Vortex shedding and hovering of a rigid body in an oscillating flow
Bin Liu, Annie Weathers, Stephen Childress, Jun Zhang
TL;DR
This study demonstrates how asymmetric rigid bodies can hover in oscillating flows by shedding vortices, revealing mechanisms of vortex-induced stability and propulsion in unsteady fluid environments.
Contribution
It introduces the concept of vortex shedding enabling asymmetric bodies to hover and move in oscillating flows, with visual evidence of vortex dynamics.
Findings
Asymmetric bodies can hover in zero-mean oscillating flows.
Vortex shedding enables the ratcheting motion of the body.
Stable hovering occurs at sufficiently high peak flow speeds.
Abstract
The fluid dynamics video shows rigid, spatially asymmetric bodies interacting with oscillating background flows. A free rigid object, here a hollow "pyramid," can hover quite stably against gravity in the oscillating airflow with a zero mean, when its peak speed is sufficiently high. We further show in shadowgraph imaging how this asymmetric body sheds vortices in such an unsteady flow, thus enabling the body to "ratchet" itself through the background flow.
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Taxonomy
TopicsFluid Dynamics and Turbulent Flows · Particle Dynamics in Fluid Flows · Fluid Dynamics and Vibration Analysis
Vortex shedding and hovering of a rigid body in an oscillating flow
Bin Liu1, Annie Weathers2, Stephen Childress1, and Jun Zhang1,2
1Applied Mathematics Laboratory, Courant Institute of Mathematical Sciences,
New York University, New York, NY 10012, USA
2Department of Physics, New York University, New York, NY 10003, USA
Abstract
The fluid dynamics video shows rigid, spatially asymmetric bodies interacting with oscillating background flows. A free rigid object, here a hollow “pyramid,” can hover quite stably against gravity in the oscillating airflow with a zero mean, when its peak speed is sufficiently high. We further show in shadowgraph imaging how this asymmetric body sheds vortices in such an unsteady flow, thus enabling the body to “ratchet” itself through the background flow.
1 Introduction
In the videos (low-res. and high-res.), we first present an example of free hovering of a rigid, spatially asymmetric body in an oscillating airflow. The rigid object is a hollow “pyramid” of height cm high, constructed of carbon fibers and wax paper. The background air oscillates at a frequency Hz. When the air speed is high enough ( m/s), the paper pyramid hovers against its weight ( dynes, or g). The pyramid aligns itself spontaneously in the lifting position, exhibiting surprising stability. The video is recorded by a high speed camera and then played back at 1/13 of the real speed.
In the second part of the video, we show the vortical structures produced by the interaction between a “V”-shaped object and an oscillating background flow. The flow visualization is realized by shadowgraph imaging of an oscillating water flow around a two-dimensional pyramid of a height cm, composed of two faces. In the example, shown in real-time in the video, the oscillating frequency of the water flow is Hz, with an amplitude of 2 cm. The system thus has approximately the same Reynolds number regime as the experiments in air. Vortices generated during each period coalesce into paired vortices, before detaching and propagating downward, producing the momentum flux needed to support the body.