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Extreme events in wall turbulence

Published online by Cambridge University Press:  19 November 2020

M. J. Philipp Hack*
Affiliation:
Center for Turbulence Research, Stanford University, Stanford, CA94305, USA
Oliver T. Schmidt
Affiliation:
Department of Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, CA92093, USA
*
Email address for correspondence: mjph@stanford.edu

Abstract

The mechanics of extreme intensity events in the buffer and logarithmic layers of a turbulent channel at $Re_\tau =2000$ is investigated. The 99.9th percentile of the most intense events in the dissipation of turbulent kinetic energy is analysed by means of conditional space–time proper orthogonal decomposition. The computed spatio-temporal modes are coherent in space and over the considered time frame, and optimally capture the energy of the ensemble. The most energetic mode with transverse symmetric structure describes a turbulent burst event. The underlying mechanism is a varicose instability which generates localized extrema in the dissipation and production of turbulent kinetic energy and drives the formation of a hairpin vortex. The most energetic anti-symmetric mode is related to a sinuous-type instability that is situated in the shear layer between two very-large-scale streaks. Statistical results show the energy in the symmetric mode to exceed that in the anti-symmetric mode by a near constant factor for the considered wall distances. Both mechanisms occur throughout the range of wall distances in an effectively self-similar manner that is consistent with the attached-eddy hypothesis. By analogy with transitional flows, the results suggest that the events are induced by an exponential growth mechanism.

Type
JFM Papers
Copyright
© The Author(s), 2020. Published by Cambridge University Press

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