Aerodynamic Analysis of Cup Anemometers Performance: The Stationary Harmonic Response

The effect of cup anemometer shape parameters, such as the cups’ shape, their size, and their center rotation radius, was experimentally analyzed.This analysis was based on both the calibration constants of the transfer function and the most important harmonic termof the rotor’smovement,which due to the cup anemometer design is the third one.This harmonic analysis represents a new approach to study cup anemometer performances. The results clearly showed a good correlation between the average rotational speed of the anemometer’s rotor and the mentioned third harmonic term of its movement.

Analysis of calibration results from cup and propeller anemometers. Influence on wind turbine Annual Energy Production (AEP) calculations

The calibration coefficients of several models of cup and propeller anemometers were analysed. The analysis was based on a series of laboratory calibrations between January 2003 and August 2007. Mean and standard deviation values of calibration coefficients from the anemometers studied were included. Two calibration procedures were used and compared. In the first, recommended by the Measuring network of Wind Energy Institutes (MEASNET), 13 measurement points were taken over a wind speed range of 4 to 16  m  s−1. In the second procedure, 9 measurement points were taken over a wider speed range of 4 to 23  m  s−1. Results indicated no significant differences between the two calibration procedures applied to the same anemometer in terms of measured wind speed and wind turbines' Annual Energy Production (AEP). The influence of the cup anemometers' design on the calibration coefficients was also analysed. The results revealed that the slope of the calibration curve, if based on the rotation frequency and not the anemometer's output frequency, seemed to depend on the cup center rotation radius.

Fourier analysis of the aerodynamic behavior of cup anemometers

The calibration results (the transfer function) of an anemometer equipped with several cup rotors were analyzed and correlated with the aerodynamic forces measured on the isolated cups in a wind tunnel. The correlation was based on a Fourier analysis of the normal-to-the-cup aerodynamic force. Three different cup shapes were studied: typical conical cups, elliptical cups and porous cups (conical-truncated shape). Results indicated a good correlation between the anemometer factor, K, and the ratio between the first two coefficients in the Fourier series decomposition of the normal-to-the-cup aerodynamic force

Análisis mediante cálculo numérico (CFD) del comportamiento de anemómetros de cazoletas = CFD analysis of cup anemometers performance

El presente trabajo consiste en la simulación del flujo aerodinámico alrededor de cazoletas de anemómetros. Para ello se ha utilizado un código numérico o software comercial de análisis numérico de fluidos o CFD (Computational Fluid Dynamics). Este trabajo es un aporte más en la línea de investigación acerca del comportamiento de los anemómetros de cazoletas, que viene llevándose a cabo en el Instituto Universitario de Microgravedad “Ignacio Da Riva” (IDR/UPM). La primera parte de este proyecto consistió en la realización de simulaciones de tipo estacionarias (esto es, con la cazoleta bajo un cierto ángulo de incidencia con respecto al viento pero sin movimiento de rotación). De esta forma se analiza de forma independiente y asilada la cazoleta en cada una de las diferentes posiciones a lo largo de un giro de 360 grados. Así pues, a varios modelos de cazoleta se les fue variando su posición en incrementos de 10 grados desde de la posición angular inicial q = 0º hasta q = 180º, ya que las cazoletas presentan un comportamiento simétrico. La segunda parte de este proyecto se destinó a la realización de otra serie de simulaciones de tipo no estacionarias. Este tipo de simulaciones se realizaron concretamente a sólo un modelo de cazoleta (cónica no porosa). Estas últimas simulaciones, en concreto nueve, se realizaron variando la velocidad angular de la cazoleta respecto a la velocidad del viento.

Anomaly detection on cup anemometers

The performances of two rotor-damaged commercial anemometers (Vector Instruments A100 LK) were studied. The calibration results (i.e. the transfer function) were very linear, the aerodynamic behavior being more efficient than the one shown by both anemometers equipped with undamaged rotors. No detection of the anomaly (the rotors'damage) was possible based on the calibration results. However, the Fourier analysis clearly revealed this anomaly.

Singularities and undefinitions in the calibration functions of sonic anemometers

A mathematical model of the process employed by a sonic anemometer to build up the measured wind vector in a steady flow is presented to illustrate the way the geometry of these sensors as well as the characteristics of aerodynamic disturbance on the acoustic path can lead to singularities in the transformation function that relates the measured (disturbed) wind vector with the real (corrected) wind vector, impeding the application of correction/calibration functions for some wind conditions. An implicit function theorem allows for the identification of those combinations of real wind conditions and design parameters that lead to undefined correction/ calibration functions. In general, orthogonal path sensors do not show problematic combination of parameters. However, some geometric sonic sensor designs, available in the market, with paths forming smaller angles could lead to undefined correction functions for some levels of aerodynamic disturbances and for certain wind directions. The parameters studied have a strong influence on the existence and number of singularities in the correction/ calibration function as well as on the number of singularities for some combination of parameters. Some conclusions concerning good design practices are included.

On Cup Anemometer Rotor Aerodynamics

The influence of anemometer rotor shape parameters, such as the cups’ front area or their center rotation radius on the anemometer’s performance was analyzed. This analysis was based on calibrations performed on two different anemometers (one based on magnet system output signal, and the other one based on an opto-electronic system output signal), tested with 21 different rotors. The results were compared to the ones resulting from classical analytical models. The results clearly showed a linear dependency of both calibration constants, the slope and the offset, on the cups’ center rotation radius, the influence of the front area of the cups also being observed. The analytical model of Kondo et al. was proved to be accurate if it is based on precise data related to the aerodynamic behavior of a rotor’s cup.

Cup Anemometer's loss of performance due to ageing process, and its effect on Annual energy Production (AEP) estimates

The deviation of calibration coefficients from five cup anemometer models over time was analyzed. The analysis was based on a series of laboratory calibrations between January 2001 and August 2010. The analysis was performed on two different groups of anemometers: (1) anemometers not used for any industrial purpose (that is, just stored); and (2) anemometers used in different industrial applications (mainly in the field—or outside—applications like wind farms). Results indicate a loss of performance of the studied anemometers over time. In the case of the unused anemometers the degradation shows a clear pattern. In the case of the anemometers used in the field, the data analyzed also suggest a loss of performance, yet the degradation does not show a clear trend. A recalibration schedule is proposed based on the observed performances variations

Effect of the Wake of the Transducer Supports on the Ultrasonic Anemometer Measurements

The wake produced by the structural supports of the ultrasonic anemometers (UAs)causes distortions in the velocity field in the vicinity of the sonic path. These distortions are measured by the UA, inducing errors in the determination of the mean velocity, turbulence intensity, spectrum, etc.; basic parameters to determine the effect of wind on structures. Additionally, these distortions can lead to indefinition in the calibration function of the sensors (Cuerva et al., 2004). Several wind tunnel tests have been dedicated to obtaining experimental data, from which have been developed fit models to describe and to correct these distortions (Kaimal, 1978 and Wyngaard, 1985). This work explores the effect of a vortex wake generated by the supports of an UA, on the measurement of wind speed done by this instrument. To do this, the Von Karman¿s vortex street potential model is combined with the mathematical model of the measuring process carried out by UAs developed by Franchini et al. (2007). The obtained results are the correction functions of the measured wind velocity, which depends on the geometry of the sonic anemometer and aerodynamic conditions. These results have been validated with the ones obtained in a wind tunnel test done on a single path UA, especially developed for research. The supports of this UA have been modified in order to reproduce the conditions of the theoretical model. Good agreements between experimental and theoretical results have been found.

Parameterization of the atmospheric boundary layer for offshore wind resource assessment with a limited length-scale k-ε model

The structure of the atmospheric boundary layer (ABL) is modelled with the limited- length-scale k-ε model of Apsley and Castro. Contrary to the standard k-ε model, the limited-length-scale k-ε model imposes a maximum mixing length which is derived from the boundary layer height, for neutral and unstable atmospheric situations, or by Monin-Obukhov length when the atmosphere is stably stratified. The model is first verified reproducing the famous Leipzig wind profile. Then the performance of the model is tested with measurements from FINO-1 platform using sonic anemometers to derive the appropriate maximum mixing length.

Deviation of Cup and Propeller Anemometer Calibration Results with Air Density

The effect of air density variations on the calibration constants of several models of anemometers has been analyzed. The analysis was based on a series of calibrations between March 2003 and February 2011. Results indicate a linear behavior of both calibration constants with the air density. The effect of changes in air density on the measured wind speed by an anemometer was also studied. The results suggest that there can be an important deviation of the measured wind speed with changes in air density from the one at which the anemometer was calibrated, and therefore the need to take this effect into account when calculating wind power estimations.

Mathematical Analysis of the Effect of Rotor Geometry on Cup Anemometer Response

The calibration coefficients of two commercial anemometers equipped with different rotors were studied. The rotor cups had the same conical shape, while the size and distance to the rotation axis varied.The analysis was based on the 2-cup positions analytical model, derived using perturbation methods to include second-order effects such as pressure distribution along the rotating cups and friction.Thecomparison with the experimental data indicates a nonuniformdistribution of aerodynamic forces on the rotating cups, with higher forces closer to the rotating axis. The 2-cup analytical model is proven to be accurate enough to study the effect of complex forces on cup anemometer performance.

The Cup Anemometer, a Fundamental Meteorological Instrument for the Wind Energy Industry. Research at the IDR/UPM Institute

The results of several research campaigns investigating cup anemometer performance carried out since 2008 at the IDR/UPM Institute are included in the present paper. Several analysis of large series of calibrations were done by studying the effect of the rotor’s geometry, climatic conditions during calibration, and anemometers’ ageing. More specific testing campaigns were done regarding the cup anemometer rotor aerodynamics, and the anemometer signals. The effect of the rotor’s geometry on the cup anemometer transfer function has been investigated experimentally and analytically. The analysis of the anemometer’s output signal as a way of monitoring the anemometer status is revealed as a promising procedure for detecting anomalies.

On the cup anemometer working condition monitoring

The analysis of the harmonic terms related to the rotational speed of a cup anemometer is a way to detect anomalies such as wear and tear, rotor non-symmetries (rotor damage) or problems at the output signal system. The research already done in this matter at the IDR/UPM Institute is now taken to cup anemometers working on the field. A 1-2 year testing campaign is being carried out in collaboration with Kintech Engineering. 2 Thies First Class Advanced installed at 58 m and 73 m height in a meteorology tower are constantly monitored. The results will be correlated to the anemometer performance evolution studied through several calibrations planned to be performed along the testing campaign.

Effect of the pulse trajectory on ultrasonic fluid velocity measurement

In a general situation a non-uniform velocity field gives rise to a shift of the otherwise straight acoustic pulse trajectory between the transmitter and receiver transducers of a sonic anemometer. The aim of this paper is to determine the effects of trajectory shifts on the velocity as measured by the sonic anemometer. This determination has been accomplished by developing a mathematical model of the measuring process carried out by sonic anemometers; a model which includes the non-straight trajectory effect. The problem is solved by small perturbation techniques, based on the relevant small parameter of the problem, the Mach number of the reference flow, M. As part of the solution, a general analytical expression for the deviations of the computed measured speed from the nominal speed has been obtained. The correction terms of both the transit time and of the measured speed are of M 2 order in rotational velocity field. The method has been applied to three simple, paradigmatic flows: one-directional horizontal and vertical shear flows, and mixed with a uniform horizontal flow.