Flow pattern transitions in oil-water flows past a bluff body

In this Thesis a novel approach is followed to facilitate the experimental investigations on the flow pattern transitions from separated to dispersed flows using a cylindrical bluff body in horizontal oil-water flows. A transverse cylindrical rod is used as a bluff body which is placed under the interface of the two immiscible liquids and near the test section inlet to passively generate interfacial perturbations and breaking waves. This approach was inspired from the use of hydrofoils in ships that reduce frictional drag via increased air entrainment. Studies are carried out using two flow facilities and high speed imaging combined with laser based measurements are performed at two axial locations along the test section, immediately after the cylinder and at large distance away from the cylinder. The effect of a confined geometry on the characteristics of the von Karman vortices and on the general flow behaviour immediately downstream of the cylinder are investigated in single phase water flows. It is found that the 3D pipe geometry does not affect significantly the vortex shedding behind the cylinder at least in the central plane of the pipe. The frequencies of the vortex shedding were comparable to those from a cylinder in an unconfined liquid. The results from two phase flows reveal that the cylinder reduces the mixture velocity for the transition separated to dispersed flows. It also actuates interfacial waves that are found to be non-linear and convective. In many cases the waves have the same frequencies as the von Karman vortices depending on the submergence depth of the cylinder underneath the oil-water interface and on the Froude number of the water layer. The observations suggest that strongly non-linear waves are responsible for forming thin ligaments that eventually break up into droplets