ENGINEERING AND CFD
Space Launch Vehicle Aerodynamics with Hot Plumes
Principal Investigator:
Klaus Hannemann
Affiliation:
Spacecraft Department, Institute of Aerodynamics and Flow Technology, German Aerospace Center (DLR)
Local Project ID:
pr62po
HPC Platform used:
SuperMUC and SuperMUC-NG of LRZ
Date published:
The aerodynamics around space launch vehicles determine the external forces that these vehicles experience. Hence, the accurate prediction of these loads is crucial for the structural design. Additionally, a better understanding of the fundamental physical phenomena responsible for the observed flow features helps to design the vehicle such that the experienced loads are reduced. Both the better understanding of the loads and the ability to reduce these loads allow for vehicles to be designed more efficiently and carry more payload mass into orbit.
Of particular interest for the aerodynamic research is the region at the bottom of the vehicle, displayed in Fig. 1, where a sudden change in diameter at the end of the main body can lead to high mechanical loads. This aspect of space launch vehicles is investigated in branch B of the Sonderforschungsbereich Transregio 40 [1], funded by the German Research Foundation DFG. Due to its simpler handling both numerically and experimentally, nearly all previous investigations in the literature have investigated the associated phenomena without a propulsive plume or with one resulting from expanding air. Consequently the plume properties differ significantly from those of realistic rocket plumes. Sub-project B5 of Transregio 40, which is handled by the Spacecraft Department of the Institute of Aerodynamics and Flow Technology at the German Aerospace Center DLR, is concerned in particular with the effects that the presence of hot propulsive plumes has on the wake flow field phenomena [2].
Figure 1: Visualization of the turbulent structures colored by temperature in the base region of a generic space launch vehicle. © DLR
For this purpose computational fluid dynamics (CFD) is applied to simulate the flow around a scale-model generic space launch vehicle geometry. The advantage of CFD is the ability to analyze all regions and aspects of the flow field as required with a high level of detail. To ensure the numerical algorithm accurately predicts the real flow physics the results obtained are compared to experimental investigations of key quantities such as wall pressure measurements.
In the scope of the project pr62po at LRZ, a sensitivity study based on a configuration with available comparison data in the literature was conducted first. Subsequently, it was investigated which impact changing the plume conditions and wall temperatures has on the flow field and associated vehicle loads.
Methods and Results
The CFD solver used for these simulations is the DLR TAU-Code that solves the compressible Navier-Stokes equations. TAU is a second-order accurate Finite Volume solver programmed in C and includes capabilities to handle multispecies flow and chemical reactions [3]. These capabilities are critical to simulate the flow field in the presence of hot plumes as these result from combustion processes. The flow field turbulence is modelled using a Hybrid RANS-LES method (HRLM) which allows particular regions of interest to be resolved with high accuracy Large Eddy Simulation (LES) in a time-resolved fashion whereas other regions are treated with a lower fidelity Reynolds-Averaged Navier-Stokes (RANS) approach.
The conducted investigations are the first with TAU using high-fidelity HRLM in combination with multispecies flow and finite-rate chemical reactions. In the process, several optimizations were implemented to accelerate, stabilize and improve the simulations. Additionally, optimal parameter settings in terms of computational effort and accuracy for these kinds of investigations were found. The required computational resources depend heavily on the settings used as well as the particular configuration investigated. The least resources were required for a grid in the sensitivity study (ca. 13 million grid points) with 200.000 core-h distributed on 1120 cores (on SuperMUC Phase 2). The most resources were required for simulations on a fine grid (ca. 31 million grid points) with around 4 million core-h on 9216 cores. These were more computationally expensive since multispecies flow and chemical reactions in addition to the high numerical resolution were required. Overall, about 20 different cases were simulated since the start of the project totaling about 26 million core-h. Due to the need of high time resolution the initial output of one large simulation on the SCRATCH file system is about 250TB, but this can be reduced to about 2TB per case of final data by removing quantities only required for restarts.
Scientific results
The scientific results obtained in the project include a better understanding both of the numerical methods and their requirements and sensitivities [4] as well as insights into the flow physics with different configuration changes [2]. For brevity, only the most significant of the latter are discussed below.
Figure 2: Comparison of the flow field with an air plume (top), a hot plume with cold walls (center) and a hot plume with hot walls (bottom). Mean axial velocity (in m/s, top) and instantaneous circumferential vorticity (in 1/s, bottom) are displayed. © DLR
The flow field around the investigated geometry can be found in Fig. 2. The figure shows the mean flow field with streamlines and axial velocity color contours on the top and an instantaneous view of the circumferential vorticity on the bottom for configurations with different plume conditions (top vs. center) and wall temperatures (center vs. bottom). It is visible that for a low velocity air plume the turbulent shear layer originating from the end of the main body impacts the end of the nozzle fairing, but an increase in plume velocity shifts the reattachment location further downstream onto the plume. This is attributed to a plume suction effect and increases the interaction between the plume and the external flow.
Furthermore, a similar shift in reattachment location is observed if the wall temperature is increased. This is associated with the reduction in density - and hence momentum - due to the heating of the flow. Additionally, it is found that the increased wall temperatures significantly reduce the mechanical loads experienced by the nozzle.
However, it is also found that the fundamental unsteady flow features observed for configurations with an air plume are also present in those with realistic plume conditions and higher wall temperatures. This indicates that the underlying flow phenomena dominating the flow field remain unaffected by these parameters.
In combination, this shows that for a general understanding of the flow field an exact replication of the full scale conditions is not necessarily required, but for the quantitative analysis of the mechanical loads accurate plume conditions and wall temperature descriptions are necessary since the detailed quantitative impact these phenomena have on the mechanical loads differs.
On-going Research / Outlook
Currently, the final evaluation and analysis of the results is still in progress. Additionally, remaining simulations are being conducted that tackle open questions that were raised in the process of the investigation.
While the simulations could likely have been conducted on SuperMUC Phase 2 as well, the size of SuperMUC-NG helps to significantly speed up the computations by reducing queuing time. Additionally, the required number of jobs per case is reduced significantly by the improved parallel performance among other factors due to the larger number of cores per node and hence reduced cross-node-communication.
References and Links
[1] www.sfbtr40.de
[2] Schumann, J.-E., Fertig, M., Hannemann, V., Eggers, T. and Hannemann, K., “Numerical Investigation of Space Launch Vehicle Base Flows with Hot Plumes”, In: Future Space-Transport-System Components under High Thermal and Mechanical Loads, Springer, 2021.
[3] Hannemann, K., Martinez-Schramm, J., Wagner, A., Karl, S., Hannemann, V., “A Closely Coupled Experimental and Numerical Approach for Hypersonic and High Enthalpy Flow Investigations Utilising the HEG Shock Tunnel and the DLR TAU Code”, Technical Report, German Aerospace Center, Institute of Aerodynamics and Flow Technology, 2010.
[4] Schumann, J.-E., Hannemann, V. and Hannemann, K., "Investigation of Structured and Unstructured Grid Topology and Resolution Dependence for Scale-Resolving Simulations of Axisymmetric Detaching-Reattaching Shear Layers", Progress in Hybrid RANS-LES Modelling, Springer, Cham, 2020, 169-179.
Researchers
Klaus Hannemann1 (PI), Volker Hannemann1, Jan-Erik Schumann1
1Spacecraft Department, Institute of Aerodynamics and Flow Technology, German Aerospace Center (DLR)
Scientific Contact
Dr. Volker Hannemann
German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt / DLR)
Institute of Aerodynamics and Flow Technology
Bunsenstraße 10, D-37073 Göttingen (Germany)
e-mail: Volker.Hannemann [@] dlr.de
Local project ID: pr62po
April 2021