Author(s): Victor Dupuis; Olivier Eiff
Linked Author(s): Olivier Eiff
Keywords: Compound channel flows; Shear layers; Kelvin-Helmholtz structures
Abstract: The two-stage compound channel, which is an ideal model of an overflowing river, consists of a deep main channel adjacent to a shallower floodplain. The difference in velocity between these two subsections leads to a shear layer. This shear layer can give rise to the development of large-scale structures of Kelvin-Helmholtz type. Proust et al. (2017) proposed a criterion for the onset of Kelvin-Helmholtz structures in compound channel shear layers, based on the shear parameter λ = (U2 − U1)/(U2 + U1), where U1 and U2 are the velocities at the edge of the shear layer, respectively at the low- and at the high-speed side. According to this criterion, the structures are present when the shear parameter is higher than a threshold value λcrit, empirically determined to be 0.3. This λ-criterion was confirmed by a series of works (Proust et al., 2020; Caroppi et al., 2020; Dupuis et al., 2023). While the shear layers above the λcrit-threshold are well characterized by their quasi-periodic Kelvin-Helmholtz vortex street, the instantaneous structure of the shear layers below the λcrit-threshold is unknown for now. The aim of the present study is to identify which kind of instantaneous structures are present for shear layers below the λcrit-threshold. In compound channels, the shear parameter λ is mainly governed by the relative depth Dr = h1/h2, where h1 and h2 are the flow depths in the floodplain and in the main channel respectively. For Dr = 0.5, the shear layer of the compound channel is expected to be vanishing (Stocchino et al., 2010). Therefore, compound channel flows with four relative depths Dr ranging from 0.09 to 0.5 were investigated in the laboratory.
Year: 2025