Fluid–structure interaction of a flexible membrane wing at a fixed angle of attack

Abstract

Fluid–structure interaction of a flexible membrane wing was investigated experimentally at a fixed angle of attack (α = 14°). The Reynolds numbers (Re) based on the chord length of the membrane wing were 6 × 104, 6.6 × 104, and 8 × 104, respectively, which belong to low Reynolds numbers. Membrane deformations and the global flow fields around the membrane wing were measured synchronously with two-dimensional time-resolved particle image velocimetry. The membrane performs strongly periodic standing-wave vibration modes while interacting with the surrounding flow. Although the flow fields and membrane vibrations are coupled in all Re cases, it is newly discovered that, at Re = 6 × 104, the dominant frequency of the flow is around 9.1 Hz (Strouhal number St = 0.15) instead of the dominant membrane vibration frequency of 35.1 Hz (first-order vibration mode, St = 0.56). By tracking the Lagrangian coherent structures over the membrane, it is revealed that the vortical structures at Re = 6 × 104 are affected by the flow from both the leading- and trailing-edges. The frequency of 35.1 Hz in the power spectrum of flow is attributed to the successive shedding of leading-edge vortices coupled with membrane vibration, while the dominant frequency of flow field (9.1 Hz) is resulted from the interaction of strong trailing-edge reverse flow and the leading-edge separated flow, and the mechanism is explored. For higher Re cases (Re = 6.6 × 104 and 8 × 104), the membrane presents second- and third-order vibration modes separately. The dominant frequencies of the flow structure and membrane vibration are consistent, and the periodic shedding of leading-edge vortices is the main flow characteristic in both Re cases.