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Wall confinement effects for circular cylinders at low Reynolds numbers Mitry, Raafat Tawfic

Abstract

Formation, development and instability of Foppl vortices and associated surface pressure distribution are investigated experimentally for a family of two dimensional circular cylinders in the Reynolds number range of 5 - 20,000 and the blockage ratio of 2 - 50%. In the beginning, design and constructional details of a glycerol-water solution tunnel used in the experimental programme is briefly described followed by an explanation of the models, pressure measuring instrumentation, and test procedures. An approach to the data reduction, so critical at low Reynolds number, is discussed and a new definition of the pressure coefficient, which promises to be less dependent on test facilities and pressure gradients, is explained. , Finally, the test data are analyzed as functions of the confinement condition and Reynolds number. The results suggest that influence of the Reynolds number on the surface pressure distribution is primarily confined to the range R[sub n] < 1200. However, for the model with the highest blockage ratio of 50%, the pressure continues to show Reynolds number dependency for R[sub n] as high as 3000. In general, effect of the Reynolds number is to increase the minimum as well as the wake pressures. On the other hand, the effect of an increase in the blockage ratio is just the opposite. The pressure profiles become extremely sensitive to the wall confinement at the lower end of the Reynolds number range under study. An extensive flow visualization study using dye injection in conjunction with still and high speed photography complements the test program. Photographs suggest that influence of the blockage is to retard, in terms of the Reynolds number, evolution of the near-wake. Location of the separating shear layers tends to move upstream with an increase in the Reynolds number, however, the wall confinement promotes downstream shift in the separation point, which can be as large as 25° for a blockage ratio of 501. The thesis suggests for future investigation several areas which are likely to be fruitful. In particular, aspect ratio effects in the presence of blockage are likely to be significant and should be studied in depth.

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