Unsteady Wind Research Articles

Observations of Wind Wave Development in Mixed Seas and Unsteady Wind Forcing*

Abstract Theoretical study and experimental verification of wind wave generation and evolution focus generally on ideal conditions of steady state and quiescent initial background, of which the ideal fetch-limited wind wave growth is an important benchmark. In nature, unsteady winds and swell presence are more common. Here, the observations of wind wave development in mixed seas under unsteady and quasi-steady wind forcing are presented. With reference to the ideal fetch-limited growth functions established under steady wind forcing in the absence of swell, the analysis shows that the wind-steadiness factor impacts wave growth. The wind wave variance in mixed sea is enhanced in both accelerating and decelerating phases of an unsteady wind event, with a larger enhancement in the accelerating phase than in the decelerating phase. Spatial and temporal wind wave measurements under similar environmental conditions are also compared; the quantifiable differences in the wave development are attributable to the wind-steadiness factor. Coupled with the empirical observation that the average wind stress is decreased in mixed sea, these results suggest that wind wave generation and development are more efficient in mixed sea than in wind sea. Possible causes include (i) oscillatory modulation of surface roughness increases air–sea exchanges, (ii) background surface motion reduces energy waste for cold start of wind wave generation from a quiescent state, and (iii) breaking of short waves redistributes wind input and allows more of the available wind power to be directed to the longer waves for their continuous growth.

Predictability of unsteady two-dimensional k− ε model on the aerodynamic instabilities of some rectangular prisms

It is well known that a bluff body cross-section exhibits various kinds of aerodynamic instabilities such as vortex-induced vibration, galloping and torsional flutter. Since these cross-sections are used in long-span bridges and tall buildings, it is important to predict their occurrence in wind resistant structural design. In this paper, the authors make a series of comparisons of unsteady wind forces, unsteady pressure distributions and free vibration responses between previously conducted studies and an unsteady two-dimensional k– ε model for rectangular cross-sections with cross-sectional ratios of 2 and 4 in a smooth uniform flow in order to verify computational predictability of aerodynamic instabilities. As a result, the computation successfully predicted the onset velocities and responses of these aerodynamic instabilities for these cross-sectional ratios, which are common to tall buildings and long bridges.

Dynamic Instability of Spatial Structures under Typical Wind Flow

The mechanism of dynamic instability of large span spatial structures under high wind speed should be studied carefully, but it has seldom been done before. In this paper, a double-layer cylindrical reticulated shell under a typical wind flow is taken as an example to study the dynamic instability of large span spatial structures under wind flows. The interaction of the shell and the flow is ignored and unsteady wind forces on the shell are obtained from wind tunnel tests. The dynamic instability status of the shell is determined by the Budiansky-Roth criterion. The results show that the double-layer cylindrical reticulated shell will become dynamically instable under high wind speed.

Numerical Simulation Method for Unsteady Wind Field around Building

A large eddy simulation algorithm for numerically calculating unsteady wind field around complex geometry building was given. In this algorithm, a curvilinear coordinate system was used to formulate the numerical model, so the complex wind-structure interface could be described precisely. Governing equations were discretized by central differencing schemes with co-located grids. The convection velocity was interpolated by using Rhie-Chow method in order to avoid the inherent odd-even decoupling problem. A common Maker and Cell (MAC) scheme was used to split the velocity-pressure linkage in Navier-Stockes (NS) equation. An explicit Adams-Bashforth differencing scheme was applied for time marching. A corresponding computer code was developed and was validated by performing on simulation of wind field around full-scale Texas Tech University (TTU) building model. Furthermore, the simulation results were compared with corresponding field measure data and wind-tunnel experiment results reported by other authors. The comparison showed that this algorithm is feasible for numerical simulation of unsteady wind field around buildings.

Seven-Sensor Fast-Response Probe for Full-Scale Wind Turbine Flowfield Measurements

The unsteady wind profile in the atmospheric boundary layer upstream of a modern wind turbine is measured. The measurements are accomplished using a novel measurement approach that is comprised of an autonomous uninhabited aerial vehicle (UAV) that is equipped with a seven-sensor fast-response aerodynamic probe (F7S-UAV). The autonomous UAV enables high spatial resolution (∼6.3% of rotor diameter) measurements, which hitherto have not been accomplished around full-scale wind turbines. The F7S-UAV probe developed at ETH Zurich is the key-enabling technology for the measurements. The time-averaged wind profile from the F7S-UAV probe is found to be in very good agreement to an independently measured profile using the UAV. This time-averaged profile, which is measured in moderately complex terrain, differs by as much as 30% from the wind profile that is extrapolated from a logarithmic height formula; therefore, the limited utility of extrapolated profiles, which are commonly used in site assessments, is made evident. The time-varying wind profiles show that at a given height, the velocity fluctuations can be as much as 44% of the time-averaged velocity, therefore indicating that there are substantial loads that may impact the fatigue life of the wind turbine’s components. Furthermore, the shear in the velocity profile also subjects the fixed pitch blade to varying incidences and loading. Analysis of the associated velocity triangles indicates that the sectional lift coefficient at midspan of this modern turbine would vary by 12% in the measured time-averaged wind profile. These variations must be accounted in the structural design of the blades. Thus, the measurements of the unsteady wind profile accomplished with this novel measurement system demonstrate that it is a cost effective complement to the suite of available site assessment measurement tools.

Experimental Study of the Plunging Motion with Unsteady Wind Tunnel Wall Interference Effects

A series of experiments were carried out to investigate unsteady behavior of the flow field as well as the boundary layer of an airfoil oscillating in plunging-type motion in a subsonic wind tunnel. The measurements involved surface-mounted hot films complimented with surface pressure. In addition, wind tunnel wall pressure distribution was acquired to furnish a baseline for the wall interference corrections. The airfoil is the section of a 660-kW wind turbine blade. The experiments were conducted at a Reynolds number of 0.42 million, and over two reduced frequencies of k = 0.06 and 0.085, at prestall, nearstall, and poststall regions. The unsteady aerodynamic loads were calculated from the surface pressure measurements, 64 ports, along the chord for both upper and lower surfaces of the model. The plunging displacements were transformed into the equivalent angle of attack. The surface hot-film measurements provided information about the boundary-layer events. The boundary-layer transition occurred via a laminar separation bubble. Variations of the surface pressure coefficients and aerodynamic loads with the equivalent angle of attack showed strong sensitivity to the reduced frequency and the mean angles of attack. The wall pressure distribution was affected by the model oscillation especially the region underneath the model.

Integration of Crosswind Forces into Train Dynamic Modelling

In this paper a new method is used to calculate unsteady wind loadings acting on a railway vehicle. The method takes input data from wind tunnel testing or from computational fluid dynamics simulations (one example of each is presented in this article), for aerodynamic force and moment coefficients and combines these with fluctuating wind velocity time histories and train speed to produce wind force time histories on the train. This method is fast and efficient and this has allowed the wind forces to be applied to a vehicle dynamics simulation for a long length of track. Two typical vehicles (one passenger, one freight) have been modelled using the vehicle dynamics simulation package ‘VAMPIRE®’, which allows detailed modelling of the vehicle suspension and wheel—rail contact. The aerodynamic coefficients of the passenger train have been obtained from wind tunnel tests while those of the freight train have been obtained through fluid dynamic computations using large-eddy simulation. Wind loadings were calculated for the same vehicles for a range of average wind speeds and applied to the vehicle models using a user routine within the VAMPIRE package. Track irregularities measured by a track recording coach for a 40 km section of the main line route from London to King's Lynn were used as input to the vehicle simulations. The simulated vehicle behaviour was assessed against two key indicators for derailment; the Y/ Q ratio, which is an indicator of wheel climb derailment, and the Δ Q/ Q value, which indicates wheel unloading and therefore potential roll over. The results show that vehicle derailment by either indicator is not predicted for either vehicle for any mean wind speed up to 20 m/s (with consequent gusts up to around 30 m/s). At a higher mean wind speed of 25 m/s derailment is predicted for the passenger vehicle and the unladen freight vehicle (but not for the laden freight vehicle).

Unsteady buffeting wind loads in the time domain and their effect on the life-cycle prediction of guyed masts

For predicting the life-cycle of guyed structures under turbulent wind loads, a model-chain has to be applied including models for: wind-loading; mechanical behaviour of the structure; and fatigue analysis. Each model affects the predicted lifetime. Unsteady wind load models in the frequency domain require a linearisation of the structure's nonlinearities due to the sagging cables. Quasi-steady models for the wind loading in the time domain lead to forces that correspond to measurements only in high winds. This paper gives an overview of the identification of complex aerodynamic admittances in full-scale measurements. Aerodynamic impulse response functions in combination with nonlinear numerical simulations of guyed masts in the time domain under turbulent wind are used for a life-cycle analysis. The results indicate a reduction of the predicted damage compared with common quasi-steady simulations of buffeting wind loads.

The atmospheric snow-transport model: SnowDrift3D

AbstractSnowDrift3D, a high-resolution, atmospheric snow-transport model, is presented for the first time. In contrast to most state-of-the-art snowdrift models, atmospheric particle transport, i.e. saltation and suspension, is accounted for by one passive transport equation. The model uses unsteady wind fields (spatial resolution of up to 2 m) computed with an atmospheric computational fluid dynamics model that is directly connected to the numerical weather prediction model ALADIN. Sensitivity runs show that (1) the saltation mass flux is a function of cubic shear velocity, , (2) the model is marginally sensitive to the grid spacing at high resolutions (up to 2 m), (3) the model computes the redistribution of snow at high resolution in real time on dual core personal computers and (4) the changing topography of the snow cover should be included in cases of local erosion or deposition of a large amount of snow. Finally, we present a comparison of modeled and measured snow distributions obtained by terrestrial laser scanning showing area-wide linear correlation up to R = 0.33.

Visualization of an Isolated Vortex Ring Generated from Step-Like Wind Variation and the Overshoot Phenomenon of Axial Velocity

The flow field of a free square jet generated from an unsteady wind tunnel with a step-like axial velocity variation was measured using an ultrasonic anemometer. Three-dimensional velocity vector fields were synthesized by computational modeling from the measured time-series velocity data. The vorticity vector field was calculated after appropriate interpolation and extrapolation. A three-dimensional geographical representation of the isosurface of the vorticity vector norm showed that an isolated vortex ring separated from the shear layer and changed its shape as it moved downstream. In order to elucidate the cause of the overshoot peak observed in time-dependent variations of the axial velocity for the step-like jet, the velocities induced at an observation point on the jet axis, first by the isolated vortex ring and then by the other unsteady portions, were calculated using Biot-Savart's Law. The contribution to the overshoot peak from the other portions, including the re-developing shear layer, was larger than that from the isolated vortex ring.

Water penetration resistance of residential window and wall systems subjected to steady and unsteady wind loading

This paper presents results from two interrelated studies that investigated the performance of residential window and wall systems subjected to (1) simplified load scenarios and (2) realistic, time-varying load conditions. The first project investigated the diagnostic ability of standard water penetration test methods for the evaluation of residential systems. Static and rapidly pulsed pressure test protocols were compared using 2.4 m × 2.4 m wall sections with integrated windows. The second project quantified water ingress rates of operable, sliding seal windows under time-varying wind loads derived from wind tunnel measurements. The findings of this research indicated that the application of a static air pressure differential across the specimen is a practical, effective technique to evaluate watertightness of residential windows, however enhancements to the testing approach are warranted if systems level performance is to be evaluated.

A coordinated study of field-aligned currents and Pc5 ULF waves during ejecta 1997

We show examples of long period Pc5 magnetic field pulsations near field-aligned current (FAC) regions in the high-latitude magnetosphere, observed by INTERBALL-Au, and coordinated with POLAR, GOES-9 and ground-based observations during 11 January and 11 April 1997. Identification of corresponding magnetosphere regions and subregions is provided by electrons and protons in the energy-range of 0.01–100 keV measured onboard the spacecraft. The ULF Pc5 wave occurrence is observed in both upward and downward FACs. A fairly good correlation is demonstrated between these ULF Pc5 waves and the consecutive injection of magnetosheath low energy protons. The constancy of the observed frequency peak at 1.8 mHz during quite unsteady solar wind pressure conditions could be reconciled with the surface wave mode model. The 3.1 mHz peak location area probably resembles field-line fluctuations with an interesting appearance of poloidal mode oscillation. It is suggested that the 1.3 mHz wave and its harmonic 2.6 mHz represent global compressional oscillations.

Spatially developing secondary instabilities in compressible swept airfoil boundary layers

Two-dimensional eigenvalue analysis is used on a massive scale to study the spatial instabilities of compressible shear flows with two inhomogeneous directions. The main focus of the study is crossflow dominated swept-wing boundary layers although the methodology can also be applied to study other types of flows, such as the attachment-line flow. Certain unique aspects of formulating a spatial, two-dimensional eigenvalue problem for the secondary instability of finite amplitude crossflow vortices are discussed, namely, fixing the spatial growth direction unambiguously through a non-orthogonal formulation of the linearized disturbance equations. A primary test case used for parameter study corresponds to Numerical results are presented for the low-speed, NLF-0415(b) airfoil configuration as tested in the ASU Unsteady Wind Tunnel, wherein a spanwise periodic array of roughness elements was placed near the leading edge in order to excite stationary crossflow modes with a specified fundamental wavelength. The two classes of flow conditions selected for this analysis include those for which the roughness array spacing corresponds to either the naturally dominant crossflow wavelength, or a subcritical wavelength that serves to reduce the growth of the naturally excited dominant crossflow modes. Numerical predictions are compared with the measured database, both as indirect validation for the spatial instability analysis and to provide a basis for comparison with a higher Reynolds number, supersonic swept-wing configuration. Application of the eigenvalue analysis to the supersonic configuration reveals that a broad spectrum of stationary crossflow modes can sustain sufficiently strong secondary instabilities as to potentially cause transition over this configuration. In particular, the control mode itself, if initiated with too large an amplitude, may lead to an earlier transition.

Wind buffeting of large telescopes

Unsteady wind loads due to turbulence within the telescope enclosure are one of the largest dynamic disturbances for ground-based optical telescopes. The desire to minimize the response to the wind influences the design of the telescope enclosure, structure, and control systems. There is now significant experience in detailed integrated modeling to predict image jitter due to wind. Based on this experience, a relatively simple model is proposed that is verified (from a more detailed model) to capture the relevant physics. In addition to illustrating the important elements of the telescope design that influence wind response, this model is used to understand the sensitivity of telescope image jitter to a wide range of design parameters.

902 変動風速下における可動翼風車の特性に関する研究(GS-8 動力エネルギー)

902 変動風速下における可動翼風車の特性に関する研究(GS-8 動力エネルギー)

1409 非定常風洞が発生するステップ風に伴う渦輪の挙動計測(流体力学V)

1409 非定常風洞が発生するステップ風に伴う渦輪の挙動計測(流体力学V)

New Method for Estimation of Aeolian Sand Transport Rate Using Ceramic Sand Flux Sensor (UD-101)

In this study, a new method for the estimation of aeolian sand transport rate was developed; the method employs a ceramic sand flux sensor (UD-101). UD-101 detects wind-blown sand impacting on its surface. The method was devised by considering the results of wind tunnel experiments that were performed using a vertical sediment trap and the UD-101. Field measurements to evaluate the estimation accuracy during the prevalence of unsteady winds were performed on a flat backshore. The results showed that aeolian sand transport rates estimated using the developed method were of the same order as those estimated using the existing method for high transport rates, i.e., for transport rates greater than 0.01 kg m−1 s−1.

The simulation of unsteady aerodynamic cross wind forces on trains

Abstract This paper describes an investigation of the effect of cross winds on train dynamic systems. It considers the nature of the fluctuating cross wind and how such cross winds produce fluctuating forces. A simple method for calculating the wind time history at the position of a moving vehicle is firstly set out, that is computationally much less expensive than existing methods. The concepts of aerodynamic admittance and weighting function are then introduced, and methods outlined for parameterising experimental values of aerodynamic admittance and obtaining simple closed form weighting function solutions from these expressions. The use of the weighting function in obtaining unsteady force time histories is then set out. It is shown that different formulations of this method are necessary for stationary and moving vehicles. A statistical analysis of the results produced by this method is then presented, which compares the results to those of simple quasi-steady calculations. An appendix describes how the cross wind forces develop in a variety of different idealised gust shapes. It is concluded that, if the dynamic phenomenon of interest has a time period of

Locating an atmospheric contamination source using slow manifolds

Finite-size particle motion in fluids obeys the Maxey–Riley equations, which become singular in the limit of infinitesimally small particle size. Because of this singularity, finding the source of a dispersed set of small particles is a numerically ill-posed problem that leads to exponential blowup. Here we use recent results on the existence of a slow manifold in the Maxey–Riley equations to overcome this difficulty in source inversion. Specifically, we locate the source of particles by projecting their dispersed positions on a time-varying slow manifold, and by advecting them on the manifold in backward time. We use this technique to locate the source of a hypothetical anthrax release in an unsteady three-dimensional atmospheric wind field in an urban street canyon.

EXPERIMENTAL INVESTIGATION ON A PITCHING MOTION DELTA WING IN UNSTEADY FREE STREAM

As combat aircraft becomes more and more maneuverable, the need to understand the unsteady behavior of aircraft in dynamic flow fields becomes more important. Usually researchers pay more attention to the effects on the changes of AOA, but ignore the effects of velocity variations. It is known that the velocity of aircraft changes greatly when the aircraft undergoes a high angle of attack maneuver, like "cobra" maneuver. To completely simulate and study the effect of rapid changes in both free stream velocity and angle of attack, a pitching motion setup is developed in the unsteady wind tunnel of NUAA. By measuring unsteady loads, unsteady pressure distribution and flow visualization, the unsteady aerodynamic behavior of a pitching isolated delta wing and the pitching delta wing coupled with unsteady free stream are investigated. It is found that the oscillating free stream velocity affects the hysteresis characteristics of the pitching delta wing further. The pressure distribution and flow visualization measurements show that the changes in the structure of the leading-edge vortices are the main reason. These studies conclude that a good understanding of the unsteady aerodynamics is vitally important in the design of super-maneuverable aircraft.