Influence of a propulsive jet on the wake-flow structure of an axisymmetric space-launcher model

The wake flow of a classical space launcher is dominated by the abrupt decrease in diameter at the junction between main body and rocket engine. At this discontinuity, the turbulent boundary layer on the main body separates, a shear layer starts to develop, and a large recirculation region forms downstream of the step. This separation-dominated flow field is highly unsteady and induces strong wall-pressure oscillations, which can excite structural vibrations detrimental to the launcher (Deprés et al. (2004)). The conditions and topology of the wake flow vary strongly along the flight trajectory of the launcher. Especially the influence of the propulsive jet, which becomes increasingly underexpanded with altitude, has a strong influence. The afterexpanding jet plume has a strong displacement effect on the outer flow, which enlarges the recirculation region. Depending on the conditions, the flow may not reattach on the nozzle fairing, potentially allowing hot exhaust gases from the engine nozzle to be convected upstream and harm the structure. Understanding this flow field is thus crucial to minimize those detrimental effects, ultimately contributing to more efficient launcher designs and thus affordable access to space. In the present study, we investigate the influence of an afterexpanding propulsive jet on the wake flow of a generic axisymmetric space-launcher model at a Mach number of M = 2.9 and a Reynolds number of ReD = 1.3 ·106, based on model diameter D. The propulsive jet is simulated with a cold air jet, exiting the integrated TIC-nozzle with a Mach numer of 2.5. The description and discussion of turbulent structures in the wake flow, as well as the influence of a propulsive jet on their dynamic behavior is the focus of this study. The mean and turbulent flow topology and the dynamics of the wake are analyzed based on experimental data from Particle Image Velocimetry (PIV) measurements, Schlieren visualizations, and measurements of surface-pressure fluctuations. The data are interpreted with a combination of classical statistical analysis and post processing by means of Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD). By combining the strengths of these different methods, we intend to improve the understanding of the mechanisms in the wake instability. In particular, we discuss the influence of the jet plume on the growth of vortices in the shear layer forming at the shoulder of the main body. Previous studies observed that the presence of a propulsive jet has a stabilizing effect on the wake. This particularly manifests itself in lower turbulent intensities of the velocity components in the near wake (see also Fig. 1). From our observations within this study, we conclude that several reasons contribute to this stabilizing effect: The width of the shear layer is restricted by the displacement effect exerted by the jet plume. This restricts the maximum size of the vortices in the shear layer. Due to the displacement of the shear layer away from the wall, the shear layer develops for a longer streamwise distance, and the vortices decay into smaller structures. Furthermore, the displacement leads to a flatter impingement angle of the shear layer on the nozzle surface. Therefore, the reattachment process is slower and not completed along the length of the nozzle of our model. The reattachment and the unsteadiness of the recompression shock thus have a much smaller influence than in the baseline case. In our opinion, this last mechanism is the most important contribution to the observed effect.