Hybrid CFD/DSMC modelling of high-altitude rocket plumes

Abstract: Space launches are often observed to generate waves and depletions in the ionosphere. These phenomena are caused by fluid interactions and chemical reactions of exhaust gases respectively. Accurate quantitative models of these processes may provide methods for launch detection and characterisation but require accurate treatment of exhaust plume expansion in the thermosphere. In this work, we applied a hybrid Computational Fluid Dynamics (CFD)/Direct Simulation Monte Carlo (DSMC) approach to model the exhaust plume for a Falcon 9 rocket launch at (1) 160 km altitude and (2) immediately preceding 2nd stage cut-off. The modelling was done at steady-state over a two-dimensional (2D) axisymmetric domain. The CFD method modelled the dense continuum flow region surrounding the rocket nozzle exit by solving the compressible Euler equations. A constant collision frequency surface was then extracted to represent inflow to a DSMC simulation. Therein, the collisionless Boltzmann equation was solved stochastically for a variable soft sphere collision model. Density, pressure, and temperature in the plume were obtained and transport and acceleration of exhaust gases investigated. Known features of high altitude plumes were observed, including the bow and inner shocks, indicating the technique reproduced the relevant flow physics. The bow shock produced higher temperatures in Case 2 but the inner shock enclosed a larger region in Case 1. Downstream of the nozzle, exhaust gasses increasingly became concentrated in the region between shocks. However, they experienced relatively little acceleration over 25 km, particularly in Case 1. Keywords: DSMC, atmospheric physics, aeroacoustics, rockets