Numerical Investigation of Electron Number Density Prediction in Hypersonic Weakly Ionized Reentry Flows

다국어 초록 (Multilingual Abstract)

Communication blackouts occur due to plasma sheaths generated during hypersonic reentry. Accurate prediction of electron number density is therefore essential for under- standing and mitigating blackout phenomena. Among the representative flight experi- ments related to this issue, the RAM-C II flight test provides valuable electron number density measurements under hypersonic reentry conditions. Because the forebody of RAM-C II employed a beryllium heat sink, the flowfield remained close to pure air, allowing direct comparison between numerical simulations and experimental measure- ments without considering ablation effects. In the present study, three-dimensional simulations considering the geometry of the electrostatic probe （ESP） mounted on the aftbody of RAM-C II were performed to pre- dict electron number density distributions and compare them with flight-test data. Since full-domain three-dimensional simulations are computationally expensive, a multi-scale simulation framework was developed by coupling two-dimensional axisymmetric sim- ulations with a local three-dimensional subdomain containing the ESP geometry. To provide accurate inflow conditions for the three-dimensional simulations, a shock-fitted mesh generation strategy was developed for the two-dimensional simulations and vali- dated using representative hypersonic test cases, including a Mach 8 cylinder and OREX reentry condition. In addition, an interpolation inlet boundary condition was imple- mented to transfer flow-variable profiles from the two-dimensional simulations to the three-dimensional subdomain through linear interpolation. An ion-electron recombina- tion wall boundary condition was also developed to account for plasma-wall interaction effects. Using the proposed framework, three-dimensional simulations of the RAM-C II ESP region were conducted, and the predicted electron number density and electron produc- tion rate were compared with experimental measurements. Furthermore, a post-pro- cessing methodology consistent with the ESP measurement principle was developed to reconstruct electron number density from CFD results while accounting for the physical characteristics of weakly ionized flows. The results demonstrate that the proposed framework enables accurate electron number density prediction at significantly reduced computational cost. In particular, the compression effect induced by the electrostatic probe was found to play an important role in accurately reproducing experimentally observed electron number density distributions.
Keywords: Hypersonic flow, Weakly ionized plasma, Communications blackout, Electron number density prediction, Computational fluid dynamics, Shock-fitted mesh

목차 (Table of Contents)

• 1. Introduction 1
• 1.1 Research background 1
• 1.1.1 Communication blackout of hypersonic vehicles 1
• 1.1.2 Weakly ionized hypersonic reentry flow and measurement challenges 2
• 1.2 Related works 3
• 1.3 Research outlines 7
• 2. Governing equations and numerical methods 12
• 2.1 Governing equations 12
• 2.1.1 N-S equation for two-temperature model 14
• 2.1.2 Viscous term 16
• 2.1.3 Thermodynamic properties 18
• 2.1.4 Transport properties 22
• 2.1.5 Source term 23
• 2.2 Numerical methods 24
• 2.2.1 Convective flux 24
• 2.2.2 Flux limiter 26
• 2.2.3 Time integration 27
• 3. Mesh generation strategy 28
• 3.1 2D shock-fitted mesh generation strategy 30
• 3.1.1 Improved Ducros sensor 30
• 3.1.2 Shock region clustering using DBSCAN 32
• 3.1.3 Shock curve fitting based on least-squares spline approximation 33
• 3.1.4 Removing phantom nodes from shock curves 35
• 3.1.5 Mesh reconstruction 35
• 3.1.6 Validation cases 39
• 3.2 3D mesh generation using electrostatic probe geometry 50
• 3.2.1 Electrostatic probe modeling 50
• 3.2.2 3D sub-domain mesh generation 52
• 4. Boundary conditions 59
• 4.1 Interpolation inlet boundary 59
• 4.1.1 Formulation and implementation 59
• 4.1.2 Validation case 62
• 4.2 Ion-electron recombination wall boundary 69
• 5. Numerical simulation of RAM-C II flight test 73
• 5.1 Two-dimensional results 74
• 5.2 Three-dimensional results 85
• 5.3 Analysis of electron number density 93
• 6. Conclusion 97
• 6.1 Contributions 97
• 6.2 Future works 98
• Bibliography 100