Mass Transfer Velocity and Momentum Vertical Exchange in Simulated Deep Street Canyons

Abstract

A box model to simulate mass transfer inside deep street canyons and with atmospheric flow above is introduced and discussed. Two ideal deep street canyons with aspect ratios of 3 and 5 (the aspect ratio being the ratio between building height and street width H/W ) are considered. This range of aspect ratios, found in many densely populated historical centres in Mediterranean cities as well as in other cities around the world, potentially creates high air pollutant concentration levels. Our model is based on a combination of analytical solutions and computation fluid dynamics (CFD) simulations using carbon monoxide (CO) as a tracer pollutant. The analytical part of the model is based on mass transfer velocity concepts while CFD simulations are used both for a preliminary validation of the physical hypothesis underlying the model (steady-state simulations) and to evaluate the concentration pattern with time (transient or wash-out simulations). Wash-out simulation curves were fitted by model curves, and mass transfer velocities were evaluated through a best-fitting procedure. Upon introducing into the model the contribution of traffic-produced turbulence, the modelled CO concentration levels became comparable with those obtained in real-world monitoring campaigns. The mass transfer rate between the canyon and the above atmosphere was then expressed in terms of an overall mass transfer velocity, which directly allows the evaluation of the mass transfer rate between the bottom volume of the canyon (pedestrian level) with the above atmosphere. Overall mass transfer velocities are reported as a function of the operating conditions studied (H/W = 3-5 and wind speeds = 2-8 m s −1 ). Finally, a simple expression is reported for determining pollutant concentrations at the pedestrian level based on the overall mass transfer velocity defined.

Modelling the dispersion and transport of reactive pollutants in a deep urban street canyon: Using large-eddy simulation

“…3 and 7). It is assumed that each vortex has sufficient intensity for the chemical species to be well-mixed within the corresponding box (Murena et al, 2011). The mass transfer between two adjacent boxes is expressed by the introduction of an 'exchange velocity'.…”

Modelling traffic-induced multicomponent ultrafine particles in urban street canyon compartments: Factors that inhibit mixing

“…This lower-box enhancement is less significant for both small particles (sub-10 nm with very fast evaporation due to the Kelvin effect) and very large particles (diameters above 200 nm, with more limited capacity to change size by evaporation due to Kelvin effect). Lower heterogeneity coefficients may be attributed to more local traffic-induced turbulence in the lower canyon ( Murena et al., 2011 ) and less dense tree canopy ( Gromke and Ruck, 2012 ), which would increase the ventilation between the lower and upper canyon boxes and thereby increasing the removal rate of particles from the pedestrian level to higher altitudes of the street canyon. Fig.…”

“…The microphysical and/or chemical processes associated with mixing across compartments, together with emissions and the exchange with background air can be parsimoniously represented by a two-box model. The concept of a two-box model was previously introduced and evaluated against field measurements ( Murena et al., 2011 ; Murena, 2012 ) to predict carbon monoxide (CO) concentration (taken as a passive scalar because of its long chemical lifetime) in a deep street canyon and no chemical processes were considered. The traditional one-box model (originally assuming a single vortex in a regular canyon) may not be appropriate for deep street canyon scenarios (with canyon compartmentalisation) ( Murena et al., 2011 ; Murena, 2012 ).…”

L’Aquila Smart Clean Air City: The Italian Pilot Project for Healthy Urban Air

“…Regarding CFD simulations, the commercial software ANSYS Fluent (version 13.0, ANSYS Inc., Canonsburg, PA, USA) code was employed to simulate the dynamics into a 2-D deep street canyon, like in [21]. The APA scrubber is composed of one cylinder 2.4 m high, including the chimney, with a 1.5 m diameter.…”