Shallow mixing layers over hydraulically smooth bottom in a tilted open channel

Shallow mixing layers (SMLs) behind a splitter plate were studied in a tilted rectangular open-channel flume for a range of flow depths and the initial shear parameter lambda = (U-2 - U-1)1(U-2 + U-1), where U-1 and U-2 are streamwise velocities of the slow and fast streams, respectively. The main focus of the study is on (i) key parameters controlling the time-averaged SMLs; and (ii) the emergence and spatial development of Kelvin-Helmholtz coherent structures (KHCSs) and large- and very-large-scale motions (LSMs and VLSMs) and associated turbulence statistics. The time-averaged flow features of the SMLs are mostly controlled by bed-friction length scale h/c(f) and shear parameter A as well as by time-averaged spanwise velocities V and momentum fluxes UV, where h and c(f) are flow depth and bed-friction coefficient, respectively. For all studied cases, the effect of shear layer turbulence on the SML growth is comparatively weak, as the fluxes UV dominate over the spanwise turbulent fluxes. initially, the emergence of KHCSs and their length scales largely depend on lambda. The KHCSs cannot form if lambda (sic) 0.3 and the turbulence behind the splitter plate resembles that of free mixing layers. Further downstream, shear layer turbulence becomes dependent on the bed-friction number S = c(f)delta(v)/(4h lambda), where delta(v) vorticity thickness. When S (sic) 0.01, the KHCSs are longitudinally stretched and the scaled transverse turbulent fluxes decrease with increasing S. The presence and streamwise development of LSMs/VLSMs away from the splitter plate depends on the lambda-value, particularly when lambda > 0.3, resembling LSMs/VLSMs in conventional open-channel flows when lambda is small.

Automated summary

The characteristics of shallow mixing layers behind a splitter plate were studied, focusing on the key parameters controlling the time-averaged SMLs and the formation and spatial development of Kelvin-Helmholtz coherent structures and large- and very-large-scale motions. The study found that the time-averaged flow features of the SMLs are mainly influenced by the bed-friction length scale and shear parameter, while the emergence and length scales of KHCSs depend on the value of lambda. Additionally, the presence and streamwise development of LSMs and VLSMs are affected by the lambda-value.