Numerical analysis of controlled turbulent wake flows

In this study, the intricate wake-nozzle flow interaction of space launchers and their controllability are numerically investigated using a zonal RANS/LES approach. Due to the interaction of spatio-temporal coherent structures in the wake, the nozzle structure of classical space launcher, e.g., Ariane 5, is subjected to pronounced dynamic loads. The present work aims to reduce these so-called buffet loads by means of active and passive flow control. In addition, the aerodynamic integration of the dual-bell nozzle into the launcher’s architecture is investigated to determine the influence of the novel nozzle concept on the intricate wake flow. The simulations are performed at transonic freestream condition, i.e., at a freestream Mach number of Ma = 0.8 and a freestream Reynolds number based on the launcher diameter of Re = 4.3 · 10^5 and Re = 6.0 · 10^5. Besides classical statistical analysis such as mean and root-mean-square distributions and spatio-temporal spectral analysis, i.e., Fourier transform and two-point correlation analysis, modal decomposition techniques like spectral and “classical” proper orthogonal decomposition and dynamic mode decomposition are used to analyze the flow field data. Due to the combined approach of zonal RANS/LES followed by modal decomposition, time-resolved highly-accurate flow field data are obtained which are then used to isolate spatial modes from the complex flow field to get insight into the fundamental wake flow phenomena. After a short description of the mathematical and numerical methodology, i.e., the governing equations of fluid motion, the zonal RANS/LES method, and the modal decomposition techniques, the results of the flow control and dual-bell nozzle investigations are presented. First, the effect of two passive flow control devices on the wake of a planar configuration is examined. By semi-circular lobes integrated at the base shoulder, streamwise vortices are generated in the separated shear layer increasing the turbulent mixing downstream of the base. As a result, the reattachment length and thus the lever arm of the dynamic forces are decreased by about 75%. Next, a planar and an axisymmetric space launcher with a dual-bell nozzle operating at sea-level mode are analyzed to examine the influence of the dual-bell nozzle jet on the wake flow dynamics. Before a detailed analysis is performed on the generic axisymmetric free-flight configuration, the planar space launcher configuration is used to enable a straightforward validation of the zonal RANS/LES results with corresponding experimental measurements carried out within the framework of the Collaborative Research Center Transregio 40. By spectral analysis and modal decomposition, a low-frequency pumping motion of the recirculation region and an antisymmetric flapping motion of the shear layer at the buffet frequency are detected. The investigated dual-bell nozzle configurations feature a flow topology and dynamics similar to configurations with conventional nozzles indicating that in the present flow parameter regime and operation mode the impact of the dual-bell nozzle is insignificant. Finally, the axisymmetric duel-bell nozzle configuration is equipped with active flow control devices. The findings show that the antisymmetric coherent mode is weakened by the flow control leading to a reduction of the undesired buffet loads on the nozzle fairing