  Soft robots, in contrast to their rigid counter parts, have infinite degrees
of freedom that are coupled with their interaction with the environment. We
consider the locomotion of an untethered robot, in the granular medium,
comprised of multiple flexible flagella that rotate about an axis by a motor.
Drag from the grains causes the flagella to deform and the deformed shape
generates a net forward propulsion. This external drag force depends on the
shape of the flagella, while the change in flagellar shape is the result of the
competition between the external loading and elastic forces. We introduce a
numerical tool that couples discrete differential geometry based simulation of
elastic rods - our model for flagella - and a resistive force theory based
model for the drag. In parallel with simulations, we conduct experiments to
quantify the propulsive speed of this class of robots. We find reasonable
quantitative agreement between experiments and simulations. Owing to a
rod-based kinematic representation of the robot, the simulation runs faster
than real-time, and, therefore, we can use it as a design tool for this class
of soft robots. We find that there is an optimal rotational speed at which
maximum efficiency is achieved. Moreover, both experiments and simulations show
that increasing the number of flagella decreases the speed of the robot. We
also gain insight into the mechanics of granular medium - while resistive force
theory can successfully describe the propulsion at low number of flagella, it
fails when more flagella are added to the robot.
