Initial Twist Research Articles

HELICAL KINK INSTABILITY IN A CONFINED SOLAR ERUPTION

ABSTRACT A model for strongly writhing confined solar eruptions suggests an origin in the helical kink instability of a coronal flux rope that remains stable against the torus instability. This model is tested against the well observed filament eruption on 2002 May 27 in a parametric MHD simulation study that comprises all phases of the event. Good agreement with the essential observed properties is obtained. These include the confinement, terminal height, writhing, distortion, and dissolution of the filament, and the flare loops. The agreement is robust against variations in a representative range of parameter space. Careful comparisons with the observation data constrain the ratio of the external toroidal and poloidal field components to and the initial flux rope twist to . Different from ejective eruptions, two distinct phases of strong magnetic reconnection can occur. First, the erupting flux is cut by reconnection with overlying flux in the helical current sheet formed by the instability. If the resulting flux bundles are linked as a consequence of the erupting rope’s strong writhing, they subsequently reconnect in the vertical current sheet between them. This reforms the overlying flux and a far less twisted flux rope, offering a pathway to homologous eruptions.

Helical kink instability in the confined solar eruption on 2002 May 27

AbstractThis paper presents an improved MHD modeling of the confined filament eruption in solar active region NOAA 9957 on 2002 May 27 by extending the parametric studies of the event in Török & Kliem (2005) and Hassanin & Kliem (2016). Here the initial flux rope equilibrium is chosen to possess a small apex height identical to the observed initial filament height, which implies a more realistic inclusion of the photospheric line tying. The model matches the observations as closely as in the preceding studies, with the closest agreement again being obtained for an initial average flux rope twist of about 4π. Thus, the model for strongly writhing confined solar eruptions, which assumes that a kink‐unstable flux rope in the stability domain of the torus instability exists at the onset of the eruption's main acceleration phase, is further substantiated. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Lateral stability of reinforced-concrete beams with initial imperfections

The paper presents the results of an analytical study aimed at investigating the lateral stability of reinforced-concrete (RC) beams with initial geometric imperfections. Based on the experimental results in the literature, a general buckling moment expression was developed, linking the limit moment of a RC beam with an initial sweep and twist to the critical moment of its geometrically perfect configuration. This expression takes into account the initial geometric and material non-linearities of the beams, the elastic–inelastic material properties of concrete and steel, the cracking of concrete due to restrained shrinkage and in-plane flexure, and the contribution of longitudinal reinforcement to stability. The formula was applied to three standard types of Aashto I-girders to determine the critical unbraced length of each type. Finally, an analytical equation was developed for estimating the lateral deflections of RC beams with initial geometric imperfections. The limit moment and lateral deflection equations were shown to provide close and conservative estimates for RC beams.

Sunspot rotation

Context. Observations of flux emergence indicate that rotational velocities may develop within sunspots. However, the dependence of this rotation on sub-photospheric field strength and twist remains largely unknown. Aims. We investigate the effects of varying the initial field strength and twist of an emerging sub-photospheric magnetic flux tube on the rotation of the sunspots at the photosphere. Methods. We consider a simple model of a stratified domain with a sub-photospheric interior layer and three overlying atmospheric layers. A twisted arched flux tube is inserted in the interior and is allowed to rise into the atmosphere. To achieve this, the MHD equations are solved using the Lagrangian-remap code, Lare3d. We perform a parameter study by independently varying the sub-photospheric magnetic field strength and twist. Results. Altering the initial field strength and twist significantly affects the tube's evolution and the rotational motions that develop at the photosphere. The rotation angle, vorticity, and current show a direct dependence on the initial field strength. We find that an increase in field strength increases the angle through which the fieldlines rotate, the length of fieldlines extending into the atmosphere, and the magnetic energy transported to the atmosphere. This also affects the amount of residual twist in the interior. The length of the fieldlines is crucial as we predict the twist per unit length equilibrates to a lower value on longer fieldlines. No such direct dependence is found when we modify the twist owing to the complex effect this has on the tension force acting on the tube. However, there is still a clear ordering in quantities such as the rotation angle, helicity, and free energy with higher initial twist cases being related to sunspots that rotate more rapidly, transporting more helicity and magnetic energy to the atmosphere.

Strain recovery analysis and stiffness verification of non-uniform composite beam with arbitrary cross section and material distribution using VABS

This paper presents a theory related to a two-dimensional linear cross-sectional analysis, recovery relationship, and a one-dimensional nonlinear beam analysis for composite wing slender structure with initial twist. Using VABS including a related theory, the design process of the composite rotor blade has been described. Cross-sectional analysis was performed at cutting point including all the details of geometry and material. Stiffness matrix and mass matrix were linked to each section to make 1D beam model. The 3D strain distributions within the structure were recovered based on the global behavior and load of the 1D beam analysis and visualize numerical results. Comparison between the analytical and experimental results shows that the proposed analytical procedure can provide an accurate and efficient prediction of the both deflection, flexural stiffness, strain of multilayer composite slender structure. Verified comparison results can be used to efficiently design accurate complex slender structure properties for detail design and manufacture. These sentences will appear after the conclusion when an article is finally published.

Transient response of delaminated composite conical shells due to multiple low velocity impacts in hygrothermal environment

This paper presents a finite element based method to investigate the hygrothermal effects on the transient dynamic response of delaminated composite pretwisted conical shells with initial twist impacted at arbitrary locations by multiple spherical impactors. An eight-noded shell element has been used in the present analysis based on the Mindlin’s theory. The generalized dynamic equilibrium equation is derived from Lagrange’s equation of motion neglecting Coriolis effect for moderate rotational speeds. The multi-point constraint algorithm has been incorporated to ensure the compatibility of deformation and equilibrium of resultant forces and moments at the delamination crack front. The indentation laws as proposed by Hertz have been implemented to compute the contact force and displacement arising from each impact on specific locations of delaminated conical shells already prestressed by residual thermal or moisture strains. Results have been presented to depict the influence of important parameters like, twist angle, size and location of delamination, velocity and mass of the impactors and the time-delay between the impactors on the multiple impact response of delaminated composite conical shells in hygrothermal environment.

An efficient 3D numerical beam model based on cross sectional analysis and Ritz approximations

A 3D beam model, i.e. a beam that may deform in space and experience longitudinal and torsional deformations, is developed considering Timoshenko's theory for bending and assuming that the cross section rotates as a rigid body but may deform in longitudinal direction due to warping. The cross sectional properties are firstly calculated and then inserted at the equation of motion. The beam is assumed to be with an arbitrary cross section, with linearly varying thickness and width, and with an initial twist. The model is appropriate for open and closed thin-walled cross sections, and also for solid cross sections. The objective of the current research is to demonstrate that complex beam structures can be modeled accurately with reduced number of degrees of freedom.

VABS를 이용한 임의의 단면과 재료 분포를 가진 비균일 복합재료 보의 변형률 복원 해석

본 논문에서는 블레이드와 같이 세장비가 크고 초기 비틀림이 존재하는 복합재료로 구성된 날개 구조물에 대한 이차원 단면의 차원축소와 복원관계를 이론적으로 기술하고 이를 이용한 복합재료 로터 블레이드의 효율적인 설계 과정을 기술하였다. 차원축소와 복원해석 수치해석 프로그램인 VABS를 이용하여 단면해석을 수행하고 연성이 고려된 강성행렬과 질량행렬을 중립점에 연결하여 블레이드의 특성을 포함한 1차원 보 모델링을 구성하였다. 1차원 보 모델을 통해 블레이드의 거동을 확인하고 내부하중을 계산하여 단면위치에서 변형률 복원을 수치적으로 계산하고 이산화 된 단면에 수치적으로 매칭하여 시각적으로 형상화하였다. This paper presents a theory related to a two-dimensional linear cross-sectional analysis, recovery relationship and a one-dimensional nonlinear beam analysis for composite wing structure with initial twist. Using VABS including a related theory, the design process of the composite rotor blade has been described. Cross-sectional analysis was performed at cutting point including all the details of geometry and material. Stiffness matrix and mass matrix were linked to each section to make 1D beam model. The 3D strain distributions within the structure were recovered based on the global behavior of the 1D beam analysis and visualize numerical results.

Bending characteristics of delaminated cross-ply composite shallow conical shells in hygrothermal environment

In the present study, the influence of temperature and moisture concentration on the bending characteristics of delaminated rotating composite conical shells has been investigated. The shells with initial twist are considered for moderate rotational speeds wherein Coriolis effect is assumed to be absent. The effects of rotary inertia and transverse shear deformation have been incorporated in the present finite element formulation along with the multi-point constraint algorithm for modeling the delamination. Graphical results have been presented depicting the contours of deflection and relative moment along the planform of the conical shell resulting from centrifugal and hygrothermal forces taking into account the influence of twist angle, stacking sequence, rotational speed, and size of delamination present inside the laminate.

A Minimal POE-Based Model for Robotic Kinematic Calibration With Only Position Measurements

This paper proposes an algorithm for robotic kinematic calibration based on a minimal product of exponentials (POE)-based model for the applications where only position measurements are required. Both joint zero-offset errors and initial frame twist error can be involved in this model. Analysis of the identifiability of these errors shows that at most six elements of these parameters can be identified. It also suggests that at least three noncollinear points on the end-effector should be measured to maximize the identifiability. Compared with the traditional POE-based model with full pose (position and orientation) measurements, the minimal model with only position measurements outperforms in terms of convenience, efficiency, and accuracy.

Method for studying the rapidity fluctuation and de-correlation of harmonic flow in heavy-ion collisions

An "event-shape twist" technique is proposed to study the longitudinal dynamics of harmonic flow, in particular the effects of rapidity fluctuation and event-plane decorrelation. This technique can distinguish between two types of rapidity decorrelation effects: a systematic rotation versus a random fluctuation of flow angles along the rapidity direction. The technique is demonstrated and the magnitude of the two decorrelation effects is predicted using the AMPT model via a single particle analysis and two-particle correlation analysis. An observed decorrelation can be attributed to a systematic rotation of event-plane angle along the pseudorapidity, consistent with a collective response to an initial state twist of the fireball proposed by Bozek {\it et.al.}. This rotation is also observed for several higher-order harmonics with the same sign and similar magnitudes.

FAST MAGNETIC TWISTER AND PLASMA PERTURBATIONS IN A THREE-DIMENSIONAL CORONAL ARCADE

We present results of 3-D numerical simulations of a fast magnetic twister excited above a foot-point of the potential solar coronal arcade that is embedded in the solar atmosphere with the initial VAL-IIIC temperature profile, which is smoothly extended into the solar corona. With the use of the FLASH code, we solve 3-D ideal magnetohydrodynamic equations by specifying a twist in the azimuthal component of magnetic field in the solar chromosphere. The imposed perturbation generates torsional Alfv\'en waves as well as plasma swirls that reach the other foot-point of the arcade and partially reflect back from the transition region. The two vortex channels are evident in the generated twisted flux-tube with a fragmentation near its apex that results from the initial twist as well as from the morphology of the tube. The numerical results are compared to observational data of plasma motions in a solar prominence. The comparison shows that the numerical results and the data qualitatively agree even though the observed plasma motions occur over comparatively large spatio-temporal scales in the prominence.

The evolution of writhe in kink-unstable flux ropes and erupting filaments

The helical kink instability of a twisted magnetic flux tube has been suggested as a trigger mechanism for solar filament eruptions and coronal mass ejections (CMEs). In order to investigate if estimations of the pre-emptive twist can be obtained from observations of writhe in such events, we quantitatively analyze the conversion of twist into writhe in the course of the instability, using numerical simulations. We consider the line tied, cylindrically symmetric Gold–Hoyle flux rope model and measure the writhe using the formulae by Berger and Prior which express the quantity as a single integral in space. We find that the amount of twist converted into writhe does not simply scale with the initial flux rope twist, but depends mainly on the growth rates of the instability eigenmodes of higher longitudinal order than the basic mode. The saturation levels of the writhe, as well as the shapes of the kinked flux ropes, are very similar for considerable ranges of initial flux rope twists, which essentially precludes estimations of pre-eruptive twist from measurements of writhe. However, our simulations suggest an upper twist limit of ∼6π for the majority of filaments prior to their eruption.

Design optimization of slender elastic beam with initial twist using genetic algorithms and finite element analysis

This paper aims to obtain the optimal composite slender beam design for a rotor blade. The cross-sectional dimensions are considered as design variables. The objective is to optimize the box beam to attain a target matrix of stiffness values and maximum elastic coupling. The optimization process is performed using a genetic algorithm (GA), associated with a variational asymptotic method for the structural analysis. The stiffness control of arbitrary, complex section under some design condition is performed in composite beam where the geometrically nonlinear characteristic of the structure is considered. The objective function is defined as the weight, strength and fatigue life. The laminate thicknesses are to be determined optimally by defining a design index comprising a weighted average of the objective functions and determining the minimum.

임플란트 시술용 드릴의 가공 성능 평가에 관한 연구

Skull Melted 3.2YSZ has good physical properties and does not undergo low temperature degradation. Due to these excellent physical and mechanical properties, Skull Melted 3.2YSZ has been studied for use in dental implants. In this study, a o2.2mm Initial Twist Drill was made using Skull Melted 3.2YSZ; the drilling characteristics were compared with those of the traditional SUS420J drill. The experimental results indicate that the Skull Melted 3.2YSZ drill requires similar thrust forces and has a slightly higher temperature.

Convergence of Yang-Mills-Higgs flow for twist Higgs pairs on Riemann surfaces

We consider the gradient flow of the Yang-Mills-Higgs functional of twist Higgs pairs on a Hermitian vector bundle (E,H) over Riemann surface X. It is already known the gradient flow with initial data (A0, ϕ0) converges to a critical point (A∞, ϕ∞). Using a modified Chern-Weil type inequality, we prove that the limiting twist Higgs bundle (E, \(d''_{A_\infty }\)ϕ∞) coincides with the graded twist Higgs bundle defined by the Harder-Narasimhan-Seshadri filtration of the initial twist Higgs bundle (E, \(d''_{A_0 }\), ϕ0), generalizing Wilkin’s results for untwist Higgs bundle.

Mechanical Strain Generated by RNA Polymerase during Transcription Initiation can Drive Structural Changes in DNA Topology that Relieve Repression

DNA in cells is often topologically closed, and in some cases, tightly looped by proteins typically associated with transcriptional repression. During transcription initiation, RNA polymerase (RNAP) is challenged to open duplex DNA, wherein RNAP generates torque that consequently overtwists DNA flanking the melted promoter. As initiation proceeds, the transcription bubble expands as RNAP synthesizes downstream RNA while maintaining upstream DNA contacts. We hypothesized that the mechanical strain imparted to DNA during initiation would repress transcription from DNA templates that restrict the relief of torsional stress. To test our hypothesis, we constructed circular DNA templates that are 100 to 108 bp in size (i.e., each initially twisted to various degrees) and quantified transcription initiation by the bacteriophage T7 RNAP. We find that transcriptional repression during initiation is dependent on the sign and magnitude of initial twist within the DNA templates. Surprisingly, however, we observe that for the most overtwisted templates, repression is relieved at positions beyond the promoter that are dependent on the initial DNA twist. To interpret these results, we used elastic rod and molecular dynamics simulations to predict the structures of both the RNAP and the circular DNA template during initiation. Our modeling studies confirm that RNAP is capable of overtwisting the circular DNA template to the point of buckling from a planar into a supercoiled conformation. Further analysis reveals that initial DNA twist determines at what position along the template supercoiling will occur during initiation and that adopting a supercoiled structure substantially relieves the torque encountered by the RNAP. Our results demonstrate that repression of RNAP during initiation is determined not only by the initial mechanics of the DNA template, but also by the torque generated by RNAP itself.

Study on the vibration characteristics of the pre-twisted pillar and beam

For the past several years, large scale earthquakes happened. A lot of industrial facilities were damaged. For example, a lot of power plants were stopped in The 2011 off the Pacific coast of Tohoku Earthquake or The Southern Hyogo prefecture earthquake in 1995. These earthquakes have larger acceleration level than the seismic design level. Therefore, the earthquake proof measures are required. In this study, earthquake proof effect of the beam and the long column having initial twist is focused. The design guidelines for application to the pre-twisted beam structure and the long column structure is proposed. The pre-twisted beam and long column are modelled by FEM and calculated. The natural frequency of the pre-twisted beam and long column are related to the Young's modulus, density, length and cross-sectional area. The tendency of the natural frequency respected to the total twisted angle and the aspect ratio of the cross-section is obtained considering the vibration mode and the natural frequency. Furthermore, when attention is focused on the vibration mode, the influence of the total twisted angle is different for the modal shapes, and the case where the modal shapes are interchanged is shown. The acceleration response to the vibration input is determined by the natural frequency of the primary cross-section, the aspect ratio and boundary conditions. In order to evaluate the response reduction effect, the acceleration amplification respected to the total twisted angle and the 1st natural frequency of the system is analyzed. And the response reduction map is proposed. Using this map, it is possible to estimate the advantage of the pre-twisted effect for the vibration characteristics in a simple manner.

Experimental and analytical assessment of lateral torsional buckling of laminated glass beams

Due to their increasing use in contemporary architecture, the lateral torsional buckling performance of laminated structural glass beams represents a topic of great interest for researchers. Although several analytical models and design approaches have been recently proposed, various aspects complicate the realistic prediction of this phenomenon. Based on experimental results of a large campaign of lateral torsional buckling tests (55 laminated beams), the paper investigates analytically the effects of various mechanical (e.g. the stiffness of interlayer) and geometrical properties (e.g. initial twist, production tolerances) on the typical lateral torsional buckling response of laminated glass beams.

Generation of twist on magnetic flux tubes at the base of the solar convection zone

Using two-dimensional magnetohydrodynamics calculations, we investigate a twist gen- eration mechanism on a magnetic flux tube at the base of the solar convection zone based on the idea of Choudhuri, 2003, Sol. Phys., 215, 31 in which a toroidal mag- netic field is wrapped by a surrounding mean poloidal field. During generation of the twist, the flux tube follows four phases. (1) It quickly splits into two parts with vortex motions rolling up the poloidal magnetic field. (2) Owing to the physical mechanism similar to that of the magneto-rotational instability, the rolled-up poloidal field is bent and amplified. (3) The magnetic tension of the disturbed poloidal magnetic field re- duces the vorticity, and the lifting force caused by vortical motion decreases. (4) The flux tube gets twisted and begins to rise again without splitting. Investigation of these processes is significant because it shows that a flux tube without any initial twist can rise to the surface in relatively weak poloidal fields.