Elastic Vortex Research Articles

Thickness dependence of the second magnetization peak effect in Ba0.6K0.4Fe2As2 single crystals

The second magnetization peak (SMP) effect has been observed widely in many type-II superconductors, but the reason remains elusive. This effect manifests an enhanced critical current density with magnetic field and should be very useful for applications. By measuring the magnetization of optimally doped Ba0.6K0.4Fe2As2 single crystals with different thickness, it is found the SMP effect exists in thick samples and gradually becomes invisible when the sample thickness is reduced to the scale of micrometer. Detailed investigation on the vortex dynamics on samples with different thickness clearly show that there is a common behavior of vortex dynamics in the low field region, which may be characterized by the Bragg glass like elastic vortex motion. This feature holds on in the whole field region for the thin samples, while it turns into the SMP effect for thicker samples when the field is increased. The results suggest that the SMP effect may be induced by the entanglement of the vortex system, and the absence of the SMP effect in thin samples is attributed to the cutoff of the entangled vortex length along c-axis.

Numerical Investigation of Flow Inside a Channel with Elastic Vortex Generator and Elastic Wall for Heat Transfer Enhancement

In the present investigation, a detailed numerical investigation of the flow and heat transfer characteristics of a channel with an elastic fin (vortex generator) and an elastic wall has been carried out using finite element method. The Fluid-Structure Interaction (FSI) model is used to capture the interaction between the fluid and the solid structure. A sinusoidal time dependent velocity profile has been imposed at the inlet of the channel and the right half of the upper wall of the channel is heated and exposed to constant temperature boundary condition. Due to the sinusoidal velocity profile at the inlet, the elastic fin oscillates periodically and act as a vortex generator, which causes more turbulence in the flow. The obtained results showed that the Nusselt number over the heated wall is affected by the position of the flexible fin, height of flexible fin and elasticity modulus of elastic fin. Moreover, due to the elasticity of the elastic wall and sinusoidal behavior of the inlet velocity, the elastic wall oscillates periodically upward and downward. The Nusselt number values over the heated wall are increased with decrease of the elastic modulus value of the elastic wall. However, the decrease in elastic modulus value of the elastic wall contributes to an increase in the pressure drop inside the channel. It should be added that the interplay between the fluid motion and the deformable structures leads to enhanced turbulence, as the flexible fin and elastic wall introduce additional disturbances and fluctuations into the flow regime. Consequently, this heightened turbulence level has profound implications for heat transfer processes within the system.

Rotor Power Savings and Pitch-Link Load Reductions with Spanwise-Varying Active Camber Morphing

The effect of spanwise-varying active-camber morphing on helicopter rotor power and pitch-link loads was investigated computationally for advance ratios ranging from hovering to high-speed forward flight (μ=0.35) and was compared to results obtained with spanwise uniform camber actuation. Different actuation strategies were applied, such as constant (0P) deflection and superimposed harmonic actuation based on a 0P deflection combined with 1P (one per revolution) and 2P harmonics. A comprehensive rotor aeromechanics model of an isolated Bo-105 rotor, including elastic blade modeling and free vortex wake aerodynamics, was used for this analysis. Two different active-camber section sizes were investigated, one ranging from 0.22R to 0.9R and one from 0.5R to 0.9R. The greatest benefits from active-camber actuation were found in hover and high-speed flight. It was possible to reduce both the rotor power consumption and the peak-to-peak pitch-link loads. A relevant contribution of the spanwise variability to these power gains was obtained in hovering flight. Toward higher flight speeds, the effect of spanwise variable camber morphing became less important in terms of rotor power savings. However, it allowed a notable reduction of the actuation amplitude far outboard on the rotor blade.

Plastic vortex creep and dimensional crossovers in the highly anisotropic superconductor HgBa2CuO4+x

In type-II superconductors exposed to magnetic fields between upper and lower critical values, ${H}_{c1}$ and ${H}_{c2}$, penetrating magnetic flux forms a lattice of vortices whose motion can induce dissipation. Consequently, the magnetization $M$ of superconductors is typically progressively weakened with increasing magnetic field $B\ensuremath{\propto}{n}_{v}$ (for vortex density ${n}_{v}$). However, some materials exhibit a nonmonotonic $M(B)$, presenting a maximum in $M$ at what is known as the second magnetization peak. This phenomenon appears in most classes of superconductors, including low-${T}_{c}$ materials, iron-based, and cuprates, complicating pinpointing its origin and garnering intense interest. Here we report on vortex dynamics in optimally doped and overdoped ${\mathrm{HgBa}}_{2}{\mathrm{CuO}}_{4+x}$ crystals, with a focus on a regime in which plastic deformations of the vortex lattice govern magnetic properties. Specifically, we find that both crystals exhibit conspicuous second magnetization peaks and, from measurements of the field- and temperature-dependent vortex creep rates, identify and characterize phase boundaries between elastic and plastic vortex dynamics, as well as multiple previously unreported transitions within the plastic flow regime. We find that the second magnetization peak coincides with the elastic-to-plastic crossover for a very small range of high fields and a sharp crossover within the plastic flow regime for a wider range of lower fields. Moreover, we find evidence that this transition in the plastic flow regime is due to a dimensional crossover, specifically, a transition from three to two-dimensional plastic dynamics.

Fluid elastic instability and vortex shedding in finned tube arrays: the effects of tube material and fin density

PurposeThe purpose of this paper is to present investigations on the significant influence of the tube material and fin density on fluid elastic instability and vortex shedding in a parallel triangular finned tube array subjected to water cross flow.Design/methodology/approachThe experiment was conducted on finned tube arrays with a fin height of 6 mm and fin density of 3 fins per inch (fpi) and 9 fpi. A dedicated setup has been developed to examine fluid elastic instability and vortex shedding. Nine parallel triangular tube arrays with a pitch to tube diameter ratio of 1.78 were considered. The plain tube arrays, coarse finned tube arrays and fine finned tube arrays each of steel, copper and aluminium materials were tested. Plain tube arrays were tested to compare the results of the finned tube arrays having an effective tube diameter same as that of the plain tube.FindingsA significant effect of fin density and tube material with a variable mass damping parameter was observed on the instability threshold. In the parallel triangular finned tube array subjected to water cross flow, a delay in the instability threshold was observed with an increase in fin density. For steel and aluminium tube arrays, the natural frequency is 9.77 Hz and 10.38 Hz, which is close to each other, whereas natural frequency of the copper tubes is 7.40 Hz. The Connors’ stability constant K for steel and aluminium tube arrays is 4.78 and 4.87, respectively, whereas it is 5.76 for copper tube arrays, which increases considerably compared to aluminum and steel tube arrays. The existence of vortex shedding is confirmed by comparing experimental results with Owen’s hypothesis and the Strouhal number and Reynolds number relationship.Originality/valueThis paper’s results contribute to understand the effect of tube materials and fin density on fluid elastic instability threshold of finned tube arrays subjected to water cross flow.

Investigation on vibration parameters in aluminum finned tube arrays subjected to water cross flow

PurposePredicting the critical velocity is crucial at the instability threshold for shell and tube heat exchangers in order to prevent tube failure due to vibrations. In this study, the vibration response of an aluminum tube bundle subjected to water cross flow was analyzed experimentally. Aluminum tubes are preferred over steel tubes because of aluminum tubes' excellent corrosion resistance, ease of manufacture, and high thermal efficiency.Design/methodology/approachThe fluid elastic instability and vortex shedding mechanisms in a finned tube array of aluminum tubes with a base tube diameter of 19.05 mm and pitch of 34 mm were investigated. The current study considers parallel triangular finned tube arrays with fin heights of 3 mm and 6 mm with a uniform fin thickness and fin pitch. The plain tube array was tested to compare the finned tube array results. The tube vibration response was measured using an accelerometer mounted on the middle tube of the third row. In order to define the fluid elastic instability behavior of various tube arrays, the critical velocity at the instability threshold is measured. By finding the Strouhal number at the small peaks before instability, the vortex shedding behavior of the tube arrays is examined.FindingsThe results reveal that the critical velocity at instability for coarse finned tube arrays increases as the fin height increases. The effect of the tube material is evaluated by comparing the results with those previously reported for parallel triangular tube arrays made of steel. Finally, the occurrence of vortex shedding in a tube array is confirmed based on the Reynolds number and Strouhal number relationship. The instability constant K for the plain tube array of steel and aluminum material are 4.97 and 4.87, respectively.Originality/valueThis paper provides the research findings on the effect of fin height on coarse density finned tube array. This will add substantial knowledge to the literature in the field of fluid elastic instability and vortex shedding, which is needed for the safe functioning of shell and tube heat exchangers.

Vortex merging and splitting events in viscoelastic Taylor–Couette flow

Recent experiments have reported a novel transition to elasto-inertial turbulence in the Taylor–Couette flow of a dilute polymer solution. Unlike previously reported transitions, this newly discovered scenario, dubbed vortex merging and splitting (VMS) transition, occurs in the centrifugally unstable regime and the mechanisms underlying it are two-dimensional: the flow becomes chaotic due to the proliferation of events where axisymmetric vortex pairs may be either created (vortex splitting) or annihilated (vortex merging). In this paper, we present direct numerical simulations, using the finitely extensible nonlinear elastic-Peterlin (FENE-P) constitutive equation to model the polymer dynamics, which reproduce the experimental observations with great accuracy and elucidate the reasons for the onset of this surprising dynamics. Starting from the Newtonian limit and increasing progressively the fluid's elasticity, we demonstrate that the VMS dynamics is not associated with the well-known Taylor vortices, but with a steady pattern of elastically induced axisymmetric vortex pairs known as diwhirls. The amount of angular momentum carried by these elastic vortices becomes increasingly small as the fluid's elasticity increases and it eventually reaches a marginal level. When this occurs, the diwhirls become dynamically disconnected from the rest of the system and move independently from each other in the axial direction. It is shown that vortex merging and splitting events, along with local transient chaotic dynamics, result from the interactions among diwhirls, and that this complex spatio-temporal dynamics persists even at elasticity levels twice as large as those investigated experimentally.

Fluid mixing behind a branched elastic flag connecting to a cylinder in channel flow

This paper proposes a mixer with an elastic vortex generator consisting of a branched elastic flag connected to the rear of a cylinder in the mixing channel and studies the effects of the branching angle of the branching elastic flag and Reynolds number based on the cylinder diameter Red on the mixing modes of the fluid flow in the mixer. One free diffusion-induced mixing mode and two different vortex-induced mixing modes are found, and a phase diagram regarding the mixing modes of the fluid flow behind the elastic vortex generator is established. It is found that the elastic vortex generator is helpful for the transition of the mixing mode from free diffusion-induced mixing to vortex-induced mixing with the increasing branching angle. Furthermore, the rising Reynolds number results in the transition of mixing mode from free diffusion-induced mixing to vortex-induced mixing. In addition, the present work quantitatively studies the effects of the branching angle of the branched elastic flag and Reynolds number on the pressure loss and the outlet mixing efficiency of the mixer. It is found that the increase in pressure loss and the outlet mixing efficiency are 141.41% and 613.70% as the branching angle increases from 0° to 180° when Red = 90. In addition, the pressure loss and outlet mixing efficiency of the mixer with the branched elastic flag of branching angle θ = 180° can be 227.66% and 601.36% higher than those of the fluid flow around the cylinder without the elastic flag in the mixing channel when Red = 50.

Space-Dependent Symmetries and Fractons

There has been a surge of interest in effective non-Lorentzian theories of excitations with restricted mobility, known as fractons. Examples include defects in elastic materials, vortex lattices or spin liquids. In the effective theory novel coordinate-dependent symmetries emerge that shape the properties of fractons. In this review we will discuss these symmetries, cover the effective description of gapless fractons via elastic duality, and discuss their hydrodynamics.

Fluid elastic instability and vortex induced vibration parameters in finned tube arrays with P/D ratio 1.79

PurposeThis paper aims to analyze the effect of fin geometry on mechanisms of flow induced vibration. Finned tube arrays are used in a heat exchanger to increase its efficiency. Therefore, it is necessary to investigate the effect of geometric parameters of the fin fluid elastic instability and vortex shedding. In this paper, the effect of fin height, fin density and tube pitch ratio for parallel triangular tube array on fluid elastic instability and vortex shedding is analyzed.Design/methodology/approachExperimental analysis was carried out on a parallel triangular finned tube array with a pitch ratio of 1.79 subjected to water crossflow. The experimentation aims to study fluid elastic instability and vortex-induced vibration mechanism responsible for flow induced vibration for finned tube array. A fully flexible finned tube array of the copper tube was used with its base diameter of 19.05 mm and thickness of 2 mm. Over the tube surface, crimped fins of height 6 mm and the same material are welded spirally with fin density 8.47 mm and 2.82 mm. Experimental analysis was carried out on a test setup developed for the same. The results obtained for the finned tube array were compared with those for the plain tube array with the same base tube diameter.FindingsFor parallel triangular tube array of copper material, test results show that critical velocity increases with an increase in fin pitch density for low pitch tube array. Before the occurrence of instability, the rate of growth in tube vibrations is high for plain tubes compared to that with fin tubes. The results based on Owen’s hypothesis show vortex shedding before the occurrence of fluid elastic instability. The effect of fin geometry on vortex-induced forces is analyzed. For the tube array pattern understudy, the values of Conner’s constant K for coarse fin-tube and fine fin tube array are obtained, respectively, 6.14 and 7.25.Originality/valueThis paper fulfills the need for research on the effect of fin geometry on fluid elastic instability and Vortex shedding on a tube array subjected to water cross flow when the pitch ratio is less than two, i.e. with a low pitch ratio.

Analysis of vortices in viscoelastic fluid flow through confined geometries at low Reynolds numbers

Understanding the behavior of viscoelastic (VE) fluids in confined geometries is crucial for applications in biologic systems, heat transfer devices, enhanced oil recovery, and many others. Here, we perform a systematic steady-state simulation of a VE fluid at low Reynolds numbers through a channel with successive smooth contractions and expansions. We analyze the hydrodynamic performance of the fluid with particular attention to vortex patterns that develop downstream of the contractions. We show that elastic vortices form at higher contraction ratios and that there are critical Weissenberg numbers (Wic) unique to each contraction ratio where the flow shifts from non-vortical to vortical. This Wic increases with an increasing contraction length. The coexistence of elongational-, shear-, and rotational-flow is essential for vortex development and evolution. We also analyzed the effect of the Deborah number (De) on the vortex pattern in a multiple contraction system and observed that the vortex area significantly depends on the distance between the contractions. We show that there are three distinctly different regions in De, in which the flow characteristics change in successive contractions. For high De, the flow in the downstream contraction is significantly affected by the upstream contraction. Our results have implications for the use of VE fluids with various VE properties in different types of porous media.

The Role of Elasticity in the Vortex Formation in Polymeric Flow around a Sharp Bend

Fluid dynamic simulations using the FENE-P model of polymer physics are compared to those of an incompressible Newtonian fluid base case in order to understand the role of elasticity in the formation of vortices in a 90° bend narrow channel. The analysis bridges the flow behavior of a purely elastic fluid and that of a Newtonian fluid. We evaluated how four dimensionless numbers—Reynolds number (Re), Weissenberg number (Wi), viscosity ratio (β), and elasticity number (El)—affect the formation of vortices. It is shown that increasing Re and Wi, or lowering β will cause vortices to grow in size. Two phase space diagrams, β vs. El and β vs. Re, were created to show the range of values where inertial and elastic vortices form. Both diagrams have three zones. Depending on the polymer viscosity ratio and the elasticity number, the vortices form either upstream of the bend (elasticity driven) or form downstream of the bend (inertia driven), are suppressed. Our predictions are in good agreement with previous experimental and numerical works.

Effect of random pinning on nonlinear dynamics and dissipation of a vortex driven by a strong microwave current

We report numerical simulations of a trapped elastic vortex driven by a strong ac magnetic field $H(t)=H\sin\omega t$ parallel to the surface of a superconducting film. The surface resistance and the power dissipated by an oscillating vortex perpendicular to the film surface were calculated as functions of $H$ and $\omega$ for different spatial distributions, densities, and strengths of pinning centers, including bulk pinning, surface pinning, and cluster pinning. Our simulations were performed for both the Bardeen-Stephen viscous vortex drag and the Larkin-Ovchinnikov (LO) drag coefficient $\eta(v)$ decreasing with the vortex velocity $v$. The local residual surface resistance $R_i(H)$ calculated for different statistical realizations of the pinning potential exhibits strong mesoscopic fluctuations caused by local depinning jumps of a vortex segment as $H$ increases, but the global surface resistance $\bar{R}_i(H)$ obtained by averaging $R_i(H)$ over different pin configurations increases smoothly with the field amplitude at small $H$ and levels off at higher fields. For strong pinning, the LO decrease of $\eta(v)$ with $v$ can result in a nonmonotonic field dependence of $R_i(H)$ which decreases with $H$ at higher fields, but cause a runaway instability of the vortex in a thick film for weak pinning. It is shown that overheating of a single moving vortex can produce the LO-like velocity dependence of $\eta(v)$, but can mask the decrease of the surface resistance with $H$ at a higher density of trapped vortices.

Effect of material on fluid elastic instability of parallel triangular tube array subjected to water cross

The effect of tube material and structure on fluid elastic instability is examined experimentally using a parallel triangular finned tube array with a P/D ratio of 1.78. Flow-induced vibration is a common cause of failure in shell and tube heat exchangers, and it may result in substantial harm to the heat exchanger as well as significant financial loss. There are different mechanisms responsible for flow induced vibration out of which fluid elastic instability and vortex shedding are the most severe due to their sudden occurrence and high amplitude of vibration. Different parameters affect the tube vibration when the tube array is subjected to water crossflow. In this paper effect of tube material and fin are examined for fluid elastic instability. Experimentation was performed to measure demanding velocity at fluid elastic instability for plain and finned tubes for steel, aluminum and copper material. A fully flexible tube array with a cantilever end condition and with a constant pitch ratio subjected to water cross flow was considered. Testing was done with a gradually increasing water flow rate from 5 m3/hr and increases up to 75 m3/hr to obtain fluid elastic instability. The relationship between the critical velocity at fluid elastic instability and the mass damping parameter was investigated using Connor's equation. The amplitude response of vibration concerning the change in velocity shows that the addition of fins, as well as the material change, greatly affects the fluid elastic instability. A small peak before the occurrence of fluid elastic instability was observed. This is maybe due to vortex shedding which further required to be verified using the Strouhal number.

On the stability of two-dimensional nonisentropic elastic vortex sheets

<p style="text-indent:20px;">We are concerned with the stability of vortex sheet solutions for the two-dimensional nonisentropic compressible flows in elastodynamics. This is a nonlinear free boundary hyperbolic problem with characteristic discontinuities, which has extra difficulties when considering the effect of entropy. The addition of the thermal effect to the system makes the analysis of the Lopatinski<inline-formula><tex-math id="M1">\begin{document}$ \breve{{\mathrm{i}}} $\end{document}</tex-math></inline-formula> determinant extremely complicated. Our results are twofold. First, through a qualitative analysis of the roots of the Lopatinski<inline-formula><tex-math id="M2">\begin{document}$ \breve{{\mathrm{i}}} $\end{document}</tex-math></inline-formula> determinant for the linearized problem, we find that the vortex sheets are weakly stable in some supersonic and subsonic regions. Second, under the small perturbation of entropy, the nonlinear stability can be adapted from the previous two-dimensional isentropic elastic vortex sheets [<xref ref-type="bibr" rid="b6">6</xref>] by applying the Nash-Moser iteration. The two results confirm the strong elastic stabilization of the vortex sheets. In particular, our conditions for the linear stability (1) ensure that a stable supersonic regime as well as a stable subsonic one always persist for any given nonisentropic configuration, and (2) show how the stability condition changes with the thermal fluctuation. The existence of the stable subsonic bubble, a phenomenon not observed in the Euler flow, is specially due to elasticity.</p>

An immersed boundary-lattice Boltzmann method with multi relaxation time for solving flow-induced vibrations of an elastic vortex generator and its effect on heat transfer and mixing

Enhancement of heat transfer and mixing via oscillation of an elastic vortex generator (EVG) is studied using combined immersed boundary-lattice Boltzmann method with multi relaxation time. The EVG is mounted inside a microchannel and vibrates due to the forces applied by the periodic flow on it without exerting any external power input. The results are compared with those of the channel with rigid vortex generator (RVG) and the channel without vortex generator. In the RVG case, only a stationary vortex is generated behind the RVG, which contributes insignificantly to heat and mass transfer augmentation. But the flapping motion of the EVG leads to shedding of sequential vortices from the back of the EVG, which swirl the flow and sweep the thermal boundary layer as they move toward the channel outlet. This difference in the flow pattern behind the EVG and RVG cases augments the Nusselt number by 31.4% and the mixing index by 14.5% in the EVG case compared to the corresponding RVG case. Meanwhile, the pressure drop is lower in the EVG case and hence thermohydraulic performance factor elevates by 32%. The results show that the use of the EVG rather than the RVG has higher potential to improve the performance and efficiency of multifunctional heat exchangers, mixers and reactors.

Flux expulsion in niobium superconducting radio-frequency cavities of different purity and essential contributions to the flux sensitivity

Magnetic flux trapped during the cooldown of superconducting radio-frequency cavities through the transition temperature due to incomplete Meissner state is known to be a significant source of radio-frequency losses. The sensitivity of flux trapping depends on the distribution and the type of defects and impurities which pin vortices, as well as the cooldown dynamics when the cavity transitions from a normal to superconducting state. Here we present the results of measurements of the flux trapping sensitivity on 1.3 GHz elliptical cavities made from large-grain niobium with different purity for different cooldown dynamics and surface treatments. The results show that lower purity material results in a higher fraction of trapped flux and that the trapped flux sensitivity parameter $S$ is significantly affected by surface treatments but without much change in the mean free path $l$. We discuss our results within an overview of published data on the dependencies of $S(l,f)$ on $l$ and frequency $f$ using theoretical models of rf losses of elastic vortex lines driven by weak rf currents in the cases of sparse strong pinning defects and collective pinning by many weak defects. Our analysis shows how multiscale pinning mechanisms in cavities can result in a maximum in $S(l)$ similar to that observed by the FNAL and Cornell groups and how pinning characteristics can be extracted from the experimental data. Here the main contribution to $S$ come from weak pinning regions at the cavity surface, where dissipative oscillations along trapped vortices perpendicular to the surface propagate into the bulk well beyond the layer of rf screening current. However, the analysis of $S$ as a function of only the mean free path is incomplete since cavity treatments change not only $l$ but pinning characteristics as well. The effect of cavity treatments on pinning is primarily responsible for the change of $S$ without much effect on $l$ observed in this work. It also manifests itself in different magnitudes and peak positions in $S(l)$, and scatter of the $S$-data coming from the measurements on different cavities which have undergone different treatments affecting both $l$ and pinning. Optimizations of flux pinning to reduce flux sensitivity at low rf fields is discussed.

Experimental analysis of fluid elastic vibrations in rotated square finned tube arrays subjected to water cross flow

The majority of the failures of shell and tube heat exchanger tubes are reported due to the flow-induced vibration caused by shell side cross flow. Fluid elastic instability, vortex shedding, and turbulent buffeting are the excitation mechanisms responsible for the failure of the tubes. The failure occurs due to tube-to-tube impacts leading to impaction marks on the tube surface and, subsequently, leading to the failure due to fretting wear and fatigue. The present research work deals with the determination of critical velocity at instability for rotated square finned tube arrays subjected to water cross flow. In all, total six tube arrays are tested with two different pitch ratios, each with a plain tube array, a coarse finned tube array, and a fine finned tube array. Pitch ratios considered in the study are 2.1 and 2.6, while fin densities considered are coarse (4 fpi = 6.35 mm) and fine (10 fpi = 2.54 mm). The effect of array pattern, pitch ratio, and fin density on the onset of instability is studied by conducting experiments in the water cross flow. The effect of tube array pattern is studied by comparing the results of the present study with authors' published results for parallel triangular finned tube arrays in the water cross flow. The study led to the conclusion that the instability threshold is delayed for rotated square tube arrays compared to parallel triangular tube arrays. It is also observed that instability thresholds for coarse and fine finned tubes are delayed compared to plain tubes and is found to be more for finned tubes with higher fin densities.

Effects of flexibility and entanglement of sodium hyaluronate in solutions on the entry flow in micro abrupt contraction-expansion channels

In this study, the effects of polymer flexibility and entanglement on elastic instability were investigated by observing sodium hyaluronate (hyaluronic acid sodium salt, Na-HA) solution in planar abrupt contraction-expansion microchannels. As the rigidity of Na-HA depends on the ionic strength of a solvent, Na-HA was dissolved in water and phosphate buffered saline with concentrations from 0.15 wt. % to 0.45 wt. %. The rheological properties were measured and analyzed to detect the Na-HA overlap and entanglement concentrations. The flow regimes of the Na-HA solutions in several planar abrupt contraction-expansion channels were characterized in the Reynolds number and Weissenberg number space. The effects of the solvent, solution concentration, and channel geometry on the elastic corner vortex growth curve and flow regimes characterized by the Weissenberg number were analyzed. It was found that the entanglement of Na-HA in the solution is a more dominant factor affecting the flow regimes than the solution relaxation time and polymer rigidity.

Highly Stretchable and Durable Conductive Knitted Fabrics for the Skins of Soft Robots.

Stretchability and durability are imperative features for many electronic skins of soft robots, particularly those involving high deformability, cyclic gaits, confined space traverse, rough terrain navigation, and frequent human-robot interaction. This article reports on the design, fabrication, and characterization of highly stretchable and durable interconnections based on conductive knitted fabrics for the skins of soft robots. The core-spun yarn containing an ultrafine metal wire (core diameter: 50 μm) fabricated using a newly developed vortex spinning technology is employed as the conductive trace and is integrated into rib-knitted fabrics together with two types of elastic composite yarns-an elastic filament yarn and an elastic vortex core-spun yarn, respectively. Owing to the structures and properties of the yarns and fabrics, the electrical resistance of the fabrics remains stable at a maximum strain of 425% in unidirectional tensile test and a maximum average membrane strain of 300% in three-dimensional deformation. The fabrics exhibit a fatigue life greater than 1,200,000 loading cycles at 20% tensile strain and 10,000 abrasion cycles. Application of the fabrics is demonstrated by covering an origami paper-fabric composite-based soft extension actuator with the fabric. Performance of the developed conductive knitted fabrics indicates that they have potential to find application in the electronic skins of soft robots.