Unifying fluidization and defluidization of granular columns

We study air pressure during the fluidization and defluidization of granular columns with the aim to unify the description of these processes. To this end, we perform experiments with different air flow velocities and columns of different heights. The experimental measurements evidence a nonlinear relationship between pressure and the depth in the column. This complex pressure profile challenges the conventional framework of Darcy's law, typically used to analyze airflow in granular columns. We introduce a law that effectively captures these nonlinear effects, occurring even at low particle Reynolds numbers. An advantage of our approach is its applicability to defluidization-a key phenomenon in gravity-driven gas-particle flows. Using this law, we present a model to describe the decrease of air pressure in a defluidizing column, with numerical simulations aligning well with experimental results.