HyperCODA -- Extension of Flow Solver CODA Towards Rocket Flows

Kurzfassung

For the development of modern reusable rocket launch and reentry vehicles, accurate simulation is imperative throughout the whole design phase. The SpaceX Falcon 9 rocket demonstrated feasibility of reusing first and second stage of a rocket, which lowers operating costs for target-to-space operations. This led to a boom of research effort in this sector, aiming to prevent the spacecraft structures from failing during the takeoff and reentry. For the involved aerodynamic maneuvers, it is essential to reliably and accurately estimate the maximum heat flux through the body surface during takeoff and reentry. However, high temperatures, prevalent strong shocks and, in turn, dissociation of the gas complicate matters. For these applications the DLR flow solver TAU was extended with the spacecraft extensions4 which include suitable fluid models, shock limiters, and shock-stable flux functions to cope with these types of applications. However, TAU was designed 30 years ago. Modern computer hardware incorporates performance enhancements, such as multiple cores per socket, deep cache hierarchies with non-uniform memory access, and accelerator cards. Taking advantage of optimizations for these kinds of hardware, can significantly impact the structure of a code and a full redesign is not feasible for the legacy code base. Thus, one European effort to improve the solver basis in Europa is based on a initiative of Airbus, DLR and ONERA to develop the next generation CFD solver CODA (short for "CFD for ONERA, DLR and Airbus"). This modularized flow solver is based on common framework and architecture of the code Flucs11 and features finite volume as well as Discontinuous Galerkin solvers with RANS and DES turbulence models. The last four years of common solver development focused on subsonic and transonic flows around aircraft. In this flow regime only weak shocks are present and the assumption of one perfect gas as fluid model proves sufficiently accurate. In contrast, the conditions in which spacecraft engines operate are more extreme: High altitude flow conditions as well as high velocities during takeoff and reentry maneuvers need to be simulated. Mach numbers in the order of 10 or above mandate specialized flow solvers able to deal with the involved strong shocks and, in turn, high temperatures and, thus, temperature-dependent ideal gas mixture equation of states (EOS). In a first contribution about HyperCODA7 we validated the HyperCODA extension of the flow solver CODA for conditions at high Mach number together with a perfect gas EOS. In this contribution we extend the description using a more realistic ideal gas EOS that includes high-temperature effects. This additional physical model allows us to make quantitative comparisons of the flow solver HyperCODA to the validated flow solver TAU for representative flight conditions of spacecraft vehicles during takeoff, reentry, and landing. The paper is organized as follows: In the first section we introduce briefly the flow solver as well as the physical models, which is followed by a validation of the solver using basic examples. Then, industry-relevant test cases are shown and the paper is closed with a short summary and outlook to further work.