13,916 research outputs found
Radar exploration of Venus
Spacecraft radar systems for exploration of Venu
Quenching of the beam-plasma instability by 3-D spectra of large scale density fluctuations
A model is presented to explain the highly variable yet low level of Langmuir waves measured in situ by spacecraft when electron beams associated with Type III solar bursts are passing by; the low level of excited waves allows the propagation of such streams from the Sun to well past 1 AU without catastrophic energy losses. The model is based, first, on the existence of large scale density fluctuations that are able to efficiently diffuse small k beam unstable Langmuir waves in phase space, and, second, on the presence of a significantly isotropic nonthermal tail in the distribution function of the background electron population, which is capable of stabilizing larger k modes. The strength of the model lies in its ability to predict various levels of Langmuir waves depending on the parameters. This feature is consistent with the high variability actually observed in the measurements
Flight Flutter Testing of the P6M
On the P6M the shake behavior, i.e., the response to random excitation at subcritical speeds of lowly damped airplane modes, is as important as the actual flutter speed. The approach is to first study the problem by means of analyses and wind-tunnel tests. These predictions are compared with flight test data obtained by spectral analysis of tape recordings of the airplane vibration responses to random aerodynamic turbulence. A similar spectrum analysis approach was used in high speed wind-tunnel tests. A resonance excitation technique was developed for low speed wind-tunnel testing, and well defined V-g curves were obtained. The effect of various parameters on both shake and flutter of T-tails with and without dihedral were studied. Preliminary flight tests yielded good correlation; they also yielded interesting information concerning a low frequency transonic snaking mode, and excitation by shed vortices
Buneman instability in a magnetized current-carrying plasma with velocity shear
Buneman instability is often driven in magnetic reconnection. Understanding
how velocity shear in the beams driving the Buneman instability affects the
growth and saturation of waves is relevant to turbulence, heating, and
diffusion in magnetic reconnection. Using a Mathieu-equation analysis for weak
cosine velocity shear together with Vlasov simulations, the effects of shear on
the kinetic Buneman instability are studied in a plasma consisting of strongly
magnetized electrons and cold unmagnetized ions. In the linearly unstable
phase, shear enhances the coupling between oblique waves and the sheared
electron beam, resulting in a wider range of unstable eigenmodes with common
lower growth rates. The wave couplings generate new features of the electric
fields in space, which can persist into the nonlinear phase when electron holes
form. Lower hybrid instabilities simultaneously occur at
with a much lower growth
rate, and are not affected by the velocity shear.Comment: Accepted by Physics of Plasm
Evaluation of the cardiovascular system during various circulatory stresses Progress report, 1 Sep. 1968 - 1 May 1969
Cardiac response to chemotherapy after myocardial infraction and diagnostic methods of heart disease in man and animal
A computer program for the calculation of laminar and turbulent boundary layer flows
The results are presented of a study to produce a computer program to calculate laminar and turbulent boundary layer flows. The program is capable of calculating the following types of flow: (1) incompressible or compressible, (2) two dimensional or axisymmetric, and (3) flows with significant transverse curvature. Also, the program can handle a large variety of boundary conditions, such as blowing or suction, arbitrary temperature distributions and arbitrary wall heat fluxes. The program has been specialized to the calculation of equilibrium air flows and all of the thermodynamic and transport properties used are for air. For the turbulent transport properties, the eddy viscosity approach has been used. Although the eddy viscosity models are semi-empirical, the model employed in the program has corrections for pressure gradients, suction and blowing and compressibility. The basic method of approach is to put the equations of motion into a finite difference form and then solve them by use of a digital computer. The program is written in FORTRAN 4 and requires small amounts of computer time on most scientific machines. For example, most laminar flows can be calculated in less than one minute of machine time, while turbulent flows usually require three or four minutes
Multi-beam Energy Moments of Multibeam Particle Velocity Distributions
High resolution electron and ion velocity distributions, f(v), which consist
of N effectively disjoint beams, have been measured by NASA's Magnetospheric
Multi-Scale Mission (MMS) observatories and in reconnection simulations.
Commonly used standard velocity moments generally assume a single
mean-flow-velocity for the entire distribution, which can lead to
counterintuitive results for a multibeam f(v). An example is the (false)
standard thermal energy moment of a pair of equal and opposite cold particle
beams, which is nonzero even though each beam has zero thermal energy. By
contrast, a multibeam moment of two or more beams has no false thermal energy.
A multibeam moment is obtained by taking a standard moment of each beam and
then summing over beams. In this paper we will generalize these notions,
explore their consequences and apply them to an f(v) which is sum of
tri-Maxwellians. Both standard and multibeam energy moments have coherent and
incoherent forms. Examples of incoherent moments are the thermal energy
density, the pressure and the thermal energy flux (enthalpy flux plus heat
flux). Corresponding coherent moments are the bulk kinetic energy density, the
RAM pressure and the bulk kinetic energy flux. The false part of an incoherent
moment is defined as the difference between the standard incoherent moment and
the corresponding multibeam moment. The sum of a pair of corresponding coherent
and incoherent moments will be called the undecomposed moment. Undecomposed
moments are independent of whether the sum is standard or multibeam and
therefore have advantages when studying moments of measured f(v).Comment: 27 single-spaced pages. Three Figure
Characterizing the Hofstadter butterfly's outline with Chern numbers
In this work, we report original properties inherent to independent particles
subjected to a magnetic field by emphasizing the existence of regular
structures in the energy spectrum's outline. We show that this fractal curve,
the well-known Hofstadter butterfly's outline, is associated to a specific
sequence of Chern numbers that correspond to the quantized transverse
conductivity. Indeed the topological invariant that characterizes the
fundamental energy band depicts successive stairways as the magnetic flux
varies. Moreover each stairway is shown to be labeled by another Chern number
which measures the charge transported under displacement of the periodic
potential. We put forward the universal character of these properties by
comparing the results obtained for the square and the honeycomb geometries.Comment: Accepted for publication in J. Phys. B (Jan 2009
Anomalous latent heat in non-equilibrium phase transitions
We study first-order phase transitions in a two-temperature system, where due
to the time-scale separation all the basic thermodynamical quantities (free
energy, entropy, etc) are well-defined. The sign of the latent heat is found to
be counterintuitive: it is positive when going from the phase where the
temperatures and the entropy are higher to the one where these quantities are
lower. The effect exists only out of equilibrium and requires conflicting
interactions. It is displayed on a lattice gas model of ferromagnetically
interacting spin-1/2 particles.Comment: 4 pages, 2 figure
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