“These three images are snapshots of a spark-ignited expanding flame in different environments of the same hydrogen-air mixture. The top flame shows the ideal, reference case of a stable, smooth flame surface in a quiescent environment at atmospheric pressure. The middle flame is taken under elevated pressure simulating that within an internal combustion engine. The bottom flame is taken in a highly turbulent environment simulating another aspect of the engine interior. All images were taken at 8000 frames per second, using schlieren photography. The radius of the top flame is 11.4 millimeters.” C.K. Law, Swetaprovo Chaudhuri, and Fujia Wu (Princeton University).