Large Eddy Simulation of the aerodynamic effects of trees on pollutant concentrations in street canyons

AbstractThis paper presents numerical simulations of the aerodynamic effects of trees on the flow field and dispersion of trafficoriginated pollutants in an urban street canyon of W/H=1 with a perpendicular approach flow. Large Eddy Simulation (LES) is employed for the investigation and is validated against wind tunnel (WT) experiment. Comparisons is made between an empty street canyon and one containing avenue-like tree planting of pore volume, Pvol=96%. In the presence of trees, both measurements and simulations show considerably larger pollutant concentrations near the leeward wall and slightly lower concentrations near the windward wall in comparison to the tree-free case

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

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

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

Laser induced fluorescence - particle tracking velocimetry (lif-ptv) measurements of water flow through snow

LIF-PTV-measurements of the micro-scale water flow through the pore space of a wet snow sample driven by either gravitational or capillary forces are presented. For the measurements, fluorescent micron-sized particles in the water are illuminated with a laser light sheet and tracked with a high-speed camera. The results show the existence of a potential preferential flow path and a loop flow in a pore space in case of a gravity driven flow. Generally, the water flow is found to be highly 3-dimensional. The average flow velocities in the pore spaces are 11.3 mm/s for the gravity driven flow and 9.6 mm/s for the upward flow driven by capillary forces. Flow acceleration and deceleration was stronger for the gravity driven flow with particle decelerations stronger than accelerations in both cases

Aerodynamic Roughness Length of Fresh Snow

This study presents the results from a series of wind-tunnel experiments designed to investigate the aerodynamic roughness length z 0 of fresh snow under no-drift conditions. A two-component hot-film anemometer was employed to obtain vertical profiles of velocity statistics in a zero pressure gradient turbulent boundary layer for flow over naturally deposited snow surfaces. The roughness of these snow surfaces was measured by means of digital photography to capture characteristic length scales that can be related to z 0. Our results show that, under aerodynamically rough conditions, the mean value of the roughness length for fresh snow is $${\langle{z}_{0}\rangle= 0.24}$$ mm with a standard deviation σ(z 0)= 0.05 mm. In this study, we show that variations in z 0 are associated with variations in the roughness geometry. The roughness measurements suggest that the estimated values of z 0 are consistent with the presence of irregular roughness structures that develop during snowfalls that mimic ballistic deposition processe