Turbulence Coherent Structures and Scalar Dispersion over Heated Urban Surfaces

Abstract

Turbulent flow and scalar dispersion over heated urban surfaces are numerically investigated using the parallelized large-eddy simulation model (PALM). To investigate thermal effects of urban surface heating on turbulent flow and scalar dispersion, a method that calculates heat fluxes at building wall surfaces based on the Monin-Obukhov (MO) similarity is developed and validated. Thermal effects on turbulent flow and dispersion in and above an idealized street canyon with a street aspect ratio of one are examined when each of upwind building wall, street bottom, and downwind building wall is heated. Compared with the neutral (no heating) case, the upwind building wall or street bottom heating strengthens a primary vortex in the street canyon, while the downwind building wall heating induces a shrunken primary vortex and a winding flow between the vortex and the downwind building wall. Heating also induces higher turbulent kinetic energy and stronger turbulent fluxes at the rooftop height. In the neutral case, turbulent eddies generated by shear instability dominate mixing at the rooftop height and appear as band-shaped perturbations in the time–space plots of instantaneous turbulent momentum and scalar fluxes. In all the heating cases, buoyancy-generated turbulent eddies as well as shear-generated turbulent eddies contribute to vertical turbulent momentum and scalar fluxes and they appear as band-shaped and lump-shaped perturbations at the rooftop height. A quadrant analysis shows that sweeps are less frequent but contribute more to turbulent momentum flux than ejections at the rooftop height in the neutral and upwind building wall heating cases. In contrast, in the street bottom and downwind building wall heating cases, the frequency of sweeps is similar to that of ejections and the contribution of ejections to turbulent momentum flux is comparable to that of sweeps. Turbulent flow and scalar dispersion over an idealized building array are investigated using the LES model. Two cases (no heating and bottom heating) are simulated and compared to each other. Above the building array, streaky structures of low-speed regions appear and ejections on the regions play a dominant role in transporting momentum downward. When the bottom is heated, plume-shaped structures appear with the streaky structures and the magnitude of vertical turbulent momentum flux, averaged over the low-speed regions, increases. Elliptical structures of negative streamwise velocity perturbation and vortical structures similar to hairpin vortices appear in the conditionally averaged fields and the vortical structure above the bottom-heated building array expands in the vertical direction. At the rooftop height and in the intersections, sweeps, induced by downward extending high-speed regions, are dominant and they transport momentum into the building array. Below the rooftop height, momentum is actively transported by the spanwise turbulent motions, especially from the intersections to the street canyons. The spanwise turbulent motions are induced by sweeps passing through the intersections or by the large secondary circulation strengthened by the bottom heating. Large-scale turbulent motions play a dominant role in vertical transport of scalar in and above the building array. Above the building array, most scalar transport events appear on the low-speed regions where ejections are dominant. Coherent structures of scalar concentration perturbation are tilted downstream and strong spanwise converging flow appears around them especially when the bottom is heated. At the rooftop height, both ejection and sweep are important to the vertical transport of scalar. Ejections pull scalar out of the street canyons and sweeps put above-canyon air into the street canyons and the coherent structures related with sweeps are quite dependent on the adjacent buildings. In the building array, time-averaged scalar concentration is high below the upper low-speed regions and it is low below the upper high-speed regions, confirming the dominant influence of upper turbulence coherent structures on scalar dispersion in the urban canopy. The effects of urban-like surface with a block array on the dry convective boundary layer (CBL) are investigated using the LES model. Four cases representing a free CBL, a sheared CBL, and a strongly sheared CBL over flat surfaces and a sheared CBL over a block array are simulated and compared to each other. In the sheared CBL over a block array, the mean flow in the mixed layer is quite decelerated due to the increased surface shear and horizontal convective rolls appear in the mixed layer. In contrast, convection cells and intermediate structures between cells and rolls occur in the free and sheared CBLs over flat surfaces, respectively. Convective rolls and the traces of block-induced turbulent eddies are detected in the spectrum density fields of vertical velocity and the vertical profiles of vertical velocity skewness in the sheared CBL over a block array. Decelerated mixed-layer flow in the sheared CBL over a block array leads to stronger wind shear in the entrainment zone than in the other cases, resulting in streamwise-alternating updrafts and downdrafts there. Due to the enhanced turbulence activity and wave-like motions in the entrainment zone, the magnitude of the entrainment heat flux in the CBL over a block array is larger than that in the other cases. Turbulent flow in a densely built-up area of Seoul, South Korea, is numerically investigated using the LES model. Based on the analysis of column-averaged vertical turbulent momentum flux, three areas of interest are selected: a downstream area of an apartment complex, an area behind high-rise buildings, and a park area. In the downstream area of the apartment complex, a large wake develops and a region of strong vertical turbulent momentum flux appears above the wake. In the area behind the high-rise buildings, fluctuating wakes and vortices are distinct flow structures around the top height of the tallest building and updrafts induced by the flow structures appear as strong ejections just behind the high-rise buildings or farther downstream. While strong ejections are dominant at building-top heights, downdrafts along the windward walls of high-rise buildings are distinct below building-top heights and they induce high turbulent kinetic energy and winding flow around the high-rise buildings near the ground surface, transporting momentum downward and intermittently into nearby streets. In the park area located downstream in the main domain, turbulent eddies exist quite above the ground surface, and the thickness of the interfacial region between low-speed air and high-speed air increases and complex turbulent flow appears in the interfacial region. Turbulent flow in a densely built-up area of Seoul for 0900–1500 LST 31 May 2008 is simulated using the LES model coupled to a mesoscale model (WRF). Time-varying turbulent inflow data (including mesoscale wind) drives quite different turbulence structures depending on time. While upper flow induces strong sweeps for 0900–0910 LST, weaker sweeps and strong ejections are dominant for 1450–1500 LST at z = 200 m and the ejections seem to be induced by buildings or building-induced flow structures. The velocity ratio of pedestrian wind speed to ambient wind speed indicates ventilation in the urban area, and it is high on broad streets and intersections. While the velocity ratio shows a distinct spatial variation, the temporal variation of the velocity ratio is quite complex, partially depending on mesoscale wind.