Flow Through Granular Beds

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Oblique image of grains packed into a narrow, clear-walled apparatus resembling an ant farm. Text reads, "To understand the movement of water permeate through granular bed, we filled experimental jig with particles ranging in size from 600 to 1200 microns. Then we varied different parameters of the setting, such as packing density, to observe how the patterns of wicking, draining, and void formations are affected."

Black and white image showing dark air bubble forming in the grain bed. Text reads, "The hydraulic pressure of the inflowing water drives the trapped air downward, overriding its natural buoyant tendency."

Black and white image of the grain bed, with dark bubbles branching on the left side and bright voids forming on the right. Text reads, "The bed is only as strong as its weakest point."

We often rely on water draining through beds of grains, whether it’s the soil foundation beneath a building or the sand-and-gravel-filter used in water treatment. But how does water move through these tortuous porous passages? That’s what we see in this video, which places grains in a jig resembling an ant farm and lets us watch as water–and air–drain through the grains. The result is more complicated than you might imagine, with dry pockets, weak spots, and developing sinkholes. (Video and image credit: J. Choi et al.)