Showing papers by "A-Man Zhang published in 2017"

Smoothed particle hydrodynamics and its applications in fluid-structure interactions

Abstract: 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.

Nonlinear interaction between underwater explosion bubble and structure based on fully coupled model

Abstract: The interaction between an underwater explosion bubble and an elastic-plastic structure is a complex transient process, accompanying violent bubble collapsing, jet impact, penetration through the bubble, and large structural deformation. In the present study, the bubble dynamics are modeled using the boundary element method and the nonlinear transient structural response is modeled using the explicit finite element method. A new fully coupled 3D model is established through coupling the equations for the state variables of the fluid and structure and solving them as a set of coupled linear algebra equations. Based on the acceleration potential theory, the mutual dependence between the hydrodynamic load and the structural motion is decoupled. The pressure distribution in the flow field is calculated with the Bernoulli equation, where the partial derivative of the velocity potential in time is calculated using the boundary integral method to avoid numerical instabilities. To validate the present fully coupled model, the experiments of small-scale underwater explosion near a stiffened plate are carried out. High-speed imaging is used to capture the bubble behaviors and strain gauges are used to measure the strain response. The numerical results correspond well with the experimental data, in terms of bubble shapes and structural strain response. By both the loosely coupled model and the fully coupled model, the interaction between a bubble and a hollow spherical shell is studied. The bubble patterns vary with different parameters. When the fully coupled model and the loosely coupled model are advanced with the same time step, the error caused by the loosely coupled model becomes larger with the coupling effect becoming stronger. The fully coupled model is more stable than the loosely coupled model. Besides, the influences of the internal fluid on the dynamic response of the spherical shell are studied. At last, the case that the bubble interacts with an air-backed stiffened plate is simulated. The associated interesting physical phenomenon is obtained and expounded.

Numerical investigation of rising bubbles bursting at a free surface through a multiphase SPH model

Abstract: As a Lagrangian meshless method, smoothed particle hydrodynamics (SPH) method is robust in modelling multi-fluid flows with interface fragmentations. However, the application for the simulation of a rising bubble bursting at a fluid surface is rarely documented. In this paper, the multiphase SPH model is extended and applied to simulate this challenging phenomenon. Different numerical techniques developed in different SPH models are combined in the present SPH model. The adoption of a background pressure determined based on the surface tension can help to avoid tensile instability and interface penetrations. An accurate surface tension model is employed. This model is suitable for bubble rising problems of small scales and high density ratios. An interface sharpness force is adopted to achieve a smoother bubble surface. A suitable formula of viscous force, which is proven to be able to accurately capture the bubble splitting and small bubble detachment, is employed. Moreover, a modified prediction-correction time-stepping scheme for a better numerical stability and allows a relatively larger CFL factor is adopted. It is also worthwhile to mention that the particle shifting technique, which helps to make the particle distribute in an arrangement of lower disorder, can significantly improve the numerical accuracy. Regarding the treatment of the fluid surface, particles of lighter phase are arranged above the free surface of the denser phase to avoid the kernel truncation in the density approximation. Furthermore, this technique also allows an accurate calculation of the surface tension on the fluid surface. A number of cases of bubbly flows are presented, which confirms the capability of the present multiphase SPH model in modelling complex bubble-surface interactions with the density ratio and viscosity ratio up to 1000 and 100 respectively.

Numerical analysis of the jet stage of bubble near a solid wall using a front tracking method

Abstract: The dynamics of a toroidal bubble near a solid wall for a large part of stand-off parameters γ (γ=d/Rmax, d is the distance between the solid wall and the bubble centre at the moment of formation and Rmax is the maximum bubble radius) have been extensively studied, but some mechanics of a toroidal bubble are not completely clear, especially for the small stand-off parameters γ ≤ 0.8. In the present study, on the basis of the finite volume method, the Navier-Stokes equations with inviscid and incompressible assumption are directly solved using a staggered grid on the fixed grid. The dynamics of the toroidal bubble near the solid for different stand-off parameters (γ = 0.4, 0.6, 0.8, and 0.97, respectively) are simulated by a front tracking method. Initial conditions of numerical simulation are estimated through the Rayleigh–Plesset equation, based on the maximum size and collapse time of a spark-generated bubble. One of the numerical results is compared with a spark-generated bubble experiment, showing tha...

Dynamic characteristics of large scale spark bubbles close to different boundaries

Abstract: Experiments on the pulsation of the high-voltage electrical-spark bubbles near different boundaries are conducted by means of high-speed photography. Some intriguing details are observed clearly, such as the formation of the jet (especially the contact jet formed when a bubble is quite close to the rigid boundary) and bubble splitting. The variation of the maximum radius of the bubble, bubble period, jet tip velocity, and bubble center migration is investigated with the presence of different boundaries. In the study of the bubble period, two fitting curves are obtained from the data by the author and previous references; one is for the bubble generated beneath the free surface and the other is for the bubble generated above the rigid boundary. In the study of the maximum jet tip velocity, a possible trend line is proposed to describe the variation of the jet tip velocity with γb (the non-dimensional standoff distance from the bubble center to the rigid boundary). Finally, the critical value of γb is studi...

Experimental and numerical study on bubble-sphere interaction near a rigid wall

Abstract: This study is concerned with the interaction between a violently oscillating bubble and a movable sphere with comparable size near a rigid wall, which is an essential physical phenomenon in many applications such as cavitation, underwater explosion, ultrasonic cleaning, and biomedical treatment. Experiments are performed in a cubic water tank, and the underwater electric discharge technique (580 V DC) is employed to generate a bubble that is initiated between a rigid wall and a sphere in an axisymmetric configuration. The bubble-sphere interactions are captured using a high-speed camera operating at 52 000 frames/s. A classification of the bubble-sphere interaction is proposed, i.e., “weak,” “intermediate,” and “strong” interactions, identified with three distinct bubble shapes at the maximum volume moment. In the numerical simulations, the boundary integral method and the auxiliary function method are combined to establish a full coupling model that decouples the mutual dependence between the force and t...

Study on coupled dynamics of ship and flooding water based on experimental and SPH methods

Abstract: The present paper studies the fluid dynamics during the flooding of a damaged ship numerically and experimentally. Attention is focused on the fluid flow characteristics and the fluid-structure interactions. The Smoothed Particle Hydrodynamics (SPH) method with an improved boundary treatment is established, which is able to capture the flow behaviors effectively. Fairly good agreement is obtained between the computational and experimental results. Based on the SPH method, the simulations are carried out for the flooding of a damaged ship with different opening sizes, opening positions, and numbers of the flooding cabins. Besides, the effects of the wave are also taken into account. The fluid behaviors are described and analyzed in detail. It is found that, during the first phase of flooding, an inflow jet with a large velocity is formed, significantly influencing the inner flows and the ship responses. During the progressive flooding phase, sloshing, crushing of the free surface, wave breaking, and vortex shedding are observed which are coupled with the ship motions. In addition, some relevant conclusions are enclosed for the motion laws of the damaged ship. This work provides physical insight into the flooding of the damaged ship, which is helpful to understand the coupled dynamics of the ship and flooding water.

Experimental and numerical investigation on bubble dynamics near a free surface and a circular opening of plate

Abstract: Dynamics behavior for a submerged oscillating bubble is closely related to the physical properties of boundaries adjacent to the bubble. For an incomplete boundary, such as a ship structure broken by the shock wave from underwater explosion attack, the bubble load formed after the shock wave may cause secondary damage to it, and the jet characteristics generated during the bubble collapse are worth exploring. Based on the incompressible potential flow method, a boundary integral method with axisymmetric code is employed to investigate the jet features of the bubble under the combined action of a free surface above and a fixed plate with a centric circular opening below. For the numerical divergence caused by the expanding bubble infinitely approaching the rim of the circular opening, the bubble walls above and below the opening are, respectively, fused with the rigid wall, and the initial infinite flow field surrounding the bubble is then divided into two independent basins that are both required to be solved. Several experiments of a spark-generated bubble oscillating near a free surface and a rigid plate with a circular opening are conducted and the numerical results are validated. Following that, two typical cases with a dominant jet, respectively, directing upward and downward are analysed in detail, and some new phenomena are discovered. Additionally, another particular case that the upward and downward jet collides in the vicinity of the opening centroid is also presented. Finally, the initial bubble-free surface distance effects upon the bubble movement and the jet developments are analysed and discussed.

Study on the interactions between two identical oscillation bubbles and a free surface in a tank

Abstract: A boundary element method based on the incompressible potential flow theory is adopted to investigate the interaction between two identical oscillating bubbles and a free surface in a tank An axisymmetric numerical model is established, and certain numerical techniques are proposed to address coefficient matrix singularity and fluid-structure intersection Experiments with spark-generated bubbles in a cylindrical tank recorded by a high-speed camera are conducted, and the numerical results are validated On this basis, a typical case of bubbles interacting with a free surface in a tank with relatively small inter-bubble and bubble-free surface distances is carefully studied A crown-shaped water column at the free surface is observed both numerically and experimentally The maximum volume of the lower bubble is found to be much larger than that of the upper one The effects of the inter-bubble and bubble-wall distances on bubble dynamics and free surface motion are analyzed The results can provide a use

Dynamic buckling of stiffened plates subjected to explosion impact loads

Abstract: The dynamic buckling characteristics and criteria of a ship’s structural stiffened plate subjected to underwater explosion impact loads are investigated in this study. Using the structural deformations observed in the experiments of underwater explosions against a plated grillage model, the mode shapes of the dynamic buckling were obtained. Through the construction of a computational model of stiffened plates subjected to an underwater explosion shock wave, the impact load was theoretically calculated and transformed into a rectangular pulse. According to the different response patterns of stiffened plates under different impact loads, a dynamic buckling criterion for the stiffened plates subjected to an explosion shock wave was proposed. Additionally, the static buckling phenomenon in the stiffened plates was analysed based on the minimum excess principle. In combination with the dynamic buckling criterion, the effects of various stiffening configurations on the dynamic and static buckling loads are discussed. The calculation results show that when the equivalent rectangular pulse is 2–3 times that of the static buckling load, the responses of the stiffened plates under the original shock load and the equivalent rectangular pulse are virtually identical. The impact load amplitude is the primary influencing factor in the dynamic buckling of stiffened plates subjected to underwater explosive impact loads. The stiffened plate aspect ratio has a substantial influence on the dynamic load factor. The analytical method and results are presented, which can be used to design stiffened optimum hull structures to enhance the dynamic load carrying capacity to withstand underwater shock damage.