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 derive

Einfluss von Bäumen auf die Durchlüftung von innerstädtischen Straßenschluchten

Die Auswirkungen von Baumpflanzungen auf die Durchlüftung von innerstädtischen Straßenzügen wurden in Windkanalexperimenten und in numerischen Simulationen untersucht. Die Baumpflanzungen führten zu geringeren Windgeschwindigkeiten und höheren verkehrsbedingten Schadstoffbelastungen aufgrund eines verminderten Luftaustausches mit der Umgebung. Ein Modell zur Bestimmung der Auswirkungen von Baumpflanzungen auf die verkehrsbedingte Schadstoffbelastung in Straßenschluchten wurde entwickelt

Shear-Stress Partitioning in Live Plant Canopies and Modifications to Raupach's Model

The spatial peak surface shear stress $${\tau _S^{\prime\prime}}$$ on the ground beneath vegetation canopies is responsible for the onset of particle entrainment and its precise and accurate prediction is essential when modelling soil, snow or sand erosion. This study investigates shear-stress partitioning, i.e. the fraction of the total fluid stress on the entire canopy that acts directly on the surface, for live vegetation canopies (plant species: Lolium perenne) using measurements in a controlled wind-tunnel environment. Rigid, non-porous wooden blocks instead of the plants were additionally tested for the purpose of comparison since previous wind-tunnel studies used exclusively artificial plant imitations for their experiments on shear-stress partitioning. The drag partitioning model presented by Raupach (Boundary-Layer Meteorol 60:375-395, 1992) and Raupach etal. (J Geophys Res 98:3023-3029, 1993), which allows the prediction of the total shear stress τ on the entire canopy as well as the peak $${(\tau _S ^{\prime\prime}/\tau )^{1/2}}$$ and the average $${(\tau _S^{\prime}/\tau )^{1/2}}$$ shear-stress ratios, is tested against measurements to determine the model parameters and the model's ability to account for shape differences of various roughness elements. It was found that the constant c, needed to determine the total stress τ and which was unspecified to date, can be assumed a value of about c =0.27. Values for the model parameter m, which accounts for the difference between the spatial surface average $${\tau _S^{\prime}}$$ and the peak $${\tau _S ^{\prime\prime}}$$ shear stress, are difficult to determine because m is a function of the roughness density, the wind velocity and the roughness element shape. A new definition for a parameter a is suggested as a substitute for m. This a parameter is found to be more closely universal and solely a function of the roughness element shape. It is able to predict the peak surface shear stress accurately. Finally, a method is presented to determine the new a parameter for different kinds of roughness element

Application and assessment of a GPU-based LES method for predicting dynamic wind loads on buildings

This study presents the assessment of a fast Large Eddy Simulation method for estimating dynamic wind loads on buildings using a GPU-based CFD software, which produces statistically converged results on a nine-million-cell mesh in approximately 6 hours. The surface pressure distribution of a cuboid building model was validated with experimental data obtained in an atmospheric boundary layer wind tunnel and compared with field measurements. Although due to the applied equidistant Cartesian grid the large gradients near the edges are not fully resolved, good overall agreement was found for the mean and fluctuating pressure distributions (correlation coefficient: 0.90/0.73, FAC2: 0.92/0.98, FB: −/0.06, MG: −/0.95, NMSE: −/0.10, VG: −/1.08). It was shown that the numerical model is able to produce matching turbulent spectra in an intermediate frequency range within the inertial subrange, limited by the domain size and the spatial resolution. Mesh refinement for capturing large gradients as well as for expanding the frequency limits can be achieved by using a GPU with higher VRAM capacity for the simulation. The continuing advancement of the presented model is a promising development for estimating dynamic wind loads on buildings and identifying design problems fast enough for the engineering practice, without high-performance computing

Die Simulation atmosphärischer Grenzschichten in Windkanälen

Ergebnisse von LDA-Messungen, durchgeführt in einem Grenzschichtwindkanal des Labora-toriums für Gebäude- und Umweltaerodynamik (LGU) am Institut für Hydromechanik der Universität Karlsruhe, werden vorgestellt. In Diagrammen werden die für das Windfeld der Anströmung charakteristischen Größen dargestellt und mit den Naturvorgaben verglichen um die Ähnlichkeit beurteilen zu können

Der Einfluss von Bäumen auf das Strömungs- und Konzentrationsfeld in Straßenschluchten

Windgeschwindigkeits- und Konzentrationsmessungen an einem Windkanalmodell einer typischen innerstädtischen Straßenschlucht mit einer mittig angebrachten Baumreihe bei gleichzeitiger Beachtung einer verkehrsinduzierten Luftbewegung werden vorgestellt. Bei den Versuchen wurde der Baumkronendurchmesser und somit der Versperrungsgrad des Straßenraumvolumens ebenso wie die Stärke der turbulenzinduzierenden Verkehrsbewegung variiert

Strömungsfelder in Strassenschluchten mit und ohne Baumpflanzungen - Vergleich zwischen LDA-Messungen und CFD-Simulationen

Zur Klärung der Strömungsverhältnisse in städtischen Straßenschluchten mit alleenartiger Baumpflanzung wurden Laser-Doppler-Anemometer Messungen an einem Windkanalmodell durchgeführt. Parallel wurden numerische Berechnungen mit einem kommerziellen CFD-Code (FLOVENT) bei Verwendung eines modifizierten k- Turbulenzmodells zur Schließung der Reynolds-Averaged-Navier-Stokes (RANS) Gleichungen ausgeführt. Die Ergebnisse zeigen eine verminderte Durchlüftung des Straßenraums in Gegenwart von Baumpflanzun-gen. Typische Wirbelstrukturen, die sich in der leeren, baumfreien Straßenschlucht ausbil-den, sind nur noch rudimentär, in abgeschwächtem Ausmaß präsent. Ein Vergleich der ex-perimentellen und numerischen Ergebnisse liefert nur qualitative Übereinstimmungen zwischen beiden Methoden. In den CFD-Simulationen werden geringere Strömungsgeschwin-digkeiten und somit ein reduzierter Luftaustausch im Straßenraum prognostiziert

Influence of trees on the dispersion of pollutants in an urban street canyon - experimental investigation of the flow and concentration field

Flow field and concentration measurements have been performed in an idealized model of an urban street canyon with one row of trees arranged along the center axis. The model was set up in an atmospheric boundary layer wind tunnel and the approach flow was directed perpendicular to the street axis. A line source embedded in the bottom of the street was used to release tracer gas for the simulation of traffic exhaust emissions. Trees with spherical crowns were modeled and positioned inside the street canyon, varying crown diameter, crown permeability, trunk height and tree spacing. Traffic-induced turbulence was simulated by rotating belts with thin plates. Concentrations were measured at the facades of the street canyon. For small tree crowns, only little changes in concentration were measured, however, increasing crown diameters led to increasing concentrations at the leeward street canyon wall associated with a reduction of local concentrations at the windward wall. For some cases, a variation of trunk height led to a modification of the concentration pattern on the walls. Increasing the tree spacing resulted in a noticeable concentration decrease. When compared to the situation with standing (but emitting) traffic, the traffic-induced turbulence by two-way car movements always contributed to a more homogenous concentration field inside the street canyon yielding to reduced mean concentration levels

Trees in urban street canyons and their impact on the dispersion of automobile exhausts

The aim of the present study is to clarify the influence of trees on the dispersion of automobile exhausts in urban street canyons. For this purpose, measurements have been performed with a small scale wind tunnel model of an idealized, isolated street canyon with model trees placed along the canyon center axis. Sulfur hexafluoride (SF6) was released from a line source embedded in the street surface, simulating vehicle exhaust emissions. The influence of various tree planting arrangements on the concentrations at the canyon walls was investigated with an approaching boundary layer flow perpendicular to the canyon axis. Increasing pollutant concentrations at the leeward wall and decreasing pollutant concentrations at the windward wall were found for increasing plant density. At the ends of the street canyon, i.e. towards the intersections, a remarkable relative increase of concentrations at both canyon walls was observed. The results indicate that due to tree planting, typical vortex structures observed in empty street canyons were either significantly weakened or no longer present

Die Simulation atmosphärischer Grenzschichten in Windkanälen

Ergebnisse von LDA-Messungen, durchgeführt in einem Grenzschichtwindkanal des Labora-toriums für Gebäude- und Umweltaerodynamik (LGU) am Institut für Hydromechanik der Universität Karlsruhe, werden vorgestellt. In Diagrammen werden die für das Windfeld der Anströmung charakteristischen Größen dargestellt und mit den Naturvorgaben verglichen um die Ähnlichkeit beurteilen zu können