Wave angle for oblique detonation waves

et al. 2017

Physics of Fluids

The understanding of oblique detonation dynamics has both inherent basic research value for highspeed compressible reacting flow and propulsion application in hypersonic aerospace systems. In this study, the oblique detonation structures formed by semi-infinite cones are investigated numerically by solving the unsteady, two-dimensional axisymmetric Euler equations with a one-step irreversible Arrhenius reaction model. The present simulation results show that a novel wave structure, featured by two distinct points where there is close-coupling between the shock and combustion front, is depicted when either the cone angle or incident Mach number is reduced. This structure is analyzed by examining the variation of the reaction length scale and comparing the flow field with that of planar, wedge-induced oblique detonations. Further simulations are performed to study the effects of chemical length scale and activation energy, which are both found to influence the formation of this novel structure. The initiation mechanism behind the conical shock is discussed to investigate the interplay between the effect of the Taylor-Maccoll flow, front curvature, and energy releases from the chemical reaction in conical oblique detonations. The observed flow fields are interpreted by means of the energetic limit as in the critical regime for initiation of detonation.

Section: B Verification and Validationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Initiation structure of oblique detonation waves behind conical shocks

Yang

et al. 2017

Physics of Fluids

“…In the literature, there have been extensive investigations of ODWs beginning with the early theoretical studies of steady ODWs initiated from semi-infinite wedges using shock and detonation polar curves for determining different wave angles (e.g. Gross 1963; Pratt, Humphrey & Glenn 1991; Ashford & Emanuel 1994; Emanuel & Tuckness 2004). Later studies then focused on the formation structure of ODWs and their subsequent evolution.…”

Section: Introductionmentioning

confidence: 99%

Numerical study of wedge-induced oblique detonations in unsteady flow

Yang

2019

J. Fluid Mech.

Oblique detonation waves (ODWs) have been studied widely to facilitate their employment in hypersonic propulsion, but the effects of continuous unsteady inflow have never been addressed so far. Thus, the present study investigates wedge-induced oblique detonations in unsteady flow via numerical simulations based on the reactive Euler equations with a two-step induction–reaction kinetic model. As a first step, the chemical and flow parameters are chosen for the simplest structure such that the ODW initiation occurs under a smooth transition with a curved shock. After a steady ODW with smooth initiation transition is established, the inflow is then subject to a continuous sinusoidal density/temperature disturbance. Cases with single-pulse inflow variation are also simulated to clarify whether the observed phenomena are derived solely from the continuous disturbance. Two aspects are analysed to investigate the features of ODWs in unsteady flow, namely, the formation of triple points on the surface, and the movement of the reactive front position. On the formation of triple points, the continuous disturbance generates at most one pair of triple points, less than or equal to the number of triple points in single-pulse cases. This indicates that the effects of continuous disturbance weaken the ability to generate the triple points, although there appear more triple points convected downstream on the surface at any given instant. On the movement of the reactive front, oscillatory behaviours are induced in either single-pulse or continuous disturbance cases. However, more complicated dynamic displacements and noticeable effects of unsteadiness are observed in the cases of continuous disturbance, and are found to be sensitive to the disturbance wavenumber, $N$. Increasing $N$ results in three regimes with distinct behaviours, which are quasi-steady, overshooting oscillation and unstable ODW. For the quasi-steady case with low $N$, the reactive front oscillates coherently with the inflow disturbance with slightly higher amplitude around the initiation region. The overshooting oscillation generates the most significant variation of downstream surface in the case of modest $N$, reflecting a resonance-like behaviour of unsteady ODW. In the case of high $N$, the disturbed ODW surface readjusts itself with local unstable features. It becomes more robust and the reactive front of the final unstable ODW structure is less susceptible to flow disturbance.

“…Classical theory on ODWs simplifies the structure into an oblique shock with a post-shock energy release zone attached to the wedge [4,5] . From shock polar analysis, the relationship between different flow parameters and oblique shock/detonation properties can be determined, e.g., [6][7][8] . The formation of oblique detonations was first analyzed numerically by Li et al [9] , who observed a structure composed of a nonreactive oblique shock, an induction region followed by a reaction region, along the wall, and the oblique detonation surface.…”

Section: Introductionmentioning

confidence: 99%

Initiation characteristics of wedge-induced oblique detonation waves in a stoichiometric hydrogen-air mixture

Proceedings of the Combustion Institute

Jiang

2017

104

The initiation features of two-dimensional, oblique detonations from a wedge in a stoichiometric hydrogen-air mixture are investigated via numerical simulations using the reactive Euler equations with detailed chemistry. A parametric study is performed to analyze the effect of inflow pressure P 0 , and Mach number M 0 on the initiation structure and length. The present numerical results demonstrate that the two transition patterns, i.e., an abrupt transition from a multi-wave point connecting the oblique shock and the detonation surface and a smooth transition via a curved shock, depend strongly on the inflow Mach number, while the inflow pressure is found to have little effect on the oblique shock-to-detonation transition type. The present results also reveal a slightly more complex structure of abrupt transition type in the case of M 0 = 7.0, consisting of various chemical and gasdynamic processes in the shocked gas mixtures. The present results show quantitatively that the initiation length decreases with increasing M 0 , primarily due to the increase of post-shock temperature. Furthermore, the effect of M 0 on initiation length is independent of P 0 , but given the same M 0 , the initiation length is found to be inversely proportional to P 0 . Theoretical analysis based on the constant volume combustion (CVC) theory is also performed, and the results are close to the numerical simulations in the case of high M 0 regardless of P 0 , demonstrating that the post-oblique-shock condition, i.e., post-shock temperature, is the key parameter affecting the initiation. At decreasing M 0 , the CVC theory breaks down, suggesting a switch from chemical kinetics-controlled to a wave-controlled gasdynamic process. For high inflow pressure P 0 at decreasing M 0 , the CVC theoretical estimations depart from numerical results faster than those of low P 0 , due to the presence of the non-monotonic effects of chemical kinetic limits in hydrogen oxidation at high pressure.