275,445 research outputs found
Particle Resuspension in Turbulent Boundary Layers and the Influence of Non-Gaussian Removal Forces
The work described is concerned with the way micron-size particles attached
to a surface are resuspended when exposed to a turbulent flow. An improved
version of the Rock'n'Roll model (Reeks and Hall, 2001) is developed where this
model employs a stochastic approach to resuspension involving the rocking and
rolling of a particle about surface asperities arising from the moments of the
fluctuating drag forces acting on the particle close to the surface. In this
work, the model is improved by using values of both the streamwise fluid
velocity andacceleration close to the wall obtained from Direct Numerical
Simulation (DNS) of turbulentchannel flow. Using analysis and numerical
calculations of the drag force on a sphere near a wall in shear flow (O'Neill
(1968) and Lee and Balachandar (2010)) these values are used to obtain the
joint distribution of the moments of the fluctuating drag force and its time
derivative acting on a particle attached to a surface. In so doing the
influence of highly non-Gaussian forces (associated with the sweeping and
ejection events in a turbulent boundary layer) on short and long term
resuspension rates is examined for a sparse monolayer coverage of particles,
along with the dependence of the resuspension upon the timescale of the
particle motion attached to the surface, the ratio of the rms/ mean of the
removal force and the distribution of adhesive forces. Model predictions of the
fraction resuspended are compared with experimental results.Comment: 31 pages 21 figure
Incompleteness of Representation Theory: Hidden symmetries and Quantum Non-Integrability
Representation theory is shown to be incomplete in terms of enumerating all
integrable limits of quantum systems. As a consequence, one can find exactly
solvable Hamiltonians which have apparently strongly broken symmetry. The
number of these hidden symmetries depends upon the realization of the
Hamiltonian.Comment: 4 pages, Revtex, Phys. Rev. Lett. , July 27 (1997), in pres
Correlation between floppy to rigid transitions and non-Arrhenius conductivity in glasses
Non-Arrhenius behaviour and fast increase of the ionic conductivity is
observed for a number of potassium silicate glasses with
potassium oxide concentration larger than a certain value .
Recovering of Arrhenius behaviour is provided by the annealing that enhances
densification. Conductivity furthermore obeys a percolation law with the same
critical concentration . These various results are the manifestation of
the floppy or rigid nature of the network and can be analyzed with constraint
theory. They underscore the key role played by network rigidity for the
understanding of conduction and saturation effects in glassy electrolytes.Comment: 4 pages, 4 EPS figure
The correlations between the twin kHz QPO frequencies of LMXBs
We analyzed the recently published kHz QPO data in the neutron star low-mass
X-ray binaries (LMXBs), in order to investigate the different correlations of
the twin peak kilohertz quasi-eriodic oscillations (kHz QPOs) in bright Z
sources and in the less luminous Atoll sources. We find that a power-law
relation \no\sim\nt^{b} between the upper and the lower kHz QPOs with
different indices: 1.5 for the Atoll source 4U 1728-34 and
1.9 for the Z source Sco X-1. The implications of our results for
the theoretical models for kHz QPOs are discussed.Comment: 6 pages, accepted by MNRA
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