The influence of an obstacle on flow and pollutant dispersion in neutral and stable boundary layers

Abstract

h i g h l i g h t s g r a p h i c a l a b s t r a c tThe combined effect of shear and buoyancy on dispersion behind a fence is examined. Stable turbulent boundary layers up to Ri grad ¼ 0.2 are generated by a recycle method. The fence affects concentrations up to at least 100 obstacle heights downstream.The decay of maximum velocity and temperature deficit is independent of stability.The decay in maximum concentration excess decreases appreciably with stability. a b s t r a c tPredicting pollutant dispersion in urban environments requires accurate treatment of obstacle geometry, inflow turbulence and temperature differences. This paper considers both the influence of thermal stratification and the presence of a single obstacle on pollutant dispersion in turbulent boundary layers (TBLs). Turbulent flow over a fence with line sources of pollutant in its vicinity is simulated by means of Large-Eddy Simulations. Separate 'driver' simulations are done to generate the inflow TBL for several levels of stratification. Using these inflow TBLs the flow development and pollutant dispersion behind the fence, up to 100 fence heights, h, is investigated. It is shown that the decay of velocity and temperature deficit is independent of stability, while the decay of Reynolds stress and concentration excess decreases with increasing stability. For neutral cases the influence of the obstacle is gone after approximately 75h, while for stable cases near the ground the flow is still accelerated compared to the undisturbed case. The fence does cause a local reduction of stratification and thereby increased pollutant dispersion. However, neglecting the effect of buoyancy results in an underestimation of pollutant concentration by a factor 2.5 at 75h downstream of the emission source for the most stable case.

Stable Stratification Effects on Flow and Pollutant Dispersion in Boundary Layers Entering a Generic Urban Environment

“…The only differences are that the buoyancy force was taken into account in the simulations and a mass-flux correction was applied such that the resulting inflow satisfied a constant mass flux. This procedure has been applied successfully before when considering a smooth-wall turbulent boundary layer approaching a single fence (Tomas et al 2015b). At the outlet a convective outflow boundary condition was applied for all variables.…”

“…However, because of potentially decreased air quality in stable conditions it is important to determine when the 'neutral urban boundary-layer assumption' is valid. Tomas et al (2015b) show that the turbulence added by the presence of a single two-dimensional obstacle is not enough to diminish buoyancy effects. Whether the transition of stable flow over a rural environment to a generic urban roughness consisting of multiple cubes does result in near-neutral flow is the subject of the present study.…”

“…Table 1 summarizes the domain size and number of grid points used in each type of simulation. The boundary layers were generated using a horizontal domain size of L x × L y = 20δ ×1.8δ, which is sufficiently large; in Tomas et al (2015b) it was shown that a smaller horizontal domain size of L x × L y = 10δ × 1.57δ was adequate to capture the largest flow structures in the boundary layer. The PR and V simulations used a smaller domain size than the RT simulations; decreasing the domain width did not result in differences in mean statistics.…”

The Impact of CCN Concentrations on the Thermodynamic and Turbulent State of Arctic Mixed-Phase Clouds

“…It was also widely used and in studies of neutral boundary layers (Ouwersloot et al, 2016) as well as stable boundary layers (Tomas et al, 2015). Furthermore, the scalability of DALES allow it to be utilised for simulating with various domain sizes 190 (Griewank., 2018) DALES has taken part in various model intercomparison studies, such as stratocumulus-cumulus transition (Dussen et al, 2013) (de Roode et al, 2016 and the CONSTRAIN experiment (de Roode et al, 2019) of the GRAYZONE project.…”

Three-dimensional study of parallel shear flow around an obstacle in water channel and air tunnel

“…The shear flows are heterogeneous flow with average velocity gradients, such as the jet of an erupted volcano and the exhaust gases of reaction engines in motion. Environmental and hydrodynamic assessment of the coastal fringe area degraded by rivers estuaries, pollution dispersion [1] as well as the wide use of obstacles in industrial and energy applications lead us to study the hydrodynamic phenomena that are formed when a shear flow encounters a fixed obstacle at the bottom of the water channel or a wind tunnel. In fact, many studies have been conducted to develop numerical models of shear flows [2].…”