A Numerical Assessment of Hypersonic Raindrop Interaction: Understanding Cavitation-Induced Fragmentation*

In the evolving landscape of hypersonic re-entry, consideration of environmental elements such as rain droplets becomes crucial. These droplets can potentially exert a force of up to 40 kN on a small area, necessitating a comprehensive understanding of their state during atmospheric penetration and the mechanisms of shock-induced breakup. The proposed research investigates the multifaceted physical phenomena related to droplet loading, which includes deformation, evaporation/boiling, and cavitation induced by shock interaction, leading to droplet disintegration. Multiscale, multiphysics simulations are employed to evaluate the loading experienced by a reentry vehicle under such conditions.

The investigation recognizes two distinct scales within this system: the macro-scale of the reentry vehicle and the micro-scale of the droplet. Conventional methods struggle with the accurate representation of these scales due to the stark contrast in their sizes. Therefore, an novel multiscale method is proposed to address this challenge. Compressible, fully coupled computational fluid dynamics are used to simulate the gas motion surrounding the vehicle. This flow is then linked to a trajectory prediction model for various droplet sizes relevant to rain, to determine the direct or unbroken impact speed. The model further integrates with a secondary domain-bound model that addresses the breakup mechanism using a direct simulation, volume-of-fluid method. This method traces droplet evolution, evaporation, and cavitation processes, effectively capturing the scales of both the vehicle and the droplet without the need for extensive meshing or computational power.

 

The anticipated results of this research involve an in-depth examination of the breakup mechanisms within the droplet, analyzing the scaling of these mechanisms based on droplet size and mach number. Secondly, the investigation is expected to scrutinize the Rayleigh-Taylor and Kelvin-Helmholtz instabilities that precipitate sheet stripping. Lastly, the study intends to probe the impact of cavitation on the natural frequency of the droplet and its role in modifying the breakup mechanism.