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 Title
 Intermittency Enhancement In Quantum Turbulence In Superfluid He4.
 Creator

Varga, Emil, Gao, Jian, Guo, Wei, Skrbek, Ladislav
 Abstract/Description

Intermittency is a hallmark of turbulence, which exists not only in turbulent flows of classical viscous fluids but also in flows of quantum fluids such as superfluid He4. Despite the established similarity between turbulence in classical fluids and quasiclassical turbulence in superfluid He4, it has been predicted that intermittency in superfluid He4 is temperature dependent and enhanced for certain temperatures, which is in striking contrasts to the nearly flowindependent intermittency...
Show moreIntermittency is a hallmark of turbulence, which exists not only in turbulent flows of classical viscous fluids but also in flows of quantum fluids such as superfluid He4. Despite the established similarity between turbulence in classical fluids and quasiclassical turbulence in superfluid He4, it has been predicted that intermittency in superfluid He4 is temperature dependent and enhanced for certain temperatures, which is in striking contrasts to the nearly flowindependent intermittency in classical turbulence. Experimental verification of this theoretical prediction is challenging since it requires wellcontrolled generation of quantum turbulence in He4 and flow measurement tools with high spatial and temporal resolution. Here we report an experimental study of quantum turbulence generated by towing a grid through a stationary sample of superfluid He4. The decaying turbulent quantum flow is probed by combining a recently developed He*(2) molecular tracerline tagging velocimetry technique and a traditional secondsound attenuation method. We observe quasiclassical decays of turbulent kinetic energy in the normal fluid and of vortex line density in the superfluid component. For several time instants during the decay, we calculate the transverse velocity structure functions. Their scaling exponents, deduced using the extended selfsimilarity hypothesis, display nonmonotonic temperaturedependent intermittency enhancement, in excellent agreement with a recent theoretical and numerical study
Show less  Date Issued
 20180904
 Identifier
 FSU_libsubv1_wos_000443685600007, 10.1103/PhysRevFluids.3.094601
 Format
 Citation
 Title
 Timeaveraged Threedimensional Flow Topology In The Wake Of A Simplified Car Model Using Volumetric Piv.
 Creator

Sellappan, Prabu, McNally, Jonathan, Alvi, Farrukh S.
 Abstract/Description

Fuel shortages and air pollution are two major incentives for improving the aerodynamics of vehicles. Reducing wakeinduced aerodynamic drag, which is strongly dependent on flow topology, is important for improving fuel consumption rates which directly affect the environment. Therefore, a comprehensive understanding of the baseline flow topology is required to develop targeted drag reduction strategies. In this research, the near wake of a generic ground vehicle, a slant Ahmed model at a flow...
Show moreFuel shortages and air pollution are two major incentives for improving the aerodynamics of vehicles. Reducing wakeinduced aerodynamic drag, which is strongly dependent on flow topology, is important for improving fuel consumption rates which directly affect the environment. Therefore, a comprehensive understanding of the baseline flow topology is required to develop targeted drag reduction strategies. In this research, the near wake of a generic ground vehicle, a slant Ahmed model at a flow Reynolds number of , is investigated and its flow topology elucidated. The flow field of this canonical bluff body is extremely rich, with complex flow features such as spanwise trailing wake and streamwise Cpillar vortices. The flow is characterized through stereoscopic and tomographic velocity field measurements. The largescale, horseshoe vortex structures in the trailing wake, conventionally denoted as A and Bvortices, are found to vary in size and shape along the spanwise direction, which in turn influence the pressure distribution on the rear vertical surface. The longitudinal Cpillar vortices are found to extend far downstream and also influence the trailing wake structures through a complex, threedimensional interaction. The accuracy and cost of obtaining volumetric information in this complex flow field, by means of volume reconstruction, through Stacked StereoscopicParticle Image Velocimetry (PIV) and Tomographic PIV are also investigated.
Show less  Date Issued
 20180801
 Identifier
 FSU_libsubv1_wos_000437789100001, 10.1007/s0034801825815
 Format
 Citation
 Title
 Exploration Of Thermal Counterflow In He Ii Using Particle Tracking Velocimetry.
 Creator

Mastracci, Brian, Guo, Wei
 Abstract/Description

Flow visualization using particle image velocimetry (PIV) and particularly particle tracking velocimetry (PTV) has been applied to thermal counterflow in He II for nearly two decades now, but the results remain difficult to interpret because tracer particle motion can be influenced by both the normal fluid and superfluid components of He II as well as the quantized vortex tangle. For instance, in one early experiment it was observed (using PTV) that tracer particles move at the normal fluid...
Show moreFlow visualization using particle image velocimetry (PIV) and particularly particle tracking velocimetry (PTV) has been applied to thermal counterflow in He II for nearly two decades now, but the results remain difficult to interpret because tracer particle motion can be influenced by both the normal fluid and superfluid components of He II as well as the quantized vortex tangle. For instance, in one early experiment it was observed (using PTV) that tracer particles move at the normal fluid velocity v(n), while in another it was observed (using PIV) that particles move at v(n)/2. Besides the different visualization methods, the range of applied heat flux investigated by these experiments differed by an order of magnitude. To resolve this apparent discrepancy and explore the statistics of particle motion in thermal counterflow, we apply the PTV method to a wide range of heat flux at a number of different fluid temperatures. In our analysis, we introduce a scheme for analyzing the velocity of particles presumably moving with the normal fluid separately from those presumably influenced by the quantized vortex tangle. Our results show that for lower heat flux there are two distinct peaks in the streamwise particle velocity probability density function (PDF), with one centered at the normal fluid velocity v(n) (named G2 for convenience) while the other is centered near v(n)/2 (G1). For higher heat flux there is a single peak centered near v(n)/2 (G3). Using our separation scheme, we show quantitatively that there is no size difference between the particles contributing to G1 and G2. We also show that nonclassical features of the transverse particle velocity PDF arise entirely from G1, while the corresponding PDF for G2 exhibits the classical Gaussian form. The G2 transverse velocity fluctuation, backed up by second sound attenuation in decaying counterflow, suggests that largescale turbulence in the normal fluid is absent from the twopeak region. We offer a brief discussion of the physical mechanisms that may be responsible for our observations, revealing that G1 velocity fluctuations may be linked to fluctuations of quantized vortex line velocity, and suggest a number of numerical simulations that may reveal the underlying physics in detail.
Show less  Date Issued
 20180622
 Identifier
 FSU_libsubv1_wos_000436043200001, 10.1103/PhysRevFluids.3.063304
 Format
 Citation
 Title
 Networkedoscillatorbased Modeling And Control Of Unsteady Wake Flows.
 Creator

Nair, Aditya G., Brunton, Steven L., Taira, Kunihiko
 Abstract/Description

A networkedoscillatorbased analysis is performed to examine and control the transfer of kinetic energy for periodic bluff body flows. The dynamics of energy fluctuations in the flowfield are described by a set of oscillators defined by conjugate pairs of spatial proper orthogonal decomposition (POD) modes. To extract the network of interactions among oscillators, impulse responses of the oscillators to amplitude and phase perturbations are tracked. Tracking small energy inputs and using...
Show moreA networkedoscillatorbased analysis is performed to examine and control the transfer of kinetic energy for periodic bluff body flows. The dynamics of energy fluctuations in the flowfield are described by a set of oscillators defined by conjugate pairs of spatial proper orthogonal decomposition (POD) modes. To extract the network of interactions among oscillators, impulse responses of the oscillators to amplitude and phase perturbations are tracked. Tracking small energy inputs and using linear regression, a networkedoscillator model is constructed that reveals energy exchange among the modes. The model captures the nonlinear interactions among the modal oscillators through a linear approximation. A large collection of system responses is aggregated to capture the general network structure of oscillator interactions. The present networkedoscillator model describes the modal perturbation dynamics more accurately than the empirical Galerkin reducedorder model. The linear network model for nonlinear dynamics is subsequently utilized to design a modelbased feedback controller. The controller suppresses the modal amplitudes that result in wake unsteadiness leading to drag reduction. The strength of the proposed approach is demonstrated for a canonical example of twodimensional unsteady flow over a circular cylinder. The present formulation enables the characterization of modal interactions to control fundamental energy transfers in unsteady bluff body flows.
Show less  Date Issued
 20180618
 Identifier
 FSU_libsubv1_wos_000435445900023, 10.1103/PhysRevE.97.063107
 Format
 Citation
 Title
 Network Communitybased Model Reduction For Vortical Flows.
 Creator

Meena, Muralikrishnan Gopalakrishnan, Nair, Aditya G., Taira, Kunihiko
 Abstract/Description

A network communitybased reducedorder model is developed to capture key interactions among coherent structures in highdimensional unsteady vortical flows. The present approach is datainspired and founded on networktheoretic techniques to identify important vortical communities that are comprised of vortical elements that share similar dynamical behavior. The overall interactionbased physics of the highdimensional flow field is distilled into the vortical community centroids,...
Show moreA network communitybased reducedorder model is developed to capture key interactions among coherent structures in highdimensional unsteady vortical flows. The present approach is datainspired and founded on networktheoretic techniques to identify important vortical communities that are comprised of vortical elements that share similar dynamical behavior. The overall interactionbased physics of the highdimensional flow field is distilled into the vortical community centroids, considerably reducing the system dimension. Taking advantage of these vortical interactions, the proposed methodology is applied to formulate reducedorder models for the intercommunity dynamics of vortical flows, and predict lift and drag forces on bodies in wake flows. We demonstrate the capabilities of these models by accurately capturing the macroscopic dynamics of a collection of discrete point vortices, and the complex unsteady aerodynamic forces on a circular cylinder and an airfoil with a Gurney flap. The present formulation is found to be robust against simulated experimental noise and turbulence due to its integrating nature of the system reduction.
Show less  Date Issued
 20180611
 Identifier
 FSU_libsubv1_wos_000434765500015, 10.1103/PhysRevE.97.063103
 Format
 Citation
 Title
 Dissipation In Quantum Turbulence In Superfluid He4 Above 1 K.
 Creator

Gao, J., Guo, W., Yui, S., Tsubota, M., Vinen, W. F.
 Abstract/Description

There are two commonly discussed forms of quantum turbulence in superfluid He4 above 1 K: in one there is a random tangle of quantized vortex lines, existing in the presence of a nonturbulent normal fluid; in the second there is a coupled turbulent motion of the two fluids, often exhibiting quasiclassical characteristics on scales larger than the separation between the quantized vortex lines in the superfluid component. The decay of vortex line density, L, in the former case is often...
Show moreThere are two commonly discussed forms of quantum turbulence in superfluid He4 above 1 K: in one there is a random tangle of quantized vortex lines, existing in the presence of a nonturbulent normal fluid; in the second there is a coupled turbulent motion of the two fluids, often exhibiting quasiclassical characteristics on scales larger than the separation between the quantized vortex lines in the superfluid component. The decay of vortex line density, L, in the former case is often described by the equation dL/dt = chi 2 (kappa/2 pi)L2, where kappa is the quantum of circulation and chi(2). is a dimensionless parameter of order unity. The decay of total turbulent energy, E, in the second case is often characterized by an effective kinematic viscosity, v', such that dE/dt = v'kappa L2(2). We present values of chi(2 )derived from numerical simulations and from experiment, which we compare with those derived from a theory developed by Vinen and Niemela. We summarize what is presently known about the values of v' from experiment, and we present a brief introductory discussion of the relationship between chi(2 )and v', leaving a more detailed discussion to a later paper.
Show less  Date Issued
 20180529
 Identifier
 FSU_libsubv1_wos_000433287200005, 10.1103/PhysRevB.97.184518
 Format
 Citation
 Title
 Effects Of Nonuniform Viscosity On Ciliary Locomotion.
 Creator

Shoele, Kourosh, Eastham, Patrick S.
 Abstract/Description

The effect of nonuniform viscosity on the swimming velocity of a free swimmer at zero Reynolds number is examined. Using the generalized reciprocal relation for Stokes flow with nonuniform viscosity, we formulate the locomotion problem in a fluid medium with spatially varying viscosity. Assuming the limit of small variation in the viscosity of the fluid as a result of nonuniform distribution of nutrients around a swimmer, we derive a perturbation model to calculate the changes in the swimming...
Show moreThe effect of nonuniform viscosity on the swimming velocity of a free swimmer at zero Reynolds number is examined. Using the generalized reciprocal relation for Stokes flow with nonuniform viscosity, we formulate the locomotion problem in a fluid medium with spatially varying viscosity. Assuming the limit of small variation in the viscosity of the fluid as a result of nonuniform distribution of nutrients around a swimmer, we derive a perturbation model to calculate the changes in the swimming performance of a spherical swimmer as a result of positiondependent viscosity. The swimmer is chosen to be a spherical squirmer with a steady tangential motion on its surface modeling ciliary motion. The nutrient concentration around the body is described by an advectiondiffusion equation. The roles of the surface stroke pattern, the specific relationship between the nutrient and viscosity, and the Peclet number of the nutrient in the locomotion velocity of the squirmer are investigated. Our results show that for a pure treadmill stroke, the velocity change is maximum at the limit of zero Peclet number and monotonically decreases toward zero at very high Peclet number. When higher surface stroke modes are present, larger modification in swimming velocity is captured at high Peclet number where two mechanisms of thinning the nutrient boundary layer and appearance of new stagnation points along the surface of squirmer are found to be the primary reasons behind the swimming velocity modifications. It is observed that the presence of nonuniform viscosity allows for optimal swimming speed to be achieved with stroke combinations other than pure treadmill.
Show less  Date Issued
 20180424
 Identifier
 FSU_libsubv1_wos_000430691900001, 10.1103/PhysRevFluids.3.043101
 Format
 Citation
 Title
 Effect Of Synthetic Jet Modulation Schemes On The Reduction Of A Laminar Separation Bubble.
 Creator

Seo, J. H., Cadieux, F., Mittal, R., Deem, E., Cattafesta, L.
 Abstract/Description

The response of a laminar separation bubble to synthetic jet forcing with various modulation schemes is investigated via direct numerical simulations. A simple sinusoidal waveform is considered as a reference case, and various amplitude modulation schemes, including the squarewave "burst" modulation, are employed in the simulations. The results indicate that burst modulation is less effective at reducing the length of the flow separation than the sinusoidal forcing primarily because burst...
Show moreThe response of a laminar separation bubble to synthetic jet forcing with various modulation schemes is investigated via direct numerical simulations. A simple sinusoidal waveform is considered as a reference case, and various amplitude modulation schemes, including the squarewave "burst" modulation, are employed in the simulations. The results indicate that burst modulation is less effective at reducing the length of the flow separation than the sinusoidal forcing primarily because burst modulation is associated with a broad spectrum of input frequencies that are higher than the target frequency for the flow control. It is found that such highfrequency forcing delays vortex rollup and promotes vortex pairing and merging, which have an adverse effect on reducing the separation bubble length. A commonly used amplitude modulation scheme is also found to have reduced effectiveness due to its spectral content. A new amplitude modulation scheme which is tailored to impart more energy at the target frequency is proposed and shown to be more effective than the other modulation schemes. Experimental measurements confirm that modulation schemes can be preserved through the actuator and used to enhance the energy content at the target modulation frequency. The present study therefore suggests that the effectiveness of synthetic jetbased flow control could be improved by carefully designing the spectral content of the modulation scheme.
Show less  Date Issued
 20180312
 Identifier
 FSU_libsubv1_wos_000427117100001, 10.1103/PhysRevFluids.3.033901
 Format
 Citation
 Title
 The Impact Of Li Grain Size On Coulombic Efficiency In Li Batteries.
 Creator

Mehdi, B. Layla, Stevens, Andrew, Qian, Jiangfeng, Park, Chiwoo, Xu, Wu, Henderson, Wesley A., Zhang, JiGuang, Mueller, Karl T., Browning, Nigel D.
 Abstract/Description

One of the most promising means to increase the energy density of stateoftheart lithium Liion batteries is to replace the graphite anode with a Li metal anode. While the direct use of Li metal may be highly advantageous, at present its practical application is limited by issues related to dendrite growth and low Coulombic efficiency, CE. Here operando electrochemical scanning transmission electron microscopy (STEM) is used to directly image the deposition/stripping of Li at the anode...
Show moreOne of the most promising means to increase the energy density of stateoftheart lithium Liion batteries is to replace the graphite anode with a Li metal anode. While the direct use of Li metal may be highly advantageous, at present its practical application is limited by issues related to dendrite growth and low Coulombic efficiency, CE. Here operando electrochemical scanning transmission electron microscopy (STEM) is used to directly image the deposition/stripping of Li at the anodeelectrolyte interface in a Libased battery. A nonaqueous electrolyte containing small amounts of H2O as an additive results in remarkably different deposition/stripping properties as compared to the "dry" electrolyte when operated under identical electrochemical conditions. The electrolyte with the additive deposits more Li during the first cycle, with the grain sizes of the Li deposits being significantly larger and more variable. The stripping of the Li upon discharge is also more complete, i.e., there is a higher cycling CE. This suggests that larger grain sizes are indicative of better performance by leading to more uniform Li deposition and an overall decrease in the formation of Li dendrites and side reactions with electrolyte components, thus potentially paving the way for the direct use of Li metal in battery technologies.
Show less  Date Issued
 20161005
 Identifier
 FSU_libsubv1_wos_000384657800002, 10.1038/srep34267
 Format
 Citation
 Title
 Ballistic Thermal Transport in Carbyne and Cumulene with MicronScale Spectral Acoustic Phonon Mean Free Path.
 Creator

Lin, Sangchao, Wang, Mingchao
 Abstract/Description

The elastic modulus of carbyne, a onedimensional carbon chain, was recently predicted to be much higher than graphene. Inspired by this discovery and the fundamental correlation between elastic modulus and thermal conductivity, we investigate the intrinsic thermal transport in two carbon allotropes: carbyne and cumulene. Using molecular dynamics simulations, we discover that thermal conductivities of carbyne and cumulene at the quantumcorrected room temperature can exceed 54 and 148â€‰kW/m/K,...
Show moreThe elastic modulus of carbyne, a onedimensional carbon chain, was recently predicted to be much higher than graphene. Inspired by this discovery and the fundamental correlation between elastic modulus and thermal conductivity, we investigate the intrinsic thermal transport in two carbon allotropes: carbyne and cumulene. Using molecular dynamics simulations, we discover that thermal conductivities of carbyne and cumulene at the quantumcorrected room temperature can exceed 54 and 148â€‰kW/m/K, respectively, much higher than that for graphene. Such conductivity is attributed to high phonon energies and group velocities, as well as reduced scattering from nonoverlapped acoustic and optical phonon modes. The prolonged spectral acoustic phonon lifetime of 30â€“110â€‰ps and mean free path of 0.5â€“2.5â€‰Î¼m exceed those for graphene, and allow ballistic phonon transport along micronlength carbon chains. Tensile extensions can enhance the thermal conductivity of carbyne due to the increased phonon density of states in the acoustic modes and the increased phonon lifetime from phonon bandgap opening. These findings provide fundamental insights into phonon transport and band structure engineering through tensile deformation in lowdimensional materials, and will inspire studies on carbyne, cumulene, and boron nitride chains for their practical deployments in nanodevices.
Show less  Date Issued
 20151210
 Identifier
 FSU_libsubv1_scholarship_submission_1475610677, 10.1038/srep18122
 Format
 Citation