Low Rotation Rates Research Articles

ALMA Observations of the Protostellar Disk around the VeLLO IRAS 16253–2429

Abstract We present ALMA long-baseline observations toward the Class 0 protostar IRAS 16253–2429 (hereafter IRAS 16253) with a resolution down to 0.″12 (∼15 au). The 1.3 mm dust continuum emission has a deconvolved Gaussian size of (20 au × 8.8 au), likely tracing an inclined dusty disk. Interestingly, the position of the 1.38 mm emission is offset from that of the 0.87 mm emission along the disk minor axis. Such an offset may come from a torus-like disk with very different optical depths between these two wavelengths. Furthermore, through CO (2 − 1) and C18O (2 − 1) observations, we study rotation and infall motions in this disk–envelope system and infer the presence of a Keplerian disk with a radius of 8–32 au. This result suggests that the disk could have formed by directly evolving from a first core, because IRAS 16253 is too young to gradually grow a disk to such a size considering the low rotation rate of its envelope. In addition, we find a quadruple pattern in the CO emission at low velocity, which may originate from CO freeze out at the disk/envelope midplane. This suggests that the “cold disk” may appear in the early stage, implying a chemical evolution for the disk around this proto-brown dwarf (or very-low-mass protostar) different from that of low-mass stars.

Flow periodicity and convection modes in rotating Rayleigh–Bénard convection at low Rayleigh numbers

A three-dimensional numerical study on rotating Rayleigh–Bénard convection of water in a cylindrical container with a specific aspect ratio is performed in the present work. The simulations are carried out at four different Rayleigh numbers ( $$3\times 10^4, 5\times 10^4,7\times 10^4$$ and $$10^5$$ ) and a fixed Prandtl number ( $$Pr=7$$ ) for a range of rotation rates. Flow structures and their evolution with the addition of rotation to the system are studied in detail. Emphasis is given on the analysis of wall mode and bulk mode convection that appear at different rotation rates. The changes in heat transfer and stability of the system are also investigated. Heat transfer rate is measured by calculating the average Nusselt number at the hot wall. The results show that rotation primarily has an inhibiting effect on heat transfer. For $$Ra\le 7\times 10^4$$ the decrease in heat transfer is negligible at lower rotation rates, while it declines steeply for higher rotation rates. At $$Ra=10^5$$ a small increase in Nusselt number is obtained at low rotation rates before it drops at higher rotation rates. Numerous probes placed at different points within the flow domain are used to investigate the flow regimes and convection modes. The flow initially remains steady at low rotation rates and transforms to a periodic stage with bulk-mode-dominated convection at moderate rotation rates. Further increase in rotation gives a wall mode convection accompanied by a drastic drop in heat transfer rate before finally approaching a static conductive stage. The dual role of rotation on the stability of Rayleigh–Bénard convection is clearly identified in the present study. At moderate rotation rates, the rotation force destabilizes the system to reach a periodic flow whereas extremely large rotation rate stabilizes it.

Effect of Friction Stir Processing on the Microstructure, Damping Capacity, and Mechanical Properties of Al-Si Alloy

Friction stir processing (FSP) was conducted on an Al-Si casting alloy. The Si phase and Al grains of an Al-Si casting alloy were refined through FSP. Furthermore, FSP with high rotation rate led to the precipitation of Mg and Si atoms and the formation of Mg2Si phase in the Al-Si alloy. Precipitation improved the low-strain damping capacity but deteriorates high-strain damping capacity of the FSP sample at room temperature. At low rotation rate, the FSP sample exhibited excellent high-temperature damping capacity mainly because of its fine grain structure and the low density of its pinning points. The plastic instability of the Al-Si alloy was eliminated by FSP because of the refinement of the Si phase. The increase in the strength of each FSP sample was attributed to the increase in second-phase and boundary strengthening effects. Thus, the mechanical properties and damping capacity of Al-Si were enhanced after FSP.

Структуры течений во вращающемся цилиндре с жидкостью и свободным цилиндрическим ядром при вибрациях

In the present work, the modes of steady time-average flows are studied experimentally, which are realized in a quasi-coaxial layer at circular inertial oscillations of a free cylindrical core and its simultaneous differential rotation. The outer cylinder is formed by the wall of a hermetically sealed rotating container. The inner cylinder (core) is hollow and made of polyester film. The volume between the cylinders is filled with a low-viscosity incompressible fluid (water), into which small heavy particles are added, which serve for visualization. The container is installed horizontally and rotates rapidly, so that the core occupies a stable position on the cavity axis. To measure the rotation rate of the core, synchronization with the flicker frequency of a stroboscope is used; to study the flow structure by the distribution of visualizer particles, photo-recording is used. To excite the core oscillations, translational vibrations are applied to the container directed perpendicular to the axis of rotation. In this case, an intensive leading rotation of the core is generated. Depending on the mode of the core motion, various distribution structures of the visualizers are observed. At relatively low rates of the core rotation, structures with a spatial period characteristic of inertial waves are observed. In the case of moderate rates, one observes a regular system of toroidal vortices. At high rates, the flow becomes irregular. The system of toroidal vortices is associated with the centrifugal instability. The threshold of its occurrence is noticeably lower than in the case of the classical Couette–Taylor flow. Analysis of the results shows that the system under study has essential prerequisites for the lowering of the onset threshold of Taylor vortex flow.

Microstructural evolution of aluminum alloy during friction stir welding under different tool rotation rates and cooling conditions

The microstructural evolution during friction stir welding (FSW) has long been studied only using one single welding parameter. Conclusions were usually made based on the final microstructure observation and hence were one-sided. In this study, we used the “take-action” technique to freeze the microstructure of an Al-Mg-Si alloy during FSW, and then systematically investigated the microstructures along the material flow path under different tool rotation rates and cooling conditions. A universal characteristic of the microstructural evolution including four stages was identified, i.e. dynamic recovery (DRV), dislocation multiplication, new grain formation and grain growth. However, the dynamic recrystallization (DRX) mechanisms in FSW depended on the welding condition. For the air cooling condition, the DRX mechanisms were related to continuous DRX associated with subgrain rotation and geometric DRX at high and low rotation rates, respectively. Under the water cooling condition, we found a new DRX mechanism associated with the progressive lattice rotation resulting from the pinning of the second-phase particles. Based on the analyses of the influencing factors of grain refinement, it was clearly demonstrated that the delay of DRV and DRX was the efficient method to refine the grains during FSW. Besides, ultra-high strain rate and a short duration at high temperatures were the key factors to produce an ultrafine-grained material.

Effect of the crucible/crystal rotation on thermocapillary instability in a shallow Czochralski configuration

To understand the effects of crucible/crystal rotation on the instability of thermocapillary flow in a shallow Czochralski configuration, a series of linear stability analysis were performed by using Legendre spectral element method, and the energy analysis was applied to explore the instability mechanism. The results indicate that there are two types of instabilities with the increase of dimensionless crucible rotation rate ωc. Thermocapillary force and crucible rotation are the main causes of flow instability for low and high crucible rotation rate, respectively. In the interval 1627<ωc < 1822, with the increase of Marangoni number, three turning points were observed, indicating three transitions between two-dimensional axisymmetric steady flow and three-dimensional oscillatory flow due to the interaction between thermocapillary force and crucible rotation. For crystal rotation, two oscillatory modes for flow instability were observed. When the dimensionless crystal rotation rate is small (ωs < 3600), the oscillatory flow rotates in the opposite direction of the crystal rotation, but in the same direction in the case of large crystal rotation rate (ωs > 3600), and the thermocapillary force is the primary inducement of flow instability.

Effect of particle shape on fluid statistics and particle dynamics in turbulent pipe flow

.Anisotropic particles are present in many natural and industrial flows. Here we perform direct numerical simulation (DNS) of turbulent pipe flows with dispersed finite-size prolate spheroids simulated by means of the lattice Boltzmann method (LBM). We consider three different particle shapes: spheroidal (aspect ratio 2 and 3) and spherical. These three simulations are complemented with a reference simulation of a single-phase flow. For the sake of comparison, all simulations, laden or unladen have the same energy input. The flow geometry used is a straight pipe with length eight times its radius where the fluid is randomly seeded with 256 finite-size particles. The volume fraction of particles in the flow has been kept fixed at 0.48% by varying the major and minor axis of each particle such that their volume remains the same. We studied the effect of different particle shapes on particle dynamics and orientation, as well as on the flow modulation. We show that the local accumulation of spheres close to the wall decreases for spheroids with increasing aspect ratio. These spheroidal particles rotate slower than spheres near to the wall and tend to stay with their major axes aligned to the flow streamwise direction. Despite the lower rotation rates, a higher intermittency in the rotational rates was observed for spheroids and this increase at increasing the aspect ratio. The drag reduction observed for particles with higher aspect ratio have also been investigated using the one-dimensional energy and dissipation spectra. These results point to the relevance of particle shapes on their dynamics and their influence on the turbulent flow.Graphical abstract

Realising equal strength welding to parent metal in precipitation-hardened Al–Mg–Si alloy via low heat input friction stir welding

ABSTRACTTwo millimetres thick Al–Mg–Si (6061Al-T6) alloy plates were friction stir welded at various welding conditions. Under a low rotation rate of 400 rev min–1 with rapid water cooling, the softening zone in the joint disappeared and a nanostructure with an average grain size of 80 nm was obtained in the stir zone (SZ). Therefore, a weld with equal strength to the parent metal (PM) was successfully achieved with the fracture occurring in the PM. Further, the average microhardness and ultimate tensile strength (UTS) of the SZs increased with the decreasing rotation rate and increasing cooling speed. The average microhardness and UTS of the SZ with nanostructure reached up to 134 HV and 505 MPa, respectively; though the initial strengthened precipitates disappeared. This work provides an effective strategy of achieving high property joints and enhancing the mechanical properties of precipitation-hardened Al alloys.

Large load friction stir welding of Mg–6Al–0.4Mn–2Ca magnesium alloy

ABSTRACTFriction stir welded (FSW) magnesium alloys usually exhibit a lower yield strength and elongation compared with base materials. In this study, large load FSW associated with an extremely low welding speed and rotation rate were applied to a non-combustive Mg–6Al–0.4Mn–2Ca magnesium alloy to modify the microstructure and texture in the weld zone and improve the mechanical properties of the joint. The twin structure in the stir zone provided adequate barriers for dislocation motion for strengthening and created more local sites for nucleating and accommodating dislocations, thereby elevating ductility and strain hardening in the transverse tensile test. The results showed that the yield strength and elongation of the joint were enhanced to 98% and 126% of the base material levels, respectively.

An experimental study of the motion of a light sphere in a rotating viscous fluid

We present the results of an experimental investigation of the motion of a light, solid sphere in a horizontal rotating cylinder filled with viscous fluid. At high rotation rates, the sphere sits near the axis of the cylinder. At lower rotation rates, a set of off-axis fixed points are observed for a range of sphere radii. The locations of these fixed points are in quantitative agreement with the predictions of a model based on available theory. The fixed points are observed to become unstable to periodic orbits below a critical Reynolds number $Re_{c}$. The radius of the observed orbits increases with Reynolds number more slowly than a typical Hopf bifurcation, in this case, growing as $1/Re^{2}$.

Ultra-robust triboelectric nanogenerator for harvesting rotary mechanical energy

Triboelectric nanogenerators (TENGs) for harvesting rotary mechanical energy are mostly based on in-plane sliding or free-standing mode. However, the relative displacement between two contacting triboelectric layers causes abrasion, which lowers the output power and reduces service life. Therefore, it is important to develop a method to minimize abrasion when harvesting rotary mechanical energy. Here, we report a scale-like structured TENG (SL-TENG), in which two triboelectric layers work under a contact-separation mode to avoid in-plane relative sliding in order to minimize abrasion. As a result, the SL-TENG exhibits outstanding robustness. For example, the output voltage of the SL-TENG does not exhibit any measurable decay although this output has been continuously generated through more than a million cycles. Moreover, at a very low rotation rate of 120 rpm, the SL-TENG can generate a maximum short-circuit current of 78 μA, delivering an instantaneous power density of 2.54 W/m2 to an external load. In relation to this, a Li-ion battery was charged using the SL-TENG. After a 30-min charging time, the battery achieved a discharge capacity of 0.1 mAh. Through a power management circuit integrated into the SL-TENG, a continuous direct current (DC) of 5 V is outputted, providing sufficient DC power for driving a radio-frequency wireless sensor and other conventional electronics.

Effect of Processing Parameters on Plastic Flow and Defect Formation in Friction-Stir-Welded Aluminum Alloy

The effect of processing parameters on material flow and defect formation during friction stir welding (FSW) was investigated on 6.0-mm-thick 2014Al-T6 rolled plates with an artificially thickened oxide layer on the butt surface as the marker material. It was found that the “S” line in the stir zone (SZ) rotated with the pin and stayed on the retreating side (RS) and advancing side (AS) at low and high heat inputs, respectively. When the tool rotation rate was extremely low, the oxide layer under the pin moved to the RS first and then to the AS perpendicular to the welding direction, rather than rotating with the pin. The material flow was driven by the shear stresses produced by the forces at the pin–workpiece interface. With increases of the rotation rate, the depth of the shoulder-affected zone (SAZ) first decreased and then increased due to the decreasing shoulder friction force and increasing heat input. Insufficient material flow appeared in the whole of the SZ at low rotation rates and in the bottom of the SZ at high rotation rates, resulting in the formation of the “S” line. The extremely inadequate material flow is the reason for the lack of penetration and the kissing bonds in the bottom of the SZ at extremely low and low rotation rates, respectively.

Three-dimensional surfactant-covered flows of thin liquid films on rotating cylinders

The coating of discrete objects is an important but poorly understood step in the manufacturing of a broad variety of products. An important model problem is the flow of a thin liquid film on a rotating cylinder, where instabilities can arise and compromise coating uniformity. In this work, we use lubrication theory and flow visualization experiments to study the influence of surfactant on these flows. Two coupled evolution equations describing the variation of film thickness and concentration of insoluble surfactant as a function of time, the angular coordinate and the axial coordinate are solved numerically. The results show that surface-tension forces arising from both axial and angular variations in the angular curvature drive flows in the axial direction that tend to smooth out free-surface perturbations and lead to a stable speed window in which axial perturbations do not grow. The presence of surfactant leads to Marangoni stresses that can cause the stable speed window to disappear by driving flow that opposes the stabilizing flow. In addition, Marangoni stresses tend to reduce the spacing between droplets that form at low rotation rates, and reduce the growth rate of rings that form at high rotation rates. Flow visualization experiments yield observations that are qualitatively consistent with predictions from linear stability analysis and the simulation results. The visualizations also indicate that surfactants tend to suppress dripping, slow the development of free-surface perturbations, and reduce the shifting and merging of rings and droplets, allowing more time for solidifying coatings in practical applications.

Enhanced Stellar Activity for Slow Antisolar Differential Rotation?

Abstract High-precision photometry of solar-like members of the open cluster M67 with Kepler/K2 data has recently revealed enhanced activity for stars with a large Rossby number, which is the ratio of rotation period to the convective turnover time. Contrary to the well established behavior for shorter rotation periods and smaller Rossby numbers, the chromospheric activity of the more slowly rotating stars of M67 was found to increase with increasing Rossby number. Such behavior has never been reported before, although it was theoretically predicted to emerge as a consequence of antisolar differential rotation (DR) for stars with Rossby numbers larger than that of the Sun, because in those models the absolute value of the DR was found to exceed that for solar-like DR. Using gyrochronological relations and an approximate age of 4 Gyr for the members of M67, we compare with computed rotation rates using just the B − V color. The resulting rotation–activity relation is found to be compatible with that obtained by employing the measured rotation rate. This provides additional support for the unconventional enhancement of activity at comparatively low rotation rates and the possible presence of antisolar differential rotation.

Mechanical properties’ modification of large load friction stir welded AZ31B Mg alloy joint

Friction stir welded (FSW) Mg alloys usually exhibit an undesirable combination of strength and elongation due to its strong texture develops in the weld. Thus, large load FSW associated with an extremely low welding speed and rotation rate were applied to an AZ31B Mg alloy to modify the microstructure, the texture and the mechanical properties of the joint. The twin structure in the weld provided adequate barriers for dislocation motion for strengthening and created more local sites for nucleating and accommodating dislocations, thereby elevating ductility and strain hardening of the weld. This work also provided a simple and effective method to enhance the strength of an FSW Mg joint without ductility loss.

Deployable structure as architectural active structure on sports building in Bandung

Buildings not only can be resilient from environmental changing by passive design, but also have ability to adapt the environment by active system. By using deployable structure, this system can be provided by changing multiple form based on its purpose to cover the building. To provide these conditions, deployable structure need to be explored and analyzed to develop compatible structure in the building. Case of study in this research is Pajajaran sports building in Bandung. The research method is exploration of form and movement based on limited variables, there are activities and weather changing/climate condition. These variables formulated into two conditions, open and closed building. Open building state in sports building have requirement that either natural or artificial lighting should not cause glare in field area. So, natural lighting was simulated by using Ecotect Analysis to find solar path movement and shadow angle. The result of the solar path simulation shows that daily solar movement must be responded directly because high rotation rate and regular changes, while the monthly sun path does not require significant movement due to low rotation rate and insignificant changes. This study will show design process of deployable structure into sports building.

Small-scale anisotropy induced by spectral forcing and by rotation in non-helical and helical turbulence

ABSTRACTWe study the effect of large-scale spectral forcing on the scale-dependent anisotropy of the velocity field in direct numerical simulations of homogeneous turbulence. ABC-type forcing and helical or non-helical Euler-type forcing are considered. We propose a scale-dependent characterisation of anisotropy based on a modal decomposition of the two-point velocity tensor spectrum. This produces direction-dependent spectra of energy, helicity and polarisation. We examine the conditions that allow anisotropy to develop in the small scales due to forcing and we show that the theoretically expected isotropy is not exactly obtained, even in the smallest scales, for ABC and helical Euler forcings. When adding rotation, the anisotropy level in ABC-forced simulations is similar to that of lower Rossby number Euler-forced runs. Moreover, even at low rotation rate, the natural anisotropy induced by the Coriolis force is visible at all scales, and two distinct wavenumber ranges appear from our fine-grained characterisation, not separated by the Zeman scale but by a scale where rotation and dissipation are balanced.

Effects of friction stir processing and minor Sc addition on the microstructure, mechanical properties, and damping capacity of 7055 Al alloy

This study investigated the effects of friction stir processing (FSP) and addition of 0.25% (mass ratio) Sc on the microstructure, mechanical properties, and damping capacity of AA 7055 alloy. Microstructure analyses revealed that a low tool rotation rate of 300rpm effectively inhibited the grain coarsening in the recrystallization of the alloy during FSP. Sc addition prevented the coarsening of grains and precipitated η phase and the annihilation of dislocations in the FSP AA 7055 alloy. After being subjected to FSP at 300rpm, the AA 7055 alloy containing Sc exhibited good mechanical and damping properties, which were mainly attributed to the fine recrystallized grain structure, fine precipitated η phase, and large number of nano-sized Al3(Sc, Zr) particles located at the grain boundaries and dislocations. This work provides an effective strategy for preparing commercial Al alloys with excellent damping capacity and mechanical properties.

Can plume-induced internal gravity waves regulate the core rotation of subgiant stars?

Context.The seismic data provided by the space-borne missions CoRoT andKeplerenabled us to probe the internal rotation of thousands of evolved low-mass stars. Subsequently, several studies showed that current stellar evolution codes are unable to reproduce the low core rotation rates observed in these stars. These results indicate that an additional angular momentum transport process is necessary to counteract the spin up due to the core contraction during the post-main sequence evolution. For several candidates, the transport induced by internal gravity waves (IGW) could play a non-negligible role.Aims.We aim to investigate the effect of IGW generated by penetrative convection on the internal rotation of low-mass stars from the subgiant branch to the beginning of the red giant branch.Methods.A semi-analytical excitation model was used to estimate the angular momentum wave flux. The characteristic timescale associated with the angular momentum transport by IGW was computed and compared to the contraction timescale throughout the radiative region of stellar models at different evolutionary stages.Results.We show that IGW can efficiently counteract the contraction-driven spin up of the core of subgiant stars if the amplitude of the radial-differential rotation (between the center of the star and the top of the radiative zone) is higher than a threshold value. This threshold depends on the evolutionary stage and is comparable to the differential rotation rates inferred for a sample of subgiant stars observed by the satelliteKepler. Such an agreement can therefore be interpreted as the consequence of a regulation mechanism driven by IGW. This result is obtained under the assumption of a smooth rotation profile in the radiative region and holds true even if a wide range of values is considered for the parameters of the generation model. In contrast, on the red giant branch, we find that IGW remain insufficient, on their own, to explain the observations because of an excessive radiative damping.Conclusions.IGW generated by penetrative convection are able to efficiently extract angular momentum from the core of stars on the subgiant branch and accordingly have to be taken into account. Moreover, agreements with the observations reinforce the idea that their effect is essential to regulate the amplitude of the radial-differential rotation in subgiant stars. On the red giant branch, another transport mechanism must likely be invoked.

Algebraic disturbances and their consequences in rotating channel flow transition

How rotation affects transition to turbulence of pressure-driven flow in a channel is investigated. Even at extremely low rotation rates, optimal perturbations are asymmetric about the centerline. Subcritical transition features are significant even in regimes of exponentially growing instabilities.