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 10 5. 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 T b = 5.5.

Numerical Investigation of the Aerodynamic Droplet Breakup at Mach Numbers Greater Than 1

“…They identified two mechanisms that are responsible for the droplet deformation at the earlystages, namely: i) pressure mechanism, which is responsible for the droplet flattening, and ii) shear mechanism, which is responsible for the formation of the small rings and bulges. Recently, (Hébert et al 2019) studied experimentally and numerically in 2-dimensions, using an in-house code called Hesione, the breakup of water droplets exposed to gas flows of Ma=4.2-4.6 and We>10 5 . They encountered the catastrophic breakup mode and divided the breakup process into three steps: i) droplet flattening, ii) fragmentation initiation at the outer rim of the droplet, and iii) droplet takes the shape of a filament aligned with the flow.…”

Numerical investigation of the interactions between a low-hypersonic shock wave and a water droplet: VOF and DI methods comparison

“…To our knowledge, it seems that the majority of literature works (except the first experimental ones) do not deal with configurations where M is higher than 4. Especially, coupled numerical/experimental studies for low-hypersonic cases (M = 4-7) seem to be inexistent in literature, except a recent work [21] wherein the authors validate a Volume of Fluid (VOF) model by comparing the numerical results with experimental data for a M-4.25 situation.…”

“…Concerning cases where M ≤ 4, data are already substantial in the literature. For this reason, we chose in this paper the reference case recently published in [21], where a low-hypersonic shock wave (M = 4.25) hits a water droplet (Ø i = 1.135 mm). The purpose of the present article is then to make a comparison between two numerical approaches (VOF and DI), in order to determine which is most appropriate to retranscribe, interpret and understand the experimental data presented in [21].…”

“…The numerical methods of both models are presented in section 2. Results obtained with the codes are then given in section 3 and discussed in section 4 by comparing them with experimental data from [21]. Conclusion and some perspectives on this work are finally proposed.…”

“…2 NUMERICAL METHODS The first strategy to simulate the experimental work of [21] is to use a DI approach. In the literature, this method has been largely used to deal with multiphase flow configurations, and it appears that it can offer some advantages, such as its inherent conservative nature and the fact that the interfaces are simply obtained by numerical diffusion, and thus, a reconstruction algorithm is not necessary as in VOF methods.…”