Journal of Hydrodynamics最新文献

Accurate estimation of the drag forces generated by vegetation stems is crucial for the comprehensive assessment of the impact of aquatic vegetation on hydrodynamic processes in aquatic environments. The coupling relationship between vegetation layer flow velocity and vegetation drag makes precise prediction of submerged vegetation drag forces particularly challenging. The present study utilized published data on submerged vegetation drag force measurements and employed a genetic programming (GP) algorithm, a machine learning technique, to establish the connection between submerged vegetation drag forces and flow and vegetation parameters. When using the bulk velocity, U, as the reference velocity scale to define the drag coefficient, C_d, and stem Reynolds number, the GP runs revealed that the drag coefficient of submerged vegetation is related to submergence ratio (H*), aspect ratio (d*), blockage ratio (ψ*), and vegetation density (λ). The relation between vegetation stem drag forces and flow velocity is implicitly embedded in the definition of C_d. Comparisons with experimental drag force measurements indicate that using the bulk velocity as the reference velocity, as opposed to using the vegetation layer average velocity, U_v, eliminates the need for complex iterative processes to estimate U_v and avoids introducing additional errors associated with U_v estimation. This approach significantly enhances the model’s predictive capabilities and results in a simpler and more user-friendly formula expression.

The objective of this paper is to investigate the turbulent flow structures around the submarine model and evaluate the effect of the yaw angle on the turbulent flow characteristics. The large eddy simulation based on the boundary data immersion method is used to investigate. The computational domain consists of 1.2×10⁸ uniformly distributed Cartesian orthogonal grid nodes to capture the basic flow characteristics around the model. The pressure coefficient, friction coefficient and wake velocity distribution are in good agreement with the experimental data. Three different types of vortex structures were mainly captured around the model, including horseshoe vortex, sail tip vortex and crossflow separation vortex. With the increase of the yaw angle, the asymmetry of the horseshoe vortex and the tip vortex gradually increases, and the vortex strength of the vortex leg on the windward of the horseshoe vortex and the vortex strength of the tip vortex also increase gradually. For the crossflow separation vortex, the flow separation zone gradually expands and migrates downstream with the increase of the yaw angle.

In this paper, the coupled sloshing and motion characteristics of a cylindrical floating production storage offloading (CFPSO) are numerically investigated by means of computational fluid dynamics (CFD) tool. The simulations are performed using an in-house CFD solver naoe-FOAM-SJTU which is developed based on OpenFOAM. The active wave generating-absorbing boundary condition (GABC) is utilized for wave generation and absorption. The stabilized k-omega SST turbulence model are used to avoid excessive eddy viscosity near the free surface. CFPSO with and without partially filled liquid tanks in regular waves with different wave periods are simulated and vertical planar motions such as surge, heave and pitch response amplitude operators (RAOs) are compared. Forces due to liquid sloshing and wave loads are extracted and analyzed. The free surface motions inside liquid tanks in one wave period presented to explain the motion characteristics.

Plunging breaking waves play an important role in the exchange of heat, momentum, and mass between the atmosphere and ocean. In this paper, a series of direct numerical simulations is conducted to investigate the fragmentation process of the ingested main cavity in plunging breaking waves. The two-phase Navier-Stokes equations are solved using the finite-volume method based on adaptive refinement meshes. The free surface is captured using a geometrical volume of fluid method. Both 2-D, 3-D simulations are conducted. Instantaneous flow fields at different stages of wave breaking are presented and quantitative analysis for bubbles is performed. The 2-D instantaneous vorticity field and local velocity field are visualized to discuss the general flow characteristics during the fragmentation process. Then a 2-D parametric study is conducted to investigate the differences in the flow characteristics during the fragmentation process under different wave parameters including initial wave steepness (ε), Bond number (Bo), and Reynolds number (Re). 3-D vortex structures are shown to further investigate the mechanisms behind the differences in the flow characteristics. The bubble size distributions under two different initial wave steepness are also discussed with their relationship to the fragmentation process of the ingested main cavity. This research offers a significant understanding of the distinct procedures and fundamental dynamics involved in wave breaking, enhancing our comprehension of this intricate event.

The wake induced vibration (WIV) of a one- and two-degree-of-freedom (1DOF, 2DOF) downstream wave-cone cylinder (WCC) behind a stationary equal-size upstream wave-cone cylinder in the staggered arrangement is numerically investigated at subcritical Reynolds number of 3 900 by using shear stressed transfer (SST) k - ω turbulence model. The streamwise pitch ratios (P / D_m) vary from 4 to 6 with a fixed incident angle α = 8°. Experimental measurements were also performed for the validation of the present numerical models. It is found that the largest vibration amplitude in crossflow direction occurred at P / D_m = 4, U_r =8 with small difference of streamwise vibration at P / D_m = 4, 6. Different from single wavy-cone cylinder (SWCC), the downstream flexible one of a pair staggered WCCs got larger vibration amplitude during phase switching stage instead of in-phase stage. The upstream wake will suppress the triple frequency of main frequency in the power spectra density (PSD) functions of Cl but stimulate the double one of that. An intriguing vibration mechanism happened in all 2DOF cases where the trajectory of downstream WCC is a significant ellipse rather than a figure of 8. The transformation of phase switching and the variation of the main frequency of drag coefficient (Cd) can be explained by the vortex-shedding modes of downstream WCC

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.