Near-Wall Flow Development After A Step Change In Surface Roughness

Cheng, Hong Ping; Castro, Ian P.

doi:10.1023/a:1020355306788

Cited by 109 publications

(83 citation statements)

References 17 publications

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“…This region has usually been identified to extend up to five roughness heights (Cheng & Castro 2002a;Flack et al 2007). To investigate the flow heterogeneity, results from stereoscopic PIV measurements in the wall-normal-spanwise (y, z) plane are presented.…”

Section: Effect Of Surface Morphology On the Roughness Sublayermentioning

confidence: 99%

Effects of frontal and plan solidities on aerodynamic parameters and the roughness sublayer in turbulent boundary layers

Placidi

Ganapathisubramani

2015

J. Fluid Mech.

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Citation: Placidi, M. & Ganapathisubramani, B. (2015). Effects of frontal and plan solidities on aerodynamic parameters and the roughness sublayer in turbulent boundary layers. Journal of Fluid Mechanics, 782, pp. 541-566. doi: 10.1017Mechanics, 782, pp. 541-566. doi: 10. /jfm.2015 This is the accepted version of the paper.This version of the publication may differ from the final published version. Experiments were conducted in the fully-rough regime on surfaces with large relative roughness height (h/δ ≈ 0.1, where h is the roughness height and δ is the boundary layer thickness). The surfaces were generated by distributed LEGO TM bricks of uniform height, arranged in different configurations. Measurements were made with both floating-element drag-balance and high-resolution particle image velocimetry on six configurations with different frontal solidity, λ F , at fixed plan solidity, λ P , and vice versa, for a total of twelve rough-wall cases. Results indicate that the drag reaches a peak value λ F ≈ 0.21 or a constant λ P = 0.27, whilst it monotonically decreases for increasing values of λ P for a fixed λ F = 0.15. This is in contrast with previous studies in the literature based on cube roughness that show a peak in drag for both λ F and λ P variations. The influence of surface morphology on the depth of the roughness sublayer (RSL) is also investigated. Its depth is found to be inversely proportional to the roughness length, y 0 . A decrease in y 0 is usually accompanied by a thickening of the the RSL and vice-versa. Proper orthogonal decomposition analysis was also employed. The shapes of the most energetic modes calculated using the data across the entire boundary layer are found to be selfsimilar across the twelve rough-walls cases, however, when the analysis is restricted to the roughness sublayer, differences that depend on the wall morphology are apparent. Moreover, the energy content of the POD modes within the RSL suggest that the effect of increased frontal solidity is to redistribute the energy towards the larger-scales (i.e. larger portion of energy is within the first few modes) whilst the opposite is found for variation of plan solidity. Permanent

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Section: Effect Of Surface Morphology On the Roughness Sublayermentioning

confidence: 99%

Effects of frontal and plan solidities on aerodynamic parameters and the roughness sublayer in turbulent boundary layers

Placidi

Ganapathisubramani

2015

J. Fluid Mech.

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“…There are only a few recent numerical studies on turbulent flow over an explicitly resolved roughness transition (Lee et al 2011;Cheng and Porté-Agel 2015), where mostly cubical roughness elements or riblets are considered. In addition, experimental investigations with results up to the roughness elements are scarce, and if a roughness transition is considered it is to determine when the boundary layer retains an equilibrium state, without investigating pollutant dispersion (Cheng and Castro 2002a).…”

Section: Introductionmentioning

confidence: 99%

Pollutant Dispersion in Boundary Layers Exposed to Rural-to-Urban Transitions: Varying the Spanwise Length Scale of the Roughness

Tomas

Eisma

Pourquié

et al. 2017

Boundary-Layer Meteorol

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Both large-eddy simulations (LES) and water-tunnel experiments, using simultaneous stereoscopic particle image velocimetry and laser-induced fluorescence, have been used to investigate pollutant dispersion mechanisms in regions where the surface changes from rural to urban roughness. The urban roughness was characterized by an array of rectangular obstacles in an in-line arrangement. The streamwise length scale of the roughness was kept constant, while the spanwise length scale was varied by varying the obstacle aspect ratio l/ h between 1 and 8, where l is the spanwise dimension of the obstacles and h is the height of the obstacles. Additionally, the case of two-dimensional roughness (riblets) was considered in LES. A smooth-wall turbulent boundary layer of depth 10h was used as the approaching flow, and a line source of passive tracer was placed 2h upstream of the urban canopy. The experimental and numerical results show good agreement, while minor discrepancies are readily explained. It is found that for l/ h = 2 the drag induced by the urban canopy is largest of all considered cases, and is caused by a large-scale secondary flow. In addition, due to the roughness transition the vertical advective pollutant flux is the main ventilation mechanism in the first three streets. Furthermore, by means of linear stochastic estimation the mean flow structure is identified that is responsible for street-canyon ventilation for the sixth street and onwards. Moreover, it is shown that the vertical length scale of this structure increases with increasing aspect ratio of the obstacles in the canopy, while the streamwise length scale does not show a similar trend.

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“…The area of the interest lies as far as possible from the channel entrance to provide a long fetch for development of a fully turbulent boundary layer ( [9]) and at the same time it far enough from the ending part of the wind-channel. Two geometries of roof are tested -triangle and flat.…”

Section: Experimental Set-upmentioning

confidence: 99%

Wavelet analysis of the turbulent flow over the very rough surface

Kellnerová

Kukačka

Nosek

et al. 2014

EPJ Web of Conferences

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Abstract. Wavelet analysis is applied to data from PIV measurement in order to recognize a specific structure in the flow. The PIV snaphots achieved on the model of the street canyon was used as a test case. Four flow characteristics that are at disposal from one-point simultaneous two-components measurement (e.g. from 2D LDA) were analyzed by Wavelet method: longitudinal and vertical velocity, momentum flux u'w' and δS -the difference between momentum fluxes associated with a sweep and an ejection. Each of characteristic is useful for detection of certain type of event. We have focused on the sweep and the ejection that seem to be the most convenient for investigation of a significant inflow or an outflow from the street canyon.

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Near-Wall Flow Development After A Step Change In Surface Roughness

Cited by 109 publications

References 17 publications

Effects of frontal and plan solidities on aerodynamic parameters and the roughness sublayer in turbulent boundary layers

Effects of frontal and plan solidities on aerodynamic parameters and the roughness sublayer in turbulent boundary layers

Pollutant Dispersion in Boundary Layers Exposed to Rural-to-Urban Transitions: Varying the Spanwise Length Scale of the Roughness

Wavelet analysis of the turbulent flow over the very rough surface

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