Bar Mode Research Articles

Global stability of self-gravitating discs in modified gravity

We study the global stability of a self-gravitating disk in the context of Modified Gravity (MOG) using N-body simulations. This theory is a relativistic scalar-tensor-vector theory of gravity and presented to address the dark matter problem. In the weak field limit MOG possesses two free parameters $\alpha$ and $\mu_0$ which have been already determined using rotation curve data of spiral galaxies. The evolution of a stellar self-gravitating disk and more specifically the bar instability in MOG is investigated and compared to a Newtonian case. Our models have exponential and Mestel-like surface densities as $\Sigma\propto \exp(-r/h)$ and $\Sigma\propto 1/r$. It is found out that, surprisingly, the disks are more stable against the bar mode in MOG than in Newtonian gravity. In other words, the bar growth rate is effectively slower than the Newtonian disks. Also we show that both free parameters, i.e. $\alpha$ and $\mu_0$, have stabilizing effects. In other words, increase in these parameters will decrease the bar growth rate.

ON THE GRAVITATIONAL STABILITY OF THE MACLAURIN DISK

ABSTRACT We study the global gravitational stability of a gaseous self-gravitating Maclaurin disk in the absence of a halo. Further, we replace Newtonian gravity with the specific modified gravity theory known as MOG in the relevant literature. MOG is an alternative theory for addressing the dark matter problem without invoking exotic dark matter particles, and it possesses two free parameters α and μ 0 in the weak field limit. We derive the equilibrium gravitational potential of the Maclaurin disk in MOG and develop a semianalytic method for studying the response of the disk to linear nonaxisymmetric perturbations. The eigenvalue spectrum of the normal modes of the disk is obtained, and its physical meaning has been explored. We show that Maclaurin disks are less stable in MOG than in Newtonian gravity. In fact, both parameters (α, μ 0) have destabilizing effects on the disk. Interestingly, μ 0 excites only the bar mode m = 2, while α affects all of the modes. More specifically, when α > 1, the bar mode is strongly unstable and unlike in Newtonian gravity cannot be avoided, at least in the weak field limit, with increasing the pressure support of the disk.

Characterization of Thermomechanical Behavior of Ti3SiC2 and Ti2AlC Ceramics Elaborated by Spark Plasma Sintering Using Ultrasonic Means

In order to better understand the thermomechanical behaviors of Ti3SiC2 and Ti2AlC ceramics, sintered by spark plasma sintering process (SPS), ultrasonic pulse echography in “long bar mode” (US) and acoustic emission (AE) techniques are applied. High‐temperature investigations of the evolution of Young's modulus (E) and AE activity are carried out up to 1 473 and 1 623 K. Results show a decrease of E with increasing temperature which is more marked around maximal temperature of the thermal cycle for Ti2AlC material. Moreover, the increase of the maximal temperature of the thermal cycle influences the number of recorded cumulated hits. Both measurements (US + AE) highlight the role of the plastic deformation mechanism (motion of dislocation) activated over the brittle‐to‐plastic transition temperature.

Simulating a slow bar in the low surface brightness galaxy UGC 628

We present a disc-halo N-body model of the low surface brightness galaxy UGC 628, one of the few systems that harbours a "slow" bar with a ratio of corotation radius to bar length of $\mathcal{R} \equiv R_c/a_b \sim 2$. We select our initial conditions using SDSS DR10 photometry, a physically motivated radially variable mass-to-light ratio profile, and rotation curve data from the literature. A global bar instability grows in our submaximal disc model, and the disc morphology and dynamics agree broadly with the photometry and kinematics of UGC 628 at times between peak bar strength and the onset of buckling. Prior to bar formation, the disc and halo contribute roughly equally to the potential in the galaxy's inner region, giving the disc enough self gravity for bar modes to grow. After bar formation there is significant mass redistribution, creating a baryon dominated inner and dark matter dominated outer disc. This implies that, unlike most other low surface brightness galaxies, UGC 628 is not dark matter dominated everywhere. Our model nonetheless implies that UGC 628 falls on same the relationship between dark matter fraction and rotation velocity found for high surface brightness galaxies, and lends credence to the argument that the disc mass fraction measured at the location where its contribution to the potential peaks is not a reliable indicator of its dynamical importance at all radii.

Sensitivity of free bar morphology in rivers to secondary flow modeling: Linear stability analysis and numerical simulation

A number of numerical models have been proposed to understand and simulate fluvial river morphodynamics; however, it is somewhat unclear whether all the models are able to consistently simulate flow-bed instability phenomena. This study investigates the sensitivity of free bar morphology in rivers to secondary flow models used in depth-averaged models using linear stability analyses and numerical simulations. Both the linear analyses and numerical simulations suggest that under certain hydraulic conditions, an equilibrium-type secondary flow model, which has been widely used in river morphodynamic models, fails to generate a finite wavelength and bar mode, allowing the inception of bars of infinitely short scale and infinitely high mode. Using a nonequilibrium-type secondary flow model avoids the unphysical formation of these incipient free bars, and gives better solutions regarding finite amplitude bars. Since free bars are essential, intrinsic river morphological features, the findings of this study can be applied to a wide range of river morphodynamic calculations.

Dynamic behavior of cement-mortar cavern reinforced by bars

This paper conducts a numerical study on the failure modes of aluminium bar reinforced cement-mortar cavern under blast loads. A simplified softening model is proposed to simulate the bond–slip behavior between the aluminium bar and the cement-mortar materials. Comparisons between the numerical results and the test ones demonstrate the accuracy of the proposed numerical model. According to the numerical simulation, three failure modes of the aluminium bar reinforced cement-mortar cavern are identified: (1) failure in reinforced zone, (2) failure in unreinforced zone, and (3) a combining mode of failure in reinforced and unreinforced zone. The results of the numerical simulations indicate that there exists a critical bar spacing, which controls these failure modes: the failure zone transits from the reinforced zone to the unreinforced one by decreasing the bar spacing. A parametric study is also conducted to give a better understanding of the dynamic responses of the aluminium bar reinforced cement-mortar cavern.

Vibrational modes of a thin bar

Although most traditional acoustics textbooks that treat vibration (e.g., Rayleigh, Lamb, Morse, Morse and Ingard, Kinsler and Frey) address the longitudinal, torsional, and flexural modes of a thin bar, they do not stress the importance of sound speed measurements for the experimental determination of elastic moduli. This presentation will demonstrate a simple and inexpensive apparatus that uses two coils of copper wire and permanent magnets, developed by Bob Leonard and Izzy Rudnick at UCLA, that can be used to excite and detect all three modes of a free-free bar, accurately measure their modal frequencies, and use those results to provide accurate and precise values for the elastic moduli of the bar material [JASA 88(1), 210 (1990)].

How can double-barred galaxies be long-lived?

Double-barred galaxies account for almost one third of all barred galaxies, suggesting that secondary stellar bars, which are embedded in large-scale primary bars, are long-lived structures. However, up to now it has been hard to self-consistently simulate a disc galaxy that sustains two nested stellar bars for longer than a few rotation periods. N-body/hydrodynamical simulations including star formation recipes have been performed. Their properties have been compared with the most recent observational data in order to prove that they are representative of double-barred galaxies, even SB0. Overlaps in dynamical resonances and bar modes have been looked for using Fourier spectrograms. Double-barred galaxies have been successfully simulated with lifetimes as long as 7 Gyr. The stellar population of the secondary bar is younger on average than for the primary large-scale bar. An important feature of these simulations is the absence of any resonance overlap for several Gyr. In particular, there is no overlap between the primary bar ILR and the secondary bar corotation. Therefore, mode coupling cannot sustain the secondary bar mode. Star formation is identified here as possibly being responsible for bringing energy to the nuclear mode. Star formation is also responsible for limiting the amount of gas in the central region which prevents the orbits sustaining the secondary bar from being destroyed. Therefore, the secondary bar can dissolve but reappear after approx. 1 Gyr. When star formation is switched off the dynamical perturbation associated with the secondary bar needs several Gyr to fully vanish. Double-bars can be long-lived in numerical simulations with a gaseous component, even in the absence of overlap of resonances or mode coupling, provided that star formation remains active in the central region where the nuclear bar lies.

Stress wave analysis of thermo-piezoelectric solid bar of polygonal cross-sections immersed in fluid

Purpose – The purpose of this paper is to study the problem of wave propagation in an infinite, homogeneous, transversely isotropic thermo-piezoelectric solid bar of polygonal (triangle, square, pentagon and hexagon) cross-section immersed in fluid is using Fourier expansion collocation method, with in the frame work of linearized, three-dimensional theory of thermo-piezoelectricity. Design/methodology/approach – A mathematical model is developed to study the wave propagation in an infinite, homogeneous, transversely isotropic thermo-piezoelectric solid bar of polygonal cross-sections immersed in fluid is studied using the three-dimensional theory of elasticity. Three displacement potential functions are introduced, to uncouple the equations of motion and the heat and electric conductions. The frequency equations are obtained for longitudinal and flexural (symmetric and antisymmetric) modes of vibration and are studied numerically for triangular, square, pentagonal and hexagonal cross-sectional bar immersed in fluid. Since the boundary is irregular in shape; it is difficult to satisfy the boundary conditions along the curved surface of the polygonal bar directly. Hence, the Fourier expansion collocation method is applied along the boundary to satisfy the boundary conditions. The roots of the frequency equations are obtained by using the secant method, applicable for complex roots. Findings – From the literature survey, it is clear that the free vibration of an infinite, homogeneous, transversely isotropic thermo-piezoelectric solid bar of polygonal cross-sectional bar immersed in fluid have not been analyzed by any of the researchers, also the previous investigations in the vibration problems of transversely isotropic thermo-piezoelectric solid bar of circular cross-sections only. So, in this paper, the wave propagation in thermo-piezoelectric cylindrical bar of polygonal cross-sections immersed in fluid are studied using the Fourier expansion collocation method. The computed non-dimensional frequencies are plotted in the form of dispersion curves and its characteristics are discussed, also a comparison is made between non-dimensional wave numbers for longitudinal and flexural modes piezoelectric, thermo-piezoelectric and thermo-piezoelectric polygonal cross-sectional bars immersed in fluid. Research limitations/implications – Wave propagation in an infinite, homogeneous, transversely isotropic thermo-piezoelectric solid bar of polygonal cross-sectional bar immersed in fluid have not been analyzed by any of the researchers, also the previous investigations in the vibration problems of transversely isotropic thermo-piezoelectric solid bar of circular cross-sections only. So, in this paper, the wave propagation in thermo-piezoelectric cylindrical bar of polygonal cross-sections immersed in fluid are studied using the Fourier expansion collocation method. The computed non-dimensional frequencies are plotted in the form of dispersion curves and its characteristics are discussed, also a comparison is made between non-dimensional wave numbers for longitudinal and flexural modes of piezoelectric, thermo-piezoelectric and thermo-piezoelectric polygonal cross-sectional bars immersed in fluid. Originality/value – The researchers have discussed the wave propagation in thermo-piezoelectric circular cylinders using three-dimensional theory of thermo-piezoelectricity, but, the researchers did not analyzed the wave propagation in an arbitrary/polygonal cross-sectional bar immersed in fluid. So, the author has studied the free vibration analysis of thermo-piezoelectric polygonal (triangle, square, pentagon and hexagon) cross-sectional bar immersed in fluid using three-dimensional theory elasticity. The problem may be extended to any kinds of cross-sections by using the proper geometrical relations.

Adventures of a tidally induced bar

Using N-body simulations, we study the properties of a bar induced in a dwarf galaxy as a result of tidal interaction with the Milky Way. The dwarf is initially composed of a disk embedded in a dark matter halo and we follow its evolution on a typical orbit for 10 Gyr. It undergoes an evolution typical of tidally stirred dwarf galaxies: it loses mass, the stellar component transforms from a disk to a spheroid and the rotation of the stars is partially replaced by random motions. A tidally induced bar forms at the first pericentre passage and survives until the end of the evolution. Fourier decomposition of the distribution of stars reveals that only even modes are significant and preserve a hierarchy so that the bar mode is always the strongest. They show a characteristic profile with a maximum, similar to simulated bars forming in isolated galaxies and observed bars in real galaxies. We adopt the maximum of the bar mode as a measure of the bar strength and we estimate the bar length by comparing the density profiles along the bar and perpendicular to it. Both the bar strength and the bar length decrease with time, mainly at pericentres, as a result of tidal torques acting at those times and not to secular evolution. The pattern speed of the bar varies significantly on a time scale of 1 Gyr and seems to be fully controlled by the orientation of the tidal torque from the Milky Way. The bar is never tidally locked but we discover a hint of a 5/2 orbital resonance between the third and fourth pericentre passage. The speed of the bar expressed in terms of the ratio of the bar length to the corotation radius decreases in the long run so that the bar changes from initially rather fast to slow in the later stages. The boxy/peanut shape is clearly visible in the edge-on view between the second and third pericentre and its occurrence is preceded by a short period of buckling instability.

Investigation on Planck scale physics by the AURIGA gravitational bar detector

We have recently shown that the very low mechanical energy achieved and measured in the main vibration mode of gravitational wave bar detectors can set an upper limit to possible modifications of the Heisenberg uncertainty principle that are expected as an effect of gravity. Here we give more details on the data analysis procedure that allows one to deduce the energy of the bar mode (i.e., the meaningful parameter for our purpose). Furthermore, we extend the analysis of our results, discussing their implication for physical models that face quantum gravity from different points of view, e.g., proposing modified commutation relations or exploring spacetime discreteness.

Hardened Case Properties and Tensile Behaviours of TMT Steel Bars

Nowadays, high strength steels are getting very popular all over the world for various applications such as in auto body making, machine parts manufacturing, steel structure buildings, concrete reinforcement, etc. In Bangladesh, use of high strength TMT steels bars has got a momentum in the construction of flyovers, bridges and high rise buildings because of its good combination of the required mechanical properties. In this research work, locally produced high strength TMT reinforcing steel bars (20 mm dia.) of two different companies have been investigated. As a part of this research work, at first, details microstructural morphologies of hardened case, transition zone and core of both steels were investigated by optical microscope and they were then photographed. After that, Rockwell hardness was measured on different zones. Tensile tests were also carried by a Universal Testing Machine at room temperature. After tensile tests, details of the fracture modes of the steel bars were studied carefully. The fracture modes were then discussed in terms of chemical compositions and carbon equivalents, resulted microstructures, case and core hardness, depths of hardened case and transition zone of the steel bars.

Magnetic actuator for active damping of boring bars

This paper presents a new, noncontact magnetic actuator instrumented with fibre optic displacement sensors. The actuator is composed of four identical electromagnetic units which are linearized with a novel design strategy. The magnetic force can be delivered in two radial directions for active damping of lateral modes of the boring bar. The actuator is manufactured and mounted on a CNC lathe. The bending mode of the boring bar is actively damped, and the dynamic stiffness of the system was increased significantly, resulting in chatter-free higher depth of cuts. The actuator has been experimentally tested in boring operations.

A water-level controlled wind instrument developed by a team-based project for an undergraduate acoustic class

During an introductory undergraduate class of acoustics, a team-based project was given to design and implement their own musical instrument, after studying basic concepts of resonance, standing wave, and modes of string, pipe, bar, membrane, and plate. One of the teams designed and implemented “W.L.C WINDY (Water-Level Controlled Wind Instrument),” which the length of the pipe can be adjusted by water level inside the pipe. The lengths satisfies the modes of a stopped pipe (open-closed end) with end correction. There are several advantages of this instrument for education of acoustics; cheap, easy to make, and easy to understand the principles of modes. It costs less than one dollar (a transparency hose, a syringe, an O-ring) to make “W.L.C WINDY”. This is how to make the instrument; 1. Connect a transparent hose with a syringe without needle using an O-ring. 2. Fill the hose with colored water for easy visualization. 3. Press the thumb rest of the syringe to make various sounds by controlling the water level. Even though W.L.C WINDY is not optimized to play music, it may contribute to easy demonstration of the principles of the modes for education of acoustics. [Supporting program: NIPA-2013-H0401-13-1007.]

Development Mode of Delta Mouth Bar

Es2 8th sand group in Shengtuo oilfield is a typical braided river delta front deposition. Through depositional physical simulation, the development mode of mouth bar and its main controlling factors were summarized. Physical model and experiment scheme were set up according to similarity criterion and the depositional background in research area. One provenance and three times simulation was designed to simulate three single layers within Es2 8th sand group respectively. Development mode of mouth bar was summarized with three single mouth bars based on swing conditions of rivel estuary and two composite mouth bars according to whether they are developed at the same time.

Spinning dark matter haloes promote bar formation

Stellar bars are the most common non-axisymmetric structures in galaxies and their impact on the evolution of disc galaxies at all cosmological times can be significant. Classical theory predicts that stellar discs are stabilized against bar formation if embedded in massive spheroidal dark matter halos. However, dark matter halos have been shown to facilitate the growth of bars through resonant gravitational interaction. Still, it remains unclear why some galaxies are barred and some are not. In this study, we demonstrate that co-rotating (i.e., in the same sense as the disc rotating) dark matter halos with spin parameters in the range of $0 \le \lambda_{\mathrm{dm}} \le 0.07$ - which are a definite prediction of modern cosmological models - promote the formation of bars and boxy bulges and therefore can play an important role in the formation of pseudobulges in a kinematically hot dark matter dominated disc galaxies. We find continuous trends for models with higher halo spins: bars form more rapidly, the forming slow bars are stronger, and the final bars are longer. After 2 Gyrs of evolution, the amplitude of the bar mode in a model with $\lambda_{\mathrm{dm}} = 0.05$ is a factor of ~6 times higher, A_2/A_0 = 0.23, than in the non-rotating halo model. After 5 Gyrs, the bar is ~ 2.5 times longer. The origin of this trend is that more rapidly spinning (co-rotating) halos provide a larger fraction of trailing dark matter particles that lag behind the disc bar and help growing the bar by taking away its angular momentum by resonant interactions. A counter-rotating halo suppresses the formation of a bar in our models. We discuss potential consequences for forming galaxies at high-redshift and present day low mass galaxies which have converted only a small fraction of their baryons into stars.

GFRP Rebar 보강 콘크리트 보의 급속노화환경에서의 휨 거동에 관한 연구

철근콘크리트(reinforced concrete) 구조부재에서 철근의 부식으로 인한 문제점을 개선하고자 섬유보강 복합재료(FRP) 보강근(rebar)을 사용하는 것에 대한 연구가 꾸준히 진행되어져 오고 있다. 하지만 이러한 FRP 보강근을 사용한 콘크리트 부재의 환경에 대한 장기거동에 대한 연구가 아직도 미흡한 수준이다. 이 연구는 GFRP(glass fiber reinforced polymer) 보강근을 사용한 콘크리트 부재를 온도 약 46℃와 습도가 80%인 인위적인 실험실에서 최대 300일까지 노출시킨 후의 장기 거동에 대한 실험적 연구를 제시하였다. 비교를 위하여 두가지 서로 다른 GFRP 보강근과 철근을 보강한 콘크리트 보 시험체를 제작하였다. 실험 결과, 장기 노출환경에서도 GFRP 보강근을 보강한 콘크리트 보 시험체의 파괴형태는 철근 보강 콘크리트 보시험체와 매우 유사한 파괴형태를 나타내었으며, 노출 시간에 따른 하중저항 감소값은 철근이 보강된 경우가 GFRP 보강근이 보강된 경우보다 하중저항 감소값이 크게 일어났다. 또한 GFRP 보강근 보강 콘크리트 보 시험체를 설계할 시에는 철근 보강보다 취성파괴에 대한 충분한 대비가 요구됨을 알 수 있었다. 그리고 압축파괴에 대한 변형도 계수(deformability factor)는 모든 경우에서 노출시간에 관계없이 큰 변화가 없음을 알 수 있었다.

철근 콘크리트 벽과 강판 콘크리트 벽이 수직으로 만나는 이질접합 구조물의 면외 휨 거동 특성 연구

This paper describes the experimental study on the structural behavior of the vertical plane connection between a reinforced concrete wall and a steel plate concrete wall under out-of-plane flexural loads. The specimen was tested under a dynamic test with the use of cyclic loads. As a result of the test, ductile failure mode of vertical bars was shown under a push load and the failure load was more than that of the nominal strength of the specimen. However, the shear failure mode of the connection was confirmed in case of a pull test and thus demonstrates a need for a shear reinforcement.

Electrical and physical properties of repoled PMN–PT single-crystal sliver transducer

Effects of the repoling process on the dielectric constant (ε33T/ε0) and electromechanical coupling factor bar mode (k′33) after array dicing for lead magnesium niobate–lead titanate (PMN–PT) single-crystal (SC) transducers of a rectangular bar (sliver) form have been investigated for the medical ultrasonic probe application. After the repoling a L 12.0×W 0.15×T 0.35mm sliver showed that ε33T/ε0 and k′33 increased from 4100 to 4500 and 87.2% to 88.6% on average, respectively. Cross-sectional observation of microstructure and a change of ε33T/ε0 values of the PMN–PT SC transducer revealed that the decreased properties of the sliver are attributable to partial depolarization, residual stress, and mechanical damage of the dicing surfaces. Thus, the repoling process of medical array probe is an effective method for improving the PMN–PT SC transducer.

철근 콘크리트와 강판 콘크리트 간 이질접합부로 구성된 구조물의 휨 및 전단거동 특성 연구

본 연구에서는 철근 콘크리트 벽과 강판 콘크리트 벽이 이질접합 형태로 만나는 구조물의 휨 및 전단거동 특성을 검토하기 위하여 L형과 I형 타입의 실험체를 제작하고 구조실험을 수행하였다. 실험 시, 지진하중 등에 대한 실험체의 동적특성을 확인하기 위하여 Push 및 Pull을 반복하는 싸이클 하중을 구현하고자 하였다. L형 실험체에 대한 면외 휨 실험결과, 실험체의 공칭강도를 초과하는 휨 성능을 발휘하였으며, 이에 따라 설계에 적용된 수직철근의 미겹침 이음길이의 타당성을 확인할 수 있었다. 한편, 강판 콘크리트 벽 내에 수평철근의 배근 유무를 변수로 구성한 두 개의 I형 실험체에 대한 면내 전단 실험결과, 수평철근의 배근 유무에 상관없이 공칭강도를 초과하는 전단 성능을 보였다. This paper describes the experimental study on the structural behavior of the joint plane between a RC(Reinforced Concrete) wall and a SC(Steel Plate Concrete) wall under out-of plane flexural loads and in-plane shear loads. The test specimens were produced with L and I shape to assess efficiently flexural and shear behavior of the structures. In order to consider dynamic loads such as earthquake, cyclic loading tests were carried out. As results of the out-of plane flexural tests, ductile failure mode of vertical bars was shown under a push load and the failure load was more than nominal strength of the specimen. And the latter test was performed to verify the variation which was composition presence of horizontal bars in the SC member. The test results showed that capacity of the specimens was more than their nominal strength regardless of composition presence of horizontal bars.