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Artykuły w czasopismach na temat "Low-modes asymmetries"
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Chaplin, W. J., Y. Elsworth, G. R. Isaak, C. P. McLeod, B. A. Miller i R. New. "A Search for ℓ = 2 Asymmetries in Bison Data". Symposium - International Astronomical Union 185 (1998): 169–70. http://dx.doi.org/10.1017/s0074180900238527.
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An in-depth discussion of the analysis presented here can be found in an up-coming paper (Chaplin et al., 1997).The presence of a magnetic field will raise the degeneracy in ℓ of the resonant p-mode oscillations, via perturbations resulting from the Lorentz force. These degeneracyraising effects will give rise to asymmetric mode-multiplet structures. Both Gough & Thompson (1990). and Dziembowski & Goode (1997) have addressed the implications and potential complications that might result from such phenomena. Here, in an attempt to reveal the presence of an asymmetric frequency structure in the low-degree ℓ = 2 modes, i.e. to measure the asymmetries
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Philidet, J., K. Belkacem, H. G. Ludwig, R. Samadi i C. Barban. "Velocity-intensity asymmetry reversal of solar radial p-modes". Astronomy & Astrophysics 644 (grudzień 2020): A171. http://dx.doi.org/10.1051/0004-6361/202038222.
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The development of space-borne missions has significantly improved the quality of the measured spectra of solar-like oscillators. Their p-mode line profiles can now be resolved, and the asymmetries inferred for a variety of stars other than the Sun. However, it has been known for a long time that the asymmetries of solar p-modes are reversed between the velocity and the intensity spectra. Understanding the origin of this reversal is necessary in order to use asymmetries as a tool for seismic diagnosis. For stars other than the Sun, only the intensity power spectrum is sufficiently resolved to allow for an estimation of mode asymmetries. We recently developed an approach designed to model and predict these asymmetries in the velocity power spectrum of the Sun and to successfully compare them to their observationally derived counterpart. In this paper we expand our model and predict the asymmetries featured in the intensity power spectrum. We find that the shape of the mode line profiles in intensity is largely dependent on how the oscillation-induced variations of the radiative flux are treated, and that modelling it realistically is crucial to understanding asymmetry reversal. Perturbing a solar-calibrated grey atmosphere model, and adopting the quasi-adiabatic framework as a first step, we reproduce the asymmetries observed in the solar intensity spectrum for low-frequency modes. We conclude that, unlike previously thought, it is not necessary to invoke an additional mechanism (e.g. non-adiabatic effects, coherent non-resonant background signal) to explain asymmetry reversal. This additional mechanism is necessary, however, to explain asymmetry reversal for higher-order modes.
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Regan, Marc A., i Krishnan Mahesh. "Adjoint sensitivity and optimal perturbations of the low-speed jet in cross-flow". Journal of Fluid Mechanics 877 (22.08.2019): 330–72. http://dx.doi.org/10.1017/jfm.2019.582.
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The tri-global stability and sensitivity of the low-speed jet in cross-flow are studied using the adjoint equations and finite-time horizon optimal disturbance analysis at Reynolds number $Re=2000$, based on the average velocity at the jet exit, the jet nozzle exit diameter and the kinematic viscosity of the jet, for two jet-to-cross-flow velocity ratios $R=2$ and $4$. A novel capability is developed on unstructured grids and parallel platforms for this purpose. Asymmetric modes are more important to the overall dynamics at $R=4$, suggesting increased sensitivity to experimental asymmetries at higher $R$. Low-frequency modes show a connection to wake vortices. Adjoint modes show that the upstream shear layer is most sensitive to perturbations along the upstream side of the jet nozzle. Lower frequency downstream modes are sensitive in the cross-flow boundary layer. For $R=2$, optimal analysis reveals that for short time horizons, asymmetric perturbations dominate and grow along the counter-rotating vortex pair observed in the cross-section. However, as the time horizon increases, large transient growth is observed along the upstream shear layer. When $R=4$, the optimal perturbations for short time scales grow along the downstream shear layer. For long time horizons, they become hybrid modes that grow along both the upstream and downstream shear layers.
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Deheuvels, S., R. M. Ouazzani i S. Basu. "Near-degeneracy effects on the frequencies of rotationally-split mixed modes in red giants". Astronomy & Astrophysics 605 (wrzesień 2017): A75. http://dx.doi.org/10.1051/0004-6361/201730786.
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Context. The Kepler space mission has made it possible to measure the rotational splittings of mixed modes in red giants, thereby providing an unprecedented opportunity to probe the internal rotation of these stars. Aims. Asymmetries have been detected in the rotational multiplets of several red giants. This is unexpected since all the red giants whose rotation profiles have been measured thus far are found to rotate slowly, and low rotation, in principle, produces symmetrical multiplets. Our aim here is to explain these asymmetries and find a way of exploiting them to probe the internal rotation of red giants. Methods. We show that in the cases where asymmetrical multiplets were detected, near-degeneracy effects are expected to occur, because of the combined effects of rotation and mode mixing. Such effects have not been taken into account so far. By using both perturbative and non-perturbative approaches, we show that near-degeneracy effects produce multiplet asymmetries that are very similar to the observations. We then propose and validate a method based on the perturbative approach to probe the internal rotation of red giants using multiplet asymmetries. Results. We successfully apply our method to the asymmetrical l = 2 multiplets of the Kepler young red giant KIC 7341231 and obtain precise estimates of its mean rotation in the core and the envelope. The observed asymmetries are reproduced with a good statistical agreement, which confirms that near-degeneracy effects are very likely the cause of the detected multiplet asymmetries. Conclusions. We expect near-degeneracy effects to be important for l = 2 mixed modes all along the red giant branch (RGB). For l = 1 modes, these effects can be neglected only at the base of the RGB. They must therefore be taken into account when interpreting rotational splittings and as shown here, they can bring valuable information about the internal rotation of red giants.
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Wilcox, R. S., L. R. Baylor, A. Bortolon, M. Knolker, C. J. Lasnier, D. Shiraki, I. Bykov i in. "Pellet triggering of edge localized modes in low collisionality pedestals at DIII-D". Nuclear Fusion 62, nr 2 (22.12.2021): 026017. http://dx.doi.org/10.1088/1741-4326/ac3b8b.
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Abstract Edge localized modes (ELMs) are triggered using deuterium pellets injected into plasmas with ITER-relevant low collisionality pedestals, and the resulting peak ELM energy fluence is reduced by approximately 25%–50% relative to natural ELMs destabilized at similar pedestal pressures. Cryogenically frozen deuterium pellets are injected from the low-field side of the DIII-D tokamak at frequencies lower than the natural ELM frequency, and heat flux is measured by infrared cameras. Ideal MHD pedestal stability calculations show that without pellet injection, these low collisionality pedestals were limited by their current density (peeling-limited) rather than their pressure gradient (ballooning-limited). ELM triggering success correlates strongly with pellet mass, consistent with the theory that a large pressure perturbation is required to trigger an ELM in low collisionality discharges that are far from the ballooning stability boundary. For sufficiently large pellets, both instantaneous and time-integrated ELM energy deposition measured by infrared cameras is reduced with respect to naturally occurring ELMs at the inner strike point, which is the position where it is largest for natural ELMs. Energy fluence at the outer strike point is less effected. Cameras observing both heat flux and D-alpha emission often find significant toroidally asymmetric striations in the outboard far scrape-off layer resulting from ELMs that are triggered by pellets. Toroidal asymmetries at the inner strike point are similar between natural and pellet-triggered ELMs, suggesting that the reduction in peak heat flux and total fluence at that location is robust for the conditions reported here.
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Woo, K. M., R. Betti, C. A. Thomas, C. Stoeckl, K. Churnetski, C. J. Forrest, Z. L. Mohamed i in. "Analysis of core asymmetries in inertial confinement fusion implosions using three-dimensional hot-spot reconstruction". Physics of Plasmas 29, nr 8 (sierpień 2022): 082705. http://dx.doi.org/10.1063/5.0102167.
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Three-dimensional effects play a crucial role during the hot-spot formation in inertial confinement fusion (ICF) implosions. A data analysis technique for 3D hot-spot reconstruction from experimental observables has been developed to characterize the effects of low modes on 3D hot-spot formations. In nuclear measurements, the effective flow direction, governed by the maximum eigenvalue in the velocity variance of apparent ion temperatures, has been found to agree with the measured hot-spot flows for implosions dominated by mode [Formula: see text]. Asymmetries in areal-density ( ρR) measurements were found to be characterized by a unique cosine variation along the hot-spot flow axis. In x-ray images, a 3D hot-spot x-ray emission tomography method was developed to reconstruct the 3D hot-spot plasma emissivity using a generalized spherical-harmonic Gaussian function. The gradient-descent algorithm was used to optimize the mapping between the projections from the 3D hot-spot emission model and the measured x-ray images along multiple views. This work establishes a platform to analyze 3D low-mode core asymmetries in ICF.
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Bugnet, L., V. Prat, S. Mathis, A. Astoul, K. Augustson, R. A. García, S. Mathur, L. Amard i C. Neiner. "Magnetic signatures on mixed-mode frequencies". Astronomy & Astrophysics 650 (czerwiec 2021): A53. http://dx.doi.org/10.1051/0004-6361/202039159.
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Context. The discovery of moderate differential rotation between the core and the envelope of evolved solar-like stars could be the signature of a strong magnetic field trapped inside the radiative interior. The population of intermediate-mass red giants presenting surprisingly low-amplitude mixed modes (i.e. oscillation modes that behave as acoustic modes in their external envelope and as gravity modes in their core) could also arise from the effect of an internal magnetic field. Indeed, stars more massive than about 1.1 solar masses are known to develop a convective core during their main sequence. The field generated by the dynamo triggered by this convection could be the progenitor of a strong fossil magnetic field trapped inside the core of the star for the remainder of its evolution. Aims. Observations of mixed modes can constitute an excellent probe of the deepest layers of evolved solar-like stars, and magnetic fields in those regions can impact their propagation. The magnetic perturbation on mixed modes may therefore be visible in asteroseismic data. To unravel which constraints can be obtained from observations, we theoretically investigate the effects of a plausible mixed axisymmetric magnetic field with various amplitudes on the mixed-mode frequencies of evolved solar-like stars. Methods. First-order frequency perturbations due to an axisymmetric magnetic field were computed for dipolar and quadrupolar mixed modes. These computations were carried out for a range of stellar ages, masses, and metallicities. Conclusions. We show that typical fossil-field strengths of 0.1 − 1 MG, consistent with the presence of a dynamo in the convective core during the main sequence, provoke significant asymmetries on mixed-mode frequency multiplets during the red giant branch. We provide constraints and methods for the detectability of such magnetic signatures. We show that these signatures may be detectable in asteroseismic data for field amplitudes small enough for the amplitude of the modes not to be affected by the conversion of gravity into Alfvén waves inside the magnetised interior. Finally, we infer an upper limit for the strength of the field and the associated lower limit for the timescale of its action in order to redistribute angular momentum in stellar interiors.
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Alexander, D. E. "Wind tunnel studies of turns by flying dragonflies". Journal of Experimental Biology 122, nr 1 (1.05.1986): 81–98. http://dx.doi.org/10.1242/jeb.122.1.81.
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High-speed movies of dragonflies turning in flight on flexible tethers show that there are two distinct modes of turning. In the ‘conventional’ mode, dragonflies use left-right asymmetries in the wing stroke amplitude, and occasionally in the angle of attack, to produce more lift and thrust on one side than the other. This causes the animal to roll into a bank, so that the lift vector has a sideward component; this sideward component produces the turn, much as in an airplane. This type of turn is probably most useful during fast forward flight. The second mode of turning in the ‘yaw turn’. Yaw turns are accomplished without banking, and the dragonfly's long axis may turn more than 90 degrees in the period of two wing strokes. The kinematics of this turn could not be as closely analysed, but it appears that dragonflies use drag on the inner wing upstroke and the outer wing downstroke to turn, much like pivoting a rowing boat. This turn may be hampered by drag on the abdomen during fast forward flight and would be most useful at low speeds or during hovering.
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CAZACU, Emil, i Lucian PETRESCU. "ASPECTE CALITATIVE ȘI CANTITATIVE ALE FENOMENUL DE FEROREZONANȚĂ ÎN INSTALAȚIILE ELECTRICE DE JOASĂ TENSIUNE". "ACTUALITĂŢI ŞI PERSPECTIVE ÎN DOMENIUL MAŞINILOR ELECTRICE (ELECTRIC MACHINES, MATERIALS AND DRIVES - PRESENT AND TRENDS)" 2020, nr 1 (10.02.2021): 1–14. http://dx.doi.org/10.36801/apme.2020.1.8.
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The phenomenon of ferroresonance is generated by the interaction between nonlinear magnetic devices and capacitive elements in an electrical installation in which losses are reduced and which are constantly supplied by at least one energy source. Feroresonance is manifested by the appearance of overvoltages and overcurrents in the installation with strongly distorted waveforms. Also, the phenomenon is accompanied by other disturbances of the quality of electricity (voltage fluctuations, asymmetries, noise, etc.), which propagate in the network affecting the proper functioning of the entire installation. In addition, unlike linear resonance, ferroresonance allows the manifestation of several stable states (modes) for the same parameters of the network, these being imposed by the initial conditions in the installation and the moment of occurrence of the phenomenon. Thus, the vulnerability to low resonance of a low voltage installation has become an indicator of electricity quality. This paper presents a procedure for calculating and investigating this phenomenon based on the analysis of numerical solutions of systems of differential equations (nonlinear and non-autonomous), which models the transient phenomena that initiate the appearance of ferroresonance (usually switching processes). Also, modern means of investigation are used (3D visualizations in the phase plan or Poincaré diagrams), imposed by the difficulty of the quantitative analysis both in dynamic regime and in stationary regime of ferroresonance. In addition, methods and procedures are proposed to mitigate the effects of the ferroresonance phenomenon on equipment or network elements in electrical distribution installations.
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Hall, H. G. "Parental analysis of introgressive hybridization between African and European honeybees using nuclear DNA RFLPs." Genetics 125, nr 3 (1.07.1990): 611–21. http://dx.doi.org/10.1093/genetics/125.3.611.
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Abstract African honeybees, introduced into Brazil 33 years ago, have spread through most of South and Central America and have largely replaced the extant European bees. Due to a paucity of genetic markers, genetic interactions between European and African bees are not well understood. Three restriction fragment length polymorphisms (RFLPs), detected with random, nuclear DNA probes, are described. The polymorphisms are specific to bees of European descent, possibly specific to certain European races. Each European marker was found present at a high frequency in U.S. colonies but absent in South African bees. Previous mitochondrial DNA studies of neotropical bees have revealed negligible maternal gene flow from managed European apiaries into feral African populations. The findings reported here with nuclear DNA show paternal gene flow between the two but suggest asymmetries in levels of introgressive hybridization. Managed colonies in southern Mexico, derived from European maternal lines, showed diminished levels of the European nuclear markers, reflecting significant hybridization with African drones. The European alleles were present only at low frequencies in feral swarms from the same area. The swarms were of African maternal descent. In Venezuelan colonies, also derived from African maternal lines, the European markers were almost totally absent. The results point to limited paternal introgression from European colonies into the African honeybee populations. These findings dispute other views regarding modes of Africanization.
Rozprawy doktorskie na temat "Low-modes asymmetries"
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Viala, Diego. "Étude de la physique de l'allumage par choc". Electronic Thesis or Diss., Bordeaux, 2024. http://www.theses.fr/2024BORD0127.
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Une décennie d'expériences au "National Ignition Facility" a démontré que la fusion par confinement inertiel constitue une approche crédible pour la production d'énergie, avec des résultats allant au-delà du régime d'allumage. Cependant, l'approche par attaque indirecte se révèle inadéquate pour les implosions à gain élevé et une production d'énergie fiable. L'option d'allumage par attaque directe est privilégiée en raison de ses conceptions de cible plus simples et de son meilleur couplage énergétique. Actuellement, aucune installation laser à l'échelle d'allumage n'est configurée selon l'approche directe standard, posant ainsi un défi à relever. Les expériences intégrées d'allumage par attaque directe se sont principalement concentrées sur la compréhension de la physique à des échelles réduites, dans le but ultime de démontrer la nécessité et la faisabilité de construire une installation laser internationale dédiée à l'attaque directe.Ce manuscrit de thèse présente une étude approfondie sur la validation de codes d'hydrodynamique radiative 3D à l'état de l'art, ainsi que sur la compréhension des bas modes d'éclairement laser et du couplage laser-cible, jouant un rôle crucial dans la fusion par confinement inertiel. L'examen attentif des modèles de CBET revêt une importance capitale dans ce contexte, assurant la précision des simulations et contribuant à la conception des futures installations à attaque directe. De plus, l'investigation de l'homogénéité du laser sur la cible est impérative pour appréhender son impact global sur le systèmeA decade of experiments at the National Ignition Facility has proven that inertial confinement fusion is a credible approach to energy production, with experiments having exceeded the ignition regime. However, the indirect-drive approach is not suited for high gain implosions and reliable energy production. The direct-drive ignition approach is favoured for energy production as it features simpler target designs and couples more energy to them. There are currently no ignition-scale laser facilities configured for the standard direct-drive approach. Integrated direct-drive experiments have mostly been focused on understanding the physics at reduced scales, with the ultimate goal of demonstration of necessity and feasibility of construction of an international direct-drive laser facility.This thesis manuscript presents a study on the validation of state-of-the-art 3D radiative hydrodynamics codes and the understanding of low modes and laser coupling which play crucial roles in the study of inertial fusion energy. Careful examination of CBET models is of paramount importance in this context, ensuring the accuracy of simulations and contributing to the design of future direct-drive facilities. In addition, the investigation of laser homogeneity on target is imperative to understand its overall impact on the system
Streszczenia konferencji na temat "Low-modes asymmetries"
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Colaitis, Arnaud, Igor Igumenschev, David Turnbull, Rahul Shah, Dana Edgell, Owen Mannion, Christian Stoeckl i in. "3D Simulations of OMEGA Implosions in Presence of Low Mode Asymmetries: Systematic Flow Anomalies and Impact of Low Modes on Implosion Performance." W Proposed for presentation at the Matter and Radiation at Extremes Young Scientist Award Event held May 16-17, 2022 in Virtual, Virtual Virtual. US DOE, 2022. http://dx.doi.org/10.2172/2003267.
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Pray, Carl M., i Stephen A. Hambric. "Finite Element Study of Harmonic Forcing Function Scattering Mechanisms for Cylindrical Structures". W ASME 2002 International Mechanical Engineering Congress and Exposition. ASMEDC, 2002. http://dx.doi.org/10.1115/imece2002-32686.
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Low frequency sound radiation from cylindrical structures is generally dominated by radiation from low order (M=0, 1) circumferential harmonic modes. The high order circumferential harmonic modes are inefficient radiators below the coincidence frequency. For cylindrical structures excited by low frequency, high circumferential order (e.g. N=10) forcing functions, the radiated sound will be dominated by radiation from the low order modes if the forcing function is scattered by structural asymmetries into these modes. In this study, cylindrical structures of varying levels of asymmetry are examined using finite element analysis to quantify the scattering of order N harmonic forcing functions into order M structural response harmonics. First, purely axially-symmetric structures are examined to verify the analysis method used. The models examined include a finite cylinder with constant cross-section and a finite cylinder with tapered cross-section. The constant cross-section cylinder is analyzed in vacuo, and the tapered cylinder is analyzed both in vacuo and with water loading. These structures show no harmonic scattering, as would be expected. Next, a finite tapered cylinder with cyclically symmetric impedance discontinuities and a finite tapered cylinder with an asymmetric large impedance discontinuities are analyzed, both with water loading, to determine the impact of structural discontinuity on the harmonic scattering and radiated sound spectra. The periodic and single discontinuities both show significant scattering into low order circumferential modes and corresponding increases in radiated sound.
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Humbert, Sylvain C., Alessandro Orchini, Christian Oliver Paschereit i Nicolas Noiray. "Symmetry Breaking in an Experimental Annular Combustor Model With Deterministic Electroacoustic Feedback and Stochastic Forcing". W ASME Turbo Expo 2022: Turbomachinery Technical Conference and Exposition. American Society of Mechanical Engineers, 2022. http://dx.doi.org/10.1115/gt2022-81729.
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Abstract In this study, we use an annular combustor experimental model with electroacoustic feedback to investigate systematically the effect of stochastic forcing and of non-uniform flame response distribution on azimuthal thermoacoustic modes. We break the symmetry of a nominally degenerate mode of azimuthal order m by imposing a non-zero 2m Fourier component of the flame gain, b2m, and of the time-delay, ε2m. Various orientations between the gain and the time-delay staging patterns are considered. In addition, stochastic forcing is introduced. First, all experiments are performed without noise, as well as at the maximum noise intensity. We observe that the mode nature that dominates in the presence of an intense noise may be far from the one observed in noise-free conditions. To better understand the effect of noise in the presence of asymmetries, we repeat some of the experiments at various noise intensities. Although our results confirm that for the axisymmetric configuration and for some asymmetric configurations pure spinning modes are never reached, we also observe some radically different behaviors. For a noise-free experiment leading to a purely standing mode, the introduction of a sufficient amount of noise can lead to beating. We also observe that, for a mode that is nearly standing in the absence of noise, an increase in the noise intensity leads to the predominance of mixed modes with a clearly favored spinning direction. We explain our experimental results with the aid of low order models.
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Darvishian, Ali, Behrouz Shiari, Jae Yoong Cho i Khalil Najafi. "Effect of Imperfections on Fused Silica Shell Resonators". W ASME 2014 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2014. http://dx.doi.org/10.1115/imece2014-39303.
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Axisymmetric shell resonators have been attractive candidates for high performance MEMS vibratory gyroscopes because of their high quality factor, low sensitivity to environmental vibrations and electrostatic tuning capability. Fused silica shell resonators made by blow torch molding with high quality factor could perform as high performance MEMS gyroscopes. Despite such advantageous features, the performance of these shell resonators is limited by geometric imperfections that occur during fabrication. This paper investigates effect of geometric asymmetries such as height and radius imperfections, notch, and mass imbalance in the rim of gyroscopes on the split in natural frequencies of the n=2 wineglass modes. Numerical simulation shows that perfect fused silica shell has 13929 Hz natural resonance frequencies without any frequency split. Analysis of imperfect shell reveals that frequency split is very sensitive to edge geometric imperfection and mass imbalance in the rim. On the other hand, Δf is not very sensitive to the notch in the rim of shell and height imperfection less than 40μm.
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O’Connor, Jacqueline, i Tim Lieuwen. "Influence of Transverse Acoustic Modal Structure on the Forced Response of a Swirling Nozzle Flow". W ASME Turbo Expo 2012: Turbine Technical Conference and Exposition. American Society of Mechanical Engineers, 2012. http://dx.doi.org/10.1115/gt2012-70053.
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This study describes continued investigations of the response of a swirling flow to transverse acoustic excitation. This work is motivated by transverse combustion instabilities in annular gas turbine engine architectures. This instability provides a spatially varying acoustic velocity disturbance field around the annulus, so that different nozzles encounter different acoustic disturbance fields. In this study, we simulate this effect by looking at a standing wave acoustic field where the nozzle is located at either a velocity anti-node, referred to as out-of-phase forcing, and a velocity node, referred to as in-phase forcing. The out-of-phase forcing condition provides an asymmetric forcing field about the center plane of the flow and excites an asymmetric response in the flow field; the in-phase forcing provides a symmetric forcing condition and results in symmetric flow response near the nozzzle. The symmetric versus asymmetric flow response was measured in two ways. First, in the r-x plane where axial and radial components of velocity are measured using high-speed particle image velocimetry (PIV), a helical and ring vortex rollup of the shear layers is evident in the asymmetric and symmetric forcing condition, respectively. Additionally, the swirling motion of the jet is measured in the r-θ plane at two downstream locations and a spatial decomposition is used to calculate the strength of azimuthal modes in radial velocity fluctuations. At the forcing frequency, the m = 0 mode is strongly excited at the nozzle exit with symmetric forcing, while asymmetric forcing results in a strong peak in the m = 1 mode, or the first helical mode. The results in this plane of view are congruent with those in the r-x plane. Further downstream, however, the mode strengths change as the vorticity is religned and natural asymmetries of the swirling jet set in. Finally, the low frequency self-excited motion of the vortex core was measured and characterized in the unforced flow. It is composed of an m = −1 and m = −2 mode, and the physical interpretation of these mode numbers is highlighted. High amplitude acoustic forcing decreases the amplitude of oscillation of this structure in both the in-phase and out-of-phase forcing, but to varying degrees.
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Wang, Jing, Xin Hui, Hao Qin, Chi Zhang i Yuzhen Lin. "Unsteady Heat Release of Stratified Swirling Flames in a Partially Premixed Pre-Vaporized Combustor". W ASME Turbo Expo 2016: Turbomachinery Technical Conference and Exposition. American Society of Mechanical Engineers, 2016. http://dx.doi.org/10.1115/gt2016-57061.
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Unsteady heat release of stratified swirling flames in a partially premixed pre-vaporized combustor at elevated pressure and temperature is experimentally investigated. Stratified flames produced by a pilot swirler and a main one which are fueled by different flow rates of aviation kerosene. Three fuel staging ratios (SR) of 25%, 30%, and 35% are tested to study its effect on the heat release of stratified flame. CH* chemiluminescence emissions are recorded by a high speed ICCD camera with a 430±10 nm band-pass filter. The Proper Orthogonal Decomposition (POD) analysis is employed to identify the most prominent energetic modes of flame fluctuations. Test results show that the low frequency fluctuation of the flame global heat release is weakened as the SR decreases while the high frequency fluctuation is enhanced. According to the analysis of average heat release of the flame, SR affects the structure of flame by changing the flame angle. The stratified flame dynamics mainly performs the bulk motion, asymmetric motion, and flame shedding. It is found that the flame bulk motion modes contain higher energy than those of other modes. While the other two lower energy modes would transit from asymmetric motion to flame shedding mode as the SR decreases. From the Fast Fourier Transform (FFT) analysis, the low frequency fluctuation of global heat release is dominated by modes of higher energy, while the high frequency fluctuation is related to all the modes.
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Zhang, Huanyu, Xinliang Tian, Yakun Zhao, Xin Li i Wenyue Lu. "Symmetry Breaking of a Rigid-Flexible Coupling System at Low Reynolds Numbers". W ASME 2023 42nd International Conference on Ocean, Offshore and Arctic Engineering. American Society of Mechanical Engineers, 2023. http://dx.doi.org/10.1115/omae2023-102999.
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Abstract Flow past a rigid-flexible coupling system consisting of a rigid plate and a trailing closed flexible filament was simulated numerically. An immersed boundary method was developed based on OpenFOAM to solve this fluid-structure interaction (FSI) problem. Three simulations including flow past a rigid circular cylinder, a rigid plate, and two flexible loops were conducted to validate our code. The motion modes transition and dynamic performance of the coupling system were investigated at low Reynolds numbers (Re ≤ 80), where the tension coefficient and bending coefficient of the filament were fixed. According to the symmetry of the filament motion, six motion modes have been identified for different filament lengths and Reynolds numbers, i.e. symmetric and stationary (SS) mode, asymmetric and stationary (AS) mode, regular and unilateral flap (RUF) mode, transition motion (TM) mode, symmetric and bilateral flap (SBF) mode, and asymmetric and bilateral flap (ABF) mode. Moreover, symmetry breaking was discovered in the AS, RUF, and ABF modes. The positions of the filament at different times were drawn to describe its movement and distinguish motion modes. Meanwhile, the drag and lift of the coupling system were investigated and a significant drag reduction was found. In addition, The time-averaged lift directly depended on the symmetry of the filament and rapidly changed with a sudden alteration in symmetry. Our research is helpful to understand the change of motion behavior of this coupling system at low Reynolds numbers.
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Park, Jung Seo, I. Y. Shen i C. P. Roger Ku. "Rocking Vibration of Rotating Disk and Spindle Systems With Asymmetric Bearings". W ASME 2001 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference. American Society of Mechanical Engineers, 2001. http://dx.doi.org/10.1115/detc2001/vib-21566.
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Abstract This paper studies how bearing asymmetry affects natural frequencies and mode shapes of a rotating disk/spindle system through a numerical simulation and a perturbation analysis. Existing literature has shown that rocking motion of a rotating disk/spindle system with symmetric bearings consists of rigid body rocking of the spindle, one-nodal-diameter modes of each disk, and deformation of spindle bearings. The rocking motion, characterized by (0,1) unbalanced modes, has repeated natural frequencies when the spindle is stationary, because the disk/spindle system is axisymmetric. For a rotating spindle, (0,1) unbalanced modes evolve into forward and backward precession with circular orbits. In this paper, the numerical simulation shows that bearing asymmetry splits a pair of repeated (0,1) unbalanced modes into two modes with distinct frequencies when the spindle is stationary. Moreover, when the rotational speed increases from zero, the (0,1) unbalanced mode with lower frequency evolves into backward precession and the (0,1) unbalanced mode with higher frequency evolves into forward precession. The precession orbits are elliptical because of the bearing asymmetry. Two perturbation schemes are developed to prove the phenomena observed in the numerical simulation. For low rotational speed, a stationary disk/spindle system with symmetric bearings serves as the unperturbed system. Both the bearing asymmetry and gyroscopic effects from rotation form the perturbation. A contraction iteration predicts the effects of bearing asymmetry on natural frequencies and mode shapes. For high rotational speed, a rotating (gyroscopic) disk/spindle system with symmetric bearings serves as the unperturbed system. The bearing asymmetry forms the perturbation. To obtain a perturbation solution, the solvability condition is first derived for the unperturbed gyroscopic system. Lindsted-Poincaré approach then predicts the effects of bearing asymmetry on natural frequencies and mode shapes of the rotating disk/spindle system.
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Chiriac, Victor Adrian, i Alfonso Ortega. "Control of Convective Heat Transfer in a Confined Laminar Impinging Jet by Low Amplitude Forcing". W ASME 2002 International Mechanical Engineering Congress and Exposition. ASMEDC, 2002. http://dx.doi.org/10.1115/imece2002-32909.
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A numerical finite-difference model, derived using a control-volume approach, was used to compute the flow and heat transfer characteristics in a two-dimensional confined laminar air jet impinging on an isothermal surface. Several cases were considered with Re=650, 750, and nozzle to plate spacing, H/W=5. The behavior of the jet and the attendant heat transfer from the target wall were investigated when the jet was forced by fluidic excitation at the nozzle exit. At Re between 585 and 610, the unforced jet exhibits a transition to an unsteady regime leading to asymmetric vortex shedding and jet flapping [1, 2]. Investigation of the velocity spectra indicate three distinct dominant modes; the lowest frequency is associated with the jet flapping while the highest frequency is associated with the asymmetric vortex formation which causes buckling of the jet column. As a result of the two combined modes, the peak heat transfer is enhanced and the lateral cooling extent is broadened. The jet was subjected to forcing by introduction of numerical excitation at each side of the jet that modeled fluidic excitation. The jet was forced on both opposing sides at its exit, both with in-phase and out-of-phase modes. Under some conditions, out of phase forcing at Re=650 at the highest frequency leads to stabilization of the normally separated flow on one side only. This unusual asymmetric flow field is unsteady but repeatable, and results in an enhancement of the heat transfer. At Reynolds number of 750, forcing with an out of phase mode at the highest frequency leads to a complete stabilization of the jet. The forcing suppresses the high-amplitude low frequency flapping mode leaving only a high frequency vortex formation mode. The suppression of the jet flapping leads to a decrease in the peak heat transfer, but because separation is suppressed, the average wall heat transfer is enhanced.
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Barbosa Moreira, Alysson Bruno, i Fabrice Thouverez. "Influence of Blade Flexibility on the Dynamic Response Simulation of a Turbomolecular Pump on Magnetic Bearings". W ASME Turbo Expo 2019: Turbomachinery Technical Conference and Exposition. American Society of Mechanical Engineers, 2019. http://dx.doi.org/10.1115/gt2019-90990.
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Abstract This paper proposes the simulation of a complete mechanical model of a turbomolecular pump rotor, including rotor and blades flexibility, suspended by controlled active magnetic bearings. The mechanical model is composed of an eight stage blisk, attached to a shaft. Magnetic forces are linearized by first order Taylor expansion around a given point. Including blades and rotor flexibility makes the mechanical system asymmetric, so the equations of motion for the coupled system have periodic terms. A modal controller was designed to control rigid body modes, since the number of sensors is limited and no state observer is implemented. PID controllers are used for low frequency modes combined with second order filters to damp high frequency modes. First of all, stability analysis was carried out for the axisymmetric case. Secondly, blades flexibility was included. Forced response of the whole system to an impulsive force was studied. Divergent responses for the system in rotation were obtained as a second order filter pole possibly interacts with blades modes. Taking second order filters off the control loop allowed the system to be stable. These results show that the analysis method developed here is efficient to evaluate the performance of a controller in closed loop with the complete flexible system. This method may be used in industrial design processes as computation times for the complete system are very short.