Japan Korea CFD Seminar 2007

	Unconditionally Stable Explicit CFD Scheme Based on Boltzmann Equation: the Cell-Boltzmann Method T.Matsuda and H.Isaka (Kobe Univ.) We propose a new class of time explicit CFD scheme, which is not restricted by the CFL condition. Space is divided into cells, which is filled by gas molecules having Maxwellian velocity distribution at each time step tⁿ. These molecules fly freely without collisions for Δt, and they relax into a new local thermodynamic equilibrium state at a new time tⁿ⁺¹. We effectively solve the Boltzmann equation, rather than hydrodynamic equations to simulate continuum flow, by setting the mean free time of molecular particles to Δt. We term the scheme the Cell-Boltzmann method (CB). The time step in the CB is not restricted by the CFL condition. Only a penalty using a time step longer than the CFL time is a gradual degrading of a solution. The CB scheme is very robust and can handle flow with infinitely large Mach number. We show numerical examples of shock tube problems and Sjoegreen problems. abstract(pdf)
	A Subgrid Scale Model Based on Explicitly Filtered Velocity Fluctuation J.H.Baek (POSTECH) A large eddy simulation with explicit filters on unstructured mesh is presented. The flow filed is semi-implicitly marched by a fractional step method. Spatial discretization of the solver is designed to guarantee the second order accuracy. An isotropic explicit filter is adopted for measuring the level of subgrid scale velocity fluctuation. The filter is linearity-preserving and has second order commutation error. The developed subgrid scale model is basically eddy viscosity model which depends on the explicitly filtered fields and needs no additional ad hoc wall treatment, such as van Driest damping function. For the validation, the flows in a channel and a pipe are calculated and compared to experimental data and numerical results in the literature. abstract(pdf)
	*Implicit LES using Weighted Compact Nonlinear Scheme* K.Ishiko, N.Ohnishi and K. Sawada (Tohoku Univ.) An implicit Large Eddy Simulation (LES) code has been developed for compressible turbulent flowfield, based on the idea of Monotonically Integrated LES (MILES). The Weighted Compact Nonlinear Scheme (WCNS) is chosen to meet the requirements that the method should be highly accurate for LES and, at the same time, robust at possible shock waves. The developed code is verified for typical test problems of 1D Burgers turbulence, and 2D incompressible homogeneous turbulence. Computed results are compared with those given by Direct Numerical Simulation (DNS). It is shown that the present implicit LES can reproduce the inertial range in the energy spectrum given by DNS results favorably well. A truncation of energy spectrum occurs naturally at high wave number limit, suggesting that the present scheme contains moderate amount of dissipative effect. In the computed results, vortices are sharply captured without any numerical oscillations. **presentation(pdf)**
	Flow Pattern past Two Nearby Circular Cylinders K.Lee and K.S. Yang (Inha Univ.) In this investigation, flow patterns past two identical nearby circular cylinders at Re=100 are numerically studied as a basic model for laminar wake interaction. An immersed boundary method is employed for effective treatment of the cylinders on a Cartesian grid system. We consider all possible arrangements of the two circular cylinders in terms of the distance between them and the inclination angle of the line connecting their centers with respect to the main flow direction. It is found that eight patterns of distinct flow characteristics are identified by vorticity contours and streamlines. Collecting all the results obtained, we propose the flow-pattern diagram (“map”) for the two cylinders to provide an overall picture on the wake interaction. The perfect geometrical symmetry implied in the flow configuration allows one to use this diagram to distinguish flow pattern past two identical circular cylinders arbitrarily positioned in physical space with respect to the main flow direction. abstract(pdf)
	*Numerical Simulation of Nonlinear Wave Systems around the Bow and Stern of Ships* Y.G. Lee, S.Y. Choi, J.W. Yu and NC Kim (Inha Univ.) This paper investigates the nonlinear wave phenomena generated by the ship hull through numerical simulations employing Marker-density Method (IUBW) to determine the complex free surfaces developed near the bow and stern of the ship in navigation. The Navier-Stokes equations with the usual continuity equations are the governing equations. This equation set with the modeled boundary conditions applying the Marker-density are numerically solved by a slightly modified finite difference method based on the rectangular variable staggered mesh system described in the contents. The free surfaces were tracked to be determined by the Marker-density Method. A Series 60(CB=0.6) hull form was used for the validation of the calculated solutions as well as for the numerical simulations. In addition, a preliminary hull form design having the Series 60(CB=0.8) was performed applying this numerical simulation technique. The results are compared with the experimental results tested in a towing tank. full paper(pdf)
	Automatic Triangular Surface Grid Generation on 3-D Surfaces Described in the Triangulated STL Format B.J. Lee and B. Kim (Chungnam National Univ.) In this paper an automatic triangular surface grid generation method on general 3-D surfaces is proposed, where the 3-D surfaces are approximately defined in the STL format.The STL format is one of standard formats for the exchange of surface shape data, especially in rapid prototyping field, which replaces the original surface with a collection of triangulated surface segments. Triangulated surface data described in the STL format, which can be easily exported by most of CAD systems or 3-D modeling programs, replace the exact surface definition with air-tight triangular cells with some degree of approximation. The STL surface data, however, keeps the curvature information by putting a large number of smaller cells near the highly-curved region while using rather bigger cells near the relatively flat area. The proposed algorithm for automatic surface grid generation is based on Delaunay triangulation method. Starting with the surface data given in the STL format, target cell size distribution is first determined by checking the curvature distribution along the surfacewhich can be estimated by the initial surface cell size distribution of the STL file. Once the target cell size distribution is decided, the node insertion followed by cell division is performed and edge-swapping is applied for high-aspect-ratio cells so that Delaunay criteria are satisfied. One of difficulties in applying Delaunay triangulation for 3-D surface grid generation is keeping triangular cells to stay on the given surface data, and the idea to overcome this issue is also introduced. In this paper detailed description of the current approach is given, and results of automatic surface grid generation are demonstrated by using general 3-D surface shapes defined by CAD system and imported in the STL format. abstract(pdf) full paper(pdf)
	*Cartesian Approach for Large-scale CFD; Issues and Advantages* K.Ono (RIKEN) The practical system for a fluid simulation, which can help to design industrial products, is required to have functionalities and performance such as the handling of large-scale data set, usability, accuracy of prediction, scalability, and so on. Especially, pre/post process will be a bottleneck in the simulation task. In this presentation, a voxel based large-scale CFD system will be discussed as a prime candidate for the peta-scale computation.
	*A Computational Study on Internal Ventilation System of Thermal Flow in Complex Automotive Part Plants* D.H.Koh, J.H.Lee, S.Lim, S.D.Kim, D.J.Song and S.E.Cha (Youngman Univ.) The increasing demand on the comfortable working conditions in automotive part plant pushes the company to improve working environment of workers. The displacement ventilation supplies air with a low velocity diffuser located at floor level and is based on using natural convection flows produced by people and equipment. The objective of this ventilation principle is to create air supply conditions in the occupied zone, while the objective of traditional mixing systems is to create extract-air conditions in the whole room(1)(2)(3)(4). In this study, we analyzed thermal flow of an automotive part factory by using CFX code. Internal working environment has been analyzed with/without ventilation system and computational results were validated by using wet bulb globe temperature. abstract(pdf)
	*Numerical Simulations for Store Separation Problems* Y.H.Yoon (Korea Air Force Academy) The prediction of the separation trajectories of the external stores carried out on military aircraft is an important task in the aerodynamic design area having the objective to define the operational, release envelopes. This work consists of concept and some results for external store configurations. A Computational Fluid Dynamics technique is applied to calculate the aerodynamic forces. The CFD-FASTRAN with an implicit Euler solver is used for the present simulations. The computational results are validated against the experimental data. full paper(pdf)
	*Prediction of Helicopter Blade-Vortex Interaction Noise using Motion Data from Experiment* T.Aoyama, C.Yang and Y.Inada (JAXA) JAXA has been developing its own CFD codes to simulate full helicopter using accurate flow solver and acoustic solver. The next challenging topic is to include aero-elastic deformation analysis for more realistic noise prediction. In the present research, the CFD simulation with elastic deformation of blade is conducted using HART II motion data as the middle step for full coupling aero-elastic analysis. The HART II data including flapping, lead-lag, and torsion deformations are interpolated and reconstructed as a continuous blade motion for one revolution. From the calculated results, we can validate the ability of present CFD code for capturing tip vortex exactly to predict BVI noise when the blade motion is prescribed from the measured data. Two approaches are suggested according to the research purpose. For low cost calculation, a CFD code with a single blade grid is applied using a vortex prescription model called Beddoes generalized wake model to prescribe the position of vortex shed from preceding blades. For high resolution calculation, a CFD code with a moving overlapped grid system is applied for direct vortex capturing. As a result, the calculations show good agreement with the HART II experiment in the baseline, minimum noise, and minimum vibration conditions. Especially, the consideration of blade elastic deformation contributes to the improvement of precision in blade loading calculation. presentation(pdf)
	*A Computational Study on the Characteristics of Asymmetric Vortex and SideForce of Tangent-Ogive-Cylinder Flight Vehicle at Various Angles of Attack* S.Lim, S.D. Kim and D.J.Song （Youngman Univ.) Flight vehicles such as tangent-ogive-cylinders encounter frequently high angles of attack flight conditions. Under very high angles of attack flight conditions, asymmetric vortices and side force are generated causing unwanted side slip forces and moments, and thus additional yaw control methods may have to be required for stable vehicle operation(1)(2)(3). The flow characteristics of asymmetric vortex and side force of tangent-ogive-cylinder flight vehicle at high angles of attack have been studied by using upwind Navier-Stokes method with the k-ω turbulence model(4) along with simple laminar-turbulent transition model(5). In this study, the asymmetric vortex and side force were introduced by using artificially different asymmetric turbulent transition location. And the effects of changing angles of attack(Fig. 1) are studied by using the same asymmetric turbulent transition condition and modified condition by streamwise Reynoldes number(6). abstract(pdf)
	*Multi-Objective Design Exploration (MODE) of Low-Boom and Low-DragConfiguration for Supersonic Business Jet* S.Obayashi, K.Sato, T.Kumano, M.Yonezawa, H.Yamashita and S.Jeong (Tohoku Univ.) Multi-Objective Optimization have been applied to a design problem of the twin engine concept for Supersonic Business Jet. This problem aims to find main wing, body, tail wing and engine nacelle configurations, which can minimize both sonic boom and drag in a supersonic cruising flight. The multi-objective genetic algorithm (MOGA) and the Kriging model has been used to search for globally optimal design candidates in the multi-objective problem, as well as reduce function evaluation time. The sonic boom and drag have been evaluated by using the waveform parameter method, coupled with the computational fluid dynamics (CFD) simulation. As a result, the present optimization can successfully obtain lowest-boom and lowest-drag design candidates, which are better than the baseline design by more than 40 %. Moreover, general information about the design space can be interpreted through visual data-mining using the self-organizing maps (SOMs) applied to all the searching points during the optimization. abstract(pdf)
**Presentation* will be uploaded after the seminar (last update: Jan. 04 2008)

12/21(Fri) 13:00-16:30
13:00- Schemes and Basic flow	Unconditionally Stable Explicit CFD Scheme Based on Boltzmann Equation: the Cell-Boltzmann Method T.Matsuda and H.Isaka (Kobe Univ.) A Subgrid Scale Model Based on Explicitly Filtered Velocity Fluctuation J.H. Baek (POSTECH) *Implicit LES using Weighted Compact Nonlinear Scheme* K.Ishiko，N. Ohnishi and K.Sawada (Tohoku Univ.) *Flow Pattern past Two Nearby Circular Cylinders* K.Lee and K.S. Yang (Inha Univ.) Numerical Simulation of Nonlinear Wave Systems around the Bow and Stern of Ships Y.G. Lee, S.Y. Choi, J.W. Yu and NC. Kim (Inha Univ.)
Complex configuration	Automatic Triangular Surface Grid Generation on 3-D Surfaces Described in the Triangulated STL Format B.J. Lee and B. Kim (Chungnam National Univ.) *Cartesian Approach for Large-scale CFD; Issues and Advantages* K.Ono (RIKEN) A Computational Study on Internal Ventilation System of Thermal Flow in Complex Automotive Part Plants D.H.Koh, J.H.Lee, S.Lim, S.D.Kim, D.J.Song and S.E.Cha (Youngman Univ.)
Aerodynamics -16:30	*Numerical Simulations for Store Separation Problems* Y.H.Yoon (Korea Air Force Academy) Prediction of Helicopter Blade-Vortex Interaction Noise using Motion Data from Experiment T.Aoyama, C.Yang and Y.Inada (JAXA) A Computational Study on the Characteristics of Asymmetric Vortex and Side Force of Tangent-Ogive-Cylinder Flight Vehicle at Various Angles of Attack S.Lim, S.D. Kim and D.J.Song （Youngman Univ.) Multi-Objective Design Exploration (MODE) of Low-Boom and Low-Drag Configuration for Supersonic Business Jet S.Obayashi, K.Sato, T.Kumano, M.Yonezawa, H.Yamashita and S.Jeong (Tohoku Univ.)