永田 靖典

Abstract Communication blackouts during atmospheric reentry pose significant challenges to the safety and adaptability of spacecraft missions. This phenomenon, caused by the attenuation of electromagnetic waves by the plasma surrounding the spacecraft, disrupts communication with ground stations or orbiting satellites. Therefore, it is crucial to decrease the plasma density in the vicinity of the spacecraft to ensure an unobstructed electromagnetic wave communication path. This study proposes a methodology that involves the injection of gas from the vehicle’s wall to create an insulating layer near the surface. This thin layer maintains lower temperatures and reduced plasma density, enabling electromagnetic wave propagation without attenuation. Practical experiments were conducted in an arc-heating facility to simulate atmospheric reentry conditions. The results of the experiments provided empirical evidence of the effectiveness of the technique in mitigating communication blackout phenomena. Numerical fluid analysis within the wind tunnel chamber validated the formation of an air film layer near the experimental model owing to the injected gas. Schlieren imaging revealed distinctive jet shapes, which corroborated the findings of the numerical analysis. The wind tunnel tests that simulated atmospheric reentry environments confirmed the formation of an air film layer through gas injection, which substantiates the reduction in communication blackout. These results have the potential to improve communication reliability in space transport.

<p>Numerical prediction of air-entraining and submerged vortices in pump sumps is important for engineering applications. The validation of pump sump simulations, however, still is not enough, because the simulations is very complicated; for examples, treatment of gas-liquid interface, detection method of the vortices and selection of turbulence model etc. We conducted numerical simulations of the benchmark experiments of the pump sump conducted by Matsui et al. (2006, 2016) and compared the simulation with the experimental data to investigate the effects of turbulence model, grid density and detection method of the vortices. We determined a threshold of the gas-liquid fraction function of VOF method (<i>α</i>) and the second invariant of velocity gradient tensor (<i>Q</i><sub>2</sub>) to detect the air-entraining and submerged vortices by using vorticity, respectively. This method well detected the vortices and well reproduced the experiments for the RANS simulation using SST <i>k-ω</i> model. Large eddy simulation using Smagorinsky model, however, was sensitive to the grid system and difficult to reproduce the experimental vortex structures even for the finest grid system having 3.7 million cells.</p>

Fluctuating pressure (p' ) of a large-scale vortical structure generated in a semiconductor single wafer spin cleaner was detected by using microphone array. Twelve microphones were installed on the exhaust cover under the rotating disk of the cleaner with their interval of 7.5° or 15°. Power spectrum densities (PSD) of p' were compared with those of fluctuating velocity measured by PIV for various rotation angular velocities to identify fluctuations due to convection of the large-scale vortical structure. Good agreement of PSDs indicates that the large-scale structure could be detected by using microphone. Cross-correlation of p' measured at different positions revealed that the large-scale structure convected to the downstream in the rotational direction of the disk. The convection speed was about 12 % of the angular velocity of the rotating disk. Number of the vortex in the large-scale structure was also evaluated from the time-series p' data. Time-space contour map was made for p' based on the data measured at the different angular position, and showed periodical swept strip patterns. Presences of the strip patterns indicate the pressure disturbances were stably convected to the downstream. From this time-space map, two-dimensional Fourier transform efficiently extracted the number of vortices in the large-scale structure.

<p>In the electrodynamic flow control, weakly-ionized plasma flow behind the strong shock wave could be controlled by the applied magnetic field around a reentry vehicle. To control the flow field, a very strong magnetic field is required and it could be applied by the superconducting magnet, which is too large and heavy for a reentry capsule. Thus, in the present study, to avoid the use of the superconducting magnet, the electrodynamic effect from the combination of multiple weaker magnetic source such as permanent magnets is numerically investigated. According to the MHD simulation, the influence on the drag force caused by the multiple magnetic source, which is placed equiangularly around the body axis, could be diminished by the Hall effect. When the Hall effect is significant, the induced electric current intensity is very small because the electric field is generated much weaker than the one of the single magnet case. Therefore, the present magnetic configuration using multiple magnetic source might not be effective under the high Hall parameter condition such as the reentry flight.</p>

We experimentally and numerically investigated large-scale structures formed by vortices in a single wafer spin cleaner. The Q-criterion identified the vortices developed in the cleaner as the flow regions with positive second invariant of the velocity gradient tensor obtained by both the PIV and LES. The time-series two-components PIV data shows that small-vortices were clustered near and under the edge of the rotating disk and were periodically emanated from there to the housing wall of the cleaner. The emanation frequency was increased with increasing in the angular velocity of the rotating disk. Three-dimensional LES reveal that six longitudinal vortices were spirally developed from under the edge of the rotating disk to the housing wall. This structure stably rotated slower than the disk speed. Fourier analysis of the LES data agreed with that of the PIV data. This supports that the passages of the stable spiral vortices on the PIV measurement region resulted in the periodical emanation of the clustered small-vortices observed in the PIV. Such a very large-scale spiral structure will induce reattachment of contaminants on the wafer surface, and should be destructed for development of much higher efficient cleaner.