We study the displacement flow of two Newtonian fluids in an inclined pipe. The fluids have the same viscosity but different densities. The displacing fluid is denser than the displaced fluid and is placed above the displaced fluid (i.e., a density-unstable configuration). Three dimensionless groups describe these flows: a densimetric Froude number Fr, a Reynolds number Re, and the pipe inclination β. Our experiments cover fairly broad ranges of these parameters: 0 ⩽ Fr ⩽ 9; 0 ⩽ Re ⪅ 2400; 0 ⩽ β ⩽ 85°. Phenomenologically, our experimental flow observations vary from well mixed fully diffusive regimes, through buoyancy-dominated inertial exchange regimes, to laminar viscous flows, all with varying degrees of stability. We characterize the different flow regimes observed in terms of the three dimensionless groups and provide leading order approximations to the velocity of the displacement front and the macroscopic diffusion in each regime.
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