Observation of acoustic turbulence in a system of nonlinear second sound waves in superfluid ⁴He

We discuss the results of recent studies of acoustic turbulence in a system of nonlinear second sound waves in a high-quality resonator filled with superfluid ⁴He. It was found that, when the driving amplitude was sufficiently increased, a steady-state direct wave cascade is formed involving a flux of energy towards high frequencies. The wave amplitude distribution follows a power law over a wide range of frequencies. Development of a decay instability at high driving amplitudes results in the formation of subharmonics of the driving frequency, and to a backflow of energy towards the low-frequency spectral domain, in addition to the direct cascade

Stationary nonlinear waves at the surface of a thin liquid layer under inverted gravitation conditions

Instability of the flat surface of a thin liquid layer wetting a solid substrate under inverted gravitation conditions is discovered. The development of this instability leads to formation of a new stationary nonuniform liquid surface state. It looks like a solitary hill with characteristics sensitive to the liquid film parameters, particularly to the layer thickness at which the instability begins to develop. By application of a variational approach the mechanical stability of such a hill (droplet) in the one-dimensional approximation is proved. A variational picture of the shape evolution for a cylindrical charged droplet in an external electric field is constructed, too. The results obtained are compared with an experiment on liquid hydrogen droplets [A.A. Levchenko, G.V. Kolmakov, L.P. Mezhov-Deglin, M.G. Mikhailov, and A.B. Trusov, Low Temp. Phys. 25, 242 (1999)]. The theory developed is in good agreement with the results of experiments

Turbulence of second sound waves in superfluid He II

We communicate the results of numerical studies of acoustic turbulence in a system of slightly dissipating, nonlinear second sound waves in superfluid He II. It is shown that at sufficiently high amplitude of the external driving force the power-like energy distribution over frequency is formed in the system of second sound waves. This distribution is attributed to formation of the acoustic turbulence regime in the system. The interval of frequencies in which the distribution has a power-like form is expanded to high frequencies with increasing the amplitude of the driving force. The distribution of the energy inside this interval is close to Eω~ 2. It is shown that the distribution of energy Eω depends on the value of the nonlinearity coefficient of the second sound, but does not depend on the sign of the coefficient, i.e., the coherent structures (shock waves) do not contribute to the statistical properties of the turbulent state

Nonlinear and shock waves in superfluid He II

We review studies of the generation and propagation of nonlinear and shock sound waves in He II (the superfluid phase of ⁴He), both under the saturated vapor pressure (SVP) and at elevated pressures. The evolution in shape of second and first sound waves excited by a pulsed heater has been investigated for increasing power W of the heat pulse. It has been found that, by increasing the pressure P from SVP up to 25 atm, the temperature Tα, at which the nonlinearity coefficient of second sound reverse its sign, is decreased from 1.88 to 1.58 K. Thus at all pressures there exists a wide temperature range below Tλ where α is negative, so that the temperature discontinuity (shock front) should be formed at the center of a propagating bipolar pulse of second sound. Numerical estimates show that, with rising pressure, the amplitude ratio of linear first and second sound waves generated by the heater at small W should increase significantly. This effect has allowed us to observe at P 133. atm a linear wave of heating (rarefaction) in first sound, and its transformation to a shock wave of cooling (compression). Measurements made at high W for pressures above and below the critical pressure in He II, Pcr 22. atm, suggest that the main reason for initiation of the first sound compression wave is strong thermal expansion of a layer of He I (the normal phase) created at the heater-He II interface when W exceeds a critical value. Experiments with nonlinear second sound waves in a high-quality resonator show that, when the driving amplitude of the second sound is sufficiently high, multiple harmonics of second sound waves are generated over a wide range of frequencies due to nonlinearity. At sufficiently high frequencies the nonlinear transfer of the wave energy to sequentially higher wave numbers is terminated by the viscous damping of the waves

Active Brownian Particles. From Individual to Collective Stochastic Dynamics

We review theoretical models of individual motility as well as collective dynamics and pattern formation of active particles. We focus on simple models of active dynamics with a particular emphasis on nonlinear and stochastic dynamics of such self-propelled entities in the framework of statistical mechanics. Examples of such active units in complex physico-chemical and biological systems are chemically powered nano-rods, localized patterns in reaction-diffusion system, motile cells or macroscopic animals. Based on the description of individual motion of point-like active particles by stochastic differential equations, we discuss different velocity-dependent friction functions, the impact of various types of fluctuations and calculate characteristic observables such as stationary velocity distributions or diffusion coefficients. Finally, we consider not only the free and confined individual active dynamics but also different types of interaction between active particles. The resulting collective dynamical behavior of large assemblies and aggregates of active units is discussed and an overview over some recent results on spatiotemporal pattern formation in such systems is given.Comment: 161 pages, Review, Eur Phys J Special-Topics, accepte

Experimental studies of the particles’ crystallization process of refractory material under centrifugal-arc dispersion

Backround. The research deals with the study of safe laminated glass of modern cars, consisting of two The object of the study is the method of dispersing a refractory material into a trap with a trapping material. The subject of the study is the process of volumetric crystallization of tungsten carbide particles when trapped with a heat-trapping material. The purpose of the research is to determine the necessary conditions for the crystallization of particles of refractory material and obtain the main characteristics for the design of an industrial trap, their experimental confirmation and interval evaluation. Materials and methods. Experimental studies, photometry methods, collecting dispersed tungsten carbide particles into a trap of an experimental stand filled with heat-trapping material, obtaining cross sections by electron microscopy. Results. A method for evaluating the crystallization conditions of particles of refractory material is proposed, an experiment is conducted to test the criterion of crystallization conditions described in the article. It has been established and confirmed experimentally that violation of this condition leads to a dominant fraction of particles with a quality unacceptable for further use. Conclusions. The experiment on the capture of tungsten carbide particles confirmed the adequacy of the theory determining the conditions of capture and the thermophysical characteristics of the trap material for the formation of spherical, dense, homogeneous powder particles from the tungsten carbide melt

Reconstruction of the charged surface of liquid hydrogen

The evolution of shape of the surface of equipotentially charged liquid hydrogen film condensed on the lower or upper plate of a horizontally placed diode in external electric fields has been studied experimentally under the condition of total compensation of the applied field by the surface charge. Reconstruction phenomenon - the formation of a solitary wave (soliton) - has been observed in an electric field higher than some critical value for the film that covers the lower plate

Rogue waves in superfluid helium.

Rogue waves have been observed in superfluid helium. The experimental system consists of high intensity second sound (temperature-entropy) waves within a resonant cavity. Under steady state conditions, with a constant oscillatory driving force at the resonant frequency, the waves are turbulent and there are fluxes of energy towards both high and low frequencies. Rogue waves appear under the nonequilibrium conditions that prevail shortly after the drive has been switched on, prior to establishment of the steady state. The experiment is described briefly, relevant results are presented and discussed theoretically in terms of nonlinear wave interactions, and possible connections to rogue waves on the ocean are considered

Propagation of short, nonlinear, second-sound pulses through He-II in one- and three-dimensional geometry

The results or an experimental study ot the evolution ot the shape ot nonlinear second-sound pulses in superfluid He-II are reported The pulses propagate in the bulk (3D geometry) and along a cryoacoustic waveguide tilled with liquid helium (quasi-ID geometry) at temperatures corresponding to the negative, positive, or zero nonlinearity coefficient. A strong dependence of the shape of the propagating pulse on the dimensionality of the wave was observed. The finite size of the heater (generator ot a sound) affects the profile ot a short 3D pulse even at distances many times greater than the heater size, which restricts the minimal width of the excited pulse. The experimental data are compared with the results of numerical simulations

Turbulence of Second Sound Waves in Superfluid 4He: Effect of Low-Frequency Resonant Perturbations.

We report the results of investigations of acoustic turbulence in a system of nonlinear second sound waves in a high-quality resonator filled with superfluid 4He. It was observed that subharmonics of a periodic driving force applied to the system may be generated via a parametric instability. We find that application of an additional low-frequency pumping to the turbulent system results in the generation of waves at combination frequencies of the driving forces and also leads to substantial changes in the energy spectrum of the acoustic oscillations