Statistical Characteristics Of Turbulence Research Articles

Characteristics of the SOL turbulence structure in the first experimental campaign on W7-X with limiter configuration

In the first experimental campaign of Wendelstein 7-X (W7-X), a combined probe head mounted on the multi-purpose manipulator has been used to measure the scrape-off layer (SOL) turbulence characteristics. The preliminary experimental results are summarized to illustrate the SOL turbulence properties in the limiter configuration on W7-X. In a standard limiter configuration, significant electrostatic fluctuations can be found in the near SOL, and the dominant frequency of fluctuation power is below 100 kHz. The auto-correlation spectrum power law decay factor is α ≈ −1 below 40 kHz and α ≈ –2 between 50 and 200 kHz. A broadband spectrum appears between 240 and 380 kHz with a low spectral power density, but a high cross-correlation coefficient. The statistical characteristics of turbulence are calculated by the two-point cross-correlation technique. A clear poloidal dispersion relation is found in the spectrum S(kθ, f), propagating along the ion diamagnetic drift direction with a group velocity (below 100 kHz) about 0.56 km/s in the near SOL in the laboratory frame. The poloidal correlation length is around 5–10 mm in SOL. The turbulence phase velocity is about 0.5–1 km/s when close to the last closed flux surface, which is comparable with the poloidal E × B drift speed.

NUMERICAL SIMULATION OF FLOW FEATURES AND ENERGY EXCHANGE PHYSICS IN NEAR-WALL REGION WITH FLUID-STRUCTURE INTERACTION

Large eddy simulation is used to explore flow features and energy exchange physics between turbulent flow and structure vibration in the near-wall region with fluid–structure interaction (FSI). The statistical turbulence characteristics in the near-wall region of a vibrating wall, such as the skin frictional coefficient, velocity, pressure, vortices, and the coherent structures have been studied for an aerofoil blade passage of a true three-dimensional hydroturbine. The results show that (i) FSI greatly strengthens the turbulence in the inner region of y+ < 25; and (ii) the energy exchange mechanism between the flow and the vibration depends strongly on the vibration-induced vorticity in the inner region. The structural vibration provokes a frequent action between the low- and high-speed streaks to balance the energy deficit caused by the vibration. The velocity profile in the inner layer near the vibrating wall has a significant distinctness, and the viscosity effect of the fluid in the inner region decreases due to the vibration. The flow features in the inner layer are altered by a suitable wall vibration.

Measurements of the statistical characteristics of the turbulence of an optical system with hybrid fiber-optic sensor

An optical system for diagnostics of turbulence with hybrid fiber-optic sensor is considered. The system functions in a digital mode of photographic recording of a two-dimensional accumulated optical signal directly associated with the two-dimensional probability density of turbulent fluctuations of the flow in the measurement region. From the values of the probability density, estimators of the statistical characteristics of turbulence are given in the form of digital matrices.

An experimental study of the turbulence statistics in a lagoon

Simultaneous measurements of three-dimensional velocity components and free-surface elevations were carried out in the lagoon of Fogliano, to investigate the statistical characteristics of turbulence produced in the flow by the action of wind over the free surface. Measurements were made in typical spring–summer environmental conditions, characterized by a gentle sea-breeze wind. Two measurement points were chosen far from the only tidal channel of the lagoon to evidence the role of the wind in generating turbulence. The analysis of velocity data has proved: (1) the presence of a significant oscillatory motion due to wind waves, which affects the entire water column and interferes with the lagoon bottom; (2) an evident degree of anisotropy between larger horizontal and vertical turbulent scales; (3) the existence of a scaling range where the relative scaling exponents, calculated using the Extended Self Similarity, have a scale-independent behavior.

Statistical structure of high-Reynolds-number turbulence close to the free surface of an open-channel flow

Statistical characteristics of turbulence in the near-surface region of a steady open- channel flow are examined using new data obtained in a high-Reynolds-number large-eddy simulation using a dynamic subgrid-scale model. These data, which correspond to a Reynolds number Re* = 1280 based on the total depth and shear velocity at the bottom wall, are systematically compared with those found in available direct numerical simulations in which Re* is typically one order of magnitude smaller. Emphasis is put on terms involved in the turbulent kinetic energy budget (dominated by dissipation and turbulent transport), and on the intercomponent transfer process by which energy is exchanged between the normal velocity component and the tangential ones. It is shown that the relative magnitude of the pressure–strain correlations depends directly on the anisotropy of the turbulence near the bottom of the surface-influenced layer, and that this anisotropy is a strongly decreasing function of Re*. This comparison also reveals the Re*-scaling laws of some of the statistical moments in the near-surface region, especially those involving vorticity fluctuations. Velocity variances, length scales and one-dimensional spectra are then compared with predictions of the rapid distortion theory elaborated by Hunt & Graham (1978) to predict the effect of the sudden insertion of a flat surface on a shearless turbulence. A very good agreement is found, both qualitatively and quantitatively, outside the thin viscous sublayer attached to the surface. As the present high-Reynolds-number statistics have been obtained after a significant number of turnover periods, this agreement strongly suggests that the validity of the Hunt & Graham theory is not restricted to short times after surface insertion.

The use of a model of axisymmetric turbulence for analysis of statistical characteristics of pulsation motion in a uniform-mean-shear-flow

This paper continues the description of investigations started in [1], where we substantiate the gradient correlation between the second and third two-point moments of isotropic turbulence and give the results of calculations of its degeneration, as well as the results of calculation of turbulence in a uniform shear flow, with the turbulence assumed to be isotropic. A semiempirical model of turbulence in a uniform shear flow is constructed which is based on the equations for the second two-point moments assuming the axial symmetry of the statistical characteristics of turbulence. Use is made of the theory of axisymmetric turbulence [2]. The derived equations are closed using the gradient formula suggested in [1]. The suggested model enables one to find the kinetic energy of pulsation motion and its components (on the assumption of axial symmetry), two-point correlation moments, and all known scales of turbulence that are determined on the basis of two-point moments. The calculation results are in adequate agreement with the experimental data.

Modeling of the limiting regime of stabilization of the average velocity of a turbulent flow in a rotating straight circular tube

Results of calculations of the limiting regime of stabilization of the average velocity of a flow and an investigation of the behavior of statistical characteristics of turbulence in a straight circular tube that rotates about the longitudinal axis based on a differential model of transfer of Reynolds stresses are presented. It is shown that, in a correct description of transformation of the structure of swirling-flow turbulence (Reynolds stresses) with simultaneous attainment of a regime of stabilization by the profiles of the axial and circumferential components of the average flow velocity, we need to allow for the action of the rotation on the spectral expenditure of kinetic energy of turbulence.

Statistical characteristics of wall turbulence

The statistical turbulence characteristics obtained by direct numerical simulation of two flows, namely, flow in a circular pipe and flow in a channel with parallel walls, are given. The velocity profiles and the distributions of the statistical moments up to the fourth order inclusive are analyzed. The calculation results are compared with known experimental and numerical data.

Statistical Characteristics of Turbulence in a Periodically Diverging-Converging Channel Flow.

The statistical properties of a periodically diverging-converging turbulent channel flow are examined experimentally. Air flow is alternately decelerated through a linearly diverging section, and accelerated through a converging section. Measurements on the energy spectrum, auto-correlation, probability density, skewness/flatness factors and space-time correlation are made in two channels with different diverging-converging angle. The inertial subrange can be recognized for the diverging and converging sections. Large differences are observed in the skewness and flatness factors compared with the equilibrium turbulent flows with no pressure gradient. The eddy scales in the converging section are considerably smaller than those in the diverging section.

The effects of intermittency on statistical characteristics of turbulence and scale similarity of breakdown coefficients

The influence of intermittency on turbulent diffusion is expressed in terms of the statistics of the dissipation field. The high-order moments of relative diffusion are obtained by using the concept of scale similarity of the breakdown coefficients (bdc). The method of bdc is useful for obtaining new models and general results, which then can be expressed in terms of multifractals. In particular, the concavity and other properties of spectral codimension are proved. Special attention is paid to the logarithmically periodic modulations. The parametrization of small-scale intermittent turbulence, which can be used for large-eddy simulation, is presented. The effect of molecular viscosity is taken into account in the spirit of the renorm group, but without spectral series, ε expansion, and fictitious random forces.

Probability distribution of velocity difference in the inertial interval of a turbulence spectrum

The statistical turbulence characteristics are analyzed in the inertial interval of a spectrum. An equation is obtained for the probability distribution of the velocity difference at two points. Parameters characterizing the turbulence spectrum in the inertial interval enter into the equation in the role of unknown constants. These constants are calculated from the condition that a solution exists which has a physical meaning.

A note on the interpretation of acoustic sounder returns

A method is outlined for estimating statistical characteristics of turbulence in the atmospheric boundary layer using pairs of observables derived from scattered acoustic energy. The procedure involves determining, from the observable pair, a local stability parameter and deducing from related similarity functions certain turbulence statistics not measured directly. The results are limited to heights small compared to the Ekman-layer depth and to flows which are neither fully developed convectively nor have imbedded gravity waves.

Empirical data on turbulence in the surface layer of the atmosphere

The statistical characteristics of turbulence in the atmospheric boundary layer are relatively stable when averaged over a time of the order of 10 to 20 minutes. The large-scale turbulence is related to a set of ‘external parameters': g/T¯, q/cpp, and v*, where T is the average temperature, q the vertical component of the turbulent heat flux, and v* the friction velocity. The smallerscale turbulence is related to a set of external parameters consisting of the rate of dissipation of turbulent energy, the rate of decay of inhomogeneities of the temperature field, and the buoyancy parameter g/T¯. For each of these ranges of the turbulence spectrum, two similarity expressions which are functions of these external parameters are determined. Three limiting cases of turbulence in the atmospheric boundary layer are considered: (a) neutral stratification where q must be eliminated from the similarity expressions; (b) strong instability where v* is eliminated; (c) strong stability where the height above the ground z must be eliminated from the similarity expressions. The time spectra of vertical velocity and of temperature are found and compared with experimental data. Spectra of turbulent stress and heat flux are also considered, and experimental data available about these spectra show that, at a height of 1 meter, the main contribution to the turbulent friction and heat flux is due to wavelengths of horizontal inhomogeneities of the order of some meters or even tens of meters.

DIFFUSION OF MATTER EMITTED FROM A LINE SOURCE IN A NON-ISOTROPIC TURBULENT FLOW

Abstract Turbulent diffusion from an infinite line source in a homogeneous, stationary, non-isotropic turbulent flow is studied. Spatial mean-concentration distributions are determined as functions of the statistical characteristics of turbulence. Particular attention is given to the evaluation of mean concentrations at relatively small distances from the source both upstream and downstream. At high intensities of turbulence, the resulting mean-concentration values may differ considerably from those obtained when the turbulent diffusion in the wind direction is neglected.

APPLICATION OF THE STATISTICAL THEORY OF TURBULENT DIFFUSION TO MICROMETEOROLOGY

Abstract Some elements of the statistical theory of turbulence are briefly reviewed from the viewpoint of their application to micrometeorology.Several relations concerning turbulent diffusion in a field of homogeneous and isotropic turbulence are then discussed. The mean-concentration distributions for an instantaneous and for a continuous point-source are investigated, with particular attention to the case when the intensity of turbulence cannot be considered as small in comparison with the mean velocity. The time history of mean-concentration distributions in a diffusing spherical cloud is then studied. The relation between Fick's differential equation of diffusion and the statistical theory of turbulence is studied, and the limits of validity of Fick's equation are determined. Finally, the relation between the coefficient of eddy diffusion and the statistical characteristics of turbulence is discussed.