Vortex transition and thermal mixing by pitching a perforated flexible panel
Phys. Rev. Fluids 11, 034703 – Published 23 March, 2026
DOI: https://doi.org/10.1103/txkf-nsst
Abstract
The effective transport of heat and mass is crucial to both industrial applications and physiological processes. Recent research has evaluated the benefit of using flexible reeds for triggering the vortex-induced vibration to enhance mixing, as opposed to traditional rigid or static techniques. Inspired by the soft, porous, and moving fish gill lamellae, we proposed a concept of thermal dispenser that prescribes active pitching motion to the leading edge of an otherwise passively flapping perforated panel. Fundamentally, we identified different vortex creation and shedding mechanisms by the perforated panel than the conventionally studied nonperforated panel. We experimentally documented the effect of chordwise flexibility on inducing vortex transitions across a range of frequencies, primarily the change of dipole orientation and wake bifurcation. A semi-empirical simulation of thermal convection and diffusion took the experimentally obtained flow field as an input and yielded the temperature mixing results. We numerically showed that the actuation and chordwise flexibility both enhance mixing. Specifically, active pitching resulted in a 37–94% higher average temperature increment than statically deflected panels. The chordwise flexible panel achieved an overall higher mixing index and was able to maintain similar mixing performances across the entire tested kinematic range due to vortex transitions, whereas the rigid panel suffers from increasing pitching frequency. To synthesize the understanding of vortex dynamics and thermal mixing, we examined the Lagrangian coherent structures to elucidate the lateral entrainment, convective transport barriers, and mixing homogeneity. Our work provides a foundational understanding of the fluid-structure interactions of perforated bendable panels under active control, which has not been described before in the intermediate Reynolds number range. It provides insights for developing an innovative bio-inspired heat or mass dispenser potentially suitable for subtle and small-scale applications.
