Influence of self-adaptive hairy flaps on the stall delay of an airfoil in ramp-up motion

It is known in the case of some birds that the coverts on the upper side of their wings pop-up under critical flight conditions such as the landing approach, thus acting like a brake on the spread of flow separation. Taking experimental investigations as its basis, this paper deals with the influence of various configurations of self-adaptable hairy flaplets located on the lower half of the wing and with chord-length c (dense rows of slender elastomeric flaps, L=0.05c, 0.1c, 0.2c) on the flow around an NACA0020 airfoil at low Reynolds number flow (Re=77×103). Flow evolution along the airfoil when in ramp-up motion (α0=0, αs=20°, reduced frequency k=0.12) was measured with and without hairy flaps, with growth in the chord-normal thickness of the separation region above the airfoil investigated in order to determine stall onset time Ts. Whereas small flaps with L=0.05c do not change the overall stall process, it was possible to use configurations with L=0.1c (double-row, triple-row configuration) to delay stall onset Ts by a factor of around 2-4 when compared with the clean airfoil. The motion of the flaps and the flow field were measured simultaneously at high temporal resolution using high-speed PIV. Correlation between flap motion and velocity distribution showed that backflow induced by vortex structures is indeed prevented by the hairy flaps. A significant difference was identified in the shear-layer roll-up process, which was almost regular and locked with the fundamental frequency on the covered airfoil with no signs of non-linear growth over longer periods. By way of contrast, in the case of the clean airfoil the early merging of the shear-layer vortices and a rapid increase in the thickness of the separation region were observed. It is therefore concluded that mode locking is achieved between flap rows with an interspacing of 0.15c-0.2c, while the fundamental shear-layer roll-up wavelength measured (λ0≈0.15c-0.2c) indicates the relevance of flap row arrangement at the specific Reynolds number. Furthermore, interaction between shear-layer vortices and flaps in the row furthest downstream leads to the beneficial modification of the trailing edge flow in a way which increases bound circulation. © 2014 Elsevier Ltd

IMPACTS OF ENVIRONMENTAL VARIABLES ON DEER MOUSE SURVIVAL IN THE SOUTHWESTERN UNITED STATES

North American deer mice (Peromyscus maniculatus) are the primary reservoir for Sin Nombre orthohantavirus, and they play a significant role in the maintenance and transmission of disease across the landscape. Vital rates, such as survival, are a key component to understanding how disease spreads in a population. Understanding environmental factors that influence survival may allow for development of a predictive model that can assess disease risk in deer mice and, thus, a corresponding increased disease risk for humans. Our work explored the relationship between deer mouse survival and environmental variables at three long-term small mammal trapping sites in the United States Southwest. Using Bayesian variable selection, we assessed support for normalized difference vegetation index, precipitation, temperature minima and maxima, and snow-water equivalent at various time lags. From the selection process we formulated a robust-design capture-mark-recapture model in a Bayesian framework to quantify the effect of the selected variables on deer mouse survival. We found that survival varied by location and no one set of variables best explained survival across all sites. Consistencies between sites indicate that survival of deer mice follows a seasonal trend and does not vary by sex. Some of our results contrast previous work focused on use of environmental variables to predict deer mouse abundance and did not provide a consistent finding around which to formulate a predictive model. Future modeling efforts should focus on assessing both survival and reproduction as well as consideration of a more place-specific approach that includes additional variables that influence survival in different ecological contexts

The [alpha/Fe] Ratios in Dwarf Galaxies: Evidence for a Non-universal Stellar Initial Mass Function?

It is well established that the [alpha/Fe] ratios in elliptical galaxies increase with galaxy mass. This relation holds also for early-type dwarf galaxies, although it seems to steepen at low masses. The [alpha/Fe] vs. mass relation can be explained assuming that smaller galaxies form over longer timescales (downsizing), allowing a larger amount of Fe (mostly produced by long-living Type Ia Supernovae) to be released and incorporated into newly forming stars. Another way to obtain the same result is by using a flatter initial mass function (IMF) in large galaxies, increasing in this way the number of Type II Supernovae and therefore the production rate of alpha-elements. The integrated galactic initial mass function (IGIMF) theory predicts that the higher the star formation rate, the flatter the IMF. We have checked, by means of semi-analytical calculations, that the IGIMF theory, combined with the downsizing effect (i.e. the shorter duration of the star formation in larger galaxies), well reproduces the observed [alpha/Fe] vs. mass relation. In particular, we show a steepening of this relation in dwarf galaxies, in accordance with the available observations.Comment: 4 pages, 2 figures; to appear in the proceedings of the JENAM 2010 Symposium on Dwarf Galaxies (Lisbon, September 9-10, 2010

Star Formation in a Cosmological Simulation of Reionization

We study the luminosity functions of high-redshift galaxies in detailed hydrodynamic simulations of cosmic reionization, which are designed to reproduce the evolution of the Lyman-alpha forest between z=5 and z=6. We find that the luminosity functions and total stellar mass densities are in agreement with observations when plausible assumptions about reddenning at z=6 are made. Our simulations support the conclusion that stars alone reionized the universe.Comment: Accepted for publication in Ap

Escaping stars from young low-N clusters

With the use of N-body calculations the amount and properties of escaping stars from low-N (N = 100 and 1000) young embedded star clusters prior to gas expulsion are studied over the first 5 Myr of their existence. Besides the number of stars also different initial radii and binary populations are examined as well as virialised and collapsing clusters. It is found that these clusters can loose substantial amounts (up to 20%) of stars within 5 Myr with considerable velocities up to more than 100 km/s. Even with their mean velocities between 2 and 8 km/s these stars will still be travelling between 2 and 30 pc during the 5 Myr. Therefore can large amounts of distributed stars in star-forming regions not necessarily be counted as evidence for the isolated formation of stars.Comment: 10 pages, 10 figures, accepted for publication by MNRA