The Ejection-Sweep Character of Scalar Fluxes in the Unstable Surface Layer

Katul, Gabriel G.; Kuhn, Greg; Schieldge, J.; Hsieh, Chee-Ruey

doi:10.1023/a:1000293516830

Cited by 137 publications

(88 citation statements)

References 49 publications

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“…Analogous to the quadrant analysis of u w , the vertical component of the turbulent pollutant flux c w is partitioned into four quadrants depending on the signs of w and c (Table 3; Chen 1990; Katul et al 1997;Katsouvas et al 2007). In contrast to the downward-moving u w , the pollutant emitted at the street level is carried upward.…”

Section: Quadrant Analysis Of the Vertical Turbulent Pollutant Flux C Wmentioning

confidence: 99%

Large-Eddy Simulation of Flow and Pollutant Transports in and Above Two-Dimensional Idealized Street Canyons

Cheng

Liu

2011

Boundary-Layer Meteorol

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A large-eddy simulation (LES) model, using the one-equation subgrid-scale (SGS) parametrization, was developed to study the flow and pollutant transport in and above urban street canyons. Three identical two-dimensional (2D) street canyons of unity aspect ratio, each consisting of a ground-level area source of constant pollutant concentration, are evenly aligned in a cross-flow in the streamwise direction x. The flow falls into the skimming flow regime. A larger computational domain is adopted to accurately resolve the turbulence above roof level and its influence on the flow characteristics in the street canyons. The LES calculated statistics of wind and pollutant transports agree well with other field, laboratory and modelling results available in the literature. The maximum wind velocity standard deviations σ i in the streamwise (σ u ), spanwise (σ v ) and vertical (σ w ) directions are located near the roof-level windward corners. Moreover, a second σ w peak is found at z ≈ 1.5h (h is the building height) over the street canyons. Normalizing σ i by the local friction velocity u * , it is found that σ u /u * ≈ 1.8, σ v /u * ≈ 1.3 and σ w /u * ≈ 1.25 exhibiting rather uniform values in the urban roughness sublayer. Quadrant analysis of the vertical momentum flux u w shows that, while the inward and outward interactions are small, the sweeps and ejections dominate the momentum transport over the street canyons. In the x direction, the two-point correlations of velocity R v,x and R w,x drop to zero at a separation larger than h but R u,x (= 0.2) persists even at a separation of half the domain size. Partitioning the convective transfer coefficient T of pollutant into its removal and re-entry components, an increasing pollutant re-entrainment from 26.3 to 43.3% in the x direction is revealed, suggesting the impact of background pollutant on the air quality in street canyons.

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Section: Quadrant Analysis Of the Vertical Turbulent Pollutant Flux C Wmentioning

confidence: 99%

Large-Eddy Simulation of Flow and Pollutant Transports in and Above Two-Dimensional Idealized Street Canyons

Cheng

Liu

2011

Boundary-Layer Meteorol

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“…In turbulent boundary-layer ows, coherent structures are associated with the sweep/ ejection nature of the¯ow, and their presence in the atmospheric surface layer is easily identi®ed through ramps in the measured temperature signal [15,16,22,23,50,53,54,58]. Such coherent motions are well known to be responsible for a very important fraction (75% and more) of the total turbulent¯uxes [15], and therefore it is important that SGS models used in LES of the ABL capture the relationship between the coherent structures and the dynamics of the non-resolved scales.…”

Section: Statistics Of the Sgs Variablesmentioning

confidence: 99%

Atmospheric stability effect on subgrid-scale physics for large-eddy simulation

Porté‐Agel

Pahlow

Meneveau

et al. 2001

Advances in Water Resources

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Field measurements in the atmospheric boundary layer were carried out to identify the eect of atmospheric stability on subgridscale physics for large-eddy simulation. The basic instrumentation setup consisted of 12 three-dimensional sonic anemometers arranged in two parallel horizontal arrays (seven sensors in the lower array and ®ve sensors in the upper array). Data from this setup are used to compute the subgrid-scale (SGS) heat¯uxes and SGS dissipation of the temperature variance under stable and unstable stability conditions. The relative contribution of the SGS vertical¯ux to the total turbulent¯ux increases when going from unstable to stable conditions. The relative importance of negative SGS dissipation (backscatter) events becomes larger under stable conditions. The model coecients for two well-known SGS models (eddy-viscosity and non-linear) are computed. Model coecients are found to depend strongly on stability. Under both stable and unstable conditions, large negative SGS dissipation is associated with the onset of ejection events while large positive SGS dissipation tends to occur during the onset of sweep events. These ®ndings are also supported by conditionally sampled 2D velocity and temperature ®elds obtained using the 12 anemometers placed in a vertical array. Ó

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“…There have been only several studies using quadrant analysis for turbulent scalar flux investigation, but only in the flow above relatively homogeneous surface, e.g. [9,10]. The quadrant analysis is applied newly to the turbulent scalar flux in highly turbulent a tree dimensional flow in this work.…”

Section: Introductionmentioning

confidence: 99%

Quadrant analysis of turbulent pollution flux above the modelled street intersection

Kukačka

Nosek²,

Kellnerová³

et al. 2013

EPJ Web of Conferences

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Abstract. The objective of this experimental study is to determine processes of a vertical turbulent pollution transport above the X-shaped street intersection in an idealised symmetric urban area for several approach flow directions. An experimental set-up for simultaneous measurement of the flow velocity and the tracer gas concentration in a high temporal resolution is assembled. Vertical turbulent scalar fluxes are computed from the measured data in a horizontal plane above the street intersection. The quadrant analysis was applied to the vertical turbulent pollution fluxes data. Events with dominant contribution to vertical turbulent pollution flux were detected. The mean duration, repetition frequency and the duration percentage were computed for these events. A strong influence of the approach flow direction on the the type of dominant events and their characteristics was resolved.

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The Ejection-Sweep Character of Scalar Fluxes in the Unstable Surface Layer

Cited by 137 publications

References 49 publications

Large-Eddy Simulation of Flow and Pollutant Transports in and Above Two-Dimensional Idealized Street Canyons

Large-Eddy Simulation of Flow and Pollutant Transports in and Above Two-Dimensional Idealized Street Canyons

Atmospheric stability effect on subgrid-scale physics for large-eddy simulation

Quadrant analysis of turbulent pollution flux above the modelled street intersection

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