Vortex-induced interfacial waves in liquid–liquid flows across cylindrical bluff bodies of various sizes

The use of transverse cylinders of various sizes for generating vortex-induced waves in stratified oil–water pipe flows is investigated in this work. From CFD simulations in single phase flows, cylinders with 2 mm and 8 mm in diameter were selected which are found to generate vortices with frequencies in the range of 1 to 100 Hz. The experimental work was conducted in a 8 m long acrylic test section with 37 mm internal diameter. High speed imaging was used to study the flow patterns and the interfacial wave characteristics while velocity fields and the characteristics of the vortices shed by the cylinders in the water phase were obtained with Particle Image Velocimetry. It was found that the vortex properties generated by different cylinder sizes are reflected on the interfacial wave characteristics. The frequencies of both the waves and the vortices agreed for all cases studied, while the wave amplitudes increased with increasing cylinder diameter. The Strouhal number for the 2 mm cylinder had the same value of 0.20 as in unbounded flows, while for the 8 mm cylinder and small distance from the wall, the Strouhal number was equal to 0.27. The presence of the transverse cylinders modified the two-phase flow pattern maps and reduced the transition from stratified to dispersed flows to lower mixture velocities compared to flows without cylinders. The transition boundaries were shifted further towards lower mixture velocity with increasing cylinder diameter.