A method of automatic image segmentation for gas bubbles in water

This paper presents an algorithm of automatic bubble image segmentation using the improved ant colony optimization methodology. The ant colony optimization method is a metaheuristic algorithm, and has been applied in many fields. To reveal the versatility and appropriateness of automatic bubble image segmentation, the fuzzy clustering analysis method is employed in ant colony optimization algorithm. Compared with the well-known image feature extraction operators such as SUSAN and Canny, the proposed method can comparatively suitable to extract the gas bubbles image edge features. The experimental results show that the proposed method is effective and reliable, and can achieve satisfactory image edge extraction effect.

Effect of residual stress on the vibration failure of dissimilar steels joint

Elasto-plastic finite element method is used for the welding residual stress calculation in butt welded joint of carbon steel and stainless steel, fluctuating pressure on the ship hull induced by a propeller is calculated from the actual measurement data. As the result of the superposition of vibration load and welding residual stress, the stress amplitude is reduced slightly in high stress area near the welding line, while the value of stress reduction becomes larger when the welding current increases. On the contrary, the stress amplitude is increased clearly far from the welding line. The welding residual stress obviously reduced the fatigue life of ship structure with the vibration load. The fatigue lives of ship structures under vibration are - 1.75 × 105 （ 110 000 ton product carrier） and - 2.43 × 105 （52 000 ton all-purpose cargo ship） without welding residual stress, while the residual stress exists, the fatigue life is down to - 3 × 104.

Numerical research on slamming characteristics of aircraft landing on water

In order to investigate the impact characteristic of an aircraft landing on water,a computational fluid dynamics(CFD)simulation is conducted to explore the slamming characteristics of the NACA TN 2929 A model.The flow around the model is solved by using Reynolds-averaged Navier-Stokes equations with the shear stress transport(SST)k—ωturbulence model,based on finite volume method(FVM).The free surface is captured by using the volume of fluid(VOF)method,and the aircraft impact process is realized with help of overset mesh technology.Then,the effects of horizontal and vertical velocities and initial pitch angle on the slamming load,attitude change,impact pressure and flow field evolution are investigated.The results reveal that the horizontal velocity has a considerable influence on whether the aircraft’s horizontal tail hits the water,and further affects the maximum vertical load as well as the maximum pitch angle throughout the impact process.The vertical velocity determines the slamming load before the horizontal tail strikes the water,while the horizontal velocity has a significant effect on the load after the horizontal tail hits the water.A smaller initial pitch angle results in not only a heavier slamming load but also a more dramatic change of the posture after the aircraft impacts the water.The impact pressure of the aircraft is maximized at the junction of the approaching surface of the fuselage and the free surface.In some cases,the pressure is also concentrated on the undersurface of the horizontal tail.

Controllability and observability measures of structural vibration and acoustic radiation

This paper studies the controllability and observability measures of structural vi- bration and acoustic radiation and proposes a measure of modal observability based on acoustic pressure outputs and measures of controllability and observability of acoustic radiation by combining the measures of modal controllability and observability of vibration modes with corresponding modal acoustic radiation efficiencies. A plate with control force inputs and vibrational response outputs and acoustic pressure outputs is involved to show the modal controllability and observability measures of vibration modes and measures of controllability and observability of structural vibration and acoustic radiation. Observability measures based on vibrational response outputs and acoustic pressure outputs are compared and the emphasis is on the measures based on the acoustic pressure outputs. Finally the application and usefulness of the controllability and observability measures to active control of structural vibration and acoustic radiation are illustrated and compared. It shows that the controllability and observability measures provide useful guidance for active control design especially in optimal actuator and sensor placement.

基于浸没边界-多相格子玻尔兹曼通量法的圆柱体出入水时自由液面形态的数值模拟

In this work,the deformation of free interface during water entry and exit of a circular cylinder is investigated numerically by using the two-dimensional(2D)immersed boundary-multiphase lattice Boltzmann flux solver(IB-MLBFS).The fluid domain is discretized by finite volume discretization,and the flux on the grid interface is evaluated by lattice Boltzmann equations.Both the implicit velocity correction and the surface flux correction are implemented by using the immersed boundary-method to consider the fluid-structure interaction and the contact interface between the multiphase fluids and the structure.First,the water entry of a circular cylinder is simulated and the results are compared with the experiment,which considered the length-diameter ratio of the circular cylinder.The reliability of 2D simulation is verified and the deformation of the free interface is well investigated.Afterward,the water exit of a circular cylinder with constant velocity is simulated,which is less researched.In addition,the results show the advantage of present IB-MLBFS to some extent.Finally,the water exit and re-entry of a circular cylinder are presented,and the results present the complex deformation of the free interface and the dynamic response of the moving structure.Based on the numerical results,the free interface of the multiphase fluids is well captured,and the contact interface on the boundary of the moving structure is accurately presented by the IB-MLBFS.

Numerical investigation and its application on the falling motion of freefall lifeboat

In order to improve the simulation accuracy for free-fall lifeboat in ship life-saving training system,this paper analyzes and models the motion of boat’s launching from the skid.The whole launching is divided into four phases,namely:sliding down,rotation,free fall and water entry.According to the theory of momentum and strip theory,hydrodynamic forces of the boat at water entry are calculated under the effect of waves.The method of interpolation is used for calculating the half width and added mass of cross-sections at water entry.The model is used for numerical investigation about the boat launching from skid under different conditions and applied to ship life-saving simulation training system.The following conclusions are finally obtained:(1)When the initial inclination angle is 30◦,the horizontal distance between the point of water entry of the boat and the lower end of the slide is about 7.2 m.The horizontal distance will be smaller,when the initial inclination angle increases.There is no obvious law between forward distance and waves.(2)When the initial inclination angle is 45◦,the setback may occur after the boat entering the wave.When the initial angle is 60◦,the setback occurs after the boat entering the water.(3)When the center of gravity is 1.5 m in front of the midship of the boat,the boat will turn over.

A sharp-interface immersed smoothed point interpolation method with improved mass conservation for fluid-structure interaction problems

To solve the problem of inaccurate boundary identification and to eliminate the spurious pressure oscillation in the previously developed immersed smoothed point interpolation method(IS-PIM),a new sharp-interface IS-PIM combining mass conservation algorithm,called Sharp-ISPIM-Mass,is proposed in this work.Based on the so called sharp-interface method,the technique of quadratic local velocity reconstruction has been developed by combining with the mass conservation algorithm,which enables the present method improve the accuracy of the velocity field and satisfy the mass conservation condition near the boundary field.So the proposed method would not encounter the problem of spurious mass flux.In addition,a new form of FSI force evaluation considering pressure and viscous force to perform a whole function from the fluid domain to fictitious fluid domain is introduced,which makes the present method obtain more accurate results of FSI force than the original one.Through the numerical studies of a number of benchmark examples,the performance of the Sharp-ISPIM-Mass has been examined and illustrated.