Investigation of mechanisms leading to water drop breakup at Mach 4.4 and Weber numbers above 105

Abstract

The paper presents an investigation on water drop breakup in the ‘catastrophic’ mode at Weber numbers above 105. Experimental data have been obtained on a detonation shock tube operated at a Mach number between 4.2 and 4.6. Displacement and deformation of the mist cloud generated around the droplet were observed with a shadowgraph system, and Schlieren imagery was used to visualise the bow and wake shocks around the droplet. We observe that all measured quantities scale with the initial drop diameter. In order to analyse the experimental results and relate these observables to the breakup process, numerical hydrodynamic simulations have been performed. Once validated by direct comparison with our experimental observations, the simulation results are used to see “through” the mist and infer the droplet evolution with dimensionless time T. According to our results, the breakup mechanism can be divided into 3 steps. First (T < 1), most of the liquid mass remains in one main drop, whose shape flattens due to the hydrodynamic forces; then (1 < T < 2) fragmentation begins along the outer rim of the liquid drop and splits the corresponding mass into two parts; the first spreads out radially in small fragments while the remains finally (2 < T < 3.5) take the shape of a filament aligned with the flow. Our results are consistent with a complete breakup time Tb = 5.5.