A Statistical investigation of near-wall quasi-streamwise vortices with extreme events of turbulent kinetic energy dissipation in turbulent channel flow*

The investigation of near-wall vortical structures with extreme Turbulent Kinetic Energy (TKE) dissipation rate events using Direct Numerical Simulation (DNS) of turbulent channel flow across various Reynolds number conditions, based on friction velocity, is presented. The simulation results are validated against previous DNS studies and experimental data by comparing turbulent statistics. Statistical evidence confirms the presence of quasi-streamwise (QS) vortices near regions with extreme TKE dissipation rates, consistent with earlier analyses from Zaripov's experimental studies, including joint probability density functions of velocity fluctuation components and TKE dissipation rates. The third-generation vortex identification method, Rortex, is employed to elucidate the statistical morphological characteristics of QS vortices, with a conditional average on spatial subdomains where the TKE dissipation rate exceeds tenfold the standard deviation. The temporal behavior of QS vortices during extreme TKE dissipation events is investigated using Connected Component Labeling and a temporal tracking algorithm based on the velocity of labeled regions. The lifecycle of QS vortices, including the stages of coupled counter-rotating streamwise vortices, tilting, and dissipation, is analyzed. A roll-up phenomenon is observed at the tail of QS vortices when the TKE dissipation rate peaks during the temporal evolution.