Efficient cruising for swimming and flying animals is dictated by fluid drag

Many swimming and flying animals are observed to cruise in a narrow range of Strouhal numbers, where the Strouhal number ${St = 2fA/U}$ is a dimensionless parameter that relates stroke frequency $f$, amplitude $A$, and forward speed $U$. Dolphins, sharks, bony fish, birds, bats, and insects typically cruise in the range $0.2 < St < 0.4$, which coincides with the Strouhal number range for maximum efficiency as found by experiments on heaving and pitching airfoils. It has therefore been postulated that natural selection has tuned animals to use this range of Strouhal numbers because it confers high efficiency, but the reason why this is so is still unclear. Here, by using simple scaling arguments, we argue that the Strouhal number for peak efficiency is largely determined by fluid drag on the fins and wings.Comment: 10 page

Scaling the propulsive performance of heaving and pitching foils

Scaling laws for the propulsive performance of rigid foils undergoing oscillatory heaving and pitching motions are presented. Water tunnel experiments on a nominally two-dimensional flow validate the scaling laws, with the scaled data for thrust, power, and efficiency all showing excellent collapse. The analysis indicates that the behaviour of the foils depends on both Strouhal number and reduced frequency, but for motions where the viscous drag is small the thrust closely follows a linear dependence on reduced frequency. The scaling laws are also shown to be consistent with biological data on swimming aquatic animals.Comment: 11 page

Forces and energetics of intermittent swimming

Experiments are reported on intermittent swimming motions. Water tunnel experiments on a nominally two-dimensional pitching foil show that the mean thrust and power scale linearly with the duty cycle, from a value of 0.2 all the way up to continuous motions, indicating that individual bursts of activity in intermittent motions are independent of each other. This conclusion is corroborated by PIV flow visualizations, which show that the main vortical structures in the wake do not change with duty cycle. The experimental data also demonstrate that intermittent motions are generally energetically advantageous over continuous motions. When metabolic energy losses are taken into account, this conclusion is maintained for metabolic power fractions less than 1.Comment: 9 page

Building bridges: experiences and lessons learned from the implementation of INSPIRE and e-reporting of air quality data in Europe

The collection, exchange and use of air quality data require diverse monitoring, processing and dissemination systems to work together. They should supply data, which can afterwards be used in different contexts such as planning, population exposure and environmental impact assessment. As air quality is not dependant on national borders this would only be feasible on an international level. This manuscript reports on the lessons learned from using the world’s largest data harmonization effort for environmental information infrastructure - INSPIRE as a backbone of a European wide spatial data reporting system which involves an unprecedented number of actors and volumes of data. It is important in the context of Digital Earth, and the establishment of a global SDI through the Global Earth Observation System of Systems (GEOSS), as the quality of ambient air is among the pressing environmental problems of today. We summarize our findings from the perspective of national public authorities, obliged by law to transmit spatio-temporal data in order to streamline reporting and facilitate the use of information, while keeping public expenditure at minimum. To identify what works in this type of reporting we established a cross-border case study, looking at the process of harmonization and exchange of data in Belgium and the Netherlands based on interoperable standards. Our results cover the legal, semantic, technological and organizational aspects of reporting. They are relevant to a cross-thematic audience, having to undergo similar processes of reporting, such as climate change, but also environmental noise, marine, biodiversity, and water management.JRC.H.6-Digital Earth and Reference Dat

The evolution of large-scale motions in turbulent pipe flow

A dual-plane snapshot POD analysis of turbulent pipe flow at a Reynolds number of 104,000 is presented. The high-speed PIV data were simultaneously acquired in two planes, a crossstream plane (2D-3C) and a streamwise plane (2D-2C) on the pipe centerline. The cross-stream plane analysis revealed large structures with a spatio-temporal extent of 1-2R, where R is the pipe radius. The temporal evolution of these large-scale structures is examined using the timeshifted correlation of the cross-stream snapshot POD coefficients, identifying the low energy intermediate modes responsible for the transition between the large-scale modes. By conditionallyaveraging based on the occurrence/intensity of a given cross-stream snapshot POD mode, a complex structure consisting of wall-attached and detached large-scale structures is shown to be associated with the most energetic modes. There is a pseudo-alignment of these large structures, that together create structures with a spatio-temporal extent of about 6R, which appears to explain the formation of the very large scale motions previously observed in pipe flow