Session: 03-06 Fluid-Structure Interaction

Paper Number: 86969

86969 - Application of Proper Orthogonal Decomposition to Study the Flow Over an Oscillating Flag 

Fluid-Structure-Interaction (FSI) is the coupling of a flowing fluid with a structure that results in static or dynamic deformation of the structure. FSI behavior has an impact in several fields like fluid-induced aircraft wing oscillation, bio-propulsion where flexible bodies generate thrust or fliers and use flapping wings for flight, energy harvesting from fluttering flags via a piezoelectric material, and passive heat transfer through the generation of a vortex in the wake of a fluttering flag. Due to the non-linear nature of FSI, the coupling between the structure and flow behavior is not entirely understood. The focus of this study was to understand the impact of fluttering on the fluid flow around the flag and observe changes in flag fluttering behavior and the aerodynamic performance of the flag. For this investigation, experiments were conducted for a flag with a mass ratio of 1.48, dimensionless rigidity varied between 4.1x10^-3 to 2.1x10^-3, and Reynolds number (Re) varied between 6x10⁴ to 9.4x10⁴. Three different measurements were performed in this study: 1) aerodynamic load, 2) membrane location imaging, and 3) Particle Image Velocimetry (PIV). The aerodynamic load data helped in quantifying the impact of observed oscillation modes on the load experienced by the flag due to changes in flow behavior around the flag. The aerodynamic load measurements showed three regions of drag behavior for the studied flag. In the first region, there was an increase in drag with increase in Re, the second region showed an unexpected decrease in drag with increase in Re and drag again started to increase with increase in Re in the third region. The membrane location data provided crucial information regarding oscillation modes and observed drag behavior. The flag imaging data clearly showed that the flag transitioned to different oscillation modes in the three regions of drag behavior. The PIV data provided crucial information regarding the velocity flow field around the flag and were used for Proper Orthogonal Decomposition (POD) analysis. The average velocity data showed a distinct flow pattern in the three regions of flag behavior. POD was used to estimate the temporally resolved flow field using the phase portrait of the energetic flow modes. The low-order reconstructed flows using the first three POD modes provided information on the impact of large-scale flow structures on the flag fluttering behavior. The phase portraits clearly showed a difference in the coupling between fluid behavior and flag fluttering in the three regions of observed drag behavior. Using only higher-order POD modes (i.e., fourth and higher modes) for low-order reconstruction provided information about the wake vortex shed from the flag and helped in identifying the K-H vortex. These low-order reconstructions clearly showed a change in the vortex behavior with the different oscillation modes. Furthermore, the wake vortex from this low-order reconstruction was identified and tracked to quantify its impact on FSI behavior and observed drag behavior. Finally, the approach presented in this study provided a unique way for phase-locked measurements using traditional low-speed PIV systems to study flag FSI.

Presenting Author: Vibhav Durgesh University of Idaho