  We present a flow-based control strategy that enables resource-constrained
marine robots to patrol gyre-like flow environments on an orbital trajectory
with a periodicity in a given range. The controller does not require a detailed
model of the flow field and relies only on the robot's location relative to the
center of the gyre. Instead of precisely tracking a pre-defined trajectory, the
robots are tasked to stay in between two bounding trajectories with known
periodicity. Furthermore, the proposed strategy leverages the surrounding flow
field to minimize control effort. We prove that the proposed strategy enables
robots to cycle in the flow satisfying the desired periodicity requirements.
Our method is tested and validated both in simulation and in experiments using
a low-cost, underactuated, surface swimming robot, i.e. the Modboat.
