Localized Airflow Fluidization During Granular Intrusion

Resistive forces during intrusion into frictional granular media increase linearly with depth. These forces arise from the sum of external stresses needed to pass the local frictional threshold and successfully shear the media to intrude. This threshold depends heavily on the packing and lithostatic pressure of the media around the intruder. An increasing lithostatic pressure gradient can prevent unanchored intruders like autonomous robots from burrowing successfully, as the gradient pushes towards the free surface. We investigate in both experiment and CFD-DEM simulations how rapid downward airflow from an intruder’s tip can create transient cavities in the media and help reduce resistive force for cylindrical intruders of 3 cm diameter in both 425-850 micron granular sand and 3 mm glass beads. When the fluidizing intruder tip is above the granular surface, airflow-induced cavities are formed which exhibit different shapes and flow rates dependent on their distance from the free surface. Below the surface, the resistive force per unit depth on the intruder decreases as a function of airflow rate until a characteristic intrusion depth, where the force increases rapidly and approaches the resistive force for the same intruder with no airflow.