Passive scalar diffusion in and above urban-like roughness under weakly stable and unstable thermal stratification conditions

Abstract

Wind-tunnel experiments were conducted to investigate passive scalar diffusion behaviour in and above urban-like roughness under various thermal stratification conditions. Staggered arrays of rectangular blocks with aspect ratios of 0.96 or 1.92 were used to model the urban canopies. Weakly stable and unstable thermal stratification conditions were created by controlling the temperature of the windtunnel floor and the airflow. Above the modelled urban roughness, vertical profiles of the velocity and temperature were measured by a laser Doppler anemometer and a cold-wire thermometer, respectively. The estimated Obukhov length was found comparable or larger than the boundary-layer thickness, indicating very weak stratification. Passive-scalar diffusion was studied by emitting ethane gas from a point source on the wind-tunnel floor. Thermal stratification had substantial effect on diffusion even in the vicinity of the source where mechanical mixing by the model blocks was presumed to eliminate the stratification effect.

Dispersion in an array of buildings in stable and convective atmospheric conditions

“…13 confirms this behaviour, with the parameter showing a clear and progressive increment after the application of unstable stratification, more evident than the variation in the plume width. Again this result is in accordance with Briggs (1973) and in contrast with Kanda and Yamao (2016).…”

Turbulence and dispersion below and above the interface of the internal and the external boundary layers

“…A more accurate method for defining the edge of the IBL helps to analyse flow and dispersion mechanisms over such roughness transition regions, in particular in some specific scenarios such as in stable stratification (e.g. Kanda & Yamao, 2016). A step change over cuboid elements with uniform height, perpendicular to the flow direction, determines a more visible interface.…”

Stable and convective boundary-layer flows in an urban array

“…To be noted that a constant flux layer is not discernible, as expected, likely due to the non-zero pressure gradient (see Marucci et al, 2018). As support of this argumentation, Kanda and Yamao (2016) was able to obtain a constant flux layer over an array of cubic buildings, but employing a wind tunnel with adjustable ceiling height and shape to generate a zero-pressure gradient boundary layer. Nevertheless, a reasonably good linear trend, similar to what was found in Marucci et al (2018) for the approaching flow, is appreciable, suggesting the applicability of the same method also in this case.…”

“…The differences in the vertical velocity length scale are much larger than in the streamwise one. On this aspect, by comparing experiments with and without spires, Kanda and Yamao (2016) found that Λ u is very dependent on the employed turbulence generator, more than on the applied stratification.…”

“…For u w the scatter between the points at a) b) c) d) Figure 10: Profiles of non-dimensional Reynolds stresses, temperature variance and vertical kinematic heat flux at x/H = 1, y/H = −6. Data is compared with Caughey and Palmer (1979), Ohya and Uchida (2004) (case E2), Wilczak and Phillips (1986), Wood et al (2010) and Kanda and Yamao (2016). different locations but same position respect to local buildings in the CBL cases suggests a high level of unsteadiness, for which a longer measuring time would be preferable.…”