Influence of an imposed flow on the stability of a gravity current in a near horizontal duct

We study experimentally the effect of a mean flow imposed on a buoyant exchange flow of two miscible fluids of equal viscosity in a long tube oriented close to horizontal. We measure the evolution of the front velocity $Vf$ as a function of the imposed velocity $V0$⁠. At low $V0$⁠, an exchange-flow dominated regime is found, as expected, and is characterized here by Kelvin–Helmholtz-like instabilities. With increasing $V0$ we observed that the flow becomes stable. Here also $Vf$ increases linearly with $V0$ with slope of $>1$⁠. At large $V0$ we find $Vf∼V0$⁠.