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The Forefront of Space Science

Computational Science in Fluid Mechanics
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Space science and numerical simulation

The advantages of numerical simulation above seem to be true. There are many cases, however, where these advantages cannot be fully utilized in space science, e.g., fluid phenomena that need to be explored under severe conditions such as high-Mach number, high-Reynolds number, extreme temperature, and high-pressure conditions. The development of an accurate numerical algorithm and physical model is essential to fully utilize the advantages of numerical simulation. This is the same as in experiments where the development of accurate experimental techniques and measuring equipment is required to conduct difficult experiments.

We have been proposing numerical algorithms that have spectral-like spatial resolution by incorporating physical insight into the algorithms, which allows us to reproduce various complex flow phenomena accurately. Flow mechanics that is difficult to reproduce accurately by numerical simulation include: turbulent flows interacting with shock waves; supercritical fluids under extremely low temperature/high pressure conditions in relation to the design of a liquid rocket engine; and plasma flows in outer space (in this case, experiment itself was difficult). As for the numerical simulation of shock waves and turbulence interactions, we have proposed a numerical algorithm that selectively adds numerical diffusion onto the dilatational motion to capture shock waves numerically while minimally affecting the vortical structures , and thus resolving the turbulence accurately. This numerical algorithm enables to reproduce the complex phenomena in shock waves and turbulence interactions.

Figure 1 shows the turbulent mixing of under-expanded sonic fuel jets injected into a supersonic engine, which was obtained by the numerical simulation using the above algorithm. With the development of the accurate numerical algorithm, we were able to successfully and accurately simulate the shock waves and turbulence interacting phenomena inside the supersonic engine and the turbulent-mixing phenomena, which were difficult to understand in detail in the past. This numerical algorithm allows us to study the details of high-speed fluid mechanics in both high-temporal and high-spatial resolution.

Figure 1
Figure 1. Turbulent mixing of under-expanded sonic fuel jets injected in a supersonic engine

At left are the simulation results (instantaneous snapshots of density-gradient distribution of simulating Schlieren image and fuel-jet distribution). At right are the experimental visualization images (instantaneous PLIF image [Santiago & Dutton, J. Prop. Power, 1997] and OH-PLIF image [Heltsley, Stanford University, doctoral thesis, 2010]). The upper images are the results observing the cross-section of the jet center plane from the side while the lower image is the results observing from the top view horizontal to the wall surface.

JAXA's Engineering Digital Innovation (JEDI) Center uses the cost and time benefits of numerical simulation for launch-pad design. JEDI center investigates a launch-pad design that reduces the intense aeroacoustics at the launch of the Epsilon rocket (Fig. 2). It succeeded in reducing the aeroacoustics by approximately one-tenth and at approximately one-tenth the cost in comparison with previous M-V rocket launches. (For more detail, see “World’s first optimal design of a launch pad by acoustic simulation technologyEon the JEDI website)

These results suggest that development of numerical algorithms and physical models by the researchers in fluid mechanics and computational mechanics becomes indispensable for full utilization of the advantages of numerical simulation.

Figure 2
Figure 2. Aeroacoustic simulation result (left) used for the launch-pad design of the Epsilon rocket and the actual launch pad (right)

Reproducing the intense aeroacoustic wave (pressure wave) propagation generated by the exhaust jets from rocket engine by numerical simulation and applying the results to actual launch-pad design. (Left: courtesy of Dr. Seiji Tsutsumi of JEDI, and right: JAXA digital archives).

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