Unsteady Wind Research Articles

Heat and Water Vapour Diffusivities Near the Base of a Disturbed Stable Internal Boundary Layer

We present results from an experiment that wasdesigned to investigate turbulent transportrelationships in a nearly homogeneous boundary layerdisturbed by unsteady wind swings, as found at thebase of an advective inversion with a convectiveboundary layer overhead. In such a situation wemeasured vertical gradients and eddy fluxes of temperature andhumidity at two heights. From these, the turbulentdiffusivities of heat and water vapour are obtained,and compared to the predictions of Monin–Obukhovsimilarity theory and those of a numericalsecond-order closure model. It is found that themeasured diffusivities exceed both predictions. Thisis interpreted as a consequence of the unsteadyconditions. It is also found that the diffusivity forheat is roughly 10% larger than that for watervapour. This is in agreement with a theoreticaltreatment of the unsteadiness effects that wedeveloped in an earlier publication. This result isnot reproduced by the numerical model because themodel has no provision for unsteady conditions. Ourresult disagrees with that from an earlier, verysimilar, field experiment, which may be due to asystematic underestimation of sensible heat flux inthe older experiment.

Unsteady flow effects due to fluctuating wind pressures in natural ventilation design—mean flow rates

The paper presents the results of a study into the effects of unsteady wind pressures on the mean flow rates in certain types of purpose-designed naturally ventilated buildings. The study used nondimensional parameters and the results should therefore cover a wide range of conditions and should be of general application. It is concluded that unsteady effects are restricted to a relatively narrow band of conditions. These conditions have been quantified in terms of nondimensional parameters. In particular a simple procedure for calculating the mean flow rates when the unsteady effects are large has been derived. In its simplest form the procedure does not require knowledge of the instantaneous wind pressures.

Effect of Isolated Micron-Sized Roughness on Transition in Swept-Wing Flows

Boundary-layer transition-to-turbulence studies are conducted in the Arizona State University Unsteady Wind Tunnel on a 45-deg swept airfoil. The pressure gradient is designed so that the initial stability characteristics are purely crossflow dominated. Flow-visualization and hot-wire measurements show that the development of the crossflow vortices is influenced by roughness near the attachment line. Comparisons of transition location are made between a painted surface (distributed 9-μm peaks and valleys on the surface), a machine-polished surface (0.5-μm rms finish), and a hand-polished surface (0.25-μm rms finish). Then isolated 6-μm roughness elements are placed near the attachment line on the airfoil surface under conditions of the final polish (0.25-μm rms). These elements create an enhanced packet of stationary crossflow waves, which results in localized early transition. The diameter, height, and location of these roughness elements are varied in a systematic manner. Spanwise hot-wire measurements are taken behind the roughness element to document the enhanced vortices. These scans are made at several different chord locations to examine vortex growth

Wind-induced response of a large cantilevered roof

The subject of this study is to clarify aerodynamic properties of a large cantilevered roof. We conducted free vibration experiments in order to investigate effects of various parameters such as mass, pitch angle, and damping ratio to wind-induced vibration of the roof. In the experiments, we have found that the roof hardly vibrated at its natural frequency until a certain critical wind velocity, then a flutter-like vibration occurred and a sharp peak at frequency lower than the natural frequency appeared in a spectrum of the vibration. In the light of forced oscillation experiments, the frequency of the sharp peak was the natural frequency of the roof lowered by the positive aerodynamic stiffness. The lowered natural frequency synchronized with the frequency of the vortex shedding from the leading edge of the roof, which causes a negative damping effect of unsteady wind force.

Evidence for the Effects of Swell and Unsteady Winds on Marine Wind Stress

Over the past four decades much effort has been directed toward determining a parameterization of the sea surface drag coefficient on readily measurable quantities, such as mean wind speed and atmospheric stability. Although such a parameterization would have obvious operational advantages, the considerable scatter present between experiments, or within any one experiment, indicates that it is not easily achievable. One likely candidate for much of the scatter is the underlying wave field. Unfortunately, few campaigns over the years have included spectral measurements of the waves. Among those that have, the results are inconclusive. Here data are presented from the Surface Wave Dynamics Experiment and High Resolution Remote Sensing Program campaigns in which 3-m discus buoys were instrumented with K-Gill and sonic anemometers and complete motion packages to measure the direct (eddy correlation) stress and, concurrently, the directional ocean wave spectrum. These data are examined for the effects of swell on the drag coefficient. It is found that much of the scatter in the drag coefficient can be attributed to geophysical effects, such as the presence of swells or nonstationary conditions.

Application of thermal anemometry and high-frequency measurement of mass flux to aeolian sediment transport research

Recent research on sediment transport by temporally varying winds has stimulated the need for high-frequency measurements of wind velocity, turbulence and transport rate. Conventional technologies yielding only temporally-averaged estimates of wind velocity and mass flux are unable to resolve the transitional and self-regulating behaviour of the aeolian saltation system in unsteady winds. The characteristics, calibration and potential of thermal velocity probes suitable for research on aeolian transport are analysed. Recent developments in particle impact sensing and the use of sand trap/force transducer combinations are reviewed, and the design of a new optical mass flux sensor is presented. An upper frequency limit of about 1 Hz is shown to be essential to transport rate relations in unsteady forcing winds or during intermittent transport. Higher frequencies are required if Reynolds stress or potentially influential flow structures and sediment/fluid interactions in the saltation cloud are to be resolved. Armoured hot-wire sensors measure u-component air velocity to 1 Hz in grain clouds, and some less robust two-dimensional thermal sensors may be used at higher frequencies to determine bed shear from Reynolds stress u′w′ during sand transport by wind. Grain impact devices and automated sand traps provide high-frequency measurements of transport rate, but they are intrusive and their output signals may be ambiguous. This paper reports the design and testing of a new instrument system providing high-frequency measurements of aeolian sand transport. It comprises a non-intrusive, optical mass flux sensor synchronized to an array of armoured hot-wire anemometers. The sensor uses a linear image array to detect, at a sampling frequency of 4–40 Hz, the transit of wind-driven particles through a laser sheet transverse to the flow in a wind tunnel. The physics of the sensor have been rigorously explored in static grain tests and calibrations to mass flux. Its principal advantages over existing devices are its non-intrusive design and ability to measure near-bed mass flux with precision at high frequency. Example applications demonstrate that this instrumentation is able, for the first time, to recover high-frequency aspects of the temporal behaviour of aeolian saltation, previously revealed only by numerical models.

Dynamic response of an elastic fence under wind action

The along‐wind response of a surface‐mounted elastic fence under the action of wind was investigated numerically. In the computations, two sets of equations, one for the simulation of the unsteady turbulent flow and the other for the calculation of the dynamic motion of the fence, were solved alternatively. The resulting time‐series tip response of the fence as well as the flow fields were analyzed to examine the dynamic behaviors of the two. Results show that the flow is unsteady and is dominated by two frequencies: one relates to the shear layer vortices and the other one is subject to vortex shedding. The resulting unsteady wind load causes the fence to vibrate. The tip deflection of the fence is periodic and is symmetric to an equilibrium position, corresponding to the average load. Although the along‐wind aerodynamic effect is not significant, the fluctuating quantities of the tip deflection, velocity and acceleration are enhanced as the fundamental frequency of the fence is near the vortex or shedding frequency of the flow due to the occurrence of resonance. In addition, when the fence is rather soft, a higher mode response can be excited, leading to significant increases in the variations of the tip velocity and acceleration.

Roughness-induced receptivity to crossflow vortices on a swept wing

The generation of stationary crossflow vortices on the 45 degree swept NLF (2)-0415 airfoil at the Arizona State University Unsteady Wind Tunnel (AIAA Paper No. 96-0184, 1996; Ph.D. thesis, Arizona State University, 1996) is analyzed using a finite Reynolds-number linear receptivity theory. The receptivity theory is based on the locally-parallel flow approximation and also neglects surface curvature. The vortices are excited by a spanwise periodic array of circular roughness elements located near the attachment line. The amplitudes of the crossflow vortices at the source location are computed and compared with the experimental results. In general, the agreement is remarkably good. This suggests that the combined effect of nonparallel flow and surface curvature on the receptivity is not significant for the range of conditions considered. The linear analysis captures the receptivity variations with Reynolds number, roughness geometry, and roughness location. When the roughness height is small, the initial amplitudes of the roughness-generated crossflow vortices are well predicted using the finite Reynolds-number receptivity theory. Nonlinear effects appeared to become significant only when the roughness height exceeds about 10% of the local boundary layer displacement thickness.

Solar Wind Models from the Sun to 1 AU: Constraints by in situ and Remote Sensing Measurements

There are three major types of solar wind: The steady fast wind originating on open magnetic field lines in coronal holes, the unsteady slow wind coming probably from the temporarily open streamer belt and the transient wind in the form of large coronal mass ejections. The majority of the models is concerned with the fast wind, which is, at least during solar minimum, the normal mode of the wind and most easily modeled by multi-fluid equations involving waves. The in-situ constraints imposed on the models, mainly by the Helios (in ecliptic) and Ulysses (high-latitude) interplanetary measurements, are extensively discussed with respect to fluid and kinetic properties of the wind. The recent SOHO observations have brought a wealth of new information about the boundary conditions for the wind in the inner solar corona and about the plasma conditions prevailing in the transition region and chromospheric sources of the wind plasma. These results are presented, and then some key questions and scientific issues are identified.

Transitional behaviour of saltation: wind tunnel observations of unsteady winds

Aspects of the transitional behaviour of aeolian saltation are investigated in a wind tunnel using non-intrusive, high frequency (≈25Hz) optical detection of sand transport. The experiments were conducted in concert with the development of a numerical model of sand transport in unsteady winds by Spies and McEwan. Comparisons are made with model outcomes and with data collected using a sand trap equipped with a sensitive load cell. A programmable velocity control was used to generate reproducible velocity excursions and sinusoidal gust sequences. For velocities less than ≈8·0ms−1the performance of the wind tunnel meets the recommended value of the independence Froude number.Transport spikes observed at inception of mass flux are shown to be an artefact of sand trap and load cell operation. In unsteady winds such devices must be used with caution. Data from the optical sensor recover most temporal features of the numerical model, including the fast response of mass flux to velocity and the slower response of velocity to mass flux. Lack of transport overshoot and an unnaturally rapid feedback (<1s) between wind and grain cloud in velocity increments are functions of limited wind tunnel height; both features are in accord with model predictions for an equivalent simulation height. Surface creep detected by the sensor also enhances the observed primary response times.In sequences of sinusoidal velocity variations, sand transport rate is found to increase as gust frequency increases. This relation becomes more pronounced as gust amplitude increases. In the range of energy-containing atmospheric gusts with periodicities between 6 and 20s, sand transport occurs at rates in excess of that recorded for steady winds of the same mean velocity. This result has significant implications for the prediction of sand transport in unsteady winds.

Study of the Current and Divergence Fields in Meiliang Bay, Taihu Lake, China with the Air-Water Model

Research is conducted on the distribution features of flow and divergence fields under the stress of ununiform and unsteady wind at Meiliang Bay on Lake Taihu by using a 3D atmospheric boundary layer model and a 2D hydrodynamic model for the area of the lake. Some meaningful results were achieved.;Research is conducted on the distribution features of flow and divergence fields under the stress of ununiform and unsteady wind at Meiliang Bay on Lake Taihu by using a 3D atmospheric boundary layer model and a 2D hydrodynamic model for the area of the lake. Some meaningful results were achieved.

Suppression of structural vibrations of an air-inflated membrane dam by its internal pressure

The suppression of vibrations in air-inflated membrane dams is considered. The vibrations are assumed to result from disturbances that include unsteady water flow and wind pressure. A mathematical model describing the vibratory motion in the air-inflated membrane and developed in J. C. Hsieh, Free Vibrations of Inflatable Dams, Ph.D. Dissertation, Virginia Polytechnic and State University (1988), is employed for devising a strategy for vibration suppression. The governing equations of motion are discretized to include a finite number of vibrational modes. The proposed suppression strategy is developed with the aid of Lyapunov functions. It has been shown that the suppression of vibrations can be accomplished by properly varying the internal pressure. In other words, the pressure by itself is capable of suppressing the vibrations in the membrane. Possible generalization of the proposed suppression scheme to other systems of relevance is discussed.

Unsteady aerodynamic force measurements on a super-tall building with a tapered cross section

The unsteady wind loads acting on a super-tall building with a tapered cross section and beveled corners were measured as functions of reduced velocity and motion amplitude. The model amplitude was controlled by an hydraulic actuator and the unsteady sectional aerodynamic loads were measured using manifolded pressure taps. The damping components of the along-wind and cross-wind unsteady aerodynamic forces were obtained from the time histories of fluctuating aerodynamic force. The vertical distribution of damping component dependent on amplitude and reduced frequency are discussed. The obtained aeroelastic damping is compared with those from aeroelastic response measurement and those predicted by quasi-steady theory.

Analysis interpretation modeling and simulation of unsteady wind and pressure data

Abstract This paper addresses the analysis, modeling and simulation of measured full-scale wind pressure and velocity data. The records exhibit both non-Gaussian and non-stationary features, and are ideal for the application of a number of contemporary methods for handling the types of problems associated with these characteristics. Modeling the probability density function (pdf) of non-Gaussian pressure is first addressed, followed by the simulation of pressure data through new static transformation techniques. The non-stationary portion of the pressure data is isolated and decomposed into a set of localized basis functions using wavelet transform techniques. Wavelet analysis is used for the identification of energy flux in time and for simulation of the non-stationary wind velocity records.

Unsteady aerodynamic force prediction on a square cylinder using k−ε turbulence models

Abstract In designing structurally safe large-scale civil structures, the dynamic structural behavior in response to unsteady wind loadings should be investigated. Since a typical civil structure is a blunt body, flow separates, reattaches, and forms an unsteady vortex in the wake region behind the structure. Conventional k−e turbulence models have failed in predicting unsteady turbulent flows around blunt bodies, unless careful modifications suited to the specific cases were made. Systematic investigation of numerical effects on the computation of turbulent flows over a square cylinder has been made. It has been found that some conventional k−e models may give reasonable predictions when proper numerical parameters are incorporated. Critical numerical aspects are found to be the near-wall grid spacing and the choice of convection schemes, while the temporal accuracy effect is not so important. A numerical prediction is compared in detail not only with experimental measurements but also with the results from a sophisticated turbulence simulation.

Prediction of vortex-induced wind loading on long-span bridges

Computational methods have been systematically applied for vortex-induced vibrations of long-span bridges by unsteady wind loadings due to vortex-shedding. Two practical bridge cases, the Namehae and the Seohae Bridge in Korea, are analyzed. Wind loading characteristics for turbulent flows over a bridge structure are investigated through computational fluid dynamics for two-dimensional bridge deck section models. With the unsteady wind loadings obtained, three-dimensional dynamic structural analyses are also performed. Reasonable agreements are obtained between the predictions by the computational analyses and the existing experimental measurements. Even though computational wind engineering cannot predict three-dimensional flow characteristics due to current computational and physical modeling constraints, it may serve as a first-stage design tool before serious wind tunnel experiments are performed.

Airflow in a Subway Station due to Train Transit. Numerical Analysis and Comparisons with Measurement Results.

In this work, the airflow generated by several types of winds in a subway station is numerically simulated by the two-dimensional SIMPLE method. A steady wind, an unsteady wind and a wind created by movement of actual train, which are the inflow or outflow conditions in the tunnel of the station, are treated for evaluation of the steady or unsteady effects of airflow in the station and estimation of the accuracy of this analysis. The subway station model used in this analysis consists of a simplified configuration of a platform, a baffle at the front end of the platform and four tunnels. In the case of the steady inflow, the airflow circulates around the platform behind the baffle. In the case of the unsteady inflow, this circulating flow gradually extends with time from the front end of the platform to the center. However, this circulating flow does not appear in either case of the steady and the unsteady outflow. In the case of the actual train wind, the calculated velocity of time process agree well with the results of fieldwork tests in the tunnels and on the platform. This numerical analysis, therefore, can be used to evaluate the airflow in an actual subway station with an intricate configuration including for example staircases and a ventilation system.

Spectra of Unsteady Wind Models of Gamma-Ray Bursts

We calculate the spectra expected from unsteady relativistic wind models of gamma-ray bursts, suitable for events of arbitrary duration. The spectral energy distribution of the burst is calculated over photon energies spanning from eV to TeV, for a range of event durations and variability timescales. The relative strength of the emission at different wavelengths can provide valuable information on the particle acceleration, radiation mechanisms and the possible types of models.

On turbulent vortex shedding flow past 2D square cylinder predicted by CFD

Analyses by CFD on unsteady flow fields past a two-dimensional (2D) square cylinder are reviewed. The accuracy of the predicted results by CFD is examined from various view points. In the first part of the paper, the comparison of the results of large eddy simulation (LES) for 2D and 3D computations are described. The LES results given from 3D computation agree very well with the experimental results, but the results based on 2D computation are different from those based on 3D computation as well as from those given from experiments. The second part of the paper concerns detailed testing of 3D LES computations for the analysis of periodic and stochastic fluctuations, for which detailed experimental data are available. Results of LES computations are compared with those from turbulence models based on Reynolds-averaged Navier-Stokes equations (RANS models) as well as with those from experiments. Finally, results of 3D LES of unsteady pressure field and wind force acting on an oscillating square cylinder are presented.

Self-Excited Oscillation of Wire and its Relaxation is System of Wire and Plate Electrodes.

The purpose of this paper is to examine the induced oscillation of a wire electrode in a system of wire and plate electrodes for the case of an electrostatic field with corona discharge. First, we have shown from experiment that such an induced oscillation is a nonplanar lateral oscillation, of which both horizontal and vertical components are excited with the fundamental natural frequency. It is also shown that the amplitude of the horizontal component exceeds that of the vertical component. Second, the above scillation, which is called a self-excited oscillation, is analyzed theoretically under the assumption that it is excited by the galloping of the wire electrode due to the unsteady corona wind with a time lag. In addition, an experiment is conducted to investigate the effect of the damper, attached near the end of the wire electrode, on relaxation of the self-excited wire oscillation.