2,081 research outputs found
Supersonic Jet Noise Reduction by Coaxial Jets with Coplanar and Staggered Exits
Far-field noise radiated from coaxial cold underexpanded
jet flows issuing from convergent two-nozzle
configurations with coplanar and staggered-exits
is investigated experimentally. The coaxial
jets are operated in the "inverted" mode, i.e., the
outer (annular) jet flow Mach number is higher than
that of the inner (round) jet. Keeping all other
geometrical and operating conditions the same, the
exit-stagger of the inner (round) and the outer
(annular) nozzles was varied. It is shown that the
extent of the exit-stagger affects both the flows
and the radiated noise from such coaxial underexpanded
jet flows and that comparatively, the lowest
noise levels are achieved when the coaxial nozzle-exits
are coplanar. Moreover, the effectiveness
of the co-flowing inner jet flow in reducing the
noise radiated from either the annular or the coaxial
underexpanded jet flows decreases noticeably
as the exit-stagger is increased
A Closed Class of Hydrodynamical Solutions for the Collective Excitations of a Bose-Einstein Condensate
A trajectory approach is taken to the hydrodynamical treatment of collective
excitations of a Bose-Einstein condensate in a harmonic trap. The excitations
induced by linear deformations of the trap are shown to constitute a broad
class of solutions that can be fully described by a simple nonlinear matrix
equation. An exact closed-form expression is obtained for the solution
describing the mode {n=0, m=2} in a cylindrically symmetric trap, and the
calculated amplitude-dependent frequency shift shows good agreement with the
experimental results of the JILA group.Comment: RevTex, 4 pages, 1 eps figure, identical to the published versio
Expansion of a Bose-Einstein Condensate in an atomic waveguide
The expansion of a Bose-Einstein condensate in an atomic waveguide is
analyzed. We study different regimes of expansion, and identify a transient
regime between one-dimensional and three-dimensional dynamics, in which the
properties of the condensate and its further expansion can be well explained by
reducing the transversal dynamics to a two-level system. The relevance of this
regime in current experiments is discussed.Comment: 4 pages, 3 figs, Accepted for publication in Phys. Rev.
Excitations of a Bose-condensed gas in anisotropic traps
We investigate the zero-temperature collective excitations of a
Bose-condensed atomic gas in anisotropic parabolic traps. The condensate
density is determined by solving the Gross-Pitaevskii (GP) equation using a
spherical harmonic expansion. The GP eigenfunctions are then used to solve the
Bogoliubov equations to obtain the collective excitation frequencies and mode
densities. The frequencies of the various modes, classified by their parity and
the axial angular momentum quantum number, m, are mapped out as a function of
the axial anisotropy. Specific emphasis is placed upon the evolution of these
modes from the modes in the limit of an isotropic trap.Comment: 7 pages Revtex, 9 Postscript figure
Propagation inhibition and wave localization in a 2D random liquid medium
Acoustic propagation and scattering in water containing many parallel
air-filled cylinders is studied. Two situations are considered and compared:
(1) wave propagating through the array of cylinders, imitating a traditional
experimental setup, and (2) wave transmitted from a source located inside the
ensemble. We show that waves can be blocked from propagation by disorders in
the first scenario, but the inhibition does not necessarily imply wave
localization. Furthermore, the results reveal the phenomenon of wave
localization in a range of frequencies.Comment: Typos in Fiures are correcte
Violation of self-similarity in the expansion of a 1D Bose gas
The expansion of a 1D Bose gas is investigated employing the Lieb-Liniger
equation of state within the local density approximation. We show that during
the expansion the density profile of the gas does not follow a self-similar
solution, as one would expect from a simple scaling Ansatz. We carry out a
variational calculation, which recovers the numerical results for the
expansion, the equilibrium properties of the density profile, and the frequency
of the lowest compressional mode. The variational approach allows for the
analysis of the expansion in all interaction regimes between the mean field and
the Tonks-Girardeau limits, and in particular shows the range of parameters for
which the expansion violates self-similarity.Comment: 6 pages, 5 eps figure
Mean field effects in a trapped classical gas
In this article, we investigate mean field effects for a bosonic gas
harmonically trapped above the transition temperature in the collisionless
regime. We point out that those effects can play also a role in low dimensional
system. Our treatment relies on the Boltzmann equation with the inclusion of
the mean field term.
The equilibrium state is first discussed. The dispersion relation for
collective oscillations (monopole, quadrupole, dipole modes) is then derived.
In particular, our treatment gives the frequency of the monopole mode in an
isotropic and harmonic trap in the presence of mean field in all dimensions.Comment: 4 pages, no figure submitted to Phys. Rev.
Dynamics of two colliding Bose-Einstein condensates in an elongated magneto-static trap
We study the dynamics of two interacting Bose-Einstein condensates, by
numerically solving two coupled Gross-Pitaevskii equations at zero temperature.
We consider the case of a sudden transfer of atoms between two trapped states
with different magnetic moments: the two condensates are initially created with
the same density profile, but are trapped into different magnetic potentials,
whose minima are vertically displaced by a distance much larger than the
initial size of both condensates. Then the two condensates begin to perform
collective oscillations, undergoing a complex evolution, characterized by
collisions between the two condensates. We investigate the effects of their
mutual interaction on the center-of-mass oscillations and on the time evolution
of the aspect ratios. Our theoretical analysis provides a useful insight into
the recent experimental observations by Maddaloni et al., cond-mat/0003402.Comment: 8 pages, 7 figures, RevTe
Thermal compression of atomic hydrogen on helium surface
We describe experiments with spin-polarized atomic hydrogen gas adsorbed on
liquid He surface. The surface gas density is increased locally by
thermal compression up to cm at 110 mK. This
corresponds to the onset of quantum degeneracy with the thermal de-Broglie
wavelength being 1.5 times larger than the mean interatomic spacing. The atoms
were detected directly with a 129 GHz electron-spin resonance spectrometer
probing both the surface and the bulk gas. This, and the simultaneous
measurement of the recombination power, allowed us to make accurate studies of
the adsorption isotherm and the heat removal from the adsorbed hydrogen gas.
From the data, we estimate the thermal contact between 2D hydrogen gas and
phonons of the helium film. We analyze the limitations of the thermal
compression method and the possibility to reach the superfluid transition in 2D
hydrogen gas.Comment: 20 pages, 11 figure
- …