Intermittency in buoyancy-induced mixing in tilted tubes

Intermittency is studied in buoyancy-induced mixing in tilted tubes of two fluids of different densities but the same viscosity ($\mu = 10^{-6}\,m^2\,s^{-1}$). Alternate phases of laminar counterflow and turbulent transverse mixing are observed. This is analysed by mapping the relative concentrations with a Laser Induced Fluorescence technique. At a given density contrast (characterised by an Atwood number $At=4\times 10^{-3}$), intermittency is observed at tilt angles from the vertical from $15$ to $75^o$, with a weakly varying period of the order of $30\pm10 s$. The different flow regimes are characterised by the transverse mean concentration profiles in the tube section: at low $\theta$ ($\le 45^{\circ}$), the profiles display a nearly constant gradient whose value is lower in the mixing phase than in the counterflow phase. At higher $\theta$, segregation occurs with regions of high concentration of the two original fluids near the upper and lower walls and a mixed zone in the central part whose width decreases with $\theta$.