Emergence of Kelvin-Helmholtz Instabilities in Shallow Mixing Layers: An Experimental Study

Free surface shallow mixing layers were studied in an 18 m long and 2 m wide open-channel flume, with a longitudinal bed slope of 1.1/1000. The main objectives of the experiments were to assess the influence of (a) vertical flow confinement, (b) dimensionless shear λ based on the velocities of the two ambient streams outside the mixing layers, and (c) the bed surface roughness, on the emergence of quasi-two-dimensional Kelvin-Helmholtz-type large coherent structures (KHCSs). Three flow confinements were tested, and the initial value of λ (at the flume entrance) varied between 0 and 0.5 (a threshold value of 0.3 was recently found for the emergence of KHCSs in compound open-channel flows, based on one-point (Proust et al. 2017) and two-point measurements (Proust and Nikora 2018)). Four cross-sectional configurations were investigated: 1) rectangular cross-section with smooth bed and sidewalls (glass); 2) rectangular composite cross-section presenting a lateral change in bed roughness (half-width made of glass, half-width covered by artificial dense plastic grass); 3) rectangular composite cross-section with a lateral change glass / emergent wooden circular cylinders installed on glass bed ; and (4) a rectangular composite cross-section with a lateral change plastic grass / emergent wooden circular cylinders installed on glass bed. The three velocity components were measured using two Acoustic Doppler Velocimeters with side-looking probes (including one- and two-point measurements). Particle Image Velocimetry was also used to identify and measure the initial emergence of KHCSs in a horizontal plane. This study is an attempt to get a better insight in the interplay between the constraining effect of flow depth, the damping effect of three-dimensional bed-induced turbulence, and the triggering effect of dimensionless shear on the formation of KHCSs.