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希薄実在気体流れの高精度シミュレーション

航空宇宙技術研究所　流体科学研究センター
古浦勝久

 　高層大気高速飛行体の空力特性や高真空機器の希薄気流特性などの計算は直接シミュレーション・モンテカルロ（DSMC）法を用いた希薄実在気体流れのシミュレーション（粒子模擬計算）によって広く行われている。当所においても大型電子計算機を利用して、分子衝突シミュレーション法の改良（Null-Collision法：文献1-3）、分子衝突モデルの開発・改良（Variable Soft Sphere モデル等：文献4-9）、それらを用いた大規模計算（文献10-12）などを行った。DSMC計算と実験結果との一致は概ね良好であるが、なお精度的に不十分な場合も多く、NWTの導入後、高精度シミュレーションのための分子衝突モデルの開発・改良（文献13-17）を進め、それらを用いた大規模計算（文献18,19）を行った。また、低温での量子効果を考慮した高精度DSMC計算なども実行した（文献20）。
　しかし、DSMC分子衝突モデルは有効性の検証も含めて、さらに精度を高める必要がある。最近、適切な分子間ポテンシャルを用いたHamilton運動方程式を解くことにより分子間衝突を高精度に計算するClassical Trajectory Calculations (CTC) 法をDSMC法における分子衝突計算に用いるCTC-DSMC法を開発した（文献21-25）。CTC-DSMC法は多大な計算時間を要するので、現在のところ複雑な流れのシミュレーションには適用困難であるが、近い将来における電子計算機の計算速度の増大を予想すれば、CTC-DSMC法は有力なシミュレーション法になることが期待できる。



参考文献
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  4. "Variable Soft Sphere Molecular Model for Inverse-Power-Law or Lennard-Jones Potential," Physics of Fluids A, Vol.3, No.10, October 1991, pp.2459-2465.
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  9. "Variable Soft-Sphere Molecular Model for Monte Carlo Simulation of Air Species," AIAA Progress in Astronautics and Aeronautics (18th), Vol.159, 1994, pp.459-468.
  10. "Direct Simulation of Vortex Shedding in Dilute Gas Flows past an Inclined Flat Plate," Physics of Fluids A, Vol.2, No.2, February 1990, pp.209-231.
  11. "Direct Simulation of Vortex Shedding in Dilute Gas Flows past Bluff Bodies," Rarefied Gas Dynamics (17th), 1991, pp.695-702.
  12. "Rarefied Gas Numerical Wind Tunnel," Rarefied Gas Dynamics (17th), 1991, pp.663-670.
  13. "A Set of Model Cross Sections for the Monte Carlo Simulation of Rarefied Real Gases: Atom-Diatom Collisions," Physics of Fluids, Vol.6, No.10, October 1994, pp.3473-3486.
  14. "Molecular Models for Monte Carlo Simulation of Rarefied Real Gases," Rarefied Gas Dynamics (19th), 1995, pp.483-489.
  15. "Monte Carlo Simulation of Hypersonic Rarefied Nitrogen Flow around a Circular Cylinder," Rarefied Gas Dynamics (19th), 1995, pp.1236-1242.
  16. "A Generalization for Paker rotational relaxation model based on variable soft sphere collision model," Physics of Fluids, Vol.8, No.5, May 1996, pp.1336-1337.
  17. "Rotationally and Vibrationally Inelastic Cross-Section Models for the Direct Simulation Monte Carlo Method," Rarefied Gas Dynamics (20th), 1997, pp.719-726.
  18. "RCSジェットと希薄気流干渉のモンテカルロ直接シミュレーション," NAL SP-34, 1997年1月, pp.209-214.
  19. "Improved Null-Collision Technique in the Direct Simulation Monte Carlo Method: Application to Vibrational Relaxation of Nitrogen," Computers and Mathematics with Applications, Vol.35, No.1/2, 1998, pp.139-154.
  20. "Direct Simulation Monte Carlo Study of Quantum Effects on the Spherical Expansion of 4He," Physics of Fluids, Vol.11, No.10, October 1999, pp.3174-3180.
  21. "Monte Carlo Direct Simulation of Rotational Relaxation of Diatomic Molecules using Classical Trajectory Calculations: Nitrogen Shock Wave," Physics of Fluids, Vol.9, No.11, November 1997, pp.3543-3549.
  22. "Monte Carlo Direct Simulation of Rotational Relaxation of Nitrogen through High Total Temperature Shock Waves Using Classical Trajectory Calculations," Physics of Fluids, Vol.10, No.10, October 1998, pp.2689-2691.
  23. "Direct Simulation Monte Carlo Method Coupled with Classical Trajectory Calculations for Rigid Diatomic Molecules: Application to Nitrogen Shock Wave," Rarefied Gas Dynamics (21st), Vol.2, 1999, pp.23-30.
  24. "Monte Carlo Direct (Test-Particle) Simulation of Rotational and Vibrational Relaxation and Dissociation of Diatomic Molecules Using Classical Trajectory Calculations," Rarefied Gas Dynamics (22nd), 2001, pp.7-16.
  25. "Direct Simulation Monte Carlo Study of Rotational Nonequilibrium in Shock Wave and Spherical Expansion of Nitrogen Using Classical Trajectory Calculations," Physics of Fluids, Vol.14, No.5, May, 2002, pp.1689-1695.
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