Condensation Modelling of Expanding Cold Gas Jets during Hypersonic Retro-Propulsion Manoeuvres within the RETPRO Project

The RETPRO project (Validation of Wind Tunnel Test and CFD Techniques for Retropropulsion), as part of ESA’s Future Launchers Preparatory Programme, aims at preparing the tools, necessary for a reliable design and simulation of future rocket launchers or spacecraft. A particular focus is assigned to vertical take-off and landing configurations using retro propulsion as part of their control concept for entry, descent, and landing manoeuvres. Wind tunnel tests and computational fluid dynamics are used to generate a comprehensive aerodynamic database, which is required for flight dynamics simulations, enabling mission and performance analyses of possible future launcher designs. Windtunnel tests are conducted in the DLR Cologne H2K facility, with room temperature dry air ejected through selected nozzles to simulate the exhaust plume. Condensation effects might occur in the plume due to the low static freestream pressure at Mach 7, combined with the expanding flow in the nozzle. This paper presents results from numerical investigations including a vapour-equilibirum model which evaluate the potential influence of plume condensation on measured data in the wind tunnel. A qualitative comparison between experimental and numerical results is presented through Schlieren photographs. Condensation was observed in the numerical results, causing the flow path in and around the plume to be altered. Surface pressure coefficients in the condensation case were observed to be approximately 5% lower than when using the standard ideal gas model. Finally, the shock stand off distance was reduced, but not significantly. The comparison with tunnel data was therefore more-or-less the same as with the ideal gas model and the use of the condensation model was not deemed necessary for subsequent computations

HyperCODA - Towards high-performing time-resolving flow simulations

The present work focuses on the performance analysis of the DG-SEM implementation of the CFD solver CODA. The turbulent Taylor-Green vortex is employed as a simple testcase for scaling behavior, while for a more detailed node-level performance analysis more granular kernel benchmarks are used. Bottlenecks in the implementation are highlighted and possible solutions proposed

Generic test case to understand cryogenic methane combustion dynamics

The accurate numerical simulation of high-pressure rocket combustion chambers is a key element for the design of future space transportation vehicles. In recent years investigations focus more and more on methane-based combustion chambers for which many thermodynamic and combustion issues are still unknown. Experimental investigations indicate that we might have to deal with a close interaction of non-ideal equation-of-state effects together with complex kinetic schemes. On the numerical side the main challenges include the consistent approximation of thermodynamic effects within a broad temperature range from cryogenic injection up to high temperature effects at combustion. Nowadays simple numerical test cases are missing to study and understand the basic effects during the injection and combustion process especially with methane as propellant. This open question is addressed by the definition of a simple test case for cryogenic methane combustion and its analysis by applying a range of thermodynamic and kinetic models

Simulation von kompressiblen Mehrphasenströmungen bei extremen Bedingungen mit einem discontinuous Galerkin Verfahren

This work provides a contribution to the approximation of compressible multi-phase flows using a high-order discontinuous Galerkin spectral element method. Compressibility effects have to be considered for operating conditions close to the critical point. Important examples for such extreme ambient conditions include fuel injection systems of aeronautical, automotive and rocket engines. The simulation of compressible multi-phase flows at these ambient conditions imposes high demands on the numerical treatment as well as the numerical method. On the one hand, due to the compressible treatment of both fluid phases and their corresponding numerical methods, especially regarding the numerical resolution of the phase interface. On the other hand, the evaluation of equation of states, that are valid in the vicinity of the critical point, is expensive. As additional challenge are hydrodynamics and thermodynamics coupled closely by the compressible flow equations. This implies that a thermodynamically consistent numerical method has to be chosen. The building blocks of the described numerical method for compressible multi-phase flows include a compressible flow solver for the bulk phases, a level-set based interface tracking method, a comprehensive description of the equation of state and a model for the interface approximation. The interaction of these parts within the solution algorithm is described and validated in the thesis.Diese Arbeit liefert einen Beitrag zur Approximation von kompressiblen Mehrphasenströmungen mit einer diskontinuierlichen Galerkin Methode hoher Ordnung. Bei Betriebsbedingungen nahe des kritischen Punktes müssen die Kompressibilitätseffekte in beiden Phasen in der numerischen Approximation berücksichtigt werden. Wichtige Beispiele für die Anwendung unter solchen Umgebungsbedingungen sind Treibstoffeinspritzsysteme, wie sie in Motoren von Automobilen, Flugzeugen und Raketen zu finden sind. Die Simulation kompressibler Mehrphasenströmungen unter diesen Bedingungen stellt hohe Anforderungen an die numerische Behandlung als auch an das numerische Verfahren. Dies ist einerseits durch die kompressible Behandlung beider Fluidphasen und der numerischen Methode bedingt, besonders aufgrund der numerischen Auflösung der Phasengrenze. Andererseits ist die Auswertung von Zustandsgleichungen, die nahe des kritischen Punktes gültig sind, teuer. Als zusätzliche Herausforderung sind die Hydrodynamik und Thermodynamik eng über die Erhaltungsgleichungen gekoppelt. Daher muss ein thermodynamisch konsistenter numerischer Ansatz gewählt werden. Die Bausteine des numerischen Verfahrens für kompressible Mehrphasenströmungen beinhalten einen kompressiblen Strömungslöser für beide Phasen, ein Level-Set basiertes Phasenverfolgungsverfahren, eine umfassende Beschreibung der Zustandsgleichung sowie ein Modell für die Approximation der Phasengrenzfläche. Die Interaktion dieser Teile im Lösungalgorithmus ist in der Arbeit beschrieben

Numerical modelling of rocket combustors

Introduction to the numerical simulation of cryogenic rocket combustion chambers and their numerical tools needed

HyperCODA -- Extension of Flow Solver CODA Towards Rocket Flows

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

HyperCODA -- Extension of Flow Solver CODA to Hypersonic Flows

This paper presents HyperCODA, the hypersonics extension to the flow solver CODA (CFD for ONERA, DLR and Airbus). Similar to the spacecraft extensions of TAU, HyperCODA extends CODA for applications at high Mach numbers, including non-ideal gas thermodynamics, gas mixtures, and chemistry. The paper includes simulations ranging from quasi one-dimensional flows proving second-order spatial convergence, over two-dimensional flows proving stability, to a full-fledged three-dimensional flow comparing DLR TAU and HyperCODA

Das Finite-Volumen-Verfahren der CFD-Software von ONERA, DLR und Airbus

Der Vortrag konzentriert sich auf die Finite-Volumen-Diskretisierung der neu entwickelten CFD-Software von ONERA, DLR und Airbus (CODA) und erläutert wesentlichen Unterschiede, die gegenüber etablierten Finite-Volumen-Verfahren hinsichtlich der zugrunde liegenden unstrukturierten, zell-zentrierten Netz-Metrik zu berücksichtigen sind. Schwerpunkt ist ein Überblick über das aktuelle Anwendungsspektrum des Finite-Volumen-Verfahrens im transsonischen und hypersonischen Strömungsbereich