Direct numerical simulation of turbulent open channel flow: Streamwise turbulence intensity scaling and its relation to large-scale coherent motions

Abstract

The well-known failure of wall-scaling of the streamwise turbulent intensity in closed channel flows (CCF) is associated with the appearance of very-large-scale motions (VLSMs, [1]). In turbulent open channel flow (OCF), VLSM are larger, more energetic and appear at lower Reynolds number than in CCF [2, 3]. Thus, to investigate the scaling of turbulence intensities and its relation to underlying coherent structures in OCF, we carried out direct numerical simulations of both OCF and CCF of friction Reynolds numbers up to Ret = 900 in large computational domains (Lx/h × Lz/h = 12pi × 4pi). Figure 1 shows the turbulent intensities normalized by the bulk flow velocity ub. Unlike CCF, where the streamwise turbulent intensity scales neither in wall nor in bulk units (figure 1b), our data suggests that urms in OCF scales with the bulk velocity ub for Ret > 400. This difference in scaling behavior of OCF with respect to CCF is presumably caused by contributions from VLSMs as depicted in figure 2(a,b). In OCF, VLSMs are linked to so-called super-streamwise vortices (SSVs), which are statistically difficult to track [6]. However, in figure 2(c,d) we visualize SSVs in terms of the two-point correlation of the streamfunction of the streamwise averaged crosssectional velocity components. Similar to VLSMs, SSVs are more intense in OCF than in CCF and they occur much more regularly in the shape of alternating positive and negative vortices. Summarizing, at the conference we are going to present new evidence of bulk scaling of the streamwise turbulence intensity in OCF and relate it to underlying coherent motions, such as VLSMs and SSVs. In addition, we are going to look at the scaling of the near-surface layers in OCF.