特別講演会のお知らせ　　　　　　　　　　　　　　　　　　　　　　　　　　　　　2017.10.5
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　北海道支部　支部長　土谷　浩昭

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日　時　：　2017年 10月　16日　（月）　　15：00～16：00
場　所　：　北海道大学工学部 大会議室　Ａ3-71　　　　（札幌市北区北１３条西８丁目）

講　演　：　[The states of flow in transitional pipes]

　　　　　　　　　　Prof. Pinaki Chakraborty 　　　　　Okinawa Inｓtitute of Science and Technology


講演概要 
In 1883 Osborne Reynolds discovered a peculiar state for flow of water in a pipe. This state corresponded to the transition from the quiescent, laminar state at low Re (Reynolds number, a dimensionless measure of the flow velocity) to the roiling, turbulent state at high Re. The laminar state is devoid of fluctuations; the turbulent state is inundated by a spectrum of fluctuations. The transitional state is distinct from the laminar and turbulent states, and consists of localized patches—Reynolds termed them “flashes”—of fluctuations alternating with plugs of laminar flow. Later work unveiled that the flashes come in two disparate states: “puffs,” which can maintain their shape or proliferate or fade away; and “slugs,” which continually expand. To summarize, puffs and slugs and turbulence appear to constitute three distinct states of flow. We take a closer look at the flashes.
First, we consider “laws of resistance” (which relate the fluid friction with Re). Introduced in Reynolds’s original work, the laws of resistance furnish a quantitative diagnostic of the state of flow. While he succeeded in determining the laws of resistance for laminar and turbulent flows, the laws for transitional flows eluded him and remain unknown to this day. By properly distinguishing between flashes and laminar plugs in the transitional regime, we show experimentally and numerically that the law of resistance for laminar plugs corresponds to the laminar law and the law of resistance for flashes is identical to that of turbulence. We conclude that flashes are turbulent sensu Reynolds.
Next we consider the nature of the fluctuations in the flashes, about which little is known. Our experimental and numerical results suggest a startling conclusion: the fluctuations in the flashes partake in the well-known Kolmogorov energy cascade, the signature of high Re turbulent state. In other words, flashes are turbulent sensu Kolmogorov.
To wit, transition is not a distinct state: flashes are turbulent.
This work is in collaboration with Rory Cerbus, Chien-chia Liu, and Gustavo Gioia.

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