Newsletter  2023.10  Index

Theme : "AJK FED 2023" Preface Hyun Jin PARK, Shoichi MATSUDA, Chungpyo HONG Evaluation of railway vehicles' resistance against strong crosswinds and its application for safe railway operation Yayoi MISU (East Japan Railway Company) Order within Turbulence Susumu GOTO (Osaka University), Yutaro MOTOORI (Osaka University) LES/Lagrangian-particle-simulation of a Reactive Turbulent Planar Jet Jiabao Xing (Nagoya University)，Tomoaki WATANABE (Nagoya University)，and Koji NAGATA (Kyoto University) Unsteady Characteristics of Tip Leakage Vortex Cavitation in the Occurrence of Cavitation Instability in Liquid Rocket Inducer Koki TAMURA (Tohoku University)，Yuto NAKURA (Tohoku University), Satoshi KAWASAKI (Japan Aerospace Exploration Agency), Yuka IGA (Tohoku University) Water Condensation in PEMFCs at Nano-scale: Insights through Lattice DFT simulations Clint John Cortes OTIC (The University of Tokyo), Masazumi ARAO (FC-Cubic), Masashi MATSUMOTO (FC-Cubic), Hideto IMAI (FC-Cubic), Ikuya KINEFUCHI (The University of Tokyo) Reconstruction of Fluid Stress Field from Flow Birefringence using Physics-Informed Convolutional Encoder-Decoder (PICED) Daichi IGARASHI (Tokyo University of Agriculture and Technology), Shun MIYATAKE (Tokyo University of Agriculture and Technology), Jingzu YEE (Tokyo University of Agriculture and Technology), Yoshiyuki TAGAWA (Tokyo University of Agriculture and Technology) Determination of Permeability in the Volume Penalisation Method with a Smooth Mask Function Taichi TSUJIMOTO (Osaka University), Yuta NAKAO (Osaka University), Takuya TSUJI (Osaka University), Toshitsugu TANAKA (Osaka University), Kimiaki WASHINO (Osaka University)

 

Order within turbulence

Abstract

Susumu GOTO，  Yutaro MOTOORI Osaka University

Order exists within turbulent flows. Conducting direct numerical simulations, we recently showed that developed turbulence at high Reynolds numbers is composed of a hierarchy of coherent tubular vortices. In this newsletter, we demonstrate this picture using visualizations of coherent vortical structures on different scales in (i) turbulence in a periodic box, (ii) turbulent wake behind a circular cylinder, and (iii) turbulent channel flow.

Here, we emphasize that we cannot capture the hierarchy of vortices on different scales by simply using a quantity related to the velocity gradient tensor, such as the vorticity magnitude (and enstrophy) or the second invariant of the tensor. This is because these quantities are determined by the smallest-scale structures. In fact, if we use these quantities to identify vortices in turbulence, we always observe a forest of worm-like fine tubular vortices.

Therefore, we need a scale decomposition to observe the hierarchy of vortices. By employing a band-pass filter of Fourier modes for the periodic turbulence and a combination of the Gaussian filters in real space for the other turbulent flows, we can identify coherent vortices at a given scale. Then, observing the identified hierarchy of coherent vortices on different scales, it is not difficult to describe their sustaining mechanism, which must be related to the physical mechanism of the energy cascade. Tubular vortices on the same scale tend to form counter-rotating pairs. Since such a pair of vortices is accompanied by a straining field around them, smaller-scale vortices are stretched and amplified there.

We also emphasize that the hierarchy of coherent vortices is useful for describing the various turbulent transport phenomena.

Key words

Turbulence, Coherent Structures, Vortices, Transport Phenomena

Figures

(a-c) Forest of fine-scale vortices identified by the magnitude of vorticity or the second invariant of the velocity gradient tensor.
(d-f) Hierarchy of coherent vortices identified by scale-decomposed fields:
Red, the largest vortices; yellow, intermediate-scale vortices; blue, small-scale vortices. (a, d) Turbulence in a periodic cube, (b, e) turbulent wake behind a cylinder, and (c, f) turbulent channel flow.