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NASA NTRS1981

Wind turbulence inputs for horizontal axis wind turbines

Wind turbine response characteristics in the presence of atmospheric turbulence was predicted using two major modeling steps. First, the important atmospheric sources for the force excitations felt by the wind turbine system were identified and characterized. Second, a dynamic model was developed which describes how these excitations are transmitted through the structure and power train. The first modeling step, that of quantifying the important excitations due to the atmospheric turbulence was established. The dynamic modeling of the second step was undertaken separately.

Holley, W. E.↗

NASA NTRS1982

Development of methodology for horizontal axis wind turbine dynamic analysis

Horizontal axis wind turbine dynamics were studied. The following findings are summarized: (1) review of the MOSTAS computer programs for dynamic analysis of horizontal axis wind turbines; (2) review of various analysis methods for rotating systems with periodic coefficients; (3) review of structural dynamics analysis tools for large wind turbine; (4) experiments for yaw characteristics of a rotating rotor; (5) development of a finite element model for rotors; (6) development of simple models for aeroelastics; and (7) development of simple models for stability and response of wind turbines on flexible towers.

Dugundji, J.↗

NASA NTRS1991

A simple, analytical, axisymmetric microburst model for downdraft estimation

A simple analytical microburst model was developed for use in estimating vertical winds from horizontal wind measurements. It is an axisymmetric, steady state model that uses shaping functions to satisfy the mass continuity equation and simulate boundary layer effects. The model is defined through four model variables: the radius and altitude of the maximum horizontal wind, a shaping function variable, and a scale factor. The model closely agrees with a high fidelity analytical model and measured data, particularily in the radial direction and at lower altitudes. At higher altitudes, the model tends to overestimate the wind magnitude relative to the measured data.

Vicroy, Dan D.↗

NASA NTRS1981

The UTRC wind energy conversion system performance analysis for horizontal axis wind turbines (WECSPER)

The theory for the UTRC Energy Conversion System Performance Analysis (WECSPER) for the prediction of horizontal axis wind turbine performance is presented. Major features of the analysis are the ability to: (1) treat the wind turbine blades as lifting lines with a prescribed wake model; (2) solve for the wake-induced inflow and blade circulation using real nonlinear airfoil data; and (3) iterate internally to obtain a compatible wake transport velocity and blade loading solution. This analysis also provides an approximate treatment of wake distortions due to tower shadow or wind shear profiles. Finally, selected results of internal UTRC application of the analysis to existing wind turbines and correlation with limited test data are described.

Egolf, T. A.↗

NASA NTRSDate not supplied

A Study of the Kinematic and Dynamic Processes associated with Mesoscale Snowbands in a Midwest United States Snowstorm on 5 February 2020

Mesoscale snowbands are frequently responsible for dumping large amounts of snow over a relatively small region and pose a considerable forecasting challenge. The present study focuses on the mesoscale snowbands observed on 5 February 2020 over the Midwest United States. In order to improve the forecasting of mesoscale snowbands, it is important to understand the processes that organize these features. To this end, we are investigating the kinematic and dynamic processes that shape the mesoscale snowbands using a combination of high-altitude airborne radar, in-situ aircraft measurements, surface observations, and model simulations. This presentation will primarily focus on the three-dimensional structure of the mesoscale snowbands in terms of EXRAD reflectivity and horizontal winds. The horizontal winds are retrieved from the EXRAD scanning-beam Doppler velocities using a VAD technique. These winds, in combination with virtual potential temperature taken from the HRRR operational analysis, are used to compute the bulk Richardson number in order to investigate any contributions to banding features that might be tied to Kelvin-Helmholtz instability. Our analysis does not support the presence of Kelvin-Helmholtz instability. That said, low, but not subcritical, Richardson numbers were found near the top of some banding features in this event and it is possible that the model analysis is smoothing out the virtual potential temperature fields. Finally, we present arguments for using a flight module designed to sample the along-band structures with particular emphasis on observing generating cells and low-level microphysics.

Charles N. Helms↗

NASA NTRS1986

Summary of tower designs for large horizontal axis wind turbines

Towers for large horizontal axis wind turbines, machines with a rotor axis height above 30 meters and rated at more than 500 kW, have varied in configuration, materials of construction, type of construction, height, and stiffness. For example, the U.S. large HAWTs have utilized steel truss type towers and free-standing steel cylindrical towers. In Europe, the trend has been to use only free-standing and guyed cylindrical towers, but both steel and reinforced concrete have been used as materials of construction. These variations in materials of construction and type of construction reflect different engineering approaches to the design of cost effective towers for large HAWTs. Tower designs are the NASA/DOE Mod-5B presently being fabricated. Design goals and requirements that influence tower configuration, height and materials are discussed. In particular, experiences with United States large wind turbine towers are elucidated. Finally, current trends in tower designs for large HAWTs are highlighted.

Frederick, G. R.↗

DOE OSTI2020

Sonic Detection and Ranging (SODAR) Wind Profiler Instrument Handbook

The SODAR (Sonic Detection and Ranging) wind profiler measures wind profiles and backscattered signal strength between (nominally) 15 meters (m) and 500 m. It operates by transmitting acoustic energy into the atmosphere and measuring the strength and frequency of backscattered energy. The strength of the backscattered signal is determined by the strength of temperature inhomogeneities with size on the order of 10 centimeters (cm). Assuming the scattering elements in the atmosphere are moving with the mean wind, the horizontal wind field can be derived. The U.S. Department of Energy (DOE)’s Atmospheric Radiation Measurement (ARM) Climate Research Facility Mobile Facility (AMF) has a system developed by Scintec, Inc. that transmits a sequence of frequencies to enhance signal determination.

17 WIND ENERGY↗

DOE OSTI2018

Mid-latitude ionospheric irregularity spectral density as determined by ground-based GPS receiver networks

In this paper, we present a new technique to experimentally measure the spatial spectrum of ionospheric disturbances in the spatial scale regime of 40 – 200 km. This technique produces a 2-dimensional (2-D) spectrum for each time snapshot over two dense GPS receiver networks (GEONET in Japan and PBO in the Western U.S.). Because this technique created the spectrum from an instantaneous time snapshot, no assumptions are needed about the speed of ionospheric irregularities. We examine spectra from three days: one with an intense geomagnetic storm, one with significant lightning activity, and one quiet day. Radial slices along the 2-D spectra provide 1-dimensional spectra that can be fit to a power law to quantify the steepness of the fall-off in the spatial scale sizes. Continuous data of this type in a stationary location allows monitoring the variability in the 2-D spectrum over the course of a day and comparing between days, as shown here, or even over a year or many years. We find that the spectra are highly variable over the course of a day and between the two selected regions of Japan and the Western U.S. When strong travelling ionospheric disturbances (TIDs) are present, the 2-D spectra provide information about the direction of propagation of the TIDs. We compare the TID propagation direction with horizontal wind directions from the Horizontal Wind Model. Finally, TID direction is correlated with the horizontal wind direction on all days, strongly indicating that the primary source of the TIDs measured by this technique is tropospheric.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

Scanning ARM Cloud Radars. Part II: Data Quality Control and Processing

The Scanning ARM Cloud Radars (SACR’s) are the primary instruments for documenting the four-dimensional structure and evolution of clouds within a 20-30 km radius from the ARM fixed and mobile sites. Here, the post-processing of the calibrated SACR measurements is discussed. First, a feature mask algorithm that objectively determines the presence of significant radar returns is described. The feature mask algorithm is based on the statistical properties of radar receiver noise. It accounts for atmospheric emission and is applicable even for SACR profiles with few or no signal-free range gates. Using the nearest-in-time atmospheric sounding, the SACR radar reflectivities are corrected for gaseous attenuation (water vapor and oxygen) using a line-by-line absorption model. Despite having a high pulse repetition frequency, the SACR has a narrow Nyquist velocity limit and thus Doppler velocity folding is commonly observed. An unfolding algorithm that makes use of a first guess for the true Doppler velocity using horizontal wind measurements from the nearest sounding is described. The retrieval of the horizontal wind profile from the HS-RHI SACR scan observations and/or nearest sounding is described. The retrieved horizontal wind profile can be used to adaptively configure SACR scan strategies that depend on wind direction. Several remaining challenges are discussed, including the removal of insect and second-trip echoes. The described algorithms significantly enhance SACR data quality and constitute an important step towards the utilization of SACR measurements for cloud research.

climate↗

DOE OSTI2015

Validation of Simplified Load Equations through Loads Measurement and Modeling of a Small Horizontal-Axis Wind Turbine Tower; NREL (National Renewable Energy Laboratory)

As part of an ongoing effort to improve the modeling and prediction of small wind turbine dynamics, NREL tested a small horizontal axis wind turbine in the field at the National Wind Technology Center (NWTC). The test turbine was a 2.1-kW downwind machine mounted on an 18-meter multi-section fiberglass composite tower. The tower was instrumented and monitored for approximately 6 months. The collected data were analyzed to assess the turbine and tower loads and further validate the simplified loads equations from the International Electrotechnical Commission (IEC) 61400-2 design standards. Field-measured loads were also compared to the output of an aeroelastic model of the turbine. Ultimate loads at the tower base were assessed using both the simplified design equations and the aeroelastic model output. The simplified design equations in IEC 61400-2 do not accurately model fatigue loads. In this project, we compared fatigue loads as measured in the field, as predicted by the aeroelastic model, and as calculated using the simplified design equations.

17 WIND ENERGY↗

NASA NTRS1985

Development of large, horizontal-axis wind turbines

A program to develop large, horizontal-axis wind turbines is discussed. The program is directed toward developing the technology for safe, reliable, environmentally acceptable large wind turbines that can generate a significant amount of electricity at costs competitive with those of conventional electricity-generating systems. In addition, these large wind turbines must be fully compatible with electric utility operations and interface requirements. Several ongoing projects in large-wind-turbine development are directed toward meeting the technology requirements for utility applications. The machines based on first-generation technology (Mod-OA and Mod-1) successfully completed their planned periods of experimental operation in June, 1982. The second-generation machines (Mod-2) are in operation at selected utility sites. A third-generation machine (Mod-5) is under contract. Erection and initial operation of the Mod-5 in Hawaii should take place in 1986. Each successive generation of technology increased reliability and energy capture while reducing the cost of electricity. These advances are being made by gaining a better understanding of the system-design drivers, improving the analytical design tools, verifying design methods with operating field data, and incorporating new technology and innovative designs. Information is given on the results from the first- and second-generation machines (Mod-OA, - 1, and -2), the status of the Department of Interior, and the status of the third-generation wind turbine (Mod-5).

Baldwin, D. H.↗

NASA NTRS1977

Variations in eddy diffusion and associated transport processes

Variation in the eddy diffusion coefficient constitutes a momentum source. This variation near the turbopause affects both the composition and the wind and temperature fields of the thermosphere. Two types of calculations are carried out, for horizontal winds considered self-consistently in a three-dimensional dynamic model and horizontal winds artificially suppressed to simulate a one-dimensional model. Momentum transfer associated with variations in eddy diffusion coefficients represents the source function in the analysis presented. It is shown that for long-term variations in the thermosphere, the compositional dependence on variations in eddy diffusion coefficients can be entirely different in one and three dimensional models with horizontal winds included. Horizontal winds can greatly reduce the effects of eddy diffusion in general and the ratio between He and O amplitudes in particular.

Mayr, H. G.↗

NASA NTRS1978

Aeroelastic stability and response of horizontal axis wind turbine blades

The coupled flap-lag-torsion equations of motion of an isolated horizontal axis wind turbine blade are formulated. Quasi-steady blade-element strip theory was applied to derive the aerodynamic operator which includes boundary layer type gradient winds. The final equations which have periodic coefficients were solved in order to obtain the aeroelastic response and stability of large horizontal axis wind turbine blade. A new method of generating an appropriate time-dependent equilibrium position (required for the stability analysis) has been implemented. Representative steady-state responses and stability boundaries, applicable mainly to an existing blade design (NASA/-ERDA MOD-0), are presented. The results indicate that the MOD-0 configuration is a basically stable design and that blade stability is not sensitive to offsets between blade elastic axis and aerodynamic center. Blade stability appears to be sensitive to precone. The tower shadow (or wake) has a considerable effect on the flap response but leaves blade stability unchanged. Finally, it was found that non linear terms in the equations of motion can significantly affect the linearized stability boundaries, however, these terms have a negligible effect on blade response at operating conditions.

Kottapalli, S. B. R.↗

NASA NTRS1980

Nonlinear Aeroelastic Equations of Motion of Twisted, Nonuniform, Flexible Horizontal-Axis Wind Turbine Blades

The second-degree nonlinear equations of motion for a flexible, twisted, nonuniform, horizontal axis wind turbine blade were developed using Hamilton's principle. A mathematical ordering scheme which was consistent with the assumption of a slender beam was used to discard some higher-order elastic and inertial terms in the second-degree nonlinear equations. The blade aerodynamic loading which was employed accounted for both wind shear and tower shadow and was obtained from strip theory based on a quasi-steady approximation of two-dimensional, incompressible, unsteady, airfoil theory. The resulting equations had periodic coefficients and were suitable for determining the aeroelastic stability and response of large horizontal-axis wind turbine blades.

Kaza, K. R. V.↗

NASA NTRS1992

Finite amplitude gravity waves in the Venus atmosphere generated by surface topography

A two-dimensional, fully nonlinear, nonhydrostatic, gravity wave model is used to study the evolution of gravity waves generated near the surface of Venus. The model extends from near the surface to well above the cloud layers. Waves are forced by applying a vertical wind at the bottom boundary. The boundary vertical wind is determined by the product of the horizontal wind and the gradient of the surface height. When wave amplitudes are small, the near-surface horizontal wind is the zonally averaged basic-state zonal wind, and the length scales of the forcing that results are characteristic of the surface height variation. When the forcing becomes larger and wave amplitudes affect the near-surface horizontal wind field, the forcing spectrum becomes more complicated, and a spectrum of waves is generated that is not a direct reflection of the spectrum of the surface height variation. Model spatial resolution required depends on the amplitude of forcing; for very nonlinear cases considered, vertical resolution was 250 m, and horizontal resolution was slightly greater than 1 km. For smaller forcing amplitudes, spatial resolution was much coarser, being 1 km in the vertical and about 10 km in the horizontal. Background static stability and mean wind are typical of those observed in the Venus atmosphere.

Young, R. E.↗

DOE OSTI2019

Shape optimization under uncertainty for rotor blades of horizontal axis wind turbines

We present a computational framework for the shape optimization of a Horizontal-Axis Wind Turbine (HAWT) rotor blade under uncertainty. Our framework integrates aerodynamic simulations based on the blade element method which utilizes reduced order models of the blade structure and wind load with design sensitivity analysis and nonlinear programming. The wind velocity is modeled as a stochastic process to account for variations in time and space. An additional stochastic process accounts for uncertainties in the structural material properties. The uncertainty propagation is based on a non-intrusive polynomial chaos expansion that allows accurate estimation of stochastic performance metrics such as generated power and structural compliance. Sensitivities of cost and constraint functions with respect to shape parameters namely twist angles are computed with an efficient scheme to enable gradient based optimization. To demonstrate the effect of uncertainty, designs obtained from optimization under uncertainty are compared to those obtained from deterministic optimization.

42 ENGINEERING↗

NASA NTRS1981

Aerodynamic performance prediction of horizontal axis wind turbines

A new method for calculating the aerodynamic performance of horizontal axis wind turbines is described. The method, entitled the helical vortex method, directly calculates the local induced velocity due to helical vortices that originate at the rotor blade. Furthermore, the method does not require a specified circulation distribution. Results of the method are compared to similar results obtained from Wilson PROP code methods as well as to existing experimental data taken from a Mod-O wind turbine. It is shown that results of the proposed method agree well with experimental values of the power output both near cut-in and at rated wind speeds. Further, it is found that the method does not experience some of the numerical difficulties encountered by the PROP code when run at low wind velocities.

Jeng, D. R.↗

NASA NTRS1986

Development and testing of vortex generators for small horizontal axis wind turbines

Vortex generators (VGs) for a small (32 ft diameter) horizontal axis wind turbine, the Carter Model 25, have been developed and tested. Arrays of VGs in a counterrotating arrangement were tested on the inbound half-span, outboard half-span, and on the entire blade. VG pairs had their centerlines spaced at a distance of 15% of blade chord, with a spanwise width of 10% of blade chord. Each VG had a length/height ratio of 4, with a height of between 0.5% and 1.0% of the blade chord. Tests were made with roughness strips to determine whether VGs alleviated the sensitivity of some turbines to an accumulation of bugs and dirt on the leading edge. Field test data showed that VGs increased power output up to 20% at wind speeds above 10 m/s with only a small (less than 4%) performance penalty at lower speeds. The VGs on the outboard span of the blade were more effective than those on inner sections. For the case of full span coverage, the energy yearly output increased almost 6% at a site with a mean wind speed of 16 mph. The VGs did reduce the performance loss caused by leading edge roughness. An increase in blade pitch angle has an effect on the power curve similar to the addition of VGs. VGs alleviate the sensitivity of wind turbine rotors to leading edge roughness caused by bugs and drift.

Gyatt, G. W.↗