Sangmook Shin

Numerical Simulation of a Viscous Flow Field Around a Deforming Foil Using the Hybrid Cartesian/Immersed Boundary Method

A code is developed to simulate a viscous flow field around a deformable body using the hybrid Cartesian/immersed boundary method. In this method, the immersed boundary(IB) nodes are defined near the body boundary then velocities at the IB nodes are reconstructed based on the interpolation along the normal direction to the body surface. A new method is suggested to define the IB nodes so that a closed fluid domain is guaranteed by a set of IB nodes and the method is applicable to a zero-thickness body such as a sail. To validate the developed code, the vorticity fields are compared with other recent calculations where a cylinder orbits and moves into its own wake. It is shown the code can handle a sharp trailing edge at Reynolds number of under moderate requirements on girds. Finally the developed code is applied to simulate the vortex shedding behind a deforming foil with flapping tail like a fish. It is shown that the acceleration of fluids near the flapping tail contributes to the generation of the thrust for propulsion.

Simulation of water entry of an elastic wedge using the FDS scheme and HCIB method

The hydroelasticity of water entry of an elastic wedge is simulated using a code developed using the flux-difference splitting scheme for immiscible and incompressible fluids and the hybrid Cartesian/immersed boundary method. The free surface is regarded as a moving contact discontinuity and is captured without any additional treatment along the interface. Immersed boundary nodes are distributed inside a fluid domain based on the edges that cross an instantaneous body boundary. Dependent variables are reconstructed at each immersed boundary node with the help of an interpolation along a local normal line for providing a boundary condition for a discretized flow problem. A dynamic beam equation is used for modeling the elastic deformation of a wedge. The developed code is validated through comparisons with other experimental and computational results for a free-falling wedge. The effects of the elastic deformation of the wedge on the pressure fields and time histories of the impact force are investigated in relation to the stiffness and density of the structure. Grid independence test is carried out for the computed time history of the force acting on an elastic wedge.

Numerical Simulation of Flow Field Around a Rotating Flexible Foil Using the 3D HCIB Method

A hybrid Cartesian/immersed boundary code is expanded to simulate flow field around a three-dimensional body which undergoes large dynamic deformation. Immersed boundary nodes are automatically distributed based on the edges crossing triangles on body boundary. Velocity vectors are reconstructed at those immersed boundary nodes along local normal lines to the boundary. The reconstruction of pressure is avoided using the hybrid staggered/non-staggered grid method. The developed code is validated through comparisons with other results on laminar flow over a sphere. The code is applied to simulate flow around a foil which is attached to a body of revolution and rotates under periodic deformation. The periodic variation of the tip vortex is observed and the effects of the deformation on hydrodynamic force acting on the body are investigated.

Computation of the Green Water Design Impact Loads Acting on the Box-Type Structure of a High-Speed Ship`s Bow

선박이 황천 중에서 운항할 때 선수부는 수면과의 과도한 상 하상대운동으로 큰 충격하중을 받게 된다. 대표적인 선수 충격현 상으로 슬래밍 충격현상과 green water 충격현상을 들 수 있다. 선수부가 파면에 돌입할 때 일차적으로 발생하는 슬래밍 과정에 서는 white water라 불리는 흰색 물보라가 발생하며, 선수 상대 운동이 매우 커서 선수갑판이 파면 아래로 내려갈 정도가 되면 흰색 물보라 하부에 해수 덩어리(green water라 불림)가 갑판 둘 레에서 순간적으로 솟아올라 일시에 수벽을 형성한다. 이 해수 덩어리 즉 green water는 곧 높은 속도로 선수갑판에 무너져 들 어오며, 이 과정에서 갑판 위 구조물과 선수갑판 자체에 큰 충격 을 가한다. 이러한 현상을 green water 충격현상이라 부른다. Green water와 관련한 다양한 연구들 (Mizoguchi, 1988; Buchner, 1995, 1996; Kim & Kim, 1996; Pekken, et al., 1999; Kim & Shin, 2005; Pham & Varyani, 2005; Jeong, et al., 2010; Lee, et al., 2012; Ha, et al., 2012; Jeong, et al., 2013)이 국내외에서 수행되어 왔으며, 이러한 연구들은 주로 가 정된 규칙파 또는 특정 해수유입조건에 의해 유발되는 green water 유동과 하중의 해석에 대한 연구들이다. Green water와 관 련하여 유체공학자들이 감당해야 하는 핵심과제 중의 하나는 green water 충격에 대비하여 구조강도 설계를 할 때 필요한 설 계하중을 제공하는 것이며, 이 과제는 주어진 파나 조건에 대한 green water 계산법만으로는 해결할 수 없다. 설계용 geen water 하중을 산출하기 위해서는 선박이 어떠한 해상상태 및 운항상태 에서 최대 green water 충격을 받게 되는지부터 검토해야 하고, 또 대개의 연구들이 구축하고자 하는 규칙파 중 green water 계 산법을 실제해상인 불규칙파 중에서의 green water 문제에 어떻 게 적용할 것인지도 기술적 측면에서 풀어야 할 중요과제가 된 다. 본 연구에서는 구조강도 설계에 필요한 green water 설계 충 격하중의 산출과정을 설계규칙파 산정과정을 포함하여 독자적으 고속선박의 선수부 상자형 구조물에 작용하는 Green

Fluid-Structure Interaction Analysis for Structure in Viscous Flow

AbstractTo calculate the fluid-structure interaction(FSI) problem rationally, it should be the basic technology to analyse each domain of fluid and structure accurately. In this paper, a new FSI analysis algorithm was introduced using the 3D solid finite element for structural analysis and CFD code based on the HCIB method for viscous flow analysis. The fluid and structural domain were analysed successively and alternatively in time domain. The structural domain was analysed by the Newmark-b direct time integration sc heme using the pressure field calculated by the CFD code. The results for example calcu lation were compared with other research and it was shown that those coincided each other. So we can conclude that the developed algorithm can be applied to the general FSI probl ems. ※Keywords: FSI(Fluid-Structure Interaction), 3D FEM, CFD, HCIB method(Hybrid Cartesian/ Immersed Boundary 법), Newmark-β method 1. 서언 수중 혹은 공기 중에서 운동하는 구조체 또는 슬로싱, 슬래밍과 같은 유제 충격에 의한 선체의 동적 거동을 정확히 평가하여 설계에 반영하기 위한 노력은 오래 전부터 지속되어 왔다. 이러한 구조와 유체의 상호작용에 관한 문제는 초기에는 구

Compressible Two-Phase Flow Computations Using One-Dimensional ALE Godunov Method

Compressible two-phase flow is analyzed based on the arbitrary Lagrangian-Eulerian (ALE) formulation. For water, Tamman type stiffened equation of state is used. Numerical fluxes are calculated using the ALE two-phase Godunov scheme which assumes only that the speed of sound and pressure can be provided whenever density and internal energy are given. Effects of the approximations of a material interface speed are Investigated h method Is suggested to assign a rigid body boundary condition effectively To validate the developed code, several well-known problems are calculated and the results are compared with analytic or other numerical solutions including a single material Sod shock tube problem and a gas/water shock tube problem The code is applied to analyze the refraction and transmission of shock waves which are impacting on a water-gas interface from gas or water medium.