Lift and drag in intruders moving through hydrostatic granular media at high speeds

TL;DR

This study uses 2D molecular dynamics simulations to analyze how intruders of different shapes experience lift and drag forces when moving at high speeds through dense granular media, revealing shape-dependent force behaviors.

Contribution

It provides new insights into the shape-dependent lift and drag forces on intruders in granular media at high speeds through detailed simulations.

Findings

Drag increases monotonically with speed and depth.

Lift force varies nonmonotonically with speed and depth.

Shape influences the lift force direction and magnitude.

Abstract

Recently, experiments showed that forces on intruders dragged horizontally through dense, hydrostatic granular packings mainly depend on the local surface orientation and can be seen as the sum of the forces exerted on small surface elements. In order to understand such forces more deeply, we perform 2D soft-sphere molecular dynamics simulation, on similar set up, of an intruder dragged through a 50-50 bi-disperse granular packing, with diameters 0.30 and 0.34 cm. We measure, for both circular and half-circle shapes, the forces parallel (drag) and perpendicular (lift) to the drag direction as functions of the drag speed, with V=10.3-309 cm/s, and intruder depths, with D=3.75-37.5 cm. The drag forces on an intruder monotonically increase with V and D, and are larger for the circle. However, the lift force does not depend monotonically on V and D, and this relationship is affected by the shape of the intruder. The vertical force was negative for the half-circle, but for a small range of V and D, we measure positive lift. We find no sign change for the lift on the circle, which is always positive. The explanation for the nonmonotonic dependence is related to the decrease in contacts on the intruder as V increases. This is qualitatively similar to supersonic flow detachment from an obstacle. The detachment picture is supported by simulation measurements of the velocity field around the intruder and force profiles measured on its surface.