Journal of Hydrodynamics最新文献

The objective is to study the vortical structural behaviors of a transient pitching hydrofoil and their effects on the hydrodynamic performance. The pitching motion of the hydrofoil is set to pitch up with an almost constant rate from 5° to 15° and then back to 5°, with the Reynolds number 4.4×10⁵ and the frequency 2 Hz. The results show that the main coherent structures around the pitching hydrofoil include small-scale laminar separation bubble (LSB), large-scale second vortex (SV) and trailing edge vortex (TEV) which are all vortical. The relationship between the vortical structure and the lift is investigated with the finite-domain impulse theory. It indicates that the major part of the lift is contributed by the LSB, whereas the shedding and the formation of the SV and TEV cause the fluctuation of the lift. The proper orthogonal decomposition (POD) method is applied to capture the most energetic modes, revealing that the LSB mode occupies a large amount of energy in the flow field. The dynamic mode decomposition (DMD) method accurately extracts the dominant frequency and modal characteristics, with the first mode corresponding to the mean flow, the second mode corresponding to the LSB structure and the third and fourth modes corresponding to the vortex shedding.

Accurate modeling for highly non-linear coupling of a damaged ship with liquid sloshing in waves is still of considerable interest within the computational fluid dynamics (CFD) and AI framework. This paper describes a data-driven Stacking algorithm for fast prediction of roll motion response amplitudes in beam waves by constructing a hydrodynamics model of a damaged ship based on the dynamic overlapping grid CFD technology. The general idea is to optimize various parameters varying with four types of classical base models like multi-layer perception, support vector regression, random forest, and hist gradient boosting regression. This offers several attractive properties in terms of accuracy and efficiency by choosing the standard DTMB 5415 model with double damaged compartments for validation. It is clearly demonstrated that the predicted response amplitude operator (RAO) in the regular beam waves agrees well with the experimental data available, which verifies the accuracy of the established damaged ship hydrodynamics model. Given high-quality CFD samples, therefore, implementation of the designed Stacking algorithm with its optimal combination can predict the damaged ship roll motion amplitudes effectively and accurately (e.g., the coefficient of determination 0.9926, the average absolute error 0.0955 and CPU 3s), by comparison of four types of typical base models and their various forms. Importantly, the established Stacking algorithm provides one potential that can break through problems involving the time-consuming and low efficiency for large-scale lengthy CFD simulations.

This paper presents an adaptive grid deformation technique for optimizing ship hull forms using computational fluid dynamics (CFD). The proposed method enables accurate and smooth updates of the hull surface and 3-D CFD grids in response to design variables. This technique incorporates a two-level point-transformation approach to move the grid points by a few design points. Initially, generic B-splines are utilized to transform grid points according to the displacements of the control points within a defined control box. This ensures surface modification accuracy and smoothness, similar to those provided by non-uniform rational B-splines. Subsequently, radial basis functions are used to interpolate the movements of the control points with a limited set of design points. The developed method effectively maintains the mesh quality and simulation efficiency. By applying this method to surface and grid adaptation, a regression model is proposed in the form of a second-order polynomial to represent the relationship between the geometric parameters and design variables. This polynomial is then used to introduce geometric constraints. Furthermore, a radial basis function surrogate model for the calm-water resistance is constructed to approximate the objective function. An enhanced optimization framework is proposed for CFD–based hull optimization and applied to KVLCC2 to validate its feasibility and efficiency.

A Reynolds averaged Navier-Stokes (RANS) computational fluid dynamics (CFD) model is built to investigate the hydrodynamic response of a circular ice floe under the influence of a passing ship in calm waters. The ship, mirroring the KRISO Container Ship’s hull design, progresses near an ice floe whose diameter is 30% of the ship’s length and its thickness is 3 m. The ship advances at a constant speed, which is handled by using the overset mesh technique. This study focuses on the ice floe’s motions and the hydrodynamic forces induced by three speeds and three transverse distances of the passing ship. Findings reveal that ship-generated wakes notably influence the ice floe’s motions, with a greater influence on sway than surge. Additionally, the ship’s speed and proximity distinctly affect the ice floe’s motions.

The impulse response method is a frequently used method to calculate ship seakeeping behavior. In this paper, the restoring and Froude-Krylov calculation is conducted with constant evaluation of panel pressures as well as Gauss quadrature and an analytical integration. The applied panel grid is coarsened by an adaptive algorithm which is based on a normal vector condition. The comparison of methods is based on grid convergence studies which are followed by a verification of forces with computational fluid dynamics (CFD) results on the fixed duisburg test case in waves. Validations with experimental results in head, oblique and following waves show that all integration methods are accurate. The exact integration is numerically sensitive in some cases. Gauss quadrature is highly accurate; however, the additional effort is not beneficial since the geometrical accuracy has-stronger influence on the force amplitudes than the integration method. Adaptive grid coarsening reduces the simulation time and is accurate up to a level, where the panel length comes close the wavelength. The added resistance at the investigated Froude number of 0.05 shows higher uncertainty levels, this applies to the results of both the numerical methods and model tests.

Suspended vegetation in rivers, lakes, reservoirs and canals can change flow structure, which will in turn affect the sediment transport and cause the variation of water ecological environment. In order to study the characteristics of bend flow through suspended vegetation, three-dimensional numerical simulations are carried out by using the multi-relaxation-time lattice Boltzmann method (MRT-LBM). The drag force induced by vegetation is added in the velocity correction in the equilibrium distribution and a hybrid format combined bounce and specular reflection scheme is applied in the solid-fluid boundaries. After the validation of this model, six cases are designed to conduct the numerical simulations according to the root depth and the arrangement of vegetation. The simulated results show that the suspended vegetation can redistribute the flow structure in curved open channels. After the arrangement of suspended vegetation, the main flow moves to the side without vegetation, and the distribution of velocity tends to be balanced when vegetation is arranged on the entire cross section, the range of circulating current is reduced from the whole cross section to the local position without vegetation, however, the circulating current can still exist in the curve where the suspended vegetation enters less than half of the water depth. In addition, it can also be concluded that the suspended vegetation can affect the lateral gradient of flow velocity, and the bed shear stress in the curved channel.

This study aimed to investigate the previously unexplored effects of ice cover and submerged vegetation on flow structure. Experiments were undertaken under both open channel and ice-covered flow conditions. The bed material consisted of three non-uniform sands. The findings revealed that when vegetation patches were present on the bed and an ice cover was present, the velocity profiles exhibited a distinctive pattern with two peak values. Turbulent kinetic energy (TKE) also exhibited two peaks, one above the vegetation bending height and another at the sheath section, with a decreasing trend towards the ice cover. Furthermore, quadrant analysis showed that when the flow surface is covered by an ice cover, the contributions of inward and outward events increased compared with those observed in an open channel flow. In most cases, these contributions surpassed the sweep and ejection events. The findings enhance our understanding of vegetation’s response to diverse surface conditions and have practical implications for river management and environmental engineering.

Numerical investigations of floating platforms with different outer column inclined angles under two operating conditions of regular wave and irregular wave are presented in this paper. A coupled aero-hydrodynamic computational fluid dynamics in-house solver FOWT-UALM-SJTU is applied for the calculation. First, the validation for wave and wind generation are conducted to determine mesh distribution strategy. Based on these, the hydrodynamic motion response, aerodynamic performance and wake flow are analyzed to explore the impact of inclined angle. Conduct spectral analysis on the motion response under wave action, discuss the aerodynamic attack angle and inflow wind velocity along the blade spanwise direction in detail, reveal different trends in wake development and recovery. The results show that for the regular wave condition with the increase of inclined angles, the equilibrium position of surge motion is constantly rising, while pitch is decreasing. The maximum root mean square (rms) value occurs at angle = 30°, compared with the original OC4 FOWT, the rms in power and thrust increase 0.35%, 0.71%. And there are two low regions of attack angle and high regions of axial inflow velocity, corresponding to aerodynamic loads. The spectral analysis indicates that the natural frequency of pitch motion will increase with inclined angle. Besides, from the middle to far region of wake flow, the velocity recovery of FOWT with inclined angle will become faster, which is beneficial for downstream turbines to enhance more wind energy.

Water entry problems represent complex multiphase flows involving air, water, and structure interaction, occurring rapidly in rough seas, and potentially effecting structural integrity of floating structures. This paper experimentally investigates asymmetric slamming loads acting on a 3-D elastic wedge section. The specimen, featuring two different bottom plates (stiffened and unstiffened), each 4 mm thick, aims to assess the effect of structural stiffness on dynamic loads. The experiments are conducted at different drop heights of 25 cm and 50 cm and varying heel angles from 5° to 25°. The paper describes the experimental conditions, including wedge geometry, material properties, and the test plan. The study explores the influence of heel angle on impact acceleration, revealing an increase in peak acceleration with a higher inclination angle, particularly in the vertical direction. Additionally, the hydrodynamic pressure resulting from asymmetric slamming is presented. The pressure results analyzed and compared at different locations along the length of the wedge. The experimental findings indicate that, despite the leeward side (stiffened) experiencing a smaller hydrodynamic load, the heel angle significantly affects pressure results on the windward side (unstiffened), leading to a more pronounced dynamic response. The time history of pressure results emphasizes the effect of elastic vibrations, particularly noticeable on the unstiffened bottom plate. This study contributes to a deeper understanding of asymmetric slamming on aluminum structures, facilitating the enhancement of mathematical models and the validation of numerical simulations.

The load rejection transient process of bulb turbine units is critical to safety of hydropower stations, and determining appropriate closing laws of guide vanes (GVs) and runner blades (RBs) for this process is of significance. In this study, we proposed a procedure to optimize the co-closing law of GVs and RBs by using computational fluid dynamics (CFD), combined with the design of experiment (DOE) method, approximation model, and genetic optimization algorithm. The sensitivity of closing law parameters on the histories of head, speed, and thrust was analyzed, and a two-stage GVs’ closing law associating with a linear RBs’ closing law was proposed. The results show that GVs dominate the transient characteristics by controlling the change of discharge. Speeding GVs’ first-stage closing speed while shortening first-stage closing time can not only significantly reduce the maximum rotational speed but also suppress the water hammer pressure; slowing GVs’ second-stage closing speed is conducive to controlling the maximum reverse axial force. RBs directly affect the runner force. Slowing RBs’ closing speed can further reduce the rotational speed and the maximum reverse axial force. The safety margin of each control parameter, flow patterns, and pressure pulsations of a practical hydropower station were all improved after optimization, demonstrating the effectiveness of this method.