Panagiotis G. Papageorgas

An experimental study of the near-wake structure of a wind turbine operating over complex terrain

Abstract Wind parks are often cited in complex terrain whose features determine the wind flow over the area. Results from a field experiment, comprising in-situ and remote sensing techniques (high-resolution acoustic sounders), concerning the upwind area and the near-wake region behind a single medium-sized wind turbine are presented. The experimental site is the Samos Island Wind Park installed on top of a 390 m-high saddle. Because of the topography, wind speed acceleration and channeling effects are expected; thus, the commonly used logarithmic profile is not valid, and the choice of a representative surface roughness length z o is difficult. Interesting features of the profiles of the standard deviation of the ambient wind speed are also presented. The obtained results reveal a nonlinear interaction of the near wake with the turbine-tower shadowing, while cross-wind wake profiles indicate a potential core structure. The effect of ambient turbulence is apparent, especially at lower wind speeds, even at a distance of one rotor diameter (1 D ) behind the turbine. The wake centerline at distances greater than 1 D is often observed at heights greater than the hub-height and attributed to the wind flow characteristics over the Wind Park. Finally, evidence of rotational motion inside the wake is identified.

Air mass exchange between the athens basin and the messogia plain of Attika, Greece

The present work investigates the interaction between the local and synoptic meteorology and the resultant air mass exchange between two major regions of the Athens Metropolitan Area (AMA); the Messogia Plain, a rural area under development and the Athens Basin which combines industrial and human activities. These are separated by the 1000 m-high Hymettos Mountain. Meteorological data from a 4-month experimental campaign are used to (a) identify and classify the principal synoptic types observed, based on past long-term observations, (b) determine the recurrent surface wind flow patterns over the Messogia Plain and the Athens Basin, (c) specify the effect of the local thermal flows on the meteorological coupling of the two areas in relation to the synoptic, background (large-scale) conditions, and (d) analyse the flow over the Hymettos Mountain and through the natural openings which link the two areas geographically.

MNISIKLIS:Indoor Location Based Services for All

MNISIKLIS is an integrated system aiming to provide universal, indoor locationbased services focusing on navigation. This paper presents the overall MNISIKLIS architecture and certain implementation details. In the context of the Design for All approach, the system targets to the support of several types of users, including persons with disabilities as well as elderly, by exploiting multimodal interaction. Moreover, the system implements effi cient path fi nding algorithms and provides advanced user experience through highly personalized services. MNISIKLIS adopts Semantic Web technologies (e.g., ontologies and reasoning methods) for representing an d managing application models. Furthermore, MNISIKLIS exploits modern positioning techniques in order to achieve high quality positioning. The paper discusses the algorithms and the models that accommodate the services provided by the system. Additionally, an analysis of the positioning subsystem, the user interaction subsystem and the peripheral infrastructure is given. Hence, a new paradigm in the area of location-based systems is presented.

Hardware implementation of a disparity estimation scheme for real-time compression in 3D imaging applications

This paper presents a novel hardware implementation of a disparity estimation scheme targeted to real-time Integral Photography (IP) image and video sequence compression. The software developed for IP image compression achieves high quality ratios over classic methodologies by exploiting the inherent redundancy that is present in IP images. However, there are certain time constraints to the software approach that must be confronted in order to address real-time applications. Our main effort is to achieve real-time performance by implementing in hardware the most time-consuming parts of the compression algorithm. The proposed novel digital architecture features minimized memory read operations and extensive simultaneous processing, while taking into concern the memory and data bandwidth limitations of a single FPGA implementation. Our results demonstrate that the implemented hardware system can successfully process high resolution IP video sequences in real-time, addressing a vast range of applications, from mobile systems to demanding desktop displays.

Photorealistic integral photography using a ray-traced model of capturing optics

We present a new approach for computer-generated integral photography (IP) based on ray tracing, for the reconstruction of high quality photorealistic 3-D images of increased complexity. With the proposed methodology, all the optical elements of a single-stage IP capturing setup are physically modeled for the production of real and virtual orthoscopic IP images with depth control. This approach is straightforward for translating a computer-generated 3-D scene to an IP image, and constitutes a robust methodology for developing modules that can be easily integrated in existing ray tracers. An extension of this technique enables the generation of photorealistic 3-D videos [integral videography (IV)] and provides an invaluable tool for the development of 3-D video processing algorithms.

Accurate lattice extraction in integral images

Integral imaging is one of the most promising techniques for delivering three-dimensional content. Most processing tasks usually require prior knowledge of the size and positions of the elemental images that comprise an integral image. In this paper we propose an automated method for calibrating the acquisition setup, by applying a preprocessing stage to an acquired integral image. The skew angle is extracted and the size and positions of the elemental images are accurately determined. For these purposes a method is developed to automatically identify an elemental image lattice that best matches the acquired integral image.

Use of an adaptive 3D-DCT scheme for coding multiview stereo images

Our work presents a novel way for compressing image sets that form multiview stereo images used in three dimensional display devices. These image sets exhibit high inter-image correlation that remains unexploited when classic image compression techniques are applied. Our efforts are focused in the development of a compression algorithm based on the three dimensional discrete cosine transform (3D-DCT) which is an extension of the two dimensional case, used widely in image compression. An adaptive 3D-DCT scheme is implemented based on the exploitation of the increased spatial redundancy of certain parts of the image set. This is primarily achieved by locating image parts that belong to the distant background and hence remain unaltered throughout the image set. This technique achieves high compression ratios while maintaining the quality needed for correct stereoscopic viewing

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.

A tethered balloon profiler system

A microprocessor-based tethered balloon sounding system has been developed at the University of Athens. It can provide accurate measurements of relative pressure, dry and wet bulb temperature, wind speed and wind direction. Certain features arise from the implementation, like easy data handling, cheap and easy construction and maintenance, increased reliability and, above all, system adaptability to different experimental needs, as well as flexibility and expandability. Advantages and disadvantages of the method are discussed, as well as the implementation of the whole sounding system.

The electronics of a control system for micromirrors in a laser-scanning device

The electronics and software developed for the control system of a micromirror-based laser-scanning endoscope are presented in conjunction with features of the micromirrors and their driving requirements. These micromirrors, which are crucial for the laser-scanning operation of the endoscope device, are embedded in the endoscope head and have been manufactured using silicon MEMS (Micro Electro-Mechanical Systems) technology. The micromirrors are electrostatically deflected and driven by appropriate high-voltage waveforms created in the control system. This computer-based system, using appropriate software, generates the control waveforms, which, after adequate amplification, drive the scanning micromirrors. The electronics developed are capable of generating control voltages with amplitudes up to 440 V pk-pk, within a bandwidth from DC to 20 kHz, and ensures an electrostatic driving of scanning micromirrors with a high positioning accuracy in the sub-pixel range.