Longitudinal Wind Component Research Articles

Variability of turbulence dispersion characteristics during heavy haze process: A case study in Beijing

The turbulent standard deviations and the turbulent third-order and fourth-order moments are the key turbulence dispersion parameters in Lagrangian dispersion models. However, the characteristics of these parameters under heavy haze conditions in urban areas have not been fully investigated, and the commonly used similarity relations of these parameters in models were based on observations in highly flat and sparsely populated areas. In this paper, the vertical profiles of these parameters and their local similarity relations under heavy haze conditions in the wintertime of Beijing have been analyzed by using data collected at a 325-m meteorological tower. The heavy haze process has been divided into three stages: transport stage (TS), cumulative stage (CS), and dispersion stage (DS). Results show that the turbulent dispersion parameters behave differently during three stages. In the TS and DS, the maxima appear in the profiles of the turbulent standard deviations above the urban canopy; in the CS, the turbulent standard deviation are almost constant with height. The analysis of the third and fourth order moments shows that the wind velocities above the urban canopy in the TS deviate from the Gaussian distribution more significantly than those in the CS and DS. The local similarity relations of the turbulent dispersion parameters in the TS, especially for the longitudinal wind components, are normally different from those in the CS and DS. Thus, different from the common assumptions in Lagrangian models, the turbulence dispersion in horizontal directions is anisotropic and should be parameterized by multiple similarity relations under heavy haze conditions.

Wind Gust Characterization at Wind Turbine Relevant Heights in Moderately Complex Terrain

AbstractImproved understanding of wind gusts in complex terrain is critically important to wind engineering and specifically the wind energy industry. Observational data from 3D sonic anemometers deployed at 3 and 65 m at a site in moderately complex terrain within the northeastern United States are used to calculate 10 descriptors of wind gusts and to determine the parent distributions that best describe these parameters. It is shown that the parent distributions exhibit consistency across different descriptors of the gust climate. Specifically, the parameters that describe the gust intensity (gust amplitude, rise magnitude, and lapse magnitude; i.e., properties that have units of length per time) fit the two-parameter Weibull distribution, those that are unitless ratios (gust factor and peak factor) are described by log-logistic distributions, and all other properties (peak gust, rise and lapse times, gust asymmetric factor, and gust length scale) are lognormally distributed. It is also shown that gust factors scale with turbulence intensity, but gusts are distinguishable in power spectra of the longitudinal wind component (i.e., they have demonstrably different length scales than the average eddy length scale). Gust periods at the lower measurement height (3 m) are consistent with shear production, whereas at 65 m they are not. At this site, there is only a weak directional dependence of gust properties on site terrain and land cover variability along sectorial transects, but large gust length scales and gust factors are more likely to be observed in unstable atmospheric conditions.

Wind characteristics at Sutong Bridge site using 8-year field measurement data

Full-scale wind characteristics based on the field measurements is an essential element in structural wind engineering. Statistical analysis of the wind characteristics at Sutong Cable-stayed Bridge (SCB) site is conducted in this study with the recorded long-term wind data from structural health monitoring system (SHMS) between 2008 and 2015. Both the mean and turbulent wind characteristics and power spectra are comprehensively investigated and compared with those in the current codes of practice, such as the measured wind rose diagram, monthly maximum mean wind speed, turbulence intensity, integral length scale. Measurement results based on the monitoring data show that winds surrounding the SCB site are substantially influenced by the southeast monsoon in summer and strong northern wind in winter. The measured turbulence intensity is slightly higher than the recommended values in specifications, while the measured ratio of lateral to longitudinal turbulence intensity is slightly lower. An approximately linear relationship between the measured turbulence intensities and gust factors is obtained. The mean value of the turbulence integral length scale is smaller than that of typical typhoon events. In addition, it is found that the Kaimal spectrum is suitable to be adopted as the power spectrum for longitudinal wind component at the SCB site. This contribution would provide important wind characteristic references for the wind performance evaluation of SCB and other civil infrastructures in adjacent regions.

Characterization of wind velocities in the upstream induction zone of a wind turbine using scanning continuous-wave lidars

As a wind turbine generates power, induced velocities, lower than the freestream velocity, will be present upstream of the turbine due to perturbation of the flow by the rotor. In this study, the upstream induction zone of a 225 kW horizontal axis Vestas V27 wind turbine located at the Danish Technical University's Risø campus is investigated using a scanning Light Detection and Ranging (lidar) system. Three short-range continuous-wave “WindScanner” lidars are positioned in the field around the V27 turbine allowing detection of all three components of the wind velocity vectors within the induction zone. The time-averaged mean wind speeds at different locations in the upstream induction zone are measured by scanning a horizontal plane at hub height and a vertical plane centered at the middle of the rotor extending roughly 1.5 rotor diameters (D) upstream of the rotor. Turbulence statistics in the induction zone are studied by more rapidly scanning along individual lines perpendicular to the rotor at different radial distances from the hub. The mean velocity measurements reveal that the longitudinal velocity reductions become greater closer to the rotor plane and closer to the center of the rotor. Velocity deficits of 1%–3% of the freestream value were observed 1 D upstream of the rotor, increasing at the rotor plane to 7.4% near the edge of the rotor and 18% near the center of the rotor while the turbine was operating with a high estimated mechanical coefficient of power (CP) of 0.56 yielding an estimated axial induction factor of 0.25. The velocity reductions relative to the freestream velocity become smaller when the turbine's coefficient of power decreases; for a low CP of 0.16 resulting in an estimated induction factor of 0.04, the velocity deficits are ∼1% of the freestream value 1 D upstream of the rotor and only 6% at the rotor plane near the center of the rotor. Additionally, the mean radial wind speeds were found to increase close to the edge of the rotor disk indicating an expansion of the incoming flow around the rotor. Radial velocity magnitudes at the edge of the rotor disk of approximately 9% and 3% of the freestream longitudinal wind speed were measured for the abovementioned high and low CP values, respectively. Turbulence statistics, calculated using 2.5-min time series, suggest that the standard deviation of the longitudinal wind component decreases close to the rotor, while the standard deviation of the radial wind component appears to increase. When the turbine was operating with a high CP of 0.54 resulting in an estimated induction factor of 0.22, standard deviation decreases of up to 22% of the estimated freestream value and increases of up to 46% were observed for the longitudinal and radial components, respectively, near the center of the rotor.

Validation of surface layer similarity theory to describe far offshore marine conditions in the Dutch North Sea in scope of wind energy research

With the ongoing development of offshore wind farms it becomes increasingly important to have an accurate description of offshore atmospheric conditions. A new meteorological mast constructed in 2011 sited 85km offshore in the Dutch North-sea area has been used to validate the relations expected from Monin–Obukhov similarity theory. It is shown that wind shear and turbulence (second order moments and turbulence spectra) are a function of the non-dimensional stability parameter ζ, which is in agreement with literature.Wind shear is well described by the stability corrected logarithmic wind shear profile, though scatter is significant for stable conditions. This is explained by considering the relative height within the boundary layer where observations are taken. The non-dimensional second order moments of the three wind components are found to be either constant (neutral), proportional to (-ζ)−1/3 (unstable) or proportional to ζ−1/2 (stable). The Kaimal spectrum has been validated for specified stability classes. The non-dimensional scaling frequency f0 for the three wind components increases for stable conditions, and decreases for unstable conditions. Besides, we find that f0 is smallest for the longitudinal wind component, and largest for the vertical wind component. These results support the validity and applicability of Monin–Obukhov similarity theory to describe atmospheric conditions far offshore.

Turbulence Spectra for Boundary-Layer Winds in Tropical Cyclones: A Conceptual Framework and Field Measurements at Coastlines

A conceptual model is proposed for the characteristic sub-ranges in the velocity and temperature spectra in the boundary layer of tropical cyclones (hurricanes or typhoons). The model is based on observations and computation of radial and vertical profiles of the mean flow and turbulence, and on the interpretation of eddy mechanisms determined by shear (namely roll and streak structures near the surface), convection, rotation, blocking and sheltering effects at the ground/sea surface and in internal shear layers. The significant sub-ranges, as the frequency increases, are associated with larger energy containing eddies, shear and blocking, inertial transfer between large and small scales, and intense small-scale eddies generated near the surface caused by waves, coastal roughness change, and the buoyancy force associated with the evaporation of spray droplets. These sub-ranges vary with the locations at which the spectra are measured, i.e. the level \(z\) in relation to the height \(z_{max}\) of the peak mean velocity and the depth \(h\) of the boundary layer, and the radius \(r\) in relation to the eyewall radius \(R_{ew}\) and the outer-vortex radius \(R_{ov}\). For two tropical cyclones (Nuri and Hagupit), experimental data were analyzed. Spectra were measured where \(r\) is near to \(R_{ew}\) and \(R_{ov}\) using four 1-h long datasets at coastal towers, at 10- and 60-m heights for tropical cyclone Nuri, and at 60-m height for tropical cyclone Hagupit at the south China coast. The field measurements of spectra within the boundary layer show significant sub-ranges of self-similar energy spectra (lying between the length scale 1,000 m and the smallest scales less than 40 m) that are consistent with the above conceptual model of the surface layer. However, with very high wind speeds near the eyewall, the energy of the independently generated intense surface eddy motions, associated with surface waves and water droplets in the airflow, greatly exceeds the energies of the small scales in the inertial sub-range of the boundary layer, over scales less than about 3–40 m depending on the height \(z\) and the radius \(r\). This rise in the small-scale frequency weighted spectra (\(nS_u (n)\), where \(n\) is natural frequency, and \(S_u (n)\) is the energy spectrum of the longitudinal wind component) is consistent with the hypothesis that these processes are only weakly correlated with the main boundary-layer turbulence.

Organised Motion in a Tall Spruce Canopy: Temporal Scales, Structure Spacing and Terrain Effects

This study investigates the organised motion near the canopy-atmosphere interface of a moderately dense spruce forest in heterogeneous, complex terrain. Wind direction is used to assess differences in topography and surface properties. Observations were obtained at several heights above and within the canopy using sonic anemometers and fast-response gas analysers over the course of several weeks. Analysed variables include the three-dimensional wind vector, the sonic temperature, and the concentration of carbon dioxide. Wavelet analysis was used to extract the organised motion from time series and to derive its temporal scales. Spectral Fourier analysis was deployed to compute power spectra and phase spectra. Profiles of tem- poral scales of ramp-like coherent structures in the vertical and longitudinal wind components showed a reversed variation with height and were of similar size within the canopy. Temporal scales of scalar fields were comparable to those of the longi- tudinal wind component suggesting that the lateral scalar transport dominates. The existence of a -1 power law in the longitudinal power spectra was confirmed for a few cases only, with a majority showing a clear 5/3 decay. The variation of effective scales of organised motion in the longitudinal velocity and temperature were found to vary with atmospheric stability, suggesting that both Kelvin-Helmholtz instabili- ties and attached eddies dominate the flow with increasing convectional forcing. The canopy mixing-layer analogy was observed to be applicable for ramp-like coherent structures in the vertical wind component for selected wind directions only. Depar- tures from the prediction of m = � wL −1 s = 8-10 (wherew is the streamwise spacing of coherent structures in the vertical wind w and Ls is a canopy shear length scale) were caused by smaller shear length scales associated with large-scale changes in the terrain as well as the vertical structure of the canopy. The occurrence of linear gravity

Drag coefficient and turbulence intensity in conifer canopies

The relationship between wind speed and drag coefficient was experimentally investigated in two mixed conifer forests of the Italian Alps. Drag coefficient was calculated from the mean momentum equation while canopy architecture was described using optical measurements of gap fraction. Two different formulations of the momentum equation were tested: the common one, where drag forces are expressed in terms of the squared mean velocity ( u ̄ 2 ), and an alternative one in which drag forces depend on the averaged product of instantaneous wind intensity and instantaneous longitudinal wind component |U|u . Values of drag coefficients computed according to the first approach, decrease with increasing wind speed in both sites and show different average values between sites (0.15±0.14 and 0.34±0.45). Drag coefficients obtained with the second approach do not show a clear dependence on wind intensity, and the mean drag coefficients of the two sites become more similar (0.09±0.06 and 0.12±0.12). According to these results the scaling based on |U|u seems more appropriate to characterise the physics of the phenomenon.

Ship-board measurements and estimations of air-sea fluxes in the western tropical pacific during TOGA COARE

Direct air-sea flux measurements were made on R/V Kexue #1 at 4 ° S, 156 ° E during the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean-Atmospheric Response Experiment (COARE) Intensive Observation Period (IOP). An array of six accelerometers was used to measure the motion of the anchored ship, and a sonic anemometer and Lyman-α hygrometer were used to measure the turbulent wind vector and specific humidity. The contamination of the turbulent wind components by ship motion was largely removed by an improvement of a procedure due to Shao based on the acceleration signals. The scheme of the wind correction for ship motion is briefly outlined. Results are presented from data for the best wind direction relative to the ship to minimize flow distortion effects. Both the time series and the power spectra of the sonic-measured wind components show swell-induced ship motion contamination, which is largely removed by the accelerometer correction scheme. There was less contamination in the longitudinal wind component than in the vertical and transverse components. The spectral characteristics of the surface-layer turbulence properties are compared with those from previous land and ocean results. Momentum and latent heat fluxes were calculated by eddy correlation and compared to those estimated by the inertial dissipation method and the TOGA COARE bulk formula. The estimations of wind stress determined by eddy correlation are smaller than those from the TOGA COARE bulk formula, especially for higher wind speeds, while those from the bulk formula and inertial dissipation technique are generally in agreement. The estimations of latent heat flux from the three different methods are in reasonable agreement. The effect of the correction for ship motion on latent heat fluxes is not as large as on momentum fluxes.

The Patos Lagoon summertime circulation and dynamics

The main aspects of the summertime circulation and dynamics of the Patos Lagoon, a system located in southern Brazil and considered as one of the world's largest choked coastal lagoons, are studied through the analysis of time series of wind stress, water level and freshwater discharge, combined with the results of a barotropic circulation model. The longitudinal wind component has been verified as the main driving force, generating a set-up/set-down mechanism of oscillation with the nodal line in the midlagoon area. The period of this oscillation coincides with the passages of frontal systems for this region. The sea breeze acts as a secondary effect, being clearly observed in the northern part of the lagoon. Freshwater discharge is expected to cause variations in water level on the seasonal band and to a lesser degree in the 8–15 day time-scale. The tidal signal is of importance only near the exit to the ocean, being strongly reduced in the interior of the lagoon. Model results suggest a wind set-up momentum balance in the longitudinal direction in the deeper parts of the lagoon; near the margins, the longitudinal momentum balance is mostly of frictional form, with the wind stress being balanced by the bottom friction. In the lateral direction, a geostrophic balance is verified in both regions. The wind forced circulation is characterized by the presence of several cells with downwind velocity near the margins and upwind return flow occurring in the central areas.

Study of the turbulent characteristics of the near-wake field of a medium-sized wind turbine operating in high wind conditions

The near-wake turbulent structure that is downwind of a medium-sized, horizontal axis wind turbine at a distance of one rotor diameter is discussed. The experimental site is the Samos Island Wind Park comprising nine wind turbines installed on the top of a 400 m-high saddle. The analysis is based on experimental data obtained mainly under strong wind conditions by two masts erected upstream and downstream of a wind turbine. The field of wind turbulence is examined both in integral and spectral form. Consideration of the perturbation produced by the tower construction is crucial in the interpretation of results. Observations show that the turbulent field varies from the edge to the center of the wake and strongly depends on the incident wind speed. Increased turbulent levels are observed near the blade tips, with evidence of a similar trend around the hub height for all wind speeds. Decreases of wind turbulence are observed in mid frequencies inside the wake due to the reduced shear associated with the flat crosswind velocity profile. This effect seems to dominate in the variation of the integral values of the longitudinal wind component variance. The low frequency portion of wind spectra reverses behavior in high wind speeds, i.e., an increase in energy relative to background values is observed. This is probably due to the shape of the turbine characteristic power curve. Cross-wind profiles of turbulent shear stresses at the lower boundary of the wake are also discussed.

An Analysis of Wind Direction and Horizontal Wind Component Fluctuations over Complex Terrain

Based on an extensive wind dataset over complex terrain, the commonly used small-angle approximation σv ≈ σθV is studied and found to overestimate over all wind speeds and σθ values observed. This should be anticipated due to the assumptions necessary to derive the approximation. Overestimation (of 10%–30%) is also observed in the small σθ range. The three parameters involved are further discussed to gain better understanding of the behavior of the approximation under different conditions. The standard deviation of wind direction σθ is shown to vary inversely with wind speed not only under stable, but also under convective conditions, reaching a site-dependent constant value at high wind speeds. The dependence of the ratio of the mean longitudinal wind component to the scalar mean wind speed on wind speed and σθ is examined, as well as that of the relevant standard deviations (σu,σVa). While the former obtains small values in the high-σθ, or low-wind range, or both, estimated values of the latter justify equivalence of σu, σVa under most conditions. Finally, the effects of wind speed and σθ on σv are examined.

A continuous three-dimensional model of short-range forecast error covariances

This paper develops a representation for the three-dimensional covariance structure of the observed residuals, or differences, between the short-range forecasts used as trial fields in an operational data assimilation system and the verifying radiosonde data. Assuming homogeneity and isotropy on pressure surfaces, geopotential and wind covariances are fitted to a series expansion employing Bessel functions in the horizontal and vertical normal modes as basis functions. The latter are obtained from linear model equations with an isothermal basic state employing a log-pressure vertical coordinate. Such a formulation allows for a closed analytic form, and therefore a spatially continuous three-dimensional covariance model. The model is used to examine covariances of geopotential as well as transverse and longitudinal wind components. Unlike commonly-used representations employing vertical/horizontal separability, the present model reproduces the observed significant increase of horizontal decorrelation length scales with height. The wind data are used to obtain divergent and solenoidal components and subsequent comparison of stream function and geopotential statistics reveals a high degree of geostrophic balance away from the upper and lower boundaries. It is suggested that the present mathematical framework could be employed in developing analytical non-separable three-dimensional correlation functions for use in operational data assimilation.

Spatial correlation functions of surface-layer atmospheric turbulence in neutral stratification

The longitudinal (i.e., in the direction of the mean wind) spectra and cospectra of wind components and temperature fluctuations in the atmospheric surface layer during neutral conditions were carefully investigated by Kader (1984, 1987) for a broad range of wave numbers which included wavelengths far beyond the large-scale limit of the inertial subrange. At the same time, some direct measurements of spatial correlation functions of the longitudinal wind component and temperature were performed by Zubkovskii and Fedorov (1986) and Zubkovskii and Sushko (1987). Section 2 of the present paper gives a review of the available results on longitudinal spectra and cospectra of wind velocity and temperature fluctuations in neutral stratification and examines the consequences of these results related to the longitudinal autocorrelation and symmetrized cross-correlation functions of surface-layer turbulence. In Section 3 it is shown that the correlation equations of Section 2 agree satisfactorily with some recent measurements of the longitudinal correlation functions in the range of distances from 3 m to 100 m. Some measurements of the lateral correlation functions of atmospheric turbulence are also presented in Section 3. It is shown that these measurements lead to some predictions concerning the never-measured lateral space spectra of surface-layer turbulence.

Vertical coherence and phase delay between wind components in strong winds below 20 m

Vertical coherence and phase delay between wind components in strong winds in the lowest 20 m over uniform terrain in New Zealand have been analysed. The vertical decay constants and slopes for the horizontal wind components could be described as linear functions of the ratio of vertical spacing to mean height (δz/z), with relatively little scatter. The decay constants for the lateral were smaller than for the longitudinal components, the slopes larger for the lateral components, as found previously. The coherence of vertical velocity is best described by exponential decay functions multiplied by a parameter which decreases linearly from 1 (for δz/z=0) to 0.5 (for δz/z=1). The decay constants increased linearly with δz/z, as in the case of the horizontal components. There were no significant phase differences for the vertical components. Lateral and vertical decay constants for the longitudinal wind component could be fitted to identical functions of δy/z and δz/z, respectively.

Lateral coherence of longitudinal wind components in strong winds

A combination of lateral coherence measurements of wind speed at five locations suggests that the ‘decay constant’ is a monotonically increasing function of the ratio of separation to height, under neutral conditions.

Horizontal coherence and Pasquill's beta

Previous work has suggested that horizontal coherence between longitudinal wind components increases with decreasing stability. Further analysis now suggests that this relation is valid only for separations of 50 m or larger. For separations of a few meters, the dependence of coherence on stability has the opposite sign. A hypothesis is suggested which accounts for the rather complex relationship between coherence, stability, roughness, and separation.

Airflow from mustard to fallow

Abstract Real terrain is complex and its effects on the atmosphere are not well understood. The two‐dimensional change‐of‐roughness problem is investigated in an experimental study of airflow from mustard to fallow. The surface friction velocity is found to overshoot and return slowly to the equilibrium value. The wind profile in the modified region is more nearly logarithmic than the Peterson model predicts. The simple Elliot theory is in fair agreement with the wind profile data. Values for the ratio of the standard deviation of the vertical wind to the friction velocity reveal no variation with stability and are in close agreement with the value for laboratory flows. Values for the ratio of the standard deviation of the longitudinal wind component to the friction velocity are comparable to other recently determined atmospheric values and show a variation attributable to large‐scale terrain properties.

Two-Point Wind Correlations on an Isobaric Surface in a Nonhomogeneous Non-Isotropic Atmosphere

The approximate relations between the correlation coefficient for geopotential, the longitudinal wind component, and the transverse wind component on an isobaric surface in the case of an homogeneous isotropic atmosphere are discussed briefly. Corresponding approximate relations for a nonhomogeneous and non-isotropic atmosphere are presented. The mixed correlations for wind component (longitudinal with transverse, and vice versa) are no longer identically zero (as was the case for the homogeneous isotropic atmosphere). Approximate expressions for these mixed correlation coefficient functions are presented in terms of the correlation coefficient for geopotential. It is shown that the general configuration of values computed from observed winds are those expected from theoretical considerations.

Estimating the longitudinal wind spectrum near the ground

AbstractA graphical procedure for estimating the spectrum of the longitudinal wind component from height, mean wind speed, roughness length, and stability data is presented, based on observed spectra at many locations.Using non‐dimensional coordinates following Monin, the amplitude of the spectrum peak in neutral stability shows no systematic variation with height, while the corresponding dimensionless frequency varies with height to the 0.75 power. At relatively high frequencies, spectra observed in unstable conditions are indistinguishable from those in neutral stratification. At lower frequencies the ratio of the spectrum in highly unstable conditions to that in the neutral case approaches 3 : 1 at low heights and decreases to less than one in stable air.