Numerical investigation on buffeting and lock-in phenomenon for biplane airfoils at [formula omitted

Abstract

Biplane airfoils at high angles of attack have interesting nonlinear characteristics, the purpose of this work is to evaluate the impacts of spacing ratio (λ=ds/c=0.25, 0.5, 1, 2 and 4, where ds is the vertical distance between biplane airfoils and c is chord length) and Reynolds number (0.87×105≤Re≤4.35×105) on buffeting and lock-in phenomenon at angle of attack α=40∘ throughout numerical investigation. Present simulation results show that biplane configuration usually can reduce buffeting frequency slightly comparing to the single airfoil, yet reasonable spacing λ=2 is helpful to suppress buffeting and continuous acceleration also promotes stability. Particular attention is paid to stagnation point region that moves away from leading edge of the upper airfoil with increasing λ. However, the lower airfoil corresponds to the fixed stagnation point region. Additionally, limit cycle oscillation (LCO) of biplane airfoils is determined by pitching angles (θ), and in consideration of spacing effect, limit cycles of the lower airfoil move downward but those of the upper airfoil move in the opposite direction. Moreover, the shape of limit cycle for the later first collapses then inflates. Unique 8-shaped hysteresis of the upper airfoil in λ=1 is caused by sporadically wake interference. On the whole, V-shaped lock-in region will expand and shift left to some extent based on λ in contrast to the single airfoil. In terms of area, lock-in region shrinks then expands with rising λ, and the proportion of the left part increases as λ increases.