Showing papers on "Wind profile power law published in 1998"

Three-Dimensional Wind Simulation

Abstract: A method for numerically simulating a three-dimensional field of turbulent windspeed (the “Sandia method”) for use in the aerodynamic and structural analyses of wind turbines is presented. The required inputs are single point power spectral densities (PSDs) and the coherence function. Suggestions for appropriate inputs and an example calculation are included. The simulation method is used to obtain “rotationally sampled” PSDs, which are compared with measurements obtained by Pacific Northwest Laboratories. The results show that the Sandia method is capable of producing simulations that agree with the measurements, especially when the coherence function is augmented from the usual form to include the ratio of spatial separation over height raised to the 0.25 power. The method is specialized for horizontal axis wind turbine analysis by phase lagging the simulations at each point in space so that wind speeds are simulated only when the turbine blade passes the point, reducing storage requirements and computation time by about an order of magnitude. For vertical axis applications, where interpolation will be required, the error induced by the interpolation is estimated and eliminated by the addition of white noise.

A wind turbine with a wind velocity measurement system

Abstract: The present invention provides a wind turbine with improved efficiency, reduced static as well as dynamic structural loads, reduced noise, and facilitating grid connection with improved power quality. According to the invention these and other features are fulfilled by a wind turbine comprising a wind velocity measurement system for determination of air velocities in front of the wind turbine and having an output for provision of determined air velocity values.

The Galileo Probe Doppler Wind Experiment: Measurement of the deep zonal winds on Jupiter

Abstract: During its descent into the upper atmosphere of Jupiter, the Galileo probe transmitted data to the orbiter for 57.5 min. Accurate measurements of the probe radio frequency, driven by an ultrastable oscillator, allowed an accurate time history of the probe motions to be reconstructed. Removal from the probe radio frequency profile of known Doppler contributions, including the orbiter trajectory, the probe descent velocity, and the rotation of Jupiter, left a measurable frequency residual due to Jupiter's zonal winds, and microdynamical motion of the probe from spin, swing under the parachute, atmospheric turbulence, and aerodynamic buffeting. From the assumption of the dominance of the zonal horizontal winds, the frequency residuals were inverted and resulted in the first in situ measurements of the vertical profile of Jupiter's deep zonal winds. A number of error sources with the capability of corrupting the frequency measurements or the interpretation of the frequency residuals were considered using reasonable assumptions and calibrations from prelaunch and in-flight testing. It is found that beneath the cloud tops (about 700 mbar) the winds are prograde and rise rapidly to 170 m/s at 4 bars. Beyond 4 bars to the depth at which the link with the probe was lost, nearly 21 bars, the winds remain constant and strong. Corrections for the high temperatures encountered by the probe have recently been completed and provide no evidence of diminishing or strengthening of the zonal wind profile in the deeper regions explored by the Galileo probe.

Stationary gravity‐wave structure in flows with directional wind shear

Abstract: Most theoretical studies of flow over mountains assume that the wind direction is constant. For two-dimensional orographic ridges this is not an issue, since only the component of the wind across the ridge forces internal gravity waves. A special case of rectilinear flow is when the wind component changes sign (i.e. the wind direction changes discontinuously through 180 degrees): such a point is a critical level for all stationary gravity waves. For isolated three-dimensional mountains the gravity-wave response comprises waves of all azimuthal orientations, and each of these may have a different critical-level height-depending on whether the wind backs or veers with height. Using a combination of ray-tracing methods and stationary-phase solutions, the structure of the stationary, hydrostatic gravity-wave field generated by an isolated mountain is studied for a simple wind field which backs with height. In contrast to the behaviour for unidirectional flow, wave action does not accumulate indefinitely beneath the critical level but is continuously advected downwind by the component of the wind parallel to the phase lines. Instead of the wave energy becoming unbounded following a ray as in the 2D case, the wave energy decays to zero along a trailing wake zone downwind of the mountain along phase lines. This is just an extension of the ‘asymptotic wake’ found in the linear solutions of Smith for constant flow over an isolated mountain. The work described here underlines the desirability of including the effects of selective critical-level absorption in the gravity-wave drag parametrization schemes of numerical weather-prediction models.

On the surface layer similarity at a complex forest site

Abstract: Eddy covariance (EC) and profile measurements from April until October 1996 have been used to analyze the variance and gradient similarity above a Scots pine forest in Hyytiala, southern Finland. The practical motivation for the study is to test the quality of the EC measurements by the similarity-predicted scaling law of variances and the potential of applying the gradient method to flux estimation. The correct stability dependence of the standard deviations (s.d.) of turbulent quantities scaled with relevant flux parameters indicates proper functioning of the EC system under unstable to moderately stable conditions. Modest variation of the nondimensional s.d. of vertical wind speed with wind direction is observed. Under strongly stable conditions the normalized standard deviations of wind speed components, but not of scalars, are substantially increased. The ratios of the nondimensional wind and temperature gradients to their corresponding values obtained from the ideal similarity formulas vary significantly with wind direction, indicating the influence of topography. No stability dependence of the ratios was observed. The profile method, applied to the measured wind and temperature profiles with the wind direction dependent corrections to flux-profile relationships, reproduced the EC-measured heat fluxes with high correlation. This indicates the possible applicability of the profile method at nonideal sites, if the modification of the profiles is studied beforehand by using independently determined fluxes.

The SEEK chemical release experiment: Observed neutral wind profile in a region of sporadic E

Abstract: Wind measurements made with the chemical release technique during a sporadic E layer event are presented. The data were obtained as part of the Sporadic E Experiment over Kyushu (SEEK) sounding rocket campaign. The winds show a strong maximum in the lower E region approaching 150 m s−1 near 105-km altitude and a large shear below the maximum. The large shear was within a few kilometers altitude of the peak in the electron densities measured on the downleg of the rocket trajectory. Calculations of the Richardson numbers for the wind profile show that the altitude range near the layer was highly unstable.

Investigation of the flicker emission by grid connected wind turbines

Abstract: Wind turbines connected to electrical grids may affect considerably the quality of the supply, due to the fluctuating character of their output power. An important effect from their connection are the rapid fluctuations of the supply voltage, usually referred to as "flicker". In this paper, an investigation is presented of the flicker emission levels from a grid connected wind turbine, using a detailed computer simulation model. The results include an analysis of the dependence of the flicker on the wind characteristics (mean value and turbulence intensity), grid parameters (short circuit capacity and impedance angle) and the type of wind turbine (constant or variable speed).

Removing wave effects from the wind stress vector

Abstract: The presence of ocean surface waves has been observed to affect both the magnitude and direction of the wind stress. Here concurrent wind and wave data are employed to study their relationship. To help isolate the influence of the waves, the wind stress is broken into three frequency bands: "low" (frequencies below 0.06 Hz), corresponding to large-scale motions in the boundary layer at frequencies below any significant wave energy; "middle" (frequencies between 0.06 and 0.16 Hz), corresponding to the frequencies of the dominant swell and wind waves; and "high" (frequencies above 0.16 Hz), corresponding to waves too short to influence coherently the wind fluctuations at the anemometer site 8 m above the surface. Most often, the low band holds the most stress. The magnitude of the wind stress within the low band increases roughly with the square of the mean wind speed, the high band appears to increase with the wind speed to the fourth power, and the middle band exhibits varied dependence. The direction of the wind stress in the low band is closely tied to the mean wind direction. In contrast, the directions in the middle and high bands are influenced by the waves and can be significantly off the mean wind direction. The middle band is biased toward the direction of long-period swell, while the high band is biased toward the direction of short- period seas, which is closer to the wind direction. Thus it is mainly within the middle band that large deviations in stress versus wind magnitude and direction are found. To further isolate the influence of waves, a wave-correlated fraction of the wind stress is estimated using direct correlations between the surface elevation and wind fluctuations. Removing this wave- correlated stress from the total results in a residual stress that is better behaved: the magnitude of the residual stress in the middle band is modeled by a simple wind speed dependent drag coefficient, and the direction is very nearly aligned with the wind in both the middle and high bands. These results indicate that waves are indeed closely associated with the observed deviations from "bulk formula" stress estimates. They also suggest a new method by which to estimate the wind stress; namely, partitioning the stress into three separately modeled parts: a low-frequency stress, a high-frequency wave-correlated stress, and a high-frequency residual stress.

Change in the aerodynamic roughness height by saltating grains: Experimental assessment, test of theory, and operational parameterization

Abstract: Data from an experiment at Owens Lake provided an opportunity to verify the validity of Raupach's formula that predicts the apparent roughness height at equilibrium with saltation during wind erosion episodes. In addition to that verification, a simplification of Raupach's formula is presented which allows the computation of the wind friction velocity affected by saltating sand grains without the need to measure the threshold wind friction velocity and to do iterative calculations. This method estimates the increase of the wind friction velocity Δu*, above the nonsaltating wind friction velocity caused by the saltating grains: Δu* = 0.3 [U − Ut]2 where U and Ut are the wind speed and the threshold wind speed, respectively, at 10m.

Urban wind flows: wind tunnel and numerical simulations—a preliminary comparison

Abstract: Previous studies have indicated that wind flow models based on the k−e formulation are able to simulate the broad features of flows as observed in field programs and wind tunnel studies. k−e models are increasingly being used to investigate the dispersion of pollutants and the transfer of heat within and out of urban canyons. In these situations realistic modelling requires greater fidelity on the part of the wind flow specification in that some confidence needs to be placed on the actual magnitude of the wind speeds and turbulent viscosities used in the dispersive components of the models. To this end a preliminary comparison of wind tunnel and numerical modelling was undertaken. It has demonstrated general agreement between the two types of modelling but has also indicated some differences. These are in the variation of concentration with height at the windward wall and in the concentration gradients away from the canyon boundaries.

Sudden narrow temperature‐inversion‐layer formation in ALOHA‐93 as a critical‐layer‐interaction phenomenon

Abstract: A sudden and dramatic mesospheric heating event was observed over Haleakala, Maui, on October 21, 1993 (day 294), during the ALOHA-93 Campaign. It consisted of a persistent, narrow temperature-inversion layer about 3-4 km wide near 87 km altitude with a peak temperature rise approaching 40 k, lasting for about 3.5 hours. Owing to the large number of ground-based instruments recording the event, it is possible to attempt to seek out a physical explanation for this temperature rise. There is powerful evidence to suggest that the temperature-inversion layer is associated with energy deposition, direct and indirect, resulting from gravity wave/critical layer interaction. Indeed, lidar wind profiles and mesospheric wave structures simultaneously observed with CCD imagers reveal the presence of a critical layer at the appropriate altitude. The data also show a narrow unstable background wind profile over a 2-3 km thickness in the immediate vicinity of the critical layer. We will show quantitatively that there is sufficient energy available from the dissipation of the observed gravity wave and from the observed instability in the background winds to account for the heating. After the critical layer has disappeared, the temperature rise subsides, and the background wind again becomes stable at all height levels of interest.

Bird Migration and Bias of WSR-88D Wind Estimates

Abstract: Migrating birds can greatly influence base velocity, velocity azimuth display (VAD), and VAD wind profile products of the WSR-88D. This is documented by comparing estimates of wind velocity and direction from these products with corresponding radiosonde and pilot balloon data. Mean absolute differences between wind velocities estimated from base velocity products and the corresponding radiosonde data were significantly greater on days with bird migration than on days with no migration. Even low-density migrations can increase VAD wind velocities relative to winds measured simultaneously by balloon. Because birds usually migrate with following winds, wind directions measured by VAD are less affected, but in some cases the difference can be as much as 128°. Consequently, when winds aloft are nearly calm and variable in direction, the data in the VAD wind profile and base velocity products may pertain almost exclusively to migrating birds.

Sensitivity of a wave model to wind variability

Abstract: A third-generation wave model was applied to the Gulf of California to hindcast wave spectra and to assess model sensitivity to wind variability on the basis of three different numerical simulations: (1) when a synthetic wind field of varying randomness is adopted and when the forcing wind field considered has an input time step of (2) 6 hours and (3) 5 min. In the first idealized simulation the wave field induced by a constant wind field is compared with the result when white noise was added to the originally constant forcing winds. Results from these numerical simulations demonstrate that wave energy increases with wind variability, even though the mean wind is kept constant. In the second and third simulations the forcing wind is averaged for periods of 6 hours and 5 min, which represent relatively low and high timescale variability, respectively. These realistic wind fields were constructed as a blend of detailed in situ measurements and analysis information, representing wind variability in both speed and direction. The results show that high-variability winds induce broad directional wave spectra and secondary peaks with similar magnitude as the main peak (bimodal spectra). With the presence of wind gustiness and a continuous spectrum of wind variability, all source terms are expected to play a significant role in the evolution of the wave spectrum.

On the solar cycle dependence of winds and planetary waves as seen from mid-latitude D1 LF mesopause region wind measurements

Abstract: At the Collm Observatory of the University of Leipzig LF D1 low-frequency total reflection nighttime wind measurements have been carried out continuously for more than two decades. Using a multiple regression analysis to derive prevailing winds, tides and the quasi-2-day wave from the half-hourly mean values of the horizontal wind components, monthly mean values of mesopause wind parameters are obtained that can be analysed with respect to long-term trends and influences of solar variability. The response of the prevailing wind to the 11-year solar cycle differs throughout the year. While in winter no significant correlation between the zonal prevailing wind and solar activity is found, in spring and summer a negative correlation between the TWC can be seen from the measurements. This is connected with stronger vertical gradients of the zonal prevailing wind during solar maximum than during solar minimum. Since the amplitude of the quasi-2-day wave is dependent on the zonal mean wind vertical gradient, this is connected with a positive correlation between solar activity and quasi-two-day wave activity.

Wind wave tank measurements of wave damping and radar cross sections in the presence of monomolecular surface films

Abstract: Measurements of the damping of small gravity and gravity-capillary water surface waves covered with monomolecular organic films of different viscoelastic properties were performed in the wind wave tank facility of the University of Hamburg. The wind speed dependence of the radar cross sections for X and Ka band was measured with upwind looking microwave antennas. It is shown that Marangoni damping theory, which describes the damping of water surface waves by viscoelastic surface films, is not the only damping mechanism in wind wave tank experiments where the wind sea is not fully developed. The other source terms of the action balance equation, i.e., the energy input into the water waves from the wind, the nonlinear wave-wave interaction, and the dissipation by wave breaking, are affected differently by the various substances. It is hypothesized that this difference is caused by the different viscoelastic properties of the substances, i.e., by the different intermolecular interactions of the film molecules. A slight dip in the wind dependence of the radar cross section at Ka band at wind speeds of 8-9 m/s was measured, which corresponds to comparable reductions of the mean squared wave height and wave slope. Polarization ratios (i.e., the ratios of the radar backscatter at vertical and horizontal polarization) higher than those predicted by simple Bragg scattering theory for X band at low wind speeds and different incidence angles are explained within a (three-scale) composite-surface model. At higher wind speeds, where the polarization ratio decreases rapidly, breaking by wedges and spilling breakers is hypothesized to become more dominant. The dependence of the polarization ratio on the coverage of the water surface with a slick is explained qualitatively by means of the composite-surface model. Finally, it is stated that wind wave tank measurements in the presence of monomolecular surface films are useful for the verification of theories concerning radar backscattering, wave damping, and wind-wave and wave-wave interactions.

Wind-Generated Waves in Lake Okeechobee

Abstract: The study of wind generated waves is important because waves affect sediment resuspension in lakes. Measurements of wind velocity and wave elevation were made at three different stations in Lake Okeechobee. Significant wave heights were computed using a direct count from the recorded data, and verified by the root-mean-square value approach. The correlation between wind stress and significant wave height also was analyzed. The data revealed a strong correlation. In addition to field measurements, a Boussinesq-type wind-wave model was developed to simulate wind-generated, long-propagating waves. This model included the effects of wind stress and bottom viscous dissipation. Wave elevation and velocity field were evaluated numerically. A six-day simulation using 1996 wind data was conducted. Simulated significant wave heights were found to agree reasonably well with measured values. A predictive wind-wave model provides information about wind generated waves, which is used to compute bottom shear stresses required for sediment resuspension studies.

Air‐sea interaction over a thermal marine front in the Denmark Strait

Abstract: An investigation was conducted into air-sea interaction in the Denmark Strait, where a distinct thermal front separates warm North Atlantic water from the cold East Greenland Current. The field data consisted of ship weather station data and rawinsonde soundings from R/V Aranda's expedition in August-September 1993. The surface energy balance differed drastically between the warm and cold side of the front (net fluxes of 95 W m−2 upward and 82 W m−2 downward, respectively). The difference resulted mostly from the contradictory turbulent fluxes. The air temperature, humidity and wind speed showed more variation on the warm side of the front. Lower wind speeds were observed on the cold side. The cross-frontal differences in the air temperature and wind speed were largest during front-parallel flow, but those in the sensible and latent heat flux were largest during cross-frontal flow. During cases of air advection across the front, the modification in the air temperature was strongest with a low wind speed. Downwind of the front, the sensible heat flux strongly depended on the south-north wind component. The rawinsonde data revealed temperature inversions and low-level jets. The wind profile was affected by the combined effects of baroclinity, surface layer stability, and stratification through the atmospheric boundary layer. The surface heterogeneity caused by the sea surface temperature front resulted in the Schmidt paradox: the area-averaged sensible heat flux was upward, while the area-averaged air temperature exceeded the area-averaged surface temperature. A mosaic method, extended by estimates of the local wind speed over the warm and cold water side, was applicable to parameterizing the area-averaged sensible heat flux.