Turbulent Wind Field Research Articles

Effects of wind turbulence on motion stability of long-span bridges

Recent results are presented for the analysis of motion stability of flexible bridges in turbulent wind flows. Two types of bridge modes are considered, the torsional modes and the bending modes. Turbulence is shown to destabilize individual modes; yet, it may serve as a conduit to transfer energy from the less stable modes to the more stable modes thus stabilize the overall systems, provided that a favorable aerodynamic coupling is present. If the wind turbulence field is span-wise uniform, then energy can only be transferred between modes of different types; namely, from a torsional mode to a bending mode or vice versa, but not between modes of the same type. However, energy transfer among same types of modes can occur when the turbulent wind field is not span-wise uniform. Numerical examples are given for illustration.

Effects of wind turbulence on motion stability of long-span bridges

Recent results are presented for the analysis of motion stability of flexible bridges in turbulent wind flows. Two types of bridge modes are considered, the torsional modes and the bending modes. Turbulence is shown to destabilize individual modes; yet, it may serve as a conduit to transfer energy from the less stable modes to the more stable modes thus stabilize the overall systems, provided that a favorable aerodynamic coupling is present. If the wind turbulence field is span-wise uniform, then energy can only be transferred between modes of different types; namely, from a torsional mode to a bending mode or vice versa, but not between modes of the same type. However, energy transfer among same types of modes can occur when the turbulent wind field is not span-wise uniform. Numerical examples are given for illustration.

Turbulent exchange above a pine forest II. Organized structures

Data from two 100-min runs on the turbulent atmospheric wind, temperature and humidity fields above a pine forest have been analysed using conditional sampling techniques. With the aid of the temperature time series, ramp events were identified and all fields were averaged in order to remove smaller scale turbulence and random low-frequency turbulence, and to map the organized structures revealed in this way. It is shown that the turbulent fluxes of momentum, heat and humidity, determined from these ramp-events, in fact constitute a large part of the total fluxes, about 90% during the actual events.

Windthrow

The factors that influence storm damage in forests are summarized and the four kinds of storm damage in forests are presented. Two equations for the estimation of windloads are explained including the problems that their use involves. By use of wind-induced bending moments of trees the occurrence of windbreakage or windthrow is discussed. The response of trees to windloads is mostly in form of damped bending sways. Based on artificial force effects, their shapes are shown to be dependent on soil conditions. If trees are continually exposed to dynamic windloads the shape of their sways is irregular at first sight. During strong winds and storms, the most important load on trees is generally stochastic and can be analysed by the spectral method. Based on this method, results from experimental investigations on wind-induced sways of tall spruce trees within a stand are shown, including power spectra of Reynold’s stress as measure of windload, stem accelerations as measures of tree response, and magnitudes of mechanical transfer functions. The main result is that conifers such as spruce trees can be compared to a narrow bandpass filter, i.e. they can take in energy from the turbulent wind field only at a certain frequency range.

Solar wind iron charge states preceding a driver plasma

During September 28 and 29, 1978, the ISEE 3 spacecraft observed several distinct types of high‐speed solar wind flows when a coronal hole‐associated high‐speed stream was followed by two interplanetary shocks, one of which was driven by flare ejecta contained in a “magnetic cloud” or interplanetary “plasmoid.” Using the University of Maryland/Max‐Planck‐Institut ultralow energy charge analyzer (ULECA) on ISEE 3, we present solar wind Fe and Si/S charge state and Fe density measurements for the different plasma regimes associated with the flare‐related shock and combine these measurements with the Los Alamos National Laboratory proton observations to obtain iron/hydrogen density ratios and iron/hydrogen velocity differences. We place special emphasis on the postshock shell of turbulent and compressed ambient solar wind and interplanetary magnetic fields constituting the “sheath region” preceding the driver plasma. It is generally expected that the abundance ratios and charge states of the solar wind ions in the driver plasma and the sheath remain distinct, reflecting their different origins in the solar corona. However, while the Fe/H abundance ratios observed in the sheath are consistent with the preshock solar wind values (and a factor of 2 to 6 times less than the values obtained in the driver plasma), the iron charge states observed in the sheath appear to indicate a transition between the lower charge states observed in the ambient (coronal hole associated) solar wind and the higher charge states observed in the driver plasma. These results may reflect X ray ionization of the solar wind plasma near the flare site, although other explanations are possible.

Comments on the infrasonic noise theory of Isakovich and Kur’yanov and its modification by Wilson

The infrasonic noise theory of Isakovich and Kur’yanov and its modification by Wilson are reviewed and evaluated. The tractability of the theory’s formulation depends upon two assumptions. The most crucial assumption is that the turbulent wind field, acting normally at the sea surface, linearly excites acoustic waves and wind waves in identical manners. This assumption is physically unreasonable. The second assumption is that the wind waves attenuate. Isakovich and Kur’yanov assume viscous attenuation, while Wilson incorporates the recent conjecture of Hasselmann that above a critical wind speed the attenuation is due to whitecapping and is quadratic in frequency. It is shown that, due to incorrect implementation of Hasselmann’s attenuation term, Wilson’s expressions for the noise source and pressure levels are too low by a factor of (2π)−2, or −16 dB.

A statistical analysis of gust characteristics

Measurements of the turbulent atmospheric wind field have been analysed using different types of gust model concepts. The data represent a large variety of surface roughnesses and also a wide range of thermal stratifications. Measuring heights were between 7 and 135 m. The resulting gust amplitudes and times have been analysed statistically. Suitable normalizations have made it possible to obtain generally valid probability distributions.

Dry deposition to a uniform canopy: Evaluation of a first-order-closure mathematical model

A mathematical model is developed for the transport of momentum and matter within a canopy consisting of identical elements protruding vertically from a smooth substrate. Turbulent flux is modelled using a mixing-length approach. The loss of momentum (or matter) to individual elements is related to the mean wind speed, and the element-element interaction via the turbulent wind field is represented by a sheltering factor. Careful consideration is given to the formulation of lower boundary conditions. The model assumptions are compared with those of other models. The model predictions are compared with measurements on a vertically- and horizontally-uniform artificial canopy in a wind-tunnel. The model reproduces well the observed relationship between the parameters of the logarithmic wind speed profile above the canopy and the observed deposition velocities of thorium-B (ThB) atoms and particles in the diameter range 0.08–32 μm, using a sheltering factor which is little dependent on wind speed and has the same magnitude for momentum, gas and particles. The predicted dependences of deposition velocity on friction velocity and, for particles, on diameter shed light on the performance of semi-empirical correlations proposed in the literature. For ThB atoms, the calculated deposition velocities are compared with those of other mathematical canopy models: a comparable degree of agreement is obtained here with fewer free parameters. The fraction of deposit on the substrate is underpredicted by an order of magnitude in some cases, pointing to the limitations of the modelling of conditions near the substrate in terms of quasi-shear flow.

Sand, wind and statistics: Some recent investigations

A survey is given of work recently carried out in the Faculty of Science at Aarhus University. The paper is in two parts. The first part concerns the dynamics of grain movement and wind flow and discusses (i) a model for the process of sand saltation together with some related experiments, (ii) the influence of thermal instabilities and surface inhomogeneities on wind profiles, and some wind tunnel experiments on the pattern of wind flow over model dunes whose crest line form an oblique angle to the wind direction, (iii) wind tunnel studies of the progress of sand grains along a sand bed, grains being marked by dyes or by a cover of radioactive gold. Part two reports on a variety of investigations that involve some use of the hyperbolic distributions but are otherwise quite distinct. The subjects are (i) the shape of sand grains, (ii) classification of sand sediments, (iii) sieve calibration, (iv) accelerations in turbulent wind fields.

A resonant test-field model of gravity waves

In this paper we propose an ‘irreversible’ resonant test-field (RTF) model to describe the statistical fluctuations of gravity waves on deep water driven by a turbulent wind field. The non-resonant interactions in the gravity-wave Hamiltonian are replaced by a Markov process in the equation of motion for the resonantly interacting gravity waves, i.e. Hamilton's equations are replaced by a Langevin equation for the RTF waves. The RTF models the irreversible energy-transfer process by a Fokker-Planck equation for the phase-space probability density, the exact steady-state solution of which is determined to be non-Gaussian. An H-theorem for the RTF predicts the monotonic approach to the asymptotic steady state near which the transport properties of the field are studied. The steady-state energy-spectral density is calculated (in some approximation) to be k−4.

Model of gravity wave growth due to fluctuations in the air‐sea coupling parameter

The linear Miles‐Phillips model of the coupling between water waves and a turbulent wind field is generalized to include fluctuations in the air‐sea coupling parameter in the dynamic equations. The model is solved exactly, assuming the fluctuations to be delta correlated in time. The energy spectrum of the water wave field is shown to grow exponentially at rates higher then those predicted by the linear Miles‐Phillips model. These increased initial growth rates are consistent with those observed in open ocean measurements.

A model on the turbulent wind field over wind-waves in curvilinear co-ordinates

The wave-induced fluctuations of wind velocity over wind-waves measured in the wind tunnel experiment (Ichikawa andImasato, 1976) are compared with the numerical results estimated by a linear model (Model II) on the turbulent wind field over a dominant component of wind-waves. In the Model II, the undulation of mean air flow is introduced by adopting the curvilinear co-ordinates, and the existence of viscous sublayer and the influence of underlying wind-waves to background atmospheric turbulence are taken into account. The numerical results estimated by the Model II are in good agreement with the experimental results. The good agreement, which was not obtained from the previous model (Model I) in the Cartesian co-ordinates, is shown to be attributed to the undulating mean flow introduced in the Model II.

The energy balance in the wind-wave spectrum

The spectral energy balance in the wind-wave spectrum is studied with taking into account the energy input from turbulent wind to waves, the energy rearrangement due to conservative nonlinear wave-wave interaction and the energy dissipation due to water turbulence. Using the Ichikawa's (1978) model on the turbulent wind field over wind-waves and assuming that the energy dissipation isβ times greater than that due to molecular viscosity of water, the energy input and dissipation are determined so as to satisfy the condition that the nonlinear-transfers of momentum and energy conserve the total momentum and energy of waves. The nonlinear energy-transfer is estimated from the energy balance at each frequency. It is found that the energy input and dissipation satisfying the condition on the conservative nonlinear-transfer are determined by the characteristic height of wind-wave field and the friction velocity of air, and that the spectral distribution of the nonlinear energy-transfer estimated in this paper is qualitatively similar to that estimated by the non-linear wave-wave interaction theory ofHasselmann (1962).

Multi-component linear digital simulation of atmospheric boundary layer turbulence

A method for the linear simulation of the multi-component structure of turbulence in the atmospheric surface boundary layer is described. The spectral matrix is first diagonalized, then spectrally factored to provide kernels of the functional representation. Some characteristics of the resulting response structure are examined, including a comparison of the joint and individual responses. In addition, the predictability and the response structure in a thermally stratified boundary layer are examined. It is suggested that the method could be extended to simulations of the non-homogeneous turbulent wind field along a glide slope but at considerable cost in numerical processing.

Atmospheric Turbulence Encountered by High-Speed Ground Transport Vehicles

The structure of atmospheric turbulence near the ground is reviewed and the statistical properties of the longitudinal, lateral and vertical velocity components of the wind presented. The statistical properties of the turbulence encountered by a high-speed ground transport vehicle travelling through a turbulent wind field of arbitrary orientation are derived. Next, the statistical properties of the atmospheric forces acting on the vehicle are determined and a method of taking account of finite vehicle length is presented. The resulting formulae are applied to a typical tracked hovercraft vehicle.

Regular drag fluctuations due to air flow normal to a plate-type structure

A model structure resembling a flat plate (or a tall prestige building) was tested in a low-turbulence wind tunnel. With flow normal to the façade the model oscillated vigorously in the flow direction when passing through a “critical” velocity range. A sufficient minimum of experimental circumstances are reported to facilitate future search for this “Big Foot” phenomenon that should interest all those who assume that the dynamic drag forces of bluff structures are soley due to the turbulent wind field.

Effects of vertical vegetation surfaces on the adjacent microclimate: the role of aspect

Vegetation can influence microclimate in a number of ways. One is when a sharp vertical border is created that exerts a shielding effect on the adjacent horizontal surface. In this case direction of exposure becomes a prime control of the microclimate.This study investigated the differences in energy balance microclimate in the vicinity of forest edges oriented in the four cardinal directions. The gradient outward from the southerly exposure was also examined. A clearing in a uniform forest stand was utilized as the study site.Exposure to direct beam solar radiation was the most important factor in determining climatic differences. Reflection from an exposed forest edge outweighed the depletion of diffuse sky radiation and gave stations nearer the edge higher totals of global solar radiation. This was counteracted by a higher surface albedo. A more favorable longwave radiation balance, therefore, was largely responsible for greater net radiation near the forest edge.Availability of surface moisture provided by dew and the turbulent wind field combined with net radiation to produce the patterns of latent and sensible heat flux. Diminished vertical humidity and temperature gradients accompanying reduced wind meant that latent and sensible heat fluxes were lower and ground storage higher in the vicinity of the forest edge. The warmest thermal environments were found on the north and east side of the clearing. The shaded south side remained cool and damp throughout the day. The microclimate of the western side of the clearing was intermediate in nature.

A criterion for determining air-flow separation from wind waves

Based on laboratory studies of wind-wave interactions, a criterion for determining air-flow separation from wind waves is proposed. The air separates from waves having a phase velocity less than the shear velocity, which characterizes the turbulent wind field. The successful application of this criterion to the analysis of oceanic data is herewith demonstrated.

Some Radar Measurements of Turbulence in Snow

The Doppler spectrum of snow contains information about the turbulent wind field in which the snow is embedded. The average spread of the spectrum is proportional to the kinetic energy per unit mass of air in turbulent eddies that are generally smaller than the radar-sampled volume. The time variations in the mean value of velocity in the Doppler spectrum can be analyzed to determine the kinetic energy in eddies that are larger than the sampled volume. Thus from the time behavior of Doppler signals from snow can be estimated the total turbulent energy and the partitioning of this energy between large and small scales. Root-mean-square turbulent velocities computed on the basis of this technique were found to range between 0.4 and 2 m sec−1, with most of the values less than 1 m sec−1. Approximately three-quarters of the energy is usually in wavelengths shorter than 100 m. The data on the partitioning of eddy energy show considerable scatter, but there is some resemblance to the relationship expected from a Kolmogorov spectral law.