Cb Christof Gromke

Aerodynamic effects of trees on pollutant concentration in street canyons

This paper deals with aerodynamic effects of avenue-like tree planting on flow and traffic-originated pollutant dispersion in urban street canyons by means of wind tunnel experiments and numerical simulations. Several parameters affecting pedestrian level concentration are investigated, namely plant morphology, positioning and arrangement. We extend our previous work in this novel aspect of research to new configurations which comprise tree planting of different crown porosity and stand density, planted in two rows within a canyon of street width to building height ratio W/H=2 with perpendicular approaching wind. Sulfur hexafluoride was used as tracer gas to model the traffic emissions. Complementary to wind tunnel experiments, 3D numerical simulations were performed with the Computational Fluid Dynamics (CFD) code FLUENT using a Reynolds Stress turbulence closure for flow and the advection-diffusion method for concentration calculations. In the presence of trees, both measurements and simulations showed considerable larger pollutant concentrations near the leeward wall and slightly lower concentrations near the windward wall in comparison with the tree-less case. Tree stand density and crown porosity were found to be of minor importance in affecting pollutant concentration. On the other hand, the analysis indicated that W/H is a more crucial parameter. The larger the value of W/H the smaller is the effect of trees on pedestrian level concentration regardless of tree morphology and arrangement. A preliminary analysis of approaching flow velocities showed that at low wind speed the effect of trees on concentrations is worst than at higher speed. The investigations carried out in this work allowed us to set up an appropriate CFD modelling methodology for the study of the aerodynamic effects of tree planting in street canyons. The results obtained can be used by city planners for the design of tree planting in the urban environment with regard to air quality issues.

Pollutant Concentrations in Street Canyons of Different Aspect Ratio with Avenues of Trees for Various Wind Directions

This study summarizes the effects of avenues of trees in urban street canyons on traffic pollutant dispersion. We describe various wind-tunnel experiments with different tree-avenue models in combination with variations in street-canyon aspect ratio W/H (with W the street-canyon width and H the building height) and approaching wind direction. Compared to tree-free street canyons, in general, higher pollutant concentrations are found. Avenues of trees do not suppress canyon vortices, although the air ventilation in canyons is hindered significantly. For a perpendicular wind direction, increases in wall-average and wall-maximum concentrations at the leeward canyon wall and decreases in wall-average concentrations at the windward wall are found. For oblique and perpendicular wind directions, increases at both canyon walls are obtained. The strongest effects of avenues of trees on traffic pollutant dispersion are observed for oblique wind directions for which also the largest concentrations at the canyon walls are found. Thus, the prevailing assumption that attributes the most harmful dispersion conditions to a perpendicular wind direction does not hold for street canyons with avenues of trees. Furthermore, following dimensional analysis, an estimate of the normalized wall-maximum traffic pollutant concentration in street canyons with avenues of trees is derived.

Numerical modeling of flow and pollutant dispersion in street canyons with tree planting

Numerical simulations of the impact of tree planting on airflow and traffic pollutant dispersion in urban street canyons have been performed using the commercial CFD (Computational Fluid Dynamics) code MISKAM. A k-e turbulence model including additional terms for the treatment of vegetation, has been employed to close the Reynolds-averaged-Navier-Stokes (RANS) equations. The numerical results were compared to wind tunnel data. In the case of the investigated wind direction perpendicular to the street axis, the presence of trees lead to increased pollutant concentrations inside the canyon. Concentrations increased strongly on the upstream side of the canyon, while on the downstream side a small concentration decrease could be observed. Lower flow velocities and higher pollutant concentrations were found in the numerical simulations when directly compared to the experimental results. However, the impact of tree planting on airflow and concentration fields when compared to the treeless street canyon as a reference configuration were simulated quite well, meaning that relative changes were similar in the wind tunnel investigations and numerical computations. This feature qualifies MISKAM for use as a tool for assessing the impacts of vegetation on local air quality.

A vegetation modeling concept for building and environmental aerodynamics wind tunnel tests and its application in pollutant dispersion studies

A new vegetation modeling concept for Building and Environmental Aerodynamics wind tunnel investigations was developed. The modeling concept is based on fluid dynamical similarity aspects and allows the small-scale modeling of various kinds of vegetation, e.g. field crops, shrubs, hedges, single trees and forest stands. The applicability of the modeling concept was validated in wind tunnel pollutant dispersion studies. Avenue trees in urban street canyons were modeled and their implications on traffic pollutant dispersion were investigated. The dispersion experiments proved the modeling concept to be practicable for wind tunnel studies and suggested to provide reliable concentration results. Unfavorable effects of trees on pollutant dispersion and natural ventilation in street canyons were revealed. Increased traffic pollutant concentrations were found in comparison to the tree-free reference case.

Measurements of the pore-scale water flow through snow using Fluorescent Particle Tracking Velocimetry

[1] Fluorescent Particle Tracking Velocimetry (FPTV) measurements of the pore-scale water flow through the pore space of a wet-snow sample are presented to demonstrate the applicability of this measurement technique for snow. For the experiments, ice-cooled water seeded with micron sized fluorescent tracer particles is either sprinkled on top of a snow sample to investigate saturated and unsaturated gravity-driven flow or supplied from a reservoir below the snow sample to generate upward flow driven by capillary forces. The snow sample is illuminated with a laser light sheet and the fluorescent light of the particles transported with the water in the pore space is recorded with a high-speed camera equipped with an optical filter. Tracking algorithms are applied to the images to obtain flow paths and flow velocities. A flow loop found in a pore space for the case of saturated gravity flow together with the tortuosity of the particle trajectories indicate the three-dimensionality of the water flow in wet snow. The average vertical flow velocities in the pore spaces were 11.2 mm s � 1 for the downward saturated gravity flow and 9.6 mm s � 1 for the upward flow that is driven by capillary forces for the limited cases presented as examples of the measurement technique. In the case of unsaturated gravity-driven flow, the average and the maximum flow velocities were found to be 30 times smaller than for the saturated gravity flow. Velocity histograms show that the fraction of the total water flowing against the main flow direction was about 3–5%, and that the horizontal velocities average to zero for both the saturated gravity-driven and the capillary flow.

Effects of trees on the dilution of vehicle exhaust emissions in urban street canyons

In order to investigate the natural ventilation and air quality of urban street canyons with trees, boundary layer wind tunnel studies at a small-scale model have been performed. Concentrations in street canyons with a tracer gas emitting line source at the ground level and one row of trees arranged along the canyon centre have been measured for several equidistant tree spacings. In the case of flow approaching perpendicular to the street axis, increases in pollutant concentration at the leeward canyon wall and decreases at the windward canyon wall were found when compared with the tree-free canyon.

Einfluss von Bäumen auf die Durchlüftung von innerstädtischen Straßenschluchten

Die Auswirkungen von Baumpflanzungen auf die Durchluftung von innerstadtischen Strasenzugen wurden in Windkanalexperimenten und in numerischen Simulationen untersucht. Die Baumpflanzungen fuhrten zu geringeren Windgeschwindigkeiten und hoheren verkehrsbedingten Schadstoffbelastungen aufgrund eines verminderten Luftaustausches mit der Umgebung. Ein Modell zur Bestimmung der Auswirkungen von Baumpflanzungen auf die verkehrsbedingte Schadstoffbelastung in Strasenschluchten wurde entwickelt.

On Wind Forces in the Forest-Edge Region During Extreme-Gust Passages and Their Implications for Damage Patterns

A damage pattern that is occasionally found after a period of strong winds shows an area of damaged trees inside a forest stand behind an intact stripe of trees directly at the windward edge. In an effort to understand the mechanism leading to this damage pattern, wind loading in the forest-edge region during passages of extreme gusts with different characteristics are investigated using a scaled forest model in the wind tunnel. The interaction of a transient extreme gust with the stationary atmospheric boundary layer (ABL) as a background flow at the forest edge leads to the formation of a vortex at the top of the canopy. This vortex intensifies when travelling downstream and subsequently deflects high-momentum air from above the canopy downwards resulting in increased wind loading on the tree crowns. Under such conditions, the decrease in wind loading in the streamwise direction can be relatively weak compared to stationary ABL approach flows. The resistance of trees with streamwise distance from the forest edge, however, is the result of adaptive growth to wind loading under stationary flow conditions and shows a rapid decline within two to three tree heights behind the windward edge. For some of the extreme gusts realized, an exceedance of the wind loading over the resistance of the trees is found at approximately three tree heights behind the forest edge, suggesting that the damage pattern described above can be caused by the interaction of a transient extreme gust with the stationary ABL flow.

Implications of Vegetation on Pollutant Dispersion in an Idealized Urban Neighborhood

Configurations of avenue-trees and a central park in an idealized urban neighborhood and their implications on traffic pollutant concentrations at the pedestrian level were investigated with Computational Fluid Dynamics (CFD). Steady state simulations were performed using a Reynolds Stress Model (RSM) extended with additional terms to represent the effects of vegetation on air flow. The results show that the type of configuration of avenue-trees and/or park has a clearly noticeable effect on the overall pollutant distribution and on the maximum concentration. The central park was found to lead to a general reduction of concentrations in its immediate vicinity and at locations downwind.

Impact of avenue-trees on traffic pollutant concentrations on the urban neighborhood scale

Traffic pollutant concentrations at the pedestrian level in a generic urban neighborhood were studied. Scenarios without and with avenue-trees were investigated with Computational Fluid Dynamics (CFD) by employing a Reynolds Stress Model (RSM) to which extra terms are added accounting for the effects of vegetation on the wind. The avenue-trees showed a significant impact on the traffic pollutant dispersion and flow fields in the street canyons and intersections. In the presence of trees, a faster build-up of concentrations was found for the wind-parallel streets. In the wind-perpendicular streets, considerably higher pollutant concentrations at the buildings’ leeward walls were found.