Atmospheric Turbulent Flow Research Articles

Wind noise and the spectrum of atmospheric turbulence pressure fluctuations

Previous research [S. Morgan and R. Raspet, J. Acoust. Soc. Am. 92, 1180–1183 (1992)] has shown that wind noise is predominantly caused by pressure fluctuations intrinsic to the turbulent atmospheric flow. Therefore it should be possible to predict wind noise from models for turbulent pressure spectra in the atmosphere. Based on simple dimensional analysis and an application of Taylor’s hypothesis, the inertial-subrange power spectrum for turbulent pressure fluctuations should be proportional to f−7/3, where f is frequency. But more recent atmospheric observations and theoretical arguments [J. D. Albertson, G. G. Katul, M. B. Parlange, and W. E. Eichinger, Phys. Fluids 10, 1725–1732 (1998)] suggest that the power spectrum actually goes as f−3/2. In this paper, it is shown that the f−3/2 dependence predicts a much slower decay in wind noise with increasing acoustic frequency than is typically observed. Possible reasons for this discrepancy are discussed.

B142 大気海洋相互作用研究データベース共有システムを用いた風波乱流場の解析

In order to grasp and understand the carbon dioxide gas transferred into the sea, a direct numerical simulation (DNS) of turbulent atmospheric gas flows interacted with the sea surface has been carried out and the computational results and the satellite measurement data can be simultaneously visualized by a Web based network collaboration system developed here. The results from this DNS can be applied to the gas transfer modeling at the sea surface of the global climate modeling and the anti-environmental material transport modeling on the earth. Moreover many researchers can conduct the DNS and access the satellite database of the wind velocity and the wave height on the sea surface, and also handle the scientific visualization software through this Web based mutual research collaboration system. This system can provide a real-time steering and tracking simulation/visualization platform for mutual scientific research collaboration not only for the environmental science but also other many scientific and engineering fields.

Motion and Sedimentation of Particles in Turbulent Atmospheric Flows above Sources of Heating

In this paper, theoretical investigation of the problem of particle evolution and sedimentation in turbulent gas flows above the zones of large-scale instabilities caused by heating from below is undertaken. The mathematical model takes into account the two-way coupling effects in gas-particle interactions and combines both deterministic and stochastic approaches. To simulate the gas-phase flow the k -epsilon model is used with accounts of the mass, momentum, and energy fluxes from the particulate phase. The equations of motion for particles take into consideration random turbulent pulsations in the gas flow. The mean characteristics of those pulsations are determined with the help of solutions obtained within the frames of the k -epsilon model. Contrary to the existing theories, the present approach enables us to take into account the polydispersed character of the mixtures. The models for phase transitions and chemical reactions take into account thermal destruction of dust particles, vent of volatiles,...

Acceleration intermittency and enhanced collision kernels in turbulent clouds

Scaling arguments suggest that in turbulent clouds, the droplet collision kernel is a fluctuating quantity with peak values at least an order of magnitude greater than the typically used kernel corresponding to droplet sedimentation in still air. These peaks are expected to occur in regions of intense fluid acceleration that are encountered in the fine scales of atmospheric turbulence, characterized by exceedingly large Reynolds numbers and strong energy-dissipation intermittency. The increase in the collision kernel is due to enhanced droplet relative velocities resulting from fluid accelerations and likely is further enhanced by increases in the droplet collision efficiency, which is a function of relative velocity. Velocity data obtained from a turbulent atmospheric flow are presented in order to support the general picture of acceleration intermittency and to estimate its peak values. Several independent estimates all point to the collision kernel as having peak magnitudes several times larger than the traditional kernel that accounts for gravitational acceleration alone.

On the dispersion of trace species in the atmospheric boundary layer: a re-formulation of the governing equations for the turbulent flow of the compressible atmosphere

Since especially the parameterisation of the vertical dispersion of trace species in the atmosphericboundary layer has controversially been discussed in the literature, the 1st-order balance equationsfor matter, momentum, and various energy forms were re-formulated with Hesselberg’sdensity-weighted averaging calculus to point out that this problem arises from averaging themacroscopic balance equations of matter, momentum and various energy forms in the sense ofReynolds, rather than from the parameterisation of the vertical dispersion by 1st-order closureprinciples, as this discussion seems to reflect. Results of the SANA field experiment “Eisd”presented here substantiate that in the case of chemically reactive trace constituents segregationeffects owing to turbulence cannot generally be neglected as usually performed in Eulerian airpollution models. Modelling such segregation effects, however, requires, at least, 2nd-orderclosureprinciples. Therefore, the 2nd-order balance equations for 2nd moments like the eddyflux densities of matter and momentum as well as covariances of scalar quantities were alsore-formulated by considering Hesselberg’s averaging procedure. This re-formulated set of governing1st-order and 2nd-order balance equations may be considered as most exact becausethe degree of simplification is reduced to a minimum. To distinguish between the Boussinesqapproximated equation set for the turbulent atmospheric flow, denoted as Boussinesq fluid, and our re-formulated one, the turbulent flow of the compressible atmosphere for which there-formulated governing balance equations are valid may be denoted as Hesselberg fluid. It isargued that averaging in the sense of Hesselberg reduces the risk to misinterpret turbulentatmospheric processes to a minimum. As exemplary shown on the basis of the balance equationsfor dry air, water vapour, and trace species, the so-called Webb correction will become insignificantif Hesselberg’s averaging calculus is considered. Based on the results obtained from the “Eisdorf” experiment and from sensitivity studies with a Seinfeld-type kinetic mechanism forphotochemical smog, it is argued that an evaluation and improvement of Eulerian air pollutionmodels require directly measured 2nd-order moments. Since the number of fast-responsephysico-chemical analysers for chemically reactive trace constituents is strongly limited, suchfast-response sensors have to be (further) developed to set-up a true platform for model evaluationthat implies not only a comparison of calculated and observed distributions of 1stmoments (necessary condition), but also a comparison of the calculated and observed distributionsof 2nd moments (sufficient condition).

The three-dimensional vortical nature of atmospheric and oceanic turbulent flows

Using a novel numerical method at unprecedented resolution, we demonstrate that structures of small to intermediate scale in rotating, stratified flows are intrinsically three-dimensional. Such flows are characterized by vortices (spinning volumes of fluid), regions of large vorticity gradients, and filamentary structures at all scales. It is found that such structures have predominantly three-dimensional dynamics below a horizontal scale L≈12LR, where LR is the so-called Rossby radius of deformation, equal to the characteristic vertical scale of the fluid H divided by the ratio of the rotational and buoyancy frequencies f/N. The breakdown of two-dimensional dynamics at these scales is attributed to the so-called “tall-column instability” [D. G. Dritschel and M. de la Torre Juárez, J. Fluid. Mech. 328, 129 (1996)], which is active on columnar vortices that are tall after scaling by f/N, or, equivalently, that are narrow compared with LR. Moreover, this instability eventually leads to a simple relationship between typical vertical and horizontal scales: for each vertical wave number (apart from the vertically averaged, barotropic component of the flow) the average horizontal wave number is equal to f/N times the vertical wave number. The practical implication is that three-dimensional modeling is essential to capture the behavior of rotating, stratified fluids. Two-dimensional models are not valid for scales below LR.

Refined similarity hypothesis and asymmetry of turbulence

An experimental study of atmospheric turbulence and turbulent pipe flow reveals some deviations from the refined similarity hypothesis (RSH), in addition to those already known. Special emphasis is placed on studying the turbulence asymmetry, as it has proved to be a sensitive indicator of this deviation. It is found that, in spite of good correlation between the velocity increments $u$ and the dissipation field $y,$ the typical amplitude of the latter is appreciably smaller than that of the velocity increments for violent events. In the framework of the RSH, the quantity $V=u/y$ is supposed to be statistically independent of the dissipation. The study shows that there is some weak dependence, manifested at least in a correlation between the sign of $V$ and the amplitude of the dissipation field $y.$ We suggest some modification of the measure in defining the dissipation field, so that the asymmetry can be treated in a self-consistent way in the framework of the RSH.

Cup Anemometer Behavior in Turbulent Environments

Abstract The behavior of the cup anemometer rotor in turbulent atmospheric flow is discussed in terms of a general equation of motion. This equates the rate of change s of the rotation rate s of the rotor to a forcing F(s, h, w), which is proportional to the torque and a function of s and of the total horizontal and the vertical wind velocity components, h, and w, respectively. To determine the so-called overspeeding, it is necessary to carry out first-and second-order perturbation calculations around the response curve obtained in a laminar flow. From this curve, which for the purpose of this paper can be considered linear, five constraints are derived between the first and second partial derivatives of F. These constraints provide sufficient information for deriving an expression for the overspeeding to which four distinctly different biases contribute—one for each of the velocity components and one from the covariance between streamwise velocity components ũ and w. A phenomenological model of...

Experimental investigation of atmospheric boundary layer turbulence

This paper describes how to measure turbulence in the atmospheric boundary layer (ABL) in order to address certain problems in modern atmospheric physics. These problems mainly relate to the Earth's energy budget (including the hydrological cycle) and biogeochemical cycles. Starting from the main characteristic numbers and the basic equations of atmospheric turbulent flow, we show what turbulence parameters are important to measure. Special attention is given to the various methods used to compute the turbulent fluxes. We analyse the range of scales which has to be measured to properly capture the eddies contributing to the turbulent transfers. This range of scales determines what sensors can be used in the atmospheric surface layer and in the ABL. We describe the most widely used instruments and their performances. The principal platforms used to deploy these instruments are examined. Aircraft are described in more details, because they allow a thorough exploration of the ABL. In the last section, some examples of ABL turbulence signals measured in various conditions are presented. These examples illustrate horizontally homogeneous turbulence as well as inhomogeneous signals for which standard analysis techniques cannot be used. We show how some recent techniques, like wavelet transforms, can help to investigate this kind of signal. At the end, we present what would be interesting to do in the near future for the study of ABL turbulence.

Mathematical modelling of aerosols evolution and sedimentation in the atmosphere of big cities

The paper presents the fundaments of numerical modelling of the dynamics of the evolution and sedimentation of clouds of dispersed particles or evaporating aerosols in turbulent atmospheric flows. A new method of modelling is described combining both deterministic approach to the description of the atmospheric flows and stochastic approach to particles' evolution modelling. The stability of the atmosphere on the top of the cities heated from below is investigated. The model takes into account the possibility of phase transition and chemical reactions on the surface of particles. The results of statistical modelling of contaminants drift in the atmosphere are appliable in making forecasts of the air pollution in the industrial zones or in accidental explosions. The worked out models can be successfully used in predicting the evolution of active aerosols in clouds causing rainfalls.

Wind tunnel simulation of atmospheric turbulent flow over a flat terrain

Wind tunnel experiment on the turbulence characteristics over a flat terrain was examined on the basis of field data. The wind tunnel results were compared with the field data in which vertical distributions of mean wind speed, turbulence intensity, Reynolds stress, and energy spectra and cospectra had been obtained from a 200 m meteorological tower at six levels under neutral atmospheric conditions. The experiment with 1 1000 scaled model was carried out under the following similarity conditions: (1) the vertical profile of mean wind speed is set to be similar to the field one, and (2) the experimental Reynolds number is set sufficiently large (i.e. Reynolds number independence). The critical Reynolds number over the flat terrain was found to be 3.7 × 10 4. The wind tunnel results of vertical profiles of turbulence intensity and normalized Reynolds stress ( − uw U 2 ) were similar to the field results. Moreover, the similarity of spectra and cospectra was obtained concerning the spectral structure and the height dependence. From the agreement between the wind tunnel and field spectra with nondimensional frequency, it follows that the field time scale, T f is reduced to the wind tunnel time scale, T w by a factor of α β , where α is the wind tunnel reduction scale and β is the ratio of the wind tunnel to field wind speeds.

Numerical Predictions of Turbulent Flow Around a Structure Via thek-ϵModel for Reactor Vessel Auxiliary Cooling System Performance Analysis

Turbulent airflows around structures are important in many engineering applications. Such flows can have a significant impact on the thermal performance of the reactor vessel auxilliary cooling system (RVACs) of advanced liquid-metal reactor designs. The adequacy of the high-Reynolds-number form of the k-e model in analyzing turbulent airflow around structures like the RVACS stacks is evaluated. An experiment of simulated atmospheric turbulent flow around a cube is analyzed with the computer code COMMIX, and numerical predictions for pressure and velocity distributions are compared with experimental measurements. Considering the complexity of the problem and the approximation involved in the k-e model, the overall agreement between numerical predictions and measurements of pressure coefficients and velocities is good. The largest discrepancies between predictions and measurements are in the pressure coefficient at the sections of the top and side cube surfaces very close to the upwind edges and in the spanwise velocity distribution downstream from the cube

Transient Ocean Currents Induced by Wind and Growing Waves

Abstract A theoretical nonlinear model for wind- and wave-induced currents in a viscous, rotating ocean is developed. The analysis is based on a Lagrangian description of motion. The nonlinear drift problem is formulated such that the solution depends on a linearly increasing eddy viscosity in the water, the wave-growth rate, and the periodic normal (or tangential) wind stress at the sea surface. Particular calculations are performed for surface-stress distributions and growth rates obtained from asymptotic analysis of turbulent atmospheric flow, where the Reynolds stress is modeled by an eddy-viscosity assumption. For growing waves the wave-induced current develops in time. The calculations are terminated when the steepness of the fastest-growing waves reach that of a saturated sea. At this point, the magnitude of the wave-induced surface current is 8–9 times larger than the friction velocity in the water, and the direction of the current is deflected 2°–10° to the right of the wave-propagation direction.

A two-dimensional computational model of turbulent atmospheric surface flows with drifting snow

A physically based computational model of drifting snow in two-dimensional terrain is developed. The model considers the case where wind speeds are low enough to neglect the transport of particles from the saltation layer into the turbulent flow field. The model has two distinct parts, one describing the turbulent airflow, and a second describing the mass-transport process and resulting snow-accumulation patterns produced by saltation transport. The turbulent-flow model consists of a general solution of the time-averaged, two-dimensional Navier-Stokes equations, where the k-∊ turbulence model is used to close the system of equations. The turbulent-flow model is coupled to a saltation model to compute the time evolution of the surface wind fields and snowdrift formation in the vicinity of a solid fence. Modeled wind fields and snow-accumulation profiles are similar to published field and experimental data.

A two-dimensional computational model of turbulent atmospheric surface flows with drifting snow

A physically based computational model of drifting snow in two-dimensional terrain is developed. The model considers the case where wind speeds are low enough to neglect the transport of particles from the saltation layer into the turbulent flow field. The model has two distinct parts, one describing the turbulent airflow, and a second describing the mass-transport process and resulting snow-accumulation patterns produced by saltation transport. The turbulent-flow model consists of a general solution of the time-averaged, two-dimensional Navier-Stokes equations, where the k-∊ turbulence model is used to close the system of equations. The turbulent-flow model is coupled to a saltation model to compute the time evolution of the surface wind fields and snowdrift formation in the vicinity of a solid fence. Modeled wind fields and snow-accumulation profiles are similar to published field and experimental data.

Reynolds number dependence of velocity structure functions in turbulent shear flows

Moments, up to order eight, of the structure function of the steamwise velocity fluctuation, have been measured in both laboratory and atmospheric turbulent shear flows. The Reynolds number dependence of structure functions evaluated for a separation equal to the Taylor microscale is closely approximated by both lognormal and β models, at least for moments up to order six. Both models predict identical inertial subrange behavior for the sixth-order structure functions. This prediction is in good agreement with experiment when the value of 0.2 is used for the universal exponent μ. This value has been obtained from correlations of the squared velocity derivative fluctuations, calculated for both laboratory and atmospheric measurements over the inertial subrange. The slope of the correlation is related to that of the sixth-order structure function, in agreement with a conjecture by Frisch, Sulem, and Nelkin. When plotted against moments of order n+2, moments of even order n exhibit a power-law behavior whose exponent is in closer agreement with the lognormal than the β model.

Stationarity of Mesoscale Airflow in Mountainous Terrain

Criteria for stationarity of turbulent atmospheric flow are examined for the mesoscale in a mountain valley in northeastern Utah. Data from four sequences of superpressure balloon flights are used. The study shows that the horizontal components of the Lagrangian velocities at levels below the ridges of the surrounding mountains are only weakly stationary; the vertical component fits the criteria better. Above the ridge levels all components of the velocities exhibit reasonable stationarity in the turbulent flow.

A Preliminary Numerical Study of Atmospheric Turbulent Flows in the Idealized Planetary Boundary Layer

Abstract A turbulent transport model is developed to study atmospheric turbulence in the planetary boundary layer. A total of nine equations governing the mean motion, mean turbulent stresses, and turbulence length scale are integrated numerically. In this preliminary study, only the ideal case of neutral lapse rate, barotropic, statistically stationary, and horizontally homogeneous conditions is treated. The height of the boundary layer is investigated and found to be about 0.5 u*/f, where u* and f are the friction velocity and Coriolis force parameter, respectively. The computed friction coefficient, the crossisobaric angle, the vertical profiles of mean wind, mean turbulent stresses, the turbulent length scale, and eddy coefficients agree well with observations and with Deardorff's results. Various terms in the turbulent stress equations, which are difficult to measure, are discussed. The direction of the stresses seems to align with the direction of the wind shear. The profiles of the turbulent diffus...

A method for measuring the spatial spectral density of refractive index in turbulent air flow using laser correlation technique

It is shown that in the case of a homogeneous and isotropic turbulence the knowledge of the temporal correlation function of the amplitude fluctuations on the laser beam axis allows the estimation of the spatial spectral density of the index of refraction. Under certain conditions this method may be used for atmospheric turbulent flow also, which represents in general a locally homogeneous and isotropic random field.

Comparison of acoustic instruments in an atmospheric turbulent flow over water

In the fall of 1968 an International Intercomparison Experiment was conducted to compare, among other things, acoustic anemometers designed for measurements of atmospheric turbulence. Excellent agreement was obtained in measurements of the vertical component, but an important discrepancy was revealed in measurements of the downwind velocity. The discrepancy proved far more significant in the cospectra than in the spectra themselves. It is evident that great care must be taken in the design of such instruments. An acoustic instrument was shown to have insufficient signal-to-noise ratio to serve as a thermometer under the near-neutral conditions often encountered over water.