The dynamics of a cantilevered elastic sheet, with a uniform steady flow impinging on its clamped end, have been studied widely and provide insight into the stability of flags and biological phenomena. Recent measurements by Kim et al. (J. Fluid Mech., vol. 736, 2013, R1) show that reversing the sheet’s orientation, with the flow impinging on its free edge, dramatically alters its dynamics. In contrast to the conventional flag, which exhibits (small-amplitude) flutter above a critical flow speed, the inverted flag displays large-amplitude flapping over a finite band of flow speeds. The physical mechanisms giving rise to this flapping phenomenon are currently unknown. In this article, we use a combination of mathematical theory, scaling analysis and measurement to establish that this large-amplitude flapping motion is a vortex-induced vibration. Onset of flapping is shown mathematically to be due to divergence instability, verifying previous speculation based on a two-point measurement. Reducing the sheet’s aspect ratio (height/length) increases the critical flow speed for divergence and ultimately eliminates flapping. The flapping motion is associated with a separated flow – detailed measurements and scaling analysis show that it exhibits the required features of a vortex-induced vibration. Flapping is found to be periodic predominantly, with a transition to chaos as flow speed increases. Cessation of flapping occurs at higher speeds – increased damping reduces the flow speed range where flapping is observed, as required. These findings have implications for leaf motion and other biological processes, such as the dynamics of hair follicles, because they also can present an inverted-flag configuration.

Large-amplitude flapping of an inverted flag in a uniform steady flow – a vortex-induced vibration

Cross axis wind turbine: Pushing the limit of wind turbine technology with complementary design

Transition of VIV-only motion of a square cylinder to combined VIV and galloping at low Reynolds numbers

Vortex streets formed in the stratocumulus‐capped wake of mountainous islands are the atmospheric analogues of the classic Karman vortex street observed in laboratory ﬂows past bluff bodies. The quantitative analysis of these mesoscale unsteady atmospheric ﬂows has been hampered by the lack of satellite wind retrievals of sufﬁciently high spatial and temporal resolution. Taking advantage of the cutting‐edge Advanced Baseline Imager, we derived kilometer‐scale cloud‐motion winds at 5‐min frequency for a vortex street in the lee of Guadalupe Island imaged by Geostationary Operational Environmental Satellite‐16. Combined with Moderate Resolution Imaging Spectroradiometer data, the geostationary imagery also provided accurate stereo cloud‐top heights. The time series of geostationary winds, supplemented with snapshots of ocean surface winds from the Advanced Scatterometer, allowed us to capture the wake oscillations and measure vortex shedding dynamics. The retrievals revealed a markedly asymmetric vortex decay, with cyclonic eddies having larger peak vorticities than anticyclonic eddies at the same downstream location. Drawing on the vast knowledge accumulated about laboratory bluff body ﬂows, we argue that the asymmetric island wake arises from the combined effects of Earth's rotation and Guadalupe's nonaxisymmetric shape resembling an inclined ﬂat plate at low angle of attack. However, numerical simulations will need to establish whether or not the selective destabilization of the shallow atmospheric anticyclonic eddies is caused by the same mechanisms that destabilize the deep columnar anticyclones of laboratory ﬂows, such as three‐dimensional vertical perturbations due to centrifugal or elliptical instabilities.

Evolution of an atmospheric Kármán vortex street from high‐resolution satellite winds: Guadalupe Island case study

Nonlinear aerodynamic forces on thin flat plate: Numerical study

:Vortex shedding flow from a flat plate inclined to a uniform flow at an angle of attack between 20o and 45o is simulated with a finite volume CFD code with RNG k-ω turbulence model. The unsteady flow simulation at Re=2X104 with RANS shows two trains of vortices shed from the two different edges of the plate forming a vortex street in the wake of the plate. The computed results provide support to previous experimental observations that in this asymmetric flow geometry, the two trains of vortices in the vortex street possess different vortex strengths. There is further evidence that the vortex from the plate leading edge is actually shed from a location near the trailing end of the plate. The computed flow at successive phases of a vortex shedding cycle show different development and shedding mechanisms for the two trains of vortices. The study also explores the generation mechanism of the fluctuating lift and drag on the plate and its relationship with the vortex shedding processes.

Numerical Simulation of Vortex Shedding from an Inclined Flat Plate

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Numerical Simulation of Vortex Shedding from an Inclined Flat Plate