Dynamic mode decomposition and reconstruction of the transient propeller wake under a light loading condition

This research aims to extend our understanding of propeller wake dynamics under a light loading condition, thereby laying a foundation for design optimization and flow control of the propeller. Dynamic mode decom-position (DMD) and reconstruction are used to analyze the transient vortical wake structures obtained by large eddy simulation. The propeller wake includes stable tip and hub vortices without interacting evolution at the light loading condition, and elliptical instabilities are observed downstream of the tip vortices. DMD describes the most energetic modes and the corresponding dominant frequencies are the blade passing frequency and its multiples. The coherent structures identified via DMD are primarily associated with the ordered convection of the tip vortices and have little correlation with the hub vortices. Additionally, the propeller wake flow is reconstructed using the first four DMD modes, and the primary wake features are well restored with a maximum reconstructed error of 7.98%. This demonstrates that the flow-field reconstruction based on the DMD reduced-order model is promising for predicting the propeller wake and controlling the propeller operation

Numerical Investigation on the Ventilated Supercavity around a Body under Free Surface Effect

Reducing vessel resistance by using ventilated cavities has been a highly researched topic in the marine industry. There is limited literature on ventilated supercavities near the free surface, which indicates that their dynamic behavior is more complex than conventional ventilated cavities due to the effect of the free surface. This paper employs numerical simulations to study the dynamic behavior of the ventilated supercavity, taking into account the effect of the free surface. Numerical simulations can predict gas leakage behaviors, cavity geometry, and internal flow structures. The influence of the free surface shortens the length of the ventilated cavity and increases the diameter. The presence of the free surface mainly changes the vertical velocity distribution between the free surface and the cavity. The results show that there are two typical gas leakage mechanisms under different immersion depths: twin-vortex tube leakage mode and re-entrant jet leakage mode. The internal flow field of ventilated supercavity is classified into three regions: the internal boundary layer, the ventilation influence region, and the reverse flow region. As the distance between the free surface and the ventilated supercavity decreases, the ventilated supercavity is affected by both the free surface effect and the gravity effect

Numerical investigations into the ventilation elimination mechanism of a surface-piercing hydrofoil

This paper investigates the ventilation elimination mechanisms during the deceleration process of a surfacepiercing hydrofoil using the unsteady Reynolds-averaged Navier-Stokes (RANS) method together with a Volume of Fluid (VOF) model. The numerical results are in good agreement with the experimental data. The ventilation elimination mechanism of the surface-piercing hydrofoil is analyzed from the perspectives of the hydrofoil hydrodynamic performance, the ventilated cavity evolution, vortex structures, and re-entrant jets. The results indicate that the ventilation elimination includes three stages, i.e. a decrease in the ventilated cavity, washout, and reattachment. The decrease in the ventilated cavity is due to the hydrofoil speed decrease in the FV flow. Washout is the transition from fully ventilated to partially ventilated flow, and reattachment is the transition from partially ventilated to fully wetted flow. The underwater vortex structures around the surfacepiercing hydrofoil are composed of a tip vortex, an unstable vortex induced by the shear layer, and a Karman vortex caused by the vortex shedding from the trailing edge of the hydrofoil. Ventilation stability strongly depends on the re-entrant jet. When Phi (the angle between the flow direction and the closure line of the ventilated cavity) is greater than 45 degrees, the re-entrant jet impinges on the ventilated cavity's leading edge and destabilizes the ventilated cavity

LES investigation into the cavity shedding dynamics and cavitation-vortex interaction around a surface-piercing hydrofoil

Recent experiments have found that there is unstable vaporous cavitation around a surface-piercing hydrofoil at high Froude numbers and small yaw angles, and it would promote ventilation formation [R. Huang et al., "Investigations into the ventilated cavities around a surface-piercing hydrofoil at high Froude numbers, " Phys. Fluids 34, 043304 (2022)], but the cavity shedding dynamics and the mechanism of cavitation-vortex interaction are still open problems. In this paper, the unstable vaporous cavities around a surface-piercing hydrofoil are numerically investigated using the large-eddy simulation coupled with the Schnerr-Sauer cavitation model. Numerical simulations can predict the cavity features, including an aerated base cavity aft of the hydrofoil trailing edge, vaporous cavitation at the hydrofoil suction surface, and tip-vortex cavitation. A U-shaped vapor cloud shedding together with a horseshoe vortex is observed during the unsteady cavitation evolution, that is, the cavity development, cutoff, and collapse. This irregular shedding is related to the three-dimensional reentrant jet induced by the velocity reflection at the vaporous cavity closure line. Furthermore, the effects of the vaporous cavitation on the vorticity generation are attributed to vortex stretching, baroclinic torque, and vortex dilatation by using the vorticity transport equation. This study could contribute to the novel hydrofoil designs and their flow control. Published under an exclusive license by AIP Publishing

Numerical investigations into supercavitating flows and hydrodynamic characteristics of a heaving hydrofoil

This paper presents the effects of heaving motions on the hydrodynamic characteristics, supercavitating flow regimes and vortex structures for a two-dimensional (2D) supercavitating hydrofoil. The sinusoidal heaving motion of the supercavitating hydrofoil is realized by overset grid technology. The lift coefficient, drag coefficient, supercavitating flow regime and vortex structures around the supercavitating hydrofoil are analyzed and compared among different amplitudes of the heaving motion. The predicted cavities and the hydrodynamic characteristics are in good accordance with the experiments at a stationary state. The lift coefficient and drag coefficient of the heaving hydrofoil present a sinusoidal law, which is related to the effective angle of attack. The heaving motion would affect the cavity length and its thickness. The greater the heaving amplitude, the greater the difference in cavity pattern at different heaving positions. The cavity variation would affect the shear layer and thus change the vortex shedding characteristics, which are different from those at a stationary state

Dynamic mode decomposition and reconstruction of the transient propeller wake under a light loading condition

This research aims to extend our understanding of propeller wake dynamics under a light loading condition, thereby laying a foundation for design optimization and flow control of the propeller. Dynamic mode decom-position (DMD) and reconstruction are used to analyze the transient vortical wake structures obtained by large eddy simulation. The propeller wake includes stable tip and hub vortices without interacting evolution at the light loading condition, and elliptical instabilities are observed downstream of the tip vortices. DMD describes the most energetic modes and the corresponding dominant frequencies are the blade passing frequency and its multiples. The coherent structures identified via DMD are primarily associated with the ordered convection of the tip vortices and have little correlation with the hub vortices. Additionally, the propeller wake flow is reconstructed using the first four DMD modes, and the primary wake features are well restored with a maximum reconstructed error of 7.98%. This demonstrates that the flow-field reconstruction based on the DMD reduced-order model is promising for predicting the propeller wake and controlling the propeller operation

Investigations into the ventilated cavities around a surface-piercing hydrofoil at high Froude numbers

This study investigates the ventilated cavities around a surface-piercing hydrofoil, aiming to extend previous studies by an in-depth understanding of the vaporous cavity behaviors and the flow-regime transition at high Froude numbers. An experiment is carried out in a constrained-launching water tank with a vertically cantilevered hydrofoil piercing a still water surface. The cavity is recorded using high-speed photography, and flow-regime maps are summarized over a broad range of Froude number and yaw angle at different immersed aspect ratios. In addition to the well-known steady flow regimes (i.e., fully wetted flow and fully ventilated flow), an unsteady vaporous cavitating flow is revealed at a very high Froude number with a small yaw angle, which exhibits cavitation shedding dynamics behaviors, including the cavity growth, destabilization, and collapse. The transition from the fully wetted flow to the fully ventilated flow is attributed to the vapor-cavitation-induced ventilation besides the tip-vortex-induced ventilation. Vaporous cavitation promotes ventilation formation, but it has to meet the criterion that air should enter the sub-atmospheric cavity through the tip-vortex path before the cavity length reaches the maximum. Moreover, an improved lifting-line model is developed with considering the effects of free surface and finite aspect ratio. Both analytical modeling and experimental measurements reveal that the vaporous cavity length follows a power relation against the cavitation parameter. Such knowledge lays a foundation for the design optimization and control strategy of high-speed hydrofoils

Multispectral Remote Sensing Image Change Detection Based on Twin Neural Networks

Remote sensing image change detection can effectively show the change information of land surface features such as roads and buildings at different times, which plays an indispensable role in application fields such as updating building information and analyzing urban evolution. At present, multispectral remote sensing images contain more and more information, which brings new development opportunities to remote sensing image change detection. However, this information is difficult to use effectively in change detection. Therefore, a change-detection method of multispectral remote sensing images based on a Siamese neural network is proposed. The features of dual-temporal remote sensing images were extracted based on the ResNet-18 network. In order to capture the semantic information of different scales and improve the information perception and expression ability of the algorithm for the input image features, an attention module network structure is designed to further enhance the extracted feature maps. Facing the problem of false alarms in change detection, an adaptive threshold comparison loss function is designed to make the threshold more sensitive to the remote sensing images in the data set and improve the robustness of the algorithm model. Moreover, the threshold segmentation method of the measurement module is used to determine the change area to obtain a better change-detection map domain. Finally, our experimental tests show that the proposed method achieves excellent performance on the multispectral OSCD detection data sets

Numerical Investigation on the Ventilated Supercavity around a Body under Free Surface Effect

Reducing vessel resistance by using ventilated cavities has been a highly researched topic in the marine industry. There is limited literature on ventilated supercavities near the free surface, which indicates that their dynamic behavior is more complex than conventional ventilated cavities due to the effect of the free surface. This paper employs numerical simulations to study the dynamic behavior of the ventilated supercavity, taking into account the effect of the free surface. Numerical simulations can predict gas leakage behaviors, cavity geometry, and internal flow structures. The influence of the free surface shortens the length of the ventilated cavity and increases the diameter. The presence of the free surface mainly changes the vertical velocity distribution between the free surface and the cavity. The results show that there are two typical gas leakage mechanisms under different immersion depths: twin-vortex tube leakage mode and re-entrant jet leakage mode. The internal flow field of ventilated supercavity is classified into three regions: the internal boundary layer, the ventilation influence region, and the reverse flow region. As the distance between the free surface and the ventilated supercavity decreases, the ventilated supercavity is affected by both the free surface effect and the gravity effect

Numerical investigations into supercavitating flows and hydrodynamic characteristics of a heaving hydrofoil

This paper presents the effects of heaving motions on the hydrodynamic characteristics, supercavitating flow regimes and vortex structures for a two-dimensional (2D) supercavitating hydrofoil. The sinusoidal heaving motion of the supercavitating hydrofoil is realized by overset grid technology. The lift coefficient, drag coefficient, supercavitating flow regime and vortex structures around the supercavitating hydrofoil are analyzed and compared among different amplitudes of the heaving motion. The predicted cavities and the hydrodynamic characteristics are in good accordance with the experiments at a stationary state. The lift coefficient and drag coefficient of the heaving hydrofoil present a sinusoidal law, which is related to the effective angle of attack. The heaving motion would affect the cavity length and its thickness. The greater the heaving amplitude, the greater the difference in cavity pattern at different heaving positions. The cavity variation would affect the shear layer and thus change the vortex shedding characteristics, which are different from those at a stationary state