A ventilated vehicle exiting water in a wave environment is a complex nonlinear process, and the mechanism by which the wave conditions influence this process remains poorly understood. This paper describes realistic simulations of a ventilated vehicle exiting a water body under various wave conditions. Comprehensive analysis is conducted for a range of distinct wave scenarios, and a machine learning-based method is developed for the rapid forecasting of vehicle-related parameters. A three-layer backpropagation neural network is constructed, and its prediction performance is verified. Subsequently, predictive and optimization procedures are employed to determine the optimal wave phase for the water exit of the vehicle. Different wave conditions are shown to significantly affect the evolution of the ventilated cavity as well as the kinematic and loading characteristics of the vehicle. The pitch angular velocity and angle at the moment when the head of the vehicle reaches the free surface exhibit a positive cosine trend under different wave conditions. No regularity of the pitch angular velocity at the moment when the tail reaches the free surface is evident. The neural network exhibits exceptional proficiency in predicting the motion parameters and load characteristics of the vehicle. The optimal point for the vehicle to exit the water is determined to be at a wave phase of 0.125π, while the most hazardous point occurs when the wave phase is 1.1875π.

The flow pattern of underwater jet under co-flow is complex, with oscillations reflected in back pressure and nozzle thrust, impacting vehicle performance. This paper presents an experimental study on jet evolution under co-flow conditions at a Froude number of 2.86, conducted in a closed-loop cavitation water tunnel. It analyzes the multiphase flow structure, back pressure Pb and nozzle thrust under various nozzle pressure ratios (NPRs) and area ratios, revealing their interrelationship. It was discovered that decreasing NPR or increasing the area ratio diminishes the jet's ability to counter buoyancy and enhances plume accumulation at the nozzle exit. The intensity of frequency shift of nozzle back pressure is positively correlated with NPR but shows limited sensitivity to the area ratio. The drift range of peak frequency is greatly affected by the area ratio. Furthermore, the back-attack always characterized as a high amplitude signal, whereas jet breaking exhibits low amplitude. A narrow bubble structure at the nozzle exit induces peak back pressure and x-direction thrust, but bubble expansion reduces these effects.

Submerged high-speed gaseous jets exhibit complex evolution and instability mechanisms due to the significant density and velocity differences between the gas and the surrounding water. This study experimentally investigates the high-speed gas jet generated by a Laval nozzle positioned behind a rotating body under various co-flow velocities. Based on the Eulerian approach to fluid dynamics, width-time maps visualizing jet width changes at specific cross-sections were generated. Utilizing high-speed shadowgraph images, pressure sensor data and the width-time maps, this study examines the evolution characteristics of jet and the pressure oscillation characteristics behind the rotating body, categorizing the jet evolution into three basic stages. This research also investigates the correlation between back pressure oscillations and jet morphology, noting that peaks in local back pressure correspond to the jet's core area being blocked. A specially developed interface tracking algorithm is used to extract the normal acceleration of the jet interface, facilitating an analysis of the impact of Kelvin-Helmholtz and Rayleigh-Taylor instabilities on jet the instability of the jet interface.

Due to the influence of initial conditions such as the tethered velocity, vertically launched underwater vehicles generate large vortex structures in the wake after leaving the launch platform. This leads to an asymmetric surface pressure distribution on the second vehicles passing through the wake, adversely affecting their attitude. The rolling of the launch platform is another critical component of the above initial conditions. This study used the Realizable k−ε model, the volume-of-fluid multiphase flow model, and overlapping grid technology to numerically simulate the launch process of sequentially launched and retrogradely launched vehicles, both with and without rolling conditions. The study conducted a comparative analysis of the evolution patterns of wake vortex intensity, pressure distribution, and air–water phase distribution for the second underwater vehicles under different conditions after their launch. The results show that under rolling conditions, the wake vortex structure exhibits curling at both the vortex head and the vortex tail, with regions of greater vorticity shifting laterally. At certain moments, rolling and vorticity have similar effects on the surface pressure of the second vehicle. The presence of rolling accelerates the expansion and compression of gas pockets within the launch tube, reducing the disturbance of gas on the retrogradely launched first vehicle and changing the rolling angle of the vehicle. In the rolling conditions, the peak water hammer force on the inner wall of the platform increases and the forces acting on the inner wall of the tube show periodic variations.

The presence of ice floes on the water surface has a significant impact on the complex hydrodynamic process of submersible ventilated vehicles exiting the water. In this paper, we propose numerical simulations based on computational fluid dynamics to investigate the process of a ventilated vehicle exiting water in an ice-water mixture. The Schnerr–Sauer model is used to describe the cavitation, while the turbulence is solved by using the k-ω shear stress transport (SST) model. We also introduce the contact coupling method to simulate the rigid collision between the vehicle and the ice floe. We calculated and analyzed the process of the vehicle exiting the water under three conditions: ice-free conditions and in the presence of regularly shaped and irregularly shaped ice floes. The findings indicate that the ice floes contributed to the rapid fragmentation of the water plume to induce the premature collapse of the ventilated cavity and alter its form of collapse. The presence of ice floes intensified the evolution of the flow field close to the vehicle, and their flipping led to a significant volume of splashing water that could have led to the localized secondary closure of the cavity. Moreover, the collision between the vehicle and the ice floes caused pressure pulsations on the surface of the former, with a more pronounced effect observed on the head compared with the cylindrical section. While crossing the ice-water mixture, the vehicle was exposed to water jets formed by the flipping ice floes, which might have led to localized high pressure.

Supersonic gas jets in conical convergent-divergent nozzles are studied numerically using the OpenFOAM rhoCentralFoam solver. The spatiotemporal evolution of the jet flow field is analyzed. The influence of the operating conditions on the flow field is studied parametrically, including the nozzle pressure ratio (NPR), area ratio, and throat position. The behaviors and mechanisms of the double-diamond structure, throat wave, and exit wave are interpreted in detail. The results show that a conical convergent-divergent nozzle always generates shock waves. The throat shock reaches its maximum length during the initial stage, then becomes slightly shorter before becoming stationary, dominated by the exit velocity. Furthermore, it is shown that the jet flow changes from overexpansion to underexpansion with increasing NPR. With an increasing area ratio, the trend is the opposite. The throat position affects the jet divergence angle at the nozzle exit, consequently causing a variation in the core radius of the jet. It is further shown that the double-diamond structure does not always appear. The throat shock angle, exit wave angle, and shear layer width directly affect the shape of the double-diamond structure. The favorable pressure gradient of the nozzle ultimately dominates the changes in the length of the throat wave and exit wave.

The effect of boundary layer oscillating suction (BLOS) on the vortex structure of high-load linear compressor cascades is studied by a high-fidelity numerical calculation. The oscillation parameters (amplitude and frequency) and suction control parameters (suction position and mass flow) are continuously adjusted. It was found that higher aerodynamic efficiency could be obtained with suitable oscillation parameters and suction control parameters. This is achieved because BLOS not only absorbs the low-energy fluid in the boundary layer but also optimizes the vortex structure in the compressor passage.

The tail cavity as induced by submerged gaseous jets on the bottom of axisymmetric vehicles is studied in a water tunnel over Froude numbers, which denote co-flow velocities, between 4.8 < Fr < 16.0, and the jet stagnation to ambient pressure ratio in a convergent–divergent nozzle between 1.7 < p0/p∞ < 14.5. The shadowgraph topology of the induced tail cavity falls into three consecutive categories on the regime map: intact cavity, partially broken cavity (PBC), and pulsating foamy cavity. Both the Fr and p0/p∞ are essential drivers for the topology and length of the tail cavity within the current scope. The length of PBC holds a logarithmic relationship to p0/p∞. The fluctuation intensity of the cavitation number is under heavy influence from Fr and p0/p∞ and is regarded as a direct and reliable measurement of jet/closure interaction intensity. Under high Fr, the Rayleigh–Taylor instability is dominant at the cavity surface, which leads to a significant amount of bubble shedding and adds to the total cavity gas leakage. From the perspective of the vehicle, the characteristics of its bottom pressure depend on the cavity regime. The characteristics of jet-induced tail cavity established in the presented study should be helpful for the design and control of submarine vehicles propelled by rockets and supersonic jets.

High-speed gaseous jets are commonly found in deep-water propulsion systems. The submerged high-speed gaseous jet becomes turbulent due to the density difference between the gas and liquid phases. In experimental studies, the interaction between the gaseous jet and the liquid has not been well characterized under co-flow conditions. In this paper, the effects of co-flow velocity, Fr, and jet pressure ratio, p0/p∞, on the flow structure of the gaseous jet and pressure on the nozzle outlet cross-section were experimentally studied. It has been observed that the increase of Fr makes the nozzle outlet cross-section gradually wrap up with transparent bubbles under subsonic gaseous jets. However, the transparent bubbles will be broken entirely under supersonic gaseous jets, and the gaseous jet will intermittently expand into bubbles downstream away from the nozzle outlet cross-section. The flow mechanism of the underwater gas jet at larger Fr seems to be different from that at smaller Fr and still water. Keeping Fr constant, the dimensionless streamwise length of the jet, Ls/LQ, increases as a power law with p0/p∞. Fr further promotes the penetration of the jet downstream under the fixed p0/p∞. In addition, the pressure on the nozzle outlet cross-section is seriously affected by p0/p∞ and Fr. The increase of Fr will seriously weaken the intensity and frequency of pressure oscillations on the nozzle outlet cross-section. This finding can provide theoretical support for the design and control of jet-propelled vehicles.

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.

The applications of freely falling bodies in steady water or air have been widely observed in nature, from the dispersing of a tree seed to the re-entry of a space shuttle. Disparate objects exhibit particular falling styles which have been studied for a long time. However, the mechanism behind this extensive phenomenon has not been clearly understood. In this paper, we investigate experimentally the dynamics of an annular disk falling freely in water in steady state. Three distinct falling motions namely, lengthways vibration motion (LV), hula-hoop motion (HH) and helical motion (HM), are examined, and Proper Orthogonal Decomposition (POD) is used to extract the coherent structures behind the falling disks. The forces and torques governing the falling motion of annular disks are determined by analyzing the recorded kinematics. The results show a rapid eigenvalue convergence where the first three modes account for the highest percentage of the total energy. Depending on the falling motions, the corresponding POD modes show different characteristics. The second and third POD mode identify the perturbations in the region downstream of the disks. The shedding pattern featured by the periodic vortex loops is extracted for the LV motion and the forces frequency generated by the vortex loops is analyzed. By comparing the flow fields in different POD modes of HH and HM motions, it is clear that the vortices break down into smaller structures due to the violent twists at the turning point of the trajectory in HH motion, while the vorticity field of HM motion shows a relatively stronger continuity in the first three POD modes. It is found that the vortical torques, i.e. the hydrodynamic torque due to wake vortices, acting on the disks of HH motion have sharp peaks compared to HM motion. This work illuminates the reason for the disparity between HH and HM motion by decomposing the flow field into distinct POD modes.

Refill friction stir spot welding was employed to produce 6061-T6 aluminum alloy joints with different sleeve plunge depths. The interface characteristics of joint-line remnant and hook are investigated by optical and scanning electron microscopy. The joint-line remnant consists of primary bonding region and secondary bonding region, and two types of hook can be identified as downward hook and upward hook. Tensile shear results demonstrate that joint-line remnant and hook make interaction effects on tensile shear properties. The optimal joint is achieved when sleeve plunge depth was 2.0 mm with the corresponding failure load of 8673.4 N. Three different types of fracture mode are exhibited in joints produced at different sleeve plunge depths, which are closely related with the morphology of interface characteristics.

Vortex structures of the separation flow fields in two-dimensional compressor cascades controlled by the boundary layer oscillating suction are numerically investigated. The proper orthogonal decomposition method is adopted to present the variation of characteristics owned by large-scale vortices. It is found that introducing unsteady excitations with proper frequencies into the steady aspiration results in a more effective control effect and the optimal oscillation frequency should be in line with the characteristic frequencies of the steady aspirated flow field. The separation flows can be decomposed in to several basic structures with the dominant one being in the manner of Karman vortex street regardless of the control method adopted. The boundary layer oscillating suction not only alleviates the separation by removing low-energy fluid, but also intensifies the harmonic flow element represented by proper orthogonal decomposition modes with others suppressed. The well-organized vortex shedding process could contribute to the loss reduction to some extent.

To understand the potential hazard and mechanism of wake vortex encounter underwater, we perform a numerical simulation on two successively launched projectiles with 6 degrees of freedom. The evolution of wake vortices and their interaction with projectiles are studied under various launcher transport speed and launch sequences. Two major wake vortex structures are discovered: the vortex ring and the counter-rotating vortex pair. As the projectile angle of attack increases, the dominant vortex inside the wake transforms from vortex rings to hairpin vortices, then counter-rotating vortex pairs. The large scale vortices in the wake are highly directional before breaking up, and the counter-rotating vortex pairs can impose significant influence on projectile attitude upon wake vortex encounter. In contrast, minor influence can be done by vortex rings under the same situation. Projectiles launched with higher platform transport speed tend to be more affected by wake vortices due to the strong counter-rotating vortex pairs generated with a high angle of attack.

为研究水下航行体俯仰运动过程中微气泡流形态及减阻特性的变化规律,利用自主设计的驱动装置、高速摄像系统和测力系统,开展水下航行体俯仰运动微气泡减阻特性试验研究.试验过程中,基于自主设计的驱动装置,实现航行体绕其头部按正弦规律作俯仰运动.研究结果表明:当体积流量系数较小时,水下航行体运动过程中离散的微气泡始终均匀分布在航行体表面;水下航行体俯仰运动过程中,轴向力系数、法向力系数、阻力系数和升力系数的变化规律类似,均呈正弦变化规律,且其变化周期与攻角变化周期基本同步;对于不同体积流量系数下俯仰运动的航行体,随着攻角的增加,其阻力系数均呈近似线性增加规律,减阻率呈逐渐线性减小规律.

为研究齐射出水过程中回转体间的相互干扰,基于高速摄像技术对小型回转体齐射出水过程开展实验研究.利用图像处理技术对采集图像序列中的回转体和空泡轮廓进行识别提取,并通过对比单发回转体和双发回转体在有无艇速下的弹道特性,分析齐射过程回转体间的流体动力干扰和艇速对其影响.实验结果表明:在回转体无艇速2发齐射出水过程中,首发回转体形成的低压区使次发回转体的内侧空泡发展较大,次发回转体的运动轨迹发生向首发回转体方向偏转;当存在艇速时,回转体空泡演化规律和弹道特性发生改变,在迎流侧高压和水流冲刷作用下,空泡均向背流侧发展较大,首发回转体发生沿流向偏转,次发回转体处于首发回转体的尾流场中,其姿态和轨迹仍存在向首发回转体方向偏转,但由于艇速作用使偏转幅度相对较小.

This current study presents a detailed analysis regarding three-dimensional separation flows controlled by steady and oscillating boundary-layer aspirations installed on suction surfaces of highly loaded planar compressor cascades. The influences of the oscillation parameters on the control effects are parametrically investigated with the objective of providing guidelines to determine the control parameters. The susceptibility to varying the aspiration locations as well as the control expenses of the steady boundary-layer suction methods are significantly improved due to the introduction of periodic oscillations into the aspirating flows. The effective excitation frequency spans a relatively wide bandwidth (from Stexcit=0.314 to Stexcit=1.411) only if the amplitude exceeds a threshold value (Delta ms over bar >0.2). Through a detailed comparative analysis on the steady and oscillating aspiration cases, the most prominent spatial change led by the oscillating aspiration is manifested by fully discretized separation vortices. These newly formed spanwise vortex tubes enhance the momentum exchange between the main flow and the recirculation zone, and hence alleviate the corner separation. Despite the increased local loss production caused by the increased vortical strength, the overall cascade performances are improved from a temporal average perspective. With the aid of the proper orthogonal decomposition method, the formation process of the discretized separation vortices is explored. It is found that the periodic disturbances originating from the suction slot are amplified, subject to the adverse pressure gradient and rollup, to form a series of vortical structures in the separated layers.

Multiphase jets and cavitation problems are inevitable for high-speed underwater vehicles propelled by jet engines. Unlike being injected into stagnant water, the gaseous jet behind a underwater vehicle is usually conjugated with a tail cavity. The pulsation and collapse of such cavities can seriously affect the vehicle performance. In this study, the shape character, forming mechanism and control conditions for the supersonic gaseous jet induced tail cavity at the wake of a revolution body are experimentally investigated in a water tunnel. The induced cavity is ventilated only by a convergent-divergent nozzle with a designed Mach number of 2.45. The form of the cavity is recorded through two high-speed cameras both horizontally and vertically under different Froude numbers and ventilation rates. The time averaged form is thus obtained through digital image processing to eliminate the transient characteristics of the cavity. The experiment is conducted with the Froude number ranging from 3.2 to 16.2, and the ventilation rate 0 to 0.5. Due to the high density and velocity ratio between water and gas, the structure of such flow is usually very complicated. Many novel phenomena of the jet-cavity interaction are observed. With increasing stagnation pressure of the central jet, the induced cavities evolves form foamy, intact, partially break, to pulsating foamy closure type. The foamy and intact tail cavities share the same profile and characteristics that of a supercavity. And the pulsating foamy closure type was never observed before in a traditional supercavitating flow. The outline of the pulsating foamy cavity is the same as the foamy cavity's, indicating that they have the similar forming mechanism. A comparison with the jet-cavity interaction model is made and the following conclusions are obtained: the real ventilation rate, which corresponds to the re-entrant jet gas blocked by the cavity boundary, is the key factor in controlling the cavity form. When the gaseous jet is completely blocked by the water-gas interface, an intact or foamy cavity will be formed. A partially break cavity appears only when some fraction of the jet is blocked and this is when some of the strongest interactions between jet and cavity occurs. When little gas was blocked by the interface, a pulsating foamy cavity forms. With the structure of gaseous jet considered, the transition of the induced cavity closure between different types is in favour of the prediction from Paryshev's model of cavity closure to a central jet. The variation of the cavity form, thus the interaction strength between jet and cavity, coincides with the real ventilation rate estimated through the theoretical model.

Advantages and disadvantages of typical aspiration configurations applied in compound lean compressor cascades are discussed in this paper. The blade leaning mechanism is elaborated first, and then a thorough study on the synergistic effect of aspiration and blade leaning on the flow field structure and the aerodynamic performance, especially the boundary layer development in blade passages, which is conducted by CFD. The calculations show that the spanwise pressure gradient in positive lean cascades is adopted to reduce the corner loss. Meanwhile, with increasing incidence, the reversal flow is accumulated near the midspan due to the redistribution of the boundary layer, which contributes to the broadening of the operation range. With the introduction of suction surface aspiration, the loss of the cascade is effectively reduced which results from the suppression of the boundary layer development on the blade suction surface. However, due to the limited controlling capacity towards the corner separation, the boundary layer thickness in the corners is almost unaffected, which leads to the restricted improvement of the operation range. Endwall aspiration is more powerful in removing the corner separation to significantly delay the occurrence of hub-stall. Nonetheless, the increased aspirated flow loss counteracts the loss reduction near the endwalls. (C) 2017 Elsevier Ltd. All rights reserved.