Permeability collapse in hopper discharge shown in a two-panel hopper diagram, with a pressure drop and rathole in Panel A, then rathole collapse, entrained air, and a discharge surge in Panel B.

A square, two-panel technical illustration of a hopper cross-section explaining permeability collapse in hopper discharge. Panel A shows restricted air flow, a pressure drop zone, and a stable rathole during discharge. Panel B shows rathole collapse with entrained air, partial fluidization, and a surging discharge stream from the outlet.