471 research outputs found
Origins and Structure of Spike-Type Rotating Stall
In this paper, we describe the structures that produce a spike-type route to rotating stall and explain the physical mechanism for their formation. The descriptions and explanations are based on numerical simulations, complemented and corroborated by experiments. It is found that spikes are caused by a separation at the leading edge due to high incidence. The separation gives rise to shedding of vorticity from the leading edge and the consequent formation of vortices that span between the suction surface and the casing. As seen in the rotor frame of reference, near the casing the vortex convects toward the pressure surface of the adjacent blade. The approach of the vortex to the adjacent blade triggers a separation on that blade so the structure propagates. The above sequence of events constitutes a spike. The computed structure of the spike is shown to be consistent with rotor leading edge pressure measurements from the casing of several compressors: the centre of the vortex is responsible for a pressure drop and the partially blocked passages associated with leading edge separations produce a pressure rise. The simulations show leading edge separation and shed vortices over a range of tip clearances including zero. The implication, in accord with recent experimental findings, is that they are not part of the tip clearance vortex. Although the computations always show high incidence to be the cause of the spike, the conditions that give rise to this incidence (e.g., blockage from a corner separation or the tip leakage jet from the adjacent blade) do depend on the details of the compressor
Origins and Structure of Spike-Type Rotating Stall
In this paper we describe the structures that produce a spiketype route to rotating stall and explain the physical mechanism for their formation. The descriptions and explanations are based on numerical simulations, complemented and corroborated by experiments. It is found that spikes are caused by a loss of pressure rise capability in the rotor tip region, due to flow separation resulting from high incidence. The separation gives rise to shedding of vorticity from the leading edge and the consequent formation of vortices that span between the suction surface and the casing. As seen in the rotor frame of reference, near the casing the vortex convects toward the pressure surface of the adjacent blade. The approach of the vortex to the adjacent blade triggers a separation on that blade so the structure propagates. The above sequence of events constitutes a spike. The simulations show shed vortices over a range of tip clearances including zero. The implication is that they are not part of the tip clearance vortex, in accord with recent experimental findings. Evidence is presented for the existence of these vortex structures immediately prior to spike-type stall and, more strongly, for their causal connection with spike-type stall inception. Data from several compressors are shown to reproduce the pressure and velocity signature of the spike-type stall inception seen in the simulations
Structure of the regulatory hyaluronan binding domain in the inflammatory leukocyte homing receptor CD44
Adhesive interactions involving CD44, the cell surface receptor for hyaluronan, underlie fundamental processes such as inflammatory leukocyte homing and tumor metastasis. Regulation of such events is critical and appears to be effected by changes in CD44 N-glycosylation that switch the receptor "on" or "off" under appropriate circumstances. How altered glycosylation influences binding of hyaluronan to the lectin-like Link module in CD44 is unclear, although evidence suggests additional flanking sequences peculiar to CD44 may be involved. Here we show using X-ray crystallography and NMR spectroscopy that these sequences form a lobular extension to the Link module, creating an enlarged HA binding domain and a formerly unidentified protein fold. Moreover, the disposition of key N-glycosylation sites reveals how specific sugar chains could alter both the affinity and avidity of CD44 HA binding. Our results provide the necessary structural framework for understanding the diverse functions of CD44 and developing novel therapeutic strategies
Direct numerical simulation of a high-pressure hydrogen micromix combustor: flame structure and stabilisation mechanism
A high-pressure hydrogen micromix combustor has been investigated using
direct numerical simulation with detailed chemistry to examine the flame
structure and stabilisation mechanism. The configuration of the combustor was
based on the design by Schefer [1], using numerical periodicity to mimic a
large square array. A precursor simulation of an opposed jet-in-crossflow was
first conducted to generate appropriate partially-premixed inflow boundary
conditions for the subsequent reacting simulation. The resulting flame can be
described as a predominantly-lean inhomogeneously-premixed lifted jet flame.
Five main zones were identified: a jet mixing region, a core flame, a
peripheral flame, a recirculation zone, and combustion products. The core
flame, situated over the jet mixing region, was found to burn as a thin
reaction front, responsible for over 85% of the total fuel consumption. The
peripheral flame shrouded the core flame, had low mean flow with high
turbulence, and burned at very lean conditions (in the distributed burning
regime). It was shown that turbulent premixed flame propagation was an
order-of-magnitude too slow to stabilise the flame at these conditions.
Stabilisation was identified to be due to ignition events resulting from
turbulent mixing of fuel from the jet into mean recirculation of very lean hot
products. Ignition events were found to correlate with shear-driven
Kelvin-Helmholtz vortices, and increased in likelihood with streamwise
distance. At the flame base, isolated events were observed, which developed
into rapidly burning flame kernels that were blown downstream. Further
downstream, near-simultaneous spatially-distributed ignition events were
observed, which appeared more like ignition sheets. The paper concludes with a
broader discussion that considers generalising from the conditions considered
here
Structural basis for complement factor H-linked age-related macular degeneration
This is the final version of the article. Available from the publisher via the DOI in this record.Nearly 50 million people worldwide suffer from age-related macular degeneration (AMD), which causes severe loss of central vision. A single-nucleotide polymorphism in the gene for the complement regulator factor H (FH), which causes a Tyr-to-His substitution at position 402, is linked to approximately 50% of attributable risks for AMD. We present the crystal structure of the region of FH containing the polymorphic amino acid His402 in complex with an analogue of the glycosaminoglycans (GAGs) that localize the complement regulator on the cell surface. The structure demonstrates direct coordination of ligand by the disease-associated polymorphic residue, providing a molecular explanation of the genetic observation. This glycan-binding site occupies the center of an extended interaction groove on the regulator's surface, implying multivalent binding of sulfated GAGs. This finding is confirmed by structure-based site-directed mutagenesis, nuclear magnetic resonance-monitored binding experiments performed for both H402 and Y402 variants with this and another model GAG, and analysis of an extended GAG-FH complex.B. Prosser is funded by the Wellcome Trust Structural Biology Training Program
(075415/Z/04/Z). S. Johnson and P. Roversi were funded by grants to S.M. Lea from
the Medical Research Council (MRC) of the United Kingdom (grants G0400389 and
G0400775). D. Uhrin and P.N. Barlow were funded by the Wellcome Trust (078780/
Z/05/Z). S.J. Clark was funded by an MRC Doctoral Training Account (G78/7925),
and R.B. Sim and A.J. Day were funded by MRC core funding to the MRC
Immunochemistry Unit
A Remarkable Three Hour Thermonuclear Burst From 4U 1820-30
We present a detailed observational and theoretical study of a ~3 hr long
X-ray burst (the ``super burst'') observed by the Rossi X-ray Timing Explorer
(RXTE) from the low mass X-ray binary (LMXB) 4U 1820-30. This is the longest
X-ray burst ever observed from this source, and perhaps one of the longest ever
observed in great detail from any source. We show that the super burst is
thermonuclear in origin. The level of the accretion driven flux as well as the
total energy release of ~1.5 x 10^{42} ergs indicate that helium could not be
the energy source for the super burst. We outline the physics relevant to
carbon production and burning on helium accreting neutron stars and present
calculations of the thermal evolution and stability of a carbon layer and show
that this process is the most likely explanation for the super burst. We show
that for large columns of accreted carbon fuel, a substantial fraction of the
energy released in the carbon burning layer is radiated away as neutrinos, and
the heat that is conducted from the burning layer in large part flows inward,
only to be released on timescales longer than the observed burst. Thus the
energy released possibly exceeds that observed in X-rays by more than a factor
of ten. Spectral analysis during the super burst reveals the presence of a
broad emission line between 5.8 - 6.4 keV and an edge at 8 - 9 keV likely due
to reflection of the burst flux from the inner accretion disk in 4U 1820-30. We
believe this is the first time such a signature has been unambiguously detected
in the spectrum of an X-ray burst.Comment: AASTEX, 44 pages, 14 figures. Accepted for publication in the
Astrophysical Journa
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