Seminars

Direct numerical simulations (DNS) of turbulent channel flow at friction Reynolds numbers up to 5200 have recently been performed to study high Reynolds number wall-bounded turbulence. DNS result have shown that this Reynolds number is high enough to exhibit scale separation between the near-wall and outer regions, and other high-Reynolds-number features (Lee & Moser J. Fluid Mech. vol. 774, 2015). A spectral analysis of simulation results, particularly the terms in the evolution equation for two-point correlation, have been performed to study the interaction of the near-wall turbulence with that of the outer flow. In this analysis, the turbulent transport terms that arise from the non-linear terms in the Navier-Stokes equations can be decomposed into a part describing transfer between scales at constant distance from the wall and a part describing interactions across the wall-normal direction.

The results show that there is a distinct difference in the structure of the scale transfer and wall-normal (y) transport terms between the near-wall and outer layers. In the latter (above y+=200) there is Kolmogorov style transfer of energy from large scale to dissipative small scales and a scale-similar, scale-local and y-local transport in y. This is as would be expected in the overlap (or log) region. In the near wall region, the structure is more complex. However, it appears that the interaction between the near-wall and outer layers is relatively simple, with evidence of transport of energy from the outer region to the near wall primarily at large scales. These results are consistent with the ideas of autonomous near-wall dynamics as described by Jimenez & Pinelli (J. Fluid Mech. vol. 389 1999) and of modulation of the near-wall layer by the outer layer as discussed, for example, by Marusic et al (Science vol. 329, 2010).