Microrheology, stress fluctuations, and active behavior of living cells

We report the first measurements of the intrinsic strain fluctuations of living cells using a recently developed tracer correlation technique along with a theoretical framework for interpreting such data in heterogeneous media with nonthermal driving. The fluctuations' spatial and temporal correlations indicate that the cytoskeleton can be treated as a course-grained continuum with power-law rheology, driven by a spatially random stress tensor field. Combined with recent cell rheology results, our data imply that intracellular stress fluctuations have a nearly 1/omega2 power spectrum, as expected for a continuum with a slowly evolving internal prestress.     

Fluctuations and Correlations of Pure Quantum Turbulence in Superfluid 3He-B

We describe the first measurements of line-density fluctuations and spatial correlations of quantum turbulence in superfluid 3He-B. All of the measurements are performed in the low-temperature regime, where the normal-fluid density is negligible. The quantum turbulence is generated by a vibrating grid. The vortex-line density is found to have large length-scale correlations, indicating large-scale collective motion of vortices. Furthermore, we find that the power spectrum of fluctuations versus frequency obeys a -5/3 power law which verifies recent speculations that this behavior is a generic feature of fully developed quantum turbulence, reminiscent of the Kolmogorov spectrum for velocity fluctuations in classical turbulence.     

Quantum fluctuations of vacuum stress tensors and spacetime curvatures

We analyze the quantum fluctuations of vacuum stress tensors and spacetime curvatures, using the framework of linear response theory which connects these fluctuations to dissipation mechanisms arising when stress tensors and spacetime metric are coupled. Vacuum fluctuations of spacetime curvatures are shown to be a sum of two contributions at lowest orders; the first one corresponds to vacuum gravitational waves and is restricted to light-like wavevectors and vanishing Einstein curvature, while the second one arises from gravity of vacuum stress tensors. From these fluctuations, we deduce noise spectra for geodesic deviations registered by probe fields which determine ultimate limits in length or time measurements. In particular, a relation between noise spectra characterizing spacetime fluctuations and the number of massless neutrino fields is obtained.     

Zur spektralen Verteilung von Temperatur- und Konzentrationsschwankungen in turbulenten Strömungen

On the Spectrum of Turbulent Fluctuations of Temperature and Concentration in Turbulent Flows The form of the turbulent transfer of a passive scalar, which is equivalent to the ?5/3 power law of turbulent temperature (concentration) fluctuations, is derived and shown to confirm an ansatz by CORRISIN. On this basis we obtain the spectrum of temperature fluctuations for the transition from the inertial and convective range to the viscous and diffusive range dependent on the Prandtl number (analogue to KOVASZNAY'S formula for the energy spectrum). Qualitative agreement is obtained with measurements of the one-dimensional spectrum in the inertial-diffusive and viscousconvective subrange.     

Power spectrum crossover in sediments of a paleolake disturbed by volcanism

We study density fluctuations from sediments of a paleolake in central Mexico that was subjected to volcanic perturbations by means of computed tomography (CT) measurements on blocks chiselled out of mines at the lake's bed. The mine walls show laminations corresponding to the alternation of low density diatom sediments and high density volcanic ash depositions. We have previously shown that there is a range of scales where these fluctuations present a self-similar behavior [1]. Here we relate density correlation calculations to the power spectrum of the fluctuations. We show that a scaling region in the power spectrum coincides with the scaling region in the correlations produced by relaxation from intense volcanic perturbations to steady state fluctuations. There appears to be a kink-like crossover in the power spectrum from mid range scaling to a shorter range scale invariance. This, together with the density probability distribution of the fluctuations, draws attention to the dominant role of rare events. We believe that our analysis may be useful for the understanding of other phenomena with similar power spectrum properties, in which a scale invariance in the unperturbed system is altered by external perturbations that induce an additional scaling behavior.     

Millimetric flux densities as a test of atmospheric turbulence

In this paper it is shown how submillimetre and millimetre flux density measurements of sky fluctuations can be used to infer the characteristics of atmospheric turbulence.Two different approaches have been followed: first, by considering the observed process as due to the atmospheric turbulence, we used the r.m.s. sky fluctuations to derive some physical parametres characterizing the phenomenon; second, by investigating the power spectrum and the autocorrelation function, we derive an empirical model (the AR model) to try to deduce the physical process. In this case, the fitted power spectrum agrees strikingly with that predicted by the theory of a stationary turbulent flow for the atmosphere in the sampled frequency range. This kind of modeling can help to disentangle the atmospheric fluctuations from the extraterrestrial signal in ground-based astrophysical measurements.Measurements of the water vapour content and atmospheric transmission at the Italian Base in Antarctica, where the data have been taken, at the wavelengths of 350 μm, 1 and 2mm are also reported.     

Multifractal dimension of Lagrangian turbulence

We report experimental measurements of the Lagrangian multifractal dimension spectrum in an intensely turbulent laboratory water flow by the optical tracking of tracer particles. The Legendre transform of the measured spectrum is compared with measurements of the scaling exponents of the Lagrangian velocity structure functions, and excellent agreement between the two measurements is found, in support of the multifractal picture of turbulence. These measurements are compared with three model dimension spectra. When the nonexistence of structure functions of order less than -1 is accounted for, the models are shown to agree well with the measured spectrum.     

微波多普勒反射计数据分析的一种优化方法

结合对多普勒频谱的分析,提出一种基于双高斯拟合的轴对称-非对称谱(SAS)分析方法。在此方法下,功率谱主要由轴对称功率谱和非轴对称功率谱两部分组成。前者主要由等离子体截止层密度的扰动调制决定,后者主要由波数为k _⊥=2k₀sinθ 的湍流的极向密度扰动决定。在分析多普勒反射计的数据时,相较于常用的频谱重心(COG)分析方法和相位微分(δ-phase)法,SAS 谱分析不仅提高了求取多普勒频移的准确性,同时还可以获取等离子体中波数为k_⊥ 的极向湍流的密度扰动强度和截止层的密度扰动强度。     

Characterization of the noise in continuously operating mode-locked lasers

An experimental and theoretical investigation of the fluctuations of the pulses from continuous-wave mode-locked lasers is presented. It is shown that these fluctuations can be detected and quantitatively characterized from measurements of the power spectrum of the light intensity. Such power spectra can be measured with great accuracy by shining the laser output on a suitable photodetector and by processing the detector signal with the use of an electronic spectrum analyzer. Different types of noise such as fluctuations of the pulse energy, pulse repetition time, and pulse duration, can be readily recognized from their characteristic spectral signature. Experimental results of noise measurements are presented for a synchronously mode-locked dye laser pumped by an acousto-optically mode-locked argon ion laser, and also for a colliding pulse passively mode-locked dye laser.     

Combustion noise—Measurements on single oil drops and other diffusion flames

Measurements were taken on four oil/air or gas/air diffusion flames to investigate the connection between fluctuations in air velocity in or near the flame zone, fluctuations in burning rate and combustion noise output. In all cases the combustion noise spectrum was found to be approximately 6 dB per octave up on the spectrum of air velocity fluctuations. The simple model of the noise generation mechanism suggested by this result was not supported by measurements on burning rate fluctuations. However, in almost all cases the transfer functions between fluctuations in air velocity, burning rate and sound output could be expressed as a function of frequency by a simple power law.     

Spatial filtering of wall pressure fluctuations. Methods of direct measurements of wave number-frequency spectra

The problems of spatial filtering of turbulent aerohydrodynamic noise sources are considered in connection with the problem of the direct measurements of wave number-frequency spectra of turbulent pressure fluctuations. The methods of wave-vector filtering of turbulent pressure fluctuations with the use of an acoustic array, i.e., a periodic structure with a finite number of elementary rectangular pressure transducers, are analyzed. Original versions of the wave number-frequency spectrum analyzer that allows the reconstruction of the wave number spectrum from the results of measurements are developed. The filtering characteristics of such analyzers are studied, and the relation between the wave number characteristic of an acoustic array and the wave number spectrum of the amplitude distribution of transducer’s local sensitivity over the aperture is determined.     

Active and passive scalar intermittent statistics in turbulent atmospheric convection

We study the small-scale statistics of active and passive scalar fields, obtained from 3D large-eddy simulations of the atmospheric boundary layer turbulence. The velocity field is anisotropic and inhomogeneous, due to the action of both buoyancy and shear. We focus on scalar field rare fluctuations dominated by the so-called fronts. Temperature, coupled to the velocity field by the Boussinesq equations, exhibits anomalous scaling and saturation of the scaling exponents to a constant value, due to the presence of thermal fronts. Although qualitatively similar, the small-scale statistics of a passive tracer advected by the convective flow shows quantitative differences: the large fluctuations of the tracer concentration field distribute differently and appear to be less intermittent than the temperature ones. To better understand these results, the role of boundaries in this problem is discussed.     

Effect of non-kolmogorov turbulence on angle-of-arrival fluctuations of starlight

We revised a non-Kolmogorov turbulent power spectrum for the refractive-index fluctuations based on the consistency between the structure function and its power spectrum and the experimental data of recent lidar measurements. We investigate the joint influence of Kolmogorov turbulence from the ground up to 6 km and non-Kolmogorov turbulence above 6 km on the fluctuations in the angle of arrival (AOA) of starlight. WE show that the AOA fluctuations of starlight are mainly determined by Kolmogorov turbulence nearby the receiver. Non-Kolmogorov turbulence is responsible for 20–40% of the total AOA fluctuations for different apertures of the receiver. In addition, the AOA fluctuations induced by non-Kolmogorov turbulence depend on the receiver aperture, outer scale, and intensity of non-Kolmogorov turbulence.     

On an investigation of slow frequency fluctuations

The possibilities of an experimental investigation of the frequency fluctuations of the oscillations are considered by means of a digital electronic frequency meter. A comparison is made of the results of the measurements with the spectrum of the fluctuations, and an analysis of the errors of the measurements is likewise carried out.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 15, No. 4, pp. 573–579, April, 1972.     

Superconductivity induced by longitudinal ferromagnetic fluctuations in UCoGe

From detailed angle-resolved NMR and Meissner measurements on a ferromagnetic (FM) superconductor UCoGe (T(Curie)～2.5 K and T(SC)～0.6 K), we show that superconductivity in UCoGe is tightly coupled with longitudinal FM spin fluctuations along the c axis. We found that magnetic fields along the c axis (H∥c) strongly suppress the FM fluctuations and that the superconductivity is observed in the limited magnetic-field region where the longitudinal FM spin fluctuations are active. These results, combined with model calculations, strongly suggest that the longitudinal FM spin fluctuations tuned by H∥c induce the unique spin-triplet superconductivity in UCoGe. This is the first clear example that FM fluctuations are intimately related with superconductivity.     

Wave number-frequency spectrum of turbulent pressure fluctuations: Methods of measurement and results

The methods proposed earlier for measuring the wave number-frequency spectrum of wall pressure fluctuations beneath a turbulent boundary layer are considered: the spatial filtering of the pressure field components by special-purpose transducers (wave filters) and the digital processing of signals obtained from an array of transducers. It is shown that, for the wave filters, transducers with a rectangular shape of sensitive surface rather than those with a circular one are necessary. Results of measuring the wave number-frequency spectrum of turbulent pressure fluctuations in a low-noise wind tunnel are presented. The measurements are performed with the use of four wave filters consisting of rectangular transducers with a constant sensitivity distribution over their surfaces. The mathematical model of the wave number-frequency spectrum proposed earlier by the authors is compared with the measurement data reported by Abraham and Keith. The model is used for processing the results of measurements in the wind tunnel. The measured spectra are compared with the data obtained by Martin and Leehey.     

The scaling of the wall pressure fluctuations in polymer-modified turbulent boundary layer flow

Wall pressure fluctuations and integrated skin friction were measured beneath a turbulent boundary layer that was modified by adding drag-reducing polymer to the pure water flow. The measurements were performed on an axisymmetric model, equipped with an isolated cylindrical drag balance section, and placed in the test section of the 0.3048-m-diam water tunnel at ARL Penn State. Data were acquired at a free-stream velocity of 10.7 m/s with pure water and with polymer added to the water at concentrations of 1, 5, 10, and 20 weight parts per million. Nondimensionalization of the wall pressure fluctuation frequency spectra with traditional outer, inner, and mixed flow variables failed to adequately collapse the data. The mean square wall pressure fluctuations were found to scale linearly with the wall shear stress. Polymer addition had little effect on the characteristic time scale of the flow. These properties were used to develop a novel form of the nondimensional wall pressure fluctuation spectrum that provided the best collapse of the measured data.     

Measurement of Residence Time Distribution with Radiotracers Using Periodic Pseudo-Random Binary Signal Sequences

A review about the properties of several types of tracerinput signals is given and in particular the stochastic signals are discussed. An estimation of the necessary measuring time shows, that true stochastic signals are unimportant for tracer experiments. The use of periodic pseudo-random binary signals (PRBS) permits a strong reduction of the measuring time. Therefore their practical application for tracer experiments is possible. The PRBS-method has special advantages in comparison with impulse methods, if fluctuations and disturbances exist within the investigated systems. Correlations for the estimation of parameters, needed for the realization of PRBS-measurements, are given and several measuring arrangements are discussed.

The application of the PRBS-method for measurements of residence time distributions (RDC) of a mixer-settler extractor is described.     

受激拉曼增益介质中的量子噪声特性研究

研究了三能级原子系综与相干控制场以及量子探测场的相互作用下的受激拉曼系统.运用量子理论推导出了该受激拉曼系统中由于量子场真空涨落引起的量子噪声以及受激散射的量子化光场噪声谱,并且数值计算了注入拥有非经典涨落的量子探测场,真空涨落所引起的量子噪声谱. 关键词： 受激拉曼增益 量子噪声 噪声谱