1,053 research outputs found
Theoretical study of the transonic lift of a double-wedge profile with detached bow wave
A theoretical study is described of the aerodynamic characteristics at small angle of attack of a thin, double-wedge profile in the range of supersonic flight speed in which the bow wave is detached. The analysis is carried out within the framework of the transonic (nonlinear) small-disturbance theory, and the effects of angle of attack are regarded as a small perturbation on the flow previously calculated at zero angle. The mixed flow about the front half of the profile is calculated by relaxation solution of a suitably defined boundary-value problem for transonic small-disturbance equation in the hodograph plane (i.e., the Tricomi equation). The purely supersonic flow about the rear half is found by an extension of the usual numerical method of characteristics. Analytical results are also obtained, within the framework of the same theory, for the range of speed in which the bow wave is attached and the flow is completely supersonic
Transonic flow past a wedge profile with detached bow wave
A theoretical study has been made of the aerodynamic characteristics at zero angle of attack of a thin, doubly symmetrical double-wedge profile in the range of supersonic flight speed in which the bow wave is detached. The analysis utilizes the equations of the transonic small-disturbance theory and involves no assumptions beyond those implicit in this theory. The mixed flow about the front half of the profile is calculated by relaxation solution of boundary conditions along the shock polar and sonic line. The purely subsonic flow about the rear of the profile is found by means of the method of characteristics specialized to the transonic small-disturbance theory. Complete calculations were made for four values of the transonic similarity parameter. These were found sufficient to bridge the gap between the previous results of Guderley and Yoshihara at a Mach number of 1 and the results which are readily obtained when the bow wave is attached and the flow is completely supersonic
Novel Josephson effects between multi-gap and single-gap superconductors
Multi-gap superconductors can exhibit qualitatively new phenomena due to
existence of multiple order parameters. Repulsive electronic interactions may
give rise to a phase difference of between the phases of the order
parameters. Collective modes due to the oscillation of the relative phases of
these order parameters are also possible. Here we show that both these
phenomena are observable in Josephson junctions between a single-gap and a
multi-gap superconductor. In particular, a non-monotonic temperature dependence
of the Josephson current through the junction reveals the existence of the
phase differences in the multi-gap superconductor. This mechanism may be
relevant for understanding several experiments on the Josephson junctions with
unconventional superconductors. We also discuss how the presence of the
collective mode resonantly enhances the DC Josephson current when the voltage
across the junction matches the mode frequency. We suggest that our results may
apply to MgB, 2H-NbSe, spin ladder and bilayer cuprates.Comment: 4 pages, 2 figure
Hyperbolic Balance Laws with a Non Local Source
This paper is devoted to hyperbolic systems of balance laws with non local
source terms. The existence, uniqueness and Lipschitz dependence proved here
comprise previous results in the literature and can be applied to physical
models, such as Euler system for a radiating gas and Rosenau regularization of
the Chapman-Enskog expansion.Comment: 26 page
Changes in growth of tropical forests: evaluating potential biases
Over the past century almost every ecosystem on Earth has come under the
influence of changes in atmospheric composition and climate caused by human activity.
Tropical forests are among the most productive and extensive ecosystems, and it has been
hypothesized that both the dynamics and biomass of apparently undisturbed, old-growth
tropical forests have been changing in response to atmospheric changes. Long-term forest
sample plots are a critical tool in detecting and monitoring such changes, and our recent
analysis of pan-tropical-forest plot data has suggested that the biomass of tropical forests
has been increasing, providing a modest negative feedback on the rate of accumulation of
atmospheric CO2. However, it has been argued that some of these old forest plot data sets
have significant problems in interpretation because of the use of nonstandardized methodologies.
In this paper we examine the extent to which potential field methodological errors may
bias estimates of total biomass change by detailed examination of tree-by-tree records from
up to 120 Neotropical plots to test predictions from theory. Potential positive biases on
measurements of biomass change include a bias in site selection, tree deformities introduced
by the measurement process, poor methodologies to deal with tree deformities or buttresses,
and nonrecording of negative growth increments. We show that, while it is important to
improve and standardize methodologies in current and future forest-plot work, any systematic
errors introduced by currently identified biases in past studies are small and calculable.
We conclude that most tropical-forest plot data are of useful quality, and that the evidence
does still weigh conclusively in favor of a recent increase of biomass in old-growth tropical
forests
Correlated defects, metal-insulator transition, and magnetic order in ferromagnetic semiconductors
The effect of disorder on transport and magnetization in ferromagnetic III-V
semiconductors, in particular (Ga,Mn)As, is studied theoretically. We show that
Coulomb-induced correlations of the defect positions are crucial for the
transport and magnetic properties of these highly compensated materials. We
employ Monte Carlo simulations to obtain the correlated defect distributions.
Exact diagonalization gives reasonable results for the spectrum of valence-band
holes and the metal-insulator transition only for correlated disorder. Finally,
we show that the mean-field magnetization also depends crucially on defect
correlations.Comment: 4 pages RevTeX4, 5 figures include
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