Momentum and energy transfer in open-channel flow over streamwise ridges

Zampiron, Andrea; Cameron, Stuart; Nikora, Vladimir

doi:10.1017/jfm.2021.44

Cited by 14 publications

(17 citation statements)

References 38 publications

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“…Roccon, Zonta & Soldati (2021) have recently and successfully applied it to two-phase flows with complex physics. Flows featuring large or secondary motions, such as open-channel flows (Zampiron, Cameron & Nikora 2021), are of interest as well; the same applies to to straight-duct flows with arbitrary geometry. Finally, as pointed out by Frohnapfel, Hasegawa & Quadrio (2012), generalization to external flows such as spatially developing boundary layers is also possible.…”

Section: Discussionmentioning

confidence: 99%

Global energy budgets in turbulent Couette and Poiseuille flows

2021

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Section: Discussionmentioning

confidence: 99%

Global energy budgets in turbulent Couette and Poiseuille flows

2021

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“…Zampiron, Cameron & Nikora (2021) describe a similar mechanism for the generation of secondary flows by triangular streamwise ridges. They decompose the stress balance equation for particle image velocimetry data and conclude that secondary flows extract energy from cross-flow velocity fluctuations, which in turn receive energy from turbulent streamwise fluctuations.…”

Section: Effects Of Large Riblets On the Flowmentioning

confidence: 93%

Reorganisation of turbulence by large and spanwise-varying riblets

et al. 2022

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We study the flow above non-optimal riblets, specifically large drag-increasing and two-scale trapezoidal riblets. In order to reach large Reynolds numbers and large scale separation while retaining access to flow details, we employ a combination of boundary-layer hot-wire measurements and direct numerical simulation (DNS) in minimal-span channels. Although the outer Reynolds numbers differ, we observe fair agreement between experiments and DNS at matched viscous–friction-scaled riblet spacings $s^+$ in the overlapping physical and spectral regions, providing confidence that both data sets are valid. We find that hot-wire velocity spectra above very large riblets with $s^+ \gtrsim 60$ are depleted of near-wall energy at scales that are (much) greater than $s$ . Large-scale energy likely bypasses the turbulence cascade and is transferred directly to secondary flows of size $s$ , which we observe to grow in strength with increasing riblet size. Furthermore, the present very large riblets reduce the von Kármán constant $\kappa$ of the spanwise uniform mean velocity in a logarithmic layer and, thus, reduce the accuracy of the roughness-function concept, which we link to the near-wall damping of large flow structures. Half-height riblets in the groove, which we use as a model of imperfectly repeated (spanwise-varying) riblets, impede in-groove turbulence. We show how to scale the drag optimum of imperfectly repeated riblets based on representative measurements of the true geometry by solving inexpensive Poisson equations.

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“…The effect on the VLSMs of secondary currents, generated by streamwise ridges on the bed at their spanwise spacings of and less, is even more dramatic and is expressed by complete suppression of VLSMs by ridge-induced secondary currents (Zampiron et al. 2020; Zampiron, Cameron & Nikora 2021). For a more complicated case of OCF in a straight compound channel without spanwise currents, the findings of Proust & Nikora (2020) suggest that in the initial development of horizontal KHCSs, secondary currents, LSMs and VLSMs, the latter two appear to be fairly competitive but further downstream they are quickly suppressed by the effects of either the spanwise shear layer, KHCSs or secondary currents, or their combined work.…”

Section: Large-scale Coherent Structures and Bulk Turbulence Statisticsmentioning

confidence: 99%

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

2022

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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 ${\lambda = (U_{2}-U_{1})/(U_{2}+U_{1})}$ , 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 $\lambda$ 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 ${\lambda \lessapprox 0.3}$ 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 $S = c_f \delta _v /(4 h \lambda )$ , where $\delta _v$ is vorticity thickness. When $S \gtrapprox 0.01$ , 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.

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Momentum and energy transfer in open-channel flow over streamwise ridges

Abstract: Abstract

Cited by 14 publications

References 38 publications

Global energy budgets in turbulent Couette and Poiseuille flows

Global energy budgets in turbulent Couette and Poiseuille flows

Reorganisation of turbulence by large and spanwise-varying riblets

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

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