On the role of wake-capture and resonance in spanwise-flexible flapping wings in tandem Articles uri icon

publication date

  • November 2024

start page

  • 1

end page

  • 20

volume

  • 130

International Standard Serial Number (ISSN)

  • 0889-9746

Electronic International Standard Serial Number (EISSN)

  • 1095-8622

abstract

  • Numerical simulations of the flow around spanwise-flexible flapping wings in tandem are
    reported, focusing on a thrust-generating configuration. Wings of aspect ratio 2 and 4 in forward
    flight undergo heaving and pitching motion following optimal 2D kinematics. The Reynolds
    number of the simulations is 𝑅𝑒 = 1000. The effect of flexibility is explored by varying the
    effective stiffness of the wings, while the effective inertia is kept constant. The aerodynamic
    performance of the tandem system results from a combination of unsteady aerodynamics
    mechanisms, fluid–structure resonance, vortex–wing interactions (denoted wake capture in this
    study) and aerodynamic tailoring. It is found that the aerodynamic performance and structural
    behavior of forewings are dominated by a fluid–structural resonance. The maximum mean thrust
    for the forewings is obtained when the driving frequency approaches the first natural frequency
    of the structure, 𝜔𝑛,𝑓 ∕𝜔 ≈ 1, similarly to what is observed in isolated wings undergoing the same
    kinematics. On the other hand, hindwings show optimal performance in a broad region near
    𝜔𝑛,𝑓 ∕𝜔 ≈ 2, and their aerodynamic performance seems to be dominated by wake–capture and
    aerodynamic–tailoring effects. The aerodynamic performance of the hindwings is dependent
    on the flexibility of the forewing, which impacts the intensity of the vortices shed into the
    wake and the resulting effective angle of attack (i.e., wake capture). The timing between the
    effective angle of attack and the pitching motion of the hindwing controls the generation of
    thrust (or drag) of each spanwise section of the hindwing (i.e., aerodynamic tayloring). A proof
    of concept study on the aerodynamic performance of systems made of wings with different
    flexibility suggests that they could outperform tandem systems with equally flexible wings.
    Thus, the optimal mixed–flexibility tandem system is composed by a resonant forewing, which
    maximizes the thrust generation of the forewing and the intensity of the vortices shed into the
    wake, and a hindwing whose flexibility must be tuned to maximize wake capture effects.

subjects

  • Aeronautics
  • Mechanical Engineering

keywords

  • spanwise-flexible wings; flapping wings; fluid–structure interaction; numerical simulations; wing–wing interactions