Volume 26, Issue 10, October 2014
Index of content:
 AWARD AND INVITED PAPERS


Drag reduction and the dynamics of turbulence in simple and complex fluids^{a)}
View Description Hide DescriptionAddition of a small amount of very large polymer molecules or micelleforming surfactants to a liquid can dramatically reduce the energy dissipation it exhibits in the turbulent flow regime. This rheological drag reduction phenomenon is widely used, for example, in the Alaska pipeline, but it is not wellunderstood, and no comparable technology exists to reduce turbulent energy consumption in flows of gases, in which polymers or surfactants cannot be dissolved. The most striking feature of this phenomenon is the existence of a socalled maximum drag reduction (MDR) asymptote: for a given geometry and driving force, there is a maximum level of drag reduction that can be achieved through addition of polymers. Changing the concentration, molecular weight or even the chemical structure of the additives has little to no effect on this asymptotic value. This universality is the major puzzle of drag reduction. We describe direct numerical simulations of turbulent minimal channel flow of Newtonian fluids and viscoelastic polymer solutions. Even in the absence of polymers, we show that there are intervals of “hibernating” turbulence that display very low drag as well as many other features of the MDR asymptote observed in polymer solutions. As Weissenberg number increases to moderate values the frequency of these intervals also increases, and a simple theory captures key features of the intermittent dynamics observed in the simulations. At higher Weissenberg number, these intervals are altered – for example, their duration becomes substantially longer and the instantaneous Reynolds shear stress during them becomes very small. Additionally, simulations of “edge states,” dynamical trajectories that lie on the boundary between turbulent and laminar flow, display characteristics that are similar to those of hibernating turbulence and thus to the MDR asymptote, again even in the absence of polymer additives. Based on these observations, we propose a tentative unified description of rheological drag reduction. The existence of MDRlike intervals even in the absence of additives sheds light on the observed universality of MDR and may ultimately lead to new flow control approaches for improving energy efficiency in a wide range of processes.

Observations of largescale fluid transport by laserguided plankton aggregations^{a)}
View Description Hide DescriptionDiel vertical migration of plankton has been proposed to affect global ocean circulation to a degree comparable to winds and tides. This biomixing process has never been directly observed, however, due to the inability to predict its occurrence in situ or to reproduce it in a laboratory setting. Furthermore, it has been argued that the energy imparted to the ocean by plankton migrations occurs at the scale of individual organisms, which is too small to impact ocean mixing. We describe the development of a multilaser guidance system that leverages the phototactic abilities of plankton to achieve controllable vertical migrations concurrently with laser velocimetry of the surrounding flow. Measurements in unstratified fluid show that the hydrodynamic interactions between neighboring swimmers establish an alternate energy transfer route from the small scales of individually migrating plankton to significantly larger scales. Observations of laserinduced vertical migrations of Artemia salina reveal the appearance of a downward jet, which triggers a KelvinHelmholtz instability that results in the generation of eddylike structures with characteristic length scales much larger than the organisms. The measured energy spectrum is consistent with these findings and indicates energy input at large scales, despite the small individual size of the organisms. These results motivate the study of biomixing in the presence of stratification to assess the contribution of migrating zooplankton to local and global ocean dynamics. The laser control methodology developed here enables systematic study of the relevant processes.

The jet in crossflow^{a)}
View Description Hide DescriptionThe jet in crossflow, or transverse jet, is a flowfield that has relevance to a wide range of energy and propulsion systems. Over the years, our group's studies on this canonical flowfield have focused on the dynamics of the vorticity associated with equidensity and variable density jets in crossflow, including the stability characteristics of the jet's upstream shear layer, as a means of explaining jet response to altered types of excitation. The jet's upstream shear layer is demonstrated to exhibit convectively unstable behavior at high jettocrossflow momentum flux ratios, transitioning to absolutely unstable behavior at low momentum flux and/or density ratios, with attendant differences in shear layer vorticity evolution and rollup. These differences in stability characteristics are shown to have a significant effect on how one optimally employs external excitation to control jet penetration and spread, depending on the flow regime and specific engineering application. Yet recent unexpected observations on altered transverse jet structure under different flow conditions introduce a host of unanswered questions, primarily but not exclusively associated with the nature of molecular mixing, that make this canonical flowfield one that is of great interest for more extensive exploration.

Vorticity dynamics for transient highpressure liquid injection^{a)}
View Description Hide DescriptionThe liquid jet from a round orifice during the transient startup and steady mass flux periods of a high pressure injector is studied via NavierStokes and levelset computations. Via postprocessing, the role of vorticity dynamics is examined and shown to reveal crucial new insights. A brief review of relevant literature is made. An unsteady, axisymmetric fulljet case is solved. Then, a less computationally intensive case is studied with a segment of the jet core undergoing temporal instability; agreement with the fulljet calculation is satisfactory justifying the segment analysis for threedimensional computation. The results for surfaceshape development are in agreement with experimental observations and other threedimensional computations; the initial, axisymmetric waves at the jet surface created by KelvinHelmholtz (KH) instability distort to cone shapes; next, threedimensional character develops through an azimuthal instability that leads to the creation of streamwise vorticity, lobe shapes on the cones, and formation of liquid ligaments which extend from lobes on the cones. The cause of this azimuthal instability has been widely described as a RayleighTaylor instability. However, additional and sometimes more important causes are identified here. Counterrotating, streamwise vortices within and around the ligaments show a relationship in the instability behavior for jets flowing into likedensity fluid; thus, density difference cannot explain fully the threedimensional instability as previously suggested. Furthermore, the formation of ligaments that eventually break into droplets and the formation of streamwise vorticity are caused by the same vortical dynamics. Waviness is identified on the ligaments which should result in droplet formation. The nonlinear development of the shorter azimuthal waves and ligament waves explains the experimental results that droplet sizes are usually smaller than KH wavelengths. The higher the relative velocity and/or the lower the surface tension the shorter are the values of the most unstable wavelengths.

Roughness effects on wallbounded turbulent flows^{a)}
View Description Hide DescriptionThis paper outlines the authors' experimental research in roughwallbounded turbulent flows that has spanned the past 15 years. The results show that, in general, roughness effects are confined to the inner layer. In accordance with Townsend's Reynolds number similarity hypothesis, the outer layer is insensitive to surface condition except in the role it plays in setting the length and velocity scales for the outer flow. An exception to this can be twodimensional roughness which has been observed in some cases to suffer roughness effects far from the wall. However, recent results indicate that similarity also holds for twodimensional roughness provided the Reynolds number is large, and there is sufficient scale separation between the roughness length scale and the boundary layer thickness. The concept of similarity between smooth and roughwall flows is of great practical importance as most computational and analytical modeling tools rely on it either explicitly or implicitly in predicting flows over rough walls. Because of the observed similarity, the roughness function (ΔU ^{ + }), or shift in the log layer, is a useful way of characterizing the roughness effect on the mean flow and the frictional drag. In the fully rough regime, it is shown that the hydraulic roughness length scale is related to the rootmeansquare height (k rms) and skewness (s k ) of the surface elevation probability density function. On the other hand, the onset of roughness effects is seen to be associated with the largest surface features which are typified by the peaktotrough height (k t). Roughness function behavior in the transitionally rough regime varies significantly between roughness types. Since no “universal” roughness function exists, no single roughness length scale can characterize all roughness types in all the flow regimes. Despite this, research using roughness with a systematic variation in texture is ongoing in an effort to uncover surface parameters that lead to the variation in the frictional drag behavior witnessed in the transitionally rough regime.

 LETTERS


Jetted mixtures of particle suspensions and resins
View Description Hide DescriptionDropondemand (DoD) inkjetting of hard particle suspensions with volume fraction Φ ∼ 0.25 has been surveyed using 1000 ultrahigh speed videos as a function of particle size (d90 = 0.8—3.6 μm), with added 2 wt. % acrylic (250 kDa) or 0.5 wt. % cellulose (370 kDa) resin, and also compared with Newtonian analogues. Jet breakoff times from 80 μm diameter nozzles were insensitive (120 ± 10 μs) to particle size, and resin jet breakoff times were not significantly altered by >30 wt. % added particles. Different particle size grades can be jetted equally well in practice, while resin content effectively controls DoD breakoff times.

Comparison of two and threedimensional simulations of miscible RichtmyerMeshkov instability with multimode initial conditions
View Description Hide DescriptionA comparison between two and threedimensional largeeddy simulations of the planar RichtmyerMeshkov instability with multimode initial conditions is made. The threedimensional calculation achieves a turbulent state where an inertial range of length scales is present after the second shock wave impacts the interface. Grid independence of the mixing width up until the time of reshock is demonstrated through mesh refinement in both two and three dimensions. Quantitative measures of mixing are compared including the mixing width, mixedness, mixed mass, and spectra of velocity and density. A proposed approximate relation for the mixed mass is evaluated in one, two, and three dimensions and is proportional to the product of the mixing width and the mass fraction variance in the layer. The variance of the velocity field and the scalar mass fraction are compared in two and three dimensions and demonstrate large differences in behavior.

 ARTICLES

 Biofluid Mechanics

Finite element analysis of magnetohydrodynamic effects on blood flow in an aneurysmal geometry
View Description Hide DescriptionBlood flow in an aneurysmal geometry, subjected to a static and uniform magnetic field, was studied. Blood was considered as a Newtonian, incompressible, and electrically conducting fluid. The nonlinear system of partial differential equations, describing the blood flow under the presence of a magnetic field, was discretized by the Galerkin weighted residual method. The transformation in generalized curvilinear coordinates facilitates the solution of the governing equations within arbitrary geometries. Pressure and velocity fields along with wall shear stress distributions were obtained for varying magnetic field intensities and directions. The visualization of the blood streamlines in the dilatation region highlights the effect of a magnetic field on the recirculation zones. The application of static magnetic fields can yield spatiotemporal description of blood flow patterns. The current study discusses implications of the hemodynamic properties estimated by respective screening techniques since the static magnetic field might cause alterations that possibly cannot be detected and thus eliminated.

Undulatory microswimming near solid boundaries
View Description Hide DescriptionThe hydrodynamic forces involved in the undulatory microswimming of the model organism C. elegans are studied in proximity to solid boundaries. Using a micropipette deflection technique, we attain direct and timeresolved force measurements of the viscous forces acting on the worm near a single planar boundary as well as confined between two planar boundaries. We observe a monotonic increase in the lateral and propulsive forces with increasing proximity to the solid interface. We determine normal and tangential drag coefficients for the worm, and find these to increase with confinement. The measured drag coefficients are compared to existing theoretical models. The ratio of normal to tangential drag coefficients is found to assume a constant value of 1.5 ± 0.1(5) at all distances from a single boundary, but increases significantly as the worm is confined between two boundaries. In response to the increased drag due to confinement, we observe a gait modulation of the nematode, which is primarily characterized by a decrease in the swimming amplitude.
 Micro and Nanofluid Mechanics

Dynamics of a spherical particle in an acoustic field: A multiscale approach
View Description Hide DescriptionA rigid spherical particle in an acoustic wave field oscillates at the wave period but has also a mean motion on a longer time scale. The dynamics of this mean motion is crucial for numerous applications of acoustic microfluidics, including particle manipulation and flow visualisation. It is controlled by four physical effects: acoustic (radiation) pressure, streaming, inertia, and viscous drag. In this paper, we carry out a systematic multiscale analysis of the problem in order to assess the relative importance of these effects depending on the parameters of the system that include wave amplitude, wavelength, sound speed, sphere radius, and viscosity. We identify two distinguished regimes characterised by a balance among three of the four effects, and we derive the equations that govern the mean particle motion in each regime. This recovers and organises classical results by King [“On the acoustic radiation pressure on spheres,” Proc. R. Soc. A147, 212–240 (1934)], Gor'kov [“On the forces acting on a small particle in an acoustical field in an ideal fluid,” Sov. Phys.6, 773–775 (1962)], and Doinikov [“Acoustic radiation pressure on a rigid sphere in a viscous fluid,” Proc. R. Soc. London A447, 447–466 (1994)], clarifies the range of validity of these results, and reveals a new nonlinear dynamical regime. In this regime, the mean motion of the particle remains intimately coupled to that of the surrounding fluid, and while viscosity affects the fluid motion, it plays no part in the acoustic pressure. Simplified equations, valid when only two physical effects control the particle motion, are also derived. They are used to obtain sufficient conditions for the particle to behave as a passive tracer of the Lagrangianmean fluid motion.

Nonlinear electrophoresis of ideally polarizable particles
View Description Hide DescriptionWe focus in this paper on the nonlinear electrophoresis of ideally polarizable particles. At high applied voltages, significant ionic exchange occurs between the electric double layer, which surrounds the particle, and the bulk solution. In addition, steric effects due to the finite size of ions drastically modify the electric potential distribution in the electric double layer. In this situation, the velocity field, the electric potential, and the ionic concentration in the immediate vicinity of the particle are described by a complicated set of coupled nonlinear partial differential equations. In the general case, these equations must be solved numerically. In this study, we rely on a numerical approach to determine the electric potential, the ionic concentration, and the velocity field in the bulk solution surrounding the particle. The numerical simulations rely on a pseudospectral method which was used successfully by Chu and Bazant [J. Colloid Interface Sci.315(1), 319–329 (2007)] to determine the electric potential and the ionic concentration around an ideally polarizable metallic sphere. Our numerical simulations also incorporate the steric model developed by Kilic et al. [Phys. Rev. E75, 021502 (2007)] to account for crowding effects in the electric double layer, advective transport, and for the presence of a body force in the bulk electrolyte. The simulations demonstrate that surface conduction significantly decreases the electrophoretic mobility of polarizable particles at high zeta potential and at high applied electric field. Advective transport in the electric double layer and in the bulk solution is also shown to significantly impact surface conduction.
 Interfacial Flows

Integral constraints in the study of RichtmyerMeshkov turbulent mixing
View Description Hide DescriptionAn experimental study of the temporal evolution of the shockinduced RichtmyerMeshkov instability in the turbulent regime with threedimensional random interfacial perturbations is carried out. The primary interest is the growth rate of the turbulent mixing layer that develops after an impulsive acceleration of the perturbed interface between two gases (air/SF6) by a weak Ma = 1.2 incident shock wave. Planar Mie scattering is used to visualize the flow, and image sequences are captured using a highspeed video camera. The analysis of the total mixing width has been extended to study the growth behaviors of the bubbles and spikes, separately. A novel definition of the bubble and spike widths is introduced using the mass and linear momentum conservation laws. For the planar incident shock wave the newly defined bubble and spike widths increase in time as h b.s ∝ t ^{θ}, with a growth exponent θ = 1/2 that does not depend on either the initial conditions or the physical properties of the gases composing the interface.

Direct simulation of single bubble motion under vertical magnetic field: Paths and wakes
View Description Hide DescriptionMotion of single Ar bubbles rising in GaInSn under vertical magnetic fields is studied numerically using a volumeoffluid method and adaptive mesh refinement technique for twophase interface treatment; a consistent and conservative scheme calculates induced current density and Lorentz force. Numerical results are compared with published experimental data [C. Zhang, S. Eckert, and G. Gerbeth, “Experimental study of single bubble motion in a liquid metal column exposed to a DC magnetic field,” Int. J. Multiphase Flow31, 824–842 (2005)], where bubble diameters range from 2.5 to 6.4 mm, producing Reynolds numbers that vary between 2000 and 4000. Maximum experimental magnetic field strength was set to 0.3 T because of experimental restrictions, although we increased it to 0.5 T for firm conclusions. Apart from terminal rising velocity comparisons, we focused on variations in bubble motion paths and wake structures under magnetic fields, which cannot be observed experimentally because liquid metal is opaque. Magnetic field effects on bubble trajectory are exerted through vortex structure modification, which reinforced the conjecture that path instability is mainly attributed to wake instability. In bubble motion without magnetic fields, vortex threads in the bubble wake wrap around each other while vortex filaments incline parallel to the field with increasing magnetic intensity. Additionally, high magnetic fields will induce secondary bubble path instabilities, which contribute to the high Reynolds number flow that instabilities develop around the bubble, producing an asymmetrical Lorentz force distribution. This instability vanishes under higher magnetic intensities because flow instability is suppressed. Rising bubble aspect ratios decrease considerably under magnetic fields and may also contribute to smaller vorticities at the bubble surface. A close relationship between fluctuations in rising velocity and shape variations is found.
 Viscous and NonNewtonian Flows

Fluidinduced propulsion of rigid particles in wormlike micellar solutions
View Description Hide DescriptionIn the absence of inertia, a reciprocal swimmer achieves no net motion in a viscous Newtonian fluid. Here, using tracking methods and birefringence imaging, we investigate the ability of a reciprocally actuated particle to translate through a complex fluid that possesses a network. A geometrically polar particle, a rod with a bead on one end, is reciprocally rotated using magnetic fields. The particle is immersed in a wormlike micellar (WLM) solution that is known to be susceptible to the formation of shear bands and other localized structures due to shearinduced remodeling of its microstructure. Results show that the nonlinearities present in this WLM solution break timereversal symmetry under certain conditions, and enable propulsion of an artificial “swimmer.” We find three regimes dependent on the Deborah number (De): net motion towards the beadend of the particle at low De, net motion towards the rodend of the particle at intermediate De, and no appreciable propulsion at high De. At low De, where the particle time scale is longer than the fluid relaxation time, we believe that propulsion is caused by an imbalance in the fluid first normal stress differences between the two ends of the particle (bead and rod). At De ∼ 1, however, we observe the emergence of a region of network anisotropy near the rod using birefringence imaging. This anisotropy suggests alignment of the micellar network, which is “locked in” due to the shorter time scale of the particle relative to the fluid.
 Particulate, Multiphase, and Granular Flows

Particle dispersion in homogeneous turbulence using the onedimensional turbulence model
View Description Hide DescriptionLagrangian particle dispersion is studied using the onedimensional turbulence (ODT) model in homogeneous decaying turbulence configurations. The ODT model has been widely and successfully applied to a number of reacting and nonreacting flow configurations, but only limited application has been made to multiphase flows. Here, we present a version of the particle implementation and interaction with the stochastic and instantaneous ODT eddy events. The model is characterized by comparison to experimental data of particle dispersion for a range of intrinsic particle time scales and body forces. Particle dispersion, velocity, and integral time scale results are presented. The particle implementation introduces a single model parameter β p , and sensitivity to this parameter and behavior of the model are discussed. Good agreement is found with experimental data and the ODT model is able to capture the particle inertial and trajectory crossing effects. These results serve as a validation case of the multiphase implementations of ODT for extensions to other flow configurations.

Physics of puffing and microexplosion of emulsion fuel droplets
View Description Hide DescriptionThe physics of waterinoil emulsion droplet microexplosion/puffing has been investigated using highfidelity interfacecapturing simulation. Varying the dispersedphase (water) subdroplet size/location and the initiation location of explosive boiling (bubble formation), the droplet breakup processes have been well revealed. The bubble growth leads to local and partial breakup of the parent oil droplet, i.e., puffing. The water subdroplet size and location determine the afterpuffing dynamics. The boiling surface of the water subdroplet is unstable and evolves further. Finally, the subdroplet is wrapped by boiled water vapor and detaches itself from the parent oil droplet. When the water subdroplet is small, the detachment is quick, and the oil droplet breakup is limited. When it is large and initially located toward the parent droplet center, the droplet breakup is more extensive. For microexplosion triggered by the simultaneous growth of multiple separate bubbles, each explosion is local and independent initially, but their mutual interactions occur at a later stage. The degree of breakup can be larger due to interactions among multiple explosions. These findings suggest that controlling microexplosion/puffing is possible in a fuel spray, if the emulsionfuel blend and the ambient flow conditions such as heating are properly designed. The current study also gives us an insight into modeling the puffing and microexplosion of emulsion droplets and sprays.
 Laminar Flows

Flow control of a circular cylinder by using an attached flexible filament
View Description Hide DescriptionThe flow control of a circular cylinder by using a flexible filament has been numerically investigated in this work. The cylinder is either fixed or elastically mounted, and the filament is attached to the base of the cylinder. Its leading end is fixed and its trailing end is free to flap. To execute the numerical simulation and deal with the fluidstructure interaction (FSI) of the filament as well, an improved immersed boundarylattice Boltzmann method (IBLBM) is presented. As compared to the conventional IBLBM for handling the FSI of a filament, the current method can incorporate the mass effect of the filament and no userdefined spring parameter is needed to calculate the interaction force on the filament. After validating the employed method, the effects of the filament on the flow control of the cylinder are systematically examined by varying the bending coefficient (K b ) and length (L) of the filament. The laminar flow with a Reynolds number of 150 is considered in this study. Based on the numerical results obtained, it is found that the fluctuation of lift force and vortex shedding of a fixed cylinder and the vortexinduced vibration of an elastically mounted cylinder can be suppressed efficiently.
 Instability and Transition

Sensitivity of aerodynamic forces in laminar and turbulent flow past a square cylinder
View Description Hide DescriptionWe use adjointbased gradients to analyze the sensitivity of the drag force on a square cylinder. At Re = 40, the flow settles down to a steady state. The quantity of interest in the adjoint formulation is the steady asymptotic value of drag reached after the initial transient, whose sensitivity is computed solving a steady adjoint problem from knowledge of the stable base solution. At Re = 100, the flow develops to the timeperiodic, vortexshedding state. The quantity of interest is rather the timeaveraged mean drag, whose sensitivity is computed integrating backwards in time an unsteady adjoint problem from knowledge of the entire history of the vortexshedding solution. Such theoretical frameworks allow us to identify the sensitive regions without computing the actually controlled states, and provide a relevant and systematic guideline on where in the flow to insert a secondary control cylinder in the attempt to reduce drag, as established from comparisons with dedicated numerical simulations of the twocylinder system. For the unsteady case at Re = 100, we also compute an approximation to the mean drag sensitivity solving a steady adjoint problem from knowledge of only the mean flow solution, and show the approach to carry valuable information in view of guiding relevant control strategy, besides reducing tremendously the related numerical effort. An extension of this simplified framework to turbulent flow regime is examined revisiting the widely benchmarked flow at Reynolds number Re = 22 000, the theoretical predictions obtained in the frame of unsteady Reynoldsaveraged Navier–Stokes modeling being consistent with experimental data from the literature. Application of the various sensitivity frameworks to alternative control objectives such as increasing the lift and reducing the fluctuating drag and lift is also discussed and illustrated with a few selected examples.

Flowinduced vibrations of two tandem circular cylinders in a parallelwall channel
View Description Hide DescriptionFlowinduced vibrations of one and two tandem circular cylinders in the flow around cylinders in a parallelwall channel are numerically studied by the lattice Boltzmann method. Within a range of Reynolds number Re = [1, 160], the effects of streamwise separation between two cylinders S/D = [1.25, 3], mass ratio M = [0.05, 5], and blockage ratio β = [1/2, 1/8] on the motions of cylinders and fluids are investigated, respectively. For the case of an isolated cylinder, as the mass ratio is 1, no largeamplitude oscillation is observed, and as the mass ratio is 0.1, the cylinder motion translates from the steady regime to the biased periodic vibration with a large oscillation amplitude gradually as Reynolds number is increased from 1 to 160. For the case of two cylinders in tandem, two steady regimes and a variety of distinct oscillation regimes with the corresponding flow structures are observed. The critical mass ratio of the two tandem cylinders in the strong coupling regime is about an order of magnitude larger than that of an isolated cylinder. For blockage ratio is more than 1/5, the vibration type of the cylinders is exclusive, while for blockage ratio is less than 1/6, the cylinder oscillation state is bistable. The mechanisms of the observed phenomena are also discussed.
 Turbulent Flows

Modelling and analysis of turbulent datasets using Auto Regressive Moving Average processes
View Description Hide DescriptionWe introduce a novel way to extract information from turbulent datasets by applying an Auto Regressive Moving Average (ARMA) statistical analysis. Such analysis goes well beyond the analysis of the mean flow and of the fluctuations and links the behavior of the recorded time series to a discrete version of a stochastic differential equation which is able to describe the correlation structure in the dataset. We introduce a new index Υ that measures the difference between the resulting analysis and the Obukhov model of turbulence, the simplest stochastic model reproducing both Richardson law and the Kolmogorov spectrum. We test the method on datasets measured in a von Kármán swirling flow experiment. We found that the ARMA analysis is well correlated with spatial structures of the flow, and can discriminate between two different flows with comparable mean velocities, obtained by changing the forcing. Moreover, we show that the Υ is highest in regions where shear layer vortices are present, thereby establishing a link between deviations from the Kolmogorov model and coherent structures. These deviations are consistent with the ones observed by computing the Hurst exponents for the same time series. We show that some salient features of the analysis are preserved when considering global instead of local observables. Finally, we analyze flow configurations with multistability features where the ARMA technique is efficient in discriminating different stability branches of the system.