Research Highlight Archive
Modal and transient dynamics of jet flows
X. Garnaud, L. Lesshafft, P. J. Schmid, and P. Huerre
The linear stability dynamics of incompressible and compressible isothermal jets are investigated by means of their optimal initial perturbations and of their temporal eigenmodes. The transient growth analysis of optimal perturbations is robust and allows physical interpretation of the salient instability mechanisms. In contrast, the modal representation appears to be inadequate, as neither the computed eigenvalue spectrum nor the eigenmode shapes allow a characterization of the flow dynamics in these settings.
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Geostrophic turbulence near rapid changes in stratification
K. S. Smith and E. Bernard
Geostrophic turbulence near horizontal surfaces on which the vertical velocity vanishes exhibits a forward cascade of buoyancy variance, characterized by a shallow energy spectrum, secondary roll-up of filaments, and a fat-tailed vorticity probability distribution. Such surfaces occur at rigid boundaries, but also at discontinuous jumps in stratification. Here we relax this mathematical idealization and investigate geostrophic turbulence near a rapid but smooth jump in stratification, modeled by N(z) = N0[1 + αtanh (z/h)]. (Click on the link to read more)
Numerical simulation of the dynamics of freely falling discs
Marcin Chrust, Gilles Bouchet, and Jan Dušek
We present a comprehensive parametric study of the transition scenario of freely falling discs. The motion of the discs is investigated by a direct numerical simulation of the solid-fluid interaction. The discs are assumed to be homogeneous and infinitely thin. The problem is shown to depend on two independent parameters, the Galileo number expressing the ratio between effects of gravity and viscosity and the non-dimensionalized mass characterizing the inertia of the disc. (Click on the link to read more)
Effect of a finite external heat transfer coefficient on the Darcy-Bénard instability in a vertical porous cylinder
A. Barletta and L. Storesletten
The onset of thermal convection in a vertical porous cylinder is studied by considering the heating from below and the cooling from above as caused by external forced convection processes. These processes are parametrised through a finite Biot number, and hence through third-kind, or Robin, temperature conditions imposed on the lower and upper boundaries of the cylinder. (Click on the link to read more)
Inertial particle trapping in viscous streaming
Kwitae Chong, Scott D. Kelly, Stuart Smith, and Jeff D. Eldredge
The motion of an inertial particle in a viscous streaming flow of Reynolds number order 10 is investigated theoretically and numerically. The streaming flow created by a circular cylinder undergoing rectilinear oscillation with small amplitude is obtained by asymptotic expansion from previous work, and the resulting velocity field is used to integrate the Maxey–Riley equation with the Saffman lift for the motion of an inertial spherical particle immersed in this flow. It is found that inertial particles spiral inward and become trapped inside one of the four streaming cells established by the cylinder oscillation, regardless of the particle size, density and flow Reynolds number. (Click on the link to read more)
Reshaping and capturing Leidenfrost drops with a magnet
Keyvan Piroird, Baptiste Darbois Texier, Christophe Clanet, and David Quéré
Liquid oxygen, which is paramagnetic, also undergoes Leidenfrost effect at room temperature. In this article, we first study the deformation of oxygen drops in a magnetic field and show that it can be described via an effective capillary length, which includes the magnetic force. In a second part, we describe how these ultra-mobile drops passing above a magnet significantly slow down and can even be trapped. The critical velocity below which a drop is captured is determined from the deformation induced by the field. (Click on the link to read more)
Convectons and secondary snaking in three-dimensional natural doubly diffusive convection
Cédric Beaume, Alain Bergeon, and Edgar Knobloch
Natural doubly diffusive convection in a three-dimensional vertical enclosure with square cross-section in the horizontal is studied. Convection is driven by imposed temperature and concentration differences between two opposite vertical walls. These are chosen such that a pure conduction state exists. No-flux boundary conditions are imposed on the remaining four walls, with no-slip boundary conditions on all six walls. Numerical continuation is used to compute branches of spatially localized convection. Such states are referred to as convectons. (Click on the link to read more)
Transient swelling, spreading, and drug delivery by a dissolved anti-HIV microbicide-bearing film
Savas Tasoglu, Lisa C. Rohan, David F. Katz, and Andrew J. Szeri
There is a widespread agreement that more effective drug delivery vehicles with more alternatives, as well as better active pharmaceutical ingredients (APIs), must be developed to improve the efficacy of microbicide products. For instance, in tropical regions, films are more appropriate than gels due to better stability of drugs at extremes of moisture and temperature. Here, we apply fundamental fluid mechanical and physicochemical transport theory to help better understand how successful microbicide API delivery depends upon properties of a film and the human reproductive tract environment. (Click on the link to read more)
Phys. Fluids 25, 031901 (2013)
Experimental manipulation of wall turbulence: A systems approach
B. J. McKeon, A. S. Sharma, and I. Jacobi
We review recent progress, based on the approach introduced by McKeon and Sharma [J. Fluid Mech. 658, 336–382 (2010)], in understanding and controlling wall turbulence. The origins of this analysis partly lie in nonlinear robust control theory, but a differentiating feature is the connection with, and prediction of, state-of-the-art understanding of velocity statistics and coherent structures observed in real, high Reynolds number flows. (Click on the link to read more)
Reynolds-number scaling of turbulent channel flow
M. P. Schultz and K. A. Flack
Results of an experimental study of smooth-wall, fully developed, turbulent channel flow are presented. The Reynolds number (Rem) based on the channel height and the bulk mean velocity ranged from 10 000 to 300 000. (Click on the link to read more)
Nanodrop impact on solid surfaces
Joel Koplik and Rui Zhang
The impact of nanometer sized drops on solid surfaces is studied using molecular dynamics simulations. Equilibrated floating drops consisting of short chains of Lennard-Jones liquids with adjustable volatility are directed normally onto an atomistic solid surface where they are observed to bounce, stick, splash, or disintegrate, depending on the initial velocity and the nature of the materials involved.(Click on the link to read more)
Influences of boundary layer scale separation on the vorticity transport contribution to turbulent inertia
C. Morrill-Winter and J. Klewicki
In the flows of interest, the mean effect of turbulent inertia can be expressed as the difference of two velocity vorticity correlations. This difference must be sufficiently non-zero if turbulent inertia is to have a net influence on the mean dynamics. One of the correlations is physically related to change of scale effects, while the other is related to advective vorticity transport. The vorticity transport mechanism is studied under the influence of increasing scale separation.(Click on the link to read more)
Falling plumes of point particles in viscous fluid
Andrew Crosby and John R. Lister
In this paper, particle-driven Stokes flows are considered. In particular, the behavior of an infinite periodic vertical cylinder of particles falling under gravity is investigated both numerically and analytically, in order to explain the mechanism behind the formation of radial bulges on particles plumes. Numerical simulations show a behavior in the bulges formation qualitatively similar to that previously observed for a finite starting plume, illustrating that the growth of the bulges is independent from the plume head or the plume source.
Flipping, scooping, and spinning: Drift of rigid curved nonchiral fibers in simple shear flow
Jianghui Wang, Emilio J. Tozzi, Michael D. Graham, and Daniel J. Klingenberg
The dynamics of slightly curved, nonaxisymmetric fibers in shear flow has been investigated by computer simulations. Results show that the combined effects of flipping, scooping and spinning motion of the fiber contribute to the drift, while the drift rate depends on initial orientation of the fiber, fiber aspect ratio, and curvature.
Biomimetic flow control based on morphological features of living creatures
Haecheon Choi, Hyungmin Park, Woong Sagong, and Sang-im Lee
Recent successful results on biomimetic flow control are presented. In particular, biological morphology is considered with the purpose of enhancing the aerodynamic performance (lift-to-drag ratio) or reducing the drag. The differences between operating environments for living organisms and operating conditions of engineered devices are also discussed, and guidelines are suggested for effective integration of biology and fluid mechanics into biomimetic engineering.
The 2012 François Naftali Frenkiel Award for Fluid Mechanics
Folded micro-threads: Role of viscosity and interfacial tension
Thomas Cubaud, Bibin M. Jose, and Samira Darvishi
The microflow behavior of high-viscosity fluid threads is examined using both miscible and immiscible fluid pairs in confined geometries. The influence of fluid properties on buckling deformations is experimentally characterized and the phenomenon of “capillary unfolding” where a corrugated thread straightens along the flow direction is demonstrated.
Capillary-driven flow induced by a stepped perturbation atop a viscous film
Thomas Salez, Joshua D. McGraw, Oliver Bäumchen, Kari Dalnoki-Veress, and Elie Raphaël
An analytical solution of the linear thin film equation is reported for a stepped initial condition atop a flat film. The solution, obtained with Fourier analysis, was characterized and the long-term viscous dissipation law was also obtained. The solution was also compared with experimental profiles of a model system (a polystyrene stepped film above the glass transition temperature ), and excellent agreement was obtained, suggesting that such a solution is a suitable tool in the physics of capillary-driven thin viscous films.
Nonlinear dynamo in a short Taylor-Couette setup
C. Nore, J.-L. Guermond, R. Laguerre, J. Léorat, and F. Luddens
A model in the Taylor-Couette geometry has been analyzed in this paper . The model was implemented with a body force sharing the same fundamental symmetry properties as the geodynamo, making it a candidate for fluid dynamos experimental setups. The critical magnetic Reynolds number for the setup was found in the same range of what was previously obtained in kinematic studies of spherical containers and Taylor-Couette analysis with pure viscous driving. Moreover a simulation in the established non-linear regime indicated the excitation of an axial asymmetric component exhibiting aperiodic reversals.
Special Topic: The 14th biennial Center for Turbulence Research Summer Program.
The 14th Summer Program of the Center for Turbulence Research (CTR) was held from June 25 to July 20, 2012 at Stanford University. The program assembled 91 participants from 15 countries, and focused on the investigation of the fundamental physics of turbulent and transitional flows with the support of existing numerical databases, experimental data and large-scale simulation. A brief account of all the projects conducted during the Summer Program is presented in the paper, accompanied by selected highlights.
A low-dimensional deformation model for cancer cells in flow
A. M. Lee, M. A. Berny-Lang, S. Liao, E. Kanso, P. Kuhn, O. J. T. McCarty, and P. K. Newton
In order to gain insight into the metastatic spread of circulating tumor cells, a new cell deformation model is developed in which the cell surface is allowed to deform parametrically as a family of Gaussian surfaces. The active shape model (ASM) does a good job of capturing the principle deformations of a cell under realistic flow conditions when the parameters are chosen to match the experiment.
Free vibrations of spherical drop constrained at an azimuth
Santhosh Ramalingam, Doraiswami Ramkrishna, and Osman A. Basaran
The subject of this paper is the infinitesimal-amplitude, axisymmetric oscillations of a drop of an inviscid fluid that is constrained by a ring of infinitesimal thickness at a latitude and surrounded by another inviscid fluid. The integro-differential eigenvalue problem governing the linear oscillations of a spherical drop that is constrained at an azimuth is analyzed. Two analytical solution methodologies are used to determine the eigenvalues and eigenfunctions of this linear operator problem.
A Lagrangian subgrid-scale model with dynamic estimation of Lagrangian time scale for large eddy simulation of complex flows
Aman Verma and Krishnan Mahesh
A dynamic Lagrangian averaging approach is developed for the dynamic Smagorinsky model for large eddy simulation of complex flow on unstructured grids. The present work shows that a “surrogate-correlation” of the Germano-identity error (GIE) based time scale is a more apt choice for Lagrangian averaging. The model is applied to flow over a cylinder at two Reynolds numbers and results show good agreement with previous computations.
A model for the human tear film with heating from within the eye
Longfei Li and R.J. Braun
The following study focuses on the fluid dynamics of tear film during the interblink and how it is affected by heat transfer beneath the tear films. The introduction of heat diffusion into the model improves existing models on predicting the thermal dynamics of the tear film. Three different domains for heat transfer are examined: no substrate, thin substrate, or thick substrate in order to identify which case correlates best with experimentally determined temperature drops on the ocular surface.
The effects of flagellar hook compliance on motility of monotrichous bacteria: A modeling study
H. Shum and E.A. Gaffney
The motility of flagellated bacteria is examined through a new model that explores the function of the hook during swimming. Elastic hook behavior is incorporated into the mathematical model due to the fact that flagellar filaments are must stiffer than hooks making most of the bending observed associated with the hook. Biological implications of the findings are also discussed.
Resuspension onset and crater erosion by a vortex ring interacting with a particle layer
N. Bethke and S. B. Dalziel
Particle image velocimetry is used to investigate the flow dynamics during the onset of particle resuspension. The final eroded crater shape is also analysed for a range of impact velocities and different particle sizes. Experimental results suggest that the particle layer permeability influences the induced bed velocity. The results also suggest an effect of bed permeability on the interaction of the vortex ring with the particle layer.
Acoustically enhanced boiling heat transfer
Zachary Douglas, Thomas R. Boziuk, Marc K. Smith, and Ari Glezer
In the following study, an acoustic driver is used to create an acoustic field in a liquid that induces volume oscillations and surface waves in the vapor bubbles. The Bjerknes forces created on the bubble helps to detach and move the bubble away from a heated surface. Three experimental setups are discussed in order to explore the mechanisms associated with these interactions.
Flow mediated interactions between two cylinders at finite Re numbers
Mattia Gazzola, Chloe Mimeau, Andrew A. Tchieu, Petros Koumoutsakos
This investigation examines the dramatic differences in the behavior of two interacting bodies at moderate and finite Reynolds numbers when immersed in a two-dimensional incompressible, viscous flow. The simulations are performed using a wavelet adapted remeshed vortex method combined with Brinkman penalization and projection approach. Results presented indicate that in certain simulations the finite size of the slave cylinders enhances the transport of the cylinders when only taking into account interactions that are purely flow mediated.
A numerical investigation of the fluid mechanical sewing machine
P.-T. Brun, N.M. Ribe, B. Audoly
A new numerical algorithm is used to simulate the variety of patterns generated by a thin thread of viscous fluid falling on a surface. The discrete viscous thread (DVT) method proves successful in simulating a range of interesting configurations when compared to experimental results previously seen in the laboratory. The simulations of the viscous sewing machine reveals unique patterns that were characterized using a new identification method that computes the Fourier spectra of the longitudinal and transverse components of the motion of the thread’s contact point with the belt.
Linear oscillations of constrained drops, bubbles, and plane liquid surfaces
This paper demonstrates a method of analysis for shape oscillations in which a constraint prevents use of the standard approaches. The new method of analysis is demonstrated on 5 systems including a constrained drop, gravity-capillary waves on a plane liquid surface, and a simple instance of the Rayleigh-Taylor instability.
Cross-waves induced by the vertical oscillation of a fully immersed vertical plate
Frédéric Moisy, Guy-Jean Michon, Marc Rabaud, and Eric Sultan
The authors investigate the parametric instability leading to the generation of cross-waves excited by a fully immersed wavemaker. Their results suggest that the resonance responsible for the growth of the cross-wave may be simply described by a three-wave interaction mechanism, in which the oscillating flow above the wavemaker is modeled by a pseudo-third wavevector.
Rheological measurements of large particles in high shear rate flows
Erin Koos, Esperanza Linares-Guerrero, Melany L. Hunt, and Christopher E. Brennen
The authors explore the rheology of an inertial suspension under conditions in which collisions may become important. The authors use a Couette concentric cylinder rheometer designed to reduce the effect of Taylor vortices to measure the torque and shear stress on mixtures of neutrally and slightly non-neutrally buoyant particles in a Newtonian fluid. The results are presented for Reynolds numbers between 20 and 800 and Stokes numbers from 3 to 90. The presentation also includes measurements for both smooth and rough walls and examines the effect of wall slip in determining the relative viscosities of the suspensions.
Developed quantum turbulence and its decay
L. Skrbek and K. R. Sreenivasan
The authors attempt to review the correspondence between classical and quantum turbulence with particular attention to the conceptually simplest case of zero temperature limit where quantum turbulence consists of a tangle of quantized vortex line and represents a simple prototype of turbulence. At finite temperature, the authors focus at the level of two-fluid description of the superfluid state - consisting of a normal viscous fluid and a frictionless superfluid - and review much of the available knowledge on quantum turbulence in liquid helium (both He II and 3He-B). The authors consider counterflows in which the normal and superfluid components flow against each other, as well as co-flows in which the direction of the two fluids is the same.
Dispersion of ferrofluid aggregates in steady flows
Alicia M. Williams and Pavlos P. Vlachos
The authors present a study of ferrofluid aggregates interacting with a magnetically non-susceptible fluid in the presence of two different applied magnetic fields. Focused shadowgraphs of the interaction reveals three regimes of aggregate dynamics over the span of Reynolds numbers studied: stable, transitional, and shedding.
The relationship between the velocity skewness and the amplitude modulation of the small scale by the large scale in turbulent boundary layers
Romain Mathis, Ivan Marusic, Nicholas Hutchins, and K. R. Sreenivasan
In this study, the authors assess this apparent relationship and show that the Reynolds number trend in the skewness profile of u is strongly related to the amplitude modulation effect of the small scales by the large. This observation also leads to an alternative diagnostic for the amplitude modulation effect, which is one component of the skewness factor based on a scale decomposition.
Orientational order in concentrated suspensions of spherical microswimmers
Arthur A. Evans, Takuji Ishikawa, Takami Yamaguchi, and Eric Lauga
The authors use numerical simulations to probe the dynamics of concentrated suspensions of spherical microswimmers interacting hydrodynamically. Unlike previous work, it is shown that isotropic suspensions of spherical swimmers are also always unstable.
Rheology of binary granular mixtures in the dense flow regime
Anurag Tripathi and D. V. Khakhar
Results are presented for a single component system and binary mixtures with particles of different size and density. Inclination angles, composition, size ratios and density ratios are varied to obtain different segregated configurations at equilibrium.
Maximum speed of dewetting on a fiber
Tak Shing Chan, Thomas Gueudré, and Jacco H. Snoeijer
A solid object can be coated by a nonwetting liquid since a receding contact line cannot exceed a critical speed. The authors theoretically investigate this forced wetting transition for axisymmetric menisci on fibers of varying radii.
Spatiotemporal persistence of spectral fluxes in two-dimensional weak turbulence
Douglas H. Kelley and Nicholas T. Ouellette
Using a recently developed filtering technique, the authors study the spatiotemporal properties of the scale-to-scale fluxes of energy and enstrophy in a weakly turbulent experimental quasi-two-dimensional flow.
Shock-wave solutions in two-layer channel flow. II. Linear and nonlinear stability
A. Mavromoustaki, O. K. Matar, and R. V. Craster
The results of this analysis reveal that increasing the density and/or the viscosity of the upper layer, and/or increasing the counter-current nature of the flow configuration exerts a stabilising influence. These results are used to guide our transient numerical simulations aimed at studying the nonlinear development of fingering phenomena.
Modal versus nonmodal linear stability analysis of river dunes
C. Camporeale and L. Ridolfi
When a free surface turbulent shear flow interacts with a deformable cohesionless sediment bottom, dune patterns can arise and perform a complex time evolution. These bed forms are very widespread in fluvial environments and have catalyzed an intense research activity of both an applicative and theoretical nature.
On the probability distribution function of the velocity field and its derivative in multi-scale turbulence
Garrett H. Good and Zellman Warhaft
The work herein is motivated by the complex wind fields, and associated, intermittent high stresses, encountered by wind turbines. The authors also draw comparisons to recent studies of multi-scale turbulence produced by fractal grids.
Stretching liquid bridges with moving contact lines: The role of inertia
Shawn Dodds, Marcio Carvalho, and Satish Kumar
Liquid bridges with moving contact lines are found in a variety of settings such as capillary feeders and high-speed printing. Although it is often assumed that the length scale for these flows is small enough that inertial effects can be neglected, this is not the case in certain applications. To address this issue, we solve the Navier-Stokes equations with the finite element method for the stretching of a liquid drop between two surfaces for non-zero Reynolds numbers.
The influence of walls on Lagrangian statistics in two-dimensional turbulence
B. Kadoch, W. J. T. Bos, and K. Schneider
It is shown that the influence of walls is not confined to a small near-wall region but alters the statistics in the entire flow domain. This can be explained by the vorticity generation in the turbulent boundary layer which destabilizes and leads to the formation of vortices that subsequently detach and travel into the bulk flow. The enstrophy level is thus increased with respect to the one in the unbounded periodic domain.
Particle accumulation on periodic orbits by repeated free surface collisions
Ernst Hofmann and Hendrik C. Kuhlmann
The motion of small particles suspended in cylindrical thermocapillary liquid bridges is investigated numerically in order to explain the experimentally observed particle accumulation structures in steady two- and time-dependent three-dimensional flows.
The effect of surface shear viscosity on the damping of oscillations in millimetric liquid bridges
Miguel A. Herrada, José M. Montanero, and José M. Vega
The authors provide a theoretical framework for some experimental measurements of the damping ratio in millimetric liquid bridges using hexadecane. Comparison with current theories showed that they predict well the natural frequencies of the free oscillations, but underestimate the damping ratio by a factor of about 0.6.
Model reduction for fluids using frequential snapshots
G. Dergham, D. Sipp, J.-C. Robinet, and A. Barbagallo
The authors describe how the use of frequential responses of a flow to a given actuator enables to compute the basis of the most controllable modes (POD modes). Analogously, the harmonic flow states yielding the maximum contribution to the sensor energy have been introduced to compute the most and equally controllable and observable modes: the balanced modes (BPOD modes).
Two-dimensional streaming flows in high-intensity discharge lamps
Thomas D. Dreeben and Gregory P. Chini
The authors examine streaming flows in high-intensity discharge (HID) lamps, in which acoustic resonance has been known for some years to impact lamp behavior. With the help of computer simulations and experimental evidence, the authors identify which aspects of classical theory do and do not apply, and offer a modified view that highlights what is different about streaming in these lamps.
Multiple scaling in the ultimate regime of thermal convection
Siegfried Grossmann and Detlef Lohse
The authors present a theory for the experimentally observed effective multiple scaling of Nu with Ra in the beginning of the ultimate Ra regime Ra≳1014 and made predictions for the corresponding effective Re-scaling.
Instability regimes in flowing suspensions of swimming micro-organisms
Amir Alizadeh Pahlavan and David Saintillan
The work investigates the effects of an externally imposed simple shear flow on the instabilities, dynamics, and pattern formation that are known to arise in suspensions of micro-organisms. This study, and previous ones, have found that suspensions of self-propelled particles should exhibit decreased viscosities in the case of pushers, but increased viscosities for pullers, which is in qualitative agreement with experiments.
Observation and regime classification of pulsation patterns in expanding spherical flames
G. Jomaas and C. K. Law
The development of pulsating instability in expanding spherical premixed flames was experimentally studied, leading to the observation and quantification of spiral waves and target patterns over the flame surfaces in rich hydrogen-air, rich hydrogen-oxygen, and lean butane-oxygen-helium mixtures at elevated pressures.
Shear-flow excitation mechanisms of recessed localized arc-filament plasma actuators
R.R. Kleinman, D.J. Bodony, and J.B. Freund
A localized arc-filament plasma actuator, which in this application is recessed in a small cavity near the nozzle lip, causes intense local heating. This heating is thought to be the root mechanism of its influence on the flow, but how this principally entropic thermal source couples with the vortical jet shear layer turbulence downstream is unclear. We investigate this using direct numerical simulations, the results of which have illuminated several features of the actuation.
Efficiency optimization and symmetry-breaking in a model of ciliary locomotion
Sébastien Michelin and Eric Lauga
Ciliates exploit the bending of a large number of small and densely packed organelles, termed cilia, in order to propel themselves in a viscous fluid. In this research, a complete optimization diagram for swimming efficiencies, swimming speeds, and amplitudes of surface deformation can be reached, with the mathematically optimal swimmer, of efficiency one-half, being a singular limit.
Jet propulsion without inertia
Saverio E. Spagnolie and Eric Lauga
This paper considers this mechanism of jet propulsion without inertia in the case of spheroidal bodies and derive both the swimming velocity and the hydrodynamic efficiency. Elementary examples are presented and exact axisymmetric solutions for spherical, prolate spheroidal, and oblate spheroidal body shapes are provided. In each case, entirely and partially porous (i.e., jetting) surfaces are considered and the optimal jetting flow profiles at the surface for maximizing the hydrodynamic efficiency are determined computationally.
Self-similar bending in a flow: The axisymmetric case
How sheets roll up into conical configurations when exposed to fluid flows is studied using simulations and analysis. The simulations couple the bending of thin sheets to axisymmetric flows with vortex shedding.
Transitional and turbulent boundary layer with heat transfer
Xiaohua Wu and Parviz Moin
A direct numerical simulation of an incompressible, nominally zero-pressure-gradient flat-plate boundary layer from momentum thickness Reynolds number 80–1950 is reported. Heat transfer between the constant-temperature solid surface and the free-stream is also simulated with molecular Prandtl number Pr = 1. Mean velocity and Reynolds stresses agree with experimental data over an extended turbulent region downstream of transition. In the transitional region, turbulent spots are tightly packed with hairpin vortices. With the advection and merging of the turbulent spots, these young isolated hairpin forests develop into the downstream turbulent region. Isosurfaces of temperature display strong and sharp signatures of hairpin vortices, indicating the persistence of hairpin forests up to Reθ = 1900.
Axial and lateral particle ordering in finite Reynolds number channel flows
Katherine J. Humphry, Pandurang M. Kulkarni, David A. Weitz, Jeffrey F. Morris, and Howard A. Stone
Inertial focusing in a pressure-driven flow refers to the positioning of particles transverse to the mean flow direction that occurs as a consequence of a finite particle Reynolds number. In channels with rectangular cross-sections, and for a range of channel aspect ratios and particle confinement, experimental results are presented to show that both the location and the number of focusing positions depend on the number of particles per unit length along the channel. This axial number density is a function of both the channel cross-section and the particle volume fraction. These results are rationalized using simulations of the particle-laden flow to show the manner in which hydrodynamic interactions set the preferred locations in these confined flows. A criterion is presented for the occurrence of a stepwise transition from one to two or more trains of particles.
High Rayleigh number convection with double diffusive fingers
An electrodeposition cell is used to sustain a destabilizing concentration difference of copper ions in aqueous solution between the top and bottom boundaries of the cell. The resulting convecting motion is analogous to Rayleigh–Bénard convection at high Prandtl numbers. In addition, a stabilizing temperature gradient is imposed across the cell. Even for thermal buoyancy two orders of magnitude smaller than chemical buoyancy, the presence of the weak stabilizing gradient has a profound effect on the convection pattern. Double diffusive fingers appear in all cases.
Wall-bounded turbulent flows at high Reynolds numbers: Recent advances and key issues
Salient advances of recent origin are distilled, particularly those that challenge textbook orthodoxy. Some of the outstanding questions, such as the extent of the logarithmic overlap layer, the universality or otherwise of the principal model parameters such as the von Kármán “constant,” the parametrization of roughness effects, and the scaling of mean flow and Reynolds stresses, are highlighted. Research avenues that may provide answers to these questions, notably the improvement of measuring techniques and the construction of new facilities, are identified. Aspects where differences of opinion persist are also highlighted, with the expectation that this discussion might mark the beginning of their resolution.
Flow, turbulence, and pollutant dispersion in urban atmospheres
The fluid dynamics of the urban atmospheric boundary layer and its prediction is the theme of this overview paper, where it is advocated that decision and policymaking in urban atmospheric management must be based on integrated models that incorporate cumulative effects of anthropogenic forcing, atmospheric dynamics, and social implications (e.g., health outcomes). An integrated modeling system juxtaposes a suite of submodels, each covering a particular range of scales while communicating with models of neighboring scales. Unresolved scales of these models need to be parametrized based on flow physics, for which developments in fluid dynamics play an indispensible role. Illustrations of how controlled laboratory, outdoor (field), and numerical experiments can be used to understand and parametrize urban atmospheric processes are presented, and the utility of predictive models is exemplified.
Effect of the prosthetic mitral valve on vortex dynamics and turbulence of the left ventricular flow
Comparison of the ventricular flow with mitral inflow generated by mechanical valves of different design is presented. The experiments and measurements were designed to capture the general features of the flow, not the details related to the particular valve model, with the aim of understanding the effects caused by the different overall valve structure. Analysis of the flow patterns indicates that modifications in the transmitral flow affect deeply the interaction between the coherent structures generated during the first filling phase and the inflow corresponding to the atrial contraction at the end of the diastole.
Investigating slippage, droplet breakup, and synthesizing microcapsules in microfluidic systems
Liquid slippage at solid walls, droplet breakup in microfluidic systems, and capsule generation in microfluidic devices are discussed. The analysis of the physical processes implied in these situations led to improve our knowledge on the importance of slippage phenomena in electroosmotic flows, the effect of the confinement in droplet breakup processes, and the effect of recirculating flows on the morphology of multiple droplets.
The art of mixing with an admixture of art: Fluids, solids, and visual imagination
The Otto Laporte Lecture provides a forum to cover some of this ground—an opportunity to offer observations about scientific imagination, the role of collaborators and environment, creative processes in general, and even how science evolves. Very little has been written about these topics in the context of fluid dynamics, but the author argues that such a viewpoint—understanding why and how scientific discoveries are made—provides significant insights which go far beyond my specific work.
Large eddy simulation study of fully developed wind-turbine array boundary layers
Marc Calaf, Charles Meneveau, and Johan Meyers
A suite of large eddy simulations (LES), in which wind turbines are modeled using the classical “drag disk” concept, is performed for various wind-turbine arrangements, turbine loading factors, and surface roughness values. The results are used to quantify the vertical transport of momentum and kinetic energy across the boundary layer. It is shown that the vertical fluxes of kinetic energy are of the same order of magnitude as the power extracted by the forces modeling the wind turbines.
Evolution and lifetimes of flow topology in a turbulent boundary layer
G. E. Elsinga and I. Marusic
The average rates of change in the invariants of the velocity gradient tensor (Q and R) have been determined experimentally in the outer layer of a turbulent boundary layer as a function of the invariants themselves. Subsequent integration yields trajectories in the QR plane describing the average evolution of the local flow topology following a fluid particle.
Direct numerical simulation of canonical shock/turbulence interaction
Johan Larsson and Sanjiva K. Lele
A set of direct numerical simulations of isotropic turbulence passing through a nominally normal shock wave is presented. It is shown that the Kolmogorov scale decreases during the shock interaction, which implies that the grid resolution needed to resolve the viscous dissipation is finer than that used inprevious studies. This leads to some qualitative differences with previous work, e.g., a rapid increase in the streamwise vorticity variance behind the shock and large anisotropy of the postshock Reynolds stresses.
Localized edge states in plane Couette flow
Yohann Duguet, Philipp Schlatter, and Dan S. Henningson
The dynamics at the threshold of transition in plane Couette flow is investigated numerically in a large spatial domain for a certain type of localized initial perturbation, for Re between 350 and 1000. The corresponding edge state is an unsteady spotlike structure, localized in both streamwise and spanwise directions, which neither grows nor decays in size. It is shown that the localized nature of the edge state is numerically robust, and is not influenced by the size of the computational domain. The edge trajectory appears to transiently visit localized steady states. This suggests that basic spatiotemporally intermittent features of transition to turbulence, such as the growth of a turbulent spot, can be described as a dynamical system.
Stability of relative equilibria of three vortices
Three point vortices on the unbounded plane have relative equilibria wherein the vortices either form an equilateral triangle or are collinear. While the stability analysis of the equilateral triangle configurations is straightforward, that of the collinear relative equilibria is considerably more involved.This paper gives analysis based on explicit formulas for the three eigenvalues determining the stability, including a new formula for the angular velocity of rotation of a collinear relative equilibrium.
Fluctuations and stratification in sedimentation of dilute suspensions of spheres
Daniel Chehata Gómez, Laurence Bergougnoux, Élisabeth Guazzelli, and John Hinch
Whether stratification can control velocity fluctuations in suspensions of sedimenting spheres is tested. The initial value and early decay of the velocity fluctuations are not affected by stratification. On the other hand, in the descending front where the stratification is strong and well defined, the velocity fluctuations are inhibited according to a previously proposed scaling. In between, after the initial decay and before the arrival of the front, the local value of the stratification does not always play a role.
Drag reduction in turbulent flows over superhydrophobic surfaces
Robert J. Daniello, Nicholas E. Waterhouse, and Jonathan P. Rothstein
This paper demonstrates that periodic, micropatterned superhydrophobic surfaces, previously noted for their ability to provide laminar flow drag reduction, are capable of reducing drag in the turbulent flow regime.
Stick-slip dynamics of an oscillated sessile drop
Irina S. Fayzrakhmanova and Arthur V. Straube
Theoretical consideration is given to dynamics of an oscillated sessile drop of incompressible liquid and focus on the contact line hysteresis. The frequency response of surface oscillations on the substrate and at the pole of the drop are analyzed. It is shown that novel features such as the emergence of antiresonant frequency bands and nontrivial competition of different resonances are caused by contact line hysteresis.
Poiseuille flow and thermal creep based on the Boltzmann equation with the Lennard-Jones potential over a wide range of the Knudsen number
The methodology to solve the linearized Boltzmann equation for an arbitrary potential of
intermolecular interaction described in our previous paper is used to calculate a rarefied gas flow between two parallel plates driven by pressure and temperature gradients over a wide range of the Knudsen number. As an example, the Lennard-Jones potential is applied. The calculations were carried out for all noble gases at the temperature equal to 300 K. A comparison with results for the same problem based on the kinetic model equations showed that the uncertainty of these equations has the same order that the Boltzmann equation based on the hard sphere particles.
Turbulent boundary layers up to Reθ =2500 studied through simulation and experiment
P. Schlatter,a_ R. Örlü, Q. Li, G. Brethouwer, J. H. M. Fransson, A. V. Johansson, P. H. Alfredsson, and D. S. Henningson
Direct numerical simulations (DNSs) and experiments of a spatially developing zero-pressure-gradient turbulent boundary layer are presented. Direct comparisons of DNS and experiments of turbulent boundary layers are given, showing excellent agreement in skin friction, mean velocity, and turbulent fluctuations. These results allow for a substantial reduction of the uncertainty of boundary-layer data, and cross validate the numerical setup and experimental technique.