Hourglass

An hourglass measures time through the steady flow of granular material under gravity. This simulation models individual sand grains as particles with velocity, gravity, and collision response. Particles funnel through a narrow neck, forming characteristic clogging arches, avalanches, and angle-of-repose piles. The flow rate follows the Beverloo equation: it depends on the opening width and grain diameter, but is nearly independent of the fill height—unlike liquid flow. Flip the glass when the top chamber empties and watch the process reverse.