  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.
