Abstract
The unsteady flow behind an inverted flag placed in a water channel and then excited into a self-oscillating state is measured using time-resolved particle image velocimetry. The dynamically deformed profiles of the inverted flag are determined by a novel algorithm that combines morphological image processing and principle component analysis. Three modes are discovered with the successive decrease in the dimensionless bending stiffness: the biased mode, the flapping mode, and the deflected mode. The distinctly different flow behavior is discussed in terms of instantaneous velocity field, phase-averaged vorticity field, time-mean flow field, and turbulent kinetic energy. The results demonstrated that the biased mode generated abundant vortices at the oscillating side of the inverted flag. In the deflected mode, the inverted flag is highly deflected to one side of the channel and remains almost stationary, inducing two stable recirculation zones and a considerably inversed flow between them. In the flapping mode, the strongly oscillating flag periodically provides a strengthened influence on the fluid near the two sidewalls. The reverse von Kármán vortex street is well formed and energetic in the wake, and a series of high-speed impingement jets between the neighboring vortices are directed toward the sidewalls in a staggered fashion.
Highlights
A self-oscillating flag placed in the channel has been recently proposed to substantially enhance the wall heat transfer.1–3 At appropriate flow condition, the flag generates periodic oscillations with large flapping amplitudes; the resulting well-organized vortices sweep out the thermal boundary layer and enhance the thermal mixing between the fluid near the heated wall and the channel core flow.1,4 The insightful understanding of unsteady flow behaviors excited by the self-oscillating flag would fundamentally benefit such applications.The flapping dynamics of an inverted flag placed in the uniform flow were first studied by Kim et al.5 An inverted flag was clamped at the trailing edge and free to oscillate at the leading edge
A highly unsteady flow field interacting with a selfoscillating inverted flag placed in a channel flow was experimentally measured using the timeresolved particle image velocimetry (TR-PIV) technique
The dynamically deformed profiles of the inverted flag were determined by a novel algorithm that combined morphological image processing and principle component analysis (PCA)
Summary
A self-oscillating flag placed in the channel has been recently proposed to substantially enhance the wall heat transfer. At appropriate flow condition, the flag generates periodic oscillations with large flapping amplitudes; the resulting well-organized vortices sweep out the thermal boundary layer and enhance the thermal mixing between the fluid near the heated wall and the channel core flow. The insightful understanding of unsteady flow behaviors excited by the self-oscillating flag would fundamentally benefit such applications.The flapping dynamics of an inverted flag placed in the uniform flow were first studied by Kim et al. An inverted flag was clamped at the trailing edge and free to oscillate at the leading edge. A self-oscillating flag placed in the channel has been recently proposed to substantially enhance the wall heat transfer.. The flag generates periodic oscillations with large flapping amplitudes; the resulting well-organized vortices sweep out the thermal boundary layer and enhance the thermal mixing between the fluid near the heated wall and the channel core flow.. The flapping dynamics of the inverted flag were characterized by low critical speed, along with high oscillating amplitude in water and air flow, which could generate large-scale coherent vortical structures. Sader et al. extended the measurements to study the effect of aspect ratio; the critical flow speed in the flapping mode decreased as H/L increased, and a convergence of the critical speed was reached beyond H/L = 2. In combination with theoretical analysis, they concluded that the oscillation exhibited the characteristics of vortex induced vibration
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