RCUE Oscillating Foils in Ground Effect
Wing-in-Ground Effect and Near Bottom Swimming Robots
- Dane Elles
- Amin Mivehchi
- Alex Stott
Wing-in-ground effect is a well understood phenomenon for the case of steady flight near a solid boundary. However, the overall effect of swimming close to the bottom using biologically inspired oscillatory propulsion mechanisms is still poorly characterized.
The presence of a ground effect for oscillating foils has two implications for near bottom swimming UUV designs. Decisions about both UUV design and optimal operation must take ground effect into account to achieve optimal range and swimming speeds. The strength of the ground effect will affect stability in near bottom swimming from a dynamic standpoint (i.e. is the effect stabilizing or destabilizing in pitch and altitude regulation?) and from a sensing standpoint (i.e can changes in actuator demand be used to detect altitude in the near field where traditional acoustic sensing approaches break down?)
We are pursuing two experimental approaches, using 2-D foil dynamometry and flow visualization in a small tank (courtesy of the Dahl Lab at URI) and using 3-D foil force sensing with an operational robot foil thruster.
2-D Foil Experiments
By installing a false wall and false bottom, constructed of clear acrylic, in the tank shown above we will allow the carriage to operate the foil arbitrarily close to a solid wall while still collecting imagery data. The key question is whether it is possible to detect the presence of/distance to the side wall. Does this depend on the foil kinematic? In what way, and can we use that knowledge to design kinematics which increase the probability of wall detection? Can this be done without significantly impacting the propulsion efficiency?
3-D Foil Experiments
The 3-D foil experiments take advantage of the URI adjustable depth towing tank to test an ground effect with an oscillating foil thruster taken from the RoboTurtle. The thruster apparatus, on loan from Tow Tank has been augmented with load cells, a fairing, and a new quick release foil connection.
Over the course of 2014, the thruster, mounted on the apparatus shown above, will be operated over a varying depth tank floor to see whether ground effect can be detected and used for control and stabilization of near bottom swimming AUVs. Can minimal sensing (torque at the shoulder) be used to detect altitude? Is this sensing modality effective for closed loop control of swimming altitude?