Comparing Models of Lateral Station-Keeping for Pitching Hydrofoils

Area of Science:

• Fluid dynamics
• Bio-inspired robotics
• Hydrodynamics

Background:

• Fish utilize lateral maneuvering for station-keeping in schools and near boundaries.
• Existing unsteady hydrodynamic models (Theodorsen, Garrick) predict forces on tethered hydrofoils in aligned flow.
• The predictive capability of these models for free-moving, angled hydrofoils remains unclear.

Purpose of the Study:

• To evaluate the effectiveness of five linear hydrodynamic models in predicting lateral adjustments of pitching hydrofoils.
• To compare model predictions with experimental data from hydrofoils with induced lateral freedom.

Main Methods:

• Water channel experiments were conducted using a rigid pitching hydrofoil with lateral freedom provided by air bushings.
• Five linear hydrodynamic models were tested for their ability to predict the hydrofoil's lateral response.
• Model predictions were compared against experimental measurements of passive heave oscillations, overshoot, and settling time.

Main Results:

• Linear models accurately predicted high-frequency features of the lateral response, such as the amplitude and phase of passive heave oscillations, even without fitted coefficients.
• Prediction of low-frequency response features, including overshoot and settling time, necessitated a semiempirical model derived from tethered force measurements.

Conclusions:

• Linear hydrodynamic models can partially predict lateral station-keeping for fish and bio-inspired vehicles.
• More sophisticated nonlinear or semiempirical wake models are required for simulating advanced maneuvers and achieving precise control.