Influence of permeable beds on hydraulically macro-rough flow

Abstract

In this study, macro-rough flows over beds with different permeability values are simulated using the large-eddy simulation, and the results are analysed by applying the double-averaging (DA) methodology. Spheres of different sizes and arrangements were used to form the beds, which are deemed to be permeable granular beds. The influence of bed permeability on the turbulence dynamics and structure is investigated. It was observed that the scales of the spanwise vortical structures over more permeable beds are larger than those over less permeable beds. This is attributed to large-scale spanwise-alternate strips of varying Reynolds shear stress (RSS), emerging from the surface of macro-rough elements for the permeable beds. The DA stress balance suggests that the time-averaged spanwise vortical structure leads to a damping in DA RSS and an unusual peak of the form-induced stress in the main flow. In the streamwise direction, both large turbulent structures that originate from the Kelvin–Helmholtz-type instability and small turbulent structures that are associated with the turbulent transport across the gaps of the roughness elements are more prevalent over highly permeable beds. Near the bed, the relative magnitude of turbulent events shows a transition from a ejections-dominating to sweeps-dominating zone with vertical distance. Further, several hydrodynamic characteristics normalized by inner scales (kinematic viscosity to shear velocity ratio) show a greater dependency on permeability Reynolds number than those normalized by sediment size. The study provides an insight into the mechanism of mass transfer near the fluid–particle interface, which is vital to benthic and aquatic ecology.

Gravity currents over fixed beds of monodisperse spheres

“…Interestingly, the friction coefficient strongly increases once the particle radius exceeds about twenty wall units, as a result of the increased advective momentum mixing seen earlier in figure 22. This observation is consistent with the work of Fang et al (2018) and Manes et al (2009), who investigated open channel pressure-driven flows over fixed beds of spheres, finding enhanced momentum mixing for larger bed permeabilities.

Figure 23.( a ) Spanwise- and time-averaged streamwise velocity profiles, evaluated behind the front and scaled with wall units, cf.

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“…These flow properties, along with the mixing of current and ambient fluid, will be analysed as a function of the bed structure. Some preliminary guidance is provided by previous numerical (Fang et al 2018; Leonardi et al 2018) and experimental (Manes et al 2009; Pokrajac & Manes 2009) studies of constant-density flows over beds of fixed spherical particles. These demonstrate that the bed structure can significantly affect the flow resistance, consistent with findings by Nogueira et al (2013) and Jiang & Liu (2018) for gravity currents over gravel beds of different sizes.…”

The Effect of Biofilms on Turbulent Flow Over Permeable Beds

“…, κ is the equivalent von Kármán constant (Breugem et al, 2006;Fang et al, 2018), y + is the vertical distance above the cylinder crest, y 0 + is the zero-displacement height, and k s + is the equivalent roughness height, all normalized with viscous length scale…”

Experimental evidence of amplitude modulation in permeable-wall turbulence

“…The low-frame-rate data were used to determine the global flow parameters for each wall case. Due to the spanwise flow heterogeneity induced by the relatively large surface topography and the interface porosity of the current wall models, the boundary-layer parameters were assessed via a double-averaging approach (Manes et al 2007;Mignot, Barthélemy & Hurther 2009;Fang et al 2018;Kim et al 2018Kim et al , 2019. This method was originally proposed in vegetated flow studies (Wilson & Shaw 1977;Raupach & Shaw 1982) and has been applied more recently to flow over large roughness in uniform and periodic configurations (Nikora et al 2001).…”