Establishing highly-reliable techniques for analyzing ultra-high-speed flow, essential for next-generation space development

OGINO Yousuke

Specialized field: Aerospace Engineering, Hypersonic and Plasma Flows, Numerical Evaluation of Aerodynamic Characteristics

Do capsules for atmospheric entry burn up due to extreme heat? You've probably seen scenes of atmospheric entry in sci-fi anime or other media. How great is the load during atmospheric entry? Crafts like asteroid explorers and rockets launched into space return to earth as capsules. The atmosphere is the intermediate zone between space and earth, and a capsule falling from outer space to earth enters the atmosphere at an ultra-high speed of 10 km/second or more. At this time, air at the front of the capsule undergoes intense compression, giving rise to air plasma at ultra-high temperature. The result is a severe environment around the capsule exceeding 10,000°C. "The question of how to protect the capsule from these high-temperatures is a demanding technical hurdle for space missions," says Assistant Professor Ogino. At present, the front side of the capsule surface is outfitted with shielding material to protect against extreme heat and avert melting, but this contributes to increased launch weight. To trim heavy heat shielding, we need to determine the load applied to each part of the capsule, i.e., we require a highly reliable technique for analyzing the capsule heating rate. The theme of Dr. Ogino's research is tackling this challenge through "non-equilibrium models" employing disciplines like fluid dynamics and plasma engineering.

Atmospheric entry: a period of extreme non-equilibrium conditions where multiple physical reactions occur simultaneously

Multiple physical processes occur simultaneously around a capsule during atmospheric entry, resulting in a non-equilibrium state. A balanced state, on the other hand, is said to be in "equilibrium," and sciences like foundational thermodynamics and fluid dynamics have been systematized under the assumption that nitrogen and oxygen molecules in the air are in such an equilibrium state.

"In non-equilibrium states with a microscopic imbalance, like ultra-high-speed air flow where there is a continuous influx and efflux of extreme heat and matter, the number of elements that must be considered increases, in line with the diverse dynamics, and the theoretical framework becomes far more complex than the equilibrium state."

In the problem of atmospheric entry, where extreme high temperature and low density are present together, true non-equilibrium characteristics are important, and deriving the heating rate of the capsule requires a non-equilibrium model, not just a general physical calculation of an equilibrium state. The non-equilibrium models employed worldwide so far have calculated numerical values by averaging distributions of atomic and molecular energy modes, but there is room to improve analysis precision.

Dr. Ogino: "I wanted to find solutions for these averaged physical phenomena through direct calculation, and overcome the uncertainty of the previous global standard models." His aim is to develop analytic techniques more faithful to the actual phenomena by tracking numerous molecules one by one, and directly analyzing multiple factors, such as chemical reactions and light emission/absorption.

This approach is possible due to dramatic improvements in computation speed enabled by the technological development of computers. However, if you actually try to implement Dr. Ogino's idea, many of the parameters in the calculation program can't be completely determined, and if you try to find them one by one through calculation, in some cases it can take about 80 years to get a result... Thus, Dr. Ogino is trying to achieve dynamic correction based on results of experimental measurement of any parameters that can't be completely determined.

Striving to establish non-equilibrium models with higher precision

Dr. Ogino has already developed an innovative calculation program incorporating multiple types of physics, including fluid dynamics, chemical reactions, quantum mechanics, and statistical mechanics. However, as noted above, some of the parameters used in the calculation program are uncertain. To verify the numeric values of these uncertain parameters, he plans to conduct wind tunnel tests* of ultra-high-speed air currents using an arc-heated wind tunnel capable of recreating the actual flight environment. This special wind tunnel is located at the Institute of Space and Astronautical Science and Chofu Aerospace Center of the Japan Aerospace Exploration Agency (JAXA).

The analysis precision and reliability of calculations in a non-equilibrium model can be further improved by experimentally deriving numeric values for parameters. Dr. Ogino has high expectations for these experiment results: "By realizing precision of experimental measurements and calculation results in a complementary manner, we can likely make substantial progress toward analysis techniques that enable accurate prediction of the flow around a capsule during atmospheric entry."

Precise analysis of the capsule's heating rate will allow for optimal use of heat-resistant material at specific design points, enabling a reduction in the weight of the existing heavy shielding material. This will open the door to loading more experimental equipment and crew onto rockets, and will also help to reduce launch costs.

The next step after completing a non-equilibrium model is creating a system that allows many people to use the results. Dr. Ogino: "I'm thinking, for example, of mapping heating and load for various cases of capsule flight patterns and paths and eventually publishing the results." A non-equilibrium fluid analysis technique that successfully integrates diverse physical processes without conflicts is vital for the upcoming phase of space development. There are high hopes for the establishment and widespread adoption of such a technique.

* Tests where air is streamed around a stationary model, simulating the state of flying through the atmosphere, and measurements are taken of forces acting on the model and the flow of air around it.

A theory of non-equilibrium flow, expected to have engineering applications in a wide range of fields

Non-equilibrium phenomena are not limited to atmospheric entry; they appear in various situations even in ordinary life. Plasma is one example. When heat and electric energy are applied to a gas, the gas molecules dissociate into atoms, and as the temperature rises, the electrons orbiting the atomic nuclei separate from atoms, resulting in a plasma--i.e., an extremely energetic state consisting of a mixture of neutral molecules, plasma ions, and negative ions.

"Devices like air purifiers, for example, produce plasma ions and negative ions through electric discharge, and they are based on non-equilibrium theory. Other forms of plasma production like lasers and microwaves are also used in various fields. By establishing new non-equilibrium models, we can more accurately model the fluid motion of plasma, and the models will be applicable to many advanced applied technologies where plasma is used."

As part of his exploration of ultra-high-speed air flow, Dr. Ogino is also involved in research on hypersonic passenger aircraft. These planes fly at Mach 6, allowing traversal of the Pacific Ocean from Japan to the US in 2.5 hours, and research is moving forward as an international project in collaboration with the US, Australia, UK, and other countries. Demonstration experiments employing unmanned flight are already being carried out, and since the vibrations when the plane gets into turbulence are greater than ordinary aircraft, a key point for practical use will be establishing technology to maintain stable flight even in turbulence.

Dr. Ogino: "At my last position, I was involved in the same hypersonic passenger aircraft project. The theme interested me, and I'm continuing that research today. Behind the scenes, talks are underway on joint efforts with researchers, both inside and outside Japan, engaged in applied plasma research." Research in this field, aimed at elucidating phenomena in a non-equilibrium state, is a vital key not just to next-generation aerospace development, but to advancing a broad range of fields that involve physical processes and engineering applications.

Employing knowledge of math and physics, learned in high school and college, in a way that's interesting!?

Dr. Ogino first worked in his current aerospace-related research field as an undergraduate. In choosing a research theme for his senior graduation thesis, he ventured into the area of "atmospheric entry" because he thought the words sounded cool. In analyzing ultra-high-speed air flows around a capsule entering the atmosphere, he frequently applied knowledge of mathematics, physics, and chemistry that he learned in high school and university, and he says that was a lot of fun.

Looking back on that time, Dr. Ogino says: "By using the equations and physical laws I'd learned, I solved one problem after another, and it was fun how the various concepts applied. It was like solving puzzles. It was fascinating--taking a physical phenomenon, visualizing it with science, and attaching an interpretation. Whenever I noticed something interesting, I wrote out equations, and I often passed hours that way."

When asked about the special appeal of his research, Dr. Ogino replied that it was the "thrill of pioneering new ground that our great predecessors couldn't tread by using the latest knowledge and technology." Another interesting point, he says, is fully exploiting human intelligence, in the form of equations and experiments, to elucidate real-world physical phenomena like air flow around an airplane or capsule.

General laws allowing unified understanding of extremely complex non-equilibrium states have yet to be worked out. Dr. Ogino's research, which endeavors to intricately elucidate each element of complex phenomena through direct calculation, holds a captivating allure, perhaps stemming from the grandeur of "unraveling the mysteries of the cosmos and the world with human intelligence"--a sentiment Dr. Ogino personally considers the true charm of his work.

Date of posting: December, 2023/ Date of interview: March, 2022