Three-dimensional morphology and formation mechanism of tongue-shaped oblique detonation waves in elliptical flow channels

Oblique detonation waves (ODWs) have been widely studied in nominally two-dimensional rectangular geometries, but it is rarely done in three-dimensional non-rectangular flow channels that are common in realistic hypersonic propulsion systems. In this study, hydrogen–air ODWs in elliptical flow channels with different inlet aspect ratios (AR) are investigated through numerical simulations. Particular attentions are given to the three-dimensional morphology of ODWs and the relevant formation mechanism. A novel tongue-shaped oblique detonation wave (TSODW) structure, characterized by a quasi-two-dimensional planar of the wavefront in the mid-span region and sweeping backwards near the sidewalls, is identified for the first time. Analysis reveals that its formation follows a two-stage mechanism: firstly, inward flow convergence induced by circumferentially non-uniform compression happens in the upstream, resulting in spanwise variation in ignition distances; and secondly, further amplification of such flow convergence by combustion heat release in the downstream leads to generation of localized kinks and significant flow deflection near the sidewalls, ultimately forming the backswept wavefronts. A parametric study with varying AR demonstrates that a lower AR promotes earlier onset of wavefront distortion and stronger back sweeping, whereas a higher AR delays kink formation and results in smoother wavefront transitions. These findings elucidate the role of non-uniform compression induced by three-dimensional geometries in shaping the wave structures of ODWs and provide physical insight into the spatial organization mechanisms of oblique detonative combustion in non-rectangular flow channels. Novelty and Significance Statement: Oblique detonation engines are attractive in hypersonic air-breathing propulsion, and oblique detonation waves (ODWs) in two-dimensional rectangular configurations have been commonly studied in the past decades. Non-rectangular flow channels are commonly involved in realistic propulsion systems, but the understanding of the combustion flow fields of ODWs under complex three-dimensional geometric confinement remains poor. This study reports the first high-resolution numerical investigation of ODWs in elliptical flow channels, revealing a novel tongue-shaped oblique detonation wave (TSODW) structure that has not been reported in literature. The detailed morphology, relevant formation mechanism, and variation with different flow-channel geometric parameters of this TSODW are revealed, and the important effects of circumferentially non-uniform compression are found as well. The significance of this work lies in elucidating how geometric confinement induces flow non-uniformity, which is further amplified by combustion heat release to shape the complex three-dimensional detonation front morphology. The findings advance the fundamental understanding of detonation wave dynamics in spatially converging flows and provide guidance for the design and optimization of oblique detonation flow channels for hypersonic propulsion.