On association of lift generation, wake topology and kinematics of oscillating foils

Abstract

The association of lift generation and evolution of wake topology behind an oscillating foil with combined heaving and pitching motion is investigated numerically at a range of bluereduced frequency (0.16 ≤ f * ≤ 0.48), phase offset (0 ∘ ≤ ϕ ≤ 315 ∘ ) and Reynolds number (1000 ≤ R e ≤ 4000). The pitch-dominated kinematics that coincide with the range of ϕ ≤ 120 ∘ and ϕ ≥ 225 ∘ suggests that leading edge vortices are suppressed while trailing edge vortices dominate the wake with increasing reduced frequency. This corresponds to a transition in wake topology from a 2 P to a reverse Von Kármán wake mode. Contrarily, heave dominated kinematics (120 ∘ < ϕ ≤ 225 ∘ ) did not exhibit any wake topology transition with increasing f * . The temporal lift variation associated with heave-dominated regime further revealed a symmetric feature in terms of the time taken to attain peak lift generation within an oscillation cycle. This temporal symmetry was, however, lost as kinematics transitioned from heave- to pitch-dominated regime. Analyzing the wake evolution and lift features at quarter phase of an oscillation cycle revealed the existence of a correspondence between the two processes during the heave- and pitch-dominated kinematics.