Comparison of Near Wake-Flow Structure behind a Solid Cap with an Attached Bubble and a Solid Counterpart

An experiment to study the near-wake flow structure behind an air bubble attached to a cap and separately, a solid equivalent was conducted. The objective was to elucidate the near-wake characteristics of the "cap-bubble" relative to the solid; in particular, to elucidate the role of the "moving tail". Experiments were performed in 80×80mm^2×1m tall channel with each object suspended in downward flow of water. Both the cap-bubble and solid had an approximate equivalent diameter, D_<eq>〜11.5mm (volume〜0.8mL), The average downward flow velocity (U〜25cm/s〜rise velocity of bubble) defined Reynolds number was, 2450<Re_<Deq><2890. The Eotvos and Weber numbers were, 17.8<Eo<17.85, 6.04<We<6.06. Particle Image Velocimetry using a cross-correlation method was used to generate velocity data; vorticity, turbulent kinetic energy (TKE) and other parameters we then calculated. Graphic and numerical comparisons between the objects led to the following: 1) for the cap-bubble, the influence on the flow structure due to motion of the tail is relatively minor, 2) tail motion contributes to transverse fluctuations and uniformity in TKE distribution and 3) object deformation, oscillation and gyration localizes the flow structure in contrast to a solid object. Results are discussed.