Unsteady detonation waves triggered by leading-edge separation zone

Oblique detonation engines (ODEs) demand rapid and stable initiation of an oblique detonation wave (ODW) in the combustor. However, the presence of a leading-edge separation zone in a compression-ramp configuration may compromise the stability of the ODW. This study investigates the influence of such separation on the ODW system. Simulations across a broad range of flight altitudes (15–33 km) and Mach numbers (8.5–10.5) reveal three macroscopic propagation modes of ODWs: downstream-anchored, periodic oscillation, and upstream-anchored. The shock-boundary layer interaction (SBLI) induces a vortex-rich separation zone, acting as a perturbation source. These disturbances generate hotspots on the autoignition reaction surface, thereby destabilizing the ODW. To characterize the role of heat release in this process, a dimensionless stability parameter χ , based on the activation energy and characteristic reaction lengths, is introduced, with a critical threshold identified at χ = 0.35 . When χ exceeds this value, hotspot-induced disturbances trigger macroscopic ODW motion. At lower altitudes, χ remains effective in identifying the onset of this hotspot-induced unsteadiness, whereas the subsequent wave dynamics are governed by the competition between the flow and chemical timescales. Accordingly, the Damköhler number D a is introduced to determine whether an upstream-propagating ODW undergoes periodic oscillation or remains anchored at the leading edge of the separation zone. When D a exceeds 2.9, the detonation wave becomes anchored upstream. These results offer an enhanced understanding of ODW stability and its sensitivity to flow-chemistry interactions. Novelty and significance statement Unsteady behavior of oblique detonation waves is commonly interpreted through detonation-front detachment, inflow fluctuations, or combustor choking. This work identifies the leading-edge separation zone as an active thermo-fluidic element that can both destabilize and anchor an ODW through coupled shock-boundary-layer interaction, radical-rich recirculation, heat release, and hotspot formation. Its significance lies in connecting macroscopic detonation-wave modes with local separation-zone chemistry and in providing two physically interpretable criteria: a stability parameter χ for the onset of separation-induced instability and a Damköhler number D a for upstream anchoring. This framework advances the mechanistic understanding of ODW stability in realistic compression-ramp flows and offers practical guidance for detonation-based combustor design.