Numerical investigation of aerodynamics and wake on biplane airfoils at high angles of attack

Abstract

Biplane configuration is a critical configuration for aircrafts all the time, but the dynamics and flow mechanism remain elusive, with no systematic description forthcoming. This paper reported a numerical investigation of spacing effect (λ=d/c=0.4∼5, c is the chord length and c=6inch) and the mean and fluctuating aerodynamics for biplane airfoils at angles of attack ranging from 30∘ to 90∘. Additionally, particular attention was devoted to vorticity fields and Strouhal number (St). The primary finding was that C¯L and C¯D of the lower airfoil were greater than those of the upper airfoil whatever λ was located to. And the turning points for C¯L and C¯D of the upper airfoil showed an evident regularity with increasing λ.But the maximum deviation first plungs then jumps with increasing λ. What inspired us was the fluctuating aerodynamics in λ=1.5 converged quickly which may contribute to enhancing stability. Interestingly, overall lift-to-drag ratio trends in all cases were strikingly similar to a single airfoil. Three vortices regimes were identified from the generation mechanism of traling-edge vortices (TEVs), named integrated vortices, transition with deformed vortices and coshedding vortices, respectively. And the vortices regime was also related to the angle of attack. Besides, in the integrated and coshedding vortices regimes, two Strouhal numbers of the lower airfoil and the upper airfoil were equivalent, but St can’t be estimated which one is larger in the transition regime.