The mechanics of extreme intensity events in the buffer and logarithmic layers of a turbulent channel at 𝑅𝑒𝜏=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.

Literature:

  • [PDF] [DOI] Hack, M. J. P. and O. T. Schmidt. “Extreme events in wall turbulence.” Journal of fluid mechanics 907 (2021): A9.
    [Bibtex] 
    @Article{hackschmidt_2020_jfm,
author = {Hack, M. J. P. and Schmidt, O. T.},
journal = {Journal of Fluid Mechanics},
title = {Extreme events in wall turbulence},
year = {2021},
pages = {A9},
volume = {907},
doi = {10.1017/jfm.2020.798},
file = {:HackSchmidt_2020_JFM.pdf:PDF},
publisher = {Cambridge University Press},
}