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Updated: Jun 12, 2026
06:42
Magnetically Induced Rotating Rayleigh-Taylor Instability
Published on: March 3, 2017
Oscillating pendulum decay by emission of vortex rings
Diogo Bolster1, Robert E Hershberger, Russell J Donnelly
1Department of Civil Engineering an Geological Sciences, University of Notre Dame, Notre Dame, Indiana 46556, USA.
Summary
This study quantifies drag on a pendulum bob in water, revealing drag coefficients that align with liquid-helium data. It also details vortex ring shedding at larger amplitudes, a phenomenon causing additional drag.
Area of Science:
- Fluid dynamics
- Classical mechanics
Background:
- Pendulum oscillations are influenced by fluid drag.
- Stokes drag describes linear drag at small amplitudes.
- Vortex ring shedding is a complex drag phenomenon at larger amplitudes.
Purpose of the Study:
- To measure drag coefficients of spherical pendulum bobs in water.
- To compare water-based drag data with liquid-helium experiments.
- To investigate and theoretically model vortex-ring-induced drag.
Main Methods:
- Oscillating a pendulum with spherical bobs in water.
- Measuring potential energy loss to determine drag coefficients.
- Analyzing amplitude decay over time to identify drag regimes.
- Developing theoretical models for vortex ring dynamics.
Main Results:
- Drag coefficients in water favorably compare with liquid-helium data, confirming scaling behavior.
- At small amplitudes, exponential decay consistent with Stokes drag was observed.
- At larger amplitudes, vortex ring shedding was identified as a source of discrete drag.
- Analytical estimates for critical amplitude and vortex ring radius were derived.
Conclusions:
- Fluid drag on oscillating spheres exhibits consistent scaling across different fluids.
- Vortex ring shedding introduces a significant, amplitude-dependent drag component.
- The study provides a theoretical framework for understanding vortex-ring-induced drag in oscillating systems.
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