Experimental study on vertical tandem water entry of front body and vehicle: Cavity evolution and load characteristics

The problem of load control during water entry of moving bodies is a classic and significant research topic. This study proposes a load control scheme based on the concept of forward separation of protective shield for air-breathing vehicles, utilizing a tandem water entry of a front object and a vehicle. High-speed imaging and an inertial measurement unit were employed to investigate the influence of a conical headed cylinder on a trailing cylinder during vertical water entry under low Froude number conditions. Typical tandem water entry modes, cavity evolution patterns, and load characteristics were systematically categorized. A non-dimensional parameter called Mt number suitable for tandem water entry was introduced for analysis. The results indicate that as Mt evolves, four distinct vertical tandem water entry modes emerge: “inside cavity with collision,” “inside cavity,” “inside upward cavity,” and “impacting jet.” The cavity seal mode transitions from a quasi-static seal to a transitional stage between shallow and deep seal, ultimately reaching deep seal. The “inside cavity with collision” mode should be avoided due to its potential to cause a sharp increase in cylinder's acceleration resulting from collisions between moving bodies. The other tandem water entry modes demonstrate effective load control performance, particularly the “Impacting Jet” mode, which achieves a stable load reduction rate of over 90%. These findings provide both theoretical foundations and technical reserves for the novel load-reduction scheme for cross-medium vehicles.