We study experimentally the effects of a pipe axial rotation on a stratified displacement flow, using two miscible fluids of different density, in a long inclined pipe. Increasing the pipe rotation speed induces transverse mixing and results in a complete removal of the displaced fluid above a critical transition, described by $\text{Rb}\approx 5/2\pi \text{Fr}{\left(1-\text{Fr}\right)}^{-1}$, relating the Rossby number (Rb) to a modified Froude number (Fr). In addition, we quantify the variation of the leading and trailing displacement front velocities $({\widehat{V}}_f,{\widehat{V}}_t)$, as a function of the mean displacement velocity (${\widehat{V}}_0$), showing that for $\text{Rb}/\text{Fr}\gtrsim 5/2\pi$ we find ${\widehat{V}}_f\approx 1.3{\widehat{V}}_0$ and ${\widehat{V}}_t\approx 0.8{\widehat{V}}_0$. These findings offer insights to greatly improve displacement efficiency using rotational motion.

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• Complex fluids
• Flow instability
• Interfacial flows