Bodo Ruck

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.

Particle dispersion in a single-sided backward-facing step flow

Abstract The paper describes the particle dispersion in a single-sided backward-facing step flow. Particles of well-known sizes in the diameter range from 1 to 70 μm were suspended in an air flow and the particle motion over a step was measured by mean of a laser-Doppler anemometer. Thus, the local and integral flow quantities, i.e. the mean and turbulent velocity data could be measured precisely. In the experiments, monodispersed particle size distributions were used to exclude particle size related information ambiguity, known as triggering effects or size bias. The results of this study show qualitatively and quantitatively the difference in time-averaged particle dynamics for selected particle sizes in a backward-facing step flow. The experiments show, for different sizes, the changes in the particle velocity field in comparison with the velocity field of the continuous phase deduced from the 1 μm particles, and also imply the strong influences which different particle sizes have on flow data evaluation when size effects are not taken into account with particle-related optical measuring techniques.

Das Strömungsfeld der Einzelbaumumströmung

ZusammenfassungDer vorliegende Bericht faßt Untersuchungsergebnisse zusammen, die im Rahmen eines Forschungsprojektes zur Bestimmung der Strömungsbläufe bei der Baumumströmung gewonnen werden konnten. Besondere Beachtung fanden hierbei die sich ausbildenden Turbulenzverhältnisse und der Einfluß auf die Depositionswahrscheinlichkeit von Feinsttröpfchen.Die Untersuchungen wurden in einem Niedergeschwindigkeitswindkanal durchgeführt. Das atmosphärische Grenzschichtprofil wurde für die Umströmungsversuche im Windkanal nachgebildet. Die Umströmungsgeschwindigkeiten lagen bei 0,5–1,5 m/s. Für die Geschwindigkeitsmessungen wurde ein 2-Komponenten-Laser-Doppler-Anemometer-System eingesetzt.Die Ergebnisse zeigen, daß die sich bei der Einzelbaumumströmung ausbildenden Strömungsfelder für verschiedene Baumformen phänomenologisch unterschiedliche Wirbelgebiete aufweisen. Die Abscheidung von Feinsttröpfchen erfolgt bei niederen Strömungsgeschwindigkeiten wie etwa schleichenden Nebelschwaden auf Grund des fehlenden mittleren konvektiven Transportes hauptsächlich durch turbulente Diffusion und ist somit an den lokalen Turbulenzverlauf gekoppelt. Strömungs- und Depositionssichtbarmachungsuntersuchungen bestätigen die Analogie zwischen lokaler Turbulenz und Depositionswahrscheinlichkeit für den Teilchengrößenbereich von Feinsttröpchen (Nebel).SummaryThis report summarizes research results obtained from a research project aimed at determining flow structures around trees. Special attention was focused on turbulence characteristics and the effect on the deposition probability of very fine droplets.Investigations were conducted in a low-speed wind tunnel. The atmospheric boundary layer profile for the flow experiments was simulated in the wind tunnel. Boundary flow speeds were 0.5 to 1.5 m/s. Speed measurements were made with a two-component-laser-Doppler-anemometer-system.The results show that flow profiles around single trees have phenomenologically different turbulence characteristics with different tree forms. With low flow speeds, as for example with slowly moving fog lacking average convective transport, precipitation of very fine droplets occurs mainly by turbulent diffusion, and thus it is linked with the local turbulence pattern. Investigations aimed at making visible both flow and deposition confirm the analogy between local turbulence and deposition probability for very fine droplets (fog).

Simulated Ground Deposition Of Fine Airborne Particles In An Array Of Idealized Tree Crowns

Wind-tunnel experiments were used to investigate the ground deposition of fine airborne particles in an array of idealized tree crowns. The particle ground deposition was modelled with a gaseous tracer instead of solid particles, which is an approach for very fine particles. A chemical method based on the reaction of ammonia and manganese chloride was used to quantify the mass transfer from the simulated atmospheric boundary-layer flow to the surface. Using a tracer gas instead of solid particles can be considered only if turbulent diffusion is the decisive deposition mechanism and effects of sedimentation, impaction, interception or molecular diffusion can be approximately ignored. These constraints are necessary due to scaling problems concerning particle modelling in the small-scale experiment. The intention was to determine the obstacle arrangement density in which the mean ground deposition is maximized for a defined crown form. A deposition amplification factor α was defined as the quotient of deposition efficiencies for an area with tree crowns and an open ground with identical similarity parameters. Based on this calculation an increase of the ground deposition by up to 60% should be realistic through a favourable arrangement of tree crowns and tree number density. An increase in turbulence intensity in the flow leads to a significant amplification of the mean ground deposition.

Distortion of LDA fringe pattern by tracer particles

For precise flow velocity measurements laser Doppler anemometry (LDA) is wide-spread in use in the laboratories of industry and universitarian research institutions. The LDA method has the advantage of being not intrusive and able to discriminate between forward and reverse velocities. So far, laser Doppler anemometry is believed to be one of the most accurate flow measuring techniques. However, recent investigations have shown that the period lengths of LDA signal bursts are not constant within an individual burst. This can induce an additional scatter in the signal frequency and in the determination of the flow velocity. Until now, the reason for the period variations has not been investigated in detail although the problem was observed before. This paper describes experimental investigations which show that the particle passage through the laser beams shortly before the point of superposition, i.e. the LDA measuring volume, yields a distorted LDA fringe pattern. Thus, the signal period length from an individual particle, passing the center of the measuring volume at the same time, varies according to the distortion of the fringe spacing.

On the influence of windward edge structure and stand density on the flow characteristics at forest edges

Experimental investigations in an atmospheric boundary layer wind tunnel were carried out in order to study both the influence of the windward edge structure and the stand density on the flow characteristics near the canopy top of forest edges. For the edge structure investigation, two different types of forest edges were used. The taper angle was varied three times for both edge types, and all tapered edges were combined with both dense and sparse forest stands. In addition, a configuration was investigated where the airflow in the trunk space of the dense forest was blocked completely by an impermeable wall. In order to study the influence of stand density, forest stands were varied three times by removing whole rows of the originally dense model forest. The measurements show that a forest edge induces strong disturbances in the flow field at the near-edge region leading to an increased momentum transfer and to an increased wind loading on trees. The region of significant disturbances extends to a distance of about eight tree heights. Both edge density and taper angle determine the amount of volume influx through the edge, which seems to be primarily responsible for the flow field properties near the canopy top. It was found that for sparse forest stands, sloping tapered edges reduce the wind velocities nearest to the canopy. It could also be shown that the positive skewness of the near-canopy longitudinal velocity distribution is diminished with decreasing taper angle indicating that higher wind velocity fluctuations are reduced.

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.

Effective particle size range in laser-Doppler anemometry

The present paper points out that all existing laser-Doppler anemometer systems do not only operate within a finite range of Doppler frequencies but also work within a relatively narrow range of signal amplitudes. It is shown that this corresponds to a finite, and usually to an extremely small, range of particle diameters which contributes to the final LDA measurements. Because of this, the particle size distribution has to be matched to the LDA-system used for measuring particle velocities. If this is not taken into account in particle seeding, low data rates will result in spite of very high particle passage rates through the measuring control volume. This is shown experimentally and is supported by theoretical considerations.The present investigation results in conclusions regarding optimum particle size distributions for laser-Doppler anemometry. If fluid velocity measurements are attempted rather than particle velocity measurements, the particles still have to satisfy well known size requirements that are flow, fluid and particle density dependent.The experimental study employs a combined optical system for simultaneous measurements of particle velocity, particle size and particle concentration. The system is used to measure those particles of a spectrum of oil droplets that contribute to the validated signal output of counter and transient recorder based LDA-electronic signal processing systems.