Coupled dynamic characteristics of the hull and cushioning structure of high-speed water entry vehicle

Classification of the collapse of a composite fairing during the oblique high-speed water entry

The high-speed water entry is of great significance in the development of the cross-media equipment. In this paper, the high-speed oblique water entry of a conical projectile is numerically investigated with particular emphasis on the local fluid field and trajectory behaviors. The numerical model is based on the Reynolds average Navier-Stokes (RANS) method with the Volume of Fluid (VOF) approach for capturing complicated multiphase flow characteristics. The present method is validated against the published experimental data by simulating a case of high-speed oblique water entry. The highspeed oblique water entry of the conical projectile under different impact velocities are investigated. The comprehensive process of the water entry, including the initial impact, formation, development, contraction, closure and collapse of the open cavity, have been reproduced. The flow evolution process of the high-speed water entry, which involves the cavity shape and the cavitation evolution, is analyzed comprehensively. Furthermore, the variation trends of the impulsive load, the projectile attitude and the trajectory stability are examined. The study is expected to enhance the understanding of the cavitation evolution, and is helpful for the design of the cross-media equipment.

The material testing machine and the split Hopkinson pressure bar (SHPB) were adopted, respectively, to conduct the static and dynamic compression tests on granite specimens heat treated by different temperatures. The effects of strain rate and heat-treatment temperature on the mechanism of energy evolution of the specimen during deformation and failure process were studied. The results show a significant strain rate effect on the granite, with the energy dissipation density increasing with increasing impact velocity (or strain rate), regardless of the treatment temperature. The specimens heat treated at 300 °C and 700 °C have the minimum and maximum energy dissipation densities, respectively. The specimen in the SHPB tests easily broke into pieces or even powder; while under static compression, only macroscopic fracture surfaces and spalling phenomenon on the specimen were detected. The energy dissipation density is inversely proportional to the compressive strength of the specimen. The rate of energy dissipation change is defined, which can be used to identify the stages in the deformation process of rock and to determine the position of the failure point in the stress-strain curve. For both the dynamic and static compression tests, the value of energy utilization ratio is relatively low, with a maximum value of about 35%.

During high-speed oblique water entry, the continuous impact load experienced by the vehicle can lead to structural damage and influence trajectory stability. This article investigates the load and motion response characteristics of the vehicle with different rudder angles during the entire high-speed water entry process. In the numerical methods, a quaternion-based six degrees of freedom motion system is employed to describe the rigid body motion, while a multiphase Eulerian finite element method serves as the fluid solver. An experiment is conducted to verify the accuracy of the numerical method. Furthermore, the mechanisms underlying the formation of tail slamming normal loads during high-speed oblique water entry of the vehicle at different rudder angles are explored. The loads including axial force coefficient, normal force coefficient, and pitch torque coefficient are extensively discussed. Results indicate that the tail slamming phenomenon and the vehicle's trajectory are significantly influenced by the rudder angle. The design of positive and negative rudder angles causes both the upward and downward tail slamming. The “excellent rudder angle range α̃” for the vehicle during high-speed water entry is defined. Selecting a rudder angle design within this range can effectively reduce the normal load during the tail slamming events, it can result in decreased pitch torque amplitude and form a straighter, more stable trajectory. This work provides new insights into load control during vehicle steering.

The high-speed oblique water entry of a segmented vehicle presents a complex multiphase fluid–structure interaction (FSI) problem, characterized by the evolution of the vapor–liquid cavity and the transient impact loads transmission. This study constructs a high-fidelity numerical model that integrates a multiphase flow solver with a FSI solver to investigate the underlying fluid physics and shock response during initial water entry for a segmented vehicle connected by bolts. The effects of key parameters such as entry velocity and angle on the spatiotemporal evolution of cavity, impact loads transmission, and shock response spectrum (SRS) are systematically investigated. Results show that multiphase FSI during cavity evolution notably amplifies high-frequency fluctuations in acceleration and induces asymmetry stress wave propagation. Segments with longer propagation distances exhibit higher peak axial forces, while connection interfaces experience negative normal contact forces and lateral contact moments. SRS analysis reveals resonant amplification peaks at 600 and 1500 Hz across the segments, with amplitudes reaching 100 g, and higher peaks at the monitoring points. Stress concentration and wave asymmetry generate elevated local stresses at connections and bolts, necessitating protective measures targeting segment-specific load peaks, critical frequency bands, and interface/bolt reinforcement. This study provides an effective multiphase FSI simulation framework and insights into the flow and shock characteristics of a segmented vehicle during high-speed water entry.

Dynamic crushing of a dedicated buffer during the high-speed vertical water entry process

Crushing behavior and load-reducing performance of a composite structural buffer during water entry at high vertical velocity

Autonomous underwater vehicle will be subjected to a huge impact load during high speed water entry, which will damage the structure and the internal instruments of the vehicle. Therefore, it is of great significance to study the buffer mechanism of the vehicle during the process of water-entry. In this paper, a kind of head-jetting device with disk cavitation is used. The complex cavitation forms, under the three-phase coupling of gas, liquid and solid, in the water entry process of the vehicle on which the device is installed. In this paper, the numerical simulation of high-speed water entry of the vehicle equipped with head jet device is carried out. Through the analysis of water entry cavitation under typical working conditions, the following conclusions are obtained. After the installation of head jet device, the water entry cavity of the vehicle changes gradually from cone to spindle shape. The air jet, compared with that without jet, can promote the formation of water inlet supercavitation, decrease the interaction area between the vehicle and water, and reduce the impact load during water entry. At the same water entry depth, the diameter of cavitation increases with the amount of air jet. The water entry velocity has a great influence on the difference of cavitation shape. The water entry depth closure phenomenon, when the water entry velocity is less than 100 m/s, can be observed in the depth of 3.5 times of the projectile length. The water entry angle has a significant effect on the cavitation shape. The cavity shows obvious asymmetry when the vehicle slants into the water, and the diameter and length of the bubbles decrease with the increase of the water entry angle. The research content of this paper provides technical support for the engineering practice of high-speed water entry and load reduction, and the conclusions are of great significance in related fields.

This research examines the influence of a linear buffer on the impact load experienced during the high-speed water entry of slender projectiles, employing both experimental and numerical simulation methods. Experiments were carried out using a custom-designed high-speed water entry testing platform. In this setup, projectiles were propelled by an air gun, and their water entry dynamics were recorded with a high-speed camera. The results show that implementing a linear buffer significantly reduces the impact load during water entry. Compared with the prototype projectile, the impact load of the projectile equipped with a buffer can be decreased by up to 91.34%. The investigation further determined that buffer stiffness and the mass ratio between the projectile's body and head play crucial roles in determining the impact load. Within the effective range of buffer stiffness, lower values, and mass ratios are associated with enhanced load reduction. As the velocity of water entry increases, the effect of buffer stiffness on peak impact load weakens, while the influence of mass ratio becomes more prominent. These findings have important engineering implications for designing high-speed water entry systems, providing a viable strategy for reducing impact loads and improving safety and reliability.

Dynamics analysis of high-speed water entry of axisymmetric body using fluid-structure-acoustic coupling method

Analysis of high-speed water entry in semi-sealed cylindrical shells: Cavity formation and self-disturbance characteristics

Mechanical performance and cushioning energy absorption characteristics of rigid polyurethane foam at low and high strain rates

The author has previously presented the static load-deflection relations of a toroidal shell subjected to axisymmetric compression between rigid plates and those of its outer half when subjected to lateral compression. In both these cases, the analytical method was based on the incremental Rayleigh-Ritz method. In this paper, the effects of compression angle and strain rate on the load-deflection relations of the toroidal shell are investigated for its use as a shock absorber for the radioactive material shipping cask which must keep its structural integrity even after accidental falls at any angle. Static compression tests have been carried out at four angles of compression, 10°, 20°, 50°, 90° and the applications of the preceding analytical method have been discussed. Dynamic compression tests have also been performed using the free-falling drop hammer. The results are compared with those in the static compression tests.

To investigate the high-speed water entry problem of sequential structures, a three-dimensional numerical calculation model for the high-speed water entry of sequential structures was established based on solving the Reynolds-averaged Navier–Stokes equations. The volume of fluid multiphase flow model and 6 degrees of freedom rigid body motion model were employed, and the Schnerr–Sauer cavitation model was introduced, combined with overlapping grid technology. Numerical simulations of sequential water entry for two structures under different water entry time interval were carried out, and the influence laws of different water entry time interval on the cavity evolution, vortex evolution, and motion characteristics during the water entry of sequential structures were obtained. Results show that the influence of sequential water entry on cavity evolution is mainly reflected in three aspects: disrupting cavity formation, influencing surface closure of the cavity, and inducing curvature in the cavity wall. Simultaneously, the conventional vortex evolution has been modified, with impact dissipation emerging as a novel mode of vortex evolution within the small water entry time intervals. Moreover, in comparison to the water entry of the first structure, the second structure experiences significantly less resistance upon immersion in the flowing water, leading to a more gradual deceleration.

Numerical study on the impact load characteristics of a trans-media vehicle during high-speed water entry and flat turning