Effect of urban morphology on wind condition in idealized city models

Wind conditions in urban environments are important for a number of reasons. They can serve to transport air pollutants out of the urban environment and to moderate urban microclimatic conditions if satisfactory, yet can compromise pedestrian comfort and safety if not. We aim to study experimentally and numerically the effects of urban morphology (e.g., overall city form (skyline), street orientation, and street configuration) on wind conditions in cities. This report considers our initial investigations of two idealized city forms that are coincidentally similar to ancient Roman cities that were organized on one or two primary streets – a main north–south street, the cardus maximus, and a secondary east–west street, the decumanus maximus – and contained within a well-defined perimeter.We first consider round and square city models with one main street set parallel to the approaching wind and a secondary street producing an intersection at city centre. Not surprisingly, wind conditions in the two city models are dissimilar due to their shape differences. We then consider a long rectangular city model with a fully developed steady flow region along the main street. If the main street of the round city model is narrow, the parallel approaching wind cannot blow through the entire street and a penetrating inflow exists at the leeward opening. For the round city model with two crossing streets, a slightly non-parallel wind to the main street generates a stronger wind level in the entire street volume.     

Actors and arenas in hybrid networks: implications for environmental policymaking in the Baltic Sea region

Policymaking within and among states is under pressure for change. One feature of this change is empirically observed as an activation of different network structures in the Baltic Sea Region, especially since the collapse of the Iron Curtain, the initiation of the Rio process, and the enlargement of the European Union. The contemporary theoretical debates about governance highlight the changing conditions for policymaking and implementation on all societal levels. This process of change, especially evident concerning environmental policies, includes new types of networks crossing state borders both at the supranational and the subnational levels. This article illuminates this process of change with empirical data from the project "Governing a Common Sea" (GOVCOM) within the Baltic Sea Research Program (BIREME).     

Direct measurements of the drag force over aligned arrays of cubes exposed to boundary-layer flows

Wind tunnel measurements of the total drag force for aligned arrays of cubes exposed to two different boundary-layer flows at three flow velocities are discussed. The drag force for eight different building packing densities λ _p (from 0.028 to 1) is measured with a standard load cell generating a novel dataset. Different λ _p are reproduced by increasing the number of buildings on the same lot area; this represents a real situation that an urban planner is faced with when a lot area of a given (fixed) size is allocated to the development of new built areas. It is assumed that the surrounding terrain is uniform and there is a transition from a given roughness (smooth) to a new roughness (rough). The approaching flow will adjust itself over the new surface within a distance that in general may be larger than the horizontal length covered by the array. We investigate the region where the flow adjustment occurs. The wide range of packing densities allowed us to analyse in detail the evolution of the drag force. The drag force increases with increasing packing densities until it reaches a maximum at an intermediate packing density (λ _p  = 0.25 in our case) followed by a slight decrease at larger packing densities. The value of the drag force depends on the flow adjustment along the array which is evaluated by introducing the parameter “drag area” to retrieve information about the drag distribution at different λ _p . Results clearly suggest a change of the distribution of the drag force, which is found to be relatively uniform at low packing densities, while most of the force acts on first rows of the arrays at large packing densities. The drag area constitutes the basis for the formulation of a new adjustment length scale defined as the ratio between the volume of the air within the array and the drag area. The proposed adjustment length scale automatically takes into account the change in drag distribution along the array for a better parameterization of urban effects in dispersion models.     

City breathability and its link to pollutant concentration distribution within urban-like geometries

This paper is devoted to the study of pollutant concentration distribution within urban-like geometries. By applying efficiency concepts originally developed for indoor environments, the term ventilation is used as a measure of city “breathability”. It can be applied to analyse pollutant removal within a city in operational contexts. This implies the evaluation of the bulk flow balance over the city and of the mean age of air. The influence of building packing density on flow and pollutant removal is, therefore, evaluated using those quantities. Idealized cities of regular cubical buildings were created with packing density ranging from 6.25% to 69% to represent configurations from urban sprawl to compact cities. The relative simplicity of these arrangements allowed us to apply the Computational Fluid Dynamics (CFD) flow and dispersion simulations using the standard k–? turbulence model. Results show that city breathability within the urban canopy layer is strongly dependent from the building packing density. At the lower packing densities, the city responds to the wind as an agglomeration of obstacles, at larger densities (from about 44%) the city itself responds as a single obstacle. With the exception of the lowest packing density, airflow enters the array through lateral sides and leaves throughout the street top and flow out downstream. The air entering through lateral sides increases with increasing packing density.At the street top of the windward side of compact building configurations, a large upward flow is observed. This vertical transport reduces over short distance to turn into a downward flow further downstream of the building array. These findings suggest a practical way of identifying city breathability. Even though the application of these results to real scenarios require further analyses the paper illustrates a practical framework to be adopted in the assessment of the optimum neighbourhood building layout to minimize pollution levels.     

Emissions from Slash Burning and the Influence of Flame Retardant Chemicals

Pollutants sampled during the burning of 30 lb ponderosa pine fuel beds yielded emission factors for CO, hydrocarbon gases, and par-ticulate matter of 146, 8.4, and 9.1 lb/ton of fuel, respectively. When similar beds were treated with diammonium phosphate flame retardant, these factors increased to 166, 11.7, and 19.3 lb/ton of fuel, respectively.

Gas chromatographic analysis of hydrocarbon gases showed that 15-40% of this material was composed of methane and eth-ylene. Ethane and acetylene were the next most abundant materials, with photochemically important materials constituting minor portions of this gaseous component. Fuel beds treated with flame retardant produced more oiefins, and this production lasted throughout the smoldering phase of burning.

These tests showed that the smoldering phase of combustion is of major importance to air pollutant production during slash burning. The initial 80% of the fuel burned accounted for only 20-30% of HC and CO emissions. This suggests that a rapid mop-up of slash burns could substantially reduce air pollutant production.     

Does oxygen deficiency modify the functional response of Saduria entomon (Isopoda) to Bathyporeia pilosa (Amphipoda)?

The functional response of the predatory isopod Saduria entomon to the prey amphipod Bathyporeia pilosa was measured in normoxia (95% O₂ saturation), moderate hypoxia (45% O₂ saturation) and hypoxia (35% O₂ saturation) in aquarium experiments. The prey densities tested ranged from 400 to 8000 ind m⁻². Prey density influenced consumption rates of S. entomon in normoxia and 45% O₂ saturation, but there was no difference between consumption rates at these two oxygen levels. Nevertheless the form of functional response differed. In normoxia S. entomon showed a positively density-dependent functional response to B. pilosa, indicating a potentially stabilizing effect on the prey population. In moderate hypoxia the variance in consumption increased, decreasing the statistical power to distinguish between response models. The functional response of S. entomon in moderate hypoxia was best described with a density-independent response, characterized as destabilizing for the prey population. In hypoxia (35% O₂) predation by S. entomon did not respond to increasing prey density, as almost no amphipods were eaten at this oxygen level. The results are discussed in terms of the usability of theoretical models to examine predator–prey relationships in stressful environments. Received: 26 April 1997 / Accepted: 20 May 1997     

Sunset orientation of robins,Erithacus rubecula,with different fields of sky vision

Summary Migratory orientation of robins (Erithacus rubecula) at sunset was recorded using orientation cages, under clear autumn skies. The aim of the experiments was to examine the importance of different visible sky sections for the orientation of robins. I obtained the following results: (1) Robins tested with the visible sky section limited to 90° around zenith (45° above the horizon) showed a mean orientation that coincided with the average sunset azimuth, with little scatter around the mean angle (Fig. 2). (2) When the birds were allowed a more extensive field of sky vision (maximum 160°), they chose headings on an approximate north-south axis, significantly different from tests with a restricted view of the sky (Fig. 3). (3) Experiments were also performed in which the response of robins to a mirror deflection (about 120° counterclockwise) of visual cues in the lower parts of the sunset sky was examined. The outcome indicated that visual information in the lower part of the sky may be critical for the orientation of robins (Fig. 5). These results, together with recent findings that robins captured and tested at two nearby sites show distinctly different orientation behavior in relation to experimental manipulations of the magnetic field, suggest that priorities among orientation cues may differ depending on the migratory situation encountered.