Self-Induced Flapping Dynamics of a Horizontal Flexible sheet in a Uniform Flow

TL;DR

This study experimentally investigates the self-induced flapping dynamics of a horizontal flexible sheet in a uniform flow, revealing how gravity influences (or does not influence) instability and flapping behaviour, with implications for energy harvesting.

Contribution

It provides new experimental insights into how gravity affects the flapping dynamics of flexible sheets in different orientations, which was previously not well understood.

Findings

Gravity along the bending direction does not influence flapping instability onset.

Horizontal sheets show higher amplitudes and frequencies than vertical sheets.

Results are relevant for designing smart propulsion and energy harvesting devices.

Abstract

The case of a conventional flexible sheet in a uniform flow has been of interest in understanding the underlying physics of passive coupled dynamics between a flexible structure and a flow field. Gravity is known to influence the flapping instability and post-critical dynamics. Interestingly, the flapping instability and dynamics of a thin flexible structure have been investigated either by neglecting the effects of gravity or by considering gravity along the length/span of the sheet. This study experimentally investigates the self-induced and sustained flapping dynamics of a thin flexible sheet positioned horizontally, with gravity acting along its bending direction. To explore the coupled interplay between gravitational, aerodynamic, and structural effects on the onset of instability and post-critical flapping dynamics, wind tunnel experiments are conducted across a range of physical parameters of the flexible sheet, such as its length (L), and aspect-ratio ($\AR$) for different wind speeds. To further understand the effects of gravity, the flapping behaviour of a vertically mounted flexible sheet with gravity acting along its span has been investigated, and comparisons have been drawn with its horizontal counterpart. It has been observed that gravity along the bending does not influence the onset of flapping instability. The horizontally mounted flexible structures exhibit higher flapping amplitudes and frequencies when compared to their vertical counterparts. The observations in this study have direct relevance in the field of smart propulsion and energy harvesting devices.