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An automated and robust unstructured hybrid mesh generation method comprised of tetrahedra, prisms, and pyramids has been developed. The hybrid grid generation starts with isotropic tetrahedral grids to enhance the robustness of the algorithm. The user-specified number of prismatic layers is then added keeping the mesh validity. The tetrahedra near no-slip walls are shifted inward and the resulting gap between the tetrahedra and the walls is filled up with prismatic elements step by step. The addition of prismatic elements is stopped if a local mesh has incorrect topology. The method is applied to the experimental supersonic airplane model designed at NAL. Computed results by solving the Navier-Stokes equations are compared with wind tunnel data and the validity of the method is demonstrated., JAXA Special Publication, 宇宙航空研究開発機構特別資料

JAXA Special Publication, 宇宙航空研究開発機構特別資料

The importance of accurate and fast interpolation algorithm is growing up for multi-body configuration with arbitrary overlap. One of the critical cases can be a helicopter simulation because of the complex relative movement of rotor-rotation and body motion. In this paper, new searching algorithms are implemented for the interpolation between two different grids, Cartesian grid and curvilinear grid, of which the overlapped grid system consists for the massive computation of the full helicopter configuration. These searching algorithms are proposed to make full use of (1) the characteristics of Cartesian grid, (2) special geometric configuration of helicopter, and (3) load balancing in parallel computation. In the first stage, Alternating Index Searching (AIS) algorithm, which alternates a searching direction by jumping the grid index to the searching point, is constructed to compare the iteration speed with a conventional Linear Searching (LS) algorithm. Simple two dimensional problems and three dimensional helicopter simulations are conducted to compare the efficiency of these searching algorithms. The result shows a considerable enhancement in computing time for whole computation domain. In the second stage, Reverse Index Searching (RIS) algorithm, which is developed to consider the load balance among each Processing Element (PE) during parallel computation, is proposed. By applying these searching algorithms, efficient massive computation can be achieved for the helicopter configuration., ロータの回転や6自由度の機体運動などの複雑な動きのため、メイン／テールロータおよび胴体からなるヘリコプタの数値シミュレーションにおいて、正確で速い補間アルゴリズムの重要性が高まってきている。本報告では、直交格子と曲線格子で構成される移動重合格子を利用したCFD(Computational Fluid Dynamics)コードに対して、より正確で早い補間法を提案した。新しい補間法では、(1)直交格子の特性、(2)ヘリコプタ・ブレードの特殊な幾何学的配置、および(3)並列計算時の計算負荷バランスなどを十分有効に利用できるアルゴリズムが考案されている。第1章では、Alternating Index Searching(AIS)アルゴリズムを提案し、従来のLinear Searchingアルゴリズムに対し、2次元の簡単なケースと実際のヘリコプタを模擬した3次元計算のケースで補間計算の速度を比べた。第2章では、並列計算における各計算ノードの負荷バランスを考慮したReverse Index Searching(RIS)アルゴリズムを提案した。この補間法を利用することによって、ヘリコプタのより効果的な大型計算を実現することができた。, JAXA Research and Development Report, 宇宙航空研究開発機構研究開発報告

This paper provides CFD (Computational Fluid Dynamics) flow analysis results of the JAXA NEXST-1 (National EXperimental Supersonic Transport-1) for the 4th JAXA SST (SuperSonic Transport) CFD workshop. A hybrid unstructured grid system was employed for viscous aerodynamic analysis. Computations were conducted for the flight test condition, then computed and measured results are compared. Effects of some computational conditions, such as, choice of turbulence modeling, mesh density and numerical scheme, are studied., JAXA Special Publication, 宇宙航空研究開発機構特別資料

A 3-D (three-dimensional) Navier-Stokes CFD (Computational Fluid Dynamics) code using unstructured grid has been developed for the purpose of design tool. Accuracy, robustness, efficiency, applicability to wide range of flow speeds and convenient grid generation have been emphasized. Two test cases are presented: a square-sectioned U-bends and a serpentine passage. It is demonstrated that the present code satisfactorily predicts the flow and heat transfer in the bend with moderate curvature. It is also indicated that the current code shows poor agreement with experiments in the bend with strong curvature because the model for the boundary layer is too simple., 設計ツールとして、非構造格子を用いた3次元ナビエ・ストークスCFD(数値流体力学)コードを開発した。精度、ロバスト性、効率、幅広い流速適応性および便利な格子生成を強調した。正方形断面のU字管およびサーペンタイン管の2つのテストケースを示した。提示コードは、中位の曲率を持つ曲がり管での流れおよび熱伝達を十分に予測することを立証した。現行のコードでは、境界層のモデルが余りにも単純なので、強い曲率を持つ曲がり管では実験との一致が良くないことを指摘した。

Research in automatic grid generation around realistic 3D geometry is being conducted in Japan Aerospace Exploration Agency. This research aims at reducing the time required to generate computational grid for inviscid as well as viscous flow simulations. The method is based on hexahedra grid method, as the extension of Cartesian grid method. As the result, the method inherits significant advantages of Cartesian grid: it is automatic and fast. Because an efficient viscous flow simulation requires grid with high aspect ratio near solid surface, a hybrid with prismatic grid is necessary. The hexahedra grid extends naturally to this approach, because there is consistency in grid topology (both Cartesian and prismatic grids have the same hexahedral shape). The main idea of the method is as follows: construct Cartesian grid, remove the cells near solid surface, and construct hexahedral cells based on the Cartesian cells' faces toward the surface. Note that both grids are not constructed from the solid surface. This leads to another advantage: the method is not sensitive to defects in solid surface (e.g. crack, overlap), because it actually reconstruct the solid surface grid. However, because of the way the hexahedral cells are constructed (simple projection from Cartesian grid cells' faces toward solid surface), there is a disadvantage: it can not capture sharp concave features. A method to overcome this problem is devised in the present research. This paper provides the summary of the method being developed as well as some current results., JAXA Special Publication, 宇宙航空研究開発機構特別資料

With the progress of computer performance, Computational Fluid Dynamics (CFD) is well developed and it is very promising to utilize as a design tool for the development of aircraft and spacecraft. It is usual to define a configuration by using a CAD (Computer Aided Design) tool. It is necessary to construct a grid system around it for analyzing the flow field. But this part is a bottleneck since the usual general-purpose CAD software packages do not support the generation of surface grids suitable for CFD analysis. In this paper, the preliminary work of surface gird generation based on the CAD data is reported. Conversion format from a CAD tool is the IGES (Initial Graphic Exchange Specification) format. The configuration from the section data is first reconstructed so that the isoparametric curves are quite suitable as the grid curves. Then a surface grid is generated by assigning the number of grid points needed., 計算機性能の進歩と共に、数値流体力学(CFD)が良く発展し、航空機並びに宇宙機の開発に役立つ極めて有望な設計ツールとなっている。通常は機体形状を定めるのにCAD(コンピュータ支援設計)ツールを用いる。流れ場を解析するためにはその周りに格子システムを構成する必要がある。しかし、通常の汎用CADソフトウエアパッケージではCFD解析に適する表面格子の生成をサポートしないので、この部分が障害となっている。CADデータを基に表面格子生成の予備研究を報告した。CADツールからの変換形式はIGES(初期図面交換仕様)形式を用いた。まず断面データから形状を再構築した結果、アイソパラメトリック曲線は格子曲線として極めて適切なものとなった。ついで、必要な数の格子点を割り振ることにより表面格子を生成させた。

JAXA Special Publication, 宇宙航空研究開発機構特別資料