S1-3.7 Jan. 5 Studies of surface drag in a fluidized bed to discover principles of locomotion on sand. KORFF, Wyatt L.*; GOLDMAN, Daniel I.; California Institute of Technology; Univ. of California, Berkeley firstname.lastname@example.org
Terrestrial animals that live in or on sandy habitats contend with a dynamic environment that can deform and flow in response to movement. Inspired by recent field and laboratory studies of lizard locomotion that have documented substrate deformation and drag-based propulsion, we studied the drag on a half-submerged 2cm disk using a large aspect ratio (1200X800 particle diameters) air-fluidized bed filled 200 particle diameters deep with 250 µm glass beads. The use of a fluidized bed allowed for the precise control and manipulation of the material properties of the substrate. We varied the air-flow rate (Q) to the bed and the drag velocity (Vd) (0-40 cm/s) of the disk. Below fluidization, the drag force (Fd) increased linearly with Vd, but unlike Newtonian fluids, had a nonzero intercept. As the onset of fluidization was approached, the intercept decreased yet the slope of the Fd / Vd relationship remained constant. Above fluidization, Fd was no longer linear with velocity but instead had a positive curvature. Above a critical velocity Vc, Fd increased sharply after subtracting the viscous drag. The average slope of Fd / Vd relationship above Vc was the same as the slope below the onset of fluidization. The sharp increase in drag was correlated with the formation of a wake behind the disk similar to studies of wave drag in a viscous Newtonian fluid [T. Burghelea and V. Steinberg, Phys. Rev. Lett. 86, 2557, (2001)]. The formation of surface waves above a critical velocity in Newtonian fluids is a result of a competition between gravitational and capillary restoring forces; in the fluidized bed, the mechanism that produces the sharp onset to wave drag is unknown.