Underwater Launch Research Articles

Muzzle bubble dynamics characterization of underwater launching

To comprehensively understand the dynamic behavior of muzzle bubbles during underwater launching, an emptying process aligned with the muzzle flow characteristics is established and an evaporative condensation mechanism is modeled according to the high temperature and pressure properties of the propellant gas. Utilizing the spherical bubble theory, which comprises the inflation process and evaporative condensation effects, the dynamics of muzzle bubbles and their corresponding pressure waves are investigated. The numerical simulation results well agree with the experimental observations in terms of bubble radius and near-field pressure waves. Furthermore, the influence of two key factors on the bubble dynamics is examined: underwater launching depth and initial muzzle pressures. The results illustrate that the inflation process needs to be accurately described for precise pressure wave predictions. Using the evaporation condensation model, the bubble radius and frequency can be accurately characterized. Moreover, the launching depth influences the free expansion radius and oscillation frequency mostly due to the increase in hydrostatic pressure, which decreases by 33% and increases by 150% in the 1–20 m range, respectively. The initial muzzle pressure affects the initial expansion velocity and initial shock wave mainly due to the increase in the mass flow rate, which increase by 56% and 82% in the 35–65 MPa range, respectively.

Evolution mechanism and interaction of the muzzle combustion-gas jets field of underwater dual-barrel synchronized launch

Despite a certain amount of research that has been done on the muzzle combustion–gas jet field of underwater launching in recent years, there are still few reports on the evolution mechanism and interaction of underwater dual-barrel synchronised launch flow fields. To fill this gap, this study aimed to numerically analyse the effects of different barrel intervals on the transient jet field evolution characteristics of the dual-barrel launch. The three-dimensional unsteady multi-phase mathematical model was created and compared with an underwater sealed emission experiment of data for verification. The volume of fluid multi-phase flow model is used to describe the gas–liquid two-phase flow process, and the k–ε turbulence model simulates the turbulent mixing phenomenon in the jet field. On this basis, the user-defined function coupling internal ballistics process was used to determine the transient jet field of a double-barrelled, 30 mm (d = 30) underwater artillery numerically at the barrel intervals of 3d, 4d and 5d. The calculation results indicate that the closer the interval is, the more chaotic the pressure field is, and the Mach disc structure forms later. When the barrel interval is 3d, the Mach disc formation time is ∼ 0.2 ms, while it forms at ∼ 0.15 ms when the barrel interval is 5d. The movement law of projectiles in water is consistent with the change of Mach disc displacement with time, and both satisfy the exponential growth law. The velocity field core is elliptical, and the vortices on both sides of the muzzle continue to grow within 0.4 ms when the launch interval is 3d. As the barrel interval increases, the core of the velocity field is bottle-shaped, the vortex on both sides of the muzzle increases first and then decreases within 0.4 ms, and the maximum temperature of the flow field increases by 15 %.

Effects of cross flow on the launching process of submarine-launched vehicle considering the 6-DOF motion at barrel-exit stage

Submarine-launched vehicle (SLV) is a research hotspot due to its strong strike capability, among which underwater launching technology is a key issue hindering its development. In this paper, a numerical model is proposed to simulate the launching problem of SLV. The high nonlinearity induced by the coupling of multi-degree-of-freedom motions and the interaction between fluid and structure is solved. Specifically, the 6-DOF motion model of structure considering its strong added-mass effect, cavitation model and collision model are adopted. This numerical model is firstly validated by simulating the launching of SLV in still water. And then, the applicability of this model is discussed, as well as the effect of cross flow and initial launching velocity on the launching process. The results indicate that proposed numerical model could be used under both low-speed and high-speed cross flow situations. Cross flow changes the attitude and locomotion state of SLV. When flow velocity exceeds 1.25 m/s, the collision occurs between SLV (with the initial launching velocity of 40 m/s) and its barrel. This obtained locomotion of SLV after leaving its barrel causes the launching failure. Besides, a method of increasing launching velocity is introduced to promote the success rate of launching in high-speed cross flow situation.

Experimental study of scaled-down model ejection water ballistics and two-phase flow of gas and water

There are many factors affecting the hydrodynamic and water ballistic trajectory of vehicles launched underwater, among which the ejection gas pressure, initial motion parameters of the submarine launch platform and the launching water depth have a particularly significant influence. Since it is costly and too long time to carry out a real-size study on the influence of underwater launching parameters, this paper adopts a scaled-down model ejection test to study the evolution characteristics of the flow field of vehicle launched underwater with different ejection pressures and initial ejection angles to reveal the influence of the above parameters on the characteristics of the water ballistics.

Modeling and Optimization of Interior Ballistics within Pneumatic Underwater Launchers

A new area of underwater equipment research focus is the use of underwater unmanned vehicles (UUVs) with launch mechanisms to deploy lightweight and small-sized robots for functions including communication, exploration, and detection. The internal ballistic mathematical model of the underwater launch system for small robots is established in this paper. The internal ballistic parameters and the robot displacement and velocity change rule over time are obtained. The optimization calculation of the crucial parameters to be determined by the particle swarm algorithm is completed. Following optimization, the gas cylinder’s initial pressure is 2 MPa, its capacity is 30 L, its opening area is 9.683 × 10−5 m2, and its opening time is 0.02 s. A numerical simulation is performed for the small robot’s underwater launch process, based on the mathematical and physical model supplied by Fluent 2020 software. The results yield the robot’s motion law and the properties of the flow field during the launch process. The purpose of the underwater launcher experiment is to determine the robot’s motion characteristics. The accuracy of the theoretical model is confirmed by comparing and analyzing the numerical simulation results with the actual data.

Investigation on mechanical behaviour of underwater launching process of bidirectional pigging robot in the subsea oil pipeline using Coupled Eulerian-Lagrangian method

Underwater launchers are typically employed for the deployment of pigging robots in subsea pipelines for the purposes of cleaning and inspection. Throughout the launching process, challenges related to vibration, damage, and other issues must be addressed and resolved. In this study, the launching process of the pigging robot is numerically simulated using Abaqus 2020. Utilising the Coupled Eulerian-Lagrangian (CEL) method, the Fluid-Structure-Interaction (FSI) model of the launching process was constructed. And the variation of parameters such as velocity, friction force and tilt angle at different inlet flow rates were analysed. The findings indicate a positive correlation between the average velocity of the pigging robot and the inlet flow rate. The factors such as friction, tilt angle, and offset distance are related to the motion process of the pigging robot. Furthermore, during the pigging robot’s traversal through narrow pipe sections, the sealing discs undergo irregular deformation, generating a reaction force on the robot and inducing vibration. In conclusion, the analysis of the impact of inlet flow velocity on the motion behaviour of the pigging robot offers a theoretical foundation and reference for the successful launch of the robot.

Sensitivity analysis of underwater launch based on Morris method

The discharge velocity, pressure difference and acceleration of underwater launch performance are affected by many factors, including depth, sailing speed, burning rate, water temperature, density and other operating parameters. The existing underwater launch sensitivity analysis is a local sensitivity qualitative analysis based on single-step perturbation analysis, which can only investigate the influence of a single uncertain factor on the model results in a small range. In this paper, based on the underwater launch model under multi-factor coupling, 10 input parameters were selected to calculate the multi-factor sensitivity of the underwater launch model based on the three objectives of velocity, pressure difference and acceleration at the time of discharge, and the Morris global sensitivity analysis method was used to calculate the sensitivity of the underwater launch model, and the sensitivity of the 10 input parameters to the model results was sorted. The results show that the five input parameters of burning rate, depth, propellant temperature, density and water temperature have the highest sensitivity to velocity, pressure difference and acceleration, and the sensitivity order is slightly different. The influence of water temperature on the three targets is almost evenly distributed in the range of values, and the other parameters have different sensitive ranges for different targets.

Dynamic modeling and motion prediction of two projectiles launched successively underwater

The technology of successive underwater launching of multiple projectiles is a growing trend in military development. However, the process is complex, and the wake field formed after the launch of the first projectile can significantly impact the trajectories and attitudes of the subsequent projectiles. This can potentially lead to mutual collisions or destabilization of motion among the projectiles, resulting in mission failures. In this paper, a mathematical model incorporating unknown parameters is proposed to effectively describe the underwater motion of projectiles. To refine this model, we employed a system identification method that utilizes experimentally validated numerical simulation data to solve for the unknown parameters. The model is then used to predict the trajectory and attitude changes of the projectiles across various launching conditions. The results indicate that to ensure a successful launch mission, certain adjustments must be made. These adjustments include increasing the launching velocity within acceptable limits, extending the launch time interval, increasing the spatial distances between projectiles, and reducing the platform velocity.

Cavity evolution of ventilated vehicle launch under a rolling condition

The unsteady development of the tail cavity of a vehicle after it leaves a tube often causes adverse effects, most notably an impact load on the vehicle when the cavity ruptures. The rolling of the launch platform can alter the development of the tail cavity, significantly altering the influence of the impact load on the motion and attitude of the vehicle. The present study employs the shear stress transport k-w model, the volume of fluid multiphase flow model, the Schnerr–Sauer cavity model, and the overlapping mesh technique to conduct numerical simulations of the underwater launching process of a ventilated vehicle under both stationary and rolling boundaries. A comparative analysis is conducted to examine the evolution of the cavity shape, pressure distribution, and collapse-induced load in the tail cavity under various conditions after vehicle launch. The findings suggest that the rolling of the tube induces an asymmetrical development of the shoulder cavity lengths and widths on both the windward and leeward sides, with the result of a lower peak pressure at the cavity closure position compared with that under stationary conditions. The rolling of the tube reduces the internal velocity within the tail cavity, elevates the rupture position of the tail cavity, delays the tail cavity rupture, impacts the timing of the force peak occurrence in the vertical direction of the vehicle, reduces the high pressure at the point of tail cavity rupture, and modifies the post-rupture structural characteristics of the tail cavity.

Study on the characteristics of the transient flow field under different underwater environments

The underwater muzzle transient flow field is an unsteady, multiphase complex flow field interacting with projectiles and containing various shock wave structures. The turbulent mixing of gunpowder gas and water has a significant impact on the development of the muzzle gas flow field. Moreover, the muzzle gas flow field disturbs the motion of the projectile, thereby affecting shooting accuracy. As part of this research, an unsteady multiphase flow model of the underwater muzzle transient flow field is established by combining the theories of multiphase flow and turbulent mixing. The volume of fluid model is employed to trace the two-phase interface, while the gas–liquid turbulent mixing is described by the standard k–ε turbulence model. Furthermore, the cavitation model is used to describe the cavitation phenomenon caused by the motion of the projectile. The established numerical model is validated by comparing underwater launching experimental results. Accordingly, the muzzle flow field of a 30 mm underwater gun under different water depth conditions is numerically calculated. The results demonstrate that, as the water depth increased, the gunpowder gas is exposed to relatively high water pressure during the expansion process, resulting in a continuous decrease in the core area of the gas, and the Mach disk is also increasingly closer to the muzzle. At different water depths, the diameter of the Mach disk conforms to the binomial law with time, while the displacement of the Mach disk from the muzzle increases exponentially with time.

Effect of Starting Conditions on the Internal Flow Field and Interior Ballistic Performance of an Underwater Ventilated Launch

As one of the future main directions for underwater artillery, a ventilated launch can significantly reduce the huge water resistance during the underwater launching process. This paper aims to clarify the effect of starting conditions on the internal flow field and interior ballistic performance of an underwater ventilated launcher. Firstly, a three-dimensional unsteady model of gas–liquid two-phase flow is established. Following, an interior ballistic program of the underwater ventilated launch is developed. A coupling model between interior ballistic and gas–liquid interaction is then established, accounting for the projectile’s dynamic boundary effect and gas–liquid interaction. Subsequently, the simulation accuracy of the model is confirmed. Finally, the effect of parameter adjustments on the internal flow field and interior ballistic properties are contrasted and examined by altering the starting conditions. The results indicate that adjusting the gas injection pressure and projectile starting pressure can effectively regulate the drainage and resistance reduction effect, thereby obtaining the desired interior ballistic performance of the underwater ventilated launch. The findings offer recommendations for future underwater launchers.

Trajectory prediction of underwater launch equipment based on varying flow coefficient

In the underwater launch systems, a real-time prediction of the trajectory of the equipment out of the tube can provide accurate motion feedback for the closed-loop control. In this paper, using an analytical approach, a hydrodynamic mathematical model is developed for the open pipeline underwater launch system, along with an analysis of the coupling of pressure-flow variables influence mechanism on the trajectory. A time-varying coefficient-based prediction model for the equipment trajectory of the underwater launch process is developed for the compressible fluid and the strong transient characteristics of the fluid during the underwater launch process. Through the incorporation of the time-varying component of inertia into the flow coefficient, the interlinking relationship between the fluid, the pipe structure, and the equipment motion is clarified. This addresses the problem that the traditional steady-state hydrodynamic pipeline model cannot accurately describe the strong transient launch process. The experimental results show that, under typical firing conditions, the maximum error between the equipment motion velocity predicted by the mathematical model based on the time-varying flow coefficient and the measured velocity is 8.48%, with an error root mean square of 0.3035, proving the accuracy of the model based on the time-varying flow coefficient.

Transient interactions between bubbles and a high-speed cylinder in underwater launches: An experimental and numerical study

The underwater launch of high-speed vehicles involves complex bubble-structure interactions, which are not currently well understood. In this study, two small-scale experiments are carried out involving transient bubble-cylinder interactions. We adopt the underwater electric discharge method to generate a high-pressure bubble that drives a cylinder to a maximum velocity of about 25 m/s within 1 ms. A tail bubble forms as the cylinder is ejected from the launch tube. Moreover, we observe a shoulder cavity around the head of the cylinder due to the pressure reduction in the flow. To better understand the complex interaction between bubbles and the high-speed cylinder, we use the boundary element method to establish a bubble-structure interaction model. Our numerical model reproduces the experimental observations quite well, including the cylinder motion and the transient evolution of the bubbles. Thereafter, a systematic study is carried out to reveal the dependence of the bubble-cylinder interactions on the initial pressure of the tail bubble P0. We obtain a scaling law for the maximum velocity of the cylinder with respect to P0. The findings from this study may provide a reference for subsequent research into underwater launches.

Подводный старт суперкавитирующего ударника из лабораторной баллистической установки

This paper considers the investigation of the high-speed motion of specifically-shaped inert bodies (projectiles) in water. The main purpose is to determine the peculiar properties of underwater launching of supercavitating projectiles out of the bore of a test ballistic setup. Preliminary calculations of the inner ballistic parameters of a shot are per-formed for the given launcher and then compared with the corresponding experimental data. Examples of experimentally fired underwater shots are presented. The high-speed high-energy processes associated with the projectile released out of the bore and entering water are described. It is shown that during the underwater launching, the complex gas-dynamic conditions of multi-phase processes following the projectile release out of the bore can significantly affect the trajectory of its further motion in water. Using the experi-mental and calculation methods with the classical loading scheme and an acceleration bore 0.5 mm long or less under given conditions, it has been determined that the pressure at the accelerator exit increases stepwise and can significantly change the flow conditions of the projectile at the start and its further trajectory. Taking into consideration the revealed peculiarities of underwater shots and a set of technical specifications and mass-size parameters, a perspective launching ballistic setup has been designed and used to estimate the motion range of the released supercavitating projectile.

Prediction of the hydrodynamic disturbance characteristics for two projectiles launched successively underwater based on the radial basis function neural network

When two projectiles are successively launched under different launch parameters, the motion of the first projectile affects the hydrodynamic characteristics of the second projectile. To predict and study such disturbances, a radial basis function (RBF) neural network model is established in this paper. Compared with the underwater launch of a single projectile, the hydrodynamic loads for two projectiles successively launched are more complex and severe. When the first projectile is launched, it will affect the forces and moments of subsequent projectiles, leading to launch failure. Thus, we apply a numerical simulation method that is verified through experiments to simulate two projectiles successively launched underwater. Then, we use the generated data to train the RBF neural network. The results show that vortices will form at the tail of the first projectile after launch due to viscous effects, which is the main reason for the hydrodynamic disturbance that affects the second projectile. Compared with numerical simulations and experimental methods, the RBF neural network model can more effectively predict the disturbance of the hydrodynamic characteristic variables of the first projectile to the second projectile. This disturbance can be reduced by increasing the spatial distance of the two projectiles, increasing the time interval between launches, and reducing the platform velocity. However, the launch time interval is the most sensitive factor affecting the hydrodynamic characteristics of projectiles.

Simulation research on the outlet cavity features in the underwater launching process

This paper analyzes the evolution of cavity features and pressure characteristics during the underwater launching process. The improved delayed eddy simulation (IDDES), energy equation, and overlapping grid technique are adopted. In addition, validation of the numerical approach and grid independence is carried out. The evolution of the multiphase flow field and pressure fluctuations near the launcher outlet are studied. Results show that the gas mass escaping from the tube expands and contracts, eventually breaking off with a jet that acts on the tail of the vehicle after the vehicle leaves the tube. Four major pressure fluctuations occurred near the launcher outlet, the first two of which are generated by the alternate expansion and contraction of the high-pressure gas mass, while the subsequent fluctuations mainly originate from the intermittent escape of the remaining gas mass inside the tube, the magnitude of the pressure fluctuations decreases gradually. As the Froude number increases, the scale of the escaping high-pressure gas mass expands significantly, while the dissipation rate of the fused gas mass at the launch tube outlet also increases, the evolution mechanism of multiphase is more complex.

A study on the performance of the cavitating flow structure and load characteristics of the vehicle launched underwater

This paper analyzes cavitating flow structure and load characteristics of vehicles launched underwater for different cavitation numbers and different angles of attack. The improved delayed detached-eddy simulation model and volume of fluid, as well as overlapping mesh technique, are adopted. Additionally, a verification of the underwater launch simulation method and cavitation model is presented. Cavitating flow structure, wall vortex structures, and load characteristics are studied with a focus on the evolution mechanism of the cavitation flow field during the water-exit process. The results show that the attached cavitation rapidly collapses from top to bottom under the combined effect of large–medium density difference and reentry jet. Due to the presence of attachment cavitation, the development of the wall vortex structure represented by the hairpin vortex is inhibited. Considering the compressibility of the vapor phase, the peak of the synchronous collapse pressure is much larger than the collapse pressure with incompressibility. The pressure appears to be characterized by short widths and high peaks during the collapse of the water-exit. As the vehicle exits the water with a certain angle of attack, the range and peak of the cavitation collapse pressure rapidly reduce. In particular, the pressure side cavitation shedding and collapse behavior at the initial moment may lead to a larger pressure peak.

Experimental Study on Gas–Liquid–Solid Interaction Characteristics in the Launch Tube

In the present study, a visual experimental system was built to explore the multiphase hydrodynamic features in the underwater launching process. The whole processes of gas-curtain generation produced by multichannel jet convergence, gas-curtain expansion, and projectile movement were captured using direct photography. The experimental results show that as the area of a single groove grows from 6.25 mm2 to 11.25 mm2, the gas-curtain displacement grows by 47.5%, and the projectile’s speed reduces by 34.1%. The expansion of the gas curtain can be aided by 36.0% by increasing the number of sidewall grooves within a specified range (4 to 8), but the vehicle’s speed is reduced by 53.8%. While increasing the maximum injection pressure from 9.9 MPa to 18.2 MPa, the gas curtain’s draining capability is improved by 29.6%, and the projectile speed increment diminishes (only 10.0%) as the amount of gas flowing into the front of the projectile grows. The impact of jet parameters on gas-curtain displacement and projectile speed is revealed in this study, which is of utmost significance to the parameter-matching design of underwater low-resistance launchers.

Research on the interference characteristics of successively launched underwater projectiles

In the successive underwater launch of two projectiles, wake vortices shed from the leading projectile have significant effects on the subsequent projectile's hydrodynamic forces, attitude, and trajectory. Herein, experimental research is conducted by successively launching two underwater projectiles at prescribed transport velocities using embedded inertial measurement units to record acceleration data simultaneous with a high-speed camera to visualize the cavity behaviors. After a trial error evaluation, the interference between two conical-nosed projectiles is shown to increase as wake vortices develop from dispersed vortex rings at low transport velocities into continuous counterrotating vortex pairs at high transport velocities. Thus, the second projectile experiences less lateral movement and rotation than the first projectile at u > 0.1 m s−1. The effect of the nose shape is also studied by conducting experiments on ellipsoidal-nosed projectiles, whose trends are similar to those of the conical-nosed projectiles. A quantitative comparison of the interference shows that the ellipsoidal-nosed projectiles experience more interference than the conical-nosed projectiles due to the absence of the shoulder cavity.

Experimental and theoretical investigation of the cavity dynamics of underwater launched projectiles

During the underwater launch of a projectile using compressed air, a bubble emerges from the launch tube before the projectile. Upon being pierced by the projectile, this bubble has significant effects on the formation and evolution of the projectile’s shoulder cavity. This paper presents experimental and theoretical analyses of the dynamics of such a projectile’s shoulder cavity launched at low Froude numbers. In the experiments, a dynamic sensor and a camera are used to record the internal pressure and profile of the cavity, respectively. The cavity pressure oscillations explain the cavity ripples observed during the experimental sequence. Accordingly, we establish an independent bubble evolution model to predict the internal cavity pressure based on the assumption of spherical expansion; we also separately construct a shoulder cavity evolution model that treats the cavity as a slender body. The matched asymptotic method is used to solve the shoulder cavity evolution model, and quantitative comparisons between the theoretical and experimental results, including the cavity shape and size, show favourable agreement. Finally, the effects of cavity pressure oscillations and of the nose shape and projectile velocity on the cavity’s behaviour are studied.