371 research outputs found
Three-dimensional analysis of the Pratt and Whitney alternate design SSME fuel turbine
The three dimensional viscous time-mean flow in the Pratt and Whitney alternate design space shuttle main engine fuel turbine is simulated using the average passage Navier-Stokes equations. The migration of secondary flows generated by upstream blade rows and their effect on the performance of downstream blade rows is studied. The present simulation confirms that the flow in this two stage turbine is highly three dimensional and dominated by the tip leakage flow. The tip leakage vortex generated by the first blade persists through the second blade and adversely affects its performance. The greatest mixing of the inlet total temperature distortion occurs in the second vane and is due to the large leakage vortex generated by the upstream rotor. It is assumed that the predominant spanwise mixing mechanism in this low aspect ratio turbine is the radial transport due to the deterministically unsteady vortical flow generated by upstream blade rows. A by-product of the analysis is accurate pressure and heat loads for all blade rows under the influence of neighboring blade rows. These aero loads are useful for advanced structural analysis of the vanes and blades
Vortex trapping and expulsion in thin-film YBCO strips
A scanning SQUID microscope was used to image vortex trapping as a function
of the magnetic induction during cooling in thin-film YBCO strips for strip
widths W from 2 to 50 um. We found that vortices were excluded from the strips
when the induction Ba was below a critical induction Bc. We present a simple
model for the vortex exclusion process which takes into account the vortex -
antivortex pair production energy as well as the vortex Meissner and
self-energies. This model predicts that the real density n of trapped vortices
is given by n=(Ba-BK)/Phi0 with BK = 1.65Phi0/W^2 and Phi0 = h/2e the
superconducting flux quantum. This prediction is in good agreement with our
experiments on YBCO, as well as with previous experiments on thin-film strips
of niobium. We also report on the positions of the trapped vortices. We found
that at low densities the vortices were trapped in a single row near the
centers of the strips, with the relative intervortex spacing distribution width
decreasing as the vortex density increased, a sign of longitudinal ordering.
The critical induction for two rows forming in the 35 um wide strip was (2.89 +
1.91-0.93)Bc, consistent with a numerical prediction
Behavior of vortices near twin boundaries in underdoped
We use scanning SQUID microscopy to investigate the behavior of vortices in
the presence of twin boundaries in the pnictide superconductor
Ba(Fe1-xCox)2As2. We show that the vortices avoid pinning on twin boundaries.
Individual vortices move in a preferential way when manipulated with the SQUID:
they tend to not cross a twin boundary, but rather to move parallel to it. This
behavior can be explained by the observation of enhanced superfluid density on
twin boundaries in Ba(Fe1-xCox)2As2. The observed repulsion from twin
boundaries may be a mechanism for enhanced critical currents observed in
twinned samples in pnictides and other superconductors
Limits on the Superconducting Order Parameter in NdFeAsOF from Scanning SQUID Microscopy
Identifying the symmetry of the superconducting order parameter in the
recently-discovered ferro-oxypnictide family of superconductors,
RFeAsOF, where is a rare earth, is a high priority. Many of
the proposed order parameters have internal phase shifts, like the d-wave
order found in the cuprates, which would result in direction-dependent phase
shifts in tunnelling. In dense polycrystalline samples, these phase shifts in
turn would result in spontaneous orbital currents and magnetization in the
superconducting state. We perform scanning SQUID microscopy on a dense
polycrystalline sample of \NdFeAsOF with K and find
no such spontaneous currents, ruling out many of the proposed order parameters.Comment: 10 pages, 5 figures; to appear in JPS
Vortex trapping and expulsion in thin-film type-II superconducting strips
Vortex trapping is investigated in thin-film strips of superconducting
material. We present a model for the critical field above which vortex trapping
occurs in these strips. This model includes the pairing energy of
vortex-antivortex pairs in addition to the energy of single vortices.
Experimental verification of the model with a scanning SQUID microscope shows
very good agreement between the model and experiments on YBa2Cu3O7-delta and Nb
strips. Statistical analysis of the vortex distribution in the strips above the
critical field has been performed and a comparison has been made between Nb and
YBa2Cu3O7-delta for the distributions in the lateral and longitudinal
directions
Induced paramagnetic states by localized -loops in grain boundaries
Recent experiments on high-temperature superconductors show paramagnetic
behavior localized at grain boundaries (GB). This paramagnetism can be
attributed to the presence unconventional d-wave induced -junctions. By
modeling the GB as an array of and conventional Josephson junction we
determine the conditions of the occurrence of the paramagnetic behavior.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
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