22,381 research outputs found
On the formation time scale and core masses of gas giant planets
Numerical simulations show that the migration of growing planetary cores may
be dominated by turbulent fluctuations in the protoplanetary disk, rather than
by any mean property of the flow. We quantify the impact of this stochastic
core migration on the formation time scale and core mass of giant planets at
the onset of runaway gas accretion. For standard Solar Nebula conditions, the
formation of Jupiter can be accelerated by almost an order of magnitude if the
growing core executes a random walk with an amplitude of a few tenths of an au.
A modestly reduced surface density of planetesimals allows Jupiter to form
within 10 Myr, with an initial core mass below 10 Earth masses, in better
agreement with observational constraints. For extrasolar planetary systems, the
results suggest that core accretion could form massive planets in disks with
lower metallicities, and shorter lifetimes, than the Solar Nebula.Comment: ApJL, in pres
A simple demonstration of corrosion cells
The objective is to reinforce and enhance the understanding of galvanic cells, anode cathode reactions and polarization phenomena. Complete instructions are given for laboratory demonstration to be performed by students
Magnetically driven accretion in protoplanetary discs
We characterize magnetically driven accretion at radii between 1 au and 100
au in protoplanetary discs, using a series of local non-ideal
magnetohydrodynamic (MHD) simulations. The simulations assume a Minimum Mass
Solar Nebula (MMSN) disc that is threaded by a net vertical magnetic field of
specified strength. Confirming previous results, we find that the Hall effect
has only a modest impact on accretion at 30 au, and essentially none at 100 au.
At 1-10 au the Hall effect introduces a pronounced bi-modality in the accretion
process, with vertical magnetic fields aligned to the disc rotation supporting
a strong laminar Maxwell stress that is absent if the field is anti-aligned. In
the anti-aligned case, we instead find evidence for bursts of turbulent stress
at 5-10 au, which we tentatively identify with the non-axisymmetric Hall-shear
instability. The presence or absence of these bursts depends upon the details
of the adopted chemical model, which suggests that appreciable regions of
actual protoplanetary discs might lie close to the borderline between laminar
and turbulent behaviour. Given the number of important control parameters that
have already been identified in MHD models, quantitative predictions for disc
structure in terms of only radius and accretion rate appear to be difficult.
Instead, we identify robust qualitative tests of magnetically driven accretion.
These include the presence of turbulence in the outer disc, independent of the
orientation of the vertical magnetic fields, and a Hall-mediated bi-modality in
turbulent properties extending from the region of thermal ionization to 10 au.Comment: accepted to MNRAS after very minor revision
Piloted-simulation study of effects of vortex flaps on low-speed handling qualities of a Delta-wing airplane
A piloted-simulation study was conducted to investigate the effects of vortex flaps on low-speed handling qualities of a delta-wing airplane. The simulation math model was developed from wind tunnel tests of a 0.15 scale model of the F-106B airplane. Pilot evaluations were conducted using a six-degree-of-freedom motion base simulator. The results of the investigation showed that the reduced static longitudinal stability caused by the vortex flaps significantly degraded handling qualities in the approach-to-landing task. Acceptable handling qualities could be achieved by limiting the aft center-of-gravity location, consequently reducing the operational envelope of the airplane. Further improvement were possible by modifying the flight control force-feel system to reduce pitch-control sensitivity
Supersonic combustor modeling
The physical phenomena involved when a supersonic flow undergoes chemical reaction are discussed. Detailed physical models of convective and diffusive mixing, and finite rate chemical reaction in supersonic flow are presented. Numerical algorithms used to solve the equations governing these processes are introduced. Computer programs using these algorithms are used to analyze the structure of the reacting mixing layer. It is concluded that, as in subsonic flow, exothermic heat release in unconfined supersonic flows retards fuel/air mixing. Non mixing is shown to be a potential problem in reducing the efficiency of supersonic as well as subsonic combustion. Techniques for enhancing fuel/air mixing and combustion are described
Non-collinear single-electron spin-valve transistors
We study interaction effects on transport through a small metallic cluster
connected to two ferromagnetic leads (a single-electron spin-valve transistor)
in the "orthodox model" for the Coulomb blockade. The non-local exchange
between the spin accumulation on the island and the ferromagnetic leads is
shown to affect the transport properties such as the electric current and
spin-transfer torque as a function of the magnetic configuration, gate voltage,
and applied magnetic field.Comment: 4 pages, 3 figure
- …