Bubble dynamics and bubble-induced agitation in the homogeneous bubble-swarm past a circular cylinder at small to moderate void fractions

Our study involved an experimental investigation of a homogeneous bubble-swarm past a circular cylinder, of which the size is more than five times larger than that of a bubble, while varying the upstream volume void fraction, <(alpha) over bar >, in a small-to-moderate range and cylinder diameter, D. The Reynolds number of rising bubbles is approximately 10(3). We focused on the way in which the preferred concentration of bubbles past a cylinder is established and how its wake is induced. We measured the gas- and liquid-phase velocity fields simultaneously by using a high-speed two-phase particle image velocimetry technique. Depending on the void distribution in the wake region, two regimes are classified. In the first regime (smaller <(alpha) over bar >), the bubbles tend to accumulate downstream of the side of the cylinder (along the shear-layer region), which is attributed to the mitigated lateral migration of bubbles via the balance between the drag force with the pressure gradient and shear-induced lift forces acting on them. As <(alpha) over bar > increases, in the second regime, the shear-induced lift force becomes sufficiently strong to move the bubbles toward the center of the cylinder, resulting in the void fraction exhibiting a broad peak at this position. Based on the void distribution, the liquid flow forms an interesting wake structure behind the cylinder. For example, a wake-defect-like streamwise velocity profile and skew-symmetric Reynolds stress distribution are induced; however, the distortion by the circular cylinder is restored quite fast (within 5D along the streamwise direction). Increasing the cylinder size, however, delays the recovery of uniform flow statistics and reduces the turbulence level in the wake. Finally, we propose a model for bubble-induced fluctuation (Reynolds stress) that includes the contributions of the gradients of liquid velocity, void distribution, and bubble velocity. The model, which is an extension of the well-known eddy-diffusivity model, considers the effects of bubble-induced flow and multiple bubbles (void fraction). The bubble-induced Reynolds stress estimated by the present model is in a reasonably good agreement with the data measured downstream of the circular cylinder.