Extreme events in wall turbulence. (19th November 2020)

• Record Type: Journal Article
• Authors: Hack, M. J. Philipp; Schmidt, Oliver T.
• Is Part Of: Journal of fluid mechanics. Volume 907(2021)
• Publication Date: 2020-11-19
• Subjects: intermittency, -- turbulent boundary layers; Fluid mechanics -- Periodicals; 532.005
• DOI: 10.1017/jfm.2020.798
• Languages: English
• ISSNs: 0022-1120

Abstract

Abstract: 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.