526 research outputs found
Von K\'arm\'an vortex street in a Bose-Einstein condensate
Vortex shedding from an obstacle potential moving in a Bose-Einstein
condensate is investigated. Long-lived alternately aligned vortex pairs are
found to form in the wake, as for the von K\'arm\'an vortex street in classical
viscous fluids. Various patterns of vortex shedding are systematically studied
and the drag force on the obstacle is calculated. It is shown that the
phenomenon can be observed in a trapped system.Comment: 4 pages, 5 figure
Order-disorder oscillations in exciton-polariton superfluids
The dynamics of an exciton-polariton superfluid resonantly pumped in a
semiconductor microcavity are investigated by mean-field theory. Modulational
instability develops into crystalline order and then ordered and disordered
states alternately form. A supersolid-like state is also found, in which
superflow coexists with crystalline order at rest.Comment: 5 pages, 3 figures, 6 movie
B\'enard-von K\'arm\'an vortex street in an exciton-polariton superfluid
The dynamics of an exciton--polariton superfluid resonantly injected into a
semiconductor microcavity are investigated numerically. The results reveal that
a B\'enard--von K\'arm\'an vortex street is generated in the wake behind an
obstacle potential, in addition to the generation of quantized vortex dipoles
and dark solitons. The vortex street is shown to be robust against a disorder
potential in a sample and it can be observed even in time-integrated
measurements.Comment: 4 pages, 3 figure
Pattern formation without heating in an evaporative convection experiment
We present an evaporation experiment in a single fluid layer. When latent
heat associated to the evaporation is large enough, the heat flow through the
free surface of the layer generates temperature gradients that can destabilize
the conductive motionless state giving rise to convective cellular structures
without any external heating. The sequence of convective patterns obtained here
without heating, is similar to that obtained in B\'enard-Marangoni convection.
This work present the sequence of spatial bifurcations as a function of the
layer depth. The transition between square to hexagonal pattern, known from
non-evaporative experiments, is obtained here with a similar change in
wavelength.Comment: Submitted to Europhysics Letter
Enhancement of thrust reverser cascade performance using aerodynamic and structural integration
This paper focuses on the design of a cascade within a cold stream thrust reverser during the early, conceptual stage of the product development process. A reliable procedure is developed for the exchange of geometric and load data between a two dimensional aerodynamic model and a three dimensional structural model. Aerodynamic and structural simulations are carried out using realistic operating conditions, for three different design configurations with a view to minimising weight for equivalent or improved aerodynamic and structural performance. For normal operational conditions the simulations show that total reverse thrust is unaffected when the performance of the deformed vanes is compared to the un-deformed case. This shows that for the conditions tested, the minimal deformation of the cascade vanes has no significant affect on aerodynamic efficiency and that there is scope for reducing the weight of the cascade. The pressure distribution through a two dimensional thrust reverser section is determined for two additional cascade vane configurations and it is shown that with a small decrease in total reverse thrust, it is possible to reduce weight and eliminate supersonic flow regimes through the nacelle section. By increasing vane sections in high pressure areas and decreasing sections in low pressure areas the structural performance of the cascade vanes in the weight reduced designs, is improved with significantly reduced levels of vane displacement and stress
Apparatus for real-time acoustic imaging of Rayleigh-Benard convection
We have designed and built an apparatus for real-time acoustic imaging of
convective flow patterns in optically opaque fluids. This apparatus takes
advantage of recent advances in two-dimensional ultrasound transducer array
technology; it employs a modified version of a commercially available
ultrasound camera, similar to those employed in non-destructive testing of
solids. Images of convection patterns are generated by observing the lateral
variation of the temperature dependent speed of sound via refraction of
acoustic plane waves passing vertically through the fluid layer. The apparatus
has been validated by observing convection rolls in both silicone oil and
ferrofluid.Comment: 20 pages, 11 figures, submitted to the Review of Scientific
Instrument
Phase instabilities in hexagonal patterns
The general form of the amplitude equations for a hexagonal pattern including
spatial terms is discussed. At the lowest order we obtain the phase equation
for such patterns. The general expression of the diffusion coefficients is
given and the contributions of the new spatial terms are analysed in this
paper. From these coefficients the phase stability regions in a hexagonal
pattern are determined. In the case of Benard-Marangoni instability our results
agree qualitatively with numerical simulations performed recently.Comment: 6 pages, 6 figures, to appear in Europhys. Let
Rhombic Patterns: Broken Hexagonal Symmetry
Landau-Ginzburg equations derived to conserve two-dimensional spatial symmetries lead to the prediction that rhombic arrays with characteristic angles slightly differ from 60 degrees should form in many systems. Beyond the bifurcation from the uniform state to patterns, rhombic patterns are linearly stable for a band of angles near the 60 degrees angle of regular hexagons. Experiments conducted on a reaction-diffusion system involving a chlorite-iodide-malonic acid reaction yield rhombic patterns in good accord with the theory.Energy Laboratory of the University of HoustonOffice of Naval ResearchU.S. Department of Energy Office of Basic Energy SciencesRobert A. Welch FoundationCenter for Nonlinear Dynamic
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