自相似声压驱动下气泡的振动

单泡声致发光来自于气泡经过缓慢膨胀之后的急剧塌缩的过程,而这种急剧塌缩是高阶谐波振动的结果。基于线性叠加的思想,提出了自相似驱动的概念,即用与半径振动相似的声压来驱动气泡。数值模拟表明,自相似驱动声压能够有效地加剧气泡的塌缩过程。与正弦驱动比较,包含基波和二次谐波的自相似驱动显著地提高光辐射强度。这为多频最优化驱动提升发光强度提供了一个直观的物理图景,也给出了一种可用于实验的提升声致发光的方法。     

Thermally driven josephson oscillations in superfluid 4He

We find that a temperature differential can drive superfluid oscillations in 4He. The oscillations are excited by a heater which causes a time dependent temperature differential across an array of 70 nm apertures. By measuring the oscillation frequency and simultaneously determining both temperature and pressure differentials we prove the validity of the most general form of the Josephson frequency relation. These observations were made near saturated vapor pressure, within a few mK of the superfluid transition temperature.     

Transient solar oscillations driven by primordial black holes

Stars are transparent to the passage of primordial black holes (PBHs) and serve as seismic detectors for such objects. The gravitational field of a PBH squeezes a star and causes it to ring acoustically. We calculate the seismic signature of a PBH passing through the Sun. The background for this signal is the observed spectrum of solar oscillations excited by supersonic turbulence. We predict that PBHs more massive than 10(21) g (comparable in mass to an asteroid) are detectable by existing solar observatories. The oscillations excited by PBHs peak at large scales and high frequencies, making them potentially detectable in other stars. The discovery of PBHs would have profound implications for cosmology and high-energy physics.     

Universal phase shift and nonexponential decay of driven single-spin oscillations

We study, both theoretically and experimentally, driven Rabi oscillations of a single electron spin coupled to a nuclear-spin bath. Because of the long correlation time of the bath, two unusual features are observed in the oscillations. The decay follows a power law, and the oscillations are shifted in phase by a universal value of approximately pi/4. These properties are well understood from a theoretical expression that we derive here in the static limit for the nuclear bath. This improved understanding of the coupled electron-nuclear system is important for future experiments using the electron spin as a qubit.     

Modeling non-spherical oscillations and stability of acoustically driven shelled microbubbles

The oscillation and destruction of microbubbles under ultrasound excitation form the basis of contrast enhanced ultrasound imaging and microbubble assisted drug and gene delivery. A typical microbubble has a size of a few micrometers and consists of a gas core encapsulated by a shell. These bubbles can be driven into surface mode oscillations, which not only contribute to the measured acoustic signal but can lead to bubble destruction. Existing models of surface model oscillations have not considered the effects of a bubble shell. In this study a model was developed to study the surface mode oscillations in shelled bubbles. The effects of shell viscosity and elasticity on the surface mode oscillations were modeled using a Boussinesq-Scriven approach. Simulation was conducted using the model with various bubble sizes and driving acoustic pressures. The occurrence of surface modes and the number of ultrasound cycles needed for the occurrence were calculated. The simulation results show a significant difference between shelled bubbles and shell free bubbles. The shelled bubbles have reduced surface mode amplitudes and a narrower bubble size range within which these modes develop compared to shell free bubbles. The clinical implications were also discussed.     

Self-induced oscillations in an optomechanical system driven by bolometric backaction

We have explored the nonlinear dynamics of an optomechanical system consisting of an illuminated Fabry-Perot cavity, one of whose end mirrors is attached to a vibrating cantilever. The backaction induced by the bolometric light force produces negative damping such that the system enters a regime of nonlinear oscillations. We study the ensuing attractor diagram describing the nonlinear dynamics. A theory is presented that yields quantitative agreement with experimental results. This includes the observation of a regime where two mechanical modes of the cantilever are excited simultaneously.     

Chemically driven nonlinear waves and oscillations at an oil-water interface

An experimental investigation is presented of chemically driven dynamic instability of an oil-water interface in a cylindrical and annular glass container. The immiscible liquids are water containing a surfactant (TSAC) and nitrobenzene containing iodine. The reaction at the interface leads to complex deformation patterns including rotating solitary waves, multiple wave trains, periodic and nonperiodic oscillations, source-to-sink propagation, and intermittent behavior. A phase diagram is established for the two solute concentrations ranging from 10^-4 to 10^-1 M. It shows five distinct regions of different dynamic regimes, determined by interaction of at least three mechanisms: dynamic wetting of the container wall, capillary effect, and Marangoni instability of the liquid-liquid interface. The influences of aspect ratio, concentration product, and temperature are investigated and local time traces are derived from electropotential differences. Their spectral analysis reveals details of periodic or irregular motion. Transitions between the dynamic modes of the system during its temporal evolution are recorded. Quantitatively determined profiles of regular waves are analyzed by exponential functions and a simple model for this is proposed.     

Magnetization switching and microwave oscillations in nanomagnets driven by spin-polarized currents

A novel theoretical approach to magnetization dynamics driven by spin-polarized currents is presented. Complete stability diagrams are obtained for the case where spin torques and external magnetic fields are simultaneously present. Quantitative predictions are made for the critical currents and fields inducing magnetization switching, for the amplitude and frequency of magnetization self-oscillations, and for the conditions leading to hysteretic transitions between self-oscillations and stationary states.     

Combustion oscillations in gas fired appliances: Eigen-frequencies and stability regimes

This paper presents a one-dimensional acoustic model for prediction of the frequencies of self-excited oscillation and acoustic mode shapes in combustion systems. The impedance of the combustion system is represented in terms of a frequency response function (FRF). Impedances of the settling and combustion chambers are predicted by using the acoustic model, taking into account the temperature distribution in the combustion chamber. Reasonably good agreement between measured and predicted acoustic resonance frequencies and mode shapes was achieved. Some data on stability regimes are discussed.     

Resonant behavior of stochastic oscillations of general relativistic disks driven by a memory-damped friction

By using a generalized Langevin equation to describe the vertical oscillations of a general relativistic disk subjected to a memory-damped friction and a stochastic force, we derive the power spectrum density(PSD) of accretion disk oscillating luminosity by the method of Laplace transform, and discuss the influence of the system parameters on the resonant behavior in PSD curves. The results show that as the damping strength α and memory time τ of the friction increase, the variation of PSD with spectrum frequency f from monotonous decreasing to occurring maximums, and the phenomenon of a general stochastic resonance(SR) with a single peak and multi-peaks can be found in PSD curves. The radial distance parameter n, the mass M, and spin parameter a* of the black hole determine the inherent frequency of vertical oscillations in the disk,and they have significant influences on the SR phenomena in a system of black hole binaries.     

Mesino-antimesino oscillations

The phenomenology of supersymmetric theories with low scale supersymmetry breaking and a squark as the lightest standard model superpartner are investigated. Such squarks hadronize with light quarks, forming sbaryons and mesinos before decaying. Production of these supersymmetric bound states at a high energy collider can lead to displaced jets with large negative impact parameters. Neutral mesino-antimesino oscillations are not forbidden by any symmetry and can occur at observable rates. Stop mesino-antimesino oscillations would give a sensitive probe of up-type sflavor violation, and can provide a discovery channel for supersymmetry through events with a same-sign top-top topology.     

Proton-positron oscillations

The possibility that baryonic and leptonic matter in nature can oscillate into one another is considered. Consistent with electric charge conservation this leads to proton-positron (or anti-proton-electron) oscillations that conserve baryon (B) minus lepton (L) number. Other equally likely oscillations are neutron-neutrino oscillations that conserve (B+L)-number and neutron-antineutrino oscillations that conserve (B?L) number. Simple theories like the SU(5) and SO(10) predict the transition probability of such modes to be ? 10^?60.     

Undamped fritting oscillations

Fritting oscillations in a glasslike film of methane and chlorine rapidly attenuate. A change in the boundary condition makes them weakly damped, while dosed synchronized injections of vacancies with high-energy particles make it possible to obtain a self-oscillatory system. The mechanism of fritting oscillations is described in detail. An oscillating dissipative structure is formed in the active medium of nonequilibrium glass supersaturated with vacancies and exhibiting a liquid-like behavior. A capillary flow of the medium plays a special role in its evolution.     

不同燃烧状态飞火颗粒自由燃烧规律研究

飞火是开放空间中大尺度火灾非连续性蔓延的主要形式.本文通过不同热流下的木质飞火颗粒自由燃烧实验,揭示不同燃烧状态飞火颗粒的结构变形、质量损失及温度分布的变化规律.研究表明,颗粒结构变形受材料化学反应机制和热机械力作用共同影响;颗粒燃烧反应易造成热解气体的内部积聚,以致内压激增、诱发喷射或喷溅细小颗粒的现象;阴燃过程颗粒...