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Hein, Stefan J.; Theiss, Alexander; Di Giovanni, Antonio; Stemmer, Christian; Schilden, Thomas; Schroeder, Wolfgang; Paredes, Pedro; Choudhari, Meelan M.; Li, Fei; Reshotko, Eli 
Numerical Investigation of Roughness Effects on Transition on Spherical Capsules 
The state of the boundary layer on space reentry vehicles significantly affects the design of the thermal protection system. However, the physical mechanism that leads to the laminar-turbulent boundary-layer transition on blunt spherical capsules remains an open question in literature. This work numerically assesses the potential of roughness-induced nonmodal disturbance growth on reentry capsules with a spherical-section forebody by optimal transient-growth theory and direct numerical simulation. Two different sets of wind-tunnel experiments are considered. Optimal transient-growth studies have been performed for the blunt capsule experiments at Mach 5.9 in the Hypersonic Ludwieg tube Braunschweig (HLB) of the Technische Universität Braunschweig. In some of these measurements, the capsule model was equipped with a specifically designed patch of distributed micron-sized surface roughness. The transient-growth results for the HLB capsule are compared to corresponding numerical data for a Mach 6 blunt capsule experiment in the Adjustable Contour Expansion (ACE) facility of the Texas A&M University (TAMU) at lower Reynolds number. Similar trends are observed for both configurations. In particular, a rather low maximum energy gain is noted for the surface temperature conditions of the experiments. The surface temperature dependence of the optimal transient-growth results is very similar for both capsule configurations. Moreover, the generation of stationary disturbances by well-defined roughness patches on the capsule surface is studied for the conditions of the HLB experiment by using direct numerical simulations (DNS). To help explain the observed laminar-turbulent transition downstream of the roughness patch in some of the HLB capsule experiments, additional simulations were carried out to study the evolution of unsteady perturbations within the steady disturbance flow field due to the roughness patch. However, the DNS did not provide any indication of modal or nonmodal disturbance growth in the wake of the roughness patch; and hence, the physical mechanism underlying the observed onset of transition remains unknown. © 2018, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. 
Dewey Decimal Classification:
620 Ingenieurwissenschaften 
Book / Congress title:
2018 AIAA Aerospace Sciences Meeting 
American Institute of Aeronautics and Astronautics 
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Part of this work has been funded by the German Research Foundation (DFG) within the HYPTRANS PAK742. The support is gratefully acknowledged. Computing resources were provided by the High Performance Computing Center Stuttgart (HLRS), the Jülich Supercomputing Center and the Leibniz Supercomputing Centre Munich (LRZ). The work of the NASA authors was performed jointly under the NASA Transformational Tools and Technologies project and the NASA Hypersonic Technologies Project. 
TUM Institution:
Lehrstuhl für Aerodynamik und Strömungsmechanik