2024-07-25 カリフォルニア工科大学(Caltech)
Credit: Shutterstock
＜関連情報＞
- https://www.caltech.edu/about/news/a-swinging-showerhead-leads-to-discovery-of-a-new-mode-of-vibration-in-nature
- https://www.pnas.org/doi/abs/10.1073/pnas.2311412120
- https://www.pnas.org/doi/10.1073/pnas.2314442120
定常的なゼロ周波数の力を動力源とする二峰性振動のクラスについて-エネルギー変換と構造安定性への示唆 On a class of bimodal oscillations powered by a steady, zero-frequency force—Implications to energy conversion and structural stability
Amnon Zalman Yariv
Proceedings of the National Academy of Sciences Published:September 11, 2023
DOI:https://doi.org/10.1073/pnas.2311412120
Significance
I view the main importance of my work in its impact on the following three areas: a category of modes in man-made and in natural environments, implications to energy harvesting, for example—direct DC to AC conversion in hydromechanical systems, implications to structural stability (bridge and building collapse). The abstract and introduction to the article provide a more detailed description of the impact and implications of the proposed mode of oscillation.
Abstract
I propose that there exists in natural and artificial environments a class of resonant oscillations that can be excited directly by a steady, zero-frequency force such as that of wind, water, electric field. A member of this class comprises two normally independent oscillating modes of a system, for example, a building or bridge, which, separately, cannot be driven by a zero-frequency force. Agreeing on terms of collaboration, the two modes engage in a joint oscillation powered by the steady zero-frequency force in which they drive each other, one directly and the other parametrically. I observed a bimodal vibration belonging to this class in a home shower where the two modes are the pendulum excursion and the torsional twisting of a freely suspended showerhead which break into a joint oscillation above a threshold value of the water flow rate. I advance a theoretical model which predicts and explains the main features of the observations. The model constitutes an extension to two modes of a proposal and demonstration in 1883 by Lord Rayleigh and Michael Faraday for the excitation of a single resonant mode by modulating a system parameter at twice the resonance frequency. The proposal is credited with the launching of parametric physics. The Experiments section of this report consists of three linked video clips photographed in the home shower which support the basic theoretical assumptions. The ubiquity of zero-frequency forces, such as that of wind, and their direct conversion to alternating on-resonance system vibrations endows the class with an amplified destructive potential with implications for structural stability.
デュアルダイナミクスのダンスが、定常的な力によるパラメトリック振動を可能にする Dances of dual dynamics enable parametric oscillations under a steady force
Demetrios Christodoulides
Proceedings of the National Academy of Sciences Published:October 4, 2023
DOI:https://doi.org/10.1073/pnas.2314442120
Parametric oscillations occur ubiquitously in nature. From an early age, a child unconsciously learns to move its body at twice the frequency of a swing to sustain oscillations (1). In astrophysics, the parametric excitation of stellar modes in close binary systems is known to lead to tidal forces and oscillation resonances in the stars’ surfaces and magnetic fields. Parametric interactions were first documented by Michael Faraday in 1831 when he noticed them in mechanical settings (2). Half a century later, Lord Rayleigh put this class of phenomena on a firm ground by providing a systematic analysis of this general class of parametric effects (3). Yet, it was during the 20th century that parametric oscillators and amplifiers reached their zenith by enabling a host of far-reaching and widespread applications (4–6). Such systems are nowadays extensively deployed in realizing low-noise electronic parametric amplifiers and detectors and in building highly versatile optical parametric oscillators, to mention a few. Looking at the enormous success this field has enjoyed for more than a century or so, it is perhaps tempting to think that little if any is left to learn. In this edition of PNAS, Amnon Yariv proves otherwise. In ref. 7, he introduces for the first time a new paradigm of bimodal oscillations that surprisingly are enabled by a constant (zero-frequency) force. The author is by no means a stranger to this landscape—being himself one of the early pioneers in the area of parametric quantum electronics. In this study, resonant parametric oscillations that could be powered by steady forces, such as, for example, wind or water, are experimentally observed and explored. Such oscillations, though absent in single-mode configurations, do manifest in dual-mode systems where two independent oscillating modes synergize. This concept is vividly demonstrated in a mechanical/fluid arrangement, where two modes—a pendulum excursion and a torsional twist—interact in response to a steady water flow, yielding a parametric bimodal vibration.
