Experimental investigation of flexible trailing edge effects on airfoil turbulent-boundary-layer noise

Abstract

The acoustic characteristics of the NACA 0012 airfoil with a flexible trailing edge (TE) are experimentally investigated. Measurements are conducted at chord-based Reynolds numbers ranging from $1.93\times 10^5$ and $1.16\times 10^6$, with the airfoil set at a zero angle of attack. Flexible TEs made of polyethylene terephthalate, polypropylene, and polyvinyl chloride strips are employed to assess their impact on noise reduction. To elucidate the effect of TE deformation on acoustic performance, the out-of-plane deformation is recorded using high-speed cameras and analyzed with a digital image correlation algorithm. The acoustic results demonstrate that flexible TEs reduce noise by 1–3 dB within the mid-frequency range, with its upper limit shifting to higher frequencies as the free-stream velocity increases, following a scaling law of $f\sim U^{1.5}$. However, noticeable tonal noise persists across various free-stream velocities, with its frequency remaining relatively consistent. Deformation analysis shows that the vibration of the flexible strip plays a key role in generating vortex shedding, which in turn leads to tonal noise. This is further supported by proper orthogonal decomposition (POD) analysis, which indicates that bending deformation, especially near the tip of the strip, is the primary mechanism driving vortex shedding. This bending deformation, associated with lower-order POD modes, generates large-scale flow structures that propagate acoustic waves to the far field, thereby contributing to the observed tonal noise.