Contrasts between momentum and scalar transport over very rough surfaces

Abstract

Large-eddy simulations are conducted to contrast momentum and passive scalar transport over large, three-dimensional roughness elements in a turbulent channel flow. Special attention is given to the dispersive fluxes, which are shown to be a significant fraction of the total fluxes within the roughness sublayers. Based on point-wise quadrant analysis, the turbulent components of the transport of momentum and scalars are found to be similar in general, albeit with increasing dissimilarity for roughnesses with low frontal blockage. However, strong dissimilarity is noted between the dispersive momentum and scalar fluxes, especially below the top of the roughness elements. In general, turbulence is found to transport momentum more efficiently than scalars, while the reverse applies for the dispersive contributions. The effects of varying surface geometries, measured by the frontal density, can be pronounced on both turbulent and dispersive fluxes. Increasing frontal density induces a general transition in the flow from a rough boundary-layer type to a mixed-layer-like type. This transition results in an increase in the efficiency of turbulent momentum transport, but the reverse occurs for scalars due to reduced contributions from large scale motions in the roughness sublayers. This study highlights the need for distinct parameterizations of the dispersive scalar fluxes and the importance of considering the contrasts between momentum and scalar transport for turbulent flows over very rough surfaces.

Direct numerical simulation of stratified Ekman layers over a periodic rough surface

“…This decomposition identifiesũ i (x, y) as the spatially coherent part of the time-averaged flow, e.g. Finnigan (2000), Sullivan et al (2000), Poggi et al (2004), Li & Bou-Zeid (2019). Physically,ũ i (x, y) in the present problem is the time-mean velocity associated with the roughness bumps.…”

“…(2000), Poggi et al. (2004), Li & Bou-Zeid (2019). Physically, in the present problem is the time-mean velocity associated with the roughness bumps.…”

“…Turbulent flow over rough surfaces has a component in the time-averaged field which is coherent with the surface structure and gives rise to a dispersive component of the turbulent fluxes. The coherent velocity can be extracted using a triple decomposition, and the so-called dispersive component is a significant contributor to turbulent fluxes in the near-surface layer (Finnigan 2000;Sullivan et al 2000;Poggi, Katul & Anderson 2004;Li & Bou-Zeid 2019).…”

“…The coherent velocity can be extracted using a triple decomposition, and the so-called dispersive component is a significant contributor to turbulent fluxes in the near-surface layer (Finnigan 2000; Sullivan et al. 2000; Poggi, Katul & Anderson 2004; Li & Bou-Zeid 2019).

Figure 1.Schematic of the computational domain and surface roughness: ( a ) 2Bump and ( b ) 4Bump.

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Neighbourhood-Scale Flow Regimes and Pollution Transport in Cities

“…8a), occurring at x ≈ 12H = 0.73 L p and x ≈ L p +12H = 1.77 L p , respectively, for the dense and sparse patches. As turbulent scalar fluxes adjust to flow variations more slowly than momentum fluxes, the turbulence tends to transfer momentum more efficiently than scalar quantities (Kanani-Sühring and Raasch 2015; Li and Bou-Zeid 2019;Ma et al 2020). This can be seen in Fig.…”