Ellipsoidal bubble diffusion in a turbulent shear layer

The objective of this research was to document the relative diffusion width s of ellipsoidal air bubbles in water at very low void fraction for a turbu lent mixing layer. This was accomplished by recording the transverse concen tration distribution of bubbles in an upward-flowing two-stream, turbulent, planar free shear layer of tap water. Ellipsoidal air bubbles with nominal equivalent diameters of 2.5, 3.5 and 4.5 mm were injected directly into th e shear layer and illuminated by a scanning laser beam 12-36 cm downstream. For each test condition, three image intensity distributions (based on lon g-time exposures which resolved individual bubbles in the shear layer) were recorded. From the digitized images, the average Gaussian dispersion width s of the bubble concentration distributions were measured for three bubble sizes, two sheer layer speeds, and six streamwise locations. The results de monstrate increased bubble concentration width at larger bubble sizes and a t higher flow speeds. The former is primarily attributed to the interaction between turbulent diffusion and the deformation-induced trajectory oscilla tions (inherent to rising ellipsoidal bubbles) while the latter is attribut ed to the increased ratio of fluid turbulent velocity fluctuations to bubbl e terminal velocity. (C) 2000 Elsevier Science Ltd. All rights reserved.