Speaker
Description
During an atmospheric re-entry, a vehicle crosses at very high speeds the distinct atmospheric layers characterised by large density variations. The vehicle thus experiences several flow regimes, ranging from free molecular, rarefied (transition, slip) and continuous regimes. These regimes are commonly characterised by a Knudsen number (Kn) range [Kn is defined as the ratio of the mean free path and the vehicle’s characteristic dimension]. We focus on rarefied flows (0.001 < Kn < 0.01 for slip flow and 0.01 < Kn < 10 for transition flow). Such flows are mainly characterised by a non-zero slip velocity and a temperature jump in the vicinity of the wall as well as a diffuse aspect of the shocks. These flow features may strongly affect the aerodynamic properties of the vehicle compared to continuous flow. Besides, the Navier-Stokes equations - which assume small deviation from equilibrium - are no longer valid and a molecular approach is then required to describe such flows. Rarefied flows are therefore governed by the Boltzmann equation which takes into account both free flight and collisions of the particles. Simplified modelling strategies such as BGK-type models are also considered. These models are often solved using DSMC (Direct Simulation Monte Carlo) solvers such as SPARTA (Sandia National Laboratory), or deterministic discrete-ordinate solvers such as the CEA in-house K solver. It is crucial to ensure the validity of these numerical methods and modelling.
Since 2014, CEA-CESTA and CNRS/ICARE gather their respective expertise in experimental observations and numerical simulation/modelling of rarefied flows. On the one hand, this collaboration aims at taking profit of the experimental measurements for simulation validation and rarefied flow modelling purposes. On the other hand, the numerical simulations are used to complement the experimental observations. Therefore, the purpose of this presentation is to provide an overview of the latest studies performed in this framework, especially within the CHYP and APHYRA projects. As part of the CHYP project, measurements of Pitot pressure profiles, drag coefficient and glow discharge visualisations were performed at Mach 4 in the MARHy wind-tunnel around cone-cylinder geometries with 3 distinct aft-body (straight, rounded and flared) shapes. In the framework of the APHYRA project, measurements of aerodynamic forces and glow discharge visualisations were performed at Mach 4 and 20 in the same wind tunnel around a waverider model at several angle of attack. For both projects the main goal was to assess the influence of the rarefaction degree on the aerodynamic features. In the present work, numerical simulations for each geometry and flow condition with the CEA in-house K solver and SPARTA. In the presentation, comparisons between experimental and numerical results will be performed to assess the validity of the solvers for such various flow conditions and to illustrate the possible discrepancies between deterministic (K solver) and stochastic (SPARTA) approaches. Such an analysis is not commonly found in the literature. The simulation results will then be analysed to complement the experimental observations.
