sci Technol & Med Dept Aeronaut Turbulence Mixing & Flow Control Grp London England Univ Buenos Aires Fac Ciencias Exactas & Nat Dept Fis RA-1428 Buenos Aires DF Argentina Consejo Nacl Invest Cient & Tecn IFIBA RA-1033 Buenos Aires DF Argentina Univ Grenoble Lab Ecoulements Geophys & Ind CNRS UJF G INP UMR 5519 Grenoble 09 France

Particle-laden flows are of relevant interest in many industrial and natural systems. When the carrier flow is turbulent, a striking feature is the phenomenon called preferential concentration: particles denser than the fluid have the tendency to inhomogeneously distribute in space, forming clusters and depleted regions. We present an investigation of clustering of small water droplets in homogeneous and isotropic active-grid-generated turbulence. We investigate the effect of Reynolds number (R-lambda) and Stokes number (St) on particles clustering in the range R-lambda similar to 200-400 and St similar to 2-10. Using Voronoi diagrams, we characterise clustering level and cluster properties (geometry, typical dimension and fractality). The exact same Voronoi analysis is then applied to investigate clustering properties of specific topological points of the velocity field of homogeneous isotropic turbulence obtained from direct numerical simulations at R-lambda similar to 220 and 300. The goal is to compare clustering properties of actual particles with those of such points in order to explore the relevance of possible clustering mechanisms, including centrifugal effects (heavy particles sampling preferentially low-vorticity regions) and sweep-stick mechanisms (heavy particles preferentially sticking to low-acceleration points). Our study points towards a leading role of zero-acceleration points and sweep-stick effects, at least for the experimental conditions considered in this study.

关键词： turbulent mixing turbulent multi-phase flows

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A numerical strategy to combine high-order schemes, complex geometry and parallel computing for high resolution DNS of fractal generated turbulence

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COMPUTERS & FLUIDS 2010年第3期39卷 471-484页

作者： Laizet, S. Lamballais, E. Vassilicos, J. C. Univ London Imperial Coll Sci Technol & Med Dept Aeronaut Turbulence Mixing & Flow Control Grp London SW7 2AZ England Univ London Imperial Coll Sci Technol & Med Inst Math Sci London SW7 2AZ England Univ Poitiers Lab Etud Aerodynam UMR 6609 ENSMACNRS F-86962 Futuroscope France

Impressive advances in parallel platform architectures over the past decade have made Direct Numerical Simulation (DNS) a powerful tool which can provide full access to the spatial structure of turbulent flows with complex geometries. An innovative approach which combines high-order schemes and a dual domain decomposition method is presented in this paper and is applied to DNS of multiscale-generated turbulent flows by a fractal grid. These DNS illustrate the applicability of our approach to the simulation of complex turbulent flows and provide results which are compared with recent laboratory experiments thus providing new insights for the interpretation of the experimental measurements. (C) 2009 Elsevier Ltd. All rights reserved.

关键词： NUMERICAL analysis PARALLEL algorithms FRACTALS turbulence SIMULATION methods & models DECOMPOSITION (Mathematics)

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High-order compact schemes for incompressible flows: A simple and efficient method with quasi-spectral accuracy

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JOURNAL OF COMPUTATIONAL PHYSICS 2009年第16期228卷 5989-6015页

作者： Laizet, Sylvain Lamballais, Eric Univ Poitiers Lab Etud Aerodynam UMR 6609 ENSMA F-86962 Futuroscope France Univ London Imperial Coll Sci Technol & Med Turbulence Mixing & Flow Control Grp Dept Aeronaut London SW7 2AZ England Univ London Imperial Coll Sci Technol & Med Inst Math Sci London SW7 2AZ England

In this paper, a finite difference code for Direct and Large Eddy Simulation (DNS/LES) of incompressible flows is presented. This code is an intermediate tool between fully spectral Navier-Stokes solvers (limited to academic geometry through Fourier or Chebyshev representation) and more versatile codes based on standard numerical schemes (typically only second-order accurate). The interest of high-order schemes is discussed in terms of implementation easiness, computational efficiency and accuracy improvement considered through simplified benchmark problems and practical calculations. The equivalence rules between operations in physical and spectral spaces are efficiently used to solve the Poisson equation introduced by the projection method. It is shown that for the pressure treatment, an accurate Fourier representation can be used for more flexible boundary conditions than periodicity or free-slip. Using the concept of the modified wave number, the incompressibility can be enforced up to the machine accuracy. The benefit offered by this alternative method is found to be very satisfactory, even when a formal second-order error is introduced locally by boundary conditions that are neither periodic nor symmetric. The usefulness of high-order schemes combined with an immersed boundary method (IBM) is also demonstrated despite the second-order accuracy introduced by this wall modelling strategy. In particular, the interest of a partially staggered mesh is exhibited in this specific context. Three-dimensional calculations of transitional and turbulent channel flows emphasize the ability of present high-order schemes to reduce the computational cost for a given accuracy. The main conclusion of this paper is that finite difference schemes with quasi-spectral accuracy can be very efficient for DNS/LES of incompressible flows, while allowing flexibility for the boundary conditions and easiness in the code development. Therefore, this compromise fits particularly well for ve

关键词： Direct numerical simulation Compact schemes Spectral methods Incompressible flows Immersed boundary method

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Comparison of thermo-hydraulic performances of large scale vortex flow (LSVF) and small scale vortex flow (SSVF) mixing vanes in 17x17 nuclear rod bundle

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NUCLEAR ENGINEERING AND DESIGN 2007年第24期237卷 2322-2331页

作者： Lee, C. M. Choi, Y. D. Korea Univ Dept Mech Engn Seoul 136701 South Korea Korea Univ Grad Sch Turbulence & Thermal Flow Control Lab Seoul 136701 South Korea

Spacer grids in the nuclear fuel rod assembly maintain a constant distance between rods, secure flow passage and prevent the damage of the rod bundle from flow-induced vibration. The mixing vanes attached to the spacer grids generate vortex flows in the subchannels and enhance the heat transfer performance of the rod bundle. Various types of mixing vanes have been developed to produce cross flows between subchannels as well as vortex flows in the subchannels. The shapes of the mixing vane have been improved to generate larger turbulence and cross flow mixing. In the present study, two types of large scale vortex flow (LSVF) mixing vanes and two types of small scale vortex flow (SSVF) mixing vanes are examined. SSVF-single is conventional split type and SSVF-couple is split type with different arraying method. LSVF mixing vane has different geometry and arraying method to make large scale vortex. 17 x 17 rod bundle with eight spans of mixing vanes is simulated using the IBM 690 supercomputer. The FLUENT code and IBM supercomputer is employed to calculate the flow field and heat transfer in the subchannels. turbulence intensities, maximum surface temperatures of the rod bundle, heat transfer coefficients and pressure drops of the four kinds of mixing vanes are compared. LSVF mixing vanes produced higher turbulence intensity and heat transfer coefficient than SSVF mixing vanes. Consequently, LSVF mixing vane increases the thermal efficiency and safety of the rod bundle. (C) 2007 Elsevier B.V. All rights reserved.

关键词： Vortex flow Blending Mixing heat transfer property Blades FUEL ELEMENT CLUSTERS subchannel turbulence intensity

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Effects of large density variation on strongly heated internal air flows

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PHYSICS OF FLUIDS 2006年第7期18卷 75102-1-75102-页

作者： Bae, Joong Hun Yoo, Jung Yul Choi, Haecheon McEligot, Donald M. Seoul Natl Univ Sch Mech & Aerosp Engn Seoul 151744 South Korea Seoul Natl Univ Inst Adv Machinery & Design Natl CRI Ctr Turbulence & Flow Control REs Seoul 151744 South Korea Idaho Natl Engn Lab Idaho Falls ID 83415 USA Univ Arizona Tucson AZ 85721 USA Univ Stuttgart IKE D-70550 Stuttgart Germany

Direct numerical simulation (DNS) is employed to examine the effects of large density variation of strongly heated air flowing in a vertical pipe on turbulent heat and momentum transfer. The predictions of the heat transfer and skin friction coefficients as well as the mean velocity and temperature profiles are in excellent agreement with the existing experimental data. Like some previous studies on heated air flows, the present study shows that the flow is laminarized with heat flux because a large reduction occurs in turbulence intensity. Unlike the velocity fluctuations, however, the thermal turbulence intensities such as the normalized density and temperature fluctuations remain as relatively insensitive to the heating conditions. Moreover, it is observed that the mean velocity and temperature profiles become dissimilar to each other during the process of laminarization with heating. In the present study we elucidate these anomalous behaviors of the strongly heated air flows with a notion of similarity breakdown between the mean velocity and temperature profiles, which occurs due to large density variations, and by examining the production rates of the turbulence kinetic energy and thermal turbulence fluctuation variance.

关键词： DIRECT NUMERICAL-SIMULATION FORCED GAS-flowS TURBULENT-BOUNDARY-LAYERS LARGE-EDDY SIMULATION CHANNEL flow CIRCULAR TUBES WALL TEMPERATURE PIPE

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Observation of yeast cell movement and aggregation in a small-scale MHz-ultrasonic standing wave field

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BIOSEPARATION 2000年第6期9卷 329-341页

作者： Spengler, JF Jekel, M Christensen, KT Adrian, RJ Hawkes, JJ Coakley, WT Cardiff Univ Sch Biosci Cardiff CF10 3TL S Glam Wales Tech Univ Berlin Dept Water Qual Control KF 4 D-10623 Berlin Germany Univ Illinois Lab Turbulence & Complex Flow Talbot Lab 216 Urbana IL 61801 USA

Aggregation of suspended yeast cells in a small-scale ultrasonic standing wave field has been monitored and quantified. The aggregation effect is based on the acoustic radiation force, which concentrates the cells in clumps. The ultrasonic chamber employed (1.9 MHz, one wavelength pathlength) had a sonication volume of 60 mul. The aggregation process was observed from above the transducer through a transparent glass reflector. A distinct, reproducible, pattern of clumps formed rapidly in the sound field. The sound pressure was estimated experimentally to be of the order of 1 MPa. Microscopic observations of the formation of a single clump were recorded onto a PC. The time dependent movement patterns and travelling velocities of the cells during the aggregation process were extracted by particle image velocimetry analysis. A time dependent change was seen in the particle motion pattern during approach to its completion of clump formation after 45 s. Streaming eddies were set-up during the first couple of seconds. The scale of the eddies was consistent with Rayleigh micro-streaming theory. An increase in the travelling velocity of the cells was observed after 30 s from initially about 400 mum s(-1) to about 1 mm s(-1). The influence of a number of mechanisms on particle behaviour (e.g. micro-streaming, particle interactions and convective flow) is considered. The experimental set-up introduced here is a powerful tool for aggregation studies in ultrasonic standing waves and lays the foundation for future quantitative experiments on the individual contributions of the different mechanisms.

关键词： aggregation micro-streaming standing wave radiation force ultrasound

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