Ryoji Takaki

This study presents a fully automated Cartesian-grid-based compressible flow solver, named FrontFlow/Violet Hierarchical Cartesian for Aeronautics based on Compressible-flow Equations (FFVHC-ACE), for large-eddy simulation (LES) and its aeronautical applications. FFVHC-ACE enables high-fidelity LES of high-Reynolds-number flows around complex geometries by adopting three key numerical methods: hierarchical Cartesian grids, wall modeling in LES, and the kinetic-energy and entropy preserving (KEEP) scheme. The hierarchical Cartesian grids allow fully automated grid generation for complex geometries in FFVHC-ACE, and high-fidelity LES of high-Reynolds-number flows around complex geometries is realized by the wall modeling and the KEEP scheme on the non-body-fitted hierarchical Cartesian grids. We apply FFVHC-ACE to wall-modeled LES around high-lift aircraft configurations at wind-tunnel-scale Reynolds number [Formula: see text] and real-flight Reynolds number [Formula: see text], demonstrating the capability of FFVHC-ACE for fully automated grid generation and high-fidelity simulations around complex aircraft configurations. The computed flowfield and aerodynamic forces at the wind-tunnel-scale Reynolds number agree well with the experimental data provided in the past AIAA High Lift Prediction Workshop (Rumsey et al., Journal of Aircraft, Vol. 56, No. 2, 2019, pp. 621–644). Furthermore, the wall-modeled LES at the real-flight Reynolds number shows good agreement of the lift coefficient with flight-test data.

<p>An adaptive estimation method for nonlinear structural deformations is presented. The method is based on the ensemble Kalman filter (EnKF), which can effectively handle nonlinearities in structural models by using the Monte-Carlo simulation. In this study, a self-tuning algorithm for the system noise in the conventional Kalman filter is extended and applied for tuning in the EnKF. To verify the effectiveness of the presented method, a numerical experiment was performed for a deployable frame structure system that contains the typical nonlinearities of space deployable structures, that is, the geometrical nonlinearity in flexible members and the cable nonlinearity resulting from its slackened state.</p>

The thermal condition of ASTRO-H under air-cooled environment before launch is investigated with a thermal testing and a computational analysis. The thermal testing shows that the temperatures of devices are confirmed to be within the operating range if the additional fans are used. Moreover, the results of the thermal testing are compared with those of computational results. The computational results of temperature of the devices around the dewar with the additional fans are in good agreement with those of the thermal testing. The good agreement in the condition with the additional fans is because the forced convection, which is a dominant effect, is well captured in the computational analysis. Meanwhile, the computational results of temperature on the side panels are in very good agreement with thermal testing despite the difference in the flow outside satellite by air conditioner: computational analysis models the air flow from the air-conditioner while thermal testing does not. This is because the air-flow is very slow (0.1[m/s] at the side panel locations) and forced convection effects are very small.

Experiment using the 2m× 2m transonic wind tunnel in JAXA and numerical simulation based on the RANS are carried out to investigate transonic flowfield influenced by a protuberance attached to shoulder of a generic cone-cylinder type rocket faring. Thickness of the protuberance considered here is about 0.1% of the fairing diameter, which is 1.8 times thicker than the displacement thickness of the incoming boundary layer. Validation and verification of the numerical method are firstly performed based on the experimental result of the clean fairing without any protuberance. It is found that effect of the protuberance changes depending on the flow speed. If the free-stream Mach number is less than 0.8, separation shock wave is generated at the protuberance on the shoulder, although the separation shock wave appears at the cylinder section of the clean fairing. On the other hand, little influence on the flow structure is observed in the flow condition with higher free-stream Mach number. The effect revealed in this study indicates the importance of the protuberance to the design of rocket fairing.

The accurate evaluation of the ground reflection effect is of importance for acoustic measurement conducted during static-firing tests of rocket motors. The effectiveness of existing acoustic impedance models was examined by comparing experimental data collected from acoustically hard surfaces. Through these comparisons and the subsequent evaluation of the effect of meteorological conditions, it was confirmed that the existing impedance models are not suitable for the evaluation of long-distance propagation over a hard surface, which corresponds to the far-field conditions in the present static-firing tests of solid rocket motors. A new acoustic impedance model is proposed in this paper. Comparisons of the acoustic measurement data indicated that the new model is effective for both near- and far-field propagation of acoustic waves. The proposed model was applied to acoustic data collected during static-firing tests of solid rocket motors assuming distributed acoustic sources along the exhaust jet axis in order to remove the ground reflection effect.

Activity on numerical plasma simulations by JAXA&rsquo;s Engineering Digital Innovation (JEDI) Center is overviewed. Currently, R&D of two major numerical tools is conducted. First one is spacecraft charging analysis tool that can compute charging status of a spacecraft solving charged particle motions precisely. Using this information, we can evaluate onboard measurement of electrostatic probes or consider better configuration of onboard equipment of a spacecraft. Computation example of the interactions between solar wind plasma and a solar sail is shown in this paper. Second one is a numerical tool called JIEDI (JAXA&rsquo;s Ion Engine Development Initiative), which aims to reduce the cost and the time required for an ion thruster life test. The JIEDI tool can numerically estimate ion bombardment to an ion thruster&rsquo;s grid material to predict the erosion rate of the grid material, and preliminary analysis by the JIEDI tool showed good agreement with the real-time life test of a microwave ion thruster.

Evaluation of ground effect is important for acoustic measurement in static-firing tests on rocket motors. The effectiveness of existing acoustic impedance models is examined by comparing with some experimental results. Through the comparison and evaluation of effect of meteorological condition, it is confirmed that the existing impedance models are unsatisfactory for the evaluation of long distance propagation over a hard surface, which corresponds to the far field condition in the present static-firing tests of rocket motors. In this study, a new acoustic impedance model is proposed. From the comparison with the existing acoustic measurement data, it is shown that the new model is effective for both of near and far field propagation. The proposed model is applied to acoustic data measured in the static-firing tests of solid rocket motors, assuming distributed acoustic sources along the exhaust jet axis.

Aerodynamic characteristics of a reusable observation vehicle are computationally investigated under subsonic and supersonic flight conditions as a preliminary study for the concept design using a design exploration method and a light CFD tool. The results show that the simulations with a coarse grid can accurately estimate the aerodynamic characteristics like axial force coefficient and the lift to drag ratio. The results of the aerodynamic shape exploration indicate tradeoff information among objective functions, and the correlation between design variables and objective functions. The preliminary knowledge for the aerodynamic shape design is obtaind.

Attenuation of sound by water droplets in air is analytically investigated. This is one of the mechanisms of the noise reduction in actual rocket launch where water is injected to jet plume. The scattering and absorption by the droplets are considered as the attenuation mechanisms. It is found that the absorption is superior to the scattering for fine droplets, such as fog or mist. Then we estimate the attenuation of sound for the experiment using subscale solid rocket motors. The result agrees well with the experimental data within the frequency range where the assumptions of this study are valid.

Since acoustic wave radiated from the rocket plume causes severe acoustic loading on the payload at lift-off, prediction and reduction of acoustic level is an important issue in the design phase. Acoustic environment is empirically predicted as of now. However, the acoustic generation mechanism is not clearly understood, which results in unsatisfactory prediction accuracy. In order to overcome the limitations of the empirical method, CFD study is carried out in JAXA. Through the comparison with the measured result in the static-firing tests, it is found that prediction accuracy of the CFD superiors to the empirical method. In addition, the acoustic generation becomes clear, which gives us the idea of developing the launch-pad which reduces acoustic level of the vehicle.

Gas flow over a two-dimensional airfoil at very low Reynolds number is investigated in order to understand basic aerodynamic characteristics related to design of Micro Air Vehicle (MAV) for planetary exploration. Before the investigations, verification was conducted for the current numerical approach, which are commonly used and validated for high Reynolds number flow analysis, showing good applicability for low Reynolds number flow analysis. Flow around NACA4402 has been investigated for the condition of Mach number of 0.1 and Reynolds number of 1,000. Investigation shows that Reynolds number has a substantial influence on aerodynamic characteristics of the airfoil in low Reynolds number flow. In contrast, Mach number has a slight influence in comparison with Reynolds number.