Development of the bubble-trapping wake behind a bluff body in liquid-gas flows at intermediate Reynolds numbers

An upward bubbly liquid-gas flow around a cylinder results in a non-uniform distribution of the gas phase, called bubble trapping, under certain flow conditions. We have previously demonstrated that the bubble-trapping wake occurred due to strong inertial, pressure gradient, and lift forces acting on the bubbles just downstream of the cylinder when the Reynolds number (Re), based on the cylinder diameter of 9.5 mm, was 3,000. In this investigation, we show the evolution of the trapping forces at intermediate Reynolds numbers with mean bubble diameters of 3-4 mm. To resolve the forces acting on the bubbles, we performed phase-separated velocimetry of the bubbly flow. We simultaneously tracked the air bubble movement using Lagrangian Particle Tracking Velocimetry (PTV) and liquid movement by applying Particle Shadow Image Velocimetry (PSIV) to suspended tracer particles. The resolution of the two-phase velocities and bubble sizes allowed us to determine the buoyancy, inertial, drag, lift, and pressure gradient forces acting on each of the bubbles. Here, we illustrate how the trapping wake develops by comparing the force balances at Reynolds numbers ranging from 500 to 2,500, which lead up to the trapping conditions observed at higher Reynolds numbers.