Critical Rotation Research Articles

Subcritical dynamo and hysteresis in a Babcock-Leighton type kinematic dynamo model

In the Sun and Sun-like stars, it is believed that cycles of the large-scale magnetic field are produced due to the existence of differential rotation and helicity in the plasma flows in their convection zones (CZs). Hence, it is expected that for each star, there is a critical dynamo number for the operation of a large-scale dynamo. As a star slows down, it is expected that the large-scale dynamo ceases to operate above a critical rotation period. In our study, we explore the possibility of the operation of the dynamo in the subcritical region using the Babcock–Leighton type kinematic dynamo model. In some parameter regimes, we find that the dynamo shows hysteresis behavior, i.e., two dynamo solutions are possible depending on the initial parameters—decaying solution if starting with weak field and strong oscillatory solution (subcritical dynamo) when starting with a strong field. However, under large fluctuations in the dynamo parameter, the subcritical dynamo mode is unstable in some parameter regimes. Therefore, our study supports the possible existence of subcritical dynamo in some stars which was previously demonstrated in a mean-field dynamo model with distributed α and MHD turbulent dynamo simulations.

The Emergence of the Problem of Changing the Environmental Law Umbrella from the Creation of the Job Creation Act

Indonesia is a country with abundant natural resources. But according to the data obtained, Indonesia is not included in the top 10 ranking countries that are rich in natural resources. Even in terms of astronomy, geology, and waters, Indonesia should be able to enter the top 5 countries with abundant natural resources. In addition, Indonesia is also included in the top 100 richest countries with a rank of 98 with a GDP/GDP (Bruno Domestic Income) - PPP (Purchase Power Parity) of 13,998 US dollars or equivalent to Rp. 195 million. According to data, the contribution of investment to the national economy reaches 30 percent of GDP. Which indirectly invests plays a major role in improving the economy and welfare of the Indonesian people. On the other hand, investment is one of the determining factors where the exploitation of the nation's natural resources, both renewable and non-renewable, is maintained or will it disappear without a trace. This is because investment, which is one of the pillars of the nation's economy, exploits natural resources on a large scale which damages the environment and ecosystem of the area. Laws on the environment and those that regulate natural resources are also allegedly affected by the emergence of the Job Creation Law in 2020, which will be discussed in this journal, and the solutions that are expected to regulate the Environmental Law and the Job Creation Act can synergize to build the Indonesian nation to be greater again.

The evolution of lithium in FGK dwarf stars

We investigate two topics regarding solar mass FGK-type stars, the lithium-rotation connection (LRC), and the existence of the ‘lithium desert’. We determine the minimum critical rotation velocity (v sin i), related with the LRC separating slow from rapid stellar rotators, to be 5 km s−1. This value also splits different stellar properties. For the first time we explore the behaviour of the LRC for some stellar associations with ages between 45 Myr and 120 Myr. This allows us to study the LRC age dependence at the beginning of the general spin-down stage for low-mass stars, which starts at ∼30–40 Myr. We find that each stellar group presents a characteristic minimum lithium depletion connected to a specific high rotation velocity and that this minimum changes with age. For instance, the minimum changes from ∼50 km s−1 to less than 20 km s−1 in 200 Myr. This desert was described as a limited region in the A(Li)-Teff map containing no stars. Using Teff from Gaia DR2 we detect 30 stars inside and/or near the same box originally defined as the lithium desert. Due to their intrinsic Teff errors some of these stars may be inside or outside the box, implying to a high probability that the box contains several stars. This leads us to consider that the lithium desert appears to be more a statistical distribution fluctuation than a real problem. We conclude that the lithium desert is rather due to a statistical distribution fluctuation than a real physical problem.

Heat transfer in an impingement cooling channel under isothermal boundaries at high rotation numbers

The jet impingement cooling technology is usually applied in the turbine blade leading edge which suffers the highest heat flux and needs priority protection. Therefore, heat transfer performances in a rotating impingement cooling channel under an isothermal boundary are experimentally studied. Both the jet-to-target surface spacing and the jet-to-jet spacing are 3 times the jet hole diameter. The effects of Coriolis force and buoyancy force on heat transfer are focused. And the maximum rotation number and buoyancy number reaches 1.63 and 8.06, respectively. The Reynolds number varies from 7119 to 21,358. Besides, numerical simulations are conducted to predict the flow field. The results show that the heat transfer in the impingement channel is determined by the radial mass-flowrate distribution of supply channel in both static and rotating states. Moreover, the rotation enhances the heat transfer at both ends but decreases the heat transfer in the middle-radius due to the stagnant zone induced by Coriolis force in the supply channel, resulting in large thermal stress on the target surface. However, the heat transfer is insensitive to the rotational effects when the rotation number exceeds 0.42. There is a critical rotation number for each location, before which the effect of buoyancy force could be neglected. Once the rotation number is greater than this value, the effect of buoyancy force increases with the wall-to-fluid temperature ratio.

The traditional approximation of rotation for rapidly rotating stars and planets

Context. The traditional approximation of rotation (TAR) is a treatment of the hydrodynamic equations of rotating and stably stratified fluids in which the action of the Coriolis acceleration along the direction of the entropy and chemical stratifications is neglected because it is weak in comparison with the buoyancy Archimedean force. This leads to the neglect of the horizontal projection of the rotation vector in the equations for the dynamics of gravito-inertial waves (GIWs). The dependent variables in those equations then become separable into radial and horizontal parts as in the non-rotating case. The TAR is built on the assumptions that the star is spherical (i.e., its centrifugal deformation is neglected) and uniformly rotating. However, it has recently been generalised to include the effects of a moderate centrifugal deformation using a perturbative approach. Aims. We study the feasibility of carrying out a new generalisation to account for the centrifugal acceleration in the case of strongly deformed uniformly and rapidly rotating stars (and planets), and to identify the validity domain of this approximation. Methods. We built a complete formalism analytically that allows the study of the dynamics of GIWs in spheroidal coordinates which take the flattening of uniformly and rapidly rotating stars into account by assuming the hierarchies of frequencies adopted within the TAR in the spherical case. Results. Using 2D stellar models, we determine the validity domain of the generalised TAR as a function of the rotation rate of the star normalised by its critical angular velocity and its pseudo-radius. Assuming the anelastic and the two-dimensional Jeffreys-Wentzel-Kramers-Brillouin approximations, we derive a generalised Laplace tidal equation for the horizontal eigenfunctions of the GIWs and their asymptotic wave periods, which can be used to probe the structure and dynamics of rotating deformed stars with asteroseismology. The generalised TAR where the centrifugal deformation of a star (or planet) is taken into account non-perturbatively allows us to identify, within the framework of 2D Evolution STEllaire en Rotation models, the validity domain of this approximation which is reduced by increasing the rate of rotation. We can affirm with a level of confidence of 90% that the TAR remains applicable in all the space domain of deformed stars rotating at a rotation rate lower than 20% of the critical rotation rate. Conclusions. A new generalisation of the TAR, which takes the centrifugal acceleration into account in a non-perturbative way, is derived. This generalisation allows us to study the detectability and the signature of the centrifugal effects on GIWs in rapidly rotating deformed stars (and planets). We found that the effects of the centrifugal acceleration in rapidly rotating early-type stars on GIWs are theoretically detectable in modern space photometry using observations from Kepler. We found also, by comparing the period spacing pattern computed with the standard and the generalised TAR, that the centrifugal acceleration affects the period spacing by increasing its values for low radial orders and by decreasing them slightly for high radial orders.

Preparing for the PICU: A Qualitative Study of Residents as They Prepare for Their First Pediatric Critical Care Rotation.

Learning critical care medicine in the pediatric intensive care unit (PICU) can be stressful. Through semistructured interviews ( n = 16), this study explored the emotions, perceptions, and motivations of pediatric medicine (PM) and emergency medicine (EM) residents, as they prepared for their first PICU rotation. Qualitative data were collected and analyzed using the grounded theory method. Three resultant themes emerged: (1) residents entered the PICU with a range of intense emotions and heightened expectations; (2) they experienced prior history of psychologically traumatic learning events (adverse learning experiences or ALEs); and (3) informed by ALEs, residents prepared for their rotation by focusing heavily on their most basic level of physiological needs and adopting a survival mindset prior to the start of the rotation. These three themes led to a substantive, or working, theory that ALE-associated events may affect how residents approach upcoming learning opportunities. Consequently, adapting a trauma-informed approach as a component of medical education may improve resident learning experiences in the PICU and beyond.

Numerical modeling of bulk flow on a pelletizing disc in different rotational regimes

Pelletizing processes are used in various industries to agglomerate fine materials. Investigation of this process is important for optimizing corresponding equipment and machines. In this article the particle-based discrete element method is used to simulate the particle behavior of a dry material on an inclined rotating disc (pelletizing disc). The process is modelled in different rotation regimes and by varying the inclination angle of the disc. Qualitative model verification is performed by a comparison of numerical simulations with experimental data. Contact data is used to analyze the flow mode of the simulated material and for detecting critical rotation speeds.

Rotating Multidimensional Quantum Droplets.

We predict a new type of two- and three-dimensional stable quantum droplets persistently rotating in broad external two-dimensional and weakly anharmonic potential. Their evolution is described by the system of the Gross-Pitaevskii equations with Lee-Huang-Yang quantum corrections. Such droplets resemble whispering-gallery modes localized in the polar direction due to nonlinear interactions and, depending on their chemical potential and rotation frequency, they appear in rich variety of shapes, ranging from nearly flat-top or strongly localized rotating wave packets, to crescentlike objects extending nearly over the entire range of polar angles. Above critical rotation frequency quantum droplets transform into vortex droplets (in two dimensions) or vortex tori (in three dimensions), whose topological charge gradually increase with the increase of the modulus of chemical potential, and therefore they belong to the family of nonlinear modes connecting fundamental and vortex quantum droplets. Rotating quantum droplets are exceptionally robust objects, stable practically in the entire range of their existence.

A computational method for rotating, multi-layer spheroids with internal jumps

Abstract We discuss the structure of differentially rotating, multilayer spheroids containing mass-density jumps and rotational discontinuities at the interfaces. The study is based upon a scale-free, numerical method. Polytropic equations of state and cylindrical rotation profiles are assumed. The Bernoulli equation and the Poisson equation for the gravitational potential are solved for each layer separately on a common computational grid. The 2-layer (core-envelope) case is first investigated in detail. We find that the core and the envelope are not, in general, homothetical in shape (cores are more than spherical than the envelope). Besides, the occurence of a mass-density jump all along the interface is prone to a rotational discontinuity (unless the polytropic indices are the same). In particular, for given rotation laws, the mass-density jump is not uniform along the interface. Tests, trends and examples (e.g. false bipolytrope, critical rotation, degenerate configurations) are given. Next, we consider the general case of systems made of ${\cal L}>2$ layers. This includes the full equation set, the virial equation, a comprehensive step-by-step procedure and two examples of tripolytropic systems. The properties observed in the 2-layer case hold for any pairs of adjacent layers. In spite of a different internal structure, two multilayer configurations can share the same mass, same axis ratio, same size and same surface velocity (which is measured through a degeneracy parameter). Applications concern the determination of the interior of planets, exoplanets, stars and compact objects.

Selection of a mother wavelet as identification pattern for the detection of cracks in shafts

Nowadays, there are many methods to detect and diagnose defects in mechanical components during operation. The newest methods that can be found in the literature are based on intelligent classification systems and evaluation of patterns to obtain a diagnosis; however, there is not any standard method to assess features. Wavelet packet transform allows to obtain interesting patterns for evaluating the condition of rotating elements. To perform this calculation, it is necessary to select a series of parameters that affect the resulting pattern. These parameters are the decomposition level and the mother wavelet function. A detailed methodology for the selection of the mother wavelet is proposed, which is the aim of this work, to obtain the most suitable patterns in the diagnostic task. This proposed methodology is applied to data obtained from a rotating shaft with a crack located at the change of section. These signals were measured at low rotation frequency (below the critical rotation frequency) and without eccentricity, where detection becomes more complex.

Stability Analysis of Composite Cylindrical Shell Containing Rotating Fluid

A semi-analytical finite element method is used to analyze the stability of composite cylindrical shells interacting with a rotating fluid inside them. A mathematical formulation of the problem of deformable structure dynamics is based on the variational principle of virtual displacements and classical shell theory. The behavior of an ideal compressible fluid is described within the framework of the potential theory. The validity of the obtained results is supported by comparing them with the known solutions. Numerical experiments were performed for two- and three-layer cross-ply shells made of boron-epoxy resin with different boundary conditions and geometrical dimensions. It is demonstrated that, for the examined configurations, an increase in the fibre angles leads to a significant increase in the critical rotation velocities of the fluid, regardless of the conditions for fixing the edges of a thin-walled structure.

Probing the temperature gradient in the core boundary layer of stars with gravito-inertial modes

Aims. We investigated the thermal and chemical structure in the near-core region of stars with a convective core by means of gravito-inertial modes. We determined the probing power of different asteroseismic observables and fitting methodologies. We focus on the case of the B-type star KIC 7760680, rotating at a quarter of its critical rotation velocity. Methods. We computed grids of 1D stellar structure and evolution models for two different prescriptions of the temperature gradient and mixing profile in the near-core region. We determined which of these prescriptions is preferred according to the prograde dipole modes detected in 4 yr Kepler photometry of KIC 7760680. We considered different sets of asteroseismic observables and compared the outcomes of the regression problem for a χ2 and a Mahalanobis distance merit function, where the latter takes into account realistic uncertainties for the theoretical predictions and the former does not. Results. Period spacings of modes with consecutive radial order offer a better diagnostic than mode periods or mode frequencies for asteroseismic modelling of stars revealing only high-order gravito-inertial modes. We find KIC 7760680 to reveal a radiative temperature gradient in models with convective boundary mixing, but less complex models without such mixing are statistically preferred for this rotating star, revealing extremely low vertical envelope mixing. Conclusions. Our results strongly suggest the use of measured individual period spacing values for modes of consecutive radial order as an asteroseismic diagnostic for stellar modelling of B-type pulsators with gravito-inertial modes.

Calibration of critical speed predictions using experimental measurements

Experimental characterization of structures provides information about the real dynamic behavior of machineries. This results in a better estimation of dynamic stress levels, used to predict lifespan, reliability and optimal operating ranges. Experimental results can also be used to improve and calibrate simulations and calculations. In particular, calculation of shaft bending frequencies is needed to define the critical rotation speed. However, natural frequency computation requires a prior knowledge on many subcomponent properties, that are not precisely known in practice. This leads to a great uncertainty on the predicted critical speed. The purpose of this study is to use experimental in-situ measurements as a means to reduce uncertainties on physical properties of the shaft line and increase confidence in the prediction of natural frequencies and critical speed. This involves ambient modal analysis, calibration techniques and statistical approaches to gain insight on the true shaft physical properties.

Constraining stellar evolution theory with asteroseismology of γ Doradus stars using deep learning

Context. The efficiency of the transport of angular momentum and chemical elements inside intermediate-mass stars lacks proper calibration, thereby introducing uncertainties on a star’s evolutionary pathway. Improvements require better estimation of stellar masses, evolutionary stages, and internal mixing properties. Aims. Our aim was to develop a neural network approach for asteroseismic modelling, and test its capacity to provide stellar masses, ages, and overshooting parameter for a sample of 37 γ Doradus stars; these parameters were previously determined from their effective temperature, surface gravity, near-core rotation frequency, and buoyancy travel time Π0. Here our goal is to perform the parameter estimation from modelling of individual periods measured for dipole modes with consecutive radial order rather than from Π0. We assess whether fitting these individual mode periods increases the capacity of the parameter estimation. Methods. We trained neural networks to predict theoretical pulsation periods of high-order gravity modes (n ∈ [15, 91]), and to predict the luminosity, effective temperature, and surface gravity for a given mass, age, overshooting parameter, diffusive envelope mixing, metallicity, and near-core rotation frequency. We applied our neural networks for Computing Pulsation Periods and Photospheric Observables (C-3PO) to our sample and compute grids of stellar pulsation models for the estimated parameters. Results. We present the near-core rotation rates (from the literature) as a function of the inferred stellar age and critical rotation rate. We assessed the rotation rates of the sample near the start of the main sequence assuming rigid rotation. Furthermore, we measured the extent of the core overshoot region and find no correlation with mass, age, or rotation. Finally, for one star in our sample, KIC 12066947, we find indications of mode coupling in the period spacing pattern which we cannot reproduce with mode trapping. Conclusions. The neural network approach developed in this study allows the derivation of stellar properties dominant for stellar evolution, such as mass, age, and extent of core-boundary mixing. It also opens a path for future estimation of mixing profiles throughout the radiative envelope, with the aim of inferring these profiles for large samples of γ Doradus stars.

The effects of process parameters on the rock-breaking efficiency of multi-nozzle jet

This paper examines the effect of multi-nozzle bit on sandstone specimens by means of numerical simulation and experiment, and the rationality is verified. Numerically, the effects of jets with different diameters and angles on rock were simulated by using LS-DYNA, and the rock-breaking shapes of jets with different angles are analyzed. Experimentally, the effects of jet pressure, impact standoff distance, erosion time and rotation speed on the crushing effect of sandstone are studied by building an experimental platform of high-pressure water jet impacting sandstone. The research shows that the rock-breaking depth decreases with the increase of the inclination angle, and the rock-breaking width increases with the increase of the inclination angle. Through comparing the rock-breaking depth and width, the reasonable structure of nozzle with diameter of 1.0 mm and inclination angle of 20°, 30° and 60° are determined. The volume of broken rock gradually increases with the increase of jet pressure, and the numerical simulation results are consistent with the experimental results. Meanwhile, the rock-breaking efficiency gradually decreases with the increase of the standoff distance, and gradually increases with the increase of the erosion time, and finally stabilizes. Through experiments, it is found that the rock-breaking depth, width and volume decrease with the increase of the rotation speed. The optimal standoff distance for the rock-breaking efficiency is 3 mm, the erosion time is no less than 60 s, and the critical rotation speed is 300 rpm.

On reducing lift and drag forces of a stationary circular cylinder using side-by-side pair of inward rotating cylinders

Effect of side-by-side pair of inwardly rotating circular cylinders on the nature of drag and lift forces of the stationary circular cylinder is numerically investigated for Reynolds number Re = 100. Non-dimensional rotation rates of cylinders in consideration are α = 0, 1, 3, and 5. Center to center gap spacing between rotating cylinders is taken as 6D (D is cylinder diameter), and the stationary cylinder is centrally placed in the gap. For the case of two side-by-side inward rotating cylinders, above a certain critical rotation rate, Chan et al. [1] reported the formation of a virtually elliptical body accompanied by vortex suppression. Drag on both the cylinders became almost zero after the formation of a virtually elliptical body. The present study aims to use the same concept of a virtually elliptical body to reduce the drag and lift forces on a stationary circular cylinder. At α = 3, complete suppression of vorticity is observed, followed by the formation of a virtual elliptical body at α = 5. With an increase in α from 0 onwards, the drag coefficient for the stationary cylinder reduces and becomes zero at α = 5.

Effects of Higher Order Interaction on Vortex Formation in Bose-Einstein Condensates

We consider theoretically the formation of vortex in rotating Bose-Einstein condensates (BECs) with higher order interaction (HOI). Our results are obtained from the twodimensional Gross-Pitaevskii equation. As the first step, for the certain number vortices, we discuss the ground state properties and show that the critical rotation frequency for HOI is smaller than those without HOI. As the increasing of HOI strength, the critical rotation frequency decreases. In addition, we verify that the Feynman rule is meet well. Moreover, we study the vortex dynamics.Numerical results indicate that the angular momentum remains almost unchanged irrespective of the HOI strength. The time taken for the nucleation of vortices pays less for strong HOI. These results suggest that the HOI is favorable to rotate the condensate, and this mechanism is useful to control the vortex number in BECs.

In-situ microstructural control of A6082 alloy to modify second phase particles by melt conditioned direct chill (MC-DC) casting process – A novel approach

• High shear melt conditioning in the sump during direct chill casting of A6082 billet. • Novelty of maintaining 2–7 °C above alloy liquidus as the sump thermal condition. • In-situ control of solidification during casting by intensive melt shearing. • Precise tailoring of as-cast microstructure to modify second phase particles. • Potential for industries to eliminate homogenisation and use recycled Al alloys. Controlling the formation of noncompact second phase particles during direct chill (DC) casting of aluminium alloys with grain refiner addition remains challenging, as it results in energy intensive homogenisation and deformation problems. In this work, we employed a novel strategy in the DC casting of A6082 alloy to produce billets with a fine-scale dispersion of second phase particles. The strategy involves maintaining 2–7 °C above alloy liquidus as a thermal condition in the sump by in-situ melt conditioning (MC) using a rotor-stator high-shear device operated at a critical rotation speed. As a result, in-situ control of solidification behaviour is achieved to precisely tailor the as-cast microstructure. The billet grain refinement is attained by MC-DC casting without the deliberate addition of chemical grain refiners. The microstructure of the MC-DC cast billet at the critical rotation speed showed a fine-scale dendritic structure with refined secondary dendrite arm spacing (SDAS). The solidification front proceeded with a shallow sump and a corresponding shorter solidification time, higher cooling rate, higher temperature gradient, and smooth solidification rate profile. The ideal fine-scale dendrites with low SDAS divided the remaining eutectic liquid into fine-scale and isolated liquid pockets, resulting in fine-scale, compact morphology, and uniform distribution of second phase particles in the as-cast microstructure. The MC-DC casting process showed the ability to increase the cast house production rate by increasing the casting speed without bleeding the billet. The present approach could be beneficial for eliminating or reducing the homogenisation practice and may also introduce significant flexibility in using recycled Al alloys in the industry.

A combination of density-based clustering method and DEM to numerically investigate the breakage of bonded pharmaceutical granules in the ball milling process

Ball milling is widely used in industry to mill particulate material. The primary purpose of this process is to attain an appropriate product size with the least possible energy consumption. The process is also extensively utilised in pharmaceuticals for the comminution of the excipients or drugs. Surprisingly, for ball mill, little is known concerning the mechanism of size reduction. Traditional prediction approaches are not deemed useful to provide significant insights into the operation or facilitate radical step changes in performance. Therefore, the discrete element method (DEM) as a computational modelling approach has been used in this paper. In previous research, DEM has been applied to simulate breaking behaviour through the impact energy of all ball collisions as the driving force for fracturing. However, the nature of pharmaceutical material fragmentation during ball milling is more complex. Suitable functional equations which link broken media and applied energy do not consider the collision of particulate media of different shapes or collisions of particulate media (such as granules) with balls and rotating mill drum. This could have a significant impact on fragmentation. Therefore, this paper aimed to investigate the fragmentation of bounded particles into DEM granules of different shape/size during the ball milling process. A systematic study was undertaken to explore the effect of milling speed on breakage behaviour. Also, in this study, a combination of a density-based clustering method and discrete element method was employed to numerically investigate the number and size of the fragments generated during the ball milling process over time. It was discovered that the collisions of the ball increased proportionally with rotation speed until reaching the critical rotation speed. Consequently, results illustrate that with an increase of rotation speed, the mill power increased correspondingly. The caratacting motion of mill material together with balls was identified as the most effective regime regarding the fragmentation, and fewer breakage events occurred for centrifugal motion. Higher quantities of the fines in each batch were produced with increased milling speed with less quantities of grain fragments. Moreover, the relationship between the number of produced fragment and milling speed at the end of the process exhibited a linear tendency.

Time-series and Phase-curve Photometry of the Episodically Active Asteroid (6478) Gault in a Quiescent State Using APO, GROWTH, P200, and ZTF

Abstract We observed the episodically active asteroid (6478) Gault in 2020 with multiple telescopes in Asia and North America and found that it is no longer active after its recent outbursts at the end of 2018 and the start of 2019. The inactivity during this apparition allowed us to measure the absolute magnitude of Gault of H r = 14.63 ± 0.02, G r = 0.21 ± 0.02 from our secular phase-curve observations. In addition, we were able to constrain Gault’s rotation period using time-series photometric lightcurves taken over 17 hr on multiple days in 2020 August, September, and October. The photometric lightcurves have a repeating ≲0.05 mag feature suggesting that (6478) Gault has a rotation period of ∼2.5 hr and may have a semispherical or top-like shape, much like the near-Earth asteroids Ryugu and Bennu. The rotation period of ∼2.5 hr is near the expected critical rotation period for an asteroid with the physical properties of (6478) Gault, suggesting that its activity observed over multiple epochs is due to surface mass shedding from its fast rotation spin-up by the Yarkovsky–O’Keefe–Radzievskii–Paddack effect.