Onset of axisymmetric sloshing in a food processor

We report on the axisymmetric oscillation at the air-water interface in a food processor and Taylor-Couette systems. To gain insights into the mechanisms of free-surface oscillation, we conduct a series of experiments on the visual observations of free-surface deformation and the velocity measurements. At relatively low rotation speeds, the surface height at the rotating object surface decreases but remains stationary. As the rotation speed increases in small increments, oscillation begins when the deformed free surface detaches from the rotating object, and its onset depends strongly on the initial surface height. To account for the similarity mechanism of the onset, a scaled surface deformation based on the bulk Froude number is introduced herein; we find that the oscillation occurs at the scaled surface deformation beyond unity. These findings explain the causal relationship between the surface deformation and the oscillation onset and help to predict its occurrence to avoid mechanical damage to an apparatus with rotating components.