A study on the coherent structures in homogeneous isotropic turbulence | [Proceedings of Pakistan Academy of Sciences • 2009]

Author(s):

1. M. Ashraf-Uddin: Department of Mathematics, Shahjalal University of Science & Technology, Sylhet-3114, Bangladesh

2. Nobuyuki Oshima: Graduate School of Engineering, Hokkaido University, Kita 13 jou Nishi 8 chome, Kita-ku, Sapporo-shi, Hokkaido 060-8628, Japan

3. Mamoru Tanahashi: Department of Mechanical and Aerospace Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan

4. Toshio Miyauchi: Department of Mechanical and Aerospace Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan

Abstract:

Coherent structures in homogeneous isotropic turbulence are investigated by using the Gridscale (GS) and Subgrid-scale (SGS) velocities, and comparisons have been made with the results of Direct Numerical Simulation (DNS) to perform a priori test. The significance of this priori test is to clarify the advantage of SGS modeling from the notion of the coherent structures in turbulence. The GS and SGS velocity fields are obtained by filtering the DNS velocity fields of Reλ = 64.9 and 87.9, using two classical filters for Large Eddy Simulation (LES): Gaussian filter and Sharp cutoff filter. Since the scaling of the coherent structures is directly related to the length of the Kolmogorov microscale (η) in turbulence, so the most important filter width, Δ, is considered as the length of η in the DNS field with a constant multiplication. For the identification of the coherent structures in the GS and SGS fields as well as in the DNS field, an auto- tracking algorithm based on ‘local flow pattern’ in turbulence is directly used. Visualization of flows revealed that the GS and SGS fields itself contain lots of distinct tube-like structures somewhat similar to the structures in the DNS field, of course different in sizes but shows strict coherency, which can be defined as coherent structures in turbulence. Coherent structures in the GS field, i.e., structures larger than filter width are considered as the GS coherent structures and others are SGS coherent structures. The characteristics of these GS and SGS coherent structures are somewhat similar to the structures in the DNS field, which indicate that the DNS velocity field may contain small-scale and large-scale structures simultaneously in turbulence.

Page(s): 145-158

DOI: DOI not available

Published: Journal: Proceedings of Pakistan Academy of Sciences, Volume: 46, Issue: 3, Year: 2009

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