Aerodynamic Excitations in the Near-Wake of Space Launchers: Insights from Hot Flow Experiments using High-Speed PIV

The catastrophic failure of the Ariane 5 ECA heavy launcher in December 2002 underscored significant gaps in understanding the dynamic loads acting on the vehicle’s base region. Investigations revealed that aerodynamic interactions between the nozzle exhaust plume and the launcher wake, particularly the phenomenon of “buffeting” led to intense oscillatory mechanical loads. This study addresses the challenges in accurately replicating these effects in both experimental and numerical models, given the wide range of time and spatial scales and the complexities of high-temperature gas flows. Traditional experimental approaches have relied on cold nozzle flows, which, while practical, fail to capture key high-temperature effects influencing aerodynamic excitations. To overcome these limitations, a novel experimental setup was developed using a wind tunnel model integrated with a solid propellant combustion chamber, producing hot exhaust gases under realistic conditions. Advanced diagnostics, including high-speed particle image velocimetry (PIV) at 8500 Hz, high-speed Schlieren imaging, and Kulite pressure sensors at 17000 Hz, enabled detailed characterization of the unsteady flow dynamics in the base region. Key findings reveal two dominant aerodynamic modes in the base region: flapping, characterized by asymmetric flow motion, and swinging, associated with symmetric oscillations. These modes are the primary drivers of the unsteady loads acting on the nozzle and adjacent structures. At a Mach number of 0.8, the flapping mode is found to resonate with the jet noise mechanism known as screeching, resulting in strong, alternating oscillatory loads on the wake region. This resonance amplifies the mechanical stresses experienced by the structure and, from the authors’ perspective, represents the most plausible explanation for the excessive loads that led to the catastrophic failure of Ariane 5 Flight 157. The study successfully replicates realistic flight conditions for investigating buffeting phenomena, bridging the gap between cold-flow experiments and actual high-temperature exhaust flows. This achievement enhances predictive modeling capabilities by providing a more accurate understanding of the dynamic interactions in the base region. To support this, a schematic model of the feedback cycle was developed to visualize and capture the interacting effects, thereby facilitating a deeper physical understanding. Consequently, the research contributes to the development of safer and more efficient space launch systems by enabling improved design strategies to mitigate such damaging aerodynamic loads. The consistency of these results with prior cold-flow studies further validates the robustness of the experimental approach and the universality of the identified aerodynamic mechanisms across varying flow conditions.