107.4 Saturday, Jan. 7 Using a sandfish simulation to compare undulatory swimming in sand and in fluids DING, Y*; SHARPE, S.S.; MALADEN, R.D; GOLDMAN, D.I.; Georgia Institute of Technology; Georgia Institute of Technology; Georgia Institute of Technology; Georgia Institute of Technology email@example.com
The sandfish lizard (Scincus scincus) swims within granular media (sand) using axial body undulations to propel itself without the use of limbs. We have developed a numerical sandfish simulation that swims within an experimentally validated discrete element model of the granular medium. The numerical sandfish is composed of 60 motor-driven segments whose angular position is controlled to reproduce a traveling wave which resembles the body kinematics of the animal. Here we use the numerical model to study the detailed mechanics of undulatory swimming in a granular medium, including the kinematics, reaction forces from the medium, power, and internal torques. The simulation reveals that in sand-swimming, the oscillation patterns of the forward velocity, lateral velocity, and yaw motion, as well as the magnitude of motor torque and motor power as a function of body position are similar to those in undulatory swimming in water. However, because in granular media forces are independent of speed in the biologically relevant range (1-4 Hz), the required mechanical power is proportional to the frequency; in fluids power increases superlinearly. Unlike in fluids where streamlining can reduce drag by an order of magnitude, streamlining the sandfish head reduces head drag by only ~30% compared to a flat head. Head drag consumes ~30% of the total mechanical energy generated by the motors. Finally, unlike undulatory motion in water, the magnitude of the force on each segment depends on the displacement after the lateral motion of the segment reverses. This effect reduces thrust forces from the steady state estimates used in a previously developed resistive force theory of sand-swimming, and explains the overestimation of the swimming speed in that theory.