  Leveraging background fluid flows for propulsion has the potential to enhance
the range and speed of autonomous aerial and underwater vehicles. In this work,
we demonstrate experimentally a fully autonomous strategy for exploiting vortex
rings for energy-efficient propulsion. First, an underwater robot used an
onboard inertial measurement unit (IMU) to sense the motion induced by the
passage of a vortex ring generated by a thruster in a 13,000-liter water tank.
In response to the sensed acceleration, an impulsive maneuver entrained the
robot into the material boundary of the vortex ring. After entrainment, the
robot was propelled across the tank without expending additional energy or
control effort. By advecting with the vortex ring, the robot achieved a near
five-fold reduction in the energy required to traverse the tank. Using the
controlled finite-time Lyapunov exponent field and corresponding Lagrangian
coherent structures, we analyze and explain the initial entrainment process and
the sensitivity to the starting time and position of the surfing maneuver.
Additionally, linear acceleration as sensed by the onboard IMU was found to
correspond with the pressure gradient of the background flow, and rotational
acceleration is suggested as a method for measuring the vorticity of the vortex
ring. This study serves as a proof-of-concept demonstration of the potential
for onboard inertial measurements to enable efficient interaction with
background fluid flows.
