Hydrodynamic structure statistics from schlieren imaging in detonations of varying regularity

Gaseous detonations exhibit a range of hydrodynamic features behind the leading shock including a reaction zone, shear layers, unburned gas pockets, and shock wave complexes. The presence of these features indicates the degree of detonation regularity: regular detonations exhibit relatively few and smoother features while irregular detonations exhibit numerous highly corrugated features. In this work, we present a detailed image processing technique to extract reaction zone features apparent in schlieren images. We apply this technique to a range of reactive mixtures, analyzing detonations from very regular (2H 2 + O 2 + 3Ar) to very irregular (CH 4 + 2O 2 ), producing a one-dimensional feature probability distribution. We confirm that increasing reduced activation energy leads to higher probability of features behind the shock, due to shear layers and unburned gas pockets in more irregular detonations. We determine the feature probability at a distance x = 0.05 times the cell width λ behind the front, P ( F ) 0.05 λ , as a sensitive metric for regularity. We approximate the effective reaction zone length by calculating the probability distribution of the final segmented pixel for each row of pixels. The median of this distribution is defined as the effective reaction zone length, with x / λ = 0.12 determined for the regular mixture, and x / λ = 0.22 for the irregular mixture. The gas appears to fully react within x / λ = 0.4 downstream of the front in regular mixtures, and approximately x / λ = 1 in irregular mixtures. By changing mixture pressure, we also observe that feature probability does not proportionally scale with cell size for a given mixture, likely due to image blurring and 3D effects. Novelty and Significance Statement Despite intense interest in detonation irregularity because of its importance in critical phenomena, methods for evaluating irregularity are largely qualitative. Some recent work has used soot foils or velocity maps to quantify regularity, but these methods are time consuming or require specialized facilities. This work provides a novel, generalizable method to quantify irregularity and reaction zone width from one of the most common detonation diagnostics: the schlieren image. We use this methodology to experimentally quantify feature probability and reaction zone width in mixtures with increasing effective activation energy and at various initial pressures. The image processing code is freely available to the community https://github.com/Rajpunna11/detonation-structure-extraction.git .