Direct Simulation Monte Carlo Modeling of Weakly Ionized Flows
When vehicles reenter a planetary atmosphere at super-orbital velocities, collisions between particles are sufficiently energetic to produce ionized species through both associative and direct ionization reactions. As a result, a weak plasma surrounds the vehicle. This plasma can prevent radio communication from reaching the vehicle, a condition commonly referred to as 'communications blackout'. Additionally, the ability to accurately simulate the flow field composition, including the ionized species, is important for the computation of both convective heating at the vehicle surface and radiative heating from the shock layer. For some flight conditions, radiative heating can be a significant portion of the total vehicle heating. For these reasons, it is necessary to incorporate thermochemical models for ionized species into numerical simulations of atmospheric entry vehicles.
The Direct Simulation Monte Carlo method (DSMC) has been used extensively to compute neutral rarefied gas flow fields. The Nonequilibrium Gas and Plasma Dynamics research group at the University of Michigan currently uses a state of the art DSMC code called MONACO to simulate neutral rarefied gas flows. This task is focused on adapting MONACO to include the capability to accurately simulate high energy, weakly ionized flows. The Flight Investigation of Reentry Environment (FIRE) project is used as the baseline case to test the modifications made to MONACO.
This research will allow the following questions to be addressed in the rarefied portion of the reentry trajectory:
- How does the presence of the weak plasma around the vehicle affect the heat transfer to the vehicle surface and the pressure distribution along the vehicle surface?
- How long is the radio blackout period and when does it occur?
- How important is the radiative heating component of the overall heat transfer to the vehicle surface?
- Is the neutral chemistry model that is typically used with the DSMC method sufficient to model ionizing reactions?
- What are the magnitudes of the self-induced electric fields within the plasma and do they significantly affect the flow field properties in an aerothermodynamics sense?
Figure 1 shows the effect of adding ionizing reactions on the energy of the FIRE II flow field at an altitude of 85km along the reentry trajectory. The translational, rotational and vibrational temperatures are reduced, and the electron temperature is in equilibrium with the vibrational temperature. Figure 2 shows the level of ionization throughout the flow field for this case.
Figure 1: Mode temperatures along the stagnation streamline, FIRE II, 85km.
Figure 2: Electron number density, FIRE II, 85km.
This research is funded by the NASA Constellation University Institutes Project (CUIP).