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Description
Atmospheric entry into a target planet is a critical phase for space missions as the spacecraft must face harsh conditions involving thermal loads in the order of Megawatts. During the entry, the atmospheric gas dissociates and (partly) ionizes. The resulting plasma sheath subjects the spacecraft to high heat fluxes and leads to communication blackout as well. Since both aspects can compromise the safety of the vehicle. Since both aspects can compromise the safety of the vehicle, a design which employs advanced protection systems is necessary to ensure the success of future planetary missions. The charged particles in a plasma flow can be manipulated by applying an adequately high electromagnetic field, which modifies the shock structure and shock standoff distance (SSD), mitigates the heat flux and creates a magnetic windowing effect that can reduce the communication blackout period.
The idea of flow control by this means of an externally applied magnetic field – i.e., the MHD approach – was proposed for the first time in the 1950s. The theoretical idea of MHD flow control is as follows: the magnetic system installed in the spacecraft produces a magnetic field which is applied to the weakly ionized plasma flow, and an electric current is produced in the shock layer. As a result of the interaction between this current and the magnetic field, a Lorentz force is induced, which decelerates the plasma flow in the shock layer and increases the shock standoff distance. This approach can be applied to atmospheric re-entry problems by means of an MHD probe. This concept was not fully developed at the time it was first formulated, because a proper technology to produce an adequate magnetic field was still missing. However, the development of superconductive coils in the last decades opens the possibility to design a flight-capable magnetic shielding device able to face the re-entry flight plasma conditions. In this new technological context, the MHD Enhanced Entry System for Space Transportation (MEESST) Horizon 2020 project will exploit MHD-effects and develop a demonstrator implementing active magnetic shielding by means of a superconductive coil system. MEESST includes experimental campaigns in the plasma wind tunnels of the Von Karman Institute (VKI) and the Institute of Space System (IRS), and numerical simulations relying upon improved models. To explore the use of MHD thermal protection system, numerical tools to predict the behaviour of atmospheric entry plasma flows in thermochemical nonequilibrium (TCNEQ) under the influence of externally applied magnetic fields have been developed as a part of MEESST project. In this work, ground experiments done by Knapp and Kranc in plasma wind tunnel using Argon with MHD have been numerically rebuilt by means of the three in-house CFD codes namely SAMSA (developed by IRS), HANSA (by University of Southampton) and COOLFluiD (by KU Leuven). The results have been compared among the codes and against the available experimental data. The key findings of this work can be summarized as :
1. All the codes present internal consistency of results, i.e. the expected MHD-effects caused by the applied magnetic field on the shock structure and heat flux can be detected.
2. The shock standoff distance increases (all the codes) and the heat flux decreases (COOLFluiD and HANSA);
3. For the Knapp test case, the computed SSDs match well for the un-magnetized case; when applying the magnetic field, the SSDs are the highest for COOLFluiD, the lowest for HANSA and in the middle for SAMSA.
4. For Knapp test case, MIG-distances computed by SAMSA match well with the experimental results from the published works of Knapp, especially for the magnetized cases.
5. For Knapp test case, the heat flux for the 0 magnets case from COOLFluiD simulations is the closest to the available experimental data from Knapp, while HANSA underestimates the value; the heat flux reduction when applying 1 and 6 magnets from COOLFluiD slightly overestimated with respect to the prediction from Knapp, while again HANSA underestimates it.
6. Results from SAMSA, HANSA, COOLFluiD for the percentage increase in SSD against the experimental values by Kranc and the numerical results with LeMans as a function of the magnetic field strength at the stagnation point are compared.
7. For the Kranc test case, the SSD percentage increase calculated by COOLFluiD matches well with the experimental results from Kranc, SAMSA has a coherent behaviour, while HANSA underestimates the results for the lowest magnetic field strength and greatly overestimates them for the highest applied fields.
The reason of the discrepancies in the results has been identified to be due to the differences in the implemented chemistry and ionization models of argon and transport properties models. Future works include the improvements of the models implemented by HANSA in order to better harmonize the results of the three codes and achieve a better match of the experimental results and testing the codes for ground experiments with MHD for air etc. , which will further benchmark them for use in re-entry applications. SAMSA will increase the resolution in the boundary layer in order to provide calculations of the heat flux. Moreover, the code will be extended in order to simulate air plasma flows. Finally, the three codes will be employed for numerically rebuilding the experimental simulations of atmospheric Earth re-entry flows performed by the MEESST consortium.
Summary
The authors aim to assess, validate and verify their in-house CFD solvers for various Argon plasma ground testing experiments in the literature. This activity helps us in understanding the differences in the respective CFD modelling and handling of plasma physics and thermochemical non equilibrium reactions. Using these results, we aim to prepare the codes to aptly handle more complex problems for future applications using air and/or carbon di oxide for atmospheric re-entry applications.
