Variable thrust and high efficiency propulsion with oscillating foils at high Reynolds numbers

Abstract

Bio-inspired oscillatory foil propulsion has the ability to traverse various propulsive modes by dynamically changing the foil's heave and pitch kinematics. This research characterizes the propulsion properties of a symmetric oscillating foil at a high Reynolds number of 10 6 , targeting specialized small to medium surface vessels whose propulsive specifications have a broad range of loads and speeds. An unsteady Reynoldsaveraged Navier-Stokes (URANS) solver with a k-ω SST turbulence model is used to sweep through pitch amplitude and frequency at two heave amplitudes of h 0 /c = 1 and h 0 /c = 2. At h 0 /c = 2, the maximum thrust coefficient is C T = 8.2 due to the large intercepted flow area of the foil, whereas at a decreased Strouhal number the thrust coefficient decreases and the maximum propulsive efficiency reaches 75%. Results illustrate the kinematics required to transition between the high-efficiency and high-thrust regimes at high Reynolds number. Reynolds number effects are explored through a comparison with direct numerical simulations (DNS) at Re = 10 3 , and demonstrate that a higher efficiency is achieved at turbulence Reynolds numbers.The unsteady vortex dynamics through the heave-pitch cycle strongly influence the characterization of thrust and propulsive efficiency, and are classified into flow regimes based on performance and vortex structure.