Investigation on three-dimensional unsteady flow characteristics during TBCC inlet mode transition using fast-responding PSP
image: Wall pressure field during mode transition process measured by fast-responding PSP (left: down-rotation processes, right: up-rotation processes)
Credit: Chinese Journal of Aeronautics
Variable-geometry combined-cycle inlets must accommodate the variations in upstream flow conditions across wide speed and altitude envelopes, while also meeting the complex demands of downstream multi-duct and multi-mode engines and ensuring smooth mode transitions. The technical challenges involved far surpass those of conventional inlets. Of particular concern, the mode transition process involves large-scale geometric adjustments, which inevitably induce highly unsteady responses and evolutions in the internal flow field, thereby directly impacting the operational stability and performance of the downstream engine. However, the current understanding of the flow mechanisms during inlet mode transition is predominantly based on two-dimensional simplified models. At the experimental level, accurately characterizing the three-dimensional flow structures, which are dominated by sidewall confinement effects, remains infeasible due to the inherent spatiotemporal resolution limitations of conventional measurement techniques, such as high-speed Schlieren photography. Fast-response PSP technology has recently emerged as a global, high spatiotemporal resolution optical measurement method.
In a recent article featured in the Chinese Journal of Aeronautics (https://doi.org/10.1016/j.cja.2025.103760), Dr. Liang Chen and Prof. Yue Zhang from Nanjing University of Aeronautics and Astronautics innovatively applies this technology to the experimental investigation of dynamic characteristics in three-dimensional internal inlet flows, aiming to overcome the dimensional and frequency constraints of traditional measurement methods. Taking a typical TBCC inlet as the research subject, this work systematically explores the shock-dominated flow response characteristics during its mode transition through wind tunnel experiments. The primary focus is to elucidate the three-dimensional unsteady flow features of the high-speed duct after it enters an unstarted state under off-design incoming Mach numbers, and to reveal the underlying physical mechanisms of its flow oscillations.
The efficacy of the fast-response PSP technique was validated, demonstrating measurement accuracy and response speed comparable to traditional dynamic transducers. The dynamic pressure fields reveal several key insights: in a started high-speed duct, the background wave system displays intrinsic spanwise three-dimensionality induced by viscous effects; during unstart and restart transients, an enlarged sidewall boundary layer promotes upstream-downstream flow coupling, thereby accelerating the response of corner separation; in a sustained unstart, the separation shock exhibits pronounced spanwise dynamic fluctuations. Thus, the PSP system, as a non-intrusive measurement tool, provides high-fidelity, spatiotemporally resolved data on the internal pressure field, uniquely capable of resolving spanwise 3D flow structures and overcoming the limitations of legacy measurement approaches.
With the high-speed duct fully open, a high internal contraction ratio (ICR) triggers throat blockage, forcing the inlet into an unstarted condition. The combined influence of downstream perturbations and sidewall constraints amplifies the scale and intensity of flow separation in both the core and corner regions, resulting in violent low-frequency oscillations. Modal analysis identifies the spanwise oscillation of the separation shock as the dominant mechanism driving this instability.
In the hysteresis state prior to restart, the transition of the incoming boundary layer from laminar to turbulent flow establishes a dual-shock separation structure. Consequently, sidewall-induced corner interference is markedly attenuated, and the separation flow achieves greater spanwise uniformity, yet the separation shock continues to exhibit strong low-frequency pulsations. Modal analysis, however, indicates a shift in the dominant instability mechanism: the primary source transitions to a streamwise oscillation, driven by the self-sustained oscillations of the downstream separation bubble and the shoulder shock train.
It is crucial to note that practical mode transitions in combined-cycle propulsion systems involve complex interactions between splitter actuation and multi-mode engine dynamics, which can produce even more intricate shock-dominated flow behaviors. The scope of the current study, constrained by available experimental and methodological capabilities, does not encompass the inlet’s response under realistic downstream engine conditions. This represents a critical area for future research.
Original Source
Liang Chen, Yue Zhang, Xu Liu, Huijun TAN, Haicheng ZHU, Mingchi PANG, Ziyun WANG. Investigation on three-dimensional unsteady flow characteristics during TBCC inlet mode transition using fast-responding PSP [J]. Chinese Journal of Aeronautics, 2025, https://doi.org/10.1016/j.cja.2025.103760.
About Chinese Journal of Aeronautics
Chinese Journal of Aeronautics (CJA) is an open access, peer-reviewed international journal covering all aspects of aerospace engineering, monthly published by Elsevier. The Journal reports the scientific and technological achievements and frontiers in aeronautic engineering and astronautic engineering, in both theory and practice. CJA is indexed in SCI (IF = 5.7, Q1), EI, IAA, AJ, CSA, Scopus.