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Exploring very-large-scale motions (VLSMs) in open channel flows (OCFs) is crucial for comprehensively understanding material transport and energy exchange. While previous studies focused on OCFs in ideal flumes with simple boundaries, this paper presents large eddy simulation (LES) results on the existence and scale characteristics of VLSMs in complex river morphologies with the Minjiang River in southwestern China as a case study. This research demonstrates that the OpenFOAM-based LES model is capable of accurately reconstructing the time-averaged flow field and providing instantaneous velocity data that capture VLSMs with sufficient resolution. Spectrum analysis of the streamwise fluctuating velocity shows that VLSMs are present in the straight segment of the river, with streamwise wavelengths approximately (16–22) times the water depth but are absent in the bend and confluence segments due to the suppression by secondary currents. This article validates the effectiveness of LES in examining the characteristics of VLSMs in natural rivers, thereby laying a good foundation for further studies on the impacts of such structures on sediment transport and pollutant dispersion. [ABSTRACT FROM AUTHOR]

Direct numerical simulations at two friction Reynolds numbers (550 and 1000) have been performed to explore the statistical behaviors of wall-attached motions (WAMs) in open- and closed-channel flows (OCFs and CCFs). To fully capture the largest energetic WAMs, superlarge domain sizes are employed (24–48 π h along streamwise direction with h being the height of OCFs). Based on the analysis of linear coherence spectrum, both geometric and kinematic characteristics of WAMs are investigated. Results revealed that obvious peaks could be found in spanwise coherence spectra of u (streamwise velocity) and w (spanwise velocity) at a wavelength close to 2h. Both the coherence and energy spectra density value of OCFs are greater than those of CCFs at large wavelengths, indicating that OCFs are more wall-attached. Within y/h = 0.2–0.7, the diagnosed streamwise and spanwise wavelengths of WAMs increase roughly linearly with increasing vertical position y. In the upper region (y / h > 0.7), the diagnosed wavelengths in OCFs are smaller than those in CCFs. In addition, the inclination angle of u fluctuates around 14° within y / h = 0.4–0.6, and angles of CCFs are slightly larger than those in OCFs when y / h > 0.6. Regarding kinematic behavior, contribution fractions of WAMs to total turbulent intensity in OCFs are always larger than those in CCFs. Additionally, the wall-attached behaviors of u were found to be more sensitive to Reynolds number than w. [ABSTRACT FROM AUTHOR]

The flows upstream of a run-of-river dam, commonly utilized as an overflow structure on rivers, are complex due to heterogeneities in both streamwise and spanwise directions. In particular, very-large-scale motions (VLSMs) are greatly influenced by the overflow structure, yet relevant understandings remain limited. Reported as novel coherent structures in turbulent flows, VLSMs are recognized with the scale up to several and tens of the outer-scaled unit, and they contribute significantly to turbulent transport and mixing. To fill the gap, experiments with particle image velocimetry were conducted to investigate the vitality of VLSMs upstream of a model dam. Measurements were designed to cover broad hydraulic scope with flow heterogeneities. The results reveal that VLSMs in the present flow scenario show noticeable characteristics in both streamwise and spanwise directions. Compared to those in uniform flows, the VLSMs in present flows are found to be more energetic and stress-active. [ABSTRACT FROM AUTHOR]

Open channel flows (OCFs) exhibit unique characteristics compared with other wall-bounded flows due to the presence of a free water surface. One of the well-known features is the turbulence kinetic energy (TKE) redistribution phenomenon in the near free-water-surface region. Unlike previous qualitative demonstrations, the present study aims providing a quantitative investigation of this phenomenon. Specifically designed super long domain direct numerical simulations (DNSs) of OCFs and closed channel flows (CCFs) are performed at three low-to-moderate friction Reynolds numbers (Reτ = 180–1000). The numerical configurations of the two flows (i.e., temporal and spatial resolution, domain size, Reynolds number, etc.) at identical Reτ are set to be the same, with the upper boundary being the only difference between them, to allow meaningful comparisons while super long domain sizes are adopted to facilitate fully spectral comparisons of the TKE features between the two flows. With such specifically designed DNS datasets, quantitative investigations of the TKE redistribution phenomenon in OCFs have been made. It is revealed that, as Reτ increases from 180 to 1000, the streamwise and spanwise TKE components of OCFs are higher than those of CCFs by 12%–23% and 28%–17%, respectively, while in the vertical direction OCFs are lower in TKE by 44%–31%. Overall, the TKE of OCFs is higher than that of CCFs by 3%–9% as Reτ increases from 180 to 1000. The comparison of OCFs and CCFs in pre-multiplied energy spectra reveals that VLSMs play a dominant role in the TKE redistribution phenomenon in OCFs. [ABSTRACT FROM AUTHOR]

The gradually varied open-channel flows (OCF) upstream of a run-of-river (RoR) dam are ubiquitous in natural rivers. In this flow type, the velocity profile shows some similarities to that in uniform open-channel flows, but the turbulence intensity and Reynolds shear stress are slightly greater. However, the presence and properties of very-large-scale motions (VLSMs) in such flows are still unclear. To fill this research gap, time-resolved particle image velocimetry measurements were performed upstream of a modeled RoR dam in an open-channel flume. Based on pre-multiplied spectra analysis, statistical evidence of the presence of VLSMs in the flow type is reported for the first time. The results reveal that although the typical streamwise wavelength of VLSMs in such gradually varied OCF is similar to that in other flows, such as turbulent boundary layers, closed-channel flows, pipe flows, and uniform OCF, the VLSMs in the present gradually varied OCF are stronger and contribute more streamwise turbulent kinetic energy as well as Reynolds shear stress than other flows. [ABSTRACT FROM AUTHOR]

Time-resolved particle image velocimetry measurements with sufficient spatial resolutions combined with coherence spectrum analysis were adopted to investigate the geometric and kinematic features of wall-attached motions in open channel flows. Results indicate that the diagnosed streamwise wavelength of wall-attached motions λxWA based on a given near zero coherence spectrum threshold exhibits a constant streamwise wavelength/wall-normal distance ratio accompanied with a roughly constant inclination angle within y+ > ∼100 and y/h ≤ 0.7 [y+ is the inner-scale normalized distance to the wall y with y+ = y/(ν/uτ) (ν is the kinematic viscosity and uτ is the friction velocity) and h is the water depth], meaning that they are geometrically self-similar. However, in the free-surface region, where y/h > 0.7, the diagnosed wall-attached motions are non-self-similar with λxWA increasing more dramatically and reaching up to ∼20h at the free surface, which is comparable to the typical scales of very-large-scale motions therein. The wall-attached motions are demonstrated to be both energetic and stress active. Within y/h < 0.7, the wall-attached motions with the streamwise wavelength greater than λxWA (including both the self-similar and non-self-similar wavelength range portions) carry more than 40% of the streamwise turbulent kinetic energy (TKE) and 30% of Reynolds shear stress. Beyond y/h = 0.7, where the self-similar portions vanish, all the wall-attached motions are non-self-similar wall-attached motions, which themselves still maintain considerable strength even at the free surface and contribute to 25% of the streamwise TKE and 10% of the Reynolds shear stress therein. [ABSTRACT FROM AUTHOR]

This study experimentally examined the turbulent characteristics of decelerating open-channel flow based on particle imaging velocimetry measurement. The decelerating flow shows a similar velocity profile to that in uniform flow, but it exhibits greater turbulence intensity and Reynolds stress. Statistical evidence of the presence of very-large-scale motions (VLSMs) in decelerating open-channel flows was presented for the first time. The results indicate that VLSMs in decelerating flows can survive further away from the wall when compared to other wall-turbulence flows. The contribution rate of the VLSMs to the turbulent kinetic energy and the Reynolds stress in the decelerating open-channel flow is slightly lower than that in channels, boundary layers, and pipe flows. [ABSTRACT FROM AUTHOR]

Swirling strength is an effective vortex indicator in wall turbulence, and it can be determined based on either two-dimensional (2D) or three-dimensional (3D) velocity fields, written as λci2D and λci3D, respectively. A comparison between λci2D and λci3D has been made in this paper in sliced XY, YZ, and XZ planes by using 3D DNS data of channel flow. The magnitude of λci2D in three orthogonal planes differs in the inner region, but the difference tends to diminish in the outer flow. The magnitude of λci3D exceeds each λci2D, and the square of λci3D is greater than the summation of squares of three λci2D. Extraction with λci2D in XY, YZ, and XZ planes yields different population densities and vortex sizes, i.e., in XZ plane, the vortices display the largest population density and the smallest size, and in XY and YZ planes the vortices are similar in size but fewer vortices are extracted in the XY plane in the inner layer. Vortex size increases inversely with the threshold used for growing the vortex region from background turbulence. When identical thresholds are used, the λci3D approach leads to a slightly smaller population density and a greater vortex radius than the λci2D approach. A threshold of 0.8 for the λci3D approach is approximately equivalent to a threshold of 1.5 for the λci2D approach. [ABSTRACT FROM AUTHOR]