49,550 research outputs found
Optimization of rotor blades for combined structural, performance, and aeroelastic characteristics
The strategies whereby helicopter rotor blades can be optimized for combined structural, inertial, dynamic, aeroelastic, and aerodynamic performance characteristics are outlined. There are three key ingredients in the successful execution of such an interdisciplinary optimization. The first is the definition of a satisfactory performance index that combines all aspects of the problem without too many constraints. The second element is the judicious choice of computationally efficient analysis tools for the various quantitative components in both the cost functional and constraints. The third element is an effective strategy for combining the various disciplines either in parallel or sequential optimizations
Evidence for a Spectroscopic Sequence Among SNe Ia
In this Letter we present evidence for a spectral sequence among Type Ia
supernovae (SNe Ia). The sequence is based on the systematic variation of
several features seen in the near-maximum light spectrum. This sequence is
analogous to the recently noted photometric sequence among SNe Ia which shows a
relationship between the peak brightness of a SN Ia and the shape of its light
curve. In addition to the observational evidence we present a partial
theoretical explanation for the sequence. This has been achieved by producing a
series of non-LTE synthetic spectra in which only the effective temperature is
varied. The synthetic sequence nicely reproduces most of the differences seen
in the observed one and presumably corresponds to the amount of 56Ni produced
in the explosion.Comment: To appear in Astrophysical Journal Letters, uuencoded, gzipped
postscript file, also available from http://www.nhn.uoknor.edu/~baron
Influence of Potamogeton crispus growth on nutrients in the sediment and water of Lake Tangxunhu
An incubation experiment was performed on Potamogeton crispus (P. crispus) using sediment collected from Lake Tangxunhu in the center of China, in order to determine the effects of plant growth on Fe, Si, Cu, Zn, Mn, Mg, P, and Ca concentrations in the sediments and overlying waters. After 3 months of incubation, Ca, Mg, and Si concentrations in the water column were significantly lower, and P and Cu concentrations were significantly higher than in unplanted controls. The effect of P. crispus growth on sediment pore waters and water-extractable elements varied. Concentrations of Ca, Mg, Si, Fe, Cu, and Zn were significantly higher, and P was significantly lower, than in pore waters of the control. Water-extracted concentrations of Fe, Mg, and Si in the sediments were lower, and P was higher, than in the control. Presence of P. crispus generally enhanced concentration gradients of elements between pore waters and overlying waters but not for P. The growth of P. crispus was associated with an increase in water pH and formation of root plaques, resulting in complex effects on the sediment nutritional status
A Handheld low-mass, impact instrument to measure nondestructive firmness of fruit
A portable, handheld impact firmness sensor was designed for nondestructive measurement of fruit firmness while the fruit remain attached to the tree or for use in other remote locations where the use of a benchtop instrument would be impractical. The instrument design was based on the low-mass, constant velocity, impact-type measurement concept. Validation tests of the handheld sensor using `Bartlett' pears from orchards in California and Washington showed excellent agreement (r2 = 0.92 and 0.96, respectively) with both ASAE Standard method S368.2 for determining the apparent modulus of intact fruit and the impact firmness scores from a commercial benchtop impact firmness instrument
Transcription factor search for a DNA promoter in a three-states model
To ensure fast gene activation, Transcription Factors (TF) use a mechanism
known as facilitated diffusion to find their DNA promoter site. Here we analyze
such a process where a TF alternates between 3D and 1D diffusion. In the latter
(TF bound to the DNA), the TF further switches between a fast translocation
state dominated by interaction with the DNA backbone, and a slow examination
state where interaction with DNA base pairs is predominant. We derive a new
formula for the mean search time, and show that it is faster and less sensitive
to the binding energy fluctuations compared to the case of a single sliding
state. We find that for an optimal search, the time spent bound to the DNA is
larger compared to the 3D time in the nucleus, in agreement with recent
experimental data. Our results further suggest that modifying switching via
phosphorylation or methylation of the TF or the DNA can efficiently regulate
transcription.Comment: 4 pages, 3 figure
Large eddy simulation and laboratory experiments on the decay of grid wakes in strongly stratified flows
A detailed analysis of the flow structure resulting from the combination
of turbulence and internal waves is carried out and visualized by means
of the Schlieren method on waves in a strongly stratified fluid at the Laboratory of the IPM in Moscow. The joint appearance of the more regular internal wave oscillations and the small-scale turbulence that is confined vertically to the Ozmidov length scale favours the use of a simple geometrical analysis to investigate their time-space span and evolution. This provides useful information on the collapse of internal wave breaking processes in the ocean and the atmosphere. The measurements were performed under a variety of linear stratifications and different grid forcing scales, combining the grid wake and velocity shear. A numerical simulation using LES on the passage of a single bar in a linearly stratified fluid medium has been compared with the experiments identifying the different influences of the environmental
agents on the actual effective vertical diffusion of the wakes. The equation of state, which connects the density and salinity, is assumed to be linear, with the coefficient of the salt contraction being included into the definition of salinity or heat. The characteristic internal waves as well as the entire beam width are related to the diameter of the bar, the Richardson number and the peak-to-peak value of oscillations. The ultimate frequency of the infinitesimal periodic internal waves is limited by the maximum buoyancy frequency relating the decrease in the vertical scale with the anisotropy of the velocity turbulent r.m.s. velocity.Peer ReviewedPreprin
Near-surface remote sensing of spatial and temporal variation in canopy phenology
There is a need to document how plant phenology is responding to global change factors, particularly warming trends. âNear-surfaceâ remote sensing, using radiometric instruments or imaging sensors, has great potential to improve phenological monitoring because automated observations can be made at high temporal frequency. Here we build on previous work and show how inexpensive, networked digital cameras (âwebcamsâ) can be used to document spatial and temporal variation in the spring and autumn phenology of forest canopies. We use two years of imagery from a deciduous, northern hardwood site, and one year of imagery from a coniferous, boreal transition site. A quantitative signal is obtained by splitting images into separate red, green, and blue color channels and calculating the relative brightness of each channel for âregions of interestâ within each image. We put the observed phenological signal in context by relating it to seasonal patterns of gross primary productivity, inferred from eddy covariance measurements of surfaceâatmosphere CO2 exchange. We show that spring increases, and autumn decreases, in canopy greenness can be detected in both deciduous and coniferous stands. In deciduous stands, an autumn red peak is also observed. The timing and rate of spring development and autumn senescence varies across the canopy, with greater variability in autumn than spring. Interannual variation in phenology can be detected both visually and quantitatively; delayed spring onset in 2007 compared to 2006 is related to a prolonged cold spell from day 85 to day 110. This work lays the foundation for regional- to continental-scale camera-based monitoring of phenology at network observatory sites, e.g., National Ecological Observatory Network (NEON) or AmeriFlux
Fire-induced Albedo Change and Its Radative Forcing at the Surface in Northern Austrailia
This paper investigates the impact of fire on surface albedo and the associated radiative forcing over 56% of continental Australia encompassing the fire-prone northern tropical savanna. Fire-affected areas and albedos are derived for the 2003 fire season using daily Moderate Resolution Imaging Spectroradiometer (MODIS) surface reflectance data. Near-infrared and total shortwave albedos are observed to generally decrease after fire occurrence. Regionally, the total shortwave albedo drops by an average of 0.024, with increasing reductions as the dry season progresses and larger reductions in grasslands than woody savannas. These fire-induced albedo changes exert a positive forcing at the surface that increases from March to November. A mean ââinstantaneousââ shortwave surface radiative forcing of 0.52 Wm -2 is estimated
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