Drag induced lift in granular media (1011.3861v2)

Abstract: Laboratory experiments and numerical simulation reveal that a submerged intruder dragged horizontally at constant velocity within a granular medium experiences a lift force whose sign and magnitude depend on the intruder shape. Comparing the stress on a flat plate at varied inclination angle with the local surface stress on the intruders at regions with the same orientation demonstrates that intruder lift forces are well approximated as the sum of contributions from flat-plate elements. The plate stress is deduced from the force balance on the flowing media near the plate.

• The paper studies drag and lift forces on objects in granular media using experiments and simulations, finding lift depends significantly on object geometry and depth.
• Contrary to fluids, drag and lift forces in granular media are asymmetric, with lift magnitude increasing with object depth due to rising stress in the granular medium.
• Findings have implications for understanding how sand-burrowing animals move and for designing control surfaces and robots operating in granular environments.

Drag Induced Lift in Granular Media: A Study on Force Dynamics

The examined paper elucidates the mechanics of lift and drag forces acting on intruders submerged in granular media. Recognizing the fundamental exploration of drag and lift forces in traditional fluids, this research extends the investigation into granular materials, which exhibit complex interactions distinctively different from liquid or gas systems.

Key Findings and Methodology

The research utilized both experimental setups and numerical simulations to probe the forces exerted on objects of varied geometric shapes as they are dragged through granular media at constant velocities. The experiments were conducted with intruders having different cross-sectional shapes dragged within a bed of glass beads. Meanwhile, simulations employed the Discrete Element Method (DEM) for a precise particle-level interaction insight. A measured drag speed of 10 cm/s was found to notably suit the quasi-static regime necessary for these investigations.

Significant Observations Include:

• The lift force $F_z$ was contingent upon the geometry of the object. The orientation and sign of the lift varied substantially, with shapes like the half-cylinder demonstrating a downward lift, while symmetric shapes like full cylinders and square rods experienced upward lift.
• The lift force magnitude increased with increased depth of the intruder. This was attributed to the rising stress in the granular media with depth.
• To understand the stresses contributing to drag and lift, analysis showed that the stresses on an intruder's surface could be approximated by summing contributions from differential flat plate elements. This was due to the predictability of surface stress distributions based on local surface orientations.

Granular Lift Versus Fluid Dynamics

Contrary to low Reynolds number fluid dynamics, where forces are symmetrically applied along the direction of motion, the paper found that in granular media:

• The drag force $F_x$ and lift force $F_z$ on plates were asymmetric about their orientation.
• This asymmetry was attributed to the granular medium's complexity, where the yield stress increases with depth, influencing flow dynamics differently than in fluid systems.

Force Modelling

The development of a force model highlighted the granular mechanics at play. By analyzing the movement of granular material as influenced by the intruder's passage, the paper delineated two flow regimes, each based on particular plate orientations. This refined understanding of slip planes and granular pressure allowed researchers to derive a stress model for plates in granular flows, addressing the discrepancy observed in traditional fluid mechanics.

Practical and Theoretical Implications

The paper's findings are significant in biological applications, particularly in examining how sand-burrowing animals negotiate their subterranean mobility. In robotics, the insights offer guidelines for developing advanced control surfaces aiding maneuverability in sandy environments.

Future Directions might involve expanding the understanding of granular force distributions by exploring more complex intruder geometries and velocities. Moreover, developing granular flow models that incorporate particle shape diversity could further detail how granular materials respond to induced disturbances.

In conclusion, this paper offers a substantial contribution to granular physics, enhancing the understanding of how complex solid-like and flowing behaviors intertwine when interacting with submerged entities. The insights extend foundational knowledge for practical applications in engineering and biological systems, paving the way for inspired systems design and theoretical advancements in granular flow mechanics.