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The science
Granular flow through an orifice does not follow the liquid rule. For water the exit speed is set by the head, v = sqrt(2 g h) (Torricelli), so the flow rate Q = Cd a sqrt(2 g h) falls as the bulb empties. For sand the pressure on the base stops growing with depth: wall friction carries the weight (the Janssen effect), P(h) = rho g R / (2 mu K) (1 - exp(-2 mu K h / R)), so the flow depends only on the orifice. The Beverloo correlation gives the mass flow W = C rho_b sqrt(g) (D - k d)^(5/2) with C about 0.58, k about 1.4, bulk density rho_b, orifice diameter D and grain diameter d.
That constant flow is why an hourglass can keep time: the sand level falls linearly (as long as the grain stream is free-falling), and the pile below grows at the angle of repose, a cone whose half-angle is set by grain friction. The exponent 5/2 means that doubling the neck diameter raises the flow about 5.7 times; the log-log graph shows the slope.
Simplifications: no clogging or arching (which appears when D is below about 5 grain diameters), no air-pressure feedback in the neck, constant grain size, perfectly smooth walls for the water case, and a fixed repose angle. The 3D page shows the grains; this page shows the measured flow law. The time axis runs faster than real time (8 times at speed 1).
What to try
- Press Water instead of sand: the level falls fast then slows, and the graph curve bends, unlike the straight sand line.
- Halve the neck width and read how the drain time grows by about 5.7 times.
- Fill the bulb deeper and watch the Janssen pressure level off while the hydrostatic line keeps rising.