Structures and statistics of an adverse pressure gradient turbulent boundary layer

Abstract

Turbulence structures and turbulence statistics in an adverse pressure gradient (APG) turbulent boundary layer (TBL) are explored. Direct numerical simulation (DNS) of the APG TBL at $Re_{\theta}$ = 6000 are performed. The presence of APG results from the enhancement of large-scale motions (LSMs) in the outer region. LSMs influence surrounding flows by amplitude modulation (AM) and footprints. These large-scale influences are extended to vortical motions. In particular, the vortical motions are closely related to the skin friction coefficient ($C_f$), which can be decomposed into several terms related to velocity–vorticity correlations (VVCs). Although total $C_f$ decreases in APG TBL, results of conditional sampling and energy spectra show that the vortical motions are amplified through AM and footprint influences of enhanced LSMs, leading to that $C_f$ terms related to VVCs increase. To investigate enhanced LSMs in detail, turbulence structures identified from instantaneous flow field are examined in the sense of Townsend’s attached-eddy hypothesis. Self-similar wall-attached structures are universal, and non-self-similar ones are closely related to outer LSMs. In addition, self-similar wall-detached structures are mainly populated in the outer region, and non-self-similar ones are the main energy-dissipating motions. Several turbulence statistics in APG TBL are first reported. The investigation of characteristics LSMs and various turbulence statistics in APG TBLs provides in-depth understanding of coherent structures and the turbulence modeling.