| Summary: | We experimentally investigated the influences of a small obstacle on the sidewall upon a detonation cellular structure using the smoked-foil technique for a stoichiometric hydrogen–oxygen mixture diluted with argon at pressures ranging from 15 to 60 kPa. The obstacles were of four types: forward-facing steps and slopes and backward-facing steps and slopes. The step height was varied and set to 1, 2, or 5 mm, but the slope height was fixed at 5 mm. The forward-facing slope angle was set to 40° or 75°, and the backward-facing slope angle was set to 20° or 40°. Under all conditions, for the forward-facing steps and slopes, the detonation cellular structure was negligibly affected, except for the region around the tip of the step and slope. However, for the backward-facing step, the detonation re-activation phenomenon occurred downstream of the obstacle. In addition, the enlarged cellular pattern due to the rarefaction wave and the trajectory of the re-activated strong transverse wave were observed downstream of the obstacle. Furthermore, a finer cellular pattern was observed downstream of the re-activation phenomenon. The distance between the backward-facing step and the re-activation position zra was fitted well by a power-law relation of the step height |h| and the cell width of the steadily propagating detonation λCJ as zra/λCJ = 2.7(|h|/λCJ)0.80. When the obstacle was the backward-facing slope, the smoked-foil records could be classified into two types based on whether the signatures of the detonation re-activation phenomena were clearly observed or not. Finally, when the finer cellular pattern was observed, the behavior of the cell-width enlargement subsequent to the re-activation phenomenon was similar in all cases, and the characteristic length required for the finer cellular pattern to relax to the steady-state pattern was approximately 10 times zra.
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