• International Journal of High-Rise Buildings
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• v.8 no.4
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• pp.255-264
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• 2019

This paper describes large eddy simulation of wind pressures on a square cylinder in a uniform flow and a high-rise building immersed in an atmospheric turbulent boundary layer. For the atmospheric boundary layer case, the inflow turbulence is generated by a numerical wind tunnel. In the numerical simulation, particular attention is devoted to the performance of an auto hexahedral non-structural mesh. Both simulations are performed for three grid systems: an auto hexahedral non-structured grid, a structured Cartesian grid and a non-structured triangular prism grid, and for three grid numbers. The present study shows that the auto hexahedral unstructured mesh achieves the best simulation results for wind pressures on the square cylinder and the high-rise building. When the grid number is sufficiently large, the differences among the results obtained from the three investigated grid systems are not significant. However, the advantage of the auto hexahedral unstructured mesh becomes clear when the grid number decreases, because it enables a balanced distribution of orthogonal grids. The results described in this paper demonstrate that the auto hexahedral non-structured mesh has good potential applicability to simulation of urban flows.

• 한국전산유체공학회:학술대회논문집
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• 2003.08a
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• pp.31-36
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• 2003

A field reconstruction scheme for a cell centered finite volume method on unstructured meshes is developed. Regardless of mesh quality, this method is exact within a machine accuracy if the solution is linear, which means it has full second order accuracy. It does not have any limitation on cell shape except convexity of the cells and recovers standard discretization stencils at structured orthogonal grids. Accuracy comparisons with other popular reconstruction schemes are performed on a simple example.

• 한국전산유체공학회:학술대회논문집
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• 2003.10a
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• pp.137-139
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• 2003

As a part of numerical and experimental research works for the prediction and improvement of ship's maneuvering performance, a study on the performance analysis of two different rudders has been carried out. While the planform shape and the aspect ratio of the rudders have been fixed, section shape has been changed. Conventional type of HMRI NP section and special type of dolphin-tail section have been employed. Performances of the rudders have been investigated by using CFD and compared with experimental data obtained in a wind tunnel. A commercial CFD program has been used to solve the RANS equations. Two-equation k-ro model has been applied to close the governing equations. Block-structured grids are used in the numerical calculation. Based upon the calculation results, the rudder with dolphin-tail section has shown a possibility of significantly improving rudder performance if utilized as the section of ship rudders.

• Journal of the Society of Naval Architects of Korea
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• v.58 no.3
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• pp.182-190
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• 2021

Computational simulations of turbulent flows around a model ship have been performed to investigate an influence of TVD schemes on the accuracy of advective terms associated with ship resistances. Several TVD schemes including upwind, second-order upwind, vanLeer, and QUICK as well as a nonTVD linear scheme were studied by examining temporal and spatial characteristics of accuracy transition in adjacent cells to the hull. Even though vanLeer scheme was the most accurate among TVD schemes in both structured and unstructured grid systems, the ratio of accuracy switch from 2^nd order to 1^st order in vanLeer scheme was considerable compared with the 2^nd order linear scheme. Also, the accuracy transition was observed to be overally scattered in the unstructured grid while the accuracy transition in the structured grid appeared relatively clustered. It concluded that TVD schemes had to be carefully used in computational simulations of turbulent flows around a model ship due to the loss of accuracy despite its attraction of numerical stability.

• Journal of Mechanical Science and Technology
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• v.20 no.12
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• pp.2218-2229
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• 2006

A conservative finite-volume numerical method for unstructured grids with the cell-centered method has been developed for computing flow and heat transfer by combining the attractive features of the existing pressure-based procedures with the advances made in unstructured grid techniques. This method uses an integral form of governing equations for arbitrary convex polyhedra. Care is taken in the discretization and solution procedure to avoid formulations that are cell-shape-specific. A collocated variable arrangement formulation is developed, i.e. all dependent variables such as pressure and velocity are stored at cell centers. For both convective and diffusive fluxes the forms superior to both accuracy and stability are particularly adopted and formulated through a systematic study on the existing approximation ones. Gradients required for the evaluation of diffusion fluxes and for second-order-accurate convective operators are computed by using a linear reconstruction based on the divergence theorem. Momentum interpolation is used to prevent the pressure checkerboarding and a segregated solution strategy is adopted to minimize the storage requirements with the pressure-velocity coupling by the SIMPLE algorithm. An algebraic solver using iterative preconditioned conjugate gradient method is used for the solution of linearized equations. The flow analysis code (PowerCFD) developed by the present method is evaluated for its application to several 2-D structured-mesh benchmark problems using a variety of unstructured quadrilateral and triangular meshes. The present flow analysis code by using unstructured grids with the cell-centered method clearly demonstrate the same accuracy and robustness as that for a typical structured mesh.

• Journal of the Korean Society for Industrial and Applied Mathematics
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• v.23 no.2
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• pp.65-92
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• 2019

We survey a recently developed immersed finite element method (IFEM) for the interface problems. The IFEM uses structured grids such as uniform grids, even if the interface is a smooth curve. Instead of fitting the curved interface, the bases are modified so that they satisfy the jump conditions along the interface. The early versions of IFEM [1, 2] were suboptimal in convergence order [3]. Later, the consistency terms were added to the bilinear forms [4, 5], thus the scheme became optimal and the error estimates were proven. For elasticity problems with interfaces, we modify the Crouzeix-Raviart based element to satisfy the traction conditions along the interface [6], but the consistency terms are not needed. To satisfy the Korn's inequality, we add the stabilizing terms to the bilinear form. The optimal error estimate was shown for a triangular grid. Lastly, we describe the multigrid algorithms for the discretized system arising from IFEM. The prolongation operators are designed so that the prolongated function satisfy the flux continuity condition along the interface. The W-cycle convergence was proved, and the number of V-cycle is independent of the mesh size.

• Journal of the Society of Naval Architects of Korea
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• v.49 no.2
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• pp.196-202
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• 2012

In this study, numerical analysis of viscous flows is carried out based on the unstructured grid. There exist some difficulties in expressing and computing numerical derivatives on the unstructured grid due to lack of the structured characteristics. The general computer algorithms are developed to perform numerical derivatives easily and extended to be applicable to various geometries composed of hybrid meshes. And the optimal method of strongly implicit procedure is newly contrived to accelerate the rate of convergence in solving the pressure Poisson equation. To verify numerical schemes, the driven cavity problems of 2 and 3 dimension are simulated. The numerical results are compared with others and our numerical schemes are shown to be valid.

• 한국전산유체공학회:학술대회논문집
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• 2009.04a
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• pp.121-124
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• 2009

A moving mesh system is one of the critical parts in a computational fluid dynamics analysis. In this study, the RBF(Radial Basis Function) which shows better performance than hybrid meshes was developed to obtain the deformed grid. The RBF method can handle large mesh deformations caused by translations, rotations and deformations, both for 2D and 3D meshes. Another advantage of the method is that it can handle both structured and unstructured grids with ease. The method uses a volume spline technique to compute the deformation of block vertices and block edges, and deformed shape.

• 한국전산유체공학회:학술대회논문집
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• 2005.10a
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• pp.131-134
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• 2005

Conventional high order interpolation schemes are limitative in several aspects mainly because they need data of neighboring cells at the reconstruction step. However, discontinuous Galerkin method and spectral volume method, two high order flux schemes which will be analyzed and compared in this paper, have an important benefit that they are not necessary to determine the flow gradients from data of neighboring cells or elements. These two schemes construct polynomial of variables within a cell so that even near wall or discontinuity, the high order does not deteriorate.

• Journal of computational fluids engineering
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• v.10 no.4 s.31
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• pp.39-50
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• 2005

An application of the KFLOW3D code which has been developed at KAIST is presented. This paper briefly describes the underlying methodology and summarizes the results for the DLR-F6 transport configuration recently presented in the second AIAA CFD Drag Prediction Workshop held in Orlando, FL, June 2003. KFLOW3D is a parallelized Reynolds averaged Navier-Stokes solver for multi-block structured grids. For the present computations, 2-equation k-$\omega$ WD+ nonlinear eddy viscosity model is used. The emphasis of the paper is placed on the implementation of the k-$\omega$ WD+ model in the multigrid framework and practicality of KFLOW3D for accurately predicting not only the integrated aerodynamic property such as the drag coefficient but pressure distributions.