Pengnan Sun

Smoothed particle hydrodynamics and its applications in fluid-structure interactions

In ocean engineering, the applications are usually related to a free surface which brings so many interesting physical phenomena (e.g. water waves, impacts, splashing jets, etc.). To model these complex free surface flows is a tough and challenging task for most computational fluid dynamics (CFD) solvers which work in the Eulerian framework. As a Lagrangian and meshless method, smoothed particle hydrodynamics (SPH) offers a convenient tracking for different complex boundaries and a straightforward satisfaction for different boundary conditions. Therefore SPH is robust in modeling complex hydrodynamic problems characterized by free surface boundaries, multiphase interfaces or material discontinuities. Along with the rapid development of the SPH theory, related numerical techniques and high-performance computing technologies, SPH has not only attracted much attention in the academic community, but also gradually gained wide applications in industrial circles. This paper is dedicated to a review of the recent developments of SPH method and its typical applications in fluid-structure interactions in ocean engineering. Different numerical techniques for improving numerical accuracy, satisfying different boundary conditions, improving computational efficiency, suppressing pressure fluctuations and preventing the tensile instability, etc., are introduced. In the numerical results, various typical fluid-structure interaction problems or multiphase problems in ocean engineering are described, modeled and validated. The prospective developments of SPH in ocean engineering are also discussed.

Multi-resolution Delta-plus-SPH with tensile instability control: Towards high Reynolds number flows

Abstract It is well known that the use of SPH models in simulating flow at high Reynolds numbers is limited because of the tensile instability inception in the fluid region characterized by high vorticity and negative pressure. In order to overcome this issue, the δ + -SPH scheme is modified by implementing a Tensile Instability Control (TIC). The latter consists of switching the momentum equation to a non-conservative formulation in the unstable flow regions. The loss of conservation properties is shown to induce small errors, provided that the particle distribution is regular. The latter condition can be ensured thanks to the implementation of a Particle Shifting Technique (PST). The novel variant of the δ + -SPH is proved to be effective in preventing the onset of tensile instability. Several challenging benchmark tests involving flows past bodies at large Reynolds numbers have been used. Within this a simulation characterized by a deforming foil that resembles a fish-like swimming body is used as a practical application of the δ + -SPH model in biological fluid mechanics.

A 3-D SPH model for simulating water flooding of a damaged floating structure

With the quasi-static analysis method, the terminal floating state of a damaged ship is usually evaluated for the risk assessment. But this is not enough since the ship has the possibility to lose its stability during the transient flooding process. Therefore, an enhanced smoothed particle hydrodynamics (SPH) model is applied in this paper to investigate the response of a simplified cabin model under the condition of the transient water flooding. The enhanced SPH model is presented firstly including the governing equations, the diffusive terms, the boundary implementations and then an algorithm regarding the coupling motions of six degrees of freedom (6-DOF) between the structure and the fluid is described. In the numerical results, a non-damaged cabin floating under the rest condition is simulated. It is shown that a stable floating state can be reached and maintained by using the present SPH scheme. After that, three-dimensional (3-D) test cases of the damaged cabin with a hole at different locations are simulated. A series of model tests are also carried out for the validation. Fairly good agreements are achieved between the numerical results and the experimental data. Relevant conclusions are drawn with respect to the mechanism of the responses of the damaged cabin model under water flooding conditions.

Investigation on charge parameters of underwater contact explosion based on axisymmetric SPH method

This paper investigates the effects of charge parameters of the underwater contact explosion based on the axisymmetric smoothed particle hydrodynamics (SPH) method. The dynamic boundary particle is proposed to improve the pressure fluctuation and numerical accuracy near the symmetric axis. An in-depth study is carried out over the influence of charge shapes and detonation modes on the near-field loads in terms of the peak pressure and impulse of shock waves. For different charge shapes, the cylindrical charge with different length-diameter ratios may cause strong directivity of peak pressure and impulse in the near field. Compared with spherical charge, the peak pressure of cylindrical charge may be either weakened or enhanced in different directions. Within a certain range, the greater the length-diameter ratio is, the more obvious the effect will be. The weakened ratio near the detonation end may reach 25% approximately, while the enhanced ratio may reach around 20% in the opposite direction. However, the impulse in different directions seems to be uniform. For different detonation modes, compared with point-source explosion, the peak pressure of plane-source explosion is enhanced by about 5%. Besides, the impulse of plane-source explosion is enhanced by around 5% near the detonation end, but close to those of the point-source explosion in other directions. Based on the material constitutive relation in the axisymmetric coordinates, a simple case of underwater contact explosion is simulated to verify the above conclusions, showing that the charge parameters of underwater contact explosion should not be ignored.

Investigation of Coalescing and Bouncing of Rising Bubbles Under the Wake Influences Using SPH Method

In the exploiting and processing of submarine energy, such as natural gas, petroleum and combustible ice, it is always accompanied with multi-phase flow of large density and viscosity ratio, like bubbly flows. The essential mentioned subjects are bubbles rising in the viscous medium, the coupling effects among rising bubbles. In this paper, SPH method is used to simulate the interaction between two bubbles, where the focusing problem is the interface between gas and liquid. The multiphase flow characteristics are greatly influenced by surface tension and viscous force especially when the characteristic length scales are relatively small. As many experiments in previous literatures indicate, the rising bubbles are often followed by a long tail which greatly affects the shape and motion path of a single bubble and bubble groups. Though Boundary Element Method (BEM) may be well used to simulate the movement and deformation of a single bubble, there are still many challenges in simulating the bubble interactions like coalescing and bouncing. The traditional Smoothed Particle Hydrodynamics (SPH) method was well employed in simulating moving boundary and large deformation problems in single-phase problems, but in the ocean engineering, the density and viscosity ratio at the gas-liquid interface may be up to nearly 1000 and 100 respectively, which will always cause unphysical penetrations and pressure fluctuations at the gas-liquid interface. The present improved SPH algorithm based on volume approximation can guarantee the continuous conditions at the gas-liquid interface. Through a staggered particle distribution and an appropriate re-mesh, the process of rising, pulsing and jet of a single bubble is simulated, which agree well with that of experiments in the existing literatures. Besides, the trails of the rising bubble and interactions among bubbles are studied. On these bases, the coalescing and bouncing of two bubbles posited at different directions are simulated, which are consistent well with the experiment carried out in previous literatures. The present studies aims to provide a reference for the industrialized productions.Copyright