Buoyant miscible viscoplastic injections

We experimentally investigate the buoyant miscible injection of a heavy viscoplastic fluid with moderate yield stresses into a lighter Newtonian fluid, via an eccentric inner pipe inside an inclined closed-end outer pipe. To capture the flow dynamics, we employ nonintrusive methods, including camera imaging and ultrasound Doppler velocimetry. Our analysis reveals that the flow behavior is governed by a set of dimensionless flow parameters: Reynolds number, Froude number, inclination angle, Bingham number, and viscosity ratio. Using a dimensionless plane that elegantly combines the aforementioned dimensionless numbers, we successfully identify and categorize various flow regimes, such as stable and unstable slumping, separation, and diffusive and nondiffusive mixing. We analyze these flow regimes and quantify their transition boundaries. Our analysis also considers the distinct characteristics of these flow regimes, focusing on interfacial velocities, including the heavy fluid front velocity, the separated piece velocity, and the backflow front velocity, and examines the flow velocity field and mixing index.