Author(s): Sebastien Proust; Celine Berni; Vladimir I. Nikora

Linked Author(s): Sébastien Proust

Keywords: Mixing layer; Coherent structures; Kelvin-Helmholtz instability; Large-scale-motions; Very-large-scale-motions

Abstract: Free-surface shallow shear flows behind a vertical splitter plate were studied in a tilted laboratory flume with a rectangular cross-section. The bed and sidewalls, made of glass, were hydraulically smooth. The vertical flow confinement (i.e., flow depth) was modified by changing the total flow rate Q (14, 30, 60, and 120 l s-1). At the flume entrance, for each Q-value, the flow partition from either side of the splitter plate was also varied to cover a wide range of the shear parameter λ = (U₂-U₁)/(U₂+U₁), where U₁ and U₂ are two velocities characterizing the low-speed and high-speed streams, respectively (with λ = 0 to 1.0). The focus of the study was on clarifying the effects on the large-scale flow structures of: (i) vertical flow confinement; (ii) bed-friction; and (iii) λ-parameter. Particular attention was paid to the emergence and development of: quasi-2D Kelvin-Helmholtz-type Coherent Structures (KHCSs); 3D Large-Scale-Motions (LSMs), e.g., packets of hairpin vortices and associated burst events; and 3D Very-Large-Scale-Motions (VLSMs), which are long meandering streamwise counter-rotating vortices. It was found that in the near-field region, the emergence and length-scales of the KHCSs were independent of (i) and (ii), but were essentially controlled by (iii). For instance, the KHCSs cannot form if λ <~ 0.3. In the far-field region, the effect of λ decreases while that of bed-friction number S increases. For S > ~ 0.04, KHCSs are fully longitudinally stretched. LSMs/VLSMs development are strongly influenced by splitter effects as well as the magnitude of velocity shear, particularly when λ > 0.3.

DOI: https://doi.org/10.3850/IAHR-39WC25217119202262

Year: 2022