On the role of turbulent large-scale streaks in generating sediment ridges

Abstract

The role of turbulent large-scale streaks or large-scale motions in forming subaqueous sediment ridges on an initially flat sediment bed is investigated with the aid of particle resolved direct numerical simulations of open channel flow at bulk Reynolds numbers up to 9500. The regular arrangement of quasi-streamwise ridges and troughs at a characteristic spanwise spacing between 1 and 1.5 times the mean fluid height is found to be a consequence of the spanwise organisation of turbulence in large-scale streamwise velocity streaks. Ridges predominantly appear in regions of weaker erosion below large-scale low-speed streaks and vice versa for troughs. The interaction between the dynamics of the large-scale streaks in the bulk flow and the evolution of sediment ridges on the sediment bed is best described as ‘top-down’ process, as the arrangement of the sediment bedforms is seen to adapt to changes in the outer flow with a time delay of several bulk time units. The observed ‘top-down’ interaction between the outer flow and the bed agrees fairly well with the conceptual model on causality in canonical channel flows proposed by Jiménez (J. Fluid Mech., vol. 842, 2018, P1, § 5.6). Mean secondary currents of Prandtl's second kind of comparable intensity and lateral spacing are found over developed sediment ridges and in single-phase smooth-wall channels alike in averages over ${O}(10)$ bulk time units. This indicates that the secondary flow commonly observed together with sediment ridges is the statistical footprint of the regularly organised large-scale streaks.

Role of the hierarchy of coherent structures in the transport of heavy small particles in turbulent channel flow

“…Recent studies (e.g. Jie, Andersson & Zhao 2021;Scherer et al 2022) have also observed the contribution to particle clustering from not only buffer-layer structures but also large-scale ones. However, we do not fully understand the transport mechanism of particles with different relaxation times in high-Reynolds-number wall turbulence.…”

Open-channel flow over evolving subaqueous ripples

“…Scherer et al. 2021) but is outside the scope of the present work.

Figure 3.Instantaneous snapshots of the flow field and particle positions of case H6 at times ( a ) , ( b ) , ( c ) and ( d ) .

…”

Bottom wall shear stress fluctuations in shallow-water Langmuir turbulence

“…( y, t) in case 1. There exists a ridge in the contours of C u L i ,τ L i2 ( y, t) with a negative slope, indicating that (Scherer et al 2022), the spatial-temporal correlation coefficients in figure 14 are larger than 0.8 for −2 < tu w /h < 2 and 0 < y/h < 0.5. This result is consistent with the observation in the works of Shrestha & Anderson (2020) and Deng et al (2020), that is, the intensities and spanwise locations of full-depth LCs are persistent for a period of 100 < tu w /h < 300.…”

“…The above triple decomposition has been used to extract the CCs in turbulent Couette flows (Papavassiliou & Hanratty 1997) and full-depth LCs in shallow-water Langmuir turbulence (Tejada-Martínez & Grosch 2007;Deng et al 2019). Similar triple decomposition techniques were also used to study the coherent motions induced by streamwise-aligned roughness and riblets (Raupach & Shaw 1982;Choi, Moin & Kim 1993;Jimenez et al 2001;Garcia-Mayoral & Jimenez 2011;Jelly, Jung & Zaki 2014;Seo, Garcia-Mayoral & Mani 2015;Scherer et al 2022).…”

“…In wall turbulence without waves, there is a time delay between LSMs and the wall shear stresses owing to the propagation of the information from the outer layer to the bottom (Mathis et al 2013;Scherer et al 2022). We also explore the time delay based on the spatial-temporal correlation coefficient, defined as…”