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Numerical investigation of effectivity indices of space-time error indicators for Navier-Stokes equations
No full textIn this paper we propose two error indicators aimed at estimating the space discretization error and the time discretization error for the unsteady Navier-Stokes equations. We define a space error indicator for evaluating the quality of the mesh and a time error indicator for evaluating the time discretization error. Moreover, we verify the reliability of the estimations through numerical experiments and we propose an effective space-time adaptive strategy for the unsteady Navier-Stokes equations. Such technique is based on two residual-based error indicators that suitably drive the mesh and the timestep-length modifications. Adaptive simulations show that the presented strategy allows to obtain accurate solutions in efficient wa
Vertical turbulent transport in an urban street canyon
Full text linkAir pollution in urban areas is a major concern for citizens' health. Rapid urbanization and climate change will further exacerbate the deterioration of air quality in the near future. As the dispersion of pollutants in the urban canopy is governed by air flows and transport phenomena, fluid mechanics plays a crucial role to predict pollutant concentration and evaluate the efficiency of mitigation strategies. The focus is on the dynamics in the street canyons, where pollution emitters and subjects exposed are extremely close and ventilation is inhibited by the presence of tall buildings on both sides.
In this framework, we propose wind tunnel experiments aimed at investigating the dynamics of mass exchange between a street canyon and the overlying atmosphere. We consider the case of a street oriented perpendicular to the wind direction. In this configuration, vertical transport is the only way out for pollutants released from a line source at street level.
Keeping the external flow unaltered, we analyse the effect of different boundary conditions at the building walls and the presence of obstacles within the canyon. The boundary conditions are modified by alternatively heating the windward and leeward walls of the canyon, by changing its aspect ratio and by introducing roughness elements at walls. Two rows of model trees are arranged at the sides of a street canyon to simulate urban vegetation.
Velocity and concentration measurements are performed within the canyon and a characteristic exchange velocity between the street canyon and the overlying atmosphere is estimated to quantify the overall canyon ventilation in the different configurations.
Results evidence that mass transfer is mainly driven by the fluctuating component of the turbulent flow. Moreover, the experimental dataset is valuable in validating numerical simulations of air pollution in cities and provides city planners with first recommendations for the sustainable design of urban environments
Atmospheric Dispersion Downstream a Two-Dimensional Obstacle: Experimental Evaluation of Turbulence Closure Models
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Turbulent transfer and concentration statistics in a street canyon with tree planting
Full text linkThe exacerbation of the urban heat island due to global warming poses a serious risk to the health of citizens. Furthermore, the alteration of the urban microclimate affects air quality with an expected increase in the concentrations of harmful pollutants. Greening cities is an effective tool to mitigate these effects. However, the effect of tree planting in urban street canyons is still a debated topic. Despite their positive effect on temperature and their filtering action, trees can hinder air circulation thus limiting pollutant removal processes. In this context, it is essential to understand and model the effect of trees on the ventilation of street pollutants, heat and moisture . To this end, we present in this work the results of an experimental campaign conducted in a wind tunnel. An urban geometry with a street canyon perpendicular to the wind direction was reproduced. A linear source of passive scalar simulated the emission of pollutants from vehicular traffic. Reduced scale trees have been conceived to mimic a realistic aerodynamic behaviour. We investigated four different configurations of vegetation density: a street with no trees, two trees in the middle of the street, two rows of scattered trees and two dense rows of trees. Concentration and velocity measurements were performed in order to characterize the transfer processes of pollutants inside the street and to estimate a bulk vertical exchange rate. Results show that the presence of trees alters the concentration field in the street with a progressive shift from a nearly two-dimensional to a three-dimensional field. Despite the significant spatial variation in concentration, the presence of trees does not alter the overall efficiency of the ventilation as the vertical bulk exchange velocity remains almost constant in the different configurations. The statistical analysis of the turbulent concentration signal gives other insights in the transfer processes. The turbulent signal measured in different positions of the cavity and for different tree density follows a Gamma distribution with constant fluctuation intensity suggesting an almost universal behaviour within the canyon and providing a powerful modelling tool. Finally, combined measurements of concentration and velocity allows to measure the turbulent mass fluxes at the roof height and investigate their spectrum therefore enlightening the effect of trees on typical scales of motion
Influence of Street Trees on Turbulent Fluctuations and Transport Processes in an Urban Canyon: A Wind Tunnel Study
Full text linkThe presence of vegetation within urban canyons leads to non-trivial patterns of the concentration
of airborne pollutants, as a result of the complex structure of the velocity field. To
investigate the relationship between concentration, velocity fields and vegetation density, we
have performed wind-tunnel experiments in a reduced-scale street canyon, oriented perpendicular
to the external wind flow, within which we placed a steady ground-level line source of
a passive tracer. The aerodynamic behavior of vegetation was reproduced by inserting plastic
miniatures of trees along the two long sides of the canyon, according to three different densities.
The canyon ventilation was investigated by acquiring one-point simultaneous statistics
of concentration and velocity over a dense grid of points within the canyon. The results show
that the presence of trees hinders the upward mean vertical velocity at the rooftop, causes a
reduction of the turbulent kinetic energy inside the canyon, and reduces the energy content of
the large scales. The scalar concentration is conversely characterized by an enhanced level of
turbulent fluctuations, whose magnitude is not dampened increasing the tree density.Within
the canyon, high tree density inhibits turbulent mass fluxes, which are instead enhanced at
roof level, where the mean component of the scalar flux is however hindered. A statistical
analysis of concentration time series reveals that the lognormal distribution is suitable
to model concentration fluctuations and extreme events, in dispersing plumes emitted by a
linear source
Wind tunnel study on the influence of vegetation density and wind direction on urban canyon ventilation
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