Hyung Taek Ahn

Multi-material interface reconstruction on generalized polyhedral meshes

We describe multi-material (more than two materials) interface reconstruction methods for 3D meshes of generalized polyhedra. The basic information used in interface reconstruction is the volume fraction of each material in mixed cells, that is, those containing multiple materials. All methods subdivide a mixed cell into a set of pure non-overlapping sub-cells, each containing just one material that have the reference volume fraction. We describe three methods. The first two methods represent an extension of standard piece-wise linear interface construction (PLIC) methods to 3D and use information only about volume fractions. The first method is first-order accurate and is based on the discrete gradient of the volume fraction as an estimate of the normal to the interface. The second method is planarity-preserving (second-order accurate) and is an extension to 3D of the least squares volume-of-fluid interface reconstruction algorithm (LVIRA, see E. Puckett, A volume-of-fluid interface tracking algorithm with applications to computing shock wave refraction, in: H. Dwyer (Ed.), Proceedings of the Fourth International Symposium on Computational Fluid Dynamics, 1991, pp. 933-938; J.E. Pilliod, E.G. Puckett, Second-order accurate volume-of-fluid algorithms for tracking material interfaces, Journal of Computational Physics 199 (2004) 465-502] for the 2D case). The third method is an extension to 3D of the so-called moment-of-fluid (MoF) method V. Dyadechko, M. Shashkov, Moment-of-fluid interface reconstruction, Tech. Rep. LA-UR-05-7571, Los Alamos National Laboratory, 2005. Also available as http://cnls.lanl.gov/~shashkov/; V. Dyadechko, M. Shashkov, Multi-material interface reconstruction from the moment data, Tech. Rep. LA-UR-06-5846, Los Alamos National Laboratory, 2006. Also available as http://cnls.lanl.gov/~shashkov/]. The MoF method is also second-order accurate. This method uses information not only about volume fractions but also about the position of the centroids of each material. In contrast to standard PLIC methods, the MoF method uses only information from the cell where reconstruction is performed; no information from neighboring cells is needed. Also, the MoF method provides automatic ordering of the materials during interface reconstruction. Optimal ordering is based on comparing the positions of the reference centroids and actual centroids of the reconstructed pure sub-cells. The performance of the methods is demonstrated with numerical examples.

Strongly coupled flow/structure interactions with a geometrically conservative ALE scheme on general hybrid meshes

A geometrically conservative finite-volume arbitrary Lagrangian-Eulerian (ALE) scheme is presented with general hybrid meshes. A moving mesh source term is derived from the geometric conservation law and physical conservation laws on arbitrarily moving meshes. The significance and effectiveness of the moving mesh source term regarding uniform flow preservation is demonstrated and also compared to a different ALE formulation without such a source term. The temporal accuracy of the current ALE scheme does not deteriorate with the use of moving meshes. The applicability of the presented ALE scheme is demonstrated by simulating vortex-induced vibrations (VIV) of a cylinder. Two different flow-structure coupling strategies, namely weak and strong, are employed and compared. The proposed strong coupling is implemented with a predictor-corrector method, and its superior stability and time accuracy over weak coupling schemes is demonstrated. The present scheme can employ general hybrid meshes consisting of four different types of elements (hexahedra, prisms, tetrahedra and pyramids) and yields good agreement with other computational and experimental results.

Adaptive moment-of-fluid method

A novel adaptive mesh refinement (AMR) strategy based on the moment-of-fluid (MOF) method for volume-tracking of evolving interfaces is presented. Moment-of-fluid method is a new interface reconstruction and volume advection method using volume fractions as well as material centroids. The mesh refinement criterion is based on the deviation of the actual centroid obtained by interface reconstruction from the reference centroid given by moment advection process. The centroid error indicator detects not only high curvature regions but also regions with complicated subcell structures like filaments. A new Lagrange+remap scheme is presented for advecting moments, which includes Lagrangian backtracking, polygon intersection-based remapping and forward tracking to define the material centroid. The effectiveness and efficiency of AMR-MOF method is demonstrated with classical test problems, such as Zalesaks disk and reversible vortex problem. The comparison with previously published results for these problems shows the superior accuracy of the AMR-MOF method over other methods. In addition, two new test cases with severe deformation rates are introduced, namely droplet deformation and S-shape deformation problems, for further demonstration of the capabilities of the AMR-MOF method.

Geometric Algorithms for 3D Interface Reconstruction

We describe geometrical algorithms for interface reconstructions for 3D generalized polyhedral meshes. Three representative piece-wise linear interface calculation methods are considered, namely gradient based method, least squares volume-of-fluid interface reconstruction algorithm, and moment-of-fluid method. Geometric algorithms for the 3D interface reconstructions are described. Algorithm for the intersection of a convex polyhedron with half-space is presented with degenerate cases. Fast iterative methods for volume matching interface computation are introduced. The numerical optimization method for interface normal computation is presented, and its super-linearly convergence is demonstrated. Finally, actual reconstruction of complex geometry is demonstrated.

Cell-centered high-order hyperbolic finite volume method for diffusion equation on unstructured grids

We apply a hyperbolic cell-centered finite volume method to solve a steady diffusion equation on unstructured meshes. This method, originally proposed by Nishikawa using a node-centered finite volume method, reformulates the elliptic nature of viscous fluxes into a set of augmented equations that makes the entire system hyperbolic. We introduce an efficient and accurate solution strategy for the cell-centered finite volume method. To obtain high-order accuracy for both solution and gradient variables, we use a successive order solution reconstruction: constant, linear, and quadratic (k-exact) reconstruction with an efficient reconstruction stencil, a so-called wrapping stencil. By the virtue of the cell-centered scheme, the source term evaluation was greatly simplified regardless of the solution order. For uniform schemes, we obtain the same order of accuracy, i.e., first, second, and third orders, for both the solution and its gradient variables. For hybrid schemes, recycling the gradient variable information for solution variable reconstruction makes one order of additional accuracy, i.e., second, third, and fourth orders, possible for the solution variable with less computational work than needed for uniform schemes. In general, the hyperbolic method can be an effective solution technique for diffusion problems, but instability is also observed for the discontinuous diffusion coefficient cases, which brings necessity for further investigation about the monotonicity preserving hyperbolic diffusion method.

Fully Unstructured Mesh based Computation of Viscous Flow around Marine Propellers

A CFD(Computational Fluid Dynamics) analysis is presented to predict hydrodynamic characteristics of a marine propeller. A commercial RANS(Reynolds Averaged Navier-Stokes equation) solver, namely FLUENT, is utilized in conjunction with fully unstructured meshes around rotating propeller. Mesh generation process is greatly accelerated by using fully unstructured meshes composed of both isotropic and anisotropic tetrahedral elements. The anisotropic tetrahedral elements were used in the flow domain near the blade and shaft, where the viscous effect is important, having complex shape yet resolving the thin boundary layers. For other regions, isotropic tetrahedral elements are utilized. Two different approaches simulating rotational effect of the propeller are employed, namely Moving reference frame technique for steady simulation, and Sliding mesh technique for unsteady simulation. Both approaches are applied to the propeller open water (POW) test simulation. The current results, which are thrust and torque coefficients, are compared with available experimental data.

Air Compressibility Effect in CFD-based Water Impact Analysis

This paper describes the air compressibility effect in the CFD simulation of water impact load prediction. In order to consider the air compressibility effect, two sets of governing equations are employed, namely the incompressible Navier-stokes equations and compressible Navier-Stokes equations that describe general compressible gas flow. In order to describe violent motion of free surface, volume-of-fluid method is utilized. The role of air compressibility is presented by the comparative study of water impact load obtained from two different air models, i.e. the compressible and incompressible air. For both cases, water is considered as incompressible media. Compressible air model shows oscillatory behavior of pressure on the solid surface that may attribute to the air-cushion effect. Incompressible air model showed no such oscillatory behavior in the pressure history. This study also showed that the CFD simulation can capture the formation of air pockets enclosed by water and solid surface, which may be the location where the air compressibility effect is dominant.This paper describes the air compressibility effect in the CFD simulation of water impact load prediction. In order to consider the air compressibility effect, two sets of governing equations are employed, namely the incompressible Navier-stokes equations and compressible Navier-Stokes equations that describe general compressible gas flow. In order to describe violent motion of free surface, volume-of-fluid method is utilized. The role of air compressibility is presented by the comparative study of water impact load obtained from two different air models, i.e. the compressible and incompressible air. For both cases, water is considered as incompressible media. Compressible air model shows oscillatory behavior of pressure on the solid surface that may attribute to the air-cushion effect. Incompressible air model showed no such oscillatory behavior in the pressure history. This study also showed that the CFD simulation can capture the formation of air pockets enclosed by water and solid surface, which may be the location where the air compressibility effect is dominant.

Self Noise Analysis of Towed Array Sonar Induced by Axisymmetric Vibrations Propagating Along Fluid-filled Elastic Hoses

Performance of array sonars towed underwater is limited due to the self-noise induced mainly by the strumming vibration of the towing cable and also turbulent flow around the acoustic sensor module. The vibration of the towing cable generates axisymmetric waves that propagate along the acoustic module of the array sonar and produce self-noise. The present study aims to investigate the characteristics of the self-noise induced by the axisymmetric vibrations of the acoustic module. The waves of interest are the bulge and extensional waves propagating along the fluid-filled elastic hose. Dispersion relations of these waves are predicted by means of the numerical simulation to evaluate the wave speeds. The self-noise induced by the axisymmetric waves are formulated taking into account the damping of the elastic hose and the effect of the damping is investigated.

6DOF Simulation and Determination of Hydrodynamic Derivatives of Underwater Tow-Fish Using CFD

최근 해저자원 개발 및 생산 활동에 따른 수중작업의 수요 증 가로 인하여 다양한 형태의 수중 운동체 관련기술이 발전되고 있 다. 또한, 해양에 존재하는 광물 자원의 탐사 및 채취 그리고 수 중탐사 및 작업 등은 수중 운동체의 개발 필요성을 더욱 증가시 키고 있다. 이에 따라 수중 운동체에 대한 연구가 활발하게 수행 되고 있다 (Jeong, 2015). 수중 운동체에 대한 일반화된 운동방정식은 The Society of Naval architects and Marine Engineers(SNAME) (1950)과 Fossen (1994)에 의해 정립되어 있기 때문에, 수중 운동체의 운 동특성 해석에 있어서 가장 중요한 것은 수중 운동체에 작용하는 유체력을 결정하는 것이다. 국내에서도 수중 운동체에 작용하는 유체력에 대한 많은 연구가 활발히 수행되었다. 이중 대표적으 로, Son, et al. (2006)은 Manta형 무인잠수정에 대한 유체력 미 계수를 이론적 접근방법과 예인수조를 활용한 정적 사항시험으 로 예측하였으며, Jeong, et al. (2016)은 강제선회시험으로 수 중 운동체에 작용하는 유체력 미계수를 추정하였다. 본 연구에서는, 일차적으로 직접 추력을 발생하지 않고 수상 선에 의해 예인되는 수중 예인체에 작용하는 유체력 미계수를 CFD 해석을 통하여 결정하고, 최종적으로 이를 이용하여 수중 예인체의 6자유도 운동을 예측하고자 한다. Ahn and Jung (2012)에 의해 CFD 해석으로 수중 운동체의 유체력 미계수중 일 부를 예측한 연구사례가 존재하지만, 해석대상이 매우 단순한 형 상이며 유체력 미계수를 예측하는 과정이 체계적으로 명시되어 CFD를 이용한 수중 예인체의 유체력 미계수 결정과

Nonlinear Analysis of Underwater Towed Cable Using Robust Nodal Position Finite Element Method

A motion analysis of an underwater towed cable is a complex task due to its nonlinear nature of the problem. The major source of the nonlinearity of the underwater cable analysis is that the motion of the cable involves large rigid-body motion. This large rigid-body motion makes difficult to use standard displacement-based finite element method. In this paper, the authors apply recently developed nodal position-based finite element method which can deal with the geometric nonlinearity due to the large rigid-body motion. In order to enhance the stability of the large-scale nonlinear cable motion simulation, an efficient time-integration scheme is proposed, namely predictor/multi-corrector Newmark scheme. Three different predictors are introduced, and the best predictor in terms of stability and robustness for impulsive cable motion analysis is proposed. As a result, the nonlinear motion of underwater cable is predicted in a very efficient manner compared to the classical finite element of finite difference methods. The efficacy of the method is demonstrated with several test cases, involving static and dynamic motion of a single cable element, and also under water towed cable composed of multiple cable elements.