共查询到20条相似文献,搜索用时 15 毫秒
1.
Vertical profiles of atmospheric turbulence characteristics, including spatial and temporal longitudinal and transverse velocity
structure functions, velocity structure characteristics, and turbulent kinetic energy dissipation rates retrieved from time
series of vertical profiles of the wind velocity vector measured with a minisodar operating at a frequency of 4900 Hz are
presented. It is established that the structure functions first increase with the separation distance between the observation
points and then are saturated; moreover, the longitudinal structure function is much greater than the transverse one, which
is in agreement with the data available from the literature. The velocity structure characteristic is well described by the
z−2/3 altitude dependence predicted theoretically. Calculated values of the turbulent energy dissipation rate are in agreement
with the data available from the literature. 相似文献
2.
The first direct determination of the inertial range energy cascade rate, using an anisotropic form of Yaglom's law for magnetohydrodynamic turbulence, is obtained in the solar wind with multispacecraft measurements. The two-point mixed third-order structure functions of Els?sser fluctuations are integrated over a sphere in magnetic field-aligned coordinates, and the result is consistent with a linear scaling. Therefore, volume integrated heating and cascade rates are obtained that, unlike previous studies, make only limited assumptions about the underlying spectral geometry of solar wind turbulence. These results confirm the turbulent nature of magnetic and velocity field fluctuations in the low frequency limit, and could supply the energy necessary to account for the nonadiabatic heating of the solar wind. 相似文献
3.
Alan Folz 《Physica D: Nonlinear Phenomena》2010,239(14):1305-1317
Measurements of the near-surface turbulence in the atmospheric boundary layer have been made using hot-wire probes above the salt flats of northwestern Utah, where the momentum thickness Reynolds number, Rθ, is O(106), and the surface is smooth and nearly devoid of flow obstructions. The measurements were made with arrays of up to 24 parallel straight sensors and with a modular 12-sensor probe capable of measuring all of the components of the instantaneous velocity vector and velocity gradient tensor. Measurements were also made in a laboratory wind tunnel at Rθ=1730 using 22 straight sensors. The data analysis focuses on the effects of the Reynolds number on turbulence properties and on the physics of the dissipation rate of turbulent kinetic energy.Some properties are found to be dependent on the Reynolds number when normalized with inner variables, while others are not. Among those that show the significant Reynolds number dependence are the rms and the skewness factor of the streamwise velocity fluctuations.Significant differences in flow structure, particularly those related to high rates of dissipation, are implied by the data. The joint PDF and covariance integrand of streamwise and wall normal vorticity fluctuations show less preferred orientation of the vorticity vector in the buffer layer at Rθ of O(106) than at Rθ=1070. The largest contribution to the dissipation rate, at O(106) is by the ∂w/∂z velocity gradient, while this term makes a quite small contribution to the dissipation rate at low Rθ. Here w and z are the spanwise velocity fluctuations and direction, respectively. Conditional analysis in the streamwise-wall normal (x−y) plane based on high instantaneous dissipation rate shows that the typical high dissipation rate events are generally similar at high and low Reynolds numbers, but display some significant differences. 相似文献
4.
5.
Inhomogeneity and anisotropy are intrinsic characteristics of daytime and nighttime atmospheric turbulence. For example, turbulent eddies are often stretched in the direction of the mean wind, and the turbulence statistics depends on the height above the ground. Recent studies have shown that the log-amplitude and phase fluctuations of plane and spherical sound waves are significantly affected by turbulence inhomogeneity and anisotropy. The present paper is devoted to studies of the mean sound field and the coherence functions of plane and spherical sound waves propagating through inhomogeneous anisotropic turbulence with temperature and velocity fluctuations. These statistical moments of a sound field are important in many practical applications, e.g., for source detection, ranging, and recognition. Formulas are derived for the mean sound field and coherence function of initially arbitrary waveform. Using the latter formula, we also obtained formulas for the coherence functions of plane and spherical sound waves. All these formulas coincide with those known in the literature for two limiting cases: homogeneous isotropic turbulence with temperature and wind velocity fluctuations, and inhomogeneous anisotropic turbulence with temperature fluctuations only. Using the formulas obtained, we have numerically shown that turbulence inhomogeneity significantly affects the coherence functions of plane and spherical sound waves. 相似文献
6.
7.
《Waves in Random and Complex Media》2013,23(1):21-29
AbstractIn a horizontal oceanic optical wireless communication link, the scintillation index (the measure for the intensity fluctuations) of the received intensity caused by the oceanic turbulence is formulated and evaluated when the source is a higher order mode laser. Variations in the scintillation index vs. the underwater turbulence parameters, size of the higher order mode laser source, link length, and the wavelength are examined. Underwater turbulence parameters are the ratio that determines the relative strength of temperature and salinity in driving the index fluctuations, the rate of dissipation of the mean squared temperature, the rate of dissipation of the turbulent kinetic energy, and the Kolmogorov microscale length. 相似文献
8.
The turbulent energy dissipation rate time-scale and length-scale has been routinely used for the prediction of noise from
turbulent flows, particularly jet streams. However, this is not the only possible choice. In general, scales evolving in a
turbulent medium are threefold. First, those associated with the mean flow; second, those attributed to the turbulence and
the mean flow interactions; and third, scales related to the turbulence-turbulence interactions. In this paper, special attention
will be paid to further study of the underlying physics of aerodynamic noise by examining various time-scales. To do so, three
time scales, namely, dissipation, production, and strain rate time scales, are defined and used in the source modelling to
emphasis the effect of the turbulence structures at different jet regions on the jet noise production mechanism. The required
mean value and turbulence parameters are obtained using a modified k − ∈ turbulence model, and Lighthill’s Acoustic Analogy is used for the prediction of the emanated noise.
The text was submitted by the authors in English. 相似文献
9.
Howes GG TenBarge JM Dorland W Quataert E Schekochihin AA Numata R Tatsuno T 《Physical review letters》2011,107(3):035004
A three-dimensional, nonlinear gyrokinetic simulation of plasma turbulence resolving scales from the ion to electron gyroradius with a realistic mass ratio is presented, where all damping is provided by resolved physical mechanisms. The resulting energy spectra are quantitatively consistent with a magnetic power spectrum scaling of k(-2.8) as observed in in situ spacecraft measurements of the "dissipation range" of solar wind turbulence. Despite the strongly nonlinear nature of the turbulence, the linear kinetic Alfvén wave mode quantitatively describes the polarization of the turbulent fluctuations. The collisional ion heating is measured at subion-Larmor radius scales, which provides evidence of the ion entropy cascade in an electromagnetic turbulence simulation. 相似文献
10.
Webster J Raspet R Yu J Prather WE 《The Journal of the Acoustical Society of America》2010,127(5):2764-2770
This paper investigates the wind noise pressure spectra measured by aerodynamically designed devices in turbulent flow. Such measurement probes are often used in acoustic measurements in wind tunnels to reduce the pressure fluctuations generated by the interaction of the devices with the incident flow. When placed in an outdoor turbulent environment however, their performance declines noticeably. It is hypothesized that these devices are measuring the stagnation pressures generated by the cross flow components of the turbulence. Predictions for the cross flow contribution to the stagnation pressure spectra based on measured velocity spectra are developed, and are then compared to the measured pressure spectra in four different probe type devices in windy conditions outdoors. The predictions agree well with the measurements and show that the cross flow contamination coefficient is on the order of 0.5 in outdoor turbulent flows in contrast to the published value of 0.15 for measurements in a turbulent jet indoors. 相似文献
11.
《中国科学:物理学 力学 天文学(英文版)》2010,(9)
A theoretical model of turbulent fiber suspension is developed by deriving the equations of Reynolds averaged Navier-Stokes,turbulence kinetic energy and turbulence dissipation rate with the additional term of fibers.In order to close the above equations,the equation of probability distribution function for mean fiber orientation is also derived.The theoretical model is applied to the turbulent channel flow and the corresponding equations are solved numerically.The numerical results are verified by comparisons with the experimental ones.The effects of Reynolds number,fiber concentration and fiber aspect-ratio on the velocity profile,turbulent kinetic energy and turbulent dissipation rate are analyzed.Based on the numerical data,the expression for the velocity profile in the turbulent fiber suspension channel flow,which includes the effect of Reynolds number,fiber concentration and aspect-ratio,is proposed. 相似文献
12.
13.
The intermittent distribution of localized turbulent structures is a key feature of the subcritical transitions in channel flows, which are studied in this paper with a wind channel and theoretical modeling. Entrance disturbances are introduced by small beads, and localized turbulent patches can be triggered at low Reynolds numbers (Re). High turbulence intensity represents strong ability of perturbation spread, and a maximum turbulence intensity is found for every test case as Re ≥ 950, where the turbulence fraction increases abruptly with Re. Skewness can reflect the velocity defects of localized turbulent patches and is revealed to become negative when Re is as low as about 660. It is shown that the third-order moments of the midplane streamwise velocities have minima, while the corresponding forth-order moments have maxima during the transition. These kinematic extremes and different variation scenarios of the friction coefficient during the transition are explained with an intermittent structure model, where the robust localized turbulent structure is simplified as a turbulence unit, a structure whose statistical properties are only weak functions of the Reynolds number. 相似文献
14.
Direct multipoint measurements of the velocity and temperature fields have been made in a turbulent Rayleigh-Bénard convection cell. In the central region of the cell it is found that both velocity and temperature exhibit the same scaling behavior that one would find for the velocity and for a passive scalar in homogeneous and isotropic Navier-Stokes turbulence. This is despite the fact that energy is pumped into the system vertically via buoyancy. Near the cell's sidewall where thermal plumes abound, vertical velocity and temperature exhibit different scalings. A model that takes into account both buoyancy and energy dissipation is proposed and its predictions agree well with the sidewall experimental results. 相似文献
15.
V. Sabel’nikov A. Chtab-Desportes M. Gorokhovski 《The European Physical Journal B - Condensed Matter and Complex Systems》2011,80(2):177-187
The experimental observations of intermittent dynamics of Lagrangian
acceleration in a “free” high-Reynolds-number turbulence are shown to be
consistent with the Kolmogorov-Oboukhov theory. In line with
Kolmogorov-Oboukhov’s predictions, a new sub-grid scale (SGS) model is
proposed and is combined with the Smagorinsky model. The new SGS model is
focused on simulation of the non-resolved total acceleration vector by two
stochastic processes: one for its norm, another for its direction. The norm
is simulated by stochastic equation, which was derived from the log-normal
stochastic process for turbulent kinetic energy dissipation rate, with the
Reynolds number, as the parameter. The direction of the acceleration vector
is suggested to be governed by random walk process, with correlation on the
Kolmogorov’s timescale. In the framework of this model, a surrogate
unfiltered velocity field is emulated by computation of the instantaneous
model-equation. The coarse-grid computation of a high-Reynolds-number
stationary homogeneous turbulence reproduced qualitatively the main
intermittency effects, which were observed in experiment of ENS in Lyon.
Contrary to the standard LES with the Smagorinsky eddy-viscosity model, the
proposed model provided: (i) non-Gaussianity in the acceleration
distribution with stretched tails; (ii) rapid decorrelation of acceleration
vector components; (iii) “long memory” in correlation of its norm. The
turbulent energy spectra of stationary and decaying homogeneous turbulence
are also better predicted by the proposed model. 相似文献
16.
A. S. Bodrova N. V. Brilliantov A. Yu. Loskutov 《Journal of Experimental and Theoretical Physics》2009,109(6):946-953
A theory is developed of Brownian motion in granular gases (systems of many macroscopic particles undergoing inelastic collisions),
where the energy loss in inelastic collisions is determined by a restitution coefficient ɛ. Whereas previous studies used
a simplified model with ɛ = const, the present analysis takes into account the dependence of the restitution coefficient on
relative impact velocity. The granular temperature and the Brownian diffusion coefficient are calculated for a granular gas
in the homogeneous cooling state and a gas driven by a thermostat force, and their variation with grain mass and size and
the restitution coefficient is analyzed. Both equipartition principle and fluctuation-dissipation relations are found to break
down. One manifestation of this behavior is a new phenomenon of “relative heating” of Brownian particles at the expense of
cooling of the ambient granular gas. 相似文献
17.
A new approach for turbulent fluxes and E×B measurements in the bulk plasma is proposed. It is based in the measurement of fluctuations in the phase velocity of fluctuations. The structure of turbulence has been investigated in the JET plasma boundary region with a fast reciprocating Langmuir probe system. Fluctuations in the radial and poloidal phase velocity have been computed from floating potential and ion saturation current measurements. The correlation between density fluctuations and fluctuations in the radial velocity of fluctuations signals show a good agreement with the turbulent transport computed from the correlation between density and poloidal electric field fluctuations. These results suggest that turbulent transport might be computed in the plasma core from measurement of density fluctuations. E×B sheared flows, both constant and varying in time, are close to the critical value to trigger the transition to improve confinement regimes below the power threshold to trigger the formation of transport barriers. 相似文献
18.
Based on the spatial power spectrum of the refractive index of ocean water, we derive analytical expressions for the effective radius of curvature and Rayleigh length of partially-coherent Hermite–Gaussian linear array (PCHGA) beams propagating through oceanic turbulence. In addition, we discuss theoretically and analyze numerically the influence of oceanic turbulence and array parameters. The effective radius of curvature and Rayleigh length of PCHGA beams increase with the relative strength of temperature and salinity fluctuations, the rate of dissipation of turbulent kinetic energy per unit mass of fluid, the array beam number, beam order, and relative separation distance, but decrease with the increase in the rate of dissipation of the mean-square temperature. The analysis provided here will help to understand the propagation of array beams through ocean turbulence. 相似文献
19.
《Revue Generale de Thermique》1996,35(412):232-242
Numerical prediction of the structure of high temperature axisymmetric turbulent jets. Turbulent axisymmetric jets at high temperature are studied numerically by using first and second order turbulence models. Regarding the temperature fields, on which we concentrate in this work, predictions with both types of models do not show large differences. In general, predictions agree well with the measurements; the existing differences are usually favorable for the second order model. The effect of solving a transport equation for the scalar dissipation rate on the prediction of the mechanical to scalar time scale ratio and on the prediction of the scalar fluctuations is studied. The influence of varying the density ratio on parameters such as the axial decay rates of the temperature and velocity and the turbulence intensity are studied. Two definitions of the mixing efficiency are introduced. According to both definitions, the mixing efficiency decreases with increasing effects of buoyancy. 相似文献
20.
High-repetition rate measurements of temperature and thermal dissipation in a non-premixed turbulent jet flame 总被引:2,自引:0,他引:2
High-repetition rate laser Rayleigh scattering is used to study the temperature fluctuations, power spectra, gradients, and thermal dissipation rate characteristics of a non-premixed turbulent jet flame at a Reynolds number of 15,200. The radial temperature gradient is measured by a two-point technique, whereas the axial gradient is measured from the temperature time-series combined with Taylor’s hypothesis. The temperature power spectra along the jet centerline exhibit only a small inertial subrange, probably because of the low local Reynolds number (Reδ ≈ 2000), although a larger inertial subrange is present in the spectra at off-centerline locations. Scaling the frequency by the estimated Batchelor frequency improves the collapse of the dissipation region of the spectra, but this collapse is not as good as is obtained in non-reacting jets. Probability density functions of the thermal dissipation are shown to deviate from lognormal in the low-dissipation portion of the distribution when only one component of the gradient is used. In contrast, nearly log-normal distributions are obtained along the centerline when both axial and radial components are included, even for locations where the axial gradient is not resolved. The thermal dissipation PDFs measured off the centerline deviate from log-normal owing to large-scale intermittency. At one-half the visible flame length, the radial profile of the mean thermal dissipation exhibits a peak off the centerline, whereas farther downstream the peak dissipation occurs on the centerline. The mean thermal dissipation on centerline is observed to increase linearly with downstream distance, reach a peak at the location of maximum mean centerline temperature, and then decrease for farther downstream locations. Many of these observed trends are not consistent with equivalent non-reacting turbulent jet measurements, and thus indicate the importance of understanding how heat release modifies the turbulence structure of jet flames. 相似文献