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Wall confinement effects for spheres in the Reynolds number range of 30-2000 Akutsu, Toshinosuke

Abstract

This thesis studies in detail: formation, developmen and instability of the vortex ring; associated surface pressure distribution; and drag for a family of spheres in the Reynolds number range of 30-2000 and the blockage ratio of 3-30%. In the beginning, a glycerol-water solution tunnel used in the experimental program is briefly described followed by an explanation of the model support system, pressure measuring instrumentation, drag balance 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 evolved. Finally, the test data are analyzed as functions of the confinement condition and Reynolds number. The results suggest that the ratio of the model to vertical stem support should be at least 10 to make stem effects negligible. Influence of Reynolds number on the surface pressure distribution is primarily confined to the range R[sub n] < 1000. However, for the model with the highest blockage ratio of 30.6%, the pressure continues to show Reynolds number dependency for R[sub n] as high as 2300 (limit of the tunnel capability for a glycerol-water concentration used). In general, the effect of Reynolds number is to increase the minimum as well as the wake pressure. On the other hand, the effect of an increase in the blockage ratio is just the opposite. The wall confinement tends to increase the drag coefficient, however, the classical dependence of skin friction on the Reynolds number, C[sub d],[sub f]αR[sub n][sup ½] , is maintained. The results vividly a, r n showed inadequacy of Maskell's correction procedure particularly at higher blockage (S/C > 5%). An extensive flow visualization study using dye injection in conjunction with high speed photography complements the test program.

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