881 research outputs found
Ring diagram analysis of near-surface flows in the Sun
Ring diagram analysis of solar oscillation power spectra obtained from MDI
data is carried out to study the velocity fields in the outer part of the solar
convection zone. The three dimensional power spectra are fitted to a model
which has a Lorentzian profile in frequency and which includes the advection of
the wave front by horizontal flows, to obtain the two components of the
sub-surface flows as a function of the horizontal wave number and radial order
of the oscillation modes. This information is then inverted using OLA and RLS
methods to infer the variation in horizontal flow velocity with depth. The
average rotation velocity at different latitudes obtained by this technique
agrees reasonably with helioseismic estimates made using frequency splitting
data. The shear layer just below the solar surface appears to consist of two
parts with the outer part up to a depth of 4 Mm, where the velocity gradient
does not show any reversal up to a latitude of 60 degrees. In the deeper part
the velocity gradient shows reversal in sign around a latitude of 55 degrees.
The zonal flow velocities inferred in the outermost layers appears to be
similar to those obtained by other measurements. A meridional flow from equator
polewards is found. It has a maximum amplitude of about 30 m/s near the surface
and the amplitude is nearly constant in the outer shear layer.Comment: aastex, 12 figures, to appear in Ap.
Hermite regularization of the Lattice Boltzmann Method for open source computational aeroacoustics
The lattice Boltzmann method (LBM) is emerging as a powerful engineering tool
for aeroacoustic computations. However, the LBM has been shown to present
accuracy and stability issues in the medium-low Mach number range, that is of
interest for aeroacoustic applications. Several solutions have been proposed
but often are too computationally expensive, do not retain the simplicity and
the advantages typical of the LBM, or are not described well enough to be
usable by the community due to proprietary software policies. We propose to use
an original regularized collision operator, based on the expansion in Hermite
polynomials, that greatly improves the accuracy and stability of the LBM
without altering significantly its algorithm. The regularized LBM can be easily
coupled with both non-reflective boundary conditions and a multi-level grid
strategy, essential ingredients for aeroacoustic simulations. Excellent
agreement was found between our approach and both experimental and numerical
data on two different benchmarks: the laminar, unsteady flow past a 2D cylinder
and the 3D turbulent jet. Finally, most of the aeroacoustic computations with
LBM have been done with commercial softwares, while here the entire theoretical
framework is implemented on top of an open source library (Palabos).Comment: 34 pages, 12 figures, The Journal of the Acoustical Society of
America (in press
Testing Helioseismic-Holography Inversions for Supergranular Flows Using Synthetic Data
Supergranulation is one of the most visible length scales of solar convection
and has been studied extensively by local helioseismology. We use synthetic
data computed with the Seismic Propagation through Active Regions and
Convection (SPARC) code to test regularized-least squares (RLS) inversions of
helioseismic holography measurements for a supergranulation-like flow. The code
simulates the acoustic wavefield by solving the linearized three-dimensional
Euler equations in Cartesian geometry. We model a single supergranulation cell
with a simple, axisymmetric, mass-conserving flow.
The use of simulated data provides an opportunity for direct evaluation of
the accuracy of measurement and inversion techniques. The RLS technique applied
to helioseismic-holography measurements is generally successful in reproducing
the structure of the horizontal flow field of the model supergranule cell. The
errors are significant in horizontal-flow inversions near the top and bottom of
the computational domain as well as in vertical-flow inversions throughout the
domain. We show that the errors in the vertical velocity are due largely to
cross talk from the horizontal velocity.Comment: 22 pages, 12 figues, accepted for publication in Solar Physic
Impact of thixotropy on flow patterns induced in a stirred tank : numerical and experimental studies
Agitation of a thixotropic shear-thinning fluid exhibiting a yield stress is investigated both experimentally and via simulations. Steady-state experiments are conducted at three impeller rotation rates (1, 2 and 8 s−1) for a tank stirred with an axial-impeller and flow-field measurements are made using particle image velocimetry (PIV) measurements. Threedimensional numerical simulations are also performed using the commercial CFD code ANSYS CFX10.0. The viscosity of the suspension is determined experimentally and is modelled using two shear-dependant laws, one of which takes into account the flow instabilities of such fluids at low shear rates. At the highest impeller speed, the flow exhibits the familiar outward pumping action associated with axial-flow impellers. However, as the impeller speed decreases, a cavern is formed around the impeller, the flow generated in the vicinity of the agitator reorganizes and its pumping capacity vanishes. An unusual flow pattern, where the radial velocity dominates, is observed experimentally at the lowest stirring speed. It is found to result from wall slip effects. Using blades with rough surfaces prevents this peculiar behaviour and mainly resolves the discrepancies between the experimental and computational results
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