Zhi Zong

NUMERICAL PREDICTION OF FATIGUE DAMAGE IN STEEL CATENARY RISER DUE TO VORTEX-INDUCED VIBRATION

For studying the characteristics of Steel Catenary Riser (SCR), a simplified pinned-pinned cable model of vibration is established. The natural frequencies, the normalized mode shapes and mode curvatures of the Scr are calculated. The fatigue damage of the SCR can be obtained by applying the modal superposition method combined with the parameters of S - N curve. For analyzing the relation between the current velocity and the SCR’s fatigue damage induced by the vortex-induced vibration, ten different current states are evaluated. Then, some useful conclusions are drawn, especially an important phenomenon is revealed that the maximum fatigue damage in the riser usually occurs near the area of the boundary ends.

Smoothed Point Interpolation Method for Elastoplastic Analysis

This work formulates the node-based smoothed radial point interpolation method (NS-RPIM), a typical model of smoothed point interpolation method, for the elastoplastic analysis of two-dimensional solids with gradient-dependent plasticity. The NS-RPIM uses radial point interpolation shape functions for field approximation and node-based gradient smoothing for strain field construction. The formulation is based on the parametric variational principle (PVP) in the form of complementarity with the gradient-dependent plasticity being represented by means of the linearization of the yield criterion and the flow rule. Numerical study results have demonstrated the accuracy and stability of the proposed approach for elastoplastic analysis.

Effect of Surface Roughness on Vortex-Induced Vibration Response of a Circular Cylinder

ABSTRACT The effects of surface roughness on the vortex-induced vibration (VIV) characteristics of a circular cylinder were studied numerically. The response amplitude, vortex shedding frequency, structural vibration frequency, lock-in region, vortex shedding flow pattern and hydrodynamic coefficient with different degrees of surface roughness were compared. The numerical results show that the reduced velocity could be divided into four parts: initial branch, upper branch, lower branch and desynchronisation region based on the VIV response amplitude and frequency. During initial branch, the obvious pure lock-in phenomenon can be found; however, in upper and lower branches, dual lock-in is detected. Rough cylinders have a smaller VIV displacement and a narrower lock-in region than a smooth cylinder. However, the vortex shedding flow pattern has not affected by the surface roughness. As surface roughness increases, the Strouhal number tends to increase; however, the mean drag coefficient has a decreasing tendency.

Numerical study of advection schemes for interface-capturing using gradient smoothing method

Abstract Based on the newly developed gradient smoothing method (GSM), three interface-capturing schemes have been implementing using unstructured mesh. The volume of fluid (VOF) model is solved without explicitly interface reconstructing in the framework of GSM. The variables on upwind points are successfully approximated using centroid GSM (cGSM) scheme, which generally cannot be clearly defined on unstructured mesh. Compressive interface-capturing scheme for arbitrary meshes (CICSAM), flux-blending interface-capturing scheme (FBICS), and cubic upwind interpolation-based blending scheme (CUIBS) have been employed through three benchmark cases. The results indicate that FBICS and CUIBS can produce more accurate predictions using unstructured meshes.

Numerical investigation of unsteady sheet/cloud cavitation over a hydrofoil in thermo-sensitive fluid

The sheet/cloud cavitation is of a great practical interest since the highly unsteady feature involves significant fluctuations around the body where the cavitation occurs. Moreover, the cavitating flows are complicated due to the thermal effects. The present paper numerically studies the unsteady cavitating flows around a NACA0015 hydrofoil in the fluoreketone and the liquid nitrogen with particular emphasis on the thermal effects and the dynamic evolution. The numerical results and the experimental measurements are generally in agreement. It is shown that the temperature distributions are closely related to the cavity evolution. Meanwhile, the temperature drop is more evident in the liquid nitrogen for the same cavitation number, and the thermal effect suppresses the occurrence and the development of the cavitating flow, especially at a low temperature in the fluoroketone. Furthermore, the cavitating flows are closely related to the complicated vortex structures. The distributions of the pressure around the hydrofoil is a major factor of triggering the unsteady sheet/cloud cavitation. At last, it is interesting to find that one sees a significant thermal effect on the cavitation transition, a small value of σ /2α is required in the thermo-sensitive fluids to achieve the similar cavitation transition that occurs in the water.

Numerical study of spike characteristics due to the motions of a non-spherical rebounding bubble

The boundary integral method (BIM) is used to simulate the 3-D gas bubble, generated within the two bubble pulsation periods in proximity to a free surface in an inviscid, incompressible and irrotational flow. The present method is well validated by comparing the calculated shapes of the bubble and the free surface with both the experimental results and the numerical ones obtained by the Axisymmetric BIM code. The expansion, the collapse of the gas bubble and the further evolution of the rebounding non-spherical bubble are simulated. The various variation patterns of the free surface spike and the bubble centroid for different standoff distances, the buoyancy parameters and the strength parameters are obtained to reveal the nonlinear interaction between the bubble and the free surface. The amplitude of the second maximum bubble volume and the four typical patterns of the bubble jet and the free surface spike are examined in the context of the standoff distance. The large buoyancy is used to elevate the spray dome rather than the free surface spike.

A Combination of Singular Cell-Based Smoothed Radial Point Interpolation Method and FEM in Solving Fracture Problem

The singular cell-based smoothed radial point interpolation method (CS-RPIM) has been previously proposed and shown good performance in solving fracture problems. Motivated from the fact that CS-RPIM performs over softly by providing an upper bound solution and the finite element method (FEM) is overly stiff by providing a lower bound solution, this work proposes a combination of singular CS-RPIM and FEM with a correlation coefficient α, and α = 0.97 has been recommended through intensive numerical studies. Several numerical examples have been studied and the proposed method has been found perform quite well from both stress intensity factors and strain energy.

A cell-based smoothed point interpolation method (CS-PIM) for 2D thermoelastic problems

Purpose n n n n nDue to the strong reliance on element quality, there exist some inherent shortcomings of the traditional finite element method (FEM). The model of FEM behaves overly stiff, and the solutions of automated generated linear elements are generally of poor accuracy about especially gradient results. The proposed cell-based smoothed point interpolation method (CS-PIM) aims to improve the results accuracy of the thermoelastic problems via properly softening the overly-stiff stiffness. n n n n nDesign/methodology/approach n n n n nThis novel approach is based on the newly developed G space and weakened weak (w2) formulation, and of which shape functions are created using the point interpolation method and the cell-based gradient smoothing operation is conducted based on the linear triangular background cells. n n n n nFindings n n n n nOwing to the property of softened stiffness, the present method can generally achieve better accuracy and higher convergence results (especially for the temperature gradient and thermal stress solutions) than the FEM does by using the simplest linear triangular background cells, which has been examined by extensive numerical studies. n n n n nPractical implications n n n n nThe CS-PIM is capable of producing more accurate results of temperature gradients as well as thermal stresses with the automated generated and unstructured background cells, which make it a better candidate for solving practical thermoelastic problems. n n n n nOriginality/value n n n n nIt is the first time that the novel CS-PIM was further developed for solving thermoelastic problems, which shows its tremendous potential for practical implications.

Simulating Ice-Structure Interaction With the Material Point Method

The process of ice-structure interaction is a complex problem which is influenced by the properties of both ice and the structure. In this paper, the material point method (MPM) is introduced to simulate the interaction between an ice sheet and a cylinder structure. MPM is efficient in solving history dependent and large deformation problems and has shown advantage in hyper-velocity impact and landslide issues, etc..The constitutive relation of ice is based on elasto-viscous-plastic model with the Drucker-Pragers yield criterion. Ice follows the Maxwell elasto-viscous model before the yield criterion is reached and fails when the plastic strain surpasses the failure strain. Meanwhile, the constitutive model used in this work considers the effect of the Young’s modulus, Poisson’s ratio, density, temperature, cohesive force and internal friction angle of ice.A series of simulations are conducted and the results are in accord with existing theories. According to the comparison, the influences of ice temperature and penetration speed of the structure on the global ice load are testified. The numerical tests have proven the feasibility of MPM in simulating the interaction between an ice sheet and a cylinder structure. Future work in ice-structure interaction problems with MPM is also discussed.Copyright

Ship hull optimization based on wave resistance using wavelet method

The ship hull surface optimization based on the wave resistance is an important issue in the ship engineering industry. The wavelet method may provide a convenient tool for the surface hull optimization. As a preliminary study, we use the wavelet method to optimize the hull surface based on the Michel wave resistance for a Wigley model in this paper. Firstly, we express the model’ s surface by the wavelet decomposition expressions and obtain a reconstructed surface and then validate its accuracy. Secondly, we rewrite the Michel wave resistance formula in the wavelet bases, resulting in a simple formula containing only the ship hull surface’ s wavelet coefficients. Thirdly, we take these wavelet coefficients as optimization variables, and analyze the main wave resistance distribution in terms of scales and locations, to reduce the number of optimization variables. Finally, we obtain the optimal hull surface of the Wigley model through genetic algorithms, reducing the wave resistance almost by a half. It is shown that the wavelet method may provide a new approach for the hull optimization.