Print Email Facebook Twitter On the effect of turbulence on bubbles: Experiments and numerical simulations of bubbles in wall-bounded flows Title On the effect of turbulence on bubbles: Experiments and numerical simulations of bubbles in wall-bounded flows Author Harleman, M.J.W. Contributor Westerweel, J. (promotor) Van Terwisga, T.J.C. (promotor) Faculty Mechanical, Maritime and Materials Engineering Department Proces and Energy Date 2012-07-02 Abstract Previous research has shown that the addition of gas bubbles to a turbulent boundary layer in water can reduce the local skin-friction drag by up to 80%. Application of this technique to ships seems promising, but to date no significant drag reductions are obtained on full-scale ships. More knowledge about the drag reduction mechanism is required to better understand its restrictions and potential. The dispersion of small bubbles (d+ of about 1) in horizontal turbulent channel flow is studied by numerical simulations and by experiments with simultaneous PIV and bubble shadowgraphy. It is shown that the equilibrium bubble concentration distribution is described by a Rouse-profile, based on a gradient-diffusion hypothesis. The combination of turbulent mixing and a wall-normal concentration gradient results in a preferential concentration, so despite the small Stokes number (St<<1) the bubbles are not ideal flow tracers. Bubbles are driven away from the wall by turbophoresis, which may create a bubble-free layer that reduces drag reduction effectiveness. Finally, the drag reducing effect of electrolysis bubbles is determined from measurements of the velocity profile. In the literature high local drag reductions are obtained with very low concentrations of small bubbles, but in our experiments neither local or global drag reductions are measured. Therefore, the bubble drag reduction effectiveness is limited and seems to be essentially caused by a density-reduction effect. Subject turbulencebubblesdrag reduction To reference this document use: http://resolver.tudelft.nl/uuid:d6229817-08eb-415b-bd6a-fbb590f366ba Publisher Uitgeverij BOXPress 's-Hertogenbosch Embargo date 2012-07-02 ISBN 9789088914379 Part of collection Institutional Repository Document type doctoral thesis Rights (c) 2012 Harleman, M.J.W. Files PDF Thesis_Marc_Harleman_20120604.pdf 23.67 MB Close viewer /islandora/object/uuid:d6229817-08eb-415b-bd6a-fbb590f366ba/datastream/OBJ/view