Search

This study investigates the discharge coefficient of a cylindrical hole with gas jets in liquid crossflow in a cavitation tunnel. Utilizing the Π theorem, the research examines the dimensionless parameters influencing the discharge coefficient. The results indicate that an increase in the Froude number notably diminishes the discharge coefficient at low-pressure ratios, primarily due to intensified crossflow shear effects, which constrain the effective flow area of the gas jet. However, as the pressure ratio increases, the gas jet velocity rises, attenuating the shear effect of the liquid crossflow and augmenting the discharge coefficient. Examining the correlation between total pressure ratio and discharge coefficient reveals that below a total pressure ratio of 1, the inhibiting effect of crossflow static pressure on the discharge coefficient is more pronounced than that of dynamic pressure. Conversely, beyond a ratio of 1, dynamic pressure imposes a more substantial inhibiting effect compared to static pressure. Additionally, the study assesses the impact of the jet Reynolds number on the discharge coefficient, demonstrating that the viscosity of gas jet has a negligible effect compared to the shear effect of the liquid crossflow. Furthermore, it is found that with an increase in the Euler number, the discharge coefficient increases at the same pressure or velocity ratio, decreases at the same momentum ratio, and remains unchanged at the same momentum flux ratio. Comparing the effects of those above non-dimensional parameters on the discharge coefficient indicates that the momentum flux ratio is the most significant dimensionless parameter that affects the discharge coefficient. • Experimentally studied the discharge coefficient of a gas jet in a liquid crossflow. • Proposed an equivalent total pressure ratio based on the incompressibility assumption. • Found that the effective flow area of the jet directly affects the discharge coefficient. • Provided a method for correlating the discharge coefficient. [ABSTRACT FROM AUTHOR]

This paper considers the problem of position tracking control of the autonomous underwater gliders, which operates under the environment of model uncertainties and external disturbances. The motion of the glider is controlled by ballast mass and moving mass, which have limited dynamic range. An adaptive control system for position tracking based on filtered backstepping is proposed, and neural networks and sliding mode control are used to improve robustness. Next, using Lyapunov stability theory, it is proved that all the signals are bounded and tracking errors eventually converge to a neighborhood of zero. The presented controller can be easily put into practice without the requirement to know the parameters of the glider and the value of external environmental disturbances. Finally, the effectiveness and good robustness of the proposed control system are verified by simulation results. • The position is taken as the control target for the six degrees of freedom movement of AUG. • The filtered backstepping method is used to solve the problem of "explosion of complexity". • The neural networks are used to solve the problem of model uncertainties. • The SMC is used to solve the problem of the disturbances and the approximation errors of neural networks. [ABSTRACT FROM AUTHOR]

A high-fidelity numerical model is employed to investigate the effect of underwater gas jet on ventilated supercavitation flow characteristics. The model includes the compressibility of gas phases and utilizes the inhomogeneous multiphase model and SST k − ω turbulence model. The developed solver is quantitatively validated by experimental results. Supercavity-jet interactions are examined under various flow conditions. The effect of underwater gas jet on the supercavitation flow is contingent upon the type of ventilated supercavities. In the case of the wake-closure flow pattern, the ventilated supercavity fully envelops the nozzle, and variations in the supercavity geometry depend on different jet momentums, the corresponding mechanism has been revealed from the perspective of the supercavitation flow structure and gas leakage behavior. When the supercavity dimension is insufficient to fully envelop the nozzle due to the angle of attack, gas jets are injected directly into the surrounding water, resulting in intense pulsations of the underwater supersonic gas jet that impact upstream dynamics of supercavitating flow fields. Increasing the ventilation rate can effectively mitigate severe oscillations in supercavitating flow fields and minimize the adverse effect caused by the gas jet. • The variations in supercavity geometry with the different gas jet momentums are obtained. • The effect of supersonic gas jet on the ventilated supercavitation flow regime is investigated and analyzed. • The mechanism of supercavity-jet interactions is revealed by analyzing gas-leaking rate. [ABSTRACT FROM AUTHOR]

The semi-sealed cylindrical shell is a hollow cylinder, of which one end is opened and the other end is sealed. In this paper, the hydrodynamic characteristics of a semi-sealed cylindrical shell at high-speed are studied experimentally. The influence of the slenderness and Froude number on the expansion characteristics of the cavity is preliminarily studied, with special attention paid to the cavity evolution, splash and kinetics of the cylindrical shell under the influence of the internal jet. The results show that, with the semi-sealed cylindrical shell penetrating into water, it forms supercavity on the outer shell surface similar to a solid cylinder. Subsequently, with the internal jet poured out the shell, a foggy cavity is formed below. As the internal jet continuously pours out of the shell, a new water surface is formed below the shell and the shell will impact on the new water surface, thus causing new cavities generated. This process will repeat continuously as the shell moves downward. Moreover, the slenderness of the shell and Froude number have remarkable influence on the motion characteristics and cavity evolution. The findings in this paper can be used as a reference for future designs of cylindrical shell projectile for water entry. • The motion mechanism of semi-sealed cylindrical shell during the vertical water entry is fully studied. • The shell structure can change significantly the evolution of the cavity shape. • A internal jet is formed upwards the shell and the free surface seal mode can be altered. • Comparison between different Froude number and slenderness ratio of semi-sealed cylindrical shell are investigated. [ABSTRACT FROM AUTHOR]

To investigate the evolution of supercavity geometry and the unsteady characteristics of the supercavitation flow from the initial generation to full development. A three-dimensional numerical model with the inhomogeneous multiphase model and SST k − ω turbulence model was established. The numerical model was validated by experimental results. The ventilated supercavitation flow was performed under the different gas entrainment coefficients and Froude numbers. The results show that the initial supercavity closure mode is the re-entrant jet, then the closure mode changes to the twin-vortex induced by the gravitational effect. For the fully developed twin-vortex supercavity, a flow structure with gas recirculation exists inside the supercavity, which includes the reverse flow region and downstream flow region. Compared to the supercavity with the twin-vortex closure mode, the increment in the supercavity dimension with the re-entrant jet closure mode increases drastically, and the oscillation characteristic of internal pressure increases, leading to the significant supercavity deformation and gas shedding. The difference of the supercavity dimension is revealed by analyzing the gas leakage behavior. Further, a theoretical model describing the strength of re-entrant jet is established to reveal the transition mechanism of the supercavity closure mode and analyze the unsteady flow characteristics during the supercavity development. • The evolution of supercavity geometry and the unsteady characteristics of the supercavitation flow are obtained. • The variation in supercavity dimension between two different closure modes is compared. • The internal gas flow structure of a twin-vortex supercavity is obtained and analyzed. [ABSTRACT FROM AUTHOR]

With the application of data-driven technologies in fluid mechanics, models based on Proper Orthogonal Decomposition (POD) and Deep Learning (DL) for predicting flow fields have gradually been used in the rapid analysis of flow fields. However, differences among predictive models have not been widely concerned. In this work, based on the dataset of the unsteady flow around a cylinder, POD-LSTM (Proper Orthogonal Decomposition - Long short-term memory) and CAE-LSTM (Convolutional Autoencoder - Long short-term memory) were designed to compare their prediction performances, which could predict flow fields of the following eight time steps from that of eight time steps before. In a prediction period, the predictive accuracy of models was compared to analyze sources of error. In the long-term, it is noticed that the predictive accuracy of POD-LSTM decreased significantly with the increase of time steps, while CAE-LSTM maintained good robustness until the last time step. In addition, the phenomena of random predictions are presented in high-order temporal coefficients, which led to POD-LSTM's worse performance. Due to the capability of fusing spatiotemporal features, CAE-LSTM shows better accuracy and robustness in the long-term prediction period. • The predictive results between POD-LSTM and CAE-LSTM are compared to analyze prediction performance. • The sources of prediction error are analyzed. • CAE-LSTM is better than POD-LSTM in a prediction period or in the long-term prediction period. • The architecture based entirely on deep learning has significant advantages. [ABSTRACT FROM AUTHOR]

The objective of this paper is to investigate the multi-factor-dependent and nonlinear hydrodynamic characteristics of a ventilated supercavitating vehicle with high Froude number, and further reveal the forming mechanisms of the complex hydrodynamic forces. A high-fidelity numerical model with the inhomogeneous multiphase model and SST k − ω turbulence model is established and validated by experimental results. Ventilated supercavitation flow has been performed over a wide range of angles of attack and gas entrainment coefficients. The results show that the wetting aft body leads to the increase of the gas-leaking rate and the decrement in the supercavity dimension, and the upper supercavity closure region gradually moves forward. Resultantly, the supercavity shape exhibits three distinct features with the angle of attack. The hydrodynamic characteristic of ventilated supercavitating vehicles depends on position relationship between the supercavity boundary and the vehicle. The hydrodynamic characteristic is more complex than that of natural supercavitating vehicles due to the joint influence of high pressure near the upper supercavity closure region, wetting area of the aft body and the variation in supercavity geometry. The hydrodynamic characteristics of the ventilated supercavitating vehicle can be controlled and improved by adjusting the gas entrainment coefficient. • The hydrodynamic characteristic of ventilated supercavitating vehicles with high Froude number is more complex. • The influences of angle of attack on the ventilated supercavity geometry and the hydrodynamic forces are obtained. • The pressure distribution of aft body and the supercavity closure mode with the angle of attack are studied. [ABSTRACT FROM AUTHOR]