Kalopedis, Achilleas; (2001) Coherent flow structures and spot formation induced by wall perturbations in a Blasius boundary layer. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Coherent flow structures and turbulent spot formation, induced by wall perturbations in a Blasius boundary layer, were studied experimentally. The study has specific engineering relevance in the area of turbulent boundary layer control to achieve drag reduction and in the wider context of improved understanding of the fundamental mechanics of turbulent generation and dynamics. Experiments were carried out in a re-circulating water channel, and the measurements and observations were made in a zero pressure gradient, laminar boundary layer, formed along the horizontal upper surface of a smooth glass plate, suspended in the channel flow. Controlled 3-D disturbances were introduced into the boundary layer flow by fixed and transitory wall perturbations. The fixed perturbations were in the form of a low-profile spherical cap. Transient perturbations were generated by the transitory inflation of a circular rubber diaphragm, 30mm in diameter, mounted flush with the wall. The diaphragm is deformed into a low profile spherical cap using a computer controlled inflation system. This allows flexible variation of the maximum inflation height (amplitude) and the height/time function shape through the variable period of the perturbation cycle. Coherent vortical structures, induced by both perturbation types are compared and vortex shedding and formation characteristics are discussed. A three-component laser Doppler anemometer has been used for the quantitative measurement of the three-dimensional velocity and vorticity field downstream of the perturbation. Special data-processing techniques have been developed to derive spatial mappings of the corresponding 3-D vorticity field. Hydrogen bubble and dye tracers were employed for flow visualization of the hairpin vortices analyzed from digital recording of high-speed video. A general parametric study of the influence of Reynolds number, perturbation diameter, maximum amplitude, height/time function shape and period on the downstream position of the induced turbulent spot is presented. The results of detailed investigations on the effect that these parameters have on the formation and intensification of the hairpin vortices shed or formed downstream from the protrusion are presented. These observations are linked to the observed subsequent chaotic breakdown of the flow to turbulence and the global behaviour of the turbulent spot formation. Experimental results are compared with previous observations obtained by analytical and numerical simulations.

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