We identify six scenarios of detonation re-initiation downstream of an obstacle with a single-centered hole in square and round tubes. The tubes have the same cross-sectional area of \(16\,\hbox {cm}^2\) , and the hole has the same shape as the tube cross section, but different open area ratios. The test mixtures investigated were 2 H2 + O2 + 2 Ar and 2 H2 + O2. We used soot foil recordings, high-speed schlieren, and chemiluminescence imaging to obtain longitudinal and frontal views of the diffraction phenomena. Depending on the initial pressure, one supercritical, four critical, and one subcritical scenarios were observed. The supercritical and critical scenarios were more likely to occur in the square tube than in the round tube, all other parameters being equal. Most of these transient, three-dimensional effects of the cross-sectional shape can be observed even at initial pressures for which there is no effect on steady propagation, e.g., without an obstacle or far from it. This raises the question of what dimensionality refers to in experiments in terms of global and local detonation dynamics.
