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The Forefront of Space Science

Computational Science in Fluid Mechanics
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At elementary school, I had my first experience playing with a NEC PC-98 series computer that my parents bought. As far as I recall, its CPU computing power was about 8MHz and hard disk capacity about 20MB. About 10 years later, I joined my university laboratory and bought a laptop computer with 266MHz CPU and 2GB hard disk. Now, another dozen years later, my favorite MacBook Air has a 1.7 GHz dual core CPU and 512GB hard disk. Amazingly, even smart phones today outperform the computers of a decade ago! The computer is advancing at a remarkable speed.

In my specialty field of fluid mechanics, numerical simulation has become the third pillar supporting research in addition to the theoretical and experimental researches. This is backed up by rapidly advancing general-purpose computers and supercomputers whose performance is being improved at a speed of almost 500 to 1,000 times per decade. In this article, I would like to introduce the fluid mechanics researches for space science using numerical simulation.

Fluid mechanics closely associated with space science

Although fluid-mechanics phenomena are often invisible, they are present and we take advantage of them in various scenes of our lives. In the same way, the fluid mechanics is inseparably bound up with space science.

The dimpling on the surface of a golf ball is closely related to fluid-mechanics phenomena (i.e., turbulence). The dimples promote turbulence and consequently improve the aerodynamic force loaded on the balls, stabilize the trajectory and extend the flight distance. This fluid mechanics for aerodynamic is deeply associated with both the aerodynamic design of the Mars exploration aircraft now being researched at ISAS and the aerodynamic stability of the reentry capsule of HAYABUSA, which returned to earth with materials from the surface of the asteroid Itokawa. By the way, have you heard the swishing sound near electrical lines on a windy day? This is also caused by the fluid mechanics phenomena called aeroacoustics. The intense aeroacoustics generated by the exhaust jet of a rocket engine is an important research theme because it affects payloads such as satellite onboard rockets. Furthermore, the overlap between space science and fluid mechanics includes: regenerative cooling and mixing/combustion of fuels related to liquid rocket engine design; interference between the solar wind and the earth’s magnetosphere in outer space; and supernova explosions.

Thus, to understand in detail the fluid dynamics phenomena that are closely related to space science is a significant element in the advance of space science generally, including space engineering and science.

What is numerical simulation?

Numerical simulation is one approach to understand the fluid mechanics in detail, in addition to theoretical and experimental approaches. The advantages of numerical simulation include: (1) Enables the close study of fluid mechanics at both high-temporal and high-spatial resolution. (2) Enables the free set up of flow conditions. In space science there are many cases where experiments are difficult or their cost too expensive, for example, experiment for high-Mach number, high-Reynolds number, extreme temperature, high-pressure conditions, and phenomena in outer space. (3) Economical and temporal advantages compared to experiments.

For better or worse, most governing equations describing fluid mechanics cannot be solved mathematically. Numerical simulation is an approach, using the arithmetical capacity of computers, to solve numerically complex fluid phenomena that cannot be solved mathematically. The simulation reproduces physical quantities representing fluid phenomena such as velocity and density as numerical values in computers, then conducts four arithmetical operations according to the governing equations describing such fluid phenomena, and tracks the phenomena along time evolution.

Numerical simulation can be considered as “numericalEexperiments on fluid mechanics using computers in place of experimental equipment. According to this concept, it is construed that a numerical simulation is the same as an laboratory experiment. Though the tool (method) used is different, the approach is the same in that it replicates fluid phenomena by setting up experimental conditions and then studies the phenomena.

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