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To shorten the time to simulate fluid flows, we develop a fast automatic grid-generation tool, HexaGrid, that produces Cartesian grids with body-fitted layers. The objective of the present study is to apply automatic grid generation to the drag prediction of the NASA Common Research Model. First, we generate a grid following the 4th Drag Prediction Workshop gridding guidelines as far as possible and discuss the capabilities and limitations of the automatic method. Then, we validate the computational-fluid-dynamics results computed with the grid by comparison with other solvers and grid generators. The HexaGrid results agree well with the other results, with the differences in predicted drag being less than five counts except at the stall angle. Additionally, we compare separated flows at the stall angle. The separation lines and Cp distribution are found to be greatly affected by grid topology., 資料番号: PA1420025000

A cell-splitting method for Cartesian grid generation that has the capability of taking into account the cases of thin body and sharp edge is proposed in this paper. Such cases are frequently found when solving the flow around a very thin wing, such as that of a supersonic transport (SST). The method has also been extended to treat the problem of multiple solid regions within a cell, which is sometimes encountered at a highly curved body surface. Validation of the method proposed here is carried out on a sharp, thin double wedge in a supersonic flow, where significant improvements in accuracy are achieved at the cost of a small increase in the number of cells. Furthermore, application of the present method to a model of SST shows its effectiveness on a three-dimensional, realistic geometry. As a result of making a pseudo-planar approximation for body surface elements, the total number of body surface elements was reduced by a factor of about 3.2 in this application. Local grid refinement by relocating grid cells to a curved surface is also proposed, so that a more accurate solution is obtained with a reasonable number of cells.

To shorten the time to simulate flow, we develop an automatic and fast grid generator, HexaGrid, that produces Cartesian/prism hybrid grids. The objective of this study is to apply it for drag prediction on NASA Common Research Model (CRM). First, we compare the generated grid with the DPW gridding guideline and discuss about the capability and limitation of HexaGrid. Then, we validate computational results with HexaGrid, comparing with other solvers and grid generators. HexaGrid results agree well with the other results, where the difference of predicted drag is less than 5 counts except for the stall angle. Additionally, we compared the separated flows at attack angle of 4°. The separation lines and Cp distribution are largely affected by grid topologies.

A hybrid gri d generation method based on prismatic and Cartesian grid is being developed. The main objective is to generate grid around c omplicated geometry for compressible and viscou s flow simulation , especially that with high Reynolds number . The method is based on unstructured grid, which treats cells as arbitrary, unstructured polyhedra. By invoking a combination of Cartesian, prismatic, and cut cell generation modules, user can produce grid for specific simulation needs. At present, development is focused on pris matic grid generation. An advancing layer strategy that incorporates a number of advancing steps to make a prismatic layer is discussed, along with a smoothing technique. Results seem to be favorable, although computation can be time consuming. 1. Introduct ion As CFD plays an increasingly greater role in flow analysis and design, it is demanded to handle even more realistic cases. One of the cases that still pose a computational challenge is simulation of viscous flow at high Reynolds number. At this condit ion isotropic grids normally employed in inviscid and low - Reynolds -number flow simulation will perform very poorly, especially in region close to body surface. When the grid is non -body -fitted, as in the case of Cartesian grid, the situation becomes worse , due to irregularity in cell size at body surface. This is true even when anisotropic grid is employed. Thus a hybrid grid method to carry out such simulation is being developed in National Aerospace Laboratory (NAL), Japan. This study is the continuatio n of the previous in anisotropic Cartesian grid for inviscid flows. 1

To improve efficiency of CFD analysis, we develop an automatic and fast grid generator, HexaGrid, that produces Cartesian/prism hybrid grids. We applied HexaGrid for the NASA common research model employed for the fourth drag prediction workshop (DPW4). We investigated the grid dependency of the separated flow at attack angle of 4°. In particular, we focus on the effect of wake resolution behind the main wing. The grid in the wake region was refined using the refinement box of HexaGrid. The wake is smeared for the original grid, whereas it is clearly captured over the span for the grid with refinement box. However, the side-of-body separation bubble and its wake are not changed significantly. Therefore, the force and moment are almost same. Finally, although the refinement box does not change the result for this problem, it is useful for the problem that includes the flow interaction between main and tail wings.

This paper presents an on-going study of grid generation within the context of CFD simulation for wind tunnel test. The objective is to devise a grid generation method that can support CFD simulation within the time scale of wind tunnel experiments. The challenges are multiple. From the geometry side, we need to handle complex geometry which consists of aircraft model, support system, and wind tunnel walls. From the physics side, we need to be able to handle Navier-Stokes equations and turbulence models. Finally, the time constraint is in the order of an hour. All these requirements pose a very tall order for grid generator. Our grid generation method uses a hybrid of Cartesian grid (for region far from body surface) and prismatic grid (to resolve boundary layer). The method is fully automatic and fast. Our previous study has shown that the resulting flow solution is competitive with that of grid generated manually by expert. The method has been improved to be even more resilient towards defects in surface geometry representation. This property has allowed us to generate grid without having to clean up the surface, which translates into significant time saving.

To shorten the time to simulate flow, we develop an automatic and fast grid generator, HexaGrid, that produces Cartesian/prism hybrid grids. Since the prism cells are also hexahedra, the generated grid is hexahedron-dominant. The objective of this study is to extend the method so that it can produce grids with sufficient quality for Navier-Stokes simulation. The transonic flows around ONERA-M6 wing and DLR-F6 wing-body configuration are computed using HexaGrid and an unstructured-grid flow solver, JTAS. The lift and drag computed with the grid are compared with others’ results. The agreement is reasonably good, which shows that HexaGrid becomes a powerful tool to accelerate a CFD process.