Ice accretion from supercooled water droplets impacting aircraft surfaces poses significant flight safety risks. As ice formation typically results from collective droplet interactions, understanding multi-droplet dynamics during impact-freezing processes is crucial. This study numerically investigated consecutive impacts of dual supercooled droplets on a cold superhydrophobic surface using the volume of fluid method coupled with a solidification/melting model. We examined the effects of droplet spacing and the Weber number on spreading behavior and heat transfer characteristics. Three distinct mixing modes emerged at low Weber numbers: internal spreading, surface spreading, and encapsulated spreading. Increasing droplet spacing sequentially triggered these modes while transforming maximum spreading behavior from constant to increasing values. Enhanced droplet aggregation accompanied larger spacing. In the internal spreading mode, vortices developing within dual droplets impeded horizontal spreading, thereby limiting spread area progression. Moreover, increasing droplet spacing enhanced the heat transfer between the droplets and the cold surface, where the droplet-surface contact area emerged as the dominant thermal factor, outweighing local heat transfer coefficient effects. This investigation advances understanding of consecutive dual-droplet dynamics and thermal transport on superhydrophobic surfaces, contributing to improved anti-/de-icing strategies.

Hypersonic Ludwieg tube tunnel is one of the most important ground facilities for hypersonic aerodynamic research. To some extent the high requirement of a large laboratory space due to the long straight storage tube of the Ludwieg tube tunnel precludes the establishment of this hypersonic aerodynamic facility for fundamental research purposes. To tackle this issue, this work reports the aerodynamic design of a hypersonic Ludwieg tube tunnel with a double-bent storage tube, and special emphasis was placed on the tunnel’s unsteady starting process and the influence of the double-bent storage tube on the flow field. Based on the aerodynamic design a Mach 6 Ludwieg tube tunnel with a double-bent storage tube has been newly built and the first shakedown results agree well with the numerical prediction. It reveals that the propagation of expansion waves in the storage tube is affected by the double-bent tube, resulting in a weak total pressure fluctuation in the storage tube. By choosing a proper diameter ratio between the bend and the storage tube, the influence of the storage tube bend can be neglected.