When a many-body system is driven away from equilibrium, order can spontaneously emerge in places where disorder might be expected. Here we report an unexpected order in the flow of a concentrated emulsion in a tapered microfluidic channel. The velocity profiles of individual drops in the emulsion show periodic patterns in both space and time. Such periodic patterns appear surprising from both a fluid and a solid mechanics point of view. In particular, when the emulsion is considered as a soft crystal under extrusion, a disordered scenario might be expected based on the stochastic nature of dislocation dynamics in microscopic crystals. However, an orchestrated sequence of dislocation nucleation and migration is observed to give rise to a highly ordered deformation mode. This discovery suggests that nanocrystals can be made to deform more controllably than previously thought. It can also lead to novel flow control and mixing strategies in droplet microfluidics.many-body system | order | microfluidic crystal | dislocation dynamics | plasticity T he emergence of order and spontaneous self-organization have been of long-standing interest (1). They are key not only to the understanding of complex phenomena from chemical oscillators to swarming behavior in animals (2, 3), but also to novel engineering solutions if harnessed (4, 5). Two-phase flow in microfluidics offers a simple platform for the study of dissipative nonequilibrium systems, where hydrodynamic interactions have led to the emergence of collective dynamics and order (6-10). Most works thus far have focused on dilute emulsions or foams in simple channel geometries. Whereas rich physics has been revealed, these phenomena have yet to find implications in the broader technological context. Nevertheless, concentrated emulsions, bubble rafts, and colloids have long been used as models of crystals for studying grain boundaries, dislocations, plasticity, and other processes central to materials science and solid mechanics (11-16). Such systems have not been applied to model the deformation modes of nanocrystals, however. Recent work on crystal plasticity in microscopic samples found that in contrast to their macroscopic counterparts, both the external geometry and internal structure of the material determine material strength (17). Furthermore, the size and timing of dislocation-induced strain bursts are found to be intermittent, stochastic, and unpredictable (17-20). The stochastic nature of dislocation dynamics complicates the control of the shape of the materials during deformation, and renders their subsequent manipulation and manufacturing challenging (18). What is unknown, however, is whether plasticity remains stochastic as the sample shrinks to the nanoscale, and whether the dimensionality and loading conditions influence the stochasticity. Given the increasing importance of low-dimensional nanodevices in applications from optoelectronics to energy conversion, it is critical to understand how nanomaterials can be shaped, manipulated, and manufactured.In t...

The formation and evolution of flow structures due to the interaction of a finite-span synthetic jet with a zero-pressure gradient laminar boundary layer were experimentally investigated using stereoscopic particle image velocimetry. A synthetic jet with three orifice aspect ratios of AR = 6, 12, and 18 was issued into a free-stream velocity of U∞ = 10 m/s (Reδ = 2000) at blowing ratios of Cb = 0.5–1.5. The interaction was found to be associated with two sets of flow structures: (1) a recirculation region downstream of the orifice due to virtual blockage, and (2) a steady streamwise vortex pair farther downstream. These two flow structures were characterized in detail. Tube-like velocity deficits in the free-stream were evident, as well as regions of increased velocity within the boundary layer. Reducing the aspect ratio of the orifice decreased the spacing of the edgewise vortices (generated due to the finite span of the orifice) as well as reducing the virtual blockage of the jet. A control volume analysis of the fluid streamwise momentum indicates that there is a momentum deficit just downstream of the jet orifice and the change in streamwise momentum is proportionally similar for all cases.

The formation and evolution of flow structures associated with a finite-span synthetic jet issued into a zero-pressure gradient boundary layer were investigated experimentally using stereoscopic particle image velocimetry. A synthetic jet with an aspect ratio of AR = 18 was mounted on a flat plate and its interaction with a free stream, having a velocity of U∞ = 10 m/s (Reδ = 2000) at momentum coefficients of Cμ = 0.08, 0.33, and 0.75, was studied. The effect of the orifice pitch (α = 20∘–90∘) and skew (β = 0∘–90∘) angles on vortex formation as well as the global impact of the synthetic jet on the flow field was explored in detail. It was found that the orifice orientation had a significant impact on the steady and unsteady flow structures. Different orifice skew and pitch angles could result in several types of vortical structures downstream, including: no coherent vortex structure, a single (positive or negative) strong vortex, or a symmetric vortex pair. In all cases, the velocity near the wall was increased; however, cases with higher blockage (i.e., more wall-normal, transverse orifice) resulted in a strong velocity deficit in the free stream where orifices with lower pitch angles yielded in an increase in velocity throughout. The analysis is concluded with a summary of quantitative metrics that allude to flow control effectiveness.

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In this study, we investigated two-degree-of-freedom (2d.f.) vortex-induced vibrations (VIVs) of a circular cylinder with a pinned attachment at its base; it had identical mass ratios and natural frequencies in both streamwise and transverse directions. The cylinder had a mass ratio, m à of 0.45, and a mass damping, (m à CC A )z, equal to 0.0841. Laserinduced fluorescence flow visualization and digital particle image velocimetry experiments were conducted over a Reynolds number range, 820%Re%6050 (corresponding to the reduced velocity range, 1.1%U à %8.3). Measurements and visualization studies were made in a fixed plane at the cylinder mid-height, providing a two-dimensional picture of a highly three-dimensional system. However, significant insights can be gained from these experiments and form the basis of this paper. A large transverse amplitude response, A à Y w2 (or four diameters peak-to-peak), in the upper branch was observed. The streamwise amplitude response exhibits an even higher peak amplitude, A à X w2:5, which is approximately 125% of peak A à Y . Results show that there is no lower branch for this system and the transverse upper branch exhibits asymptotic behaviour, i.e. a wide regime of resonance. For ReO3000, the Strouhal number for the vortex shedding was 0.16 (G9%). Both the transverse cylinder oscillation and vortex-shedding frequencies, f OS,Y and f VS , respectively, were virtually identical throughout this range. While the streamwise oscillation frequency is typically twice the transverse oscillation frequency for a 2d.f. system, this is not the case at the lowest reduced velocities where oscillations first occur. Under these conditions the streamwise and transverse oscillation frequencies were identical. Finally, we observed that the cylinder wake exhibits both the PCS vortex-shedding mode and a desynchronized vortex pattern, which are uncommon for flow past a cylinder experiment. Very interestingly, the wide U à range over which resonance occurs is dominated by a desynchronized vortex pattern. These results clearly demonstrate the differences that arise in 2d.f. VIV occurring below the critical mass ratio.

The feasibility of active flow control, via arrays of synthetic jet actuators, to mitigate hysteresis was investigated experimentally on a dynamically pitching finite span S809 blade. In the present work, a six-component load cell was used to measure the unsteady lift, drag and pitching moment. Stereoscopic Particle Image Velocimetry (SPIV) measurements were also performed to understand the effects of synthetic jets on flow separation during dynamic pitch and to correlate these effects with the forces and moment measurements. It was shown that active flow control could significantly reduce the hysteresis in lift, drag and pitching moment coefficients during dynamic pitching conditions. This effect was further enhanced when the synthetic jets were pulsed modulated. Furthermore, additional reduction in the unsteady load oscillations can be observed in post-stall conditions during dynamic motions. This reduction in the unsteady aerodynamic loading can potentially lead to prolonged life of wind turbine blades.

The feasibility of active flow control to mitigate hysteresis loop due to a dynamically pitching finite span s809 blade was investigated experimentally at a Reynolds number of 220,000. Under normal operating conditions, hysteresis loop and tip vibrations exist, which with extended exposure would cause blade fatigue and eventually translate to a reduced lifetime of wind turbines. In this regard, active flow control via arrays of synthetic jet actuators was explored as a means to control flow separation over the finite span blade, which can lead to mitigation of these undesired unsteady loads. In the present work, a six-component load cell was used to measure the aerodynamic loading of lift, drag and pitching moment. Stereoscopic Particle Image Velocimetry (SPIV) measurements were also performed to understand the effects of synthetic jets on flow separation during dynamic pitch, and to correlate these effects to the forces and moment measurements. It was shown that active flow control could delay or minimize dynamic stall through the reduction of the hysteresis loop of the aerodynamic loads. This implies less unsteady aerodynamic loadings on the blade, which can potentially lead to prolonged life of wind turbines. Nomenclature= Planform area of model = area of synthetic jet orifice = instantaneous angle of attack ̅ = mean angle of attack = amplitude of angle of attack during sinusoidal pitching oscillation AEP = Annual Energy Production c = chord length C D = drag coefficient C L = lift coefficient C m = pitching moment coefficient C µ = momentum coefficient COE = Cost of Energy DRF = Discount Rate Factor ICC = Installed Capital Costs f p = pitching frequency ̅ = time averaged jet momentum = linear reduced frequency LRC = Levelized Replacement Costs = free-stream dynamic viscosity n = number of jets O&M = Operations and Maintenance Costs Re c = chord based Reynolds number = free-stream density = density of fluid exhausted from synthetic jet during outstroke SPIV = Stereoscopic Particle Image Velocimetry t = time T = cycle period of a synthetic jet actuator τ = synthetic jet outstroke time U = streamwise velocity component U ∞ = free-stream velocity u sj (t) = jet exit velocity at the centerline V = normal velocity component W = spanwise velocity component x = streamwise direction (parallel to the free stream) y = wall-normal direction (perpendicular to the free stream) I. IntroductionWind energy is a rapidly growing sector of both the economy and research. At the end of 2009, the total capacity of wind turbines installed in the world accounted for nearly 160 GW [1]. However, in order to maintain and improve the economics of wind energy, three different factors must be addressed. These factors are the energy capture of the turbine over its lifetime, the capitol cost necessary to install the turbine, and operations and maintenance cost. This can be expressed as [2]:

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