Numerical observations of turbulence structure modification in channel flow over 2D and 3D rough walls

Marchis, Mauro De; Milici, Barbara; Napoli, Enrico

doi:10.1016/j.ijheatfluidflow.2015.07.002

Cited by 40 publications

(7 citation statements)

References 67 publications

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“…Despite both surfaces having similar measurements of drag, it was found that the two-dimensional circular rods had a larger effect on the outer region of the flow as there was a lack of collapse in both the wall-normal turbulence intensity and Reynolds shear stress profiles between the smooth and rough cases. De Marchis et al (2015) simulated two-and three-dimensional roughness by superposition of sinusoidal surfaces of varying roughness height and wavelength. From their simulations, they found that the streaky structures still persist near the wall but are selectively modified by the roughness.…”

Section: Near-wall Structuresmentioning

confidence: 99%

Secondary motion in turbulent pipe flow with three-dimensional roughness

et al. 2018

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The occurrence of secondary flows is investigated for three-dimensional sinusoidal roughness where the wavelength and height of the roughness elements are systematically altered. The flow spanned from the transitionally rough regime up to the fully rough regime and the solidity of the roughness ranged from a wavy, sparse roughness to a dense roughness. Analysing the time-averaged velocity, secondary flows are observed in all of the cases, reflected in the coherent stress profile which is dominant in the vicinity of the roughness elements. The roughness sublayer, defined as the region where the coherent stress is non-zero, scales with the roughness wavelength when the roughness is geometrically scaled (proportional increase in both roughness height and wavelength) and when the wavelength increases at fixed roughness height. Premultiplied energy spectra of the streamwise velocity turbulent fluctuations show that energy is reorganised from the largest streamwise wavelengths to the shorter streamwise wavelengths. The peaks in the premultiplied spectra at the streamwise and spanwise wavelengths are correlated with the roughness wavelength in the fully rough regime. Current simulations show that the spanwise scale of roughness determines the occurrence of large-scale secondary flows.

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Section: Near-wall Structuresmentioning

confidence: 99%

Secondary motion in turbulent pipe flow with three-dimensional roughness

et al. 2018

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“…Thus, drag is maximized at intermediate spacing where wakes associated with roughness elements interact with those located downstream, creating complex turbulent flow (wake-interference flow). Parallel to investigations of roughness element spacing, researchers have pursued an approach stemming from the work of Musker (1980), whereby drag can be related to the statistics of the elevation field (Anderson and Meneveau, 2011;Barros et al, 2018;De Marchis et al, 2015;Yuan and Piomelli, 2014). The two approaches were defined by Grinvald and Nikora (1988) as discrete element and random field approaches, respectively.…”

Section: In Fluid Dynamicsmentioning

confidence: 99%

Toward bed state morphodynamics in gravel-bed rivers

Adams

2020

Progress in Physical Geography: Earth and Environment

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In fluvial geomorphology, one of the most pervasive paradigms is that the size of the grains present in a river exercises an important effect on its character. In gravel-bed rivers, there is considerable scatter in the relations between so-called “representative grain sizes” and basic channel processes and morphologies. Under a grain size paradigm, our ability to rationalize the characteristics of a given channel and predict how it will respond to a change in conditions is limited. In this paper, I deconstruct this paradigm by exploring its historical origins in geomorphology and fluid dynamics, and identify three of its underlying premises: (1) the association between grain diameter and fluid drag derived from Nikuradse’s experiments with sand-coated surfaces; (2) the use of grain size by early process geomorphologists to describe general trends across large samples of sand-bed rivers; and (3) a classificatory approach to discerning bed structures originally developed for bed configurations found in sand-bed rivers. The conflation of sand- and gravel-bed rivers limits our ability to understand gravel-bed morphodynamics. Longstanding critique of the grain size paradigm has generated alternative ideas but, due to technological and conceptual limitations, they have remained unrealized. One such unrealized idea is the morphology-based definition of bed state – an important degree of freedom within fluvial systems, particularly in reaches where adjustments to planform are not easily achieved. By embracing recent advancements in fluid dynamics and remote sensing, I present an alternative or complementary concept of bed state based on the notion that fluvial systems act to maximize flow resistance. The proposed quantitative index represents the relative contribution of morphologic adjustments occurring at different spatial scales (discriminated using a wavelet transform) to a stable channel configuration. By explicitly acknowledging the complexity of bed adjustments we can move toward a more complete understanding of channel stability in gravel-bed rivers.

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“…Researchers have used CFD to study the fluid flow characteristics of wastewater treatment systems to optimize the design of wastewater treatment units, such as aerated grit tanks [ 27 ], multichamber tanks [ 28 , 29 ], oxidation ditches [ 30 ], grit chambers [ 31 ], bioreactors [ 32 , 33 ], membrane reactors [ 34 , 35 ], and membrane modules [ 36 , 37 ]. Numerous studies [ 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 ] have aimed at understanding the complex flow phenomena occurring in narrow flow channels with a turbulence promoter. Numerous researchers have also conducted two-dimensional [ 47 , 48 , 49 ] and three-dimensional [ 50 , 51 , 52 , 53 , 54 ] CFD studies of turbulence promoters with different shapes and structures.…”

Section: Introductionmentioning

confidence: 99%

Study of Turbulence Promoters in Prolonging Membrane Life

Jiang

Yang

et al. 2021

Membranes

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Nanofiltration membrane technology is an effective method for secondary treated sewage purification. However, membrane fouling, which is inevitable in the membrane-separation process, can reduce membrane performance and shorten membrane life. Installing a turbulence promoter is a promising means of improving the hydraulic conditions inside the membrane chamber. In this study, the effect of turbulence promoter on prolonging membrane life was studied for the first time. Flat-sheet polyethersulfone nanofiltration membrane was used to filter humic acid solution, used for simulating secondary treated sewage. By comparing photographs and SEM images of the membrane before and after the simulated secondary treated sewage filtration, it was found that humic acid tended to be deposited on the low-velocity region, which was reflected by COMSOL simulation. After incorporating a turbulence promoter, the reduction of the humic acid deposition area and membrane fouling resistance indicated that the turbulence promoter could reduce membrane fouling due to the improved hydraulic conditions. Additionally, the turbulence promoter also increased the flux and reduced the flux decay rate. The turbulence promoter was then place in the crossflow flat-sheet membrane filtration module, and the variation of flux with time was tested in simulated secondary treated sewage with different concentrations. The results showed that the membrane life for the filtration of simulated secondary treated sewage comprising 50, 250, and 500 ppm humic acid increased by 23.1%, 80.4%, and 85.7%, respectively. The results of this article can serve as a reference for the prediction of membrane life and the performance enhancement mechanism of membranes containing a turbulence promoter.

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Numerical observations of turbulence structure modification in channel flow over 2D and 3D rough walls

Cited by 40 publications

References 67 publications

Secondary motion in turbulent pipe flow with three-dimensional roughness

Secondary motion in turbulent pipe flow with three-dimensional roughness

Toward bed state morphodynamics in gravel-bed rivers

Study of Turbulence Promoters in Prolonging Membrane Life

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