Hysteresis in a swirling two-fluid flow

This experimental study describes a hysteresis—a vivid manifestation of strongly nonlinear flow physics. A sealed vertical cylindrical container of radius 45 mm and height 90 mm is filled with water and sunflower oil. The rotating lid drives swirl and the meridional circulation of both fluids. As the rotation strength Re increases, the oil-water interface rises near the axis, touches the lid at Re = Re1, and moves toward the container sidewall. Then as Re decreases, the interface returns to the axis and separates from the lid at Re = Re2 < Re1. At each Re from the range, Re2 < Re < Re1, two different stable steady flow states are observed that is typical of hysteresis. The hysteresis only occurs if a volume fraction of oil is small. The hysteresis disappears as the oil fraction exceeds a threshold, which is around 0.4.