A comparative laboratory and full-scale study of the near wake structure of a wind turbine

The operation of a wind turbine produces a downstream region of reduced wind speed, the so-called wake. The wake constitutes an important factor in determining the siting of turbines in wind farms. The mean wake characteristics, and their relation to the incident wind field and the local topography, are of primary importance for the estimation of available wind energy. The turbulent structure of the wake affects the loading and fatigue of downstream turbine rotors, and dictates the minimum spacing of the machines within a wind fam. In order, then, to achieve satisfactory performance from wind farms, especially in areas of complex terrain, a detailed knowledge of the above wake parameters is required. The present paper describes a recent investigation into the properties of the wake of a three-bladed wind turbine. Measurements were made on both a full-scale machine, and on a replica model in the laboratory, at approximately 1/100 scale. The full-scale experiments were carried out on the Greek island of Samos, by workers from the University of Athens, and were based on comprehensive anemometry measurements. The small-scale experiments were conducted by a research group at the University of Edinburgh, Scotland, using the relatively new technique of particle image velocimetry (PIV). A major objective of the work was to assess the validity of small-scale PIV measurements as a tool for investigating full-scale wind turbine phenomena. If successful, there would be significant attractions in using the PIV method, due to its ability to map the velocity in the entire rotor wake at a given instant PIV vector maps may be processed to yield both bulk wake measurements, such as velocity deficits, or data relating to the detailed structure of the wake, e.g. vorticity measurements. In the present campaign, velocity ratios measured 1.1 diameters (D) downstream of the rotor, ie. in the near wake, were compared, using data from full-scale and model scale

An experimental case study of the mean and turbulent characteristics of the vertical structure of the atmospheric boundary layer over the sea

The main characteristics of the mean and turbulent vertical structure of the Marine Atmospheric Boundary Layer (MABL) up to the height of 500 m are examined for an experimental case, using both remote and in situ sensing instrumentation. The observations indicate the existence of a very stable surface layer up to the height of 150-200 m followed by slightly stable to neutral conditions at higher levels, while a Low-Level Jet (LLJ) is developed, on the top of the intense surface based inversion. High values of the variance of the vertical wind component are observed at levels above the LLJ core, presumably associated with the shear forcing close to the LLJ. The plausible mechanism for the development of the LLJ is the inertial oscillation due to the frictional decoupling over the sea, which is associated with the strong stability of the MABL lower layer. © Gebrüder Borntraeger, Berlin, Stuttgart 2007

A study of the near wake structure of a wind turbine comparing measurements from laboratory and full-scale experiments

Wake flow measurements have been performed using the technique of particle image velocimetry (PIV) at stations downstream from a model wind turbine rotor, and evaluated against experimental data from two full-scale machines. Comparisons include both mean velocity and turbulent intensity cross-wake profiles at a range of tip speed ratios. The application of PIV to the study of wind turbine wakes is described in detail, including the steps required to ensure appropriate and accurate simulation of the flow field conditions. The results suggest that the PIV method is a potentially useful tool in the investigation of detailed wake flow, though significant differences are observed between wake velocity deficits at full- and model scale. These are discussed with regard to scale effect, the influence of terrain, model similarity, and the phenomenon of wake meandering and effective cross-wake smoothing

On the temporal distribution of bursts and events in the stable marine atmospheric surface layer

A methodology for tracing the bursting phenomenon occurring in the turbulent boundary-layer is applied to stable marine atmospheric boundary-layer (MABL) data, collected within the framework of the Coupled Boundary Layer Air-Sea Transfer, Low wind component experiment at Nantucket Island, 2003, Massachusetts, USA. For the definition of the events contributing to the bursting phenomenon, the quadrant analysis of the momentum flux with varying threshold value is utilized. The present work aims at the better understanding of the time scales of the microstructures in the MABL, by providing the number of the events and the groups and their mean duration and the time difference of all quadrants of the quadrant analysis, for different thresholds and under different meteorological conditions. The interrelation of the different time scales as well as the correlation between the quadrants are investigated and discussed. Also, it is demonstrated that the procedure of grouping events leads to bursts with a mean duration that is independent of the threshold value, for the most important quadrants of the momentum transfer (2 and 4). It is found that the combination of near-neutral conditions with low wind speed permits the development of larger duration events within the stable MABL. Finally, it is shown that the groups of events maintain adequate time and flux coherence only for quadrants 2 and 4. © Springer Science+Business Media B.V. 2008

Some aspects of the variation with time of katabatic flow over a simple slope

Features of the variations in katabatic flow along a simple slope in Athens, Greece are examined both observationally and from a simplified theoretical basis. When undisturbed periods of established flows are selected it is observed that the velocity and temperature fluctuations reveal characteristics which are in agreement with the theory proposed by Fleagle. Interesting implications of the theory are discussed, such as the effects of prescribing a decreasing cooling rate throughout the night or of taking into account the vertical structure of the katabatic layer

Automatic detection of fixed echoes in sodar facsimile records using the wavelet transform

A method to detect and remove fixed echoes in sodar facsimile records is described. This method is based on the capability of the wavelet transform to objectively detect singularities in signals, and it is adapted to two-tone facsimile records. At the present time, no other relevant method for facsimile record processing, except human observation, has been described in the literature. Automatic removal of fixed echoes is useful during computer processing of facsimile records with modern algorithms in order to prevent false layer detection when it is not possible to avoid or remove fixed echoes before recording. A test of the method on complex examples shows its success and limitations

Temporal evolution of the main processes that control indoor pollution in an office microenvironment: A case study

The aim of this study is to examine the relative contribution of the outdoor concentration, the ventilation rate, the geometric characteristics of the indoor environment (i.e., extent of indoor surfaces and indoor volume), the deposition, and chemical reactions to the indoor air quality of the office microenvironment. For this case study, the NO, NO2, and O3 concentrations indoors and outdoors and TVOCs and CO2 concentrations indoors were measured in an office microenvironment in Athens, Greece, that was ventilated both naturally and mechanically. The calculated ventilation and loss rates and the measured outdoor concentrations of NO, NO2, and O 3 were set as input to Multi-chamber Indoor Air Quality Model in order to study the temporal variation of the indoor NO, NO2, and O3 concentrations. Results showed that when the ventilation rate and outdoor concentration are high, the relative contribution of the transport process contributes significantly, while the chemical process depends on the contemporary interplay between the indoor O3, NO, and NO2 concentrations and lighting levels. The significance of each process was further examined by performing sensitivity tests, and it was found that the most important parameters were the deposition velocities, the UV infiltration rates (which determines the indoor chemical reaction rates), the ventilation rates, and the filtration (when a mechanical ventilation system is used). The effect of the hydrocarbon chemistry was not significant. © Springer Science + Business Media B.V. 2009

Flux measurements in the surface marine Atmospheric Boundary Layer over the Aegean Sea, Greece

Micro-meteorological measurements within the surface Marine Atmospheric Boundary Layer took place at the shoreline of two islands at northern and south-eastern Aegean Sea of Greece. The primary goal of these experimental campaigns was to study the momentum, heat and humidity fluxes over this part of the north-eastern Mediterranean Sea, characterized by limited spatial and temporal scales which could affect these exchanges at the air-sea interface. The great majority of the obtained records from both sites gave higher values up to factor of two, compared with the estimations from the most widely used parametric formulas that came mostly from measurements over open seas and oceans. Friction velocity values from both campaigns varied within the same range and presented strong correlation with the wind speed at 10m height while the calculated drag coefficient values at the same height for both sites were found to be constant in relation with the wind speed. Using eddy correlation analysis, the heat flux values were calculated (virtual heat fluxes varied from -60 to 40W/m2) and it was found that they are affected by the limited spatial and temporal scales of the responding air-sea interaction mechanism. Similarly, the humidity fluxes appeared to be strongly influenced by the observed intense spatial heterogeneity of the sea surface temperature. © 2014 Elsevier B.V

Deviations of the temperature models derived by remote and in situ sensing techniques for the global middle atmosphere

The intercomparison between the two recently proposed models for the vertical and latitudinal distribution of temperature in the middle atmosphere is presented. The first model is derived from all available radiance measurements made by satellites and the second one from all available rocketsonde-derived measurements. The main deviations between the two models exist (a) at high latitudes at the middle and upper strato-mesosphere, depending on season, and (b) at low and middle latitudes of the Southern hemisphere at the upper mesosphere. © 1992 Taylor and Francis Ltd

Second‐order spectral local isotropy of the humidity and temperature fields in an urban area

A theoretical and experimental analysis of the humidity and temperature spatial differentials (δ/δxj,) in an urban area is presented. The local isotropy form of the spectra of the streamwise and vertical differentials of a scalar field is derived. The experimental results for the humidity and temperature fields in the lowest part of the urban boundary layer are in satisfactory agreement (at least in form under stable conditions) with the local isotropy model indicating that these fields can be locally isotropic. A strong anisotropy was observed at small wavenumbers of the vertical differentials spectra and it is probably connected with surface‐layer plumes. The analysis of the humidity and temperature differentials is extended to their co‐spectra and the spectra of the vertical differentials are compared to the streamwise ones. The comparison of the structure parameters of the temperature or the humidity field in the streamwise and the vertical direction, estimated using a spectral and a variance (structure function) method, showed no systematic departure from the isotropic prediction of equality. Copyright © 1995 Royal Meteorological Societ