20 research outputs found
Particle Image Velocimetry Measurements over an Aerodynamically Open Two-Dimensional Cavity
An experimental study has been undertaken to characterise the low Reynolds number flow over an aerodynamically open two-dimensional cavity of rectangular cross-section. A series of water tunnel experiments have been performed in which the flow field was initially examined using dye flow visualisation, then particle image velocimetry (PIV) was used to obtain quantitative velocity measurements. For a laminar upstream boundary layer, the cavity flow structure has been characterised for a cavity length-to-depth ratio of 5 and Reynolds numbers, based on cavity depth, of 990, 1930, 3780 and 5710. Three different flow regimes were identified over the Reynolds number range tested. The mean PIV flow fields are presented and are found to compare well with the flow visualisation study and existing PIV literature
Gravitational radiation from nonaxisymmetric spherical Couette flow in a neutron star
The gravitational wave signal generated by global, nonaxisymmetric shear
flows in a neutron star is calculated numerically by integrating the
incompressible Navier--Stokes equation in a spherical, differentially rotating
shell. At Reynolds numbers \Rey \gsim 3 \times 10^{3}, the laminar Stokes
flow is unstable and helical, oscillating Taylor--G\"ortler vortices develop.
The gravitational wave strain generated by the resulting kinetic-energy
fluctuations is computed in both and polarizations as a function
of time. It is found that the signal-to-noise ratio for a coherent,
-{\rm s} integration with LIGO II scales as for a star at 1 {\rm kpc} with angular velocity
. This should be regarded as a lower limit: it excludes pressure
fluctuations, herringbone flows, Stuart vortices, and fully developed
turbulence (for \Rey \gsim 10^{6}).Comment: (1) School of Physics, University of Melbourne, Parkville, VIC 3010,
Australia. (2) Departamento de Fisica, Escuela de Ciencias,Universidad de
Oriente, Cumana, Venezuela, (3) Department of Mechanical Engineering,
University of Melbourne, Parkville, VIC 3010, Australia. Accepted for
publication in The Astrophysical Journal Letter
Superfluid spherical Couette flow
We solve numerically for the first time the two-fluid,
Hall--Vinen--Bekarevich--Khalatnikov (HVBK) equations for a He-II-like
superfluid contained in a differentially rotating, spherical shell,
generalizing previous simulations of viscous spherical Couette flow (SCF) and
superfluid Taylor--Couette flow. In axisymmetric superfluid SCF, the number of
meridional circulation cells multiplies as \Rey increases, and their shapes
become more complex, especially in the superfluid component, with multiple
secondary cells arising for \Rey > 10^3. The torque exerted by the normal
component is approximately three times greater in a superfluid with anisotropic
Hall--Vinen (HV) mutual friction than in a classical viscous fluid or a
superfluid with isotropic Gorter-Mellink (GM) mutual friction. HV mutual
friction also tends to "pinch" meridional circulation cells more than GM mutual
friction. The boundary condition on the superfluid component, whether no slip
or perfect slip, does not affect the large-scale structure of the flow
appreciably, but it does alter the cores of the circulation cells, especially
at lower \Rey. As \Rey increases, and after initial transients die away,
the mutual friction force dominates the vortex tension, and the streamlines of
the superfluid and normal fluid components increasingly resemble each other. In
nonaxisymmetric superfluid SCF, three-dimensional vortex structures are
classified according to topological invariants.Comment: Accepted for publication in the Journal of Fluid Mechanic
Global three-dimensional flow of a neutron superfluid in a spherical shell in a neutron star
We integrate for the first time the hydrodynamic
Hall-Vinen-Bekarevich-Khalatnikov equations of motion of a -paired
neutron superfluid in a rotating spherical shell, using a pseudospectral
collocation algorithm coupled with a time-split fractional scheme. Numerical
instabilities are smoothed by spectral filtering. Three numerical experiments
are conducted, with the following results. (i) When the inner and outer spheres
are put into steady differential rotation, the viscous torque exerted on the
spheres oscillates quasiperiodically and persistently (after an initial
transient). The fractional oscillation amplitude () increases
with the angular shear and decreases with the gap width. (ii) When the outer
sphere is accelerated impulsively after an interval of steady differential
rotation, the torque increases suddenly, relaxes exponentially, then oscillates
persistently as in (i). The relaxation time-scale is determined principally by
the angular velocity jump, whereas the oscillation amplitude is determined
principally by the gap width. (iii) When the mutual friction force changes
suddenly from Hall-Vinen to Gorter-Mellink form, as happens when a rectilinear
array of quantized Feynman-Onsager vortices is destabilized by a counterflow to
form a reconnecting vortex tangle, the relaxation time-scale is reduced by a
factor of compared to (ii), and the system reaches a stationary state
where the torque oscillates with fractional amplitude about a
constant mean value. Preliminary scalings are computed for observable
quantities like angular velocity and acceleration as functions of Reynolds
number, angular shear, and gap width. The results are applied to the timing
irregularities (e.g., glitches and timing noise) observed in radio pulsars.Comment: 6 figures, 23 pages. Accepted for publication in Astrophysical
Journa
Vortex pseudomomentum and dissipation in a superfluid vortex lattice
We propose an alternative approach to the dissipative vortex dynamics
occurring in a superfluid vortex lattice at finite temperatures. Focusing upon
the pseudomomentum of a vortex and its surrounding quasiparticles, we derive an
equation of motion which, in spite of yielding the same evolution as the usual
one for massless vortices, does not involve the vortex mass. This picture could
provide further insights into the controversy about the nature of the vortex
mass.Comment: 11 pages, no figures. Typo corrected in equation (28
Transitions between turbulent and laminar superfluid vorticity states in the outer core of a neutron star
We investigate the global transition from a turbulent state of superfluid
vorticity to a laminar state, and vice versa, in the outer core of a neutron
star. By solving numerically the hydrodynamic Hall-Vinen-Bekarevich-Khalatnikov
equations for a rotating superfluid in a differentially rotating spherical
shell, we find that the meridional counterflow driven by Ekman pumping exceeds
the Donnelly-Glaberson threshold throughout most of the outer core, exciting
unstable Kelvin waves which disrupt the rectilinear vortex array, creating a
vortex tangle. In the turbulent state, the torque exerted on the crust
oscillates, and the crust-core coupling is weaker than in the laminar state.
This leads to a new scenario for the rotational glitches observed in radio
pulsars: a vortex tangle is sustained in the differentially rotating outer core
by the meridional counterflow, a sudden spin-up event brings the crust and core
into corotation, the vortex tangle relaxes back to a rectilinear vortex array,
then the crust spins down electromagnetically until enough meridional
counterflow builds up to reform a vortex tangle. The turbulent-laminar
transition can occur uniformly or in patches; the associated time-scales are
estimated from vortex filament theory. We calculate numerically the global
structure of the flow with and without an inviscid superfluid component, for
Hall-Vinen and Gorter-Mellink forms of the mutual friction. We also calculate
the post-glitch evolution of the angular velocity of the crust and its time
derivative, and compare the results with radio pulse timing data, predicting a
correlation between glitch activity and Reynolds number.Comment: (1) School of Physics, University of Melbourne, Parkville, VIC 3010,
Australia. (2) Departamento de Fisica, Escuela de Ciencias,Universidad de
Oriente, Cumana, Venezuela, (3) Department of Mechanical and Manufacturing
Engineering, University of Melbourne, Parkville, VIC 3010, Australia.
Accepted for publication in The Astrophysical Journal. 30 pages, 9 figures
(in jpg format
Joint NRC/DSTO Program on Nonplanar High-Alpha Aerodynamic Modelling
NRC publication: Ye