Incomplete fluid–structure coupling mechanism of a flexible membrane wing

The effect of angles of attack (α) on the fluid–structure coupling of a flexible membrane wing is investigated experimentally at Re = 6 × 10⁴. Membrane deformations and the surrounding flow fields are measured synchronously by two-dimensional time-resolved particle image velocimetry (2D-TRPIV). Results show that, for 12 ≤ α ≤ 16°, the vertical velocities over the membrane are coupled with the membrane vibration, but the streamwise velocities present lower dominant frequencies, which means the susceptibility of the flow in vertical and streamwise directions is different. As a result, the dominant frequencies of the instantaneous flows are different from and lower than the membrane, suggesting that the flow and the membrane are incompletely coupled. Then, the incomplete fluid–structure coupling mechanism is clearly revealed by the colored virtual dye visualization as well as the instantaneous pressure gradients. The periodically alternating adverse and favorable pressure gradients near the leading edge lead to the unsteady flow separation and reattachment, resulting in periodic presence and absence of the trailing-edge reverse flow in streamwise direction. Consequently, the intermittent fluid–fluid interaction between the leading-edge flow and the trailing-edge reverse flow causes the incomplete fluid–structure coupling.