Coherent structures near the wall in a turbulent channel flow

摘要: Coherent structures (CS) near the wall (i.e. y + ≤ 60) in a numerically simulated turbulent channel flow are educed using conditional sampling scheme which extracts entire extent of dominant vortical structures. Such detected from instantaneous field our newly developed vortex definition (Jeong & Hussain 1995) - region negative λ 2 , second largest eigenvalue tensor S ik kj Ω accurately captures structure details (unlike velocity-, vorticity- or pressure-based eduction). Extensive testing has shown that correctly structures, even presence strong shear occurring boundary layer. We have near-wall ensemble averaged after proper alignment) CS highly elongated quasi-streamwise vortices; inclined 9° vertical ( x, )-plane and tilted ±4° horizontal z )-plane. The vortices alternating sign overlap x as staggered array; there is no indication hairpin vortices, not only data but also fields. Our model array reproduces nearly all experimentally observed events reported literature, such VITA, Reynolds stress distribution, pressure variation, low-speed streaks, spanwise shear, etc. In particular, phase difference (in space) between streamwise normal velocity fluctuations created by advection causes Q4 ('sweep’) to dominate Q2 ('ejection’) creates counter-gradient stresses (such Ql Q3 events) above below CS. show these effects adequately modelled half Batchelor's dipole embedded (and decoupled from) background U(y) . tilting )-plane) found be responsible for sustaining through redistribution kinetic energy components via coherent pressure-strain effects.