Spanwise Aspects of Low-Frequency Unsteadiness in Turbulent Separation Bubbles

• Applicants: Professorin Dr.-Ing. Anne-Marie Schreyer; Professor Dr.-Ing. Julien Weiss
• Subject Area: Fluid Mechanics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 564693884

Turbulent separation bubbles (TSBs) are ubiquitous in applications such as diffusers, air intakes, compressor and turbine blades, and overexpanded rocket nozzles. Due to the associated dynamic phenomenon of the low-frequency unsteadiness – occurring at a frequency low enough to match a structural resonance frequency – separation bubbles negatively affect both the aerodynamic behavior and structural integrity of these systems. This unsteadiness occurs both in low-speed and high-speed configurations, with striking similarities in characteristics and behavior. The physical mechanism driving this unsteadiness is most probably the same and inherent to the separation bubble, and some progress has been made in identifying the mechanisms. However, the underlying physics are still not yet fully understood. One reason for this is that the low-frequency unsteadiness in turbulent separation bubbles so far has primarily been documented on a streamwise axis close to the bubble centerline, both in subsonic and supersonic configurations. Any spanwise non-homogeneity has mostly been neglected. There are, however, strong indications that the unsteady features and dynamics of a separation bubble have a spanwise character and periodicity. The low-frequency breathing motion of TSBs has been observed to be coherent over a spanwise wavelength, and this seems equally to be the case in low-speed and high-speed cases. The possible existence of a spanwise wavelength associated with the breathing of TSBs has consequences on the identification of its driving mechanism. It is thus necessary for further progress on the understanding of this influential phenomenon to clarify the spanwise extent of the unsteadiness, and to take these characteristics into account in the identification and explanation of the governing dynamic mechanisms. Due to the strong similarities observed in low- and high-speed flows, studying these effects in both regimes simultaneously and interpreting the data jointly is key to an overall understanding of the underlying physics. In this project, we close this gap in knowledge by experimentally analyzing two directly comparable cases of pressure induced TSBs in the subsonic and supersonic regimes in detail. The experimental domains will be wide enough in both cases to observe spanwise periodicities – and cases with equal bubble aspect ratios in both flow regimes will be directly compared. In addition, the influence of bubble aspect ratio will be analyzed. We generate highly-resolved experimental data sets of unsteady pressure and velocity throughout the large-span separation bubbles. Two project groups closely work together in jointly analyzing the data with statistical, spectral, and modal methods. This approach provides a unique opportunity to a general understanding of the low-frequency unsteadiness, especially given the scarcity of currently available data. This research will benefit numerous aerospace, mobility, and energy engineering applications.

• DFG Programme: Research Grants