High Rotation Rates Research Articles

Tuning the rotation rate of light-driven molecular motors.

Overcrowded alkenes are among the most promising artificial molecular motors because of their ability to undergo repetitive light-driven unidirectional rotary motion around the central C═C bond. The exceptional features of these molecules render them highly useful for a number of applications in nanotechnology. Many of these applications, however, would benefit from higher rotation rates. To this end, a new molecular motor was designed, and the isomerization processes were studied in detail. The new motor comprises a fluorene lower half and a five-membered-ring upper half; the upper-half ring is fused to a p-xylyl moiety and bears a tert-butyl group at the stereogenic center. The kinetics of the thermal isomerization was studied by low-temperature UV-vis spectroscopy as well as by transient absorption spectroscopy at room temperature. These studies revealed that the tert-butyl and p-xylyl groups in the five-membered-ring upper half may be introduced simultaneously in the molecular design to achieve an acceleration of the rotation rate of the molecular motor that is larger than the acceleration obtained by using either one of the two groups individually. Furthermore, the new molecular motor retains unidirectional rotation while showing remarkably high photostationary states.

Tool wear and its effect on microstructure and properties of friction stir processed Ti–6Al–4V

Ti–6Al–4V alloy was subjected to friction stir processing at rotation rates of 400, 800 and 1200 rpm using a polycrystalline cubic boron nitride (pcBN) tool and tool wear at different travel distances was investigated. At high rotation rates of 800 and 1200 rpm, the greatest tool wear, including mechanical and chemical wear, occurred at the initial tool plunge point. Detailed microstructural examinations on the tool plunge point at 1200 rpm by transmission electron microscopy indicated that the “onion ring” structure in the stir zone was caused by a variation in the distribution of TiB particles. Two similar but not identical spatial phase sequences around BN particles, BN–TiB2–TiB–α-Ti (N) and BN–TiB2–TiB–transformed β-Ti (N), as well as Ti2N phase were identified. The reaction mechanism between the tool and the Ti matrix was discussed. Moreover, when the tool wear reached a steady-state condition, the effect of tool wear on the microstructure and mechanical properties of the stir zone was evaluated. A fully transformed β with a Widmanstatten structure was observed at all rotation rates and the average size of prior β grains increased with the rotation rate. The tool wear led to an increment in hardness and tensile strength but a loss of ductility of the stir zone.

Thin-liquid-film flow on a topographically patterned rotating cylinder

The flow of thin liquid films on rotating surfaces is directly relevant to the coating of discrete objects. To begin understanding how surface topography influences such flows, we consider a model problem in which a thin liquid film flows over a rotating cylinder patterned with a sinusoidal surface topography. Lubrication theory is applied to develop a partial differential equation that governs the film thickness as a function of time and the angular coordinate. Static situations are considered first in order to determine the parameter regime in which the lubrication approximation is expected to be valid. When gravitational forces are relatively weak, cylinder rotation leads to the formation of droplets connected by very thin films. The number of droplets is equal to the pattern frequency at low and high rotation rates, with the droplets located at the pattern troughs at low rotation rates and the pattern crests at high rotation rates. When gravitational forces become significant, the film thickness never reaches a steady state, in contrast to the case of an unpatterned cylinder. The results of this work clearly establish that the flow of thin liquid films on rotating surfaces can be very sensitive to the presence of surface topography.

Effect of rotation on ferromagnetic porous convection with a thermal non-equilibrium model

The effect of rotation on the onset of thermal convection in a horizontal layer of ferrofluid saturated Brinkman porous medium is investigated in the presence of a uniform vertical magnetic field using a local thermal non-equilibrium (LTNE) model. A two-field model for temperature representing the solid and fluid phases separately is used for energy equation. The condition for the occurrence of stationary and oscillatory convection is obtained analytically. The stability of the system has been analyzed when the magnetic and buoyancy forces are acting together as well as in isolation and the similarities as well as differences between the two are highlighted. In contrast to the non-rotating case, it is shown that decrease in the Darcy number Da and an increase in the ratio of effective viscosity to fluid viscosity Λ is to hasten the onset of stationary convection at high rotation rates and a coupling between these two parameters is identified in destabilizing the system. Asymptotic solutions for both small and large values of scaled interphase heat transfer coefficient H t are presented and compared with those computed numerically. Besides, the influence of magnetic parameters and also parameters representing LTNE on the stability of the system is discussed and the veracity of LTNE model over the LTE model is also analyzed.

Electrochemical study of the corrosion behaviour of carbon steel in fracturing fluid

Corrosion behaviour of carbon steel (K-55) in fracturing fluid was studied with a rotation cylinder electrode, under static and rotation conditions by means of several electrochemical techniques which are as follows: open circuit potential (OCP) decay, potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS). The corrosion rate was determined by weight loss measurements. The electrode surface after a prefixed immersion time was characterised by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicated that carbon steel showed anodic dissolution behaviour that increased under rotating condition. The cathodic polarisation current density also increased with the electrode rotation due to the increased oxygen diffusion on the electrode surface. Two different oxide layers were formed: a dark, thin layer of magnetite tightly adhering to the electrode surface, characterised by localised corrosion spots, and a porous reddish layer of poorly adhering hematite (Fe2O3) and maghemite (γ-Fe2O3). Under higher rotation rate, the developed oxide layer was not so stable, owing to the shear stress induced between the solution and the specimen surface, enhancing the corrosion rate.

On the impact of swirl on the growth of coherent structures

AbstractSpatial linear stability analysis is applied to the mean flow of a turbulent swirling jet at swirl intensities below the onset of vortex breakdown. The aim of this work is to predict the dominant coherent flow structure, their driving instabilities and how they are affected by swirl. At the nozzle exit, the swirling jet promotes shear instabilities and, less unstable, centrifugal instabilities. The latter stabilize shortly downstream of the nozzle, contributing very little to the formation of coherent structures. The shear mode remains unstable throughout generating coherent structures that scale with the axial shear-layer thickness. The most amplified mode in the nearfield is a co-winding double-helical mode rotating slowly in counter-direction to the swirl. This gives rise to the formation of slowly rotating and stationary large-scale coherent structures, which explains the asymmetries in the mean flows often encountered in swirling jet experiments. The co-winding single-helical mode at high rotation rate dominates the farfield of the swirling jet in replacement of the co- and counter-winding bending modes dominating the non-swirling jet. Moreover, swirl is found to significantly affect the streamwise phase velocity of the helical modes rendering this flow as highly dispersive and insensitive to intermodal interactions, which explains the absence of vortex pairing observed in previous investigations. The stability analysis is validated through hot-wire measurements of the flow excited at a single helical mode and of the flow perturbed by a time- and space-discrete pulse. The experimental results confirm the predicted mode selection and corresponding streamwise growth rates and phase velocities.

Surface enhancement of cold work tool steels by friction stir processing with a pinless tool

The microstructure and mechanical properties of enhanced tool steel (AISI D2) surfaces produced using a friction stir welding (FSW) related procedure, called friction stir processing (FSP), are analysed in this work. The surface of the tool steel samples was processed using a WC–Co pinless tool and varying processing conditions. Microstructural analysis revealed that meanwhile the original substrate structure consisted of a heterogeneous distribution of coarse carbides in a ferritic matrix, the transformed surfaces consisted of very small carbides, homogenously distributed in a ferrite- bainite- martensite matrix. The morphology of the surfaces, as well as its mechanical properties, evaluated by hardness and tensile testing, were found to vary with increasing tool rotation speed. Surface hardness was drastically increased, relative to the initial hardness of bulk steel. This was attributed to ferrite and carbide refinement, as well as to martensite formation during solid state processing. At the highest rotation rates, tool sliding during processing deeply compromised the characteristics of the processed surfaces.

Silver nanodendrite modified graphene rotating disk electrode for nonenzymatic hydrogen peroxide detection

Although silver nanostructures have been widely used for H2O2 detection, the development of silver based nanomaterials with higher performance toward H2O2 detection is still needed. In this work, metallic silver nanodendrite (AgND) modified reduced graphene oxide (rGO) electrode with open pore structure was prepared by an electrodeposition method and used for H2O2 detection. The rotating disk electrode (RDE) was used to study the convective diffusion effect of H2O2 to the electrode. The performance of the AgND/rGO electrode toward H2O2 detection significantly depends on the rotation rate of the electrode. At the rotation rates of 0 and 100rpm, the as-fabricated electrode could not detect H2O2. At higher rotation rates, wide linear ranges of detected H2O2 concentrations with a determination coefficient R2 of over 0.99 are found to be 1–100, 0.5–100, 0.1–100, and 0.05–30mM at the electrode rotation rates of 500, 1000, 2000, and 6000rpm, respectively. The LOD values determined at a signal-to-noise ratio of ca. 3 are about 100, 50, 30, and 10μM at 500, 1000, 2000, and 6000rpm, respectively. The as-fabricated electrode can detect diluted H2O2 in milk demonstrating its potential sensing application.

Utilization of omega-3 fatty acids is improved by embedding flaxseed in a matrix of dolomitic lime hydrate (2014)

Introduction Omega-3 fatty acids are essential nutrients for humans, but American diets are often deficient in these important long-chain fats. Incorporating greater proportions of omega-3 fatty acids into beef offers a means of increasing daily consumption of essential fats, while also enhancing the perceived value of beef. In cattle, dietary polyunsaturated fats are extensively hydrogenated into saturated fats by microbes in the rumen. This effectively decreases the efficiency of transfer for fats from the animal’s diet into edible beef tissues, because the bacteria convert more than 90% of the polyunsaturated fats into saturated fats before they are absorbed into the animal’s blood stream. Preventing this saturation process in the rumen would increase the proportion of dietary omega-3 fatty acids that are available for deposition into beef, thus making the production of omega-3-enriched beef more cost-effective. We have devised a method for improving the stability of fats in the rumen that effectively increases their resistance to the hydrogenating actions of rumen microbes. Sources of polyunsaturated fats are combined with dolomitic lime hydrate, water is added, and the mixture is blended at a high rate of rotation, yielding a densified matrix with improved ruminal stability. Our objective in this study was to evaluate feedlot performance, carcass characteristics, and blood profiles of long-chain fatty acids in cattle fed diets containing varying concentrations of ground flaxseed or ground flaxseed embedded in the dolomitic lime matrix.

Relationship between polishing performance and viscoelasticity of epoxy resin polishing pads

Polishing performances of glass using epoxy resin polishing pads from the aspect of their mechanical properties are described. Material removal rates are independent to the hardness of the epoxy resin pads. The different types of epoxy resin pads are produced by varying the composition of the resin and the curing agent. Dynamic mechanical analysis (DMA) of the epoxy and conventional urethane resin polishing pads was conducted to investigate the relationship between the material removal rates and the mechanical properties of the epoxy resin polishing pads. DMA measurement indicates that the epoxy resin polishing pads showed a significant difference in the storage and loss modulus compared to the conventional urethane pads. Moreover, the epoxy resin pads showed a higher loss tangent (tan δ) than the urethane resin polishing pad. From the investigation of the relationship between the material removal rates and tan δ of the polishing pads, the strong positive correlation between the material rate and tan δ is observed. Finally, the dependence of polishing conditions on the material removal rate by the epoxy and urethane resin pads is evaluated. It is found that the difference in the material removal rate between the epoxy and urethane resin pads becomes larger under a condition of a low abrasive concentration and a high rotation rate.

Fabrication of Nonporous Layer on Surface of ALPORAS by Friction Stir Incremental Forming

Porous metals having nonporous (skin) layer at the surface have a potential to improve the mechanical properties of the porous metals. To fabricate nonporous layer on surface of porous metals, friction stir incremental forming process is applied to form surface of a commercial closed-cell type aluminum foam (ALPORAS) in this study. In the process, the cell walls near the surface of the aluminum foam are plastically deformed by a rotating tool with a high rotation rate, fabricated the nonporous layer at the surface. The nonporous layer with thinner than 0.4mm is fabricated at the surface without internal fracture of the aluminum foam under forming conditions of a tool rotation rate of 8000rpm, a tool feed rate of 60mm/min, and total forming depth of 7.0mm.

Microstructure evolution and texture development in a friction stir-processed AISI D2 tool steel

Crystallographic texture developments during friction stir processing (FSP) of AISI D2 tool were studied with respect to grain sizes in different tool rotation rates. Comparison of the grain sizes in various rotation rates confirmed that grain refinement occurred progressively in higher rotation rates by severe plastic deformation. It was found that the predominant mechanism during FSP should be dynamic recovery (DRV) happened concurrently with continuous dynamic recrystallization (CDRX) caused by particle-stimulated nucleation (PSN). The developed shear texture relates to the ideal shear textures of D1 and D2 in bcc metals. The prevalence of highly dense arrangement of close-packed planes of bcc and the lowest Taylor factor showed the lowest compressive residual stress which is responsible for better mechanical properties compared with the grain-precipitate refinement.

Plantar flexor moment arm and muscle volume predict torque-generating capacity in young men

Muscle volume is known to correlate with maximal joint torque in humans, but the role of muscle moment arm in determining maximal torque is less clear. Moderate correlations have been reported between maximal isometric knee extensor torque and knee extensor moment arm, but no such observations have been made for the ankle joint. It has been suggested that smaller muscle moment arms may enhance force generation at high rates of joint rotation, but this has not yet been observed for ankle muscles in vivo. The purpose of the present study was to correlate plantar flexor moment arm and plantar flexor muscle volume with maximal plantar flexor torque measured at different rates of plantar flexion. Magnetic resonance imaging was used to quantify the plantar flexor moment arm and muscle volume of the posterior compartment in 20 healthy young men. Maximal plantar flexor torque was measured isometrically and at three plantar flexion speeds using an isokinetic dynamometer. Plantar flexor torque was significantly correlated with muscle volume (0.222 < R(2) < 0.322) and with muscle moment arm at each speed (0.323 < R(2) < 0.494). While muscle volume was strongly correlated with body mass and stature, moment arm was not. The slope of the torque-moment arm regression line decreased as the rate of joint rotation increased, indicating that subjects with small moment arms experienced smaller reductions in torque at high speeds. The findings of this study suggest that plantar flexor moment arm is a determinant of joint strength that is at least as important as muscle size.

Direct formation of millisecond pulsars from rotationally delayed accretion-induced collapse of massive white dwarfs

Abstract Millisecond pulsars (MSPs) are believed to be old neutron stars, formed via Type Ib/c core-collapse supernovae, which have subsequently been spun up to high rotation rates via accretion from a companion star in a highly circularized low-mass X-ray binary. The recent discoveries of Galactic field binary MSPs in eccentric orbits, and mass functions compatible with that expected for helium white dwarf companions, PSR J2234+06 and PSR J1946+3417, therefore challenge this picture. Here, we present a hypothesis for producing this new class of systems, where the MSPs are formed directly from a rotationally delayed accretion-induced collapse of a super-Chandrasekhar mass white dwarf. We compute the orbital properties of the MSPs formed in such events and demonstrate that our hypothesis can reproduce the observed eccentricities, masses and orbital periods of the white dwarfs, as well as forecasting the pulsar masses and velocities. Finally, we compare this hypothesis to a triple-star scenario.

Vortex plume distribution in confined turbulent rotating convection

Vortical columns are key features of rapidly rotating turbulent Rayleigh-Bénard convection. In this work we probe the structure of the sidewall boundary layers experimentally and show how they affect the spatial vortex distribution in a cylindrical cell. The cell has a diameter-to-height aspect ratio and is operated at Rayleigh number and Prandtl number 6.4. The vortices are detected using particle image velocimetry. We find that for inverse Rossby numbers (expressing the rotation rate in a dimensionless form) the sidewall boundary layer exhibits a rotation-dependent thickness and a characteristic radial profile in the root-mean-square azimuthal velocity with two peaks rather than a single peak typical for the non-rotating case. These properties point to Stewartson-type boundary layers, which can actually cover most of the domain for rotation rates just above the transition point. A zonal ordering of vortices into two azimuthal bands at moderate rotation rates can be attributed to the sidewall boundary layer. Additionally, we present experimental confirmation of the tendency of like-signed vortices to cluster on opposite sides of the cylinder for . At higher rotation rates and away from the sidewall the vortices are nearly uniformly distributed.

Single crystal growth in the Ga–Pd system

Single crystal growth of the intermetallic compounds GaPd2, GaPd, and Ga7Pd3 is described for the first time. These phases have recently been explored as highly selective heterogeneous catalysts in the semi-hydrogenation of acetylene. Well-oriented single-crystalline surfaces are needed to perform fundamental studies on catalytic processes. The three Ga–Pd phases were grown using the Czochralski method from Ga-rich solutions. Thermodynamic properties of the Ga–Pd system determine very different growth temperatures reaching from less than 460°C for Ga7Pd3 to about 1200°C for GaPd2. Avoiding mother liquid inclusion formation proved to be the key problem that could be solved by pulling rates sometimes as low as 25µm/h and forced convection by high crystal rotation rates. In the last grown part of GaPd2 crystals decomposition into GaPd2/Ga3Pd5 lamellas occurred which can be explained either by spinodal decomposition or by nucleation caused by the GaPd2 stability region that becomes narrower with decreasing temperatures.

Evolution towards and beyond accretion-induced collapse of massive white dwarfs and formation of millisecond pulsars

Millisecond pulsars (MSPs) are generally believed to be old neutron stars (NSs), formed via type Ib/c core-collapse supernovae (SNe), which have been spun up to high rotation rates via accretion from a companion star in a low-mass X-ray binary (LMXB). In an alternative formation channel, NSs are produced via the accretion-induced collapse (AIC) of a massive white dwarf (WD) in a close binary. Here we investigate binary evolution leading to AIC and examine if NSs formed in this way can subsequently be recycled to form MSPs and, if so, how they can observationally be distinguished from pulsars formed via the standard core-collapse SN channel in terms of their masses, spins, orbital periods and space velocities. Numerical calculations with a detailed stellar evolution code were used for the first time to study the combined pre- and post-AIC evolution of close binaries. We investigated the mass transfer onto a massive WD in 240 systems with three different types of non-degenerate donor stars: main-sequence stars, red giants, and helium stars. When the WD is able to accrete sufficient mass (depending on the mass-transfer rate and the duration of the accretion phase) we assumed it collapses to form a NS and we studied the dynamical effects of this implosion on the binary orbit. Subsequently, we followed the mass-transfer epoch which resumes once the donor star refills its Roche lobe and calculated the continued LMXB evolution until the end. We demonstrate that the final properties of these MSPs are, in general, remarkably similar to those of MSPs formed via the standard core-collapse SN channel. However, the resultant MSPs created via the AIC channel preferentially form in certain orbital period intervals. Finally, we discuss the link between AIC and young NSs in globular clusters. Our calculations are also applicable to progenitor binaries of SNe Ia under certain conditions. [Abridged]

FAST STAR, SLOW STAR; OLD STAR, YOUNG STAR: SUBGIANT ROTATION AS A POPULATION AND STELLAR PHYSICS DIAGNOSTIC

Stellar rotation is a strong function of both mass and evolutionary state. Missions such as Kepler and CoRoT provide tens of thousands of rotation periods, drawn from stellar populations that contain objects at a range of masses, ages, and evolutionary states. Given a set of reasonable starting conditions and a prescription for angular momentum loss, we address the expected range of rotation periods for cool field stellar populations (∼0.4–2.0 M☉). We find that cool stars fall into three distinct regimes in rotation. Rapid rotators with surface periods less than 10 days are either young low-mass main sequence (MS) stars, or higher mass subgiants which leave the MS with high rotation rates. Intermediate rotators (10–40 days) can be either cool MS dwarfs, suitable for gyrochronology, or crossing subgiants at a range of masses. Gyrochronology relations must therefore be applied cautiously, since there is an abundant population of subgiant contaminants. The slowest rotators, at periods greater than 40 days, are lower mass subgiants undergoing envelope expansion. We identify additional diagnostic uses of rotation periods. There exists a period–age relation for subgiants distinct from the MS period–age relations. There is also a period–radius relation that can be used as a constraint on the stellar radius, particularly in the interesting case of planet host stars. The high-mass/low-mass break in the rotation distribution on the MS persists onto the subgiant branch, and has potential as a diagnostic of stellar mass. Finally, this set of theoretical predictions can be compared to extensive datasets to motivate improved modeling.

Albedo and heat transport in 3-D model simulations of the early Archean climate

Abstract. At the beginning of the Archean eon (ca. 3.8 billion years ago), the Earth's climate state was significantly different from today due to the lower solar luminosity, smaller continental fraction, higher rotation rate and, presumably, significantly larger greenhouse gas concentrations. All these aspects play a role in solutions to the "faint young Sun paradox" which must explain why the ocean surface was not fully frozen at that time. Here, we present 3-D model simulations of climate states that are consistent with early Archean boundary conditions and have different CO2 concentrations, aiming at an understanding of the fundamental characteristics of the early Archean climate system. In order to do so, we have appropriately modified an intermediate complexity climate model that couples a statistical-dynamical atmosphere model (involving parameterizations of the dynamics) to an ocean general circulation model and a thermodynamic-dynamic sea-ice model. We focus on three states: one of them is ice-free, one has the same mean surface air temperature of 288 K as today's Earth and the third one is the coldest stable state in which there is still an area with liquid surface water (i.e. the critical state at the transition to a "snowball Earth"). We find a reduction in meridional heat transport compared to today, which leads to a steeper latitudinal temperature profile and has atmospheric as well as oceanic contributions. Ocean surface velocities are largely zonal, and the strength of the atmospheric meridional circulation is significantly reduced in all three states. These aspects contribute to the observed relation between global mean temperature and albedo, which we suggest as a parameterization of the ice-albedo feedback for 1-D model simulations of the early Archean and thus the faint young Sun problem.

Three-dimensional instability of the flow around a rotating circular cylinder

AbstractThe two-dimensional stationary flow past a rotating cylinder is investigated for both two- and three-dimensional perturbations. The instability mechanisms are analysed using linear stability analysis and the complete neutral curve is presented. It is shown that the first bifurcation in the case of the rotating cylinder occurs for stationary three-dimensional perturbations, confirming recent experiments. Interestingly, the critical Reynolds number at high rotation rates is lower than that for the stationary circular cylinder. The spatial characteristics of the disturbance and a qualitative comparison with the results obtained for linear flows suggest that the stationary unstable three-dimensional mode could be of hyperbolic nature.