Influence of wall-attached structures on the boundary of the quiescent core region in turbulent pipe flow

The entrainment phenomena of the quiescent core region in a turbulent pipe flow are examined by characterizing the tall wall-attached structures of the streamwise velocity fluctuations (u) extracted from direct numerical simulation data for Re-tau = 926. The quiescent core region is the uniform momentum zone with the highest streamwise velocity magnitude. The conditionally averaged turbulence statistics undergo an abrupt jump across the boundary of the core region, which indicates that there is a viscous shear layer at this interface, and that it is similar to the turbulent-nonturbulent interface. The two-point correlation of the interface height fluctuations extends along the streamwise direction in a manner similar to that of u at the core boundary. The magnitudes of the local entrainment velocity were conditionally sampled by measuring the local curvatures of the core boundary, which are associated with large-scale structures. The local entrainment velocity (v(n)) is enhanced at saddle concave and elliptic convex regions of the interface. This behavior mainly arises from small-scale motions due to viscous diffusion, which indicates that the large-scale features modulate the small scales associated with the entrainment velocity. In particular, the saddle concave shape formed by the bulgelike low-speed structures significantly strengthens the viscous component of v(n). The turbulence statistics near the core boundary were obtained based on the projection area of the tall wall-attached structures that were extracted from the instantaneous flow field. Sharp changes in the turbulence statistics are evident; the magnitude of these changes increases with increases in the height (l(y)) of the identified structures. We reconstructed the turbulence statistics by using the tall structures with l(y)(+) > 100 and found that the reconstructed profiles are remarkably similar to the turbulence statistics across the core boundary. These results confirm that the sudden changes in the averaged turbulence statistics and the modulation of v(n) are associated with the instantaneous wall-attached structures that reach the core region.