2023/07/11 by Suyash Verma, Verma, Suyash, Muhammad Saif Ullah Khalid +3
Engineering · #Fluid Dynamics and Turbulent Flows #Biomimetic flight and propulsion mechanisms #Fluid Dynamics and Vibration Analysis
paper · pdf · doi:10.48550/arxiv.2307.05823
The correspondence of secondary spanwise structures and pressure gradient is numerically evaluated for a foil, performing heaving and pitching motion, at a range of phase offsets (90^∘ ≤ ϕ≤ 270^∘) and reduced frequency (0.32 ≤ Stc ≤ 0.56). The Reynolds number is Re = 8000. The wake is shown to be dominated by secondary hairpin-like structures that are formed due to an elliptic instability prompted by the paired primary and secondary leading edge vortex (LEV). The weaker secondary LEV undergoes a core deformation, resulting in streamwise vorticity outflux across the span of the foil, and hence, the growth of hairpin-like structures. Evaluating pressure gradients on the surface of the foil reveals a unique fundamental measure to quantitatively characterize the growth of these coherent structures. Their dominant presence can be directly linked to the growth of the secondary LEV formed due to the large-scale interactions under localized adverse pressure gradients. These promote a streamwise flow compression in neighboring regions of the primary LEV. This association also presents a vivid consistency across a range of kinematics. Therefore, this correspondence provides a novel procedure to investigate the mechanisms involved in the formation of secondary structures in the wake of an oscillating foil.