There are three common methods for calculating the lift generated by a flying animal based on the measured airflow in the wake. However, these methods might not be accurate according to computational and robot-based studies of flapping wings. Here we test this hypothesis for the first time for a slowly flying Pacific parrotlet in still air using stereo particle image velocimetry recorded at 1000 Hz. The bird was trained to fly between two perches through a laser sheet wearing laser safety goggles. We found that the wingtip vortices generated during mid-downstroke advected down and broke up quickly, contradicting the frozen turbulence hypothesis typically assumed in animal flight experiments. The quasi-steady lift at mid-downstroke was estimated based on the velocity field by applying the widely used Kutta-Joukowski theorem, vortex ring model, and actuator disk model. The calculated lift was found to be sensitive to the applied model and its different parameters, including vortex span and distance between the bird and laser sheet-rendering these three accepted ways of calculating weight support inconsistent. The three models predict different aerodynamic force values mid-downstroke compared to independent direct measurements with an aerodynamic force platform that we had available for the same species flying over a similar distance. Whereas the lift predictions of the Kutta-Joukowski theorem and the vortex ring model stayed relatively constant despite vortex breakdown, their values were too low. In contrast, the actuator disk model predicted lift reasonably accurately before vortex breakdown, but predicted almost no lift during and after vortex breakdown. Some of these limitations might be better understood, and partially reconciled, if future animal flight studies report lift calculations based on all three quasi-steady lift models instead. This would also enable much needed meta studies of animal flight to derive bioinspired design principles for quasi-steady lift generation with flapping wings.

The built environment surrounding arterials affects the dispersion of vehicular emissions in urban areas, modifying the potential risks to public health. In order to study the influence of urban morphometry on flow and dispersion of vehicular fine particulate matter emissions, in the summer of 2008 field measurements were performed in major arterials located in five Southern Californian cities with different building geometries. In each city, local mean wind, turbulence, virtual temperature, roadside DustTrak Fine Particles (DTFP) concentration, and traffic flow data were collected in 2-hr measurement periods during morning and evening rush hours and lighter midday traffic, over a period of 3 days. The calculated Monin-Obukhov length, L, suggests that near-neutral and slightly unstable conditions were present at both street and roof levels. The nondimensional forms of turbulent wind and temperature fluctuations show that the data at street level within the urban canopy can be represented using the Monin-Obukhov similarity theory. Generalized additive models were applied to analyze the impact of meteorological and traffic-related variables on fine particle concentrations at street level. Compared to other variables, urban-scale background concentrations were the most important variables in all five models. The results confirmed that turbulent mixing in urban areas dominated the variation of roadside particle concentrations regardless of urban geometry. The distance from the local sites to the nearby monitoring stations affected model performance when urban-scale concentrations were used to predict middle-scale concentrations by generalized additive models (GAMs). A radius of influence for background concentrations was 6-10 km. There were also relationships between concentration and other variables affecting the local components of the concentrations, such as wind direction, sensible heat flux, and vertical wind fluctuation, although the influences were much weaker.Implications: The built environment surrounding major arterials affects the dispersion of vehicular emissions in urban areas, modifying the potential risks to public health. Dispersion of pollutants within the urban canopy is governed by flow and turbulence characteristics caused by building morphometry. Current dispersion models used for regulatory purposes have difficulties simulating the flow and dispersion for complex building cases, especially when fine resolution is needed. Urban planning strategies, such as limitation of building height, pedestrian-friendly community design, or zoning of building structures, modify concentrations of vehicular emissions in built environments surrounding major arterials, which may modify health risks for adjacent communities. IntroductionIn metropolitan cities, vehicular emissions are in close proximity to pedestrians, residences, and local businesses. Current line-source models based on the Gaussian diffusion equation are commonly applied to evaluate the air quality impact of freeways or highways passing through a...