For the aerodynamic design of aircraft, CFD has made a remarkable progress to provide lift, drag and surface pressure distribution collaborating with wing tunnel experiment. However, not much information is known about the flow phenomena in wakes of airplanes and wings. In the article, we concentrate on wake flow analysis as a post work of computations and experiments done for APC-I workshops. Through the analysis, we have found that wing tip vortices and boundary layers of a wing affect velocity distribution on a wake, while only the tip vortices do pressure. The precise prediction of wake physics deeply depends on grid resolution quality in subspaces such as cross-sectional planes in a wake away from an airplane surface, for both of experiment and computation.
The paper reports numerical simulations of aerodynamic characteristics of a commercial aircraft model known as the NASA-CRM. Specific test cases and detailed experimental results have been provided by JAXA for the CFD workshop “Aerodynamic Prediction Challenge (APC).” We employed the cell-vertex finite-volume method on unstructured grids and carried out the RANS simulations with the k-ω SST turbulence model to present results in the APC-I. After the verification of the model in a benchmark problem, both alpha-sweep simulations from low- to high-angles of attack and the grid-convergence study at a cruise conditions are carried out. The simulations well reproduced the experimental results at moderate angles of attack by both types of computational grids used in the study but the results showed the dependency on the grid type and grid resolutions at the highest angle of attack of 5.72 degrees due to different predictions of flow separation near the root chord of the main wing.
The computational grid dependency is an important problem for CFD. We have computed aerodynamics on NASA-Common Research Model (CRM) with FaSTAR and various grids to investigate the grid dependency. We employed four grids: two Cartesian-based unstructured grids, a tetrahedral unstructured grid, and a hexahedral structured grid. The computational conditions are based on the test cases of Aerodynamics Prediction Challenge (APC). First, the grid convergence at a fixed angle of attack and the trend of an angle-of-attack sweep are compared between the four grids. The lift coefficients computed with the two similar Cartesian-based grids are different, and this is caused by the grid difference around the leading edge. However, the overall trend of angle-of-attack sweep is almost same between the four grids. Next, we computed aerodynamics on NASA-CRM with a support device to investigate the support interference. It is found that the support interference on the drag and pitching moment is large and should be considered.
Ceramic/metal brazing was investigated to produce light-weight and highly-efficient ceramic thrusters. Silicon nitride ceramic and metal bars were brazed using an Ag-based brazing material. Four-point bend tests were conducted at room and high temperatures to evaluate the strength of the brazed joints. Computational fluid dynamics (CFD) and finite element method (FEM) analyses were also performed to investigate the effect of the construction and shape of the joints on the stress distribution around them. It was demonstrated that brazing was a great candidate as the joining technique, and a 20 N ceramics/metal brazed thruster was successfully produced.
The conventional spectral volume (SV) method for three-dimensional tetrahedral unstructured mesh is extended to use hexahedral mesh. In the test calculations of linear scalar advection problem and diffusion problem, the formal spatial order of accuracy is achieved even for skewed computational meshes. The Spalart Allmaras turbulence model implemented in the present code is verified by calculating the grid-converged skin friction of turbulent boundary layer on a flat plate. Then verified code is applied to compute the flowfield around the NASA-CRM. In this study, we examine the agreement of the computed aerodynamic coefficients with the corresponding experimental data for different angles of attack. We also examine the change of aerodynamic coefficients with varying Reynolds number, although the experimental data is not available. It is shown that the present SV code can predict aerodynamic coefficients around the cruise angle of attack conditions fairly well. When higher Reynolds number is assumed, the computed lift increases while the viscous drag is reduced, as is expected. On the other hand, the pressure drag is increased with increasing Reynolds number due to the shock wave on the wing which moves toward downstream.
Aerodynamics coefficients of NASA common research model (NASA-CRM) are computationally investigated by revisiting some test cases appeared in aerodynamics prediction challenge (APC) using high-order discontinuous Galerkin (DG) methods. While the employed high-order DG methods reasonably well predict the overall aerodynamics for the NASA-CRM, some discrepancies appear between the CFD and experimental data, especially in a lift and a pitching moment at low angle of attack. Although those discrepancies still exist, installing a sting enhances the reproduction of experimental data of pitching moment. Additionally, fluid-structure coupled simulations used to consider aero-elastic deformations give better agreements of the pitching moment with experimental data.
In response to the First and Second Aerodynamics Prediction Challenges, held in Tokyo, July 2015 and in Kanazawa, July 2016, respectively, computational fluid dynamics simulations were performed for the NASA Common Research Model using the Tohoku University Aerodynamic Simulation (TAS) Code. Our results were summarized in this manuscript, with an emphasis on key computational techniques and mesh generation methods. Unstructured hybrid meshes were generated using the Mixed-Element Grid Generator in 3 Dimensions (MEGG3D), and were deformed based on wing deformation data obtained during wind tunnel testing. The effects of support system interference, of mesh density and of laminar to turbulent boundary layer transition are shown to discuss the validity of computational results. Aerodynamic coefficients were well predicted at low angles of attack when the support system interference effect was considered, while an accurate prediction of pitching moment at high angles of attack was challenging because mesh density affected the shock location on the wing and the size of side-of-body separation.
In order to validate CFD results using wind tunnel test (WTT) data, it is necessary to consider what types of corrections are applied to the WTT data, such as wall interference correction, near-field support interference correction and so on. In this paper, a CFD tool called “Cflow” developed by Kawasaki Heavy Industries, Ltd. is validated using results of wind tunnel test conducted by JAXA with NASA-CRM (Common Research Model). At first, effects of near-field support interference and wing deformation on aerodynamic performance are discussed. Then it is confirmed that Cflow results are well consistent with experimental results, taking these two effects into account.
Energy absorbing system for landing gears is an important on the SLIM project. Open cell porous aluminum manufactured through 3D selective laser melting (SLM) process has been applied on the energy absorbing system. Compressive tests for cylindrical and hemispherical shaped porous aluminum with different porosities revealed the high potential as an energy absorbing component. Heat treatment after SLM processing is effective to increase the energy absorbing potential of the porous aluminum.
Inviscid and Reynolds-averaged Navier-Stokes (RANS) simulations of transonic flows around the NASA Common Research Model are conducted using the Cartesian flow solver UTCart. The immersed boundary method is used to represent the smooth geometry surfaces on the Cartesian grids. The wall function is combined with the immersed boundary method to reproduce the turbulent boundary layer on the geometry surface in the RANS simulations. In the inviscid calculations, the qualitative flow feature including the position on the shock-wave on the wing shows agreement with the reference result a body-fitted grid. In the RANS calculations, the trend of pitching moment and drag shows fair agreement with the reference result, while prediction of the flow separation at high angle of attack is still difficult. Compared with the reference result, the differences in the total drag coefficient at a moderate angle of attack on the medium grid (33 million cells) and the fine grid (99 million cells) are 31 drag counts (10%) and 20 drag counts (6.5%), respectively. Furthermore, each of the calculated aerodynamic coefficients shows a consistent trend of grid convergence toward the reference result.
Summary of First Aerodynamics Prediction Challenge (APC-I) is presented. The APC-I is a domestic CFD prediction workshop that was held on July 3, 2015. The test cases include aerodynamic prediction of NASA-CRM with and without aeroelastic effects, and its wake flow prediction. We compare the CFD results with JAXA’s wind tunnel measurements. There are 15 participants from government, academia, industry, and commercial. The CFD results submitted from the participants are compared and discussed.
Summary of Second Aerodynamics Prediction Challenge (APC-II) is presented. The APC-II is a domestic CFD prediction workshop that was held on July 6, 2016. The test cases include aerodynamic prediction of NASA-CRM with and without support effects, and buffet prediction. We compare the CFD results with JAXA’s wind tunnel measurements. There are 9 participants from national research agency, academia, industry, and commercial software vendor. The CFD results submitted from the participants are compared and discussed in the presentation.