Spin-up Process Research Articles

The rotational disruption of porous dust aggregates from ab initio kinematic calculations

The size of dust grains in the interstellar medium follows a distribution where most of the dust mass is made up of smaller grains. However, the redistribution from larger grains towards smaller sizes, especially by means of rotational disruption, is still poorly understood. We aim to study the dynamics of porous grain aggregates undergoing an accelerated rotation, namely, a spin-up process that rapidly increases the angular velocity of the aggregate. In particular, we aim to determine the deformation of the grains and the maximal angular velocity up to the rotational disruption event by caused by centrifugal forces. We precalculated the porous grain aggregate by means of ballistic aggregation analogous to the interstellar dust as input for subsequent numerical simulations. We performed three-dimensional (3D) N-body simulations, mimicking the radiative torque spin-up process up to the point where the grain aggregates become rotationally disrupted. Our simulations results are in agreement with theoretical models predicting a characteristic angular velocity, $ disr $, on the order of $ rad\ $, where grains become rotationally disrupted. In contrast to theoretical predictions, we show that for large porous grain aggregates ($ nm $), the $ disr $ values do not strictly decline. Instead, they reach a lower asymptotic value. Hence, such grains can withstand an accelerated rotation more efficiently up to a factor of 10 because the displacement of mass by centrifugal forces and the subsequent mechanical deformation supports the buildup of new connections within the aggregate. Furthermore, we report that the rapid rotation of grains deforms an ensemble with initially 50:50 prolate and oblate shapes, respectively, preferentially into oblate shapes. Finally, we present a best-fit formula to predict the average rotational disruption of an ensemble of porous dust aggregates dependent on the internal grain structure, total number of monomers, and applied material properties.

Finite orbit width effects on turbulent transport of ion parallel momentum

Abstract A kinetic model for ion turbulent parallel momentum transport is developed with finite orbit width effects for Tokamak plasmas. It is shown that the curvature and gradient drifts of ions can introduce pressure perturbations into the transport equation of ion parallel momentum, which leads to a new source term. And the source term can be understood as a Coriolis force and can play a key role in the toroidal symmetry breaking during the spontaneous spin-up process.

Sliding Mode Control Strategy of Spinning Electrodynamic Tether Formation During Its Spin-Up Process

Sliding Mode Control Strategy of Spinning Electrodynamic Tether Formation During Its Spin-Up Process

Mass measurements and 3D orbital geometry of PSR J1933–6211

PSR J1933−6211 is a pulsar with a spin period of 3.5 ms in a 12.8 d nearly circular orbit with a white dwarf companion. Its high proper motion and low dispersion measure result in such significant interstellar scintillation that detections with a high signal-to-noise ratio have required long observing durations or fortuitous timing. In this work, we turn to the sensitive MeerKAT telescope, and combined with historic Parkes data, are able to leverage the kinematic and relativistic effects of PSR J1933−6211 to constrain its 3D orbital geometry and the component masses. We obtain a precise proper motion magnitude of 12.42 (3) mas yr−1 and a parallax of 1.0 (3) mas, and we also measure their effects as secular changes in the Keplerian parameters of the orbit: a variation in the orbital period of 7 (1)×10−13 s s−1 and a change in the projected semi-major axis of 1.60 (5)×10−14 s s−1. A self-consistent analysis of all kinematic and relativistic effects yields a distance to the pulsar of $ 1.6^{+0.2}_{-0.3} $ kpc, an orbital inclination, i = 55 (1) deg, and a longitude of the ascending node, $ \Omega = 255^{+8}_{-14} $ deg. The probability densities for Ω and i and their symmetric counterparts, 180 − i and 360 − Ω, are seen to depend on the chosen fiducial orbit used to measure the time of passage of periastron (T0). We investigate this unexpected dependence and rule out software-related causes using simulations. Nevertheless, we constrain the masses of the pulsar and its companion to be $ 1.4^{+0.3}_{-0.2}\,M_\odot $ and 0.43 (5) M⊙, respectively. These results strongly disfavour a helium-dominated composition for the white dwarf companion. The similarity in the spin, orbital parameters, and companion masses of PSRs J1933−6211 and J1614−2230 suggests that these systems underwent case A Roche-lobe overflow, an extended evolutionary process that occurs while the companion star is still on the main sequence. However, PSR J1933−6211 has not accreted significant matter: its mass is still at ∼1.4 M⊙. This highlights the low accretion efficiency of the spin-up process and suggests that observed neutron star masses are mostly a result of supernova physics, with minimum influence of subsequent binary evolution.

Long-term trends of regolith movement on the surface of small bodies

This paper studies the long-term migration of disturbed regolith materials on the surface of Solar System small bodies from the viewpoint of nonlinear dynamics. We propose an approximation model for secular mass movement, which combines the complex topography and irregular gravitational field. Choosing asteroid 101955 Bennu as a representative, the global change of the dynamical environment is examined, which presents a division of the creeping-sliding-shedding regions for a spun-up asteroid. In the creeping region, the dynamical equation of disturbed regolith grains is established based on the assumption of “trigger-slide” motion mode. The equilibrium points, local manifolds and large-scale trajectories of the system are calculated to clarify the dynamical characteristics of long-term regolith movement. Generally, we find that for a low spin rate, the surface regolith grains flow toward the middle latitudes from the polar/equatorial regions, which is dominated by the gradient of the geopotential. While spun up to a high rate, regolith grains tend to migrate toward the equator, which happens in parallel with a topological shift of the local equilibrium points at low latitudes. From a long-term perspective, we find the equilibrium points dominate the global trends of regolith movements. Using the methodology developed in this paper, we give a prospect or retrospect to the secular motion of regolith materials during the spin-up process, and the results reveal a significant regulatory role of the equilibrium points. Through a detailed look at the dynamical scheme under different spin rates, we achieve a macro forecast of the global trends of regolith motion during the spin-up process, which explains the global geologic evolution driven by the long-term movements of regolith materials.

On the spinup and spreadout of a Cartesian gravity current on a slope in a rotating system

Ocean gravity currents flow along the inclined ocean floor for long times compared to the planet's rotation period. Their shape and motion is governed by the gravity buoyancy, Coriolis acceleration and friction-induced Ekman-layer spinup circulation. In order to understand this process, we consider the flow of a dense-fluid Boussinesq gravity current of fixed volume over an inclined bottom in a rotating system, in the framework of Cartesian 2.5-dimensional geometry (no dependency on the lateral direction $y$ , but with a non-trivial $y$ -component velocity $v$ due to Coriolis coupling with the main $u$ along the bottom $x$ ). After release from rest in a lock (co-rotating, with two gates creating propagation in ${\pm }x$ -directions), the current forms a quasi-steady geostrophic ‘vein’ of parabolic height profile with a significant lateral velocity $v$ . Subsequently, a spinup process, driven by the Ekman layers on the bottom and interface, appears and prevails for many revolutions, during which $v$ decays and the shape of the interface changes dramatically. We investigate the spinup motion, using an approximate model, for the case of large Rossby number, small Ekman number and small slope $\gamma$ (relevant to oceanic currents). We show that the initial shape of the natural geostrophic vein can be calculated rigorously (not an arbitrary parabola), and the initial lateral velocity $v(x,t=0)$ is counter-rotation about a fixed point (pivot) $x_{\rm \pi}$ at which $v(x_{\rm \pi},t) =0$ (at the beginning and during spinup). This point is placed excentrically, in the upper part, and this excentre, $\propto \gamma$ , plays a significant role in the process. The spinup in a rigid container is developed as the prototype process; an essential component is the edge (outer wall) where the flux of the Ekman layer is arrested (and then returned to the centre via the inviscid core). While the upper part of the vein adopts this spinup pattern, the lower part (most of the vein, $x< x_{\rm \pi}$ ) develops a leak (drainage) at the edge that (a) modifies the spinup of the vein, and (b) generates a thin tail extension downslope. The tail consists of two merged non-divergent Ekman layers, which chokes the drainage flow rate. The present model provides clear-cut insights and some quantitative predictions of the major spinup stage by analytical algebraic solutions. A comparison with a previously published simple model (Wirth, Ocean Dyn., vol. 59, 2009, pp. 551–563) is presented. We also discuss briefly stability of the initial vein.

Research on degradation prediction of rolling bearing based on adaptive multi-GA-BP

Rolling bearings are widely used in industrial equipment. It is of great significance to study the degradation trend of rolling bearings. In this paper, an Adaptive Multi-population Genetic Algorithm (AMGA) is proposed. Firstly, Kernel Principal Component Analysis (KPCA) method is used to fuse the vibration signal in both time domain and frequency domain, which uses kernel to map the sample space to higher dimensional space and uses the higher dimensional space for linear dimensionality reduction. It can effectively reduce the dimension of nonlinear correlation variables and obtain the trend signal representing the Remaining Useful Life (RUL) characteristics of rolling bearing. Moreover, AMGA is proposed to optimize the number of neurons, initial weight, and initial threshold of the Back Propagation (BP) neural network prediction model. AMGA applies chaos algorithm to Genetic Algorithm (GA) to improve the diversity of the initial population. Meanwhile, the communication frequency between different populations is controlled by judging the similarity of the optimal solution among different populations, so as to effectively jump out of the local optimum and obtain the global optimal solution. Finally, the whole life data of the spin-up process of the rolling bearing from University of Cincinnati is taken as an example to analyze the performance of the algorithm. Compared with the traditional BP, the R2-score performance and the MAPE performance of KPCA-AMGA-BP are improved by 0.297 and 2.46% respectively. Furthermore, compared with the optimized BP, this method obtains the improved R2-score performance and the MAPE performance by 0.218 and 0.46%.

Global evaluation of the Ecosystem Demography model (ED v3.0)

Abstract. Terrestrial ecosystems play a critical role in the global carbon cycle but have highly uncertain future dynamics. Ecosystem modeling that includes the scaling up of underlying mechanistic ecological processes has the potential to improve the accuracy of future projections while retaining key process-level detail. Over the past two decades, multiple modeling advances have been made to meet this challenge, such as the Ecosystem Demography (ED) model and its derivatives, including ED2 and FATES. Here, we present the global evaluation of the Ecosystem Demography model (ED v3.0), which, like its predecessors, features the formal scaling of physiological processes for individual-based vegetation dynamics to ecosystem scales, together with integrated submodules of soil biogeochemistry and soil hydrology, while retaining explicit tracking of vegetation 3-D structure. This new model version builds on previous versions and provides the first global calibration and evaluation, global tracking of the effects of climate and land-use change on vegetation 3-D structure, spin-up process and input datasets, as well as numerous other advances. Model evaluation was performed with respect to a set of important benchmarking datasets, and model estimates were within observational constraints for multiple key variables, including (i) global patterns of dominant plant functional types (broadleaf vs. evergreen), (ii) the spatial distribution, seasonal cycle, and interannual trends for global gross primary production (GPP), (iii) the global interannual variability of net biome production (NBP) and (iv) global patterns of vertical structure, including leaf area and canopy height. With this global model version, it is now possible to simulate vegetation dynamics from local to global scales and from seconds to centuries with a consistent mechanistic modeling framework amendable to data from multiple traditional and new remote sensing sources, including lidar.

Habitat Bennu: Design Concepts for Spinning Habitats Constructed From Rubble Pile Near-Earth Asteroids

The authors explore the possibility that near-earth, rubble pile asteroids might be used as habitats for human settlement by increasing their rotation to produce spin gravity. Using previously published scaling by Maindl et al. and studies of asteroid populations, it is shown that there is no class of hollowed body that would survive the spin-up process on its own without additional reinforcement. Large solid-rock asteroids (diameterD> 10 km) would not have the tensile strength to withstand the required rotation rates and would fracture and break apart. Smaller asteroids, being ‘rubble piles’, have little tensile strength and would quickly disperse. The possibility of containing the asteroid mass using higher-strength materials like carbon nanofiber is instead considered. It is found that a moderate tensile strength container can maintain the integrity of a large spinning cylinder composed of dispersed asteroid regolith. The research extends the range of possible asteroid habitat candidates, since it may become feasible to construct habitats from the more numerous smaller bodies, including NEAs (Near Earth Asteroids). The required tensile strength of the container material scales with habitat radius and thickness and is ∼ 200 MPa for a starting asteroid body of radius 300 m that is spun up to provide 0.3 g⊕while increasing its radius to 3 km and maintaining a rubble and regolith shield thickness of 2 m to protect against cosmic rays. Ambient solar power can be harvested to aid in spin-up and material processing.

Sand creep motion in slow spin-up experiment: An analog of regolith migration on asteroids.

We studied the creep motion of granular materials in a gradient potential field that is created using a slow spin-up experiment device. Natural sand confined in the acrylic box is spun up by a controlled turntable and the surface flows are captured using video-based measurements. Various spin-up accelerations were considered to understand the responses of creep motion on different accelerating paths. Convergent behaviors in the morphological change of sand surface were observed in the final steady state. To quantify the quasistatic spin-up process, we examined the net flux and the surface slope as a function of the spin rate and offset from the rotation axis. The creep motion of sand demonstrated behaviors similar to the regolith migration in numeric simulations. We have noticed the sand surface approaches criticality as the spin-up proceeding, consistent with the observation that top-shaped asteroids near limiting spin rate take on critical shapes. Comparisons to large-scale numeric simulations and analytical solutions reveal underlying similarities between the experiments and the million-year evolution of asteroid regolith under Yarkovsky-O'Keefe-Radzievskii-Paddack acceleration, which raises the possibility of studying asteroid surface processes in laboratory analog experiments.

Timing and spectral variability of the high-mass X-ray pulsar GX 301−2 over orbital phases observed by Insight-HXMT

ABSTRACT We report the orbital X-ray variability of the high-mass X-ray binary (HMXB) GX 301−2. GX 301−2 underwent a spin-up process in 2018–2020 with the period evolving from ∼685–670 s. The energy-resolved pulse profiles of the pulsar at 1–60 keV varied from single-peaked and sinusoidal shapes to multipeaked ones across different orbital phases. Pulse fractions evolving over the orbit had negative correlations with the X-ray flux. The broad-band X-ray energy spectrum of the pulsar can be described with a partially covered negative–positive cut-off power-law continuum model. Near the periastron passage of the pulsar we found strong variation in the additional column density ($N_{\mathrm{H}_{2}}$), which correlated with variation of the flux. Curves of growth for both Fe Kα and Fe Kβ lines were plotted to investigate the distribution of matter around the neutron star. We also found evidence for two cyclotron absorption lines in the phase-averaged spectra in GX 301−2, with one line of 30–42 keV and the other line varying over 48–56 keV. The centroid energies of both lines show a similar relationship with X-ray luminosity: positive correlation in the lower luminosity range, and a negative relation above a critical luminosity of $10^{37}\, \rm erg\, s^{-1}$. We estimate the surface magnetic field of the neutron star in GX 301−2 to be ∼(0.5–2) × 1013 G. The two cyclotron line energies have a nearly fixed ratio of ∼1.63 while having a low strength ratio (∼0.05), suggesting that these two features may actually be one line.

Studying magnetic fields of ultraluminous X-ray pulsars using different accretion torques

The magnetic field of ultraluminous X-ray (ULX) pulsars is the key parameter to understand the nature and accretion physics. However, the typical magnetic field values in these ULX pulsars are still under debate. We used six different torque models to study the magnetic fields of ULX pulsars, to see how derived magnetic fields change with different models, and to determine which models are more suitable for ULX pulsars. We took the currently available period, period derivative, and flux data of 7 confirmed ULX pulsars, M82 X-2, ULX NGC 7793 P13, ULX NGC 5907, NGC 300 ULX1, NGC 1313 X-2, M51 ULX-7, Swift J0243.6+6124, plus one potential ULX pulsars, SMC X-3. The magnetic fields of these ULX pulsars were constrained from two physical conditions: the spin-up process and near equilibrium. We checked possible dependence of the magnetic field estimations on the different torque models. The calculations suggest the accretion torque models by Ghosh and Lamb (1979), Wang (1995), Kluźniak and Rappaport (2007) and Campbell (2012) are more likely to support magnetar models for ULX pulsars, while Lovelace et al. (1995)'s model generally predicts the magnetic field in normal neutron stars. Implications of our results combined with other independent methods are also discussed, which will help us to understand the nature and rotational behavior of these ULX pulsars.

Understanding the Role of Mean and Eddy Momentum Transport in the Rapid Intensification of Hurricane Irma (2017) and Hurricane Michael (2018)

The prediction of rapid intensification (RI) in tropical cyclones (TCs) is a challenging problem. In this study, the RI process and factors contributing to it are compared for two TCs: an axis-symmetric case (Hurricane Irma, 2017) and an asymmetric case (Hurricane Michael, 2018). Both Irma and Michael became major hurricanes that made significant impacts in the United States. The Hurricane Weather Research and Forecasting (HWRF) Model was used to examine the connection between RI with forcing from the large-scale environment and the subsequent evolution of TC structure and convection. The observed large-scale environment was reasonably reproduced by HWRF forecasts. Hurricane Irma rapidly intensified in an environment with weak-to-moderate vertical wind shear (VWS), typically favorable for RI, leading to the symmetric development of vortical convective clouds in the cyclonic, vorticity-rich environment. Conversely, Hurricane Michael rapidly intensified in an environment of strong VWS, typically unfavorable for RI, leading to major asymmetries in the development of vortical convective clouds. The tangential wind momentum budget was analyzed for these two hurricanes to identify similarities and differences in the pathways to RI. Results suggest that eddy transport terms associated with convective processes positively contributed to vortex spin up in the early stages of RI and inhibited spin up in the later stages of RI in both TCs. In the early stages of RI, the mean transport terms exhibited notable differences in these TCs; they dominated the spin-up process in Irma and were of secondary importance to the spin-up process in Michael. Favorable aspects of the environment surrounding Michael appeared to aid in the RI process despite hostile VWS.

Creep stability of the DART/Hera mission target 65803 Didymos: II. The role of cohesion

The binary asteroid 65803 Didymos-Dimorphos is the target of the first asteroid deflection test (NASA's Double Asteroid Redirection Test, DART) and the first binary asteroid system that will be characterized by a rendezvous mission (ESA's Hera). The cohesive strength of the fast-spin-primary Didymos is a key factor that could affect the impact outcome and stability of this system. To support the preparation and data interpretation of these missions and gain a better understanding of the formation and evolution of this system, we investigate the structural stability and cohesive strength of Didymos based on current observational information. We use the Didymos radar shape model to construct rubble-pile models consisting of ~40,000 to ~100,000 particles with different arrangements and size distributions. To investigate the effect of cohesion on the structural stability and dynamical behaviors of Didymos, we explicitly simulate the YORP spin-up process of these rubble-pile models from a slow spin state to Didymos' current spin state using a high-efficiency soft-sphere-discrete-element-model code, pkdgrav. We test the creep stability of Didymos' rubble-pile representation with different values of cohesion and derive the critical amount of cohesion to maintain stability. The results show that Didymos should at least have a minimum bulk cohesion on the order of 10 Pa to maintain its structural stability if the interparticle tensile strength is uniformly distributed. Since the surface particles are less bonded by cohesive contacts than the interior particles, the internal macroscopic cohesion is about three times the surface macroscopic cohesion. We find that the bulk density and particle arrangement and size distribution of Didymos have significant influences on its critical cohesion and failure behaviors, indicating different binary formation pathways. With the critical cohesion, Didymos is at the edge of maintaining a stable shape, and a rapid small decrease in its spin period would excite its rubble-pile structure and lead to reshaping or mass shedding. Whether the DART impact could partially or globally destabilize this system requires further investigation of the full two-body gravitational dynamics and the ejecta evolution. With the expected measurements returned by DART's onboard cubesat LICIACube in 2022 and Hera in 2027, the correlations between Didymos' physical properties and failure behaviors found in this study may be possible to constrain the mechanical properties and evolutionary history of this binary system.

Accretion torque reversals in GRO J1008-57 revealed by Insight-HXMT

GRO J1008-57, as a Be/X-ray transient pulsar, is considered to have the highest magnetic field in known neutron star X-ray binary systems. Observational data of the X-ray outbursts in GRO J1008-57 from 2017 to 2020 were collected by the Insight-HXMT satellite. In this work, the spin period of the neutron star in GRO J1008-57 was determined to be about 93.28 seconds in August 2017, 93.22 seconds in February 2018, 93.25 seconds in June 2019 and 93.14 seconds in June 2020. GRO J1008-57 evolved in the spin-up process with a mean rate of −(2.10±0.05)×10−4 s/d from 2009 – 2018, and turned into a spin down process with a rate of (6.7±0.6)×10−5 s/d from Feb 2018 to June 2019. During the type II outburst of 2020, GRO J1008-57 had the spin-up torque again. During the torque reversals, the pulse profiles and continuum X-ray spectra did not change significantly, and the cyclotron resonant scattering feature around 80 keV was only detected during the outbursts in 2017 and 2020. Based on the observed mean spin-up rate, we estimated the inner accretion disk radius in GRO J1008-57 (about 1 - 2 times of the Alfvén radius) by comparing different accretion torque models of magnetic neutron stars. During the spin-down process, the magnetic torque should dominate over the matter accreting inflow torque, and we constrained the surface dipole magnetic field B≥6×1012 G for the neutron star in GRO J1008-57, which is consistent with the magnetic field strength obtained by cyclotron line centroid energy.

Implementation of the Incremental Analysis Update Initialization Scheme in the Tropical Regional Atmospheric Modeling System under the Replay Configuration

In traditional simulations of heavy rainfall events, the regional model is often initialized by using a global reanalysis dataset and a cold start method. An alternative to using global analysis data is to gradually introduce the analysis field via an incremental analysis update (IAU) method under the replay configuration. We found substantial differences in the forecast of a heavy rainfall event in southern China between a precipitation forecast using the traditional method and a forecast using the IAU method in the Tropical Regional Atmospheric Modeling System (TRAMS), based on the ECMWF global analysis. The IAU method is efficient in removing spurious high-frequency gravity wave noise, especially when the relaxation time is more than 90 min. The regional model needs to be pre-integrated for about 12 h to warm up the convective system in the background field. The improvement by the IAU method is supported by verification of simulations over 1 month (1–30 April 2019). In general, the IAU technique improves the initialization and spin-up process in the simulation of the heavy rainfall event.

Improving Nonlinear and Nonhydrostatic Ocean Lee Wave Drag Parameterizations

AbstractOcean lee waves occur on length scales that are smaller than the grid scale of global circulation models (GCMs). Therefore, such models must parameterize the drag associated with launching lee waves. This paper compares the lee wave drag predicted by existing parameterizations with the drag measured in high-resolution nonhydrostatic numerical simulations of a lee wave over periodic sinusoidal bathymetry. The simulations afford a time-varying glimpse at the nonlinear and nonhydrostatic oceanic lee wave spinup process and identify a characteristic time scale to reach steady state. The maximum instantaneous lee wave drag observed during the spinup period is found to be well predicted by linear lee wave theory for all hill heights. In steady state, the simulations demonstrate the applicability of parameterizing the drag based on applying linear theory to the lowest overtopping streamline of the flow (LOTS), as is currently employed in GCMs. However, because existing parameterizations are based only on the height of the LOTS, they implicitly assume hydrostatic flow. For hills tall enough to trap water in their valleys, the simulations identify a set of nonhydrostatic processes that can result in a reduction of the lee wave drag from that given by hydrostatic parameterizations. The simulations suggest implementing a time-dependent nonhydrostatic version of the LOTS-based parameterization of lee wave drag and demonstrate the remarkable applicability of linear lee wave theory to oceanic lee waves.

Discovery of Delayed Spin-up Behavior Following Two Large Glitches in the Crab Pulsar, and the Statistics of Such Processes

Abstract Glitches correspond to sudden jumps of rotation frequency (ν) and its derivative ( ) of pulsars, the origin of which remains not well understood yet, partly because the jump processes of most glitches are not well time-resolved. There are three large glitches of the Crab pulsar, detected in 1989, 1996, and 2017, which were found to have delayed spin-up processes before the normal recovery processes. Here we report two additional glitches of this pulsar that occurred in 2004 and 2011 for which we discovered delayed spin-up processes, and present refined parameters of the largest glitch, which occurred in 2017. The initial rising time of the glitch is determined as <0.48 hr. The two glitches that occurred in 2004 and 2011 had delayed spin-up time scales (τ 1) of 1.7 ± 0.8 days and 1.6 ± 0.4 days, respectively. We also carried out a statistical study of these five glitches with observed spin-up processes. We find that the Δν versus relation of these five glitches is similar to those with no detected delayed spin-up process, indicating that they are similar to the others in nature except that they have larger amplitudes. For these five glitches, the amplitudes of the delayed spin-up process ( ) and recovery process (Δν d2), their time scales (τ 1, τ 2), and permanent changes in spin frequency (Δν p) and total frequency step (Δν g) have positive correlations. From these correlations, we suggest that the delayed spin-up processes are common for all glitches, but are too short and thus difficult to be detected for most glitches.

Large axisymmetric surface deformation and dewetting in the flow above a rotating disk in a cylindrical tank: Spin-up and permanent regimes

In this paper, we aim at characterizing the spin-up process and the permanent regime of a rotating flow with free surface. The motion is created by the quasi-impulsive rotation of a disk located at the bottom of a cylindrical tank partially filled with a Newtonian water-glycerol mixture at a fixed initial aspect ratio (height to radius) of 0.25. Two experimental setups with different radius sizes and two numerical codes with distinct formulations are used, allowing for numerous comparisons and cross validations. Time-dependent surface height profiles and azimuthal velocity profiles are accurately measured with a laser line sensor and Laser Doppler Velocimetry, respectively. These measurements are compared to numerical results showing an overall excellent agreement and whenever a discrepancy exists, explanations are given. The spin-up and permanent regime for two typical flow cases are detailed: (i) one for which the angular speed moderately deforms the free surface and (ii) one at higher angular speed for which the disk becomes partially dewetted. The shape of the free surface in the permanent regime for intermediate rotation speeds is also reported. The flow parameters are chosen in such a way that large deformations of the free surface are achieved, yet remaining under the threshold of appearance of rotating polygonal patterns. This enables us to characterize realistic base flows on which this instability may develop.

Vortex Spinup Process in the Extratropical Transition of Hurricane Sandy (2012)

Abstract This study utilizes the quasi-Lagrangian azimuthal momentum equation (i.e., budget calculation) and 1.667-km-resolution numerical simulation data to study the intensity and structural changes in Hurricane Sandy’s extratropical transition. The results indicate that after the onset of extratropical transition, Sandy maintains an eyewall-like convection and warm core in the core region and has a frontal structure in the outer region. In the outer region, baroclinicity-driven frontal convection induces extensive planetary boundary layer (PBL) inflow, causing an inward advection of absolute angular momentum (AAM) per unit radius, which generates outer local wind maxima and expands Sandy’s outer wind field through a spinup process. Moreover, because the outer tangential wind velocity accelerates in a frontal convection, local wind maxima associated with fronts can expand to the outer sides of frontal regions. Frontal convection increases AAM in the outer region, providing the precondition for reintensification; however, the front itself cannot cause Sandy’s reintensification. The eyewall-like convection in the core region still plays an important role in Sandy’s reintensification. When the baroclinic zone, where a strong horizontal temperature gradient exists, approaches the core region, the eyewall-like convection is enhanced because the warm, moist air of the core region is lifted by the cold, dry air associated with the approaching baroclinic zone. Consequently, owing to the enhancement of eyewall-like convection, the PBL inflow, which extends from the outer region to the core region, develops. This inflow increases the inward transportation of the outer frontal region’s high-AAM air, thus leading to spinning up the core region’s wind and reintensification.