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The present work describes the results of an experimental study of coherent structures in a two-dimensional zero pressure gradient turbulent boundary layer by conditional sampling. The main aim of the work is to compare coherent events detected by various conditional sampling methods on a one-to-one basis and to investigate the degree of correspondence between them. The conditional-sampling techniques used include some existing ones, such as the uv-quadrant, the VITA, the u-level, and the pattern recognition methods, but also some suggested ones which are based on modifications of existing methods or ideas. The results are presented in two forms. First, the conditional averages of the streamwise and normal velocity fluctuations, and those of the Reynolds stress, are grouped into four classes based on the similarity of the pattern and then compared. Next, the correspondence between events detected by various methods are presented on a scale of zero to one, the latter indicating complete correspondence. The conditional-averaged patterns show very differing results for different methods. Also, very poor correspondences have been obtained between some pairs of methods. It is concluded that these methods may detect events of a different nature or different phases of the same event, and thus cause such discrepancies. The results are also discussed by relating them to the physics of the flow and coherent motions known as ejection and sweep.

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Purpose -Aims to present a methodology for analysing a solar-electric, high-altitude, long-endurance, unmanned aircraft. Design/methodology/approach -The study focuses on the aerodynamics, flight performance and power requirements of a heavier-than-air, solarelectric, HALE UAV. The methodology is founded on using an analytical approach to determine the power required to undertake various flight manoeuvres. An analytical approach is also undertaken in determining the intensity of the solar radiation available to the aircraft. Finally to demonstrate the methodology, a HALE concept was generated and evaluated. Findings -When using estimates of current solar-electric propulsion and energy conversion efficiencies, the HALE concept was only able to sustain year round, level flight up to latitudes of 108N.Research limitations/implications -Further analysis needs to be undertaken into the effect of altitude on the intensity of solar radiation, which could be as much as 25 per cent higher at an altitude of 21.3 km (70,000 ft). Further study into this subject area may provide proof that sustained flight is possible at more northerly latitudes. Originality/value -This paper provides a simple methodology for persons wishing to undertake an initial feasibility study of a solar-electric HALE concept.

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