Research on similarity of water entry load for scaled-down underwater vehicle based on different model test environments

Flow control techniques play an important role during water entry. In this paper, the idea of water entry of the projectile with single canard-wing is proposed and applied to the water entry problem. The cavity evolution and motion characteristics of projectile with canard-wing were investigated through experiments, and the cavity length, trajectory, and attitude changes of projectile with canard-wing during water entry were quantified. The results show that, different from the water entry process of projectile without wing, the projectile with canard-wing has the typical characteristics of forming the attached cavity on the wing. Due to the influence of canard-wing, the trajectory deflection is always toward the side without the wing, and the initial moment of trajectory deflection is advanced with the increase in the impact velocity. The length of the fore-end cavity and the attached cavity on the wing increases as the impact velocity increases and the pinch-off depth of the fore-end cavity also increases. Moreover, the deviation of the trajectory and the attitude angle of the projectile with canard-wing increases as the impact velocity increases during water entry. The results can provide important support for the passive flow control during the water entry of the projectile and the development of the trans-media aircraft.

Experimental study on vertical water entry of the projectile with canard-wing

Li,

Wang,

Wei

et al. 2024

Compressibility effects on cavity dynamics and shock waves in high-speed water entry

Yang,

Xiang,

Zhang

et al. 2024

The importance of high-speed water entry is acknowledged within the defense industry. This study numerically investigates the water entry of a high-speed rectangle projectile, focusing on cavity dynamics and shock wave generation. A computational model is employed to accurately simulate the intricate fluid dynamics of compressible multiphase flows. This model integrates a dual-phase flow algorithm with a thermally sensitive Tait equation of state for the liquid phase. The primary focus lies in understanding the effects of fluid compressibility on cavity evolution and shock wave propagation across different Froude numbers. The findings reveal that compressibility induces changes in cavity formation size, leading to significant variations in phase composition within the cavity. Furthermore, compressibility enhances the air cushion effect upon surface impact, resulting in delayed water entry and concurrent reduction in projectile drag. Moreover, a prognostic model is proposed, correlating shock pressure with propagation distance, thereby validating theoretical hypotheses advanced by Lee et al. [J. Fluid Struct., 11, 819–844 (1997)].

Similarity of scaled-down tests of water entry slamming considering the effects of atmospheric pressure and density

Fan,

Yao,

et al. 2024

The water entry cavity and load characteristics obtained through scaled-down tests are correlated with the atmospheric pressure and density at the free surface. The evaluation of the influence of the cavitation number and atmospheric density coefficient is highly essential for scale tests to improve the prototype prediction accuracy. Focusing on the similarity criterion simulations and load prediction of the scaled-down tests, this study conducts the simulation tests of the water entry characteristics of the scaled-down model under different environments: normal pressure, reduced pressure, and reduced pressure and heavy gas replacement. Moreover, the influence of the cavitation number and atmospheric density coefficient on the multiphase flow, slamming load, and air cushion effect is discussed. The “air cushion effect” is formed at the top of the vehicle during water entry process, which affects the peak narrow pulse width slamming load. Furthermore, the “air cushion” experiences expansion–stability–rupture–escape with increasing water invasion depth. As the atmospheric pressure decreases, the gas tends to thin and the retention inertia weakens. The decrease in the “air cushion” buffering capacity leads to the increase in the slamming load and the expansion of the cavity scale formed by the liquid. Excessive simulation of the dynamic pressure results in the delayed closure of the cavity surface and the slow fall of the water curtain. As the atmospheric density increases, the retention inertia of gas increases because of the increase in the molecular mass, the slamming load gradually decreases, and the closure time of the cavitation and water curtain decreases. The research results of this paper provide some reference for the similarity transformation of the scaled-down test and the pre-research of the prototype.