Acceleration dependence of the instability growth at a fixed frequency of 50 Hz. (a) Time evolution of the instability and the stage transitions at three different accelerations. Yellow lines trace the surface of the granular layer and the two-layer interface. Blue lines at t = 5 s indicate these heights at t = 0 s and are shown for reference. (b) Growth of amplitude δ z with time. The thin, medium, and thick lines indicate the experiments in the percolation, transition, and flame regimes, respectively, and are classified using the relative amplitude δ z′ at t = 9.9 s. A red arrow indicates a peak amplitude for the case in which a flame structure forms. Diameters of red and white particles which comprise the lower and upper layers, respectively, are indicated for comparison.